Manufacture of acetic anhydride
1. Introducti Introduction on :Acetic anhydride also known as ethanoic anhydride or methyl carboxylic anhydride is a colourless liquid, very similar to acetic acid in its pungent, acrid odour, viscosity, density & refractive index. It does not occur naturally & was 1 st synthesized by C.F.Gerhardt in 1852 by the reaction of benzoil chloride & molten potassium acetate. Today it is one of the most important organic intermediates & is widely used in both research & industry. About 40% of the acetic anhydride produced throughout the world is used for the production of Vinyl acetate monomer(VAM) which is used for the production of downstream products such as adhesives, textiles, and paints. Apart from this acetic anhydride has many other utilities such as in the manufacture of “Aspirin” an important medicine, in the production of acetanilide which is used as a starting material in the manufacture of some sulpha sulpha drugs, drugs, it is also also useful useful in manufa manufactu cture re of perfum perfumes, es, herbic herbicide ides, s, acetyl acetyl peroxide bleach & plasticizers. plasticizers.
L.I.T. Nagpur
Manufacture of acetic anhydride
Production of acetic anhydride (in kt) :Country United
1961 571
1971 686
1974 741
1979 -
1980 -
1981 567
1982 481
1989 778
1990 830
1996 1000
2001 1160
States Germany Japan
32 33
47 96
74 115
91 114
85 150
77 145
76 144
108 205
112 1160
-
-
Major producers in India :Sr. No. 1
Manufacturer VAM Organics Ltd,
Capacity(TPA) 33000
2
Andhra sugars Ltd, Andhra .
3058
3
Ashok Brothers, Mumbai
6600
4
IOL chemicals & pharmaceuticals Ltd, Punjab
12000
5
Vasantdada Shetkari SSK
3000
6
Trichy distillers & chemicals Ltd, Trichy
2,100
7
R L G Group of industries, Gujrat
10000
8
Naran Lala Private Ltd, Gujrat
-
9
FISCHER Chemic Ltd, Chennai
-
10
Mysore Acetate & chemicals Ltd., Mysore 6000
L.I.T. Nagpur
Manufacture of acetic anhydride
Markets :Vinyl Acetate Monomer (VAM) :-
It is the largest consumer of acetic anhydride and constitutes nearly 40% of the demand. VAM is usually used in the downstream products pr oducts such as adhesives, textiles, and paints. Drugs/ Pharma :-
Drugs and pharmaceuticals constitute 18% of the total production. It is used in the manufacturing of Aspirin. About 0.8 tons of anhydride is required to produce 1 ton of Aspirin. This constitutes 3% of the total production. Manufacturing 0.9 ton of Paracetamol requires 1 ton of acetic anhydride and it constitutes 9% of the total demand. Vitamins constitute nearly 1% of the total demand and intermediates like MCA account for 5% of total demand. Cellulose acetate :-
These account for nearly 9 % of the total demand for anhydride. Cellulose acetate. is used used in the the down downst strea ream m prod produc ucts ts such such as ciga cigare rett ttee filt filter ers. s. Abou Aboutt 1 ton ton of anhydride is required to produce 1 ton of the cellulose acetate.
L.I.T. Nagpur
Manufacture of acetic anhydride
Dyes and pigments :-
This constitutes about 18% of the total production.
Capacity structure :VAM organics is the largest producer with an installed capacity of 33,000 ton per annum. Ashok organic, Mysore acetate and IOL have smaller capacities. The total installed capacity is around 62,000 ton per annum whereas the demand hovers around 42000 ton per annum. Most of the consumption is captive.
Demand and pricing :Dema Demand nd has has seen seen a stea steady dy rise rise,, but but the the capa capaci city ty util utiliz izat atio ion n has has been been arou around nd 65-70%. The main reason for the slow growth has been the plant change-overs where anhydride capacity can be used for acetic acid production.
L.I.T. Nagpur
Manufacture of acetic anhydride
The prices are dependent on the prices of the raw material namely acetic acid. The import duty for acetic anhydride was reduced from 65% in 1994-95 to 40% in 1996-97. Unlike acetic acid domestic prices were pegged slightly lower than the landed costs. All the major manufacturers of acetic anhydride also manufacture acetic acid. Hence the sizeable difference between international and domestic prices of acetic acid allows the producers to peg the prices of acetic anhydride just below the landed cost. cost.
Outlook
The The dema demand nd is expe expect cted ed arou around nd 5500 55000 0 tonn tonnes es this this year year.. Howe Howeve ver, r, with with the the reducing of import duty the flexibility of the local manufacturers will be eroded. The declining international prices of acetic acid have resulted in the reduction of the prices of the anhydrid ride. This has resulted in the reduction of the competitiveness among domestic producers.
Import :Market estimated at Rs. 90 crs. Import not high - around 3%
Price :Historical (1994 - 1999): 1999): 45 Rs./kg , Current: 50 Rs/kg.
L.I.T. Nagpur
Manufacture of acetic anhydride
1.1 1. 1 Hist Histor oric ical al bac backg kgro roun und d :The oldest process for making acetic anhydride is based on the conversion of sodium acetate with the excess of an inorganic chloride such as thionyl chloride, sulfuryl chloride or phosphoryl chloride. In this process half of the sodium acetate is converted to acetyl chloride, which then reacts with the remaining sodium acetate to form acetic anhydride as follows:CH3COONa + X-Cl
→
CH 3COCl + X-ONa
CH3COONa + CH 3COCL → (CH3CO)2O + NaCl Where, X= SOCl, SO 2Cl, POCL2 A further development, the conversion of acetic acid with phosgene in the presence of aluminium chloride, has the advantage that it allows continous operation. 2CH3COOH + COCl 2
→
(CH3CO) 2O + 2HCl + CO 2
Two other methods also were used in the past: The cleavage of ethylidene diacetate to form acetaldehyde and acetic anhydride in the presence of acid catalyst such as zinc chloride & the second method is by the reaction of vinyl acetate with acetic acid on palladium to form acetaldehude and acetic anhydride. None of these processes is having any industrial importance. Today, acetic anhydride is made mostly by either the ketene process or the oxidation of acetaldehyde. Production by another process, the carbonylat carbonylation ion of methyl methyl acetate acetate (Halcon (Halcon process ) was begun in 1983. 1983. In western Europe, 77% of the acetic anhydride is made by the ketene processand rest 23% by L.I.T. Nagpur
Manufacture of acetic anhydride
the oxidation of acetaldehyde. In United States 25% of the acetic anhydride is made by the Halcon Process & the rest 75% by the ketene process.
1.2. Physical Properties of Raw Material:Acetic Acid :
Auto-ignition temperature (°C)
565
Boiling point at 760mm Hg (°C)
118.1
Colour
colourless
Critical pressure, (atm)
57.2
Critical temperature, (°C)
321.6
Molecular Weight
60.0530
Heat of Combustion (Kcal/mole)
-208.7
Heat of Formation (Kcal/mole)
-116.2
Heat of Fusion (cal/g)
44.7
Heat of Vapourisation (cal/g)
96.8
Heat of Solution at 18°C (Kcal/mole)
0.375
Refractive index
1.3718
Specific Heat at 0°C (cal/g)
0.468
Surface Tension at 20°C in air, (dyne/cm)
27.6
Taste
acrid
Viscosity (cp)
At 20°C
1.18
At 40°C
0.82
Specific Gravity
1.049
L.I.T. Nagpur
Manufacture of acetic anhydride
1.3.
Physical Properties Of The Product :-
Acetic Anhydride :-
Property
Vapour
Liquid
Molecular Weight
102.090
102.090
Melting Point (°C)
-
-73
Normal Boiling Point (°C)
-
139
Specific Gravity
Coefficient of expansion (20°C)
-
Critical Temperature (°C)
-
326
-
Critical Pressure (atm)
-
43
-
Critical Volume (cc/g-mol)
-
290
-
Surface Tension (dyne/cm, 20°C) air
-
33
Viscosity (cp, 20°C)
0.008
0.91
Specific Heat (cal/g °C)
0.23
0.434
Heat of Fusion (cal/g)
-
24.6
Heat of Vapourisation (at NBP, cal/g)
-
93
ΔHf ° (cal/g) at 25°C
-1347.8
-1460.9
ΔGf °(cal/g) °(cal/g) at 25°C
-1116.0
-1144.8
Heat of Hydrolysis (cal/g) at 25°C
-
136.9
3.52
L.I.T. Nagpur
1.084 0.00112
Manufacture of acetic anhydride
1.4. Chemical Properties Of Acetic Anhydride :
On chlorination it produces chloro-acetyl chloride. In addition small quantities of dichlo dichloro-a ro-acet cetyl yl chlori chloride, de, acetyl acetyl chlori chloride, de, chloro chloro-ac -aceti eticc acid acid & HCl are formed.
(CH3CO)2O
+
Cl2
→
Acetic anhydride
Cl-CH2COCl
+ CH3COOH
chloro-acetyl
Acetic acid
chloride
On reaction with hydrogen chloride under pressure it gives acetyl chloride.
(CH3CO)2O
+
HCl
Acetic anhydride
→
CH3OCl acetyl chloride
+ CH3COOH Acetic acid
It undergoes hydrolysis slowly with water but rapidly hydrolysed with alkali to form acetic acid.
(CH3CO)2O Acetic anhydride
+
H2 O
→ 2CH3COOH Acetic acid
L.I.T. Nagpur
Manufacture of acetic anhydride
On reaction with acetaldehyde it forms ethylidene di-acetate.
(CH3CO)2O Acetic anhydride
+
CH3CHO
→ CH3CH(OCOCH3)2
Acetaldehyde
Ethylidene di-acetate
1.5. Storage Storage & Trans Transporta portation tion ::-
For storage & transporta transportation tion of pure acetic anhydride tanks made of aluminium, stainless ste.6el ( 18% Cr, 8% Ni & 2% Mo ) or poly-ethylene are generally used. Although glass or enamel containers also may be employed. Iron is highly resistant to acetic anhydride, provided moisture is excluded. Hence it is possible to use iron in the production & workup in certain instances for example in pumps & tanks.
1.6.
Health & Safety Aspects :-
Acetic anhydride penetrates the skin quickly and painfully forming burns and blisters that are slow to heal. Anhydride is especially dangerous to the delicate tissues of the eyes, ears, nose & mouth. The threshold value for eyes is 0.36 mg/m 3. When handling acetic anhydride, rubber gloves that are free of pinholes are recommended for the hands, as well as plastic goggles for the eyes, and Face-marks to cover the face and ears.
L.I.T. Nagpur
Manufacture of acetic anhydride
Acetic acid is dangerous in combination with various oxidizing substances and strong acids. Chromium trioxide and anhydride react violently to burn, thermal decomposition of nitric acid in acetic acid is accelerated by the presence of anhydride. anhydride.
1.7.
Applications :-
The The bigg bigges estt use use of acet acetic ic anhy anhydr drid idee is in the the manuf anufac actu ture re of cell cellul ulos osee acetates( about 86% of world production ). Acetates produced include cellulose acetate, cellulose di-acetate, cellulose tri-acetate, cellulose acetate propionate & cellulose butyrate. The remaining 14% is consumed in various miscellaneous uses which are as given below.
It is used used in the produc productio tion n of polyme polymethy thyl-ac l-acyla ylamid midee or hard hard foam, foam, acetic acetic anhydride is used for binding ammonia that is liberated on conversion of two amide groups to an imide group.
It is used in dyeing industry, where acetic anhydride is used chiefly in mixtures with nitric acid as a nitrating agent. Here the solvent and dehydrating properties of acetic anhydride are used.
L.I.T. Nagpur
Manufacture of acetic anhydride
It is used in the manufacture of various organic intermediates such as chloroacetyl chloride, di-acetyl peroxide, higher carboxylic anhydrides, acetates and boron tri-fluoride complex.
It is used in the manufactu manufacture re of certain pharmaceutica pharmaceuticall products products such as acetyl salicylic salicylic acid(aspir acid(aspirin), in), p-acetyl p-acetyl amino phenol, phenol, acetanilid acetanilide, e, acetophena acetophenacetin cetin,, theophyllin, sulfamides, a number of hormones & vitamins.
It is used used in the the dete deterg rgen entt indu indust stry ry for for the the prod produc ucti tion on of cold cold-b -ble leac achi hing ng activators such as tetra-acetyl-ethylene diamine.
It is used in the manufacture of explosives, particularly hexogen production.
It is used in the manufacture of acetylated plastic auxiliaries such as glycerol tri-acetate acetyl tri-butyl citrate & acetyl ricinolate.
It is used in the food industry, mainly in the acetylation of animal & plant fats in order to obtain the desired solubilities, in the production of acetostearin, in the edible packing materials & to clarify plant oils.
It is used in the manufacture of flavours & fragrances.
It is used n metallography, etching & polishing of metals and in semiconductor manufacture.
L.I.T. Nagpur
Manufacture of acetic anhydride
Small % of acetic anhydride in acetic acid or cold water solutions are used as powerful fungicides & herbicides.
2.
Dif Differe feren nt Ro Routes tes Of Of Ma Manufa nufact ctur ure e ::-
(a) Acetaldehyde oxidation :Acetaldehyde oxidation for the production of acetic anhydride co-produces acetic acid. The reaction conditions are about 60ºC at 1atm pressure and 70ºC at 6atm. Oxygen or air is employed for the oxidation purpose in the presence of cobalt acetate catalyst promoted by copper acetate. The reactions taking place are :-
CH3CHO
+
O2
→
Acetaldehyde
CH3COOOH Peracetic acid
CH3COOOH peracetic acid
+
CH3CHO
→
Acetic acid
L.I.T. Nagpur
(CH3CO)2O
+ H2O
Acetic anhydride
Manufacture of acetic anhydride
(CH3CO)2O
+
H2 O
→
2CH3COOH
Acetic anhydride
Acetic acid
The last reaction is to be minimized if acetic anhydride yield is to be maximum. Overall selectivity of acetaldehyde plus the acetic acid is maintained at 95-98% while the weight ratio of overall yields can be from 0.5-9 (anhydride to acid ). The higher ratios require a vapor product from the reactor to rid the product mixture of the catalyst quickly. An azeotropic solvent, such as ethyl acetate also enhances water vaporization from the reaction r eaction zone. Heat of reaction is adequate to vaporize the product and unconverted acetaldehyde, but a high recycle of low oxygen content off-gas is required for stripping because of the low vapor pressure of the reaction products.
(b) Methyl Acetate Carbonylation :Acetic anhydride can be made by the carbonylation of meth methyl yl acet acetat ate. e. Meth Methan anol ol acet acetyl ylat atio ion n is an esse essent ntia iall 1 st step step in anhydr anhydride ide manufacture by carbonylation, the reactions taking place are :-
CH3COOH
+
CH3OH
→
CH 3COOCH3
+
H2O , ∆H= -4.89
KJ/mol Acetic acid
Methanol
CH3COOCH3 + CO Methyl acetate
Methyl acetate
→
(CH3CO)2O ,
∆H= -94.8 KJ/mol
Acetic anhydride
L.I.T. Nagpur
Manufacture of acetic anhydride
The catalyst stream for the methyl acetate carbonylation process involves rhodium chloride tri-hydrate, methyl iodide, chromium metal powder and alumina support or nickel carbonyl complex with tri-phenyl phosphine, methyl iodide and chromium hexa-carbonyl. In another, the alumina catalyst support is treated with an organosilicon compound having either a terminal organopho organophosphin sphinee or similar ligands ligands and rhodium rhodium or a similar similar noble metal. metal. Such a catalyst enabled methyl acetate carbonylation at 200ºC under 20 MPa pressure. Conversion is 42.8% with the 97.5% selectivity. In anhydride purification, iodide purification is of considerable significance, potassium acetate is employed for this purpose. Because of the presence of iodide in the reaction system, titanium is the most suitable material of construction.
(c) Ketene processes :(i )
Acetone Cracking :-
Acetic anhydride can be manufactured by acetone cracking. In this process acetone is 1 st cracked to ketene and in the next step ketene reacts with acetic acid to form acetic anhydride. The 1 st step of the reaction is carried out in a pyrolysis heater at about 700ºC and 1.5 atm pressure. The reaction goes to achieve 20-25% of acetone and 70-75% selectivity to ketene. The reaction taking place are :-
CH3-CO-CH3 Acetone
→
CH2=CO Ketene
L.I.T. Nagpur
+
CH4 Methane
Manufacture of acetic anhydride
CH2=CO
+ CH3COOH
Ketene
Acetic acid
→
(CH 3CO)2O +
H20
Acetic anhydride
Quenching of the high-temperature reaction by evapor evaporati ating ng an inject injected ed mixtu mixture re of acetic acetic acid acid and acetic acetic anhydr anhydride ide precee preceeds ds cooling and ketene absorption by acetic acid. At the available pressure, chilling is unnecessary, however both excess acetone and ketene must be absorbed from a relatively large volume of gas. Water formed inside the reaction leads to some hydrolysis of acetic anhydride. Water wash of the vent gas recovers acetic acid vapor and recycle acetone.
(ii)
Acetic Acid Dehydration :-
Acetic anhydride can be manufactured by the thermal decomposition of acetic acid. The 1 st step of the reaction is the dehydration of acetic acid at pressures of about 15-20 KPa and temperature of about 700ºC to form ketene, the 2 nd step involves the reaction of ketene with acetic acid to form acetic anhydride at a temperature of about 50ºC. The reactions taking place are :-
CH3COOH Acetic acid
→
CH2=CO
+
H20 , Ketene
L.I.T. Nagpur
∆H=147 KJ/mol
Manufacture of acetic anhydride
CH2=CO Ketene
+
CH3COOH
→
Acetic acid
(CH3CO)2 ,
∆H= -63 KJ/mol
Acetic anhydride
Tri-ethyl phosphate is commonly used as a dehydration catalyst for the water formed in the 1 st step. It is neutralized in the exit gases with ammonia. Aqueous 30% ammonia is employed as a solvent in the second step because water facilitates the reaction, and the small amount of water introduced introduced is not significanr significanr overall. overall. Nickel-free Nickel-free alloys for example, example, ferrochrome ferrochrome alloy, alloy, chrom chrome-al e-alumi uminiu nium m steel, steel, are needed needed for the acetic acetic acid acid pyroly pyrolysis sis tubes, tubes, because nickel promotes the formation of soot and coke, and reacts with carbon mono monoxi xide de yiel yieldi ding ng a high highly ly toxi toxicc meta metall carb carbon onyl yl.. Conv Conven enti tion onal al oper operat atin ing g conditions furnish 85-88% conversion, selectivity to ketene is 90-95%.
3.
Process Se Selection ::-
(a)
Acetone process :-
It can be used only when acetone is relatively expensive, but the major disadvantages are that methane is formed during the process which is very harmful and extra measures are to be taken to remove it. Coke formation at the severe conditions is more of a problem than with acetic acid dehydration process, also low conversion demands more heater duty for the ketene produced.
(b)
Acetaldehyde ox oxidation ::-
L.I.T. Nagpur
Manufacture of acetic anhydride
Acetic anhydride formed during the process may undergo hydrolysis to form acetic acid according to the reaction :-
(CH3CO)2
+
H2O
→
2CH3COOH
Acetic anhydride
Acetic acid
This reaction is to be minimized in order to achieve the maximum yield of acetic anhydride. Also acetaldehyde is to be manufactured 1st because it is not available directly, this makes the process uneconomical & the cost of the production increases.
(c)
Meth ethyl Ace Acetat tate Car Carbo bon nyla ylatio tion ::-
Catalyst recovery is the major operating problem because rhodium is very costly metal & every trace must be recovered, otherwise it may lead to a major economic loss. Hence additional process would be required for the recovery of the catalyst which makes the process a bit expensive. Also the process is still in a developing phase and only 15% of the acetic anhydride produced in the world is being manufactured using this process.
(d)
Acetic Ac Acid De Dehydration ::-
L.I.T. Nagpur
Manufacture of acetic anhydride
Ketene reacts readily with acetic acid to produce acetic anhydride hence the process is economically and practically more viable this is the why about 85% of the acetic anhydride produced in the world is being manufactured by this process. Also the raw material required is only acetic acid which is readily available which makes the process more economical.
Thus from above considerations it is clear that the acet acetic ic acid acid proc proces esss is the the one one whic which h is gene genera rall lly y empl employ oyed ed beca becaus usee it is econom economica ically lly and practi practical cally ly more more viabl viable. e. Thus Thus acetic acetic acid acid proces processs is being being sele select cted ed taki taking ng in to acco accoun untt its its adva advant ntag ages es than than the the othe otherr proce rocess sses es of manufacture.
4.
Process De Description ::-
(i)
Principle :-
Production of acetic anhydride from acetic acid comprises of two steps :-
(a) (a)
Pyro Pyroly lysi siss of of acet acetic ic acid acid to form form kete ketene ne..
(b) (b)
Reac Reacti tion on of of kete ketene ne obta obtain ined ed with with acet acetic ic aci acid d
L.I.T. Nagpur
Manufacture of acetic anhydride
The first conversion which is highly endothermic is carried out in vapor phase at high temperature at about 700ºC, and at reduced pressure about 10-20 KPa, very short residence time in the neighborhood of 1 sec, and the presence of catalyst, serve to limit the formation of by products.The catalyst system employed for dehydration are usually organic phosphates (triethyl, tri-cresyl, di-methyl ammonium, pyridium phosphates, etc.) which are added directly and continuously in to the gas feed stream, at the rate of 0.20.5% weight. The addition of water in small concentrations(10% weight) to the acetic acid offers similar advantage to those procured in steam cracking. In particular it slows down the formation of coke. The addition of small amounts of ammonia (< 1000 ppm) exerts an indirect inhibiting effect on the polymerization of ketene. If these precautions are taken then once-through conversion is about 85-90% and the molar yield 90-95%.
The second conversion, which is exothermic, can be can be carried out in the absence of catalyst, by absorption in acetic acid, between 45-55ºC, at reduced pressure 7-25 KPa. Higher temperatures and pressures facilitate the dimerisation of ketene to di-ketene, whose boiling point is 127.4ºC which is fairly close to that of anhydride. Less than 2% weight is generally formed, so that the yield of operation, with respect to both acid and ketene is about 95-98 molar percent.
(ii)
Industrial Manufacture :-
L.I.T. Nagpur
Manufacture of acetic anhydride
The industrial manufacture of acetic anhydride using acetic acid consists of four main sections :-
(a)
Acetic acid pyrolysis
(b)
Ketene absorption
(c)
Acetic acid purification.
(d)
Recovery of unconverted acetic acid.
The first step which is the pyrolysis of acetic acid involves the thermal decomposition of acetic acid preheated to about 110ºC and containing continuous additions of triethyl phosphate catalyst, which contains nickel and facilitate the complete cracking of the reactants and products, as well as the formation of coke, it is preferable to use high-chromium steels as the tube material, or alloys of chromium (23%), aluminium (1.5%), and silicon (1.5%). If not, coking can be slowed down by the addition of carbon-di-sulfide to the feed.
The reactor effluents, available at about 700ºC, first receive an inline injection of ammonia to neutralize the catalyst. They are then cooled rapidly to 0ºC in a series of heat exchangers. The liquid obtained by cond conden ensa sati tion on and and cont contai aini ning ng abou aboutt 35% 35% weig weight ht acet acetic ic acid acid is sent sent to the the recovery section.
Ketene absorption takes place on the off gases, with
a countercurrent of acetic acid, collecting about 95% of the available ketene. The The unit unit oper operat ates es at arou around nd 45-5 45-55º 5ºC C and and the the pres pressu sure re of abou aboutt 5-15 5-15 KPa KPa
L.I.T. Nagpur
Manufacture of acetic anhydride
absolute. The liquid leaving the absorption stage contains more than 90% acetic anhydride. It is sent to the purification system. Purification takes place by distillation in a series of
two distillation columns, the first column separates acetic acid from the top which is sent to the recovery section and acetic anhydride of about 99% purity from the bottom, the heavier components are collected at the bottom of the final fractionation. The recovered acetic acid ( unconverted acetic acid)
is reconcentrated in a distillation column which removes water from the top and acetic acid of 95% purity at the bottom.
5.
Thermodynamics :Thermodynamic properties of raw materials &
product are as given below :(i)
Acetic Acid :-
Property
A
Specific -18.944 Heat(C Heat(Cp), p), J/mol J/mol K
B
C
D
1.0971
2.8921 ×10 -3
2.9275 ×10 -6
L.I.T. Nagpur
Manufacture of acetic anhydride
∆Hf (KJ/mol)
-44.988
-0.00983
2.46×10 -6
-
∆G (KJ/mol)
-47.916
0
5.04×10 -6
-
Property
A
B
C
D
∆Hf (KJ/mol)
-44.988
-0.00983
2.46×10 -6
-
∆G (KJ/mol)
-47.916
0
5.04×10 -6
-
A
B
C
D
-3.9953×10 -2
-2.1103×10-4
5.3469×10-7
-0.01226
2.77×10 -6
-
B
C
D
(ii)
(iii)
Ketene :-
Water :-
Property
Specific 92.053 Heat(C Heat(Cp), p), J/mol J/mol K ∆Hf (KJ/mol) -238.41
Also, ∆G at 298K for Water W ater = 238.59 Kj/hr
(iv)
Acetic An Anhydride ::-
Property
A
Specific 71.831 Heat(C Heat(Cp), p), J/mol J/mol K ∆Hf (KJ/mol) -44.988
8.8879 ×10 -1
-2.6534 ×10 -3
3.3501 ×10 -6
-0.00983
2.46×10 -6
-
∆G (KJ/mol)
0
5.04×10 -6
-
-47.916
L.I.T. Nagpur
Manufacture of acetic anhydride
Reaction 1 :-
CH3COOH Acetic acid
a)
→
CH2=CO + Ketene
H2O
Heat of Reaction :-
Ketene :-
∆H0 = A + BT + CT 2 (KJ/mol) ∆H0 = -44.988 – 0.00983×T + 2.46×10 -6×T2
Therefore. at 298K ∆H0298 = -44.988 – 0.00983×298 + 2.46×10 -6×2982 = -74.063 KJ/mol
And, at 700 ºC that is at 973K ∆H0973 = -44.988 – 0.00983×973 + 2.46×10 -6×9732 = -51.23 KJ/mol Acetic acid :-
∆H0 = A + BT + CT 2 (KJ/mol) ∆H0 = -422.584 – 4.8354×10 -4×T + 2.46×10 -6×T2
Therefore. at 298K ∆H0298 = -422.584 – 4.8354×10 -4×298 + 2.3337×10 -5×2982 = -434.84 KJ/mol
L.I.T. Nagpur
Manufacture of acetic anhydride
And at 973K ∆H0973 = -422.584 – 4.8354×10 -4×973 + 2.3337×10 -5×9732 = -408.3 KJ/mol Water :-
∆H0 = A + BT + CT 2 (KJ/mol) ∆H0 = -238.41 – 0.01226×T + 2.77×10 -6×T2 Therefore. at 298K ∆H0298 = -238.41 – 0.01226×298 + 2.77×10 -6×2982 = - 241.8 KJ/mol
And at 973K ∆H0973 = -238.41 – 0.01226×973 + 2.77×10 -6×9732 = -246.4 KJ/mol Therefore, At 298K ∆H0reaction = ∑ H products – ∑ H reactants = ( ∆H0ketene + ∆H0water ) – (∆H0acetic acid ) = (-74.063-241.8) – (-434.84) = 118.977 KJ/mol
And at 973K ∆H0reaction = ∑ H products – ∑ H reactants = ( ∆H0ketene + ∆H0water ) – (∆H0acetic acid ) = (-51.23-246.4) – (-408.3) = 110.09 KJ/mol
L.I.T. Nagpur
Manufacture of acetic anhydride
(b) Feasibility of Reaction :Ketene :-
∆G0 = A + BT + CT 2 (KJ/mol) ∆G0 = -47.916 + 0×T + 5.04×10 -6×T2 Therefore, At 298K ∆G0298 = -47.916 + 0×298 + 5.04×10 -6×2982 = -47.47 KJ/mol
Acetic acid :-
∆G0 = A + BT + CT 2 (KJ/mol) ∆G0 = -435.963 + 1.9346×10 -1×T + 1.6362×10 -5×T2 Therefore, At 298K ∆G0298 = -435.963 + 1.9346×10 -1×298 + 1.6362×10 -5×2982 = -365.69 KJ/mol
Water :-
For water ∆G0298 = -238.59 Thus, ∆G0reaction = ∑ ∆G products – ∑ ∆G
reactants
= ( ∆G0ketene + ∆G0water ) – (∆G0acetic acid ) Therefore, at 298K
L.I.T. Nagpur
Manufacture of acetic anhydride
∆G0298
= (-47.47-238.59) – (-365.69) = 79.63 KJ/mol
Since, ∆G is positive, the reaction is not feasible at 298K. Now, ∆G0298 = -RT ln K ln K 1 = (-79.63×1000)/(8.314 × 298) = -32.14 -14 Therefore, K 298 298 = 1.1×10
Now,
dlnK dT
=
ΔH o RT
Therefore, ln(K 2/K 1) = (-∆H0298 /R) × [(1/T2)-(1/T1)] ln(K 2/K 1) = (-118.977×1000/ 8.314) × [(1/973)-(1/298)] = 33.314
Therefore, (K 2/K 1) = 2.938 × 10 14 K 2 = 3.232
L.I.T. Nagpur
Manufacture of acetic anhydride
∆G0973 = - RT (ln K 2) = -8.314 × 973 × ln 3.232 = -26.15 KJ/mol Since, ∆G0973 is negative, the reaction is feasible at 973K
Reaction 2 :-
CH3COOH + Acetic acid
(a)
CH2=CO Ketene
→
(CH3CO)2 Acetic Anhydride
Heat of Reaction :-
Ketene :-
∆H0 = A + BT + CT 2 (KJ/mol) ∆H0 = -44.988 – 0.00983×T + 2.46×10 -6×T2
Therefore, at 50 ºC that is at 323K ∆H0323 = -44.988 – 0.00983×323 + 2.46×10 -6×3232 = -47.9 KJ/mol
Acetic acid :-
L.I.T. Nagpur
Manufacture of acetic anhydride
∆H0 = A + BT + CT 2 (KJ/mol) ∆H0 = -422.584 – 4.8354×10 -4×T + 2.46×10 -6×T2 Therefore, at 323K ∆H0323 = -422.584 – 4.8354×10 -4×323 + 2.3337×10 -5×3232 = -420.23 KJ/mol
Acetic Anhydride :-
∆H0 = A + BT + CT 2 (KJ/mol) ∆H0 = -554.715 – 8.4124×10 -2×T + 4.3618×10 -5×T2 Therefore, at 323K ∆H0323 = -554.715 – 8.4124×10 -2×323 + 4.3618×10 -5×3232 = -577.615 KJ/mol
And at 323K ∆H0reaction = ∑ H products – ∑ H reactants = (∆H0acetic anhydride ) – ( ∆H0ketene + ∆H0acetic acid ) = (-577.615) – (-47.9 – 420.23) = -109.485 KJ/mol
(b) Feasibility of Reaction :Ketene :-
L.I.T. Nagpur
Manufacture of acetic anhydride
∆G0 = A + BT + CT 2 (KJ/mol) ∆G0 = -47.916 + 0×T + 5.04×10 -6×T2 Therefore, At 323K ∆G0323 = -47.916 + 0×323 + 5.04×10 -6×3232 = -47.39 KJ/mol
Acetic acid :-
∆G0 = A + BT + CT 2 (KJ/mol) ∆G0 = -435.963 + 1.9346×10 -1×T + 1.6362×10 -5×T2 Therefore, At 323K ∆G0323 = -435.963 + 1.9346×10 -1×323 + 1.6362×10 -5×3232 = -371.75 KJ/mol
Acetic Anhydride :-
∆G0 = A + BT + CT 2 (KJ/mol) ∆G0 = -578.076 + 3.3162 ×10 -1×T + 2.5188 ×10 -5×T2 Therefore, at 323K ∆G0323 = -578.076 + 3.3162 ×10 -1×323 + 2.5188 ×10 -5×3232 = -468.334 KJ/mol
L.I.T. Nagpur
Manufacture of acetic anhydride
Thus, ∆G0reaction = ∑ ∆G products – ∑ ∆G
reactants
= ∆G0acetic anhydride – ( ∆G0ketene + ∆G0acetic acid ) Hence, at 323K ∆G0323 = -468.334 – (–47.39–371.75) = –49.194 KJ/mol Since, ∆G0323 is negative, the reaction is feasible at 323K Now, ∆G0323 = -RT ln K ln K = (-∆G 0323) / (RT) = (49.194 × 1000)/ (8.314 (8.314 × 323) = 18.32 Therefore, K = 9.032 × 10 7
L.I.T. Nagpur
Manufacture of acetic anhydride
6.
Material Balance :-
Plant capacity = 50 tonnes/day = (50 × 1000)/ 24 = 2083.33 Kg/ hr Therefore, Acetic Anhydride to be produced = 2083.33 Kg/hr Considering 5% overall loss Hence, acetic Anhydride to be produced actually = 1.05 × 2083.33 = 2187.5 Kg/hr Molecular weight of Acetic Anhydride = 102 Therefore, Acetic Anhydride to be produced = 2187.5/102 = 21.446 Kmol/hr The main reaction is :-
CH3COOH + Acetic acid
CH2=CO Ketene
→
(CH3CO)2 Acetic Anhydride
% Yield based on ketene required is = acetic anhydride formed/ ketene required 0.9 = 21.446/ketene required Therefore, ketene required = 21.446/0.9 = 23.83 Kmol/hr
L.I.T. Nagpur
Manufacture of acetic anhydride
% yield yield base base on acetic acetic acid requir required ed = acetic acetic anhydrid anhydridee formed formed / acetic acetic acid required Therefore, acetic acid required = 21.446/0.85 = 25.23 Kmol/h
The first reaction taking place is :-
CH3COOH Acetic acid
→
CH2=CO Ketene
+
H2O
Ketene to be produced = 23.83 Kmol/hr Hence, water produced = 23.83 Kmol/hr % yield based on acetic acid required = 0.9 = ketene formed / acetic acid required Therefore, Acetic acid required = 23.83/ 0.9 = 26.478 Kmol/hr % conversion based on acetic acid = 0.88 = acetic acid required / acetic acid fed Therefore, acetic acid to be fed = 26.478/0.88 = 30.1637 Kmol/hr 33% excess of acetic acid is taken :0.33 = (fed-reacted)/fed Therefore, total acetic acid to be fed = (25.23 + 26.478) / 0.77 = 77.18 Kmol/hr
L.I.T. Nagpur
Manufacture of acetic anhydride
(i)
Pyrolysis Heater :-
Acetic acid input = 30.1637 Kmol/hr Acetic acid reacted = 26.478 Kmol/hr Hence, acetic acid remaining = 30.1637-26.478 = 3.6857 Kmol/hr 1.5% of the unreacted acetic acid gets converted in to flue gases Therefore, flue gases produced = 0.015 × 3.6857 = 0.0553 Kmol/hr Acetic acid remaining = 3.6857 – 0.0553 = 3.6304
Component
Input (Kmol/hr)
Output (Kmol/hr)
Acetic acid
30.1637
3.6304
Water
-
23.83
Ketene
-
23.83
Flue gases
-
0.0553
L.I.T. Nagpur
Manufacture of acetic anhydride
Considering overall 0.1% loss of ketene, 0.5% loss of acetic acid & 0.2% loss of water in condenser, cooler cooler & chiller. (ii)
Separator ::-
3% water goes from the top :Therefore, water in the upstream = 23.83 × 0.03 = 0.715 Kmol/ hr And, water in the downstream = 23.83 – 0.715 = 23.782 Kmol/hr 1% ketene goes in the downstream :Hence, ketene in the downstream = 0.01 × 23.86 = 0.2383 Kmol/hr And, ketene in upstream = 23.806 – 0.2383 = 23.59 Kmol/hr The bottom product contains 35% by weight acetic acid Hence, acetic acid in the bottom product = 0.35 × 3.612 = 1.2565 Kmol/hr And, acetic acid in the top product = 0.65 × 3.612 = 2.3478 Kmol/hr Component
Acetic acid acid
Input (Kmol/hr) 3.612
Output (Kmol/hr) ( Upstream ) 2.3478
L.I.T. Nagpur
Output (Kmol/hr) ( downstream ) 0.2383
Manufacture of acetic anhydride
Water
23.782
0.715
1.2565
Ketene
23.806
23.59
23.115
Flue gases
0.055
0.055
-
(iii iii)
Keten etenee Absor sorbe berr :-
Total acetic acid fed = 2.3478 + 47.09 = 49.44 (Kmol/hr) Acetic acid required = 25.23(Kmol/hr) Hence, acetic acid remaining = 49.44 – 25.23 = 24.21(Kmol/hr) 95% acetic anhydride is produced Hence, acetic anhydride formed = 0.95 × 23.59 = 22.41 Kmol/hr 12% acetic anhydride goes from the top Hence, acetic anhydride in the top product = 0.12 × 22.41 = 2.6892 Kmol/hr And, acetic anhydride in the bottom product = 0.88 × 22.41 = 19.7208 Total acetic acid fed = 2.3478 + 47.09 = 49.44 (Kmol/hr) Acetic acid required = 25.23(Kmol/hr) Hence, acetic acid remaining = 49.44 – 25.23 = 24.21(Kmol/hr) 80% acetic acid goes from the top Hence, acetic acid in top product = 24.21 × 0.8 = 19.368 Kmol/hr And, in the bottom product = 0.2 × 24.21 = 4.842 Kmol/hr L.I.T. Nagpur
Manufacture of acetic anhydride
0.4 % water goes from the bottom Hence, water in the bottom product = 0.004 × 0.715 = 0.00286 Kmol/hr
Component
Input (Kmol/hr)
Acetic acid
Output (Kmol/hr) ( downstream )
49.44
Output (Kmol/hr) ( Upstream ) 19.368
4.842
Water
0.715
12.816
0.00286
Ketene
23.59
1.18
-
Flue gases
0.055
0.055
-
-
2.6892
19.368
Acetic anhydride anhydride
(iv)
Tail ga gas sc scrubber ::-
2% Water goes from the top :Hence, Water in the top product = 0.02 × 0.712 = 0.01424 Kmol/hr 1% acetic anhydride goes from the top Hence, acetic anhydride in the top product = 0.01 × 2.6892 = 0.0268 Kmol/hr 0.5% acetic acid goes from the top Hence, acetic acid in the top product = 0.05 × 19.368 = 0.097 Kmol/hr
L.I.T. Nagpur
Manufacture of acetic anhydride
Component
Input(Kmol/hr) Output (Kmol/hr) ( Upstream )
Output (Kmol/hr) ( downstream )
Acetic acid
19.368
0.097
19.271
Water
0.712
0.01424
0.698
Ketene
1.18
1.18
-
Flue gases
0.055
0.055
-
Acetic anhydride
2.6892
0.0268
2.6623
(v)
Jet Condensor :-
Considering 0.2% loss of Acetic acid, 0.1% loss of acetic anhydride and 0.5% loss of water in jet condenser :-
Component
Input (Kmol/hr)
Output (Kmol/hr) (Upstream)
Output (Kmol/hr) (Downstream)
Acetic acid
0.097
-
0.0968
L.I.T. Nagpur
Manufacture of acetic anhydride
Water
0.01424
-
0.01417
Ketene
1.18
1.18
-
Flue gases
0.055
0.055
-
Acetic anhydride
0.0268
-
0.02677
(vi) vi)
Acet Acetiic aci acid d colu colum mn ::-
Input Feed (F):
1) Acetic acid (CH 3COOH) = 20.632 Kmol/hr 2) Water (H2O) = 23.827 Kmol/hr Total feed to the distillation column = 20.632 + 23.827 = 44.459 Kmol/hr
Mole fraction of components in the feed :-
Xf1 (CH3COOH) = 20.632/44.459 = 0.464 Xf2 (H2O) = 23.827/44.459 = 0.536
Top Product (D) :-
Mole fraction of components in the top product :-
L.I.T. Nagpur
Manufacture of acetic anhydride
Xd1 (CH3CO0H) = 0.1 Xd2 (H2O) = 0.9 Bottom Product (W):
Mole fraction of components in the bottom product :Xw1 (CH3COOH) = 0.95 Xw2 (H2O) = 0.05
Taking overall material balance :-
F= D + W Putting the values in above equation :44.459 = D + W -------------------------------------------- -------------------------------------------- (1)
Now, taking component balance :-
F Xf = D Xd +W Xw For Water :44.459 ×0.536 = D × 0.9 + W × 0.05 23.827 = 0.9 D + 0.05 W -------------------------------------------- ------------------ (2) Solving equations 1 & 2, we get :Top product (D) = 24 Kmol/hr Bottom product (W) = 20.45 Kmol/hr
L.I.T. Nagpur
Manufacture of acetic anhydride
Hence, acetic acid in the top product = 0.1 × 24 = 2.4 Kmol/hr And, Water in the top product = 0.9 × 24 = 21.6 Kmol/hr Acetic acid in the bottoms product = 0.95 × 20.45 = 19.4275 Kmol/hr Water in the bottoms bottoms product = 0.95 × 20.45 = 1.0225 Kmol/hr Reflux Ratio = 2.25 ( calculated below ) Hence, Ln = 2.25 × D = 2.25 × 24 = 54 Kmol/hr
(vii (vii)) Acet Acetic ic anh anhyd ydri ride de col colum umn n :Input Feed (F):
1) Acetic acid = 4.842 Kmol/hr 2) Acetic anhydride = 19.7208 Kmol/hr Total feed to the distillation column = 4.842 + 19.7208 = 24.563 Kmol/hr
Mole fraction of components in the feed :-
Xf1 (CH3COOH) = 4.842/24.563 = 0.197 Xf2 (Acetic anhydride) = 19.7203/24.563 = 0.803
Top Product (D) :-
L.I.T. Nagpur
Manufacture of acetic anhydride
Mole fraction of components in the top product :Xd1 (CH3CO0H) = 0.9 Xd2 (Acetic anhydride) = 0.1
Bottom Product (W):
Mole fraction of components in the bottom product :Xw1 (CH3COOH) = 0.01 Xw2 (Acetic anhydride) = 0.99
Taking overall material balance :-
F= D + W Putting the values in above equation :24.563 = D + W -------------------------------------------- -------------------------------------------- (1)
Now, taking component balance :-
F Xf = D Xd +W Xw For Acetic acid :-
24.563 ×0.197 = D × 0.9 + W × 0.01 4.842 = 0.9 D + 0.01 W ----------------------------------------------- ---------------- (2) Solving equations 1 & 2, we get :Top product (D) = 5.164 Kmol/hr
L.I.T. Nagpur
Manufacture of acetic anhydride
Bottom product (W) = 19.398 Kmol/hr
Hence, acetic acid in the top product = 0.9 × 5.164 = 4.6476 Kmol/hr = 278.856 Kg/hr And, Acetic anhydride in the top product =0.1 × 5.164 = 0.5164 Kmol/hr = 52.6728 Kg/hr Acetic acid in the bottoms product = 0.01 × 19.398 = 0.194 Kmol/hr = 11.64 Kg/hr Acetic anhydride in the bottoms product = 0.99 × 19.398 = 19.2 Kmol/hr = 1958.4 Kg/hr
7.
Energy Balance :-
Component
Specific heat (J/mol K)
Latent Heat of Vaporization ( cal/g)
Acet Acetic ic Acid Acid
-18. -18.94 944– 4–1. 1.09 0971 71×T ×T + 2.89 2.8921 21×1 ×10 0 -6×T2
96.8
Acetic Anhydride
78.831 – 8.8879×10 -1T+ 2.6534×10 -6T2
93
(i)
Pyrolysis Reactor :-
Reaction Temperature = 700ºC Temperature of acetic acid entering = 110 ºC Taking enthalpy balance around the pyrolysis Reactor :-
Heat to be supplied by the fuel gas = Heat required to raise the temperature of acetic L.I.T. Nagpur
Manufacture of acetic anhydride
acid + Heat required for the decomposition of acetic acid to form ketene. That is, ∆Hf (theoretical)
=
Q1
+
Q2
Tenter
Q1 = n ×
∫ CpdT
Tref
973
=
∫
30.1637× (− 18.944 94 4 + 1.0971× T + 2.8921× 10 − 3 × T 2 )dT 383
=
12.9 × 106 KJ/hr
Q2 = Heat of reaction reaction × amount of of acetic acid cracked = 110.09 × 10 3 × 30.1637 = 3.32 × 106 Therefore, ∆Hf (theoretical) = Q1 + Q2 = (12.9 + 3.32) × 106 =
16.22 × 106 KJ/hr
But, efficiency of the furnace = 0.65 Therefore, heat to be supplied actually = ∆H f (theoretical) /0.65 = 16.22 × 106/0.65 = 24.954 × 106 KJ/hr (ii)
Waste Heat Boiler :-
L.I.T. Nagpur
Manufacture of acetic anhydride Texit
∫ CpdT
Heat removed = nacetic acid ×
Tenter
773
94 4 + 1.0971× T + 2.8921× 10 ∫ (−18.944
= 3.6304×
−3
× T 2 )dT
973
= - 685.341 685.341 × 103 KJ/hr Heat lost lost = Heat gained gained by the cooling medium (water) (water)
Inlet temperature of water = 25ºC Outlet temperature of water = 210ºC Cooling water flowrate = ? Heat gained = m water × Cpwater × ∆T mwater = 685.341 × 10 3/[4.184 × (483-298)] = 885.4 Kg/hr
(iii)
Cooler :Texit
∫ CpdT
Heat removed = nacetic acid ×
Tenter
47 3
= 3.6304×
∫
(− 18.944 94 4 + 1.0971× T + 2.8921× 10 − 3 × T 2 ) dT
77 3
= - 726.226 726.226 × 103 KJ/hr
L.I.T. Nagpur
Manufacture of acetic anhydride
Heat lost lost = Heat gained gained by the cooling medium (water) (water) Inlet temperature of water = 25ºC Outlet temperature of water = 195ºC Cooling water flowrate = ? Heat gained = m water × Cpwater × ∆T mwater = 726.226 × 10 3/[4.184 × (468-298)] = 1021.012 Kg/hr (iv)
Chiller :Texit
∫ CpdT
Heat removed = nacetic acid ×
Tenter
273
= 3.6304×
94 4 + 1.0971× T + 2.8921× 10 ∫ (−18.944
473
= -283.495 × 103 KJ/hr Heat lost lost = Heat gained gained by the cooling medium (ammonia) (ammonia) Inlet temperature of ammonia = -33ºC Outlet temperature of ammonia = 50ºC ammonia flowrate = ? Heat gained = m ammonia × Cpammonia × ∆T mammonia = 283.495 × 10 3/[2.114 × (323-298)] = 1615.7 Kg/hr
L.I.T. Nagpur
−3
× T 2 ) dT
Manufacture of acetic anhydride
(iv)
Ktene Absorber :-
The reaction taking place in the ketene absorber is :CH3COOH + Acetic acid
CH2=CO Ketene
→
(CH3CO)2 Acetic Anhydride
Above reaction is exothermic and the amount of heat released = -109.485 KJ/mol This is the amount of heat released when one mole of acetic anhydride is formed. Hence, for the formation of 22.41 Kmol acetic anhydride, the amount of heat released wil be :∆H = -109.485 × 1000 × 22.41 = -2453.56 × 10 3 KJ/hr The negative sign shows that the heat is released, that is the reaction is exothermic.
L.I.T. Nagpur
Manufacture of acetic anhydride
(v)
Acetic Acid Column :-
Temperature of feed = 382K Temperature of distillate = 373K Temperature of the bottoms product = 392K Reference Temperature = 273K
Heat in feed (F×H F) = Heat in acetic acid + Heat in in water water Texit
= naceticacid ×
∫ CpdT +
Texit
n water ×
Tenter
∫ CpdT
Tenter
382
=
∫
− 20.632 63 2 × (− 18.944 94 4 + 1.0971× T + 2.8921× 10 × T )dT 3
2
273
+ 382
∫
23.827 82 7 × (92.053 05 3 − 3.9953× 10 − 2 T − 2.1103× 10 − 4 T 2 )dT 273
=
=
765.31 × 103 + 239.075 × 10 3
1004.385 × 10 3 KJ/hr
Heat in Distillate (D×H d) = Heat in acetic acid + Heat in water
L.I.T. Nagpur
Manufacture of acetic anhydride
Texit
∫ CpdT
= nacetic acid ×
Texit
+
nwater ×
Tenter
∫ CpdT
Tenter
373
=
∫
2.4 × (− 18.944 94 4 + 1.0971× T + 2.8921× 10 − 3 × T 2 )dT 273
+ 373
∫
21.6 × (92.053 05 3 − 3.9953× 10 − 2 T − 2.1103× 10 − 4 T 2 ) dT 273
(D×Hd)
= =
80.50 × 103 + 198.834 × 10 3 279.334 × 10 3 KJ/hr
Heat in bottoms (WH W) = Heat in acetic acid + Heat in water Texit
=
∫ CpdT
nacetic acid ×
Texit
+ nwater ×
Tenter
∫ CpdT
Tenter
392
= 19.4275×
∫
(− 18.944 94 4 + 1.0971× T + 2.8921× 10 − 3 × T 2 )dT
273
+ 392
∫
1.0225× (92.053 05 3 − 3.9953× 10 − 2 T − 2.1103× 10 − 4 T 2 )dT 273
= =
799.542 × 10 3 + 11.32 × 10 3 810.862 × 10 3 KJ/hr L.I.T. Nagpur
Manufacture of acetic anhydride
Average Latent heat ( L avg ) = Lacetic acid × Xacetic acid + Lwater × Xwater = 18 × 2200 2200 × 0.9 + 60 × 96.8 96.8 × 4.184 4.184 × 0.1 0.1 = 38.07 × 103 KJ/Kmol
Condenser duty ( Q c ) = Vn × Lavg = 78 × 38.07 × 103 = 2969.46 × 10 3 KJ/hr
Taking overall Heat balance on distillation column :-
Heat in Feed (FH F) + Reboiler duty (Q r ) = Heat Heat in dist distil illa late te (DH (DH d)+Heat )+Heat in bottoms(WHW) + Condenser duty (Q c) Hence, (Qr ) = (DHd) + (WHW) + (Qc) - (FHF) = ( 279.334 + 810.682 + 2969.46 2969.46 – 1004.385 1004.385 ) × 10 3 = 3055.091 × 10 3 KJ/hr
Therefore, quantity of steam required = (Q r ) / Lsteam = 3055.091 / 2200 = 1388.67 Kg/hr
(v)
Acet cetic anhydride column :-
Temperature of feed = 403K
L.I.T. Nagpur
Manufacture of acetic anhydride
Temperature of distillate = 392K Temperature of the bottoms product = 413K Reference Temperature = 273K
Heat in feed (F×H F) = Heat in acetic acid + Heat in in acetic acetic anhydride anhydride Texit
= naceticacid ×
∫ CpdT +
Texit
∫ CpdT
nanhydride ×
Tenter
Tenter
403
=
∫
4.842 84 2 × (− 18.944 94 4 + 1.0971× T + 2.8921× 10 − 3 × T 2 )dT 273
+ 403
∫
19.7208× (71.831 83 1+ 8.8879× 10 −1 T − 2.6534× 10 −3 T 2 )dT 273
=
221.491 × 10 3 + 202.099 × 10 3
=
423.59 × 103 KJ/hr
Heat in Distillate (D×H d) = Heat in acetic acid + Heat in anhydride
Texit
= nacetic acid ×
∫ CpdT
Texit
+
Tenter
L.I.T. Nagpur
nanhydride ×
∫ CpdT
Tenter
Manufacture of acetic anhydride
392
∫
4.6476× (−18.944 94 4 + 1.0971× T + 2.8921× 10 − 3 × T 2 )dT
=
273
+ 392
∫
0.5164× (71.831 83 1+ 8.8879× 10 −1 T − 2.6534× 10 − 3 T 2 )dT 273
(D×Hd)
= 191.272 × 10 3 + 4.844 × 103 = 196.117 × 10 3 KJ/hr
Heat in bottoms (WH W) = Heat in acetic acid + Heat in anhydride Texit
=
nacetic acid ×
∫ CpdT
Texit
+ nanhydride ×
Tenter
∫ CpdT
Tenter
413
19 4× = 0.194
∫
(− 18.944 94 4 + 1.0971× T + 2.8921× 10 − 3 × T 2 )dT
273
+ 413
∫
19.2 × (71.831 83 1+ 8.8879× 10 −1 T − 2.6534× 10 − 3 T 2 )dT 273
= =
9.704 × 103 + 211.942 × 10 3 221.647 × 10 3 KJ/hr
Average Latent heat ( L avg ) = Lacetic acid × Xacetic acid + Lanhydride × Xanhydride = 60 × 96.8 96.8 × 4.184 4.184 × 0.9 + 102 × 93 × 4.184 4.184 × 0.1 0.1
L.I.T. Nagpur
Manufacture of acetic anhydride
= 25.84 × 103 KJ/Kmol
Condenser duty ( Q c ) = Vn × Lavg = 24.227 × 25.84 × 103 = 626.025 × 10 3 KJ/hr
Taking overall Heat balance on distillation column :-
Heat in Feed (FH F) + Reboiler duty (Q r ) = Heat Heat in dist distil illa late te (DH (DH d)+Heat )+Heat in bottoms(WHW) + Condenser duty (Q c) Hence, (Qr ) = (DHd) + (WHW) + (Qc) - (FHF) = ( 196.117 + 221.647 + 626.025 626.025 – 423.59 423.59 ) × 10 3 = 620.199 × 10 3 KJ/hr
Therefore, quantity of steam required = (Q r ) / Lsteam = 621.199 / 2200 = 281.91 Kg/hr
L.I.T. Nagpur
Manufacture of acetic anhydride
8.
Equipment Design :-
(i)
Acetic Anhydride column :-
Input Feed (F):
1) Acetic acid = 4.842 Kmol/hr 2) Acetic anhydride = 19.7208 Kmol/hr Total feed to the distillation column = 4.842 + 19.7208 = 24.563 Kmol/hr Mole fraction of components in the feed :-
Xf1 (CH3COOH) = 4.842/24.563 = 0.197 Xf2 (Acetic anhydride) = 19.7203/24.563 = 0.803
Top Product (D) :-
Mole fraction of components in the top product :Xd1 (CH3CO0H) = 0.9 Xd2 (Acetic anhydride) = 0.1
L.I.T. Nagpur
Manufacture of acetic anhydride
Bottom Product (W):
Mole fraction of components in the bottom product :Xw1 (CH3COOH) = 0.01 Xw2 (Acetic anhydride) = 0.99 Taking overall material balance :-
F= D + W Putting the values in above equation :24.563 = D + W -------------------------------------------- -------------------------------------------- (1)
Now, taking component balance :-
F Xf = D Xd +W Xw
For Acetic acid :-
24.563 ×0.197 = D × 0.9 + W × 0.01 4.842 = 0.9 D + 0.01 W ----------------------------------------------- ---------------- (2) Solving equations 1 & 2, we get :Top product (D) = 5.164 Kmol/hr Bottom product (W) = 19.398 Kmol/hr Hence, acetic acid in the top product = 0.9 × 5.164 = 4.6476 Kmol/hr = 278.856 Kg/hr And, Acetic anhydride in the top product =0.1 × 5.164 = 0.5164 Kmol/hr =
L.I.T. Nagpur
Manufacture of acetic anhydride
52.6728 Kg/hr Acetic acid in the bottoms product = 0.01 × 19.398 = 0.194 Kmol/hr = 11.64 Kg/hr Acetic anhydride in the bottoms product = 0.99 × 19.398 = 19.2 Kmol/hr = 1958.4 Kg/hr
Boiling point point of acetic acetic acid = 118 ºC Boiling point point of acetic acetic acid = 139 ºC Hence, acetic acid is more volatile component ( MVC) Acetic acid- acetic anhydride (x-y data) :x y
0.1 0 .2 0.225 0.41
0.3 0.56
0.4 0.66
0 .5 0.6 0.7 0.735 0.795 0.85
ya = α xa/ 1+( α-1)xa substitute, y = 0.225 & x= 0.1 we get, α = 2.613 from graph :(xd/R+1)min = 0.26 (0.9/R+1)min = 0.26 Therefore, R min min = 2.461 But, R opt opt = 1.5 R min min Therefore, R opt opt. = 1.5 × 2.461 = 3.6915 R = Ln/ D
L.I.T. Nagpur
0.8 0.9
0.9 0.94
1 .0 1 .0
Manufacture of acetic anhydride
Hence, Ln = 3.6915 × 5.164 = 19.063 Kmol/hr Or, D = 52.6728 52.6728 + 278.856 = 331.53 331.53 Kg/hr Kg/hr Therefore, L n = 1223.842 Kg/hr Vn = Ln + D = 19.063 + 5.164 = 24.227 Kmol/hr 0r, Vn = 1223.842 + 331.53 = 1555.372 Kg/hr Lm = Ln + F = 19.063 19.063 + 24.563 = 43.626 43.626 Kmol/hr Or, Lm = 1223.842 + (19.7208 × 102 + 4.842 × 60 ) = 3525.88 Kg/hr Vm = Lm – W = 3525.88 – (11.64 + 1958.4 ) = 1555.84 Kg/hr Or, Vm = 43.626- 19.398 = 24.228 Kmol/hr Therefore, equation of upper operating line :-
y n =
x R x n +1 + d R + 1 R + 1
0 .9 3.6915 y n = x n +1 + 4.6915 4.6915 Therefore, yn = 0.7868 x n+1 + 0.192 Hence, y-intercept = 0.192 & slope = 0.7868
Equation of lower operating line (LOL) :-
y m +1 =
L M V M
x M −
Wx w V M
L.I.T. Nagpur
Manufacture of acetic anhydride
∴ y m +1 =
43 .626 24 .228
xm −
19 .398 × 0.01 24 .228
∴ y m+1 = 1.8 x m − 0.7926
Therefore, number of theoretical plates = 14 -1=13 (from graph) One plate is deducted for reboiler Efficiency of the column = 62% Therefore actual no. of plates required r equired = 13/0.62 = 23 plates Feed plate = 6/0.62 = 10 th plate. For atmospheric columns generally, tray spacing = 0.457m Therefore, height of the distillation column = (N+1) ( N+1) × 0.457 = 24 × 0.457 = 10.968m=11m 10.968m=11m
Take 100 mm of water as the pressure drop per plate in the column. Therefore, column pressure drop = 100 × 10 −3 × 1000 × 9.81× 23 = 20601 Pa = 20.601 KPa. Therefore, Top pressure = 101.325 KPa. Bottom pressure = 101.325 + 20.601= 127.926 KPa. At the bottom of the distiilation column vapor density can be calculated as follows;
L.I.T. Nagpur
Manufacture of acetic anhydride
( ρ v ) bottom =
P × M avg
∴ ( ρ v ) bottom =
RT 127 .926 × (60 × 0.9 + 102 × 0.1) × 273 101 .325 × 22 .414 × 392
∴ ( ρ v ) bottom = 1.995 Kg / m 3 .
The density of liquid at the bottom of the column can be calculated by multiplying the density of each component with the mole fraction. fr action.
( ρ l ) top = 1084 × 0.99 + 1049 × 0.01
∴ ( ρ l ) top = 1083.65 Kg / m 3 .
At the top of the distiilation column vapor density can be calculated as follows; ( ρ v ) top =
P × M avg
∴ ( ρ v ) top =
RT 101 .325 × (60 × 0.01 + 102 × 0.99 ) × 273 101 .325 × 22.414 × 413
∴ ( ρ v ) top = 3.782 Kg / m 3 .
L.I.T. Nagpur
Manufacture of acetic anhydride
The density of liquid at the bottom of the column can be calculated by multiplying the density of each component with the mole fraction. fr action.
( ρ l ) top = 1084 × 0.1 + 1049 × 0.9
∴ ( ρ l ) top = 1052.5 Kg / m 3 .
Calculation of Parachor value (P) :-
Ptop = 134.16 × 0.9 + 228.2 × 0.1 = 143.56 P bottom = 134.16 × 0.01 + 228.2 × 0.99 = 227.26
Surface tension :4
σ top
P × ( ρ L − ρ V ) = × 10 −12 M 4
∴ σ top
143 .56 × (1052.5 − 1.995 ) = × 10 −12 (60 × 0.9 + 102 × 0.1)
∴ σ top = 30 .45dyne / cm = 0.03045 N / m
Similarly :-
L.I.T. Nagpur
Manufacture of acetic anhydride 4
σ bottom
P × ( ρ L − ρ V ) −12 = × 10 M
∴ σ top = 0.03406 N / m
Liquid-vapor Liquid-vapor flowrate factor FLV is given by :-
∴ F LV ( top ) =
=
Ln
ρ V
V n
ρ L
19.063 1.995 = 0.0342. 24.227 1052.5
F LV ( bottom ) =
Lm V m
ρ V ρ L
= 0.1063 tray spacing = 0.457 m. From graph in RC-6 page-568 we get; L.I.T. Nagpur
Manufacture of acetic anhydride
Bottom k 1 = 0.086, Top k 1 = 0.096
For liquid surface tension (0.02 N/m) take K 1 as it is otherwise:-
K 1 (bottom )
σ = 0.086 0.02
0.2
= 0.1067
K 1 (TOP )
σ = 0.096 0.02
0.2
= 0.0935
Maximum velocity at top and bottom can be calculated as follows; ρ L
Bottom: u f = k 1
Top: u f = k 1
− ρ V
= 0.1067
ρ V
ρ L
− ρ V
= 0.0935
ρ V
1083.65 − 3.782 3.782
= 1.8m / s.
1052.5 − 1.995 = 2.145 m / s. 1.995
Now we will design the column for 70% flooding, therefore the velocities will be given by; Bottom: uf = 1.8 × 0.7 = 1.26 m/s. Top: uf = 2.145 × 1.5015 m/s.
Maximum volumetric flow rate of vapor can be calculated as follows:
L.I.T. Nagpur
Manufacture of acetic anhydride
Bottom Q =
Top Q =
L m × M ρ V
× 3600
=
1555.372 = 0.1143m 3 / s. 3.782 × 3600
1555.372 = 0.2165m 3 / s. 1.995 × 3600
Now area at the bottom and at the top of the tower tower can be calculated as follows: follows:
Net area(An) bottom =
Net area(An) top =
Q 0.1143 = = 0.0907 m 2 . uv 1.26
Q 0.2165 = = 0.144 m 2 . u v 1.5015
Now let us take 15% of of the area as the downcomer downcomer area, Ad = 0.15AT But, AT = An + Ad Therefore, An/0.15 = AT
L.I.T. Nagpur
Manufacture of acetic anhydride
Therefore,
0.0907
Bottom area =
Top area =
0.85
0.144 0.85
Now, area =
π
4
= 0.1067m 2 .
= 0.1694 m 2 .
× d i . 2
Column diameter can be calculated as follows; Bottom Botto m =
0.1067 × 4
= 0.3685m
π
. Top =
0.144 × 4
= 0.4644m.
π
Take 0.4644 m as inside diameter of the column.
Liquid Flow Pattern :-
Now, maximum volumetric flowrate at the bottom
L.I.T. Nagpur
Manufacture of acetic anhydride
=
3525.88 1083.65 × 3600
= 9.04 × 10 − 4 m 3 / sec .
Provisional Plate Design :-
Column diameter D c = 0.4644 m. Area of column, Ac =
π
4
× 0.4644 2 = 0.0.1694m 2 .
Now area of downcomer downcomer is 15% of the area of the column. Ad = 0.15 × Ac = 0.02541m 2 . 2 Also Net area is given by, An = Ac − Ad = 0.1694 − 0.02541 = 0.144 m . 2 Active area, Aa = Ac − 2 Ad = 0.1694 − 2 × 0.02541 = 0.11858m .
Take hole area as 10% of active area. Therefore
Ah = 0.1×0.11858 = 0.011858 m 2.
For 15% downcomer area, from graph of (A d/Ac) ×100 Vs
For, (Ad/Ac) = 0.15,
I w Dc
I w Dc
= 0.82
Therefore Length of the weir, I w = 0.82 × O.4644 = 0.38 m. Take weir height as 50 mm. Plate thickness = 5mm Hole diameter = 5mm
Check Weeping :-
Maximum liquid rate,
L.I.T. Nagpur
Manufacture of acetic anhydride
LW =
3525.88 3600
= O.9794kg / sec .
At 70% turndown the liquid rate is, = 0.7 × 0.9794 = 0.6856 kg/sec.
The height of the liquid crest over the weir can be estimated using the Francis F rancis weir formula. For a segmental downcomer this can be written as:
how = 750 ( Where,
Lw 2 / 3 ) ρ L l w
lw = weir length, m, how = weir crest, mm liquid, Lw = liquid flow-rate, kg/s.
At the maximum flow rate of liquid the liquid crest over the weir can be calculated using the maximum liquid flow rate calculated above, while for the conditions at minimum flowrate are assumed to be the 70% turndown conditions. 0.9794 ) 2 / 3 = 13.31 mm. Maximum how = 750( 1083.65 × 0.38
Minimum how = 750(
0.6856
1083.65 × 0.38
) 2 / 3 = 10.53 mm.
At minimum rate, h w + how = 50 + 10.53 = 60.53 mm. From graph of K 2 Vs (hw + how), K 2 = 30.3 Where, K 2 is a constant dependent on the depth of the clear liquid on the plate. The minimum design vapour velocity is given by , L.I.T. Nagpur
Manufacture of acetic anhydride
[ K 2 − 0.9( 25 .4 − d h )]
uh = Where,
ρ V
uh = mini minimu mum m vapo vapour ur velo veloci city ty throu through gh the the hole holes(b s(bas ased ed on the the hole area), m/s, dh = hole diameter, mm, K 2 = a constant, dependent on the depth of clear liquid on the plate
uh =
30.4 − 0.9(25.4 − 5) 3.782
= 6.14m / sec .
Actual minimum vapour velocity is given by, Actual u h = Minimum vapour rate/A h
uh =
0.7 × 0.1143 0.011858
= 6.747m / sec .
This is well above the weeping velocity(minimum vapor velocity), therefore the design is acceptable.
Plate Pressure Drop :-
Maximum vapour velocity through the holes is given by,
u h (max) =
V max hole area
=
0.1143 = 9.64 m / sec . 0.011858
For (plate thickness)/(hole dia) = 1, from graph of (A h/Aa)×100 Vs C o For, (Ah/Aa)= 0.1; L.I.T. Nagpur
Manufacture of acetic anhydride
Co = 0.84 The pressure drop through the dry plate can be estimated using expressions derived for flow through orifices.
h D = 51[
= 51[
uh C o
9.64 0.84
]2
ρ V ρ L
]2 ×
3.782 1083.65
= 23.44mm. ……..(bottom)
Residual Head :-
hr =
12 .5 × 10 3 ρ L
= 11.53 mm.
Total Head :-
ht = h D + ( hw + how ) + hr
L.I.T. Nagpur
Manufacture of acetic anhydride
= 23.44 + (50 + 10.53) + 11.53 = 95.5mm. of H 2 O
This is less than the assumed per plate pressure drop of 100 mm, thus the design is acceptable.
Downcomer Liquid Backup :-
The height above the bottom edge of the apron is calculated as follows: hap = hw − 10 = 50 − 10 = 40 mm. Thus the clearance area under the apron is given by, Aap = haplw = 0.04×0.38 = 0.0152 m 2. This is less than the area of the downcomer A d. Thus use A ap in the equation given below :-
Head loss in the downcomer can be estimated by the following equation:
hdc = 166[
L wd ρ L Am
]2
Where, Am = Ad or Aap whichever is smaller. Lwd = liquid flowrate in the downcomer.
∴ hdc = 166[
0.9794 ] 2 = 0.586 m 1083.65 × 0.0152
L.I.T. Nagpur
Manufacture of acetic anhydride
In terms of clear liquid, the downcomer backup is given by;
hb = (hw + how ) + ht + hdc = 50 + 10.53 + 95.5+ 0.586 = 156.616 mm.
= 0.156 m.
Now tray spacing + weir height height = 0.38 + 0.05 = 0.43 0.43 m.
1 hb 〈 × 0.43 2 Therefore he tray spacing is within acceptable limits.
Downcomer Residence Time :-
The residence time of the liquid over the downcomer is given by;
t r =
Ad hbc ρ L L wd
=
0.02541 × 0.156 × 1083.65 = 4.386 sec . 0.9794
This is greater than 3 seconds therefore it is acceptable.
Entrainment Entrainment :-
L.I.T. Nagpur
Manufacture of acetic anhydride
The entrai entrainme nment nt can be estima estimated ted by the follow following ing realat realation ionshi ships ps which which give give entrainment as a function of percentage flooding.
uv =
max ma x imun bottom vapor . flowrat flow ratee An
% flooding =
uv u f ( bottom )
=
0.79375 1.26
=
0.1143 = 0.79375m / sec . 0.144
× 100 = 62.996%
Therefore from graph of F LV Vs % flooding. (fractional entrainment),
ψ
=
0.015 015 , this is less than 0.1, therefore it is acceptable.
Trial Layout :-
0.38m
0.4644m
Number Of Holes :-
L.I.T. Nagpur
Manufacture of acetic anhydride
Area of one hole,
A1 =
π
4
× d 2 = 1.964 × 10 −5 m 2 / hole
Number of holes, N =
Ah A1
=
0.011858 1.964 × 10 −5
= 604 .23 ≈ 605 holes.
MECHANICAL DESIGN
Design of shell :-
Taking material of construction as Stainless Steel Maximum allowable stress f = 1420 kg/cm 2 Operating pressure = 101.325 KN/m 2 = 101325/(9.81 x 10000) = 1.033 Kg/cm 2 Design pressure is 10% excess of operating pressure = 1.033 x 1.1 = 1.1363 kg/cm 2 Thickness of shell (t s) = (P x Di)/(2fJ –P) = (1.1363 x 46.44)/(2 x 1420 x 0.85 – 1.1363) = 0.218 mm Taking allowance = 3 mm Shell thickness = 3.218 Use thickness of 3.5 mm Therefore, Outer diameter of the column (Do) = Di + 2t = 0.4714m
Design of heads :-
Elliptical heads are used
L.I.T. Nagpur
Manufacture of acetic anhydride
th
= (P DiV)/(2fJ)
Where, Di = internal diameter of the column P = design pressure = 1.1363 kg/cm 2 th = thickness of head
J = welded joint efficiency = 0.85 V = stress intensity factor And, V = (2 + k 2)/6 k = ratio of major axis to minor axis = 2:1 =2 Therefore, V = 1 th = (1.1363 x 46.44 x 1) / (2x1420 x0.85) =0.0218 cm = 0.218 mm Taking allowance = 3 mm Thickness of head = 3.218 mm Therefore, use thickness of 3.5mm
Design of gasket and bolt size :-
Gaskets are used for making leak proof joint between two surfaces Gasket: Asbestos with suitable binder (3mm thick) Gasket factor m = 2.0 Minimum design sitting stress for asbestos with suitable binder (3 mm
L.I.T. Nagpur
Manufacture of acetic anhydride
thick) is Ya = 112 kg / cm 2
Go/Gi = [(Ya – Pi x m)/(Ya – Pi (m + 1))]0.5 = [(112 – 1.1363 x 2)/(112 – 1.1363(2+1)] 0.5 = 1.010
Gi = 46.44 + 2 x 0.0816 = 46.6 cm Go = 1.010 x 46.6 = 47.07 cm Mean gasket diameter G = (G o + Gi)/2 = 46.835 cm Basic gasket sitting width bo = ( Go - Gi )/4 = 0.1175 cm = 1.2 mm Taking it as 1 mm. Effective gasket sitting width as b o is less than 6.3 mm b = bo b = 1.2 mm
Force acting on bolt under atmospheric condition Wm1 =
3.14 x b x G x Y a
= 3.14 x 0.12 x 46.835 x 112 = 1977.514 kg
L.I.T. Nagpur
Manufacture of acetic anhydride
Force acting on the bolt under operating condition Wm2 = 3.14 x 2b x G x m x Pi + 3.14 x G 2 x Pi / 4 = 3.14 x 2 x0.12 x 46,835 46,835 x2 x1.1363 + 3.14 x 46.835 2 x 1.1363 / 4 = 2037.85 kg
Maximum bolting area :-
Bolting material is rolled carbon steel. Am1 = Wm1 / f a Am2 = Wm2 / f b Where, f a = allowable stress for bolt material under atmospheric Conditions = 545 kg / cm 2 f b = allowable stress for bolt material material under operating condition condition = 545 kg / cm 2 Am1
= 1977.514/545 = 3.628 cm 2
Am2
= 2037.85/545 = 3.74 cm2
Therefore, minimum bolting area is taken as 3.74 cm 2 No. of bolts = mean diameter diameter of gasket/2.5 = 46.835/2.5 = 18.734 Since the total no. of bolts must be a multiple of 4 No. of bolts = 20 If Am is the area of one bolt then,
L.I.T. Nagpur
Manufacture of acetic anhydride
Am x 20 = 3.74 Am = 0.187 cm 2 Therefore, 3.142 x d b2 /4 = 0.187 d b = 0.487 cm Diameter of bolt = 0.487 cm
Pitch circle diameter = outside diameter of gasket + 2xdiameter of bolt + 1.2 = Go + 2 x d b + 1.2 = 47.07 + 2 x 0.487 + 1.2 = 49.244 cm
Bolt spacing = (3.14 x pitch pitch circle diameter)/no. of of bolts = 3.14 x 49.244 / 20 = 7.73 cm
Flange design :-
Outside diameter of the flange = Pitch circle diameter + 2 x diameter of bolt = 49.244 + 2 x o.487 = 50.218 cm
Thickness of the flange t f = G x (P i/kf)0.5 Where, k = 1/[0.3 + (1.5 x W m x Hg) /(HxG)] G = diameter of the gasket load reaction or mean diameter of gasket
L.I.T. Nagpur
Manufacture of acetic anhydride
Wm = maximum bolt load = 53678.106 kg Hg = radial distance from gasket load reaction to the bolt circle = (pitch circle diameter – G)/2 = (49.244 – 46.835)/2 = 1.2045 cm
H = hydrostatic end force = (3.14 x G 2 x Pi)/4 = (3.14 x 46.835 2x 1.1363)/4 = 1956.6 Kg k = 1/[0.3 +(1.5 x 53678.106 x 1.2045)/(1956.6x 46.835)] = 0.736 tf = 46.835 x (1.1363/0.736x1420) (1.1363/0.736x1420) 0.5 = 1.544 cm
Design of skirt support support :-
The stresses due to vessel dead weights, wind load and seismic load are taken into account while the column is designed to withstand maximum values of tensile or compressive stresses. The stresses are:
L.I.T. Nagpur
Manufacture of acetic anhydride
1. Due to dead weight f d = W/(3.14 x Dsk x tsk ) Dsk = outside diameter of skirt support tsk = thickness of skirt support W = total weight of the vessel including attachments
Weight of shell shell can be calculated as :lnW = 0.694 + 0.882 lnB B
= {[(L/D) + 1.82] P D3}/(25600 + 1.2P) + 20L
Where W = weight of column without internals (kg) D = diameter of column (inches) D = 0.4644 m = 18.283 inches P = design pressure (Psig) = 1.1363 kg / cm 2 = (14.7 x 1.1363)/1.1 = 15.185 Psig L = column height of cylindrical shell and heads (inches) L = 10.968 m = 431.81 inches Therefore, B = 7638.05 kg W = 5323.42 kg
Weight of the contents = volume of contents x Avg density
L.I.T. Nagpur
Manufacture of acetic anhydride
= [(3.14 x 0.4644 2 x 10.968) x (0.803 x 1084 + 0.197 x 1049)]/4 = 1790.295 kg
Assume, Weight of accessories = 200 kg ∑W = 1790.295 + 200 + 5323.42 = 7313.715 kg
Dsk = Gi = 46.6 46.6 cm f d
= ∑W/(3.14 x Dsk x tsk ) = 7313.715/(3.14 x 46.6 x t sk = 49.98 / t sk kg/cm2
2. Due to wind wind load load PLW = K 1 x K 2 X P x H x D o Where K 1 = coeff depending on shape factor = 0.7 K 2 = 1 P = wind pressure = 152 kg/m 2 (assuming) Do = 0.4714 m H = 10.968 + 2 = 12.968 m
L.I.T. Nagpur
Manufacture of acetic anhydride
PLW = K 1 x K 2 x P x H x Do = 0.7 x 1 x 152 x 0.4714 x 12.968 = 581.67 kg
The bending moment due to wind at the base of skirt is Mw = PLW x H/2 = 581.67x 12.968/2 = 3372.813 kg-m
f w = Mw / Z = 3372.813/[(3.14 D 2sk tsk )/4] )/4] = 19785.67/t sk kg / m 2 = 0.528138/t sk kg /cm2 2. Due Due to seis seismi micc load load f s = 4 Mw / (3.14 D2o tsk ) but, Mw = (2 x C x H x W)/3 therefore, f s = (2 x 4 x C x H x W)/(3 x 3.14 D 2o tsk ) Where, C = seismic coefficient = 0.8 H = 10.968 + 2 = 12.968 m W = 7313.715 kg; D o = 0.4714 m f s = (2 x4 x 0.8 x 12.968 x 7313.715)/(3 x 3.14 x 0.4714 3 x tsk ) = 25.932/t sk kg/cm2
L.I.T. Nagpur
Manufacture of acetic anhydride
Maximum tensile stress at the bottom of the skirt = f d - (f w or f s) = (49.98-25.932)/t sk = 20.048/t sk kg/ cm2
Permissible tensile stress = 1420 kg/ cm 2 tsk = 20.048 / 1420 = 0.0169 cm = 0.169 mm
f c (max) = (49.98/tsk ) + (25.932/t sk ) = 75.912/t sk kg/ cm2
fc (permissible ) = yield point stress / 3 = 2000 / 3 = 666 kg/ cm 2
tsk = 75.912/ 666 = 0.114 cm = 1.14 mm
L.I.T. Nagpur
Manufacture of acetic anhydride
Therefore, thickness of 2 mm is used.
2. Acetic acid column :Input Feed (F):
1) Acetic acid = 20.632 20.632 Kmol/hr Kmol/hr 2) water water = 23.827 23.827 Kmol/hr Kmol/hr Total feed to the distillation column = 20.632 + 23.827 = 44.459 Kmol/hr
Mole fraction of components in the feed :-
Xf1 (CH3COOH) = 20.632/44.459 = 0.464 Xf2 (water) = 23.827/44.459 = 0.803
L.I.T. Nagpur
Manufacture of acetic anhydride
Top Product (D) :-
Mole fraction of components in the top product :Xd1 (water) = 0.9 Xd2 (CH3CO0H) = 0.1
Bottom Product (W):
Mole fraction of components in the bottom product :Xw1 (CH3COOH) = 0.95 Xw2 (water) = 0.05
Taking overall material balance :-
F= D + W Putting the values in above equation :44.459 = D + W ------------------------------------------------- --------------------------------------- (1)
Now, taking component balance :-
F Xf = D Xd +W Xw For Acetic acid :-
44.459 × 0.464 0.464 = D × 0.1 + W × 0.95 20.632 = 0.1 D + 0.95 W -------------------------------------------- ------------------ (2) Solving equations 1 & 2, we get :-
L.I.T. Nagpur
Manufacture of acetic anhydride
Top product (D) = 24 Kmol/hr Bottom product (W) = 20.45 Kmol/hr Hence, acetic acid in the top product = 0.1 × 24 = 2.4 Kmol/hr = 144 Kg/hr And, water in the top product =0.9 × 24 = 21.6 Kmol/hr = 388.8 Kg/hr Acetic acid in the bottoms product = 0.95 × 20.45 = 19.4275 Kmol/hr = 1165.65 Kg/hr water in the bottoms product = 0.05 × 20.45 = 1.0225 Kmol/hr = 18.405 Kg/hr Boiling point point of acetic acetic acid = 118 ºC Boiling point of water = 100 ºC Hence, water is more volatile component ( MVC) Water-Acetic acid (x-y data) :x y
0.1 0 .2 0.173 0.32
0.3 0.4 0 .5 0.6 0.447 0.557 0.654 0.74
ya = α xa/ 1+( α-1)xa substitute, y = 0.173 & x= 0.1 we get, α = 1.89 from graph :(xd/R+1)min = 0.36 (0.9/R+1)min = 0.36 Therefore, R min min = 1.5
L.I.T. Nagpur
0.7 0.8 0.9 1 .0 0.815 0.883 0.944 1.0
Manufacture of acetic anhydride
But, R opt opt = 1.5 R min min Therefore, R opt opt. = 1.5 × 1.5 = 2.25 R = Ln/ D Hence, Ln = 2.25 × 24 = 54 Kmol/hr Vn = Ln + D = 54 + 24 = 78 Kmol/hr Lm = Ln + F = 54 + 44.459 44.459 = 98.459 98.459 Kmol/hr Vm = 98.459 – 20.45 = 78.009 Kmol/hr
Therefore, equation of upper operating line :-
y n =
x R x n +1 + d R + 1 R + 1
0 .9 2.25 y n = x n +1 + 3.25 3.25 Therefore, yn = 0.692 xn+1 + 0.277 Hence, y-intercept = 0.277 & slope = 0.692 Equation of lower operating line (LOL) :-
y m +1 =
L M V M
x M −
Wx w V M
L.I.T. Nagpur
Manufacture of acetic anhydride
∴ y m +1 =
98 .459 78 .009
xm −
20.45 × 0.05 78 .009
∴ y m +1 = 1.262 x m − 0.013 Therefore, number of theoretical plates = 14 -1=13 (from graph) One plate is deducted for reboiler Efficiency of the column = 65% Therefore actual no. of plates required = 13/0.65 = 20 plates Feed plate = 6/0.65 = 10 th plate.
9.
Material of Construction :Acetic acid, ketene & acetic anhydride require a
suitable material to be used to avoid any corrosion and spoiling of the product. Aluminium though can be used throughout the process except in the hot zones of pyrolysis where stainless steel is used. Stainless steel 316 and copper are best suitable material that can be used in the process equipment. Stainless steel for process equipment has an additional advantage over copper of causing less discolourization and moreover for pure acetic anhydride
L.I.T. Nagpur
Manufacture of acetic anhydride
conden condenser ser silver silver coils coils are used. used. In genera generall stainl stainless ess steel steel of compos compositi ition on 33% 33% chromium, 1.5% aluminium, 1.5% silicon is used. For storage & transportation of pure acetic anhydride tanks made of aluminium, stainless steel ( 18% Cr, 8% Ni & 2% Mo ) or poly-ethylene are generally used. Although glass or enamel containers also may be empl employ oyed ed.. Iron Iron is high highly ly resi resist stan antt to acet acetic ic anhy anhydr drid ide, e, prov provid ided ed moist moistur uree is excluded. Hence it is possible to use iron in the production & workup in certain instances for example in pumps & tanks. Nickel is avoided in the alloys for a high temperature applications, since it is reported to accelerate the coking on heater tube walls unless pacified periodically by special pretreatment or injection of sulfur compounds.
Cost Estimation & Economics :Determining Determining Purchased Equipment cost(PEC) :-
Acetic Anhydride column :-
Internal diameter of the column = D i = 0.4644 m External diameter of the column = D o = 0.4644 m Volume of the distillation column = volume of the cylindrical shell
L.I.T. Nagpur
Manufacture of acetic anhydride
+ volume of elliptical heads
Therefore, V =
(
π DO
2
− D I 2 4
+
π D
12
3
( 0.4744 2 − 0.4644 2 ) 3.14 × 0.4644 3 = 3.14 × + 4
2
= 0.118 m
Density of stainless steel = 8000 kg/m3
Hence, weight of the material used for the column = 8000 × 0.118
= 944 kg
Considering the weight of the accessories to be = 150 kg
Hence, total weight of the material used for the column = 1094 kg
Cost of stainless steel = 192 Rs/kg
Cost index of 2004 = 432
Cost index of 2006 = 606.5
L.I.T. Nagpur
12
Manufacture of acetic anhydride
Therefore, the cost of the material for the column = (606.5 × 192 × 1094)/432
= 294893.778/- Rs
Cost of fabrication = 50% of the cost of material
Hence, the total cost of the column = 1.5 × 294893.778
= 442340.67 /- Rs.
Considering the cost of accessories i.e. condenser and reboiler reboiler to be = 2lakh 2lakh
Hence, total cost of the column along with the accessories = 642340.67 /- Rs.
Sr. No. 1 2 3 4 5 6 7 8 9 10
Equipment Acetic acid column Waste heat boiler Cooler Chiller Separator Ketene absorber Tale gas scrubber Vacuum jet Storage tanks Pyrolysis heater
Quantity 1 1 1 1 1 1 1 1 2 1
Cost 622000 320000 370000 420000 325000 678000 288000 301000 1022000 1522000
Total PEC =642340.67 + 622000 + 320000 + 370000 + 420000 + 325000 + 678000 + 288000 + 301000 + 1022000 + 1522000 Therefore, PEC = 6510340.67 Rs.
L.I.T. Nagpur
Manufacture of acetic anhydride
For a fluid processing plant the following quantities must be added to the PEC to get the physical plant cost or the PPC. P PC. Installation charges = 50% of PEC. = 3255170.335 Piping = 70% of PEC = 4557238.469 Instrumentation = 20% of PEC = 1302068.134 Electrical 10% of PEC = 651034.067 Site development = 5% of PEC = 325517.0335 Buildings and land cost = 1 Crore. Therefore total Physical plant costs (PPC) = 26601368.71 . Design and engg. = 30% of PPC = 7980410.613 Contractor’s fees = 5% of PPC = 1330068.436 Contingency = 10% of PPC = 2660136.871 Total indirect costs = 56250000. Fixed capital investment(FCI) = Total direct costs + Total indirect costs = 26601368.71 + 11970615.92 = 38571984.63 Working capital(WC) = 40% of FCI = 15428793.85 Total Capital investment(TCI) = FCI + WC = 50400000
L.I.T. Nagpur
Manufacture of acetic anhydride
Production Costs :-
FIXED CHARGES
Depreciation is 10% on machinery and equipment and 3% on buildings. Depreciation = 0.1 × FCI + 0.03 × Buildings. = 0.1 × 38571984.63 + 0.03 × 10000000 = 4157198.463 Local taxes = 4% of FCI = 1542879.385 Insurance = 1% of FCI = 385719.8463 Therefore total fixed charges = Depreciation + Local taxes + Insurance = 6085797.694
DIRECT PRODUCTION COSTS
Total product charges (TPC): Fixed charges = 10% of TPC. Therefore TPC = 60857976.94 Cost of raw material material = 25 × 60 × 51 × 24 × 300 = 550800000 Operating labour = 15% of TPC = 9128696.541 Supervisory control and clerical labour = 25% of operating labour = 2282174.135
L.I.T. Nagpur
Manufacture of acetic anhydride
Utilities = 20% of TPC = 12171595.39 Maintenance = 10% of FCI = 3857198.463 Operating supplies = 1% of FCI F CI = 385719.8463 Patents and royalties = 5% of TPC = 3042898.847 Plant overheads cost = 15% of TCI = 8100116.772
Manufacture Cost = 650626376.9
GENERAL EXPENSES Administrative expenses = 6% of TPC = 3651478.616 Distribution and selling costs = 15% of TPC = 9128696.991 Research & development cost = 5% of TPC = 3042898.8 Financing = 10 % of TCI = 5400077.848
General Expenses = 21223152.26
Therefore, Total production production cost(TPC) cost(TPC) = Manufacture Manufacture Cost + General Expenses = 650626376.9 + 21223152.26 21223152.26 = 671849529.2
L.I.T. Nagpur
Manufacture of acetic anhydride
Selling price = Rs 56 per kg. Total income = 50 50 × 19.2 × 102 × 24 × 300 = 705024000 Total profit = Total income − TPC = 3.32 Crore Tax on income = 40% of total profit = 1.328 Crore. Net profit = 1.992 Crore. Dividend = 10% of Net profit = 0.1992 Crore. Tax on dividend = 10% = 0.01992 Crore. Therefore total dividend = 0.21912 Crore. Profit = 1.77288 Crore.
Rate Of Return =
Net Pr ofit Total Investment
=
1.77288 50400000
× 100 = 35.17%
Pay Back Period = 1/0.3517 = 2.85 YEARS = 35 MONTHS
L.I.T. Nagpur
Manufacture of acetic anhydride
Process Control & Instrumentation :Digital feedback control of pyrolysis heater :-
The direct digital feedback control of pyroysis heat heater er is show shown n in the the figu figure. re. The The tempe tempera ratu ture re insi inside de the the pyro pyroly lysi siss heat heater er is measured by the thermocouple and the signal is sampled using a sampler switch. The discrete time signal in the analog form is converted in to digital signal by analog to digital converter. The electronic error detector generates the error which is the difference between the measured value and the set point. The generated error is minimized by the digital feedback controller monitored by computer. The output command signal from the digital feedback controller is converted into analog from by digital to analog analog converter. The hold element element is used to convert the discrete discrete time analog signal into the continuous analog signal. Electro-pneumatic transducer is used to convert the electric signal in to pneumatic signal. The output from the electr electro-p o-pneu neumat matic ic transd transduce ucerr acts acts on the diaphr diaphragm agm actuat actuator or of the pneuma pneumatic tic control valve which in turn regulates the flowrate of the fuel gas in order to maintain the temperature inside the pyrolysis heater at a desired value. The signals are transmitted through the transmission lines. The flowrate of fuel gas is the manipulated variable.
L.I.T. Nagpur
Manufacture of acetic anhydride
Digital Feedforward control of distillation column :-
The direct digital feedforward control of distillation column is shown in the figure. The two principal disturbances inlet flowrate and inlet composition are measured and the signals are samples using the sampler switch. The discrete time signal is converted in to digital form by analog to digital converter.The electronic comparator generates the errors which are nothing but the difference between the set points or the desired values and the corresponding measured values. The generated errors are minimized by the digital feedforward controller monitored by computer. The output command signals from the computer are in the digital form and converted in to analog signals using digital to analog converters. The discrete time signals are converted in to the continuous analog signal using hold elements. The electro-pneumatic transducers are used to convert the electric signal in to pneumatic signal. The pneumatic signal from the electro-pneumatic tran transsduc ducer in the the trans ransm missi ission on line ine used sed for for the the inle inlett feed feed com composit ositio ion n measurement acts on the diaphragm actuator of the pneumatic control valve which
L.I.T. Nagpur
Manufacture of acetic anhydride
in turn regulates the steam pressure in the reboiler. The pneumatic signal from the electro-pneumatic transducer in the transmission line used for inlet feed flowrate measurement acts on the diaphragm actuator of the pneumatic control valve which in turn regulates the reflux ratio. The signals are carried through the transmission lines. The steam pressure in the reboiler and the reflux ratio are the manipulated variables.
SCADA Configuration :-
The SCADA system configuration consist of following control units.
1) Superv Superviso isory ry contro controll station station 2) PLC PLC & RTU 3) Comput Computer er networ network k segment segment..
The process information such as temperature, flowrate, composition etc. are communicated between the process plant and PLC’s of control system.
Sr. No. 1 2 3 4 5 6
Control Equipment Pyrolysis heater Cooler Separator Ketene absorber Acetic acid column Acetic anhydride co column
Sensor Thermocouple Thermocouple Composition measurement Thermocouple & Composition measurement Flowmeter & Composition measurement Flowmeter & Composition measurement
L.I.T. Nagpur
Manufacture of acetic anhydride
Plant Location & Layout :-
The location of the plant can have a crucial effect on the profitability of a project, and the scope for future expansion. Many factors must be considered when selecting selecting a suitable site, the principal factors to consider are: 1. Location, with respect to the marketing area. 2. Raw material supply. 3. Transport facilities. 4. Availability of labour. 5. Availability of utilities: water, fuel, power. 6. Availability of suitable land. 7. Environmental impact, and effluent disposal. 8. Local community considerations. 9. Climate. 10. Political and strategic considerations.
Marketing Area :-
Acetic anhydride is widely used in the production of dyes as an intermediate. Hence, it is necessary to locate the plant near to dye
L.I.T. Nagpur
Manufacture of acetic anhydride
factories. From this prospective Mumbai is the ideal place for acetic anhydride plant since many dye factories factories are located close to Mumbai. Mumbai.
Raw Materials :-
The raw material for the plant is acetic acid. As Most of the acetic acid is produced from petroleum, hence it can be obtained very easily from the refineries. Hence it is advised that the site of the plant should be nearer to the refineries, so that the transportation cost is reduced. From this prospective Mumbai or any place nearer to Mumbai is the most suitable region for the plant.
Transport :-
The transport of materials and products to and from the plant will be an overriding consideration in site selection. If practicable, a site should be selected that is close to at least two major forms of transport: road, rail, waterway (canal or river), or a sea port. Road transport is being increasingly used, and is suitable for local distribution from a central warehouse. Rail transport will be cheaper for the long-distance transport of bulk chemicals. Air transport is convenient and efficient for the movement of personnel and essential equipment and supplies, and the proximity of the site to a major airport should be considered. All these facilities of transport are very easily available in a place like Mumbai,
L.I.T. Nagpur
Manufacture of acetic anhydride
hence from the prospective transportation facilities Mumbai is the ideal place for the project.
Availability Of Labour :-
Labour will be needed for construction of the plant and its operation. Skilled construction workers will usually be brought in from outside the site area, but there should be an adequate pool of unskilled labour available locally; and labour suitable for training to operate the plant. Skilled tradesmen will be needed for plant maintenance. Local trade union customs and restrictive practices will have to be considered when assessing the availability and suitability of the local labour for recruitment and training.
Utilities (services)
Chemical processes invariably require large quantities of water for cooling and general process use, and the plant must be located near a source of water of suitable quality. Process water may be drawn from a river, from wells, or purchased from a local authority. At some sites, the cooling water required can be taken from a river or lake, or from the sea; at other locations cooling towers will be needed.
L.I.T. Nagpur
Manufacture of acetic anhydride
Electrical power will be needed at all sites. A competitively priced fuel must be available on site for steam and power generation.
Environmental Environmental impact, and Effluent disposal :-
All industrial processes produce waste products, and full consideration must be given to the difficulties and cost of their disposal. The disposal of toxic and harmful effluents will be covered by local regulations, and the approp appropria riate te author authoriti ities es must must be consul consulted ted during during the initia initiall site site survey survey to determine the standards that must be met. An environmental impact assessment should be made for each new project, or major modification or addition to an existing process.
Local Community Considerations :-
The proposed plant must fit in with and be acceptable to the local community. Full consideration must be given to the safe location of the plant so that it does not impose a significant additional risk to the community. On a new site, the local community must be able to provide adequate facilities for the plant personnel: schools, banks, housing, and recreational and cultural facilities
L.I.T. Nagpur
Manufacture of acetic anhydride
Land (site considerations)
Sufficient suitable land must be available for the proposed plant and for future expansion. The land should ideally be flat, well drained and have suitable load-bearing characteristics. A full site evaluation should be made to determine the need need for piling or other special special foundations.
Climate :-
Adverse climatic conditions at a site will increase costs. costs. Abnorm Abnormall ally y low temper temperatu atures res will will requir requiree the provi provisio sion n of addit addition ional al insulation and special heating for equipment and pipe runs. Stronger structures will be needed at locations subject to high winds (cyclone/hurricane areas) or earthquakes.
Political and strategic Considerations :-
Capital grants, tax concessions, and other inducements are often given by governments to direct new investment to preferred locations; such as areas of high unemployment. The availability of such grants can be the overriding consideration in site selection.
L.I.T. Nagpur
Manufacture of acetic anhydride
GENERAL PLANT LAYOUT FOR A CHEMICAL INDUSTRY:
Maintain -ance Building
Future Expa-
Storage Building
Main Plant
nsion Scrap Yard
Research and Develo pment Centre Quality Control Laboratory
ETP
Fire Station
Medical Centre
Security Cooling towers
Control Room
Tank Yard Power
Wash and Changing
Utilities
Station
Room
Genera-
L.I.T. Nagpur Security
Garden
tion
Manufacture of acetic anhydride
Parking Space
Training
Administration
Centre
Building Canteen
L.I.T. Nagpur
Manufacture of acetic anhydride
L.I.T. Nagpur
Manufacture of acetic anhydride
L.I.T. Nagpur
Manufacture of acetic anhydride
L.I.T. Nagpur
Manufacture of acetic anhydride
L.I.T. Nagpur
Manufacture of acetic anhydride
L.I.T. Nagpur
Manufacture of acetic anhydride
L.I.T. Nagpur
Manufacture of acetic anhydride
L.I.T. Nagpur
Manufacture of acetic anhydride
L.I.T. Nagpur
Manufacture of acetic anhydride
L.I.T. Nagpur
Manufacture of acetic anhydride
L.I.T. Nagpur
Manufacture of acetic anhydride
L.I.T. Nagpur
Manufacture of acetic anhydride
L.I.T. Nagpur
Manufacture of acetic anhydride
L.I.T. Nagpur
Manufacture of acetic anhydride
L.I.T. Nagpur