EKC 451:Proces Design& Analysis Mass Transfer Equipment Design
DISTILLATION COLUMN DESIGN By Prof. Subhash Bhatia Zainal Ahmad, PhD
Objectives 1. 2. 3. 4. 5. 6. 7. 8.
Be able to determine the column operating conditions of pressure and temperature and type of condenser to use. Be able to determine the number of equilibrium stages and reflux required. Be able to select an appropriate contacting method( plates or packing). Be able to determine the number of actual plates or packing height required, together with feed and product locations. Be able to determine the tower diameter. Be able to determine other factors that may influence tower operation. Be able to determine the auxiliary equipment required for the tower. Be able to determine the hole size of sieve tray, down comer size and internal details of the distillation column.
Distillation Column Design
Specify the degree of separation required: set product specifications Select the operating conditions: Batch or continuous; operating pressure Select the type of contacting device: plates or packing Determine the stage and reflux requirements: the number of equilibrium stages.
Distillation Column Design
Size the column: Diameter, number of real stages. Design the column internals: plates, distributors, packing supports. Design of condenser and reboiler. Select the material of construction of all parts. Design the suitable insulation system. Mechanical Design: Vessel, internal fittings and skirt support for the vessel. Specifications Engineering Drawing with all the dimensions and details of column and its auxiliaries using AUTOCAD.
Industrial Separation Processes
Distillation
The separation process exploits the differences in the vapor pressure of the key components in the mixture.
The relative volatilities between key components to be separated can be as low as 1.2.
The separation process has the ability to handle wide ranges of feed concentrations and throughputs while producing a high purity product.
Distillation can be at disadvantageous when temperature is too low (- 40C) or high temperature ( 250C).If operating pressure are less than about 2kPa, column size and vacuum costs also escalate rapidly. Pressure greater than 5Mpa will result in an escalation of column costs.
Distillation as Separation Process ¾ ¾
Distillation is the least expensive means of separating mixtures of liquids. If relative volatilities of two components with neighboring boiling points is less than 1.1 or so distillation becomes very expensive, i.e.: - large reflux ratio - large vapor rate - large column diameter - large condensers - large reboilers (steam consumption high)
Separation Equipment: Distillation Column
2 basic approaches: - Design: New column to determine the column diameter and height required to achieve a specific separation. Stage to stage calculations: No. of equilibrium stages. - Rating: Existing column of given diameter and height. Flow capacity and separation are to be determined.
Design of Distillation Column ( Steps)
Set the product specifications Set the operating pressure Determine the number of theoretical stages required and the energy requirements. Determine the actual number of trays or height of packing needed and the column diameter. Design the column internals, which involve determining the dimensions of the trays, packing, liquid and vapor distribution systems and so on. Carry out the mechanical design to determine the thickness of the vessel walls, internal fittings and so on.
Design of Distillation Column 1. Distribution of light and heavy key components Light Key Component: It is a component of the feed mixture which is desired to be kept out of the bottom product. Heavy key component: It is a component of feed mixture which is desired to be kept out of the top product. If the light key and heavy key components are selected as per the order of volatility then they are known as adjacent keys. If any other component lies between them, then they are known as split keys. Key components are the two components of the feed mixture between which one likes to make the sharp separation. More volatile key component is called light key and less volatile component is called heavy key.
Design Parameters -
Min. no. of theoretical trays (Underwood – Frenske Equation) Distribution of non keys between the overheads and bottom products Minimum reflux Operating reflux No. of theoretical trays Location of the feed tray Tray efficiencies
Shortcut Procedures for Equipment Design
We use shortcut procedures for equipment design when use screen process alternatives.
Distillation Column - Short cut method - Rigorous tray by tray calculation
Overhead and Bottom Compositions - Fixed by product purity specifications - Rule of thumb: - 99.5% recovery of light key in the overhead. - 99.5% of the heavy key in the bottoms. Column Pressure - Operating pressure of a column normally is fixed by the economic desirability of using a condenser supplied with cooling water (90oF) from the cooling tower. - If the bubble point of the overhead mixture from the column is greater than 130oF, which allows a temperature driving force of 10oF at the condenser outlet, then we can use a total condenser and operate the column at atmospheric pressure.
Estimating Bubble Points and Dew Points Dew Point: yi =Ki xi
Ki αi = KH K
∑y
i
= 1 = ∑ K i xi
K i xi α i xi yi = = ∑ K i xi ∑ α i xi
Bubble Point Estimate
yi = Ki = xi
αi ∑ α i xi
Calculate K value for the light key to estimate the temperature
Dew Point yi = Ki = xi
1
xi ∑ yi / α i
Estimate Ki for light key and estimate the temperature Relative Volatility
α avg = α top .α bottom
Dew point and Bubble point Top most temperature=Dew point temperature of overhead vapor Use Antoine equation: vapor pressure of any component as a function of temperature. This equation is used in calculating K values at any temperature. Bottom most temperature=Bubble point of the bottom product.
Distillation Column Design (Sieve Trays) 1)
Establish the composition and physical properties of the feed and products, the feed rate and any special constrains such as maximum temperature and pressure drop.
2)
Selection of design variables: - Operating pressure: an increase in operating pressure is reflection of an increase in separation difficulty, an increase in reboiler and condenser temperature
- The operating pressure should be selected so that the bubble point of the overhead products is at least 5 to 10oC above the cooling water temperature. - Reflux ratio: 1.2 times the minimum reflux ratio. - Feed condition: requires more heat in the reboiler and less cooling in the condenser. 3)
Physical equilibrium data - V-L-E data (binary system) - Watson, NRTL and UNIQUAC models, UNIFAC --multicomponent as well as binary mixtures.
4)
Determination of No. of equilibrium stages: Fenske – Underwood – Gilliland’s Method ( FUG Method) Minimum No. of stages, Nmin
xLK = mol fraction of the light key xHK = mol fraction of the heavy key = the average geometric relative volatility of the LK to the HK. This average value is calculated by using the dew point temperature of the assumed overhead product and bubble point temperature of the assumed bottom product. Thus:
Minimum Reflux Ratio
Where the average geometric relative volatility of component I in the mixture relative to the heavy key = mol fraction of the component i in the feed. = mols of saturated liquid on the feed tray per mol of feed value is obtained by trial and error and lies between the relative volatilities of the two key components.
Minimum reflux Rmin
And number of equilibrium stages, N s given by:
Where R is the operating reflux selected by the designer. The distribution of non key component is distillate and bottoms is given as:
Finite Reflux conditions for Multicomponent Mixtures Minimum Reflux ratio from Underwood equation. Minimum number of stages from Fenske equation. Empirical relationship of Gilliland: Y= 0.2788-1.3154X + 0.4114 X 0.291 + 0.8268ln X+ 0.902 ln ( X+ 1/X) Where X= (R- R min)/ ( R+1) and Y= ( N-Nmin)/(N+1)
The above equation can be used to estimate number of theoretical stages the column requires.
xB,i = mol fraction of component I in the bottoms. xD,i = mol fraction of component, I in the distillate.
Kirkbride’s method: the ratio of trays above and below the feed point allows estimation of the feed tray location:
Where B and D are the mol flow rates of the bottoms and distillate respectively and ND and NB are the number of equilibrium stages above and below the feed tray respectively.
5)
Selection of the Column Internals - Trays – operating pressure & liquid flow rate high, diameter is large. - Random packings – small column diameter. - Structural packings, low pressure or vacuum operation, low liquid holdup, low delta P.
6)
Diameter evaluation for columns with sieve trays Column diameter requires calculation of net vapor velocity to be maintained in the column for distillation operation and depends on the hydrodynamics of the column, tray spacing and properties of liquid and vapor.
Distillation column design ( Height of the Column) Sieve trays column use tray efficiency to obtain total number of trays in the column. Random packing: Height calculated from NTU and HTU calculations. Structured Packing: Calculation of HETP. H = N x HETP HETP= 0.3 to 0.9 for random packing = 0.2 to 0.7 for structured packing
Diameter
Diameter
Diameter evaluation for Column with Sieve Trays
Determination of the column diameter requires calculation of the net vapor (gas) velocity at flood conditions, Vnf
Where Csb: Souders and Brown factor at flood conditions in m/s (Figure 15-5, depends on tray spacing)
σ = surface tension, dyne/cm ρL = density of liquid ρV = density of vapor Standard tray spacings for large-diameter column = 0.46 or 0.61 m.
The actual vapor velocity, Vact Vn = Vact = 0.5 to 0.9 Vnf
The net column area is
and Ac = An + Ad
Where An is the net column area. Ad is the down comer area. Ac = cross sectional area of the column = volumetric flow rate of vapor
Column diameter,
Height of the Column (Sieve trays) - Overall Tray Efficiency, Eo
= Liquid viscosity of the feed mixture
The actual column height, Hc
Hs = the tray spacing = the additional height required for column operation
O’ Connell’s Correlation Overall column efficiency
O’ Connell’s Correlation
Entrainment
Example
Solution
Approximate Column Sizing
The number of actual trays in the column is then Nact = N/Eo Eo = overall efficiency (=0.5) Number of trays for a gas absorber The design of plate gas absorbers and that of distillation columns have many similarities.
Distillation Column Plate Contactors - Sieve plate or tray: Plate containing small holes but larger holes and slots are used. - Bubble cap : Bubble cap riser contains a short pipe through which vapor phases and pipe through which vapor phases and pipe covered b a cap with a serrated edge or slots. - Valve plate: These are sieve plates with large diameter holes covered by movable flaps, which lift as the vapor flow increases.
Plate Contactor
Most common plate contactor used in distillation and absorption column
Cross‐flow plates
Sieve Trays 1. 2.
Sieve tray is most versatile contacting device and considered first. Sieve trays are not recommended for the following conditions: Low pressure drop , less than 2.5 mmHg. Very Low liquid flow rates are required below 0.6m3/(h)(m2) of active tray area.
Under these conditions look for other types of contacting devices like valve trays etc.
Plate Contactor
Sieve Tray in a Distillation Column
Plate Contactor
Valve Trays
Valve trays are cheaper compared to sieve trays. The openings in valve trays are covered with liftable caps that adjust themselves to vapor flow. The common hole diameter is 1.5 inch but sizes to 6 inch are available. The spacing of the standard diameter is 3-6 inch. With 3 inch spacing, the number of valves is 12 -14/sqft of free area. Valve trays are subject to fouling and defer to sieves for such services.
Plate Contactor
Tray operating in the forth regime
Downcomer The segmental or chord downcomer is the simplest and cheapest form of construction and is satisfactory for most purposes. 1.Straight segmental downcomer 2. Inclined segmental downcomer 3. Circular downcomers or pipes.
Tray in a distillation column
Adjustable Weir in the column
Plat layout - Down comer area= 0.12 x column area (12%) - Active area - Hole area - Hole size - Weir height
Packed Columns Packed columns are used for Distillation Gas absorption Liquid liquid Extraction Stripping (desorption) is the reverse of absorption and the same design methods will apply. The gas liquid contact in a packed bed column is continuous, not stage wise, as in a plate column. Choice of Plates or Packing 1. Plate columns can be designed to handle a wider range of liquid and gas flow rates than packed columns. 2. Packed column are not suitable for very low liquid rates.
Third generation Column Random Packings
Structured Packing Sulzer Mellapack
Packed column with structured and random packings
Plate Hydraulic Design The basic requirements of a plate contacting stage are that it should: - Provide good vapor – liquid contact. - Provide sufficient liquid hold up for good mass transfer (high efficiency). - Have sufficient area and spacing to keep entrainment and delta P within limit. - Have sufficient down comer area for the liquid to flow freely from plate to plate. Operating Range: Satisfactory operation will only be achieve over a limited range of vapor and liquid flow rates.
Vapor cross flow channeling
Flooding: At flooding there is sharp drop in plate efficiency and increase in delta P. Flooding is caused by either the excessive carry over of liquid to the next plate by entrainment of by liquid back up in the downcomers. The upper limit to vapor flow is set by the condition of flooding. Weeping: The lower limit of the vapor flow is set by the condition of weeping. Weeping occurs when the vapor flow is insufficient to maintain a level of liquid on the plate.
Coning: Occurs at low liquid rates, and is the term given to the condition where the vapor pushes the liquid back from the holes and jets upward, with poor liquid contact. Tray Selection and Design 1. Set tray spacing: Tray spacing is selected to minimize entrainment. Vacuum column: A large distance between the trays. Common tray spacing: 24 in (0.61 m) Tray spacing ranges from 0.15m to 1 m. Column diameter Tray spacing Less than 1 m 0.2 to 0.3m Above 1m 0.3 to 0.6m Generally chose tray spacing as 0.3m and check the column diameter after calculation.
Tray Selection and Design 1. Estimate column diameter - Calculate the maximum and minimum vapor flow rate - Estimate system physical properties - Estimate flooding velocity - Estimate vapor velocity = 0.60 x flooding velocity 2. Estimate the required free cross sectional area to accommodate the maximum allowable vapor velocity.
Plate Design Procedure
Specification of Sieve Tray Design
Hole dia, area, pitch and pattern. Blanking of Holes for less than eventual load. Downcomer type, size, clearance, and weir height ( weir height 2 inch fairly standard with weir lengths about 75% of the tray diameter). Tray thickness and material. Pressure drop. Turndown ratio before weeping begins. Liquid gradient.
Plate Area
Liquid‐Flow Arrangement Choice of plate type (reverse? Single‐pass? Or multiple pass?)
Depend on liquid flow rate and column diameter.
Entrainment
Weep Point
Weir Liquid Crest
Weir Dimension
Perforated Area
Hole Size
Hole Pitch
Hydraulic Gradient
Liquid Throw
Plate Pressure Drop
Plate Pressure Drop
Downcomer Design (Backup)
Downcomer Design (Backup)
Downcomer Design (Backup)
Downcomer Design (Backup)
Example11.2: Recovery of acetone
Acetone to be recovered from an aqueous stream by continuous distillation. A feed will contain 10%w/w acetone. Acetone ofat least 98% purity is wanted, and the aqueous effluent must not contain 50ppm acetone. The feed will be at 20C. Estimate the number of ideal stages required.
Results Total number of stages=16 Total number of stages below feed=9 Use Sieve plates Minimum feed rate= 70% of maximum feed of 10,000kg/h.
Plate Hydraulic Design Column Diameter Liquid Flow Pattern Provisional Plate Design: check 1.Weeping 2. Plate pressure Drop 3. Downcomer Liquid Backup 4. Entrainment 5. Trial Layout: Perforated Area, Number of holes, Plate specifications
Other types of distillation Processes
Short Path Distillation (SPD): SPD is very high vacuum distillation. Operating pressure at distillation surface in short path distillation unit is as low as 10-6 bar(1 micro bar). SPD provides the distillation at the minimum possible temperature which is desirable for heat sensitive products.
Other types of distillation Processes Reactive and Catalytic Distillation: Reaction and separation in single column. Azeotropic Distillation Extractive Distillation Pressure Swing Distillation
Distillation Column Auxiliaries For distillation columns, we also must design the condenser and reboiler. -Requirements of cooling-water and steam. Column condenser and cooling water The condenser heat duty is the heat required to completed condenser the vapor passing overhead. With cooling water available at 90oF and being returned at 120 oF, heat balance give:
We normally assume that an overall heat transfer coefficient for the condenser
The required heat transfer area for the condenser is
The required flow of cooling water is
Reboiler and Steam Supply If we use steam to supply the heat to produce of vapor at the bottom of the tower, a heat balance gives:
The temperature driving force in the reboiler must be constrained to be less than about 30 to 45 oF, to prevent film boiling We expect to obtain a very high value of the overall heat transfer coefficient in the reboiler. With this approximation the required heat transfer area is
and the required steam supply is
For 25 psi steam where, The vapor rate in the bottom of the tower depends on the quality of the feed q:
Where
Which is the heat required to convert 1 mol of feed to a saturated vapor divided by the molal latent heat of vaporization
Summary
Select an appropriate operating pressure for a multistage tower and a condenser type for distillation. Determine the number of equilibrium stages required for a separation and reasonable reflux ratio for distillation. Determine whether trays, packing or both should be considered. Determine the number of actual trays or packing height required. Estimate the tower diameter. Consider other factors for successful tower operation.
References
Chapter15: Separation Equipment, Peter and Timmerhaus,5th edition Chapter 14: Separation Tower Design, Seider, Seader and Lewin Book, 2nd edition, Product and Process Design Principles Chapter 11,Separation columns, Chemical Engineering Design, R K Sinnott,4th Edition.
The End
