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CHAPTER 2
PROCESS MODELING AND FLOWSHEETING
2.1
Introduction
The process overview illustrates the urea plant process design. This diagram is a layout of all units operation, which are needed in proper sequence to convert the raw material to desired product, ethylene glycol.
The plant is to produce a total amount of 100,000 metric ton ethylene glycol per annum. The operating day for this plant is 335 days per year and 24 hours a day. The purity of the urea in the market is 99.8% for glycol industrial grades and 98% for glycol antifreeze grade.
In this ethylene glycol plant design, several main synthesis and other process will involve there are :-
Mixing section
Ethylene Glycol synthesis
Ethylene Glycol purification
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2.2 Preliminary Process Synthesis Step
2.2.1 Eliminate Differences in Molecular Type
There are several ethylene glycol synthesis methods but those synthesis methods have different raw material and yield different composition as well as the product composition.
These are the synthesis methods that have been discussed in Chapter 1:
Pathway 1 = Hydrolysis of Ethylene Oxide Pathway 2 = OMEGA Process
As been discussed in Chapter 1, eventhough, the gross profit for pathway 2 is mostly desirable compared to Pathway 1, we will be choosing the synthesis 1 that is hydrolysis of ethylene Oxide. It is because Pathway 1 is profitable, widely use and the conversion of reaction in Pathway 2 is higher compared to the conversion of reaction in Pathway 1 at optimum condition of temperature and pressure.
2.2.2 Distribute the Chemicals Reaction path: CH2CH2O + H2O → HOCH2CH2O ethylene glycol 2CH2CH2O + H2O → HOCH2CH2OCH2CH2OH diethylene glycol
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3CH2CH2O + H2O → HOCH2CH2OCH2CH2OCH2CH2O triethylene glycol
Conversion of reactant (Ethylene Oxide) =90%
From the stoichiometry of the reaction, to produce 1 mole of ethylene glycol, we need 1 mole of ethylene oxide and 1 mole of water. In this reaction, due to conversion of toluene is only 90%, the excess ethylene oxide is present in the product stream. However, for this reaction to successfully performed, there is ethylene oxide – water ratio which is 1:12. Hence, there will be several side reaction which is ethylene glycol will react with water to form diethylene glycol and the diethylene glycol will react with water to form triethylene glycol.
In order to produce 203.58 kgmol/hr of Ethylene Glycol, the flow rate of the reactant that we need to obtain the desired product is shown below : Ethylene Glycol production rate = 203.58
𝑘𝑔𝑚𝑜𝑙 ℎ𝑟 𝑘𝑚𝑜𝑙
Ethylene Oxide feed rate
= 230.73
Water feed rate
= 2768.72
ℎ𝑟 𝑘𝑚𝑜𝑙 ℎ𝑟
Hence, to produce 203.58 kmol per hour of Ethylene Glycol, we need 230.58 kmol per hour of Ethylene Oxide and 2768.72 kmol per hour of Water in the reactor feed stream.
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2.2.3
Eliminate Differences in Composition
According to the third synthesis step, there are several product stream components which involves with separation process.
Flash Drum
Monoethylene Glycol
Ethylene Oxide
Diethylene Glycol Triethylene Glycol
Flash Drum Monoethylene Glycol
Ethylene Oxide Water
Diethylene Glycol Triethylene Glycol Water
Multiple Effect Evaporator
Monoethylene Glycol
Water
Diethylene Glycol Triethylene Glycol Water
Evaporator Monoethylene Glycol Diethylene Glycol Triethylene Glycol
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Distillation Column Monoethylene Glycol Monoethylene Glycol Diethylene Glycol
FC1
Triethylene Glycol
Diethylene Glycol Triethylene Glycol
Table 2.1: Boiling Point of Product Stream Component
Boiling Point (oC)
MonoEthylene Glycol
197.0
DiEthylene Glycol
245.0
TriEthylene Glycol
288.0
Ethylene Oxide
10.7
Water
100.0
Based on the heuristic for determining favorable sequences: 1. Remove final product as a distillate one by one. 2. Sequence separation in order of decreasing relative volatility so that the most difficult splits are made in absence of the other components. 3. A sequence separation point to remove is based on the greatest molar percentage of the components in the feed. 4. Remove thermally the unstable component in the sequence.
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2.2.4
Eliminate Differences in Temperature, Pressure
Figure 2.1: Process Flow Diagram of Production of Ethylene Glycol
From the figure 2.1, we can see that, to eliminate the difference between temperature, pressure and phase, we are using temperature changer which is heat exchanger and for pressure, we are using compressor and pump to change the pressure.
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2.3
Process Description
This process produced ethylene glycol by the hydrolysis of ethylene oxide in the presence of excess water. After the hydrolysis reaction is completed the glycol is separated from the excess water and then refined to produce monoethylene glycol (MEG). Water and ethylene oxide are both fed at temperature of 25oC and 5oC respectively at pressure of 1 atm. The reaction will occur in gas phase at a temperature of 120oC and pressure of 1 atm. Before entering the reactor we need to increase temperature to reactor conditions.
2.3.1 Mixer
Figure 2.2: Schematic diagram of Mixer
The multiple input streams are mixed well before they are entering reactor to ensure that all the reactant is perfectly mix each other. The mixer helps to maximize the reaction in Plug Flow Reactor on the next stage operation point. Feed to the reactor is a fresh raw material and recycle stream of ethylene oxide and water.
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2.3.2 Reactors (PFR)
Figure 2.3: Schematic diagram of Plug Flow Reactor
The plug flow reactor model (PFR, sometimes called continuous tubular reactor, CTR) is a model used to describe chemical reactions in continuous, flowing systems of cylindrical geometry. It runs at steady state with continuous flow of reactants and products.
(R-001) Operating condition:
Operating temperature : 120˚C
Operating pressure : 1 atm
Overall conversion : 90%
Limiting reactant : ethylene oxide
Ratio ethylene oxide to water is 1:12
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2.3.3 Vapor-liquid separator (Flash Column)
Figure 2.4: Schematic diagram of Flash Column
A vapor–liquid separator may also be referred to as a flash drum to separate vapor-liquid mixture. A liquid mixture feed is pumped through a heat exchanger to meet the operating temperature required by flash drum to separate mixture. It then flows through a vertical vessel where gravity is utilized causing the liquid to vaporize.
Because the vapor and liquid are in such close contact up until the "flash" occurs, the product liquid and vapor phases approach equilibrium. So it can separate ethylene oxide from ethylene glycol and water of the stream S-7. One more flash separator is operates to separate ethylene oxide and water from mono, di and triethylene glycol of the stream S-20. The main advantage of flash drum is the relatively low cost.
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2.3.4 Distillation Column
Figure 2.5: Schematic diagram of Distillation Column
In order to separate mixture based on the component boiling point, a distillation column is used. The stream S-23 contains MEG, DEG and TEG. The boiling points of MEG, DEG and TEG are 197.3oC, 245oC and 285oC respectively. DIST-100 separates MEG from DEG and TEG. The MEG being the lighter key is obtained as distillate. The distillate stream S-26 is flow to MEG storage. The bottom stream S-25 from DIST-100 contains DEG and TEG is flow to waste treatment system.
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2.3.5
Evaporator
Figure 2.6: Schematic diagram of Evaporator
A multiple-effect evaporator is use as an apparatus for efficiently using the heat from steam to evaporate water. In a multiple-effect evaporator, water is boiled in a sequence of vessels, each held at a lower pressure than the last. In first effect evaporator EVA-01, certain amount of vapor ethylene oxide and water is evaporates and sends to waste treatment system via S-14.
While, the second effect unit evaporator evaporates the vapor ethylene oxide and water which are not evaporate in EVA-01 and send to waste treatment system via S-15. The boiling temperature of water decreases as pressure decreases, so the vapor boiled off in first effect evaporator, EVA-01 is use to heat the second effect evaporator, EVA-02.
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Same concepts apply to third effect evaporator EVA-03, the vapor boiled off in EVA-02 is use to EVA-03 because multi-effect evaporator is a sequence of vessels. The water vapor is evaporates in this unit and recycle back to S-16 before entering mixer MIX-01. Then, the liquid components stream, S-13 entering the flash column V-002.
2.3.6
Compressor
Figure 2.7: Schematic diagram of Compressor
This isothermal compressor is use to increase pressure of water recycle stream S-16 from 0.0987 atm to 1 atm by compressing the water vapor. Water stream S-16, change phase from vapor to liquid before entering cooler.
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2.3.7
Pump
Figure 2.8: Schematic diagram of Pump
This is isothermal pump increasing the pressure of stream S-13 from 0.0987 atm to 1 atm by moving the fluids. Pump is identical to compressor but differ only in the phase involves. Pump is dealing only with liquid phase while compressor is only for gas phase.
2.3.8
Heat Exchanger
Figure 2.9: Schematic diagram of Heat Exchanger
Heat exchanger used to transfer heat into or from the stream to the process fluid. In most chemical reaction process heat transfer is required to heat up or cool down the stream of fluid.
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In this plant we used several heat exchanger shell and tube type such as Heater (HX-01), (HX-05), (HX-06) and Cooler (HX-02), (HX-03), (HX-04).
Heater (HX-01): This is heater that increases the temperature of the stream S-4 to 120oC. Ethylene oxide and water stream S-4, change phase from liquid to vapor before entering plug flow reactor R-001.
Cooler (HX-02): This is cooler that decrease the temperature of the ethylene glycol stream S-6 to 98oC.
Cooler (HX-03): This is cooler that decreases the temperature of the recycle ethylene oxide stream S-8 to 5oC. Ethylene oxide stream S-9, change phase from vapor to liquid before entering mixer MIX-01.
Cooler (HX-04): This is cooler that decreases the temperature of the recycle water stream S-17 to 5oC and fed to mixer MIX-01.
Heater (HX-05): This is heater that increases the temperature of the stream S-19 to 160oC.
Heater (HX-06): This is heater that increases the temperature of the stream S-22 to 197.3oC.
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2.4
Process Flow Diagram (PFD) of Ethylene Glycol Plant
Figure 2.10: Process Flow Diagram (PFD) of Ethylene Glycol Plant
