UEMK4323 PROCESS AND PLANT DESIGN I
MAY 2014
ASSIGNMENT I
TASK II Name Lee Jian Yang Yew Ren Dieh Hua Loong Kwan Yi An Wah Tee Yan
Student ID 11UEB02057 11UEB02058 11UEB04154 11UEB01740 10UEB01875
Group 2 Lecturer/Tutor : Dr. Chin Siew Kian
Year/Trimester Y4/T1 Y4/T1 Y4/T1 Y4/T1 Y5/T2
2 Introduction
In the first task of the assignment (Task 1), it was decided that the designed plant will use the EBOneTM Process to produce ethylbenzene (EB) due to its substantially lower capital cost, along with high yield and less environmental impacts. The EBOneTM Process also has comparatively easier process control and instrumentation procedures. To recap, an overview of the EBOneTM Process is briefly discussed in this section.
The EBOneTM Process is one of the process that utilises the catalytic alkylation of benzene with ethylene using zeolite-based catalysts to produce ethylbenzene. The usage of zeolite-based catalysts (EBZ-500 and EBZ-100) is to replace the conventional Friedel – Crafts catalyst (AlCl3 and HCl) which is highly corrosive to the units of the plants. Like any other alkylation process, the EBOneTM Process consists of two main processes namely alkylation of benzene with ethylene followed by transalkylation of di-ethylbenzene with benzene. In general, benzene undergoes an alkylation reaction with ethylene to form ethylbenzene with a side product of di-ethylbenzene. The diethylbenzene is then separated from ethylbenzene and transalkylated with benzene to form ethylbenzene. The overall reaction is further illustrated using the figure below.
Figure 2: Reaction Steps in Direct Alkylation of Benzene and Ethylene. Note: From High Performance Catalyst for Liquid Phase EB Technology by Narsolis, F., Woodle, G., Gajda, G. and Gandhi, D.
3 Based on the description of the process, it can be inferred that the types of materials involved in the plant is benzene, ethylene, ethylbenzene, diethylbenzene and zeolite catalyst. Thus, it is crucial to compile a preliminary database which includes the thermophysical properties of the materials, environmental and safety data, prices and in some cases data obtained from laboratory experiment. All these data are easily made available from material safety data sheets that were compiled in Task 1. These data will be summarized in the preliminary database creation section.
Preliminary Process Synthesis Preliminary Database Creation
Table 1: Summary of relevant thermophysical, environmental and safety and chemical prices data of reactants, catalyst and products. Reactants
Catalyst
Product
Ethylene
Benzene
Zeolite
Ethylbenzene
Formula
C2H4
C6H6
-
C6H5CH2CH3
Melting point (°C)
-169
5.5
N/A
-94.9
Boiling point (°C)
-103.8
80
N/A
136
Flash point (°C)
-136
-11
N/A
15
Density (kg/m3)
156.8
880
N/A
867
Environmental and Extremely
Highly
N/A
Highly toxic
safety remarks
flammable
toxic
Price (USD/lb)
0.63
3.45
Varies
0.61
4 Identification of Desired Specifications of Raw Materials and Products
As mentioned in Task 1, ethylbenzene producing plants in the US have capacities which ranges from 133 to 1440 thousand ton per year which is equivalent to 15.2 to 164.4 ton per hour respectively. Given that the global demand and consumption of ethylbenzene is increasing annually, it is reasonable to aim to design a plant that has a capacity within the mentioned range. Thus, it is reasonable to design a start-up plant that still has a capacity that falls within the range of other well established plants. Therefore, the capacity of the plant is set to the average the value of the lower and upper boundary of the range which is approximately 50 ton per hour or 50 000 kg per hour.
Two raw materials are required to produce ethylbenzene namely ethylene and benzene. As the raw materials need to undergo a series of reactions to form the desired product, the plant capacity should be expressed with a molar basis instead of a mass basis. Since the molecular weight of ethylbenzene is 106.17 kg/kmol, the equivalent plant capacity on a molar basis is 471 kmol per hour. Due to the stoichiometry, the minimum amount of benzene and ethylene required to meet the plant capacity is 471 kmol per hour which is conveniently rounded up to 480 kmol per hour. Converting the flow rate to a mass basis yields 13 464 kg per hour of ethylene and 37 493 kg per hour of benzene respectively.
Plant capacity
: 50 000 kg/h × 106.17 kg/kmol = 471 kmol/h ≈ 480 kmol/h
Minimum ethylene rate : 480 kmol/h × 28.05 kg/kmol = 13 464 kg/h ≈ 13 500 kg/h Minimum benzene rate : 480 kmol/h × 78.11 kg/kmol = 37 493 kg/h ≈ 38 000 kg/h
5 There is however another side product namely di-ethylbenzene (DEB) which is produced due to excessive alkylation of benzene in the reaction. Although this product is recycled in the plant and reacted with benzene in a trans-alkylation process, the composition of this side product does affect the purity of the desired product. Furthermore, the determination of the composition of our product is premature in this step as detailed mass and energy balances around the reactors and distillation units of the plant is required to determine the purity of our product. Therefore, the total flow rate of the product of the plant is still uncertain due to the presence of side product although the flow rate of the desired product by itself is known.
With the flow rates of the raw materials and products determined, other parameters that needs to be determined include their respective phases, temperature and pressure. In accordance to the EBOneTM process, the raw materials and products are specified to be in liquid phase. Assuming that both raw materials are pure and supplied at atmospheric pressure and temperature ranging from 20 to 30°C, which is the average range of temperature in Dalian, China, both benzene and ethylbenzene will exist in its liquid form (vapour fraction of zero) while ethylene will exist in its gaseous form (vapour fraction of 1). Hence, the mixture of ethylene and benzene will yield an outflow of a mixture of liquid and vapour. Therefore, the outflow is typically cooled using a shell and tube heat exchanger to eliminate the vapour phase in the outflow before entering the reactors. The table below summarizes the specifications of the raw materials and products
Table 2: Summary of Specifications of Raw Materials and Products Name
Type
Total
flow Composition
rate (kg/h) Ethylene
Reactant
Phase at 25°C and 1 atm
13 500
1.0
Gas
Benzene
38 000
1.0
Liquid
Ethylbenzene Product
Unconfirmed 0.95
Liquid
6 Eliminating Differences between Molecular Types
The elimination of differences between the molecular types of the raw materials namely ethylene and benzene is achieved through two main set of reactions namely the alkylation reaction and the trans-alkylation reaction. In the alkylation part of the reaction, ethylene is reacted with benzene to form ethylbenzene as shown in the equations below. C2H4 + C6H6 → C6C5CH2CH3 = 8.4 × 10 exp (−
45.8
.
)
.
This is the main reaction that is typically depicted in the alkylation of benzene with ethylene. However, MacDonald, Roda and Beresford in 2005 suggested that there are two other reactions that occur simultaneously when the main reaction in the previous page takes place. The two reactions are listed as follow. C6H6 + 2C2H4 → C6C4(CH2CH3)2 = 6.03 × 10 exp (−
61.6
.
)
.
C6H5CH2CH3 + C2H4 → C6C4(CH2CH3)2 = 8.5 × 10 exp (−
86.4
)
.
.
Each reaction is governed by a rate law which is dependent on temperature. Besides the alkylation set of the reaction, there is another set of reaction that is reversible namely the trans-alkylation reaction. The equation that governs the reaction is as follows. C6H6 + C6C4(CH2CH3)2 ↔ C6C5CH2CH3 ln
=
276.6
− 0.3599
7 It is important to note that the alkylation and trans-alkylation reaction utilises different type of zeolite-based catalyst and hence requires different operating conditions i.e. different temperatures and pressures. Distribution of Chemicals
As ethylene is set to be the basis, we first distribute the chemicals in the process under ideal circumstances by assuming 100% of ethylene fed in is converted in accordance to stoichiometry i.e. 1 mole of ethylene produces 1 mole of ethylbenzene. Steady state conditions are also assumed by setting the production rate and the purity of the products to be constant.
The per-pass conversion of ethylbenzene strongly depends on the reactants ratio. Based on findings on the EBOneTM Process, the Benzene to Ethylene molar ratio in alkylation reactor is about 4 to 6 at temperature range from 125oC to 175oC will best fit the process in terms of overall conversion, product selectivity and yield. Excess 8~10% of fresh benzene is fed into the reaction streams in order to promote the desired product (EB)’s selectivity. However, excess benzene will compromise the final product’s purity, although this can be mitigated by the addition of a purge stream, where it will fulfil the benzene balance and avoid the accumulation of excess benzene and thus easing the separation between benzene and the final product.
The trans-alkylation reactor recovers the over-alkylised benzene (i.e. polyethylbenzene). It is assumed that only di-ethylbenzene (DEB) is formed without further alkylisation to higher degree of poly-ethylbenzene as DEB is the major byproduct formed in the process while other over-alkylised products which are known as tar in this process are of negligible amount. Moreover, to decide the recycling capacity of unreacted benzene, we further assume that the molar ratio of Benzene to DEB is about 13.
8 To deviate from ideal assumptions which assumes 100% ethylene conversion, the overall conversion of the process needs to be determined. According to Zhang, Li, Fu and Cao in 2003, the overall conversion of ethylene to ethylbenzene is 95% at minimum. Therefore, attempts were made to vary the overall conversion to 95%. In such case, there will be an excess of ethylene and benzene that needs to be removed simultaneously. Again, it is assumed that the production rate and purity is fixed. Since the overall conversion is lowered, there must be reimbursement at the fresh feed of ethylene. However, fresh feed of benzene remains unchanged.
Removal of benzene and impurities are essential for EB production process. Where Stream 13 is the benzene and ethylene (if there is any) mixture that is sent out from the process circuit to avoid accumulation of reactants. Notably, the composition and flow rate of such purge are varied while subjecting to the excess amount of fresh feed of ethylene and benzene. In both cases, it is presumed that some heavy organic impurities are obtained in the form of tar which can be removed via installation of a poly-ethylbenzene distillation column as shown. To optimize the product’s purity, the high weightage of benzene downstream of the reactors (i.e. Stream 5) has to be separated by benzene distillation column before entering product distillation column.
Table 3: Overall Reaction Summary of EB Production for 0.9999 (approximately 1.00) Conversion of Ethylene
Molecular Weight
+
B
→
Flow rate
E
EB
kg/kmol
28.05
78.11
106.165
kmol/h
480
525
0
kg/h
13500
41000
0
kg/h
13500
37500
51000
kmol/h
480
480
480
kg/h
13464
37492.8
50959.2
Initial (X=1)
(Round up)
Converted
9 kmol/h
0
45
480
kg/h
0
3514.95
50959.2
Remained
Excess Benzene needs to be removed
Annual production of EB ≈ 51000
Desired product with 95.8% purity obtained
kg kmol ≈ 480 h h
Assume EB at the product stream has purity of 95.8% with 4.2% Benzene. Since the overall conversion based on ethylene is assumed to be 100%, therefore, for overall mass balance is as follows.
Product 1
EB Production Process
13
2 16
Figure 1: Overall Mass Balance of EB Production Process
̇
= ̇ × (MW) ̇
=
̇
,
̇
̇
,
×[
,
× MW ) + (
,
× MW ]
kmol 480 h = × (0.958 × 106.165 + 0.042 × 76.11) 0.958 kg ≈ 52600 h
10
During the alkylation process, assume about 300~500kg/hr of tar is formed ∴ let ̇
= 300
kg h
For 100% conversion of ethylene, the least value of fresh ethylene feed, ̇
,
∴
= ̇ ,
= 480
,
= 480
kmol h
kmol kg kg × 28.05 ≈ 13500 h kmol h
About 9% excess of fresh benzene is fed in this process in order to promote the per pass conversion of the reactors: ̇
,
̇
,
kmol kmol = 523 h h kmol kg kg = 523 × 78.11 ≈ 41000 h kmol h
= 1.09 ̇
,
= 1.09 480
Overall mass balance: ̇ + ̇ − ̇ − ̇
− ̇
=0
For alkylation and trans-alkylation reactor: 3
Alkylation Reactor
Product 4
99.0~99.5mol%
78.5mol%
5
12
11
TransAlkylation Reactor
21.5mol%
Separation
0.5~1.0 mol% Recycle
11 Figure 2: Mass Balance on Alkylation and Trans-alkylation Reactors
̇ , kmol = 485 0.99 h
̇
,
=
̇
,
= 0.785 ̇
̇
= 0.215 ̇
,
kmol h kmol = 105 h
= 380
,
,
At 125 – 175°C, the reaction’s equilibrium conversion based on ethylene in the alkylation reactor is approximately 72.33%. ∴ ̇
,
=
̇ , kmol = 525 0.7233 h
Benzene composition in Stream 3: According to the EBOneTM process, the molar feed ratio of Benzene to Ethylene is about 4 – 6, assuming a value of 5, we obtain, ̇
,
= 5̇
where
,
kmol h 1 5 = , ≈ (0.17,0.83) 6 6
≈ 2657
,
,
,
Trans-alkylation conversion based on ethylene is approximately 17.57% ∴ ̇
,
=
0.1757 ̇ 2
,
= 46
kmol h
The unreacted ethylene and benzene at Stream 4: ̇
,
= ̇
,
(1 − 0.7233 − 0.1757) ≈ 57
kmol h
12 During the alkylation process, assume about 300 – 500 kg/h of tar is formed. In 100% of conversion from E to EB, it is assumed that 100% of tar is produced from benzene.
̇
∴ the benzene lost due to the formation of tar =
,
= ̇
,
− ̇
,
− ̇
≈ 2226
kg 300 hr kg 78.11 kmol
= 3.84
− bezene loss = 2657 − 380 − 46 − 3.84
,
kmol h
Table 4: Summary of Stream 3 Molecular Weight
̇
Composition,
̇
(kg/kmol)
X
(kmol/h)
(kg/h)
E
28.05
0.165
525
14700
B
78.11
0.835
2657
207600
EB
106.165
0
0
0
DEB
134.22
0
0
0
Total
3182
222300
kmol kg
13 Table 5: Summary of Stream 4 Molecular Weight
̇
Composition,
̇
(kg/kmol)
X
(kmol/h)
(kg/h)
E
28.05
0.021
57
1600
B
78.11
0.822
2226
173900
EB
106.165
0.14
380
40300
DEB
134.22
0.017
46
6200
Tar
-
-
-
300
Total
2713
222300
In the trans-alkylation reactor, about 84% of DEB is recovered to EB, to balance the whole recycle loop of the process, since the recovery rate of DEB in the transalkylation reactor is equivalent to the formation rate of DEB in the alkylation reactor and the recovery of DEB is approximately 84%, ̇
,
̇
,
kmol h kmol =9 h = 56
The molar ratio of Benzene to DEB in the trans-alkylation reactor is approximately equals to 13. ∴ ̇
,
And ̇
= 13 ̇
,
,
≈ 713
kmol h
≈ 0.5 mol% EB recycled from stream 5 = 0.005(485) ≈ 2
kmol h
14 1
3
4
14
Reaction 8
5 12
10 6 7
9
11
Figure 3: Ethylene Balance for the Loop ̇
= ̇
,
̇
= ̇
,
= ̇ =
− ̇
,
+ ̇
,
+
,
̇
,
= 525 − 480 = 45
,
̇
,
+ ̇
,
= ̇
,
− 45
= ̇
,
+ ̇
,
kmol h
,
− ̇
,
− 45
− 45 = ̇
,
+ 10
60% of ethylene is converted into EB in Transalkylation Reactor: 1−
̇
̇
,
̇
,
̇
,
̇
,
= 0.40
= ̇
,
∴ ̇
= 0.60
,
,
= 13
+ 10 kmol , ̇ h
,
=5
kmol h
The unreacted benzene at Stream 12: ̇
,
= ̇
̇
,
= 713 − (13 − 5) − (56 − 9)
̇
,
= 657
,
−
kmol h
̇
,
− ̇
,
−
̇
,
− ̇
,
15 Table 6: Summary of Stream 11 Molecular Weight (kg/kmol)
Composition, X
̇ (kmol/h)
̇ (kg/h)
E
28.05
0.017
13
400
B
78.11
0.908
713
55700
EB
106.165
0.003
2
200
DEB
134.22
0.072
56
7500
Total
784
63800
Table 7: Summary of Stream 12 Molecular Weight (kg/kmol)
Composition, X
̇ (kmol/h)
̇ (kg/h)
E
28.05
0.007
5
200
B
78.11
0.846
657
51300
EB
106.165
0.135
105
11100
DEB
134.22
0.012
9
1200
Total
776
63800
Table 8: Summary of Stream 5 Molecular Weight (kg/kmol)
Composition, X
̇ (kmol/h)
̇ (kg/h)
E
28.05
0.018
62
1700
B
78.11
0.826
2883
225100
EB
106.165
0.14
485
51500
DEB
134.22
0.016
56
7500
Total
3486
285800
16 Table 9: Overall Reaction Summary of EB Production for 95% Conversion of Ethylene
Molecular Weight
Flow
E
+
B
=
EB
kg/kmol
28.05
78.11
106.165
kmol/h
505
525
0
kg/h
14200
41000
0
Kg/h
13500
37500
51000
kmol/h
480
480
480
kg/h
13464
37492.8
50959.2
kmol/h
25
45
480
kg/h
701.25
3514.95
50959.2
95% conversion of ethylene. Excess ethylene needs to be removed.
Unchanged excess benzene. Needs to be removed.
Initial (X=1)
(Round Up)
Converted
Remained
Desired product with purity of 95.8% is obtained
Eliminating Differences in Composition
1. Benzene Distillation Column With the aid of the spreadsheet provided by ChemSOF, as a preliminary process synthesis, we assume the separation is a binary system consisting of the dominant keys in the separation process. In benzene distillation, based on the summary of Stream 5, the dominant keys are Benzene (14%) and Ethylbenzene (82.6%). Apart from ethylene and DEB, the binary composition of E and EB are: ( ) ,
( ) ,
0.14 = 0.145 0.826 + 0.14 0.826 = = 0.855 0.826 + 0.14
=
17 Type equation here.And the desired distillate composition of light key, B is: ( ) ,
= 0.99
Similary, desire bottom composition of light ke, B is: ( ) ,
= 0.05
General reflux ratio ≈ 1.5 Feed temperature = 107 C
Through Antoine Equation, we obtained the saturated pressure of B and EB at 107 °C as follows. ln[
(mmHg)] = A −
B (K) + C
,
= 1584.09mmHg ,
,
= 322.97 mmHg Relative volatility of B and EB,
,
=
= 4.9047
,
The values above are plugged into the spreadsheet to generate a McCabe Thiele diagram in order to show the approximate number of stages and its feed stage location. However, such diagram only indicates the fine separation between B and EB, regardless of the presence of E and DEB. Considering the existence of E and DEB in the column, it is estimated that the minor light key, ethylene would be distillated and the minor heavy keys, DEB and Tar would be liquefied. Such diagram indicates that the separation of benzene from ethylene is relatively easy, and we predict that Stream 6 and 14 has the approximate composition as shown below.
18
Table 10: Summary of Stream 6 Molecular Weight (kg/kmol)
Composition, X
̇ (kmol/h)
̇ (kg/h)
E
28.05
0.020
58
1600
B
78.11
0.976
2860
223100
EB
106.165
0.002
7
700
DEB
134.22
0
0
0
Total
2925
226500
Table 11: Summary of Stream 14 Molecular Weight (kg/kmol)
Composition, X
̇ (kmol/h)
̇ (kg/h)
E
28.05
0.001
4
100
B
78.11
0.051
26
2000
EB
106.165
0.848
480
50900
DEB
134.22
0.1
56
7500
Tar
-
-
-
300
Total
566
60800
19
Figure 4: Mc-Cabe Thiele Diagram of Binary System at 107oC of Ethylbenzene and Benzene
2. Ethylbenzene Distillation Column Similarly, as a preliminary process synthesis, we assume the separation is a binary system of the dominant keys in the separation process, for EB distillation, based on Stream 14, the dominant keys are Ethylbenzene (84.8%) and Di-ethylbenzene (10.0%). Apart from ethylene and benzene, the binary composition of EB and DEB are: ( )
( ) ,
,
0.848 = 0.8945 0.848 + 0.1 0.1 = = 0.1055 0.848 + 0.1 =
Desired distillate composition of light key, B is ( ) ,
= 0.999
20 Similary, desired bottom composition of light key, B is: = 0.005
( ) ,
General reflux ratio ≈ 1.5l Feed temperature = 195 C Through Antoine Equation we obtained the saturated pressure of B and EB at 195 °C ln[
(mmHg)] = A −
B (K) + C
= 2938.11mmHg ,
,
,
= 993.43 mmHg Relative volatility of B and EB,
,
=
= 2.9575
,
The values above are plugged into the spreadsheet to generate a McCabe Thiele Diagram in order to show the approximate number of stages and its feed stage location. However, such diagram only indicates the fine separation between EB and DEB, regardless of the presence of E and B. Considering the existence of E and B in the column, it is estimated that both the minor light keys, ethylene and benzene would be distillated and the minor heavy key, tar would be liquefied. In addition, the amount of ethylene from the feed stream is significantly small and can be negleted. Such diagram indicates that the separation of ethylbenzene from di-ethylbenzene is relatively easy. However, it is difficult to remove the remained benzene in Stream 14 as the previous benzene distillation already had its maximum distillation limit to remove benzene. Thus, we predict that the Product Stream and Stream 15 has the approximate composition as shown: Table 12: Summary of Product Stream Molecular Weight (kg/kmol)
Composition, X
̇ (kmol/h)
̇ (kg/h)
E
28.05
0
0
0
B
78.11
0.042
201
1600
EB
106.165
0.958
480
51000
DEB
134.22
0
0
0
Total
501
52600
21 Table 13: Summary of Stream 15 Molecular Weight (kg/kmol)
Composition, X
̇ (kmol/h)
̇ (kg/h)
E
28.05
0
0
0
B
78.11
0
0
0
EB
106.165
0.007
1
100
DEB
134.22
0.993
50
6700
Tar
-
-
-
300
total
51
7100
Figure 5: Mc-Cabe Thiele Diagram of Binary System at 195oC of Ethylbenzene and Diethylbenzene 3. Poly-Ethylbenzene Distillation Column The poly-ethylbenzene is estimated as a perfect separator between Tar and Polyethylbenzene, as the Tar is comparatively heavier than the distillati
22
Figure 6: Distribution of Chemicals throughout the Proces
23 Eliminating Differences in Temperature, Pressure and Phase
At the beginning of the process both raw materials are feed into the stream under surrounding temperature and pressure which are 25°C and 101.325 kPa. When it comes to stream 3, the pressure of mixture of benzene and ethylene is rapidly increased to 1500-2000kPa by compressor to change gas phase of benzene into liquid phase. Therefore, gas phase of ethylene will dissolve in benzene and feed into the alkylation reactor.
The temperature of effluent from alkylation reactor is increased to the range of 125°C - 175°C by supplying heat to the reactor. In this condition, reactants still remain in liquid phase. The reason why temperature is increased is because under that condition we can get the highest conversion of ethylbenzene compare to other temperature.
However, in stream 5 the temperature and pressure is decreased to 107°C and 470kPa respectively which is the condition of bubble point then feed into distillation column. The bottom product comes out from first distillation column with higher temperature and lower pressure due to the reboiler supplies heat during the operation and a minor pressure drop commonly occur when pass through a distillation column. Both top and bottom column of product will still remain in liquid phase. Same theory applied to second and third distillation column.
The effluent from third distillation column will pass through a shell-and-tube heat exchange first then only feed into alkylation reactor. Therefore, temperature decreases to 40°C. Same condition and reason to alkylation reactor, effluent from the transalkylation with 125°C- 175°C temperature and 1500-2000kPa
24
Figure 7: Changes in Temperature, Pressure and Phase in the Plant
25 Task Integration
1. Alkylation Reactor The first reactor that will be used in this plant design is alkylation reactor. This is because the ethylbenzene plant consists of a benzene and ethylene alkylation reactor assembly which forms the product compound of ethylbenzene and other by-products such as diethylbenzene. This reactor is to facilitate a reaction to produce ethylbenzene at the highest yield and level of safety as possible.
2. Benzene Distillation Column Benzene distillation column will be used in ethylene benzene production. The main purpose of this benzene distillation column is used to separate benzene and ethylbenzene from the main process stream. The benzene which had already separated will be recycled.
3. Ethylbenzene Distillation Column The ethylbezene and di-ethylbenzene at bottom part of benzene distillation column will be flowed to another distillation column which is ethylbenzene distillation column. The main function of this distillation column is to separate the main product (ethylbezene) and byproduct (di-ethylbenzene) from the process.
4. Polyethylbenzene Distillation Column Polyethlybenzene distillation column will be used in this process because to separate diethylbenzene and tar. Moreover, diethylbenzene will be proceeding to trans-alkylation reactor to form ethylbenzene.
26 5. Flash Drum A portion of the distillated from benzene distillation column will proceed to flash drum. Flash drum is used for gas and liquid mixture separation. The vapour travels through the gas outlet at a design velocity which minimizes the entrainment of any liquid droplets in the vapour as it exits the vessel. In this plant design project, flash drum is used to separate benzene into two streams. One of the streams will be flow to storage tank and another one stream will be proceeding to trans-alkylation reactor for trans-alkylation process. This is accomplished by invoking high pressure drop allowing some of the process stream to flash off.
6. Condenser The condenser will be used in this plant design. The purpose of the condenser is to cool down the temperature of the cool water after flowed through the shell-and-tube heat exchanger and so it can be re-used back.
7. Valves Valves are used as safety relief device and also to decrease the pressure of a liquid stream in the process.
8. Trans-alkylation Reactor Trans-alkylation reactor will be used in this plant design. The main function of this reactor is to produce ethylbenzene at low temperature from benzene and polyethylbenzene. The trans-alkylation process is independent of pressure, and as such there is no reason to lower the pressure when it will need to be increased again for the recycle loop.
27 9. Cooling Water System Cooling water at 22°C and 100 psig enters the upper header and exits the plant at 55°C and 100 psig. The outlet temperature was determined to be below 60 - 70°C to avoid scaling issues (MacDonald et. al.). An inlet temperature of 22°C was selected based on an average seasonal temperature.
10. Shell-and-tube Heat Exchanger Shell-and-tube heat exchanger will be used in this ethylbenzene production plant design. The heat exchanger is simulated with a complex shell and tube heat exchanger. The process stream flows on the tube side of the exchanger and the cooling water passes through the shell ride.
11. Centrifugal Pump Centrifugal pump is to pump require component in liquid phase at high pressure into the reactor. In this plant design, the fresh benzene is pumped to the reactor at about 1.8MPa (Zhang et. al., 2003).
28
Base Case Design Process Flow Diagram (PFD)
Figure 8: Process Flow Diagram (PFD) of Ethylbenzene Production Plant
29 Heuristics
1. Raw material and chemical reactions Heuristic 1: Select raw materials and chemical reactions to avoid, or reduce, the handling, and storage of hazardous and toxic chemicals. In recent years, there is rising awareness in the handling and storage of toxic and hazardous raw materials. This is to protect the environment and to avoid evident safety problems. Lesson was learnt from the accident occurrences especially from the 1984 accident in Bhopal, India, where water was accidentally mixed with the active intermediate, methyl isocyanate. Thus, selection of raw materials and chemical reactions is vital in reducing and minimising the loss of live and cost. In spite of that, the usage of benzene and ethylene cannot be avoided as our raw materials in ethylbenzene manufacturing since the benzene group is a major part of ethylbenzene chemical structure. Benzene is widely used as an industrial raw material to produce ethylbenzene. In fact, every industrial process reviewed in the previous task (Task 1) uses benzene as a main raw material. Although there is an experimental process that attempts use lignin and toluene as the raw materials to produce ethylbenzene, the lignin is still ultimately converted to benzene while toluene is converted to ethylene in that process. Furthermore, the yield is significantly lower than other processes with low product purity and fluctuating lignin prices. Hence, we must use benzene and ethylene as our raw materials and cannot avoid the handling of these hazardous raw materials as benzene is toxic and ethylene is flammable.
30 2. Distribution of Chemicals Heuristic 2: Use an excess of one chemical reactant in a reaction operation to completely consume a second valuable, toxic, or hazardous chemical reactant. According to this heuristic, we have to use ethylene as the excess chemical reactant to completely consume benzene, since benzene is more valuable and more toxic based on the MSDS. But, to prevent over alkylation of benzene to di-ethylbenzene and tri-ethylbenzene, we decided to make benzene as the excess chemical reactant. The over alkylation of benzene will produce more di-ethylbenzene and tri-ethylbenzene than our desired product, ethylbenzene which are side products that are not valuable. Furthermore, by adjusting the temperature and pressure will not help in avoiding the usage of benzene as an excess material as the alkylation process is more sensitive to the molar ratio of benzene to ethylene. Therefore, we have no choice but to use benzene as the excess chemical reactant.
Heuristic 7: For competing series or parallel reactions, adjust the temperature, pressure, and catalyst to obtain high yields of the desired products. In the initial distribution of chemicals, assume that these conditions can be satisfied- obtain kinetics data and check this assumption before developing a base-case design. According to the collision theory, the rate of reaction can be increased if the collisions between molecules are increased. Thus, by increasing the temperature, kinetic energy is increased allowing the molecules to move faster and collide more frequently. The temperature adjusted for the both alkylation and trans-alkylation reactions are in the range of 125 – 175°C but the catalyst used in alkylation and trans-alkylation are EBZ-500TM and EBZ-100TM respectively. A catalyst is a substance which speeds up a reaction but is chemically unchanged at the end of the reaction. In other words, after the reaction we can obtain the same mass of catalyst as the beginning of reaction. Catalyst will provide an alternative pathway for the reaction with lower activation energy while ensuring a higher successful collision rate between molecules. Hence, an approximated 99.6% yield of the desired products will be obtained from EBOneTM process.
31 Heuristic 8: For reversible reactions, especially, consider conducting them in a separation device capable of removing the products, and hence, driving the reactions to the right. Such reaction - separation operations lead to very different distribution of chemical. A reversible reaction proceeds in both directions i.e. reactant will react and form products, while the product will also react and reform into reactants. Reversible reactions will reach an equilibrium point which equalizes the concentrations of reactants and products. In the ethylbenzene production process, there is one reversible reaction that occurs in transalkylation reaction. C6H6 + C6H4(C2H5)2 ↔ 2C6H5C2H5 If a reversible reaction occurred during the process, it will cause a lower conversion of desired amount that we want. Thus, we must drive the reaction to the right to optimise the conversion. In order to solve this problem, there are two ways that can be used to shift the reaction to the right i.e. by using an excess of benzene and by removing ethylbenzene from the feed stream before entering into the trans-alkylation reactor. Le Châtelier’s principle states that the position of equilibrium point will change and shift to counteract the change to re-establish an equilibrium if the dynamic equilibrium is influenced by changing the condition. With excess benzene, the concentration of benzene will increase and thus follow the principle of Le Châtelier the reaction will tend to shift to right. Same principle apply to second method we used, the concentration of ethylbenzene will remain minimum as possible thus the reaction will tend to shift to right. If we distillation ethylbenzene out after transalkylation, the concentration of ethylbenzene will become higher and thus the reaction will reverse back and lead to a lower conversion.
32 3. Separation Heuristic 9: Separation liquid mixture using distillation and stripping towers, and liquid-liquid extractors, among similar operations Heuristic 10: Attempt to condense vapour mixtures with cooling water. Then follow the previous heuristic (heuristic 9). When we separate a mixture of component, the first things we must do is to determine the phase of mixture enter the separator. With the different phase of mixture we used different type of separator. For liquid phase of mixture, we can use distillation, enhanced distillation, stripping towers, liquid-liquid extraction and so on. However for vapour phase of mixture, we can use partial condensation, cryogenic distillation, absorption and absorption membrane. So, after determined the phase of mixture entering the separator, the second step we need to do is determine which type of separator we need to use. In the whole process we total have three distillation columns and one flash drum. The three distillation columns are actually in series order. First distillation column is installed connect to the shell-and-tube heat exchanger (E-2) which receive the effluent of alkylation reactor. In this distillation column, the main purpose is wanted to separate excess benzene out from top column of separator with other substances. Part of the excess benzene will recycle back to feed stream of benzene and ethylene. However, another part of excess benzene will flow through flash drum. Other substances like ethylbenzene, diethylbenzene and small amount of benzene in liquid phase will connect and feed into the second distillation column. In the second distillation column is wanted to extract our desire product, ethylbenzene from the process. Ethylbenzene is extracted out in this step because we want to reduce the amount of reversible reaction occur in the transalkylation section after that. So ethylbenzene will extract out from top column and other substances leftover in liquid phase will go through bottom stream and connect to third distillation column. The last distillation column is connected with the transalkylation reactor. The purpose of this distillation is wanted to purge the unwanted side product, tar out from the process. If we do not purge the tar out from the process, it will accumulate and damage to
33 our equipment. Di-ethylbenzene and benzene will combine with the feed from flash drum and feed into transalkylation reactor to futher react become ethylbenzene that we wanted.
4. Heat Transfer in Reactors Heuristic 12: To remove a highly-exothermic heat of reaction, consider the use of excess reactant, and inert diluents, and cold shoots. There affect the distribution of chemicals and should be inserted early in process synthesis. In our process, we are involving two reactors for alkylation and transalkylation reaction. Both of the reactors are high exothermic reaction. So this becomes an important issue on our process. To handle this problem, we using excess of benzene to cover out it. So, heauristic 12 is applied.
5. Heat Exchangers and Furnaces Heuristics 16: Unless required as part of the design of the separator or reactor, provide necessary heat exchange for heating or cooling process fluid streams, with or without utilities, in an external shell-and-tube heat exchanger using counter-current flow. However, if a process stream requires heating above 750⁰F, use a furnaces unless the process fluid is subject to chemical decomposition. Heuristics 27: When using cooling water to cool or condense a process stream, assume a water inlet temperature of 90⁰F (from a cooling tower) and a maximum water outlet temperature of 120⁰F. To cool the mixture of fresh feed and recycle stream to the reactor, we have design a heat exchanger E-1 to cool the feed before it entering alkylation reactor. We have to cool the feed to around 40⁰C to make sure that it is in liquid form at applied presssure. This condition is required for the benzene alkylation.
34 Moreover, there is another heat exchanger E-2 to cool down the stream before entering distillation column C-1. This heat exchanger will cool the stream from around 290-310⁰C to around 102⁰C, which a temperature suitable for distillation process. Besides these, heat exchanger E-3 is designed to cool the stream before it enters the transalkylation reactor R-2 as well. These heat exchangers are external shell-and-tube heat exchangers using countercurrent flow. They have higher efficiency as compared to parallel flow heat exchanger. Besides, we also make sure that the inlet temperature of cooling water is at 90⁰F (32.2⁰C) and the outlet is at 120⁰F (48.89⁰C). This can be done by adjusting the flow rate of cooling water. So, we applied heuristics 16 and 27.
6. Pressure Reduction Heuristics 21: For liquid flow, assume pipeline pressure drop of 2psi/100ft of pipe and a control valve pressure drop of at least 10psi. for each 10-ft rise in elevation, assume a pressure drop of 4psia. We have included a control valve in our process designing. The valve is located right after the alkylation reactor. This valve helps to reduce the pressure from around 1700kPa to around 470kPa. According to heuristics 21, a control valve pressure drop of at least 10psi (68.95kPa). Since our pressure drop across the control valve is much greater than 10psi, we applied heuristics 21.
Heuristics 20: For heads up to 3,200 ft and flow rates in the range of 10 to 5,000 gpm, use a centrifugal pump. For high heads up to 20,000 ft and flow rates up to 500 gpm, use a reciprocating pump. Less common are axial pump for heads up to 40 ft for flow rates in the range of 20 to 100,000 gpm and rotary pumps for heads up to 3,000 ft for flow rates in the range of 1 to 1,500 gpm.
35 In fact, we have three pumps in our PFD. The first one is located right at the inlet of benzene feed, which it used to pump benzene feed to the alkylation reactor. The second pump is used to pump the recycle benzene to mix with the fresh feed. The third pump is to pump outlet stream of trans-alkylation reactor to recycle back for distillation process. All the pumps are centrifugal pumps. This is because the difference in pressure of streams in our operation is lower than 3,200 ft of head and the flow rates is within the range of 10 to 5,000 gpm. Hence, we have chosen centrifugal pump to be included in our operation. We follow the heuristics 20.
List of Major Equipment and Required Specifications Table 14: List of Major Equipment and Required Specifications Equipment Type Reactor
Heat Exchanger
Required specification
Temperature
Sizing of Reactor
Pressure
Type of reactor
Flow rate
Material used to
Height
Diameter
Temperature
Diameter
Pressure
Type of heat
Flow rate
Sizing
build reactor
exchanger
Duty
Material used to build heat exchanger
36 Distillation
Temperature
Height
Column
Pressure
Diameter
Flow rate
Material used to
Type of
build heat exchanger
distillation column
Number of tray
Valve
Pump
Condenser
Temperature
Sizing
Pressure
Material used to
Flow rate
Valve type
Number of port
Temperature
Type of pump
Pressure
Material used to
Flow rate
Horsepower
Temperature
Sizing
Pressure
Entering feed phase
Flow rate
Horsepower
Type of
Material used to
build valve
build pump
condenser
Flash Drum
build condenser
Temperature
Entering feed phase
Pressure
Material used to
Flow rate
build flash drum
37
Additional Data Table 15: Summary of Additional Data of equipment Component Sizing Equipment Centrifugal pump *TBM (P-2) Alkylation 250m3 reactor (R-1) Benzene distillation *TBM column (C-1) EB distillation *TBM column (C-2) Transalkylation 30m3 (R-2) Control valve *TBM (VLV-100) Note: *TBM =To be measured.
Height
Diameter
Power needed
efficiency
Tray efficiency
Tray number
Number of tray feed (count from top)
*TBM
*TBM
10.5 kW
75%
-
-
-
-
*TBM
*TBM
*TBM
-
-
-
-
0
16.3m
3.4m
21kW
-
60%
27
11
70kPa
20.3m
2.3m
21kW
-
60%
33
19
70kPa
*TBM
*TBM
*TBM
-
-
-
-
0
*TBM
*TBM
*TBM
*TBM
-
-
-
1200kPa
Pressure drop
The data above are further details regarding conditions of the equipment used in the construction of the plant. For the data which is not inserted at the above table, it will be generated in task 3 when the stimulation is constructed out.
38 Conclusion In Task 2, the preliminary process synthesis and base-case is created and designed. A summary of all relevant data regarding raw materials, products, unit operations and equipment used are provided and summarized in this task. This serve as a detailed guideline for the team to initiate a pilot plant to test and verify the conditions given in this task. There are limitations encountered in the performance of this task. Much of the data required such as the sizing of each unit used as well as detailed kinetic and transport data can only be obtained through a thorough HYSYS simulation which will be carried out in Task 3. Therefore, certain aspects of the PFD and values of parameters such as the conversion of the reactors and temperature and pressure of the distillation columns are pending with respect to the simulation. Therefore, all data and values mentioned in this task are preliminary and should only serve as a guideline as they will be finalized only in task 3 after a thorough mass and energy balance is carried out coupled with a steady state simulation of the plant.
References Jirui, Z., Dongfeng, L., Jiquan, F., & Gang, C. (2003). US 6,504,071 B2. Process and Apparatus for Preparation of Ethylbenzene by Alkylation of Benzene with Dilute Ethylene Contained in Dry Gas by Catalytic Distillation. Retrieved from http://www.google.com/patents/US6504071 MacDonald, J., Roda, R., & Beresford, M. (2005). Liquid Phase Alkylation of Benzene with Ethylene. Retrieved from www.scribd.com/mobile/doc/106876722 Seider, W. D., Seader, J. D. & Levin, D. R. (2003). Product & Process Design Principles. (2nd ed.). U.S: Wiley. Narsolis, F., Woodle, G., Gajda, G. & Gandhi, D. (n.d.) High Performance Catalyst for Liquid Phase EB Technology. Retrieved from http://www.digitalrefining.com/data/articles/file/271533368.pdf
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