Process Optimization on Alkylation of Benzene with Propylene

Apr 20, 2009 - This work deals with the improvement of the current process of alkylation of benzene (B) with propylene (P) to produce cumene (IPB). Du...
0 downloads 24 Views 603KB Size
Download PDF
Energy & Fuels 2009, 23, 3159–3166

3159

Process Optimization on Alkylation of Benzene with Propylene Zhigang Lei,* Chengna Dai, Yuli Wang, and Biaohua Chen State Key Laboratory of Chemical Resource Engineering, Beijing UniVersity of Chemical Technology, Box 35, Beijing, 100029, China ReceiVed January 20, 2009. ReVised Manuscript ReceiVed March 21, 2009

This work deals with the improvement of the current process of alkylation of benzene (B) with propylene (P) to produce cumene (IPB). Due to high energy consumption brought by high feeding molar ratio of benzene to propylene (B/P), an optimized process design has been developed and is presented. First, a side-stream was added to the diisopropylbenzene (DIPB) distillation column in order to recover some trisopropylbenzene (TIPB), resulting in a decrease of the energy consumption per product. The total heat duty on condensers and reboilers per product decreased by 5.5 and 4.3%, respectively. On the other hand, if the original fixed-bed reactor is replaced by a bubble-point reactor, the total heat duty on condensers and reboilers will decrease by 18.5 and 22.8%, respectively. Using the above two schemes together, they decreased up to 19.4 and 23.6% per product, respectively. Then, using a fixed-bed catalytic distillation (FCD) column for producing cumene, the calculated results showed that a much lower energy consumption was required, and the total heat duty on condensers and reboilers per product decreased by 36.4 and 23.6%. The improvement of FCD column was done by carrying out alkylation and transalkylation reactions simultaneously in a single column for producing cumene, with the result that investment of equipment for transalkylation is reduced and the process is simplified. Finally, with the combination of the improved DIPB column and FCD process, the heat duty was found to be the lowest.

1. Introduction Cumene (isopropyl benzene) is a basic petrochemical material used in the production of phenol and acetone. It is usually obtained in the industry by alkylation of benzene with propylene over a zeolite catalyst. The reactions mainly include alkylation and transalkylation: 1-13 k1

B + P 98 I k2

B + I 98 D * To whom correspondence should be addressed. E-mail: leizhg@ mail.buct.edu.cn. (1) Gao, H.; Shi, J. G.; Cao, G.; Lu, G. Z. J. Chem. Ind. Eng. 2006, 24, 36–38. (2) Siffert, S.; Gaillard, L.; Su, B. L. J. Mol. Catal. A: Chem. 2000, 153, 267–279. (3) Geatti, A.; Lenarda, M.; Storaro, L.; Ganzerla, R.; Perissinotto, M. J. Mol. Catal. A: Chem. 1997, 121, 111–118. (4) Ivanova, I. I.; Brunel, D.; Nagy, J. B.; Derouane, E. G. J. Mol. Catal. A: Chem. 1995, 95, 243–258. (5) Cavani, F.; Girotti, G.; Terzoni, G. Appl. Catal. A: Gen. 1993, 97, 177–196. (6) Jian, P.; Wang, Q.; Zhu, C.; Xu, Y. Appl. Catal. A: Gen. 1992, 91, 125–129. (7) Fu, J.; Ding, C. Catal. Commun. 2005, 6, 770–776. (8) Tian, Z.; Qin, Z.; Dong, M.; Wang, G.; Wang, J. Catal. Commun. 2005, 6, 385–388. (9) Girotti, G.; Rivetti, F.; Ramello, S.; Carnelli, L. J. Mol. Catal. A: Chem. 2003, 204-205, 571–579. (10) Han, M.; Lin, S.; Roduner, E. Appl. Catal. A: Gen. 2003, 243, 175–184. (11) Abasov, S. I.; Babayeva, F. A.; Zarbaliyev, R. R.; Abbasova, G. G.; Tagiyev, D. B.; Rustamov, M. I. Appl. Catal. A: Gen. 2003, 251, 267–274. (12) Jansang, B.; Nanok, T.; Limtrakul, J. J. Phys. Chem. B 2006, 110, 12626–12631. (13) Tian, Z.; Qin, Z. F.; Wang, G. F.; Dong, M.; Wang, J. G. J. Supercrit. Fluids 2008, 44, 325–330.

k3

B + D {\} 2I k4

where B, P, I, and D represent benzene, propylene, cumene, and dialkylbenzene, respectively. The first two are called alkylation reactions, and the latter is a transalkylation reaction. In addition, diisopropylbenzene (DIPB) can further alkylate into trialkylbenzene (TIPB). The main types of reactors that can be used for producing cumene with benzene and propylene are fixed-bed reactor, fixedbed catalytic distillation (FCD) column, and suspension catalytic distillation (SCD) column. The most commonly used in industry is the fixed-bed reactor, which is easy to be implemented.14–16 The whole process for producing cumene consists of a reaction section and a separation section.17-20 In reaction section, alkylation and transalkylation reactions use the same catalysts, but take place in different reactors. The separation section includes three distillation columns, that is, benzene column, cumene column, and DIPB column. The original flowsheet is shown in Figure 1. However, in the original process, the feeding molar ratio of benzene to propylene must be large enough to maintain the catalyst actively run for a long time. Therefore, this ratio is often up to 5.0, which leads to high energy and fuel consumption in (14) Du, Z. X.; Min, E. Z. Petrochem. Tech. 1999, 28, 562–564. (15) Dai, C. N.; Lei, Z. G.; Chen, B. H.; Cao, G. Mod. Chem. Ind. 2008, 28, 144–147. (16) Lei, Z. G.; Yang, J. F.; Gao, J. J.; Chen, B. H.; Li, C. Y. Chem. Eng. Sci. 2007, 62, 7320–7326. (17) Li, D. F.; Wang, Z. W.; Jin, Y.; Cao, G. Petrochem. Tech. 2001, 30, 355–357. (18) Shi, L. H.; Chen, S. G.; Qu, B.; Cao, G. Petrochem. Ind. Tech. 2006, 13, 13–16. (19) Sotelo, J. L.; Calvo, L.; Pe´rez- Vela´zquez, A.; Capilla, D.; Cavani, F.; Bolognini, M. Appl. Catal. A: Gen 2006, 312, 194–201. (20) Sundmacher, K.; Kienle, A. ReactiVe Distillation; Chemical Industry Press: Beijing, 2005.

10.1021/ef900052j CCC: $40.75  2009 American Chemical Society Published on Web 04/20/2009

3160

Energy & Fuels, Vol. 23, 2009

Lei et al.

Figure 1. Flowsheet of the original process for producing cumene: 1, alkylation reactor; 2, transalkylation reactor; 3, benzene column; 4, cumene column; 5, DIPB column.

Figure 2. Flowsheet of the process with the improved DIPB column for producing cumene: 1, alkylation reactor; 2, transalkylation reactor; 3, benzene column; 4, cumene column; 5, DIPB column. Table 1. Comparison between the Calculated and Actual Values in Benzene Column mass fraction of the bottom stream actual values calculated values

mass fraction of the side-stream

B

IPB

DIPB

B

IPB

DIPB

0 0.008 76

0.778 82 0.764 43

0.210 76 0.210 76

0.9958 0.9913

0 0.0066

0 0

separation sections. As we know, distillation is typically an energy-intensive unit operation in the chemical industry. So, focus of this work is given to optimize the whole process with respect to energy consumption. 2. Process Optimization 2.1. The Improved DIPB Column. The DIPB column in the original process has only one side-stream, through which DIPB flows into the transalkylation reactor. However, the sideproduct TIPB leaving from the bottom of DIPB column does not flow into the transalkylation reactor, resulting in heavier component emission and also high material and energy consumption. In this work, an improved DIPB column, into which another side-stream is added to recover and utilize TIPB, is proposed and is illustrated in Figure 2. In the original process the benzene column is divided into 55 theoretical stages. The cumene and DIPB columns have 40 and 30 theoretical stages, respectively, and for all columns the total condenser is stage 1. The operating pressures of benzene column, cumene column, and DIPB column are 500, 110, and 40 kPa, respectively. The feeding flow rates of benzene and reacted liquid in benzene column are 8868.6 and 51 029.7 kg · h-1, respectively. In the improved process we add another

side-stream in the DIPB column at the 24th theoretical stage. When the amount of this side-stream changes, the reflux ratio and amount of product in benzene and cumene columns are also changed. But the other parameters, for example, stage number, operation pressure, product composition at the top of all columns, and feeding parameters, remain constant. In this work the equilibrium stage (EQ) model is established to simulate the distillation column. The equations that model EQs are known as the MESHR equations, 21-24 into which the reaction terms, including reaction rate equations and reaction heat equations, may be incorporated in the following optimization steps. MESHR is an acronym referring to the different types of equations. The M equations are the mass balance, E the phase equilibrium relations, S the summation equations, H the enthalpy balance, and R the reaction rate equations. The UNIFAC model25 is used for description of liquid phase nonideality. The (21) Lee, J. H.; Dudukovic, M. P. Comput. Chem. Eng. 1998, 23, 159– 172. (22) Sundmacher, K.; Hoffmann, U. Chem. Eng. Sci. 1996, 51, 2359– 2368. (23) Tanskanen, J.; Pohjola, V. J. Comput. Chem. Eng. 2000, 24, 81– 88. (24) Taylor, R.; Krishna, R. Chem. Eng. Sci. 2000, 55, 5183–5229. (25) Lei, Z. G.; Chen, B. H.; Li, C. Y.; Liu, H. Chem. ReV. 2008, 108, 1419–1455.

Alkylation of Benzene with Propylene

Energy & Fuels, Vol. 23, 2009 3161

Figure 3. The influence of new added side-stream in the DIPB column on the heat duty in the benzene column. ((a) on the condenser and (b) on the reboiler; side-stream ) 0, meaning the original process.)

Figure 4. The influence of new added side-stream in the DIPB column on the heat duty in the cumene column. ((a) on the condenser and (b) on the reboiler; side-stream ) 0, meaning the original process.)

comparison between the calculated and actual values from industrial scale in benzene column in the original process is listed in Table 1. It can be seen that both agree very well, which can prove the reliability of the calculation. With the amount of the new added side-stream in the DIPB column increasing, the side-stream amount from the benzene column will decrease. However, the ultimate product, that is, cumene, is obtained in greater quantity. Meanwhile, the cumene concentration at the top of cumene column remains almost stable after the DIPB column is improved. The optimization steps were done under the condition that the product quality is unchanged. Figures 3-5 show the heat duty on condensers and reboilers in benzene column, cumene column, and DIPB column. With the amount of new added side-stream in the DIPB column increasing, the heat duty on condensers and reboilers in benzene column and cumene column will gradually increase, which is due to the larger vapor load in these two columns, but the heat

duty in the DIPB column decreases rapidly and then keeps almost unchanged because of the vapor load reducing. On the other hand, with the amount of new added sidestream in DIPB column increasing, the heat duty on condensers and reboilers per product of benzene, cumene, and DIPB will all decrease, no matter whether the total heat duty decreases or not. Therefore, we use the heat duty per product as the energy index. Compared with the heat duty in the original process, the heat duty on condensers per product in benzene column, cumene column, and DIPB column decreases 5.0, 2.4, and 18.0%, respectively, whereas the heat duty on reboilers decreases 1.7, 2.5, and 22.7%, respectively. Therefore, the way to add another side-stream in the DIPB column attains the aim at energy saving since the ultimate product, that is, cumene, is obtained in much greater quantities. The heat duty in the separation section including benzene column, cumene column, and DIPB column is listed in Table

3162

Energy & Fuels, Vol. 23, 2009

Lei et al.

Figure 5. The influence of new added side-stream in the DIPB column on the heat duty in the DIPB column. ((a) on the condenser and (b) on the reboiler; side-stream ) 0, meaning the original process.) Table 2. Comparison of Heat Duty on Condensers and Reboilers between the Original and Improved Processes

original process improved process

new added side-stream (kg/h)

total condensers energy (M · kJ/h)

total reboilers energy (M · kJ/h)

condensers’ energy per product (kJ/mol)

reboilers’ energy per product (kJ/mol)

0 100 200 300 400 500 600 700

21.3 21.17 21.18 21.2 21.23 21.27 21.28 21.31

16.89 16.93 16.95 16.98 17.00 17.05 17.08 17.11

195.06 186.91 186.57 186.14 185.80 185.25 184.78 184.38

154.67 149.50 149.35 149.04 148.80 148.55 148.27 148.11

2. The total heat duty per product on all condensers and reboilers decreases by 5.5 and 4.3%, respectively. 2.2. Bubble-point Reactor for Alkylation. The traditional fixed-bed reactor is currently used for alkylation of benzene with propylene, which has the disadvantages of high feeding molar ratio of benzene to propylene and high energy consumption. Since the synthesis of cumene with benzene and propylene is an exothermic reaction, the heat of reaction can be used to partly evaporate the unreacted benzene, realizing a preliminary separation of the alkylation liquids. By this mean, the task of the separation section, especially for benzene column, is relieved and thus the final energy consumption may reduce. Therefore, in this work we use a bubble-point reactor (see Figure 6) as the replacement of the original alkylation reactor, making full use of the heat of the reaction. But the operating conditions for the streams and columns in separation section are the same as in the original process. The amount of ultimate product, that is, cumene, obtained from the top of cumene column and its concentration in the bubble-point reactor are listed in Table 3. It can be seen that both the amount and quality of product are better than the original. Moreover, the total heat duty on all condensers and reboilers per product in separation section decreases by 18.5 and 22.8%, respectively, as given in Table 4. Therefore, using a bubble-point reactor for alkylation will save energy. 2.3. Combination of Two Optimization Schemes. It is straightforward to combine the two schemes, that is, the improved DIPB column and bubble-point reactor for alkylation, as mentioned above, to optimize the original process including both reaction and separation sections. The simulated results are given in Table 5. Compared with the original process, the heat

duty on all condensers per product decreases from 195.06 to 157.27 kJ/mol, and the heat duty on all reboilers per product decreases from 154.67 to 118.13 kJ/mol. That is to say, the heat duty on all condensers and reboilers per product decreases by 23.6 and 19.4%, respectively.

Figure 6. Schematic diagram of bubble-point reactor for alkylation. Table 3. Comparison of Cumene Amount and Concentration between Bubble-point Reactor and Fixed-bed Reactor

fixed-bed reactor bubble-point reactor

cumene amount, kmol/h

mole fraction of cumene at the top of cumene column, %

109.1772 111.0618

97.28 97.33

Alkylation of Benzene with Propylene

Energy & Fuels, Vol. 23, 2009 3163

Table 4. Heat Duty on Condensers and Reboilers Using Two Kinds of Reactors for Alkylation fixed-bed reactor

bubble-point reactor

energy per energy per total energy product total energy product (M · kJ/h) (kJ/mol) (M · kJ/h) (kJ/mol) benzene column condenser reboiler cumene column condenser reboiler DIPB column condenser reboiler total heat duty on condensers total heat duty on reboilers

11.77 9.12 7.33 5.92 2.20 1.85 21.30

107.83 83.52 67.10 54.26 20.125 16.90 195.06

9.82 7.18 6.12 4.70 1.71 1.37 17.66

88.45 64.64 55.13 42.37 15.43 12.37 159.01

16.89

154.67

13.25

119.38

Table 5. Heat Duty on Condensers and Reboilers Per Product Using the Combination of Two Optimization Schemes

Figure 7. Temperature profile along the FCD column.

new added side-stream (kg/h) original process heat duty on condensers benzene column (kJ/mol) cumene column DIPB column all condensers heat duty on reboilers benzene column (kJ/mol) cumene column DIPB column all reboilers

107.83 67.1 20.13 195.06 83.52 54.26 16.9 154.67

100

200

270

87.86 87.62 87.54 54.98 54.82 54.80 14.87 14.93 14.93 157.70 157.45 157.27 64.29 64.18 64.14 42.31 42.20 42.14 11.83 11.85 11.85 118.44 118.33 118.13

2.4. The FCD Process. The distillation process coupled with reaction, namely reactive distillation (RD), has been devised for many years. The RD process has been applied in the industry,26-29 for example, etherification, esterification, hydrolysis, alkylation, and isomerization. In general, the RD process is divided into two categories: homogeneous and heterogeneous catalytic distillation. Moreover, heterogeneous catalytic distillation is a more recent development and has attracted researchers’ attention because the separation between products and catalyst is easy to accomplish. Therefore, in this work we also tried to replace the original fixed-bed reactor with a fixed-bed catalytic distillation (FCD) column. 2.4.1. Simulation of FCD Column. It is assumed that the height of catalytic distillation column, which is composed of three zonessthat is, rectifying zone, reaction zone, and stripping zonesis divided into 41 theoretical stages; the total condenser is stage 1, and the reboiler is stage 41. The operating pressure is 0.7 MPa, and the reflux rate 960 kg · h-1, while the feeding flow rates of benzene and propylene are 100 and 50 kg · h-1, respectively. The equilibrium stage (EQ) model has been established to simulate FCD column. The reaction kinetic data for alkylation reaction measured in the range 80-150 °C is adopted as below30 1.0 r1 ) k1C0.9 B CPr

(1)

0.9 r2 ) k2CI0.5CPr

(2)

where k1 ) 3.74 × exp(-7.39 × k2 ) 3.68 × 107 4 exp(-1.00 × 10 /T). However, it is known that transalkylation 104

103/T),

(26) Lei, Z. G.; Chen, B. H.; Ding, Z. W. Special Distillation Processes; Elsevier: Amsterdam, 2005. (27) An, Z. G.; Zhang, X. J.; Ren, W. Z. J. Chem. Ind. Eng. 2007, 28, 14–19. (28) Tanskanen, J.; Pohjola, V. J. Comput. Chem. Eng. 2000, 24, 81– 88. (29) Zhang, Y. X.; Xu, X. Trans. IChemE 1992, 70, 465–470. (30) Lei, Z. G.; Li, C. Y.; Li, J. W.; Chen, B. H. Sep. Purif. Technol. 2004, 34, 265–271.

Figure 8. Flow rate profile of liquid and vapor phases along the FCD column.

reaction is reversible, and thus it is supposed to reach chemical equilibrium in each stage. The chemical equilibrium constant K is written below.31 K ) 6.52 × 10-3 exp(27 240/RT) (3) Figure 7 shows the temperature profile along the FCD column. The trend is consistent with the results from ref 32, in which the measured temperatures at reaction section, stripping section, and the bottom of the column are 159.0, 167.7, and 195.2 °C, respectively. Figure 8 shows that the changes of vapor and liquid flows along the FCD column are almost flat. Therefore, the diameter of reaction and stripping sections in the FCD column can be the same, which makes a great convenience for the practical design. The change of the concentration in liquid phase along the FCD column is shown in Figure 9, where the molar fraction of benzene at the top of the FCD column is up to 100%. Therefore, the reflux liquid is pure benzene, and the temperature at the top is roughly the boiling point of benzene at the operating pressure. In addition, the molar fraction of benzene sharply decreases near the bottom (31) Gao, Z.; He, L. Y.; Dai, Y. Y. Zeolite Catalysis and Separation Technology; Petroleum Technology Press: Beijing, 1999. (32) Lin, H. F.; Han, M. H.; Wang, Z. W.; Jin, Y. Petrochem. Tech. 2000, 29, 849–852. (33) Smoot, L. D. Energy Fuels 1993, 7, 689–699. (34) Garcia, G. O.; Croiset, E.; Douglas, P.; Elkamel, A.; Gupta, M. Energy Fuels 2007, 21, 2098–2111. (35) Garcia, G. O.; Elkamel, A.; Douglas, P. L.; Croiset, E.; Gupta, M. Energy Fuels 2008, 22, 2660–2670. (36) Zhao, G. B.; Hu, X. D.; Plumb, O. A.; Radosz, M. Energy Fuels 2004, 18, 1522–1530.

3164

Energy & Fuels, Vol. 23, 2009

Lei et al.

Figure 9. Concentration profile in liquid phase along the FCD column.

of the FCD column. On the contrary, the molar fractions of cumene and DIPB increase rapidly. Moreover, in the FCD column the molar ratio of benzene to propylene in the liquid phase is much larger than the feeding molar ratio in the fixedbed reactor (e.g., feeding molar ratio of B/P ) 5) so as to prevent from polymerization of propylene and inactivity of catalyst. 2.4.2. The FCD Process for Producing Cumene. In this optimization step, the fixed-bed reactor is replaced by FCD column, in which the separation section is unchanged. The alkylation liquid concentrations are listed in Table 6. Table 7 gives the calculated results of energy consumption in separation section in the FCD process. It seems that the use of FCD column will make the energy consumption in separation section decreased greatly. The heat duty on all condensers and reboilers per product is less than that using the combination of improved DIPB column and bubble-point reactor for alkylation, and decreases by 36.4 and 23.6%, respectively. 2.4.3. The ImproVed FCD Process for Producing Cumene. In the FCD process, alkylation reaction takes place in one FCD column, but transalkylation reaction in another fixed-bed reactor. In this work an improved FCD process, in which alkylation and transalkylation reactions take place simultaneously in a single FCD column, is proposed and is illustrated in Figure 10. The distinguishable difference between a single alkylation reaction and simultaneous alkylation and transalkylation reactions in a column is that a DIPB stream is fed into the catalytic

Figure 11. The relation of F1 and F2 in the FCD column where alkylation and transalkylation reactions take place simultaneously at different feeding molar ratio of benzene to propylene.

distillation column in the improved FCD process. It is evident that the flow rate of DIPB fed into the FCD column, F1, is an important parameter influencing the column design and operation. In the simulation F1 is constantly adjusted until it is equal to F2, and thus F1 is determined. The FCD column is divided into 41 theoretical stages; the condenser is the stage 1, and the reboiler is stage 41. The operating pressure is 700 kPa and the reflux rate is 960 kmol h-1, while the feeding molar ratio of benzene to propylene is set to be 1.5, 2.5, and 3.0. The relation of F1 and F2 at different feeding molar ratio in the improved FCD column is shown in Figure 11. On the other hand, the feeding molar ratio of benzene to propylene should be below or equal to 3.0 in the improved FCD column. Otherwise, there may be no point of intersection with diagonal. 2.5. Combination of the improved DIPB column and FCD process. In the last optimization step, we combine the two schemes, i.e. the improved DIPB column and FCD process, as mentioned above, to optimize the original process including both reaction and separation sections. Figures 12- 15 show the heat duty per product on condensers and reboilers in benzene column, cumene column, and DIPB column. With the amount of new added side-stream in DIPB column increasing, the heat duty on condensers and reboilers per product of benzene, cumene, and DIPB will all decrease. Compared with the heat duty in the original process, the heat duty on condensers per

Figure 10. Flowsheet of the improved FCD process for producing cumene with simultaneous alkylation and transalkylation reactions: 1, FCD column; 2, benzene column; 3, cumene column; 4, DIPB column.

Alkylation of Benzene with Propylene

Energy & Fuels, Vol. 23, 2009 3165

Table 6. The Alkylation Liquid Concentrations at the Bottom of FCD Column composition

C6

benzene

C7/C8

cumene

NPB

C10

DIPB

TIPB

HB

mass fraction

0.000 40

0.410 37

0.002 50

0.539 13

0.000 10

0.001 45

0.041 55

0.004 20

0.000 30

Table 7. Comparison of Heat Duty on Condensers and Reboilers between the Original and FCD Processes original process

FCD process

energy per energy per total energy product total energy product (M · kJ/h) (kJ/mol) (M · kJ/h) (kJ/mol) benzene column condenser reboiler cumene column condenser reboiler DIPB column condenser reboiler total heat duty on condensers total heat duty on reboilers

11.77 9.12 7.33 5.92 2.20 1.85 21.30

107.83 83.52 67.10 54.26 20.13 16.90 195.06

6.87 8.11 6.55 5.00 1.18 1.04 14.61

58.41 68.88 55.69 42.45 10.03 8.84 124.12

16.89

154.67

14.14

120.16

product in benzene column, cumene column, and DIPB column decreases 46.9, 17.8, and 9.2%, respectively. Meanwhile, the heat duty on reboilers decreases 18.8, 21.8, and 8.0%, respectively. Figure 15 shows the total heat duty on condensers per product, including the benzene column, cumene column, and

Figure 12. The influence of new added side-stream in the DIPB column on the heat duty in the benzene column using the combination of the improved DIPB column and FCD process.

Figure 14. The influence of new added side-stream in the DIPB column on the heat duty in the DIPB column using the combination of the improved DIPB column and FCD process.

Figure 15. The influence of new added side-stream in the DIPB column on the heat duty using the combination of the improved DIPB column and FCD process.

DIPB column, decreases from 195 to 122 kJ/mol, and the total heat duty on reboilers per product decreases from 155 to 118 kJ/mol. That is to say, about 0.016 kg heating steam will be saved for 1 mol of product. 3. Conclusions

Figure 13. The influence of new added side-stream in the DIPB column on the heat duty in the cumene column using the combination of the improved DIPB column and FCD process.

Five improved methods for producing cumenes that is, the improved DIPB column, bubble-point reactor process, combination of the improved DIPB column and bubble-point reactor, FCD column, and combination of the improved DIPB column and FCD columnshave been investigated in this work in order to decrease the high energy consumption in the original process of alkylation of benzene with propylene. Among these five improved methods, the total heat duty on condensers and reboilers per product with the improved DIPB column is the highest, whereas it is the least with the combination of the improved DIPB column and FCD column. The improved DIPB column makes the amount of ultimate product, that is, cumene, much more. In the FCD column, the feeding molar ratio of benzene to propylene can be reduced. The boiling point of benzene is higher than that of propylene, and thus propylene is

3166

Energy & Fuels, Vol. 23, 2009

the light component, with benzene being the heavy component. Therefore, the molar ratio of benzene to propylene in the liquid phase along the FCD column becomes larger than the feeding molar ratio according to the distillation principle. So, it can prevent polymerization of propylene and the inactivity of catalyst. Besides, the reaction heat also can be utilized. Moreover, this work goes a step further to improve the FCD column in that alkylation and transalkylation reactions take place simultaneously in a single FCD column. Therefore, the equipment investment is reduced and the process is simplified. However, the optimized results need to be validated in actual equipment since they depend on equipment operation as simulated. In petrochemical processing, the energy required for producing cumene with alkylation of benzene with propylene

Lei et al.

comes from combustion of fossil fuels. So it is important to save energy in order to reduce CO2 emission and fuel consumption.33-36 The process optimization present in this work was done from the viewpoint of separation and reaction principles rather than system engineering. Therefore, the pinch technology methods for heat exchanger will be considered in the next work. Acknowledgment. This work is financially supported by the National Nature Science Foundation of China under Grant (No. 20821004 and No. 20625621), the Program for New Century Excellent Talents in University, and Fok Ying Tong Education Foundation (No. 111074). EF900052J