Design and Control of Reactive Distillation Sequences with Heat

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Design and control of reactive distillation sequences with heat-integrated stages to produce diphenyl carbonate Masataka Terasaki, J. Rafael Alcántara Avila, Hao-Yeh Lee, Jun-Lin Chen, Ken-Ichiro Sotowa, and Toshihide Horikawa Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b02651 • Publication Date (Web): 16 Dec 2016 Downloaded from http://pubs.acs.org on December 24, 2016

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Industrial & Engineering Chemistry Research

Design and control of reactive distillation sequences with heat-integrated stages to produce diphenyl carbonate J. Rafael Alcántara-Avila1, Masataka Terasaki1, Hao-Yeh Lee2*, Jun-Lin Chen2, Ken-Ichiro Sotowa1, Toshihide Horikawa1 1

Department of Applied Chemistry, Graduate School of Science and Technology, Tokushima University, 2-1 Minami Josanjima-cho, Tokushima 770-8506, Japan

2

Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan

ABSTRACT

Reactive distillation (RD) in quaternary systems has gained importance when one of the reagents is in excess. In this work, higher energy savings in a reactive distillation sequence is addressed to produce diphenyl carbonate, which is a crucial precursor of polycarbonate. Energy savings were attained through heat integration between the high-pressure RD column and the low-pressure separation column. The design of sequences with heat-integrated stages was done by combining simulation and optimization. The sequence with one heat-integrated stage realized the minimum energy consumption with energy savings around 24% and cost savings around 15.7% in

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comparison with the conventional reactive distillation sequence. Also, two control schemes for the best design configuration have also been studied in this work. The results showed that the control scheme that manipulates the reboiler duty and reflux ratio to control two stage temperatures can provide excellent responses under throughput and feed composition disturbances. Furthermore, the feed ratio of the RD column is not the manipulating variable like in the conventional RD process. However, an internally stoichiometric balance result can be observed under composition disturbance.

1. INTRODUCTION Diphenyl carbonate is an important precursor in the production of polycarbonate, which is an important thermoplastic with excellent mechanical, optical, electrical and heat resistance properties. Most polycarbonates are manufactured by the phosgene process, which uses carbon monoxide and chloride as raw materials. However, phosgene is highly toxic; the process uses large amounts of methylene chloride and water that must be separated from the produced polycarbonate. Thus, a green route has been proposed to produce diphenyl carbonate (DPC) and methyl acetate (MA) from the reaction between phenyl acetate (PA) and dimethyl carbonate (DMC).1 Also, the DPC synthesis method is favorable because it has high equilibrium constants, no azeotropes, and no side reactions.2 Reactive distillation (RD) is a combination of simultaneous separation and reaction in a single vessel. Through this integrative strategy, chemical equilibrium limitations can be overcome; higher selectivities can be achieved, and heat of reaction can be directly used for distillation. Thus, the advantages of RD are the increased process efficiency, the reduction of investments


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and operational costs.3 RD has been extensively researched in the last decades because it can increase the reaction conversion by continuously removing the generated products and it can simplify the separation by avoiding the formation of azeotropes. RD is attractive in those systems that remove the products from the reactants by distillation, which implies that the products should be lighter and/or heavier than the reactants.4 In terms of boiling point temperature, the previous situation is valid for the transesterification reaction of DMC and PA because the normal boiling point temperature of pure components is: DPC (302 ºC) > PA (195.7 ºC) > DMC (90.2 ºC) > MA (57.1 ºC). Typically, the light and heavy reactants are fed immediately below and above the reactive section in an RD column, respectively. This configuration is known as double feed reactive distillation, and significant energy savings are possible by altering the conventional design of RD to allow feeds into the reactive section as well as by catalyst redistribution to extend the reactive section into the stripping (rectifying) section for an exothermic (endothermic) reactions.5 Based on the boiling ranking of the DPC process, the combination of reaction and separation can take place in only one RD column. However, it is difficult to control it because it is necessary to feed the correct amount of each reactant to satisfy the stoichiometry. If one of the reactants feed is in excess, it will exit the column and the column control scheme will not be able to hold the products at their specified compositions. As an alternative, a two-column sequence can have better controllability because if one of the reactants is fed in excess to the RD column, it can be separated in the separation column and recycled to the RD column. The control of the latter system is easier because the inventory of reactant in excess can be inferred from the liquid level in the reflux drum of the separation column to adjust the flow rate of the fresh feed of the reactant in excess.4 From the economic viewpoint, the capital investment and operation costs in a


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process that use only one RD are lower than in the process using two columns. However, the controllability in the former process is harder than that in the latter process. This work aims to design RD sequences that can further reduce the energy consumption while keeping good control performance through the installation of external heat exchangers that can realize heat integration between the stages in the two columns. Although heat integration has been widely applied in non-reactive conventional or complex distillation columns, its extension to RD processes remains an active research area. Heat integration can be attained by compressing the vapor distillate or a working fluid to upgrade its heat (i.e., heat pump), or by integrating the heat between columns in which the vapor distillate of one column supplies heat to the reboiler of another column (i.e., double effect). The Reactive Heat-Integrated Distillation Column (R-HIDiC) is a kind of diabatic heat pump.611

Thermodynamic feasibility and energy savings for the R-HIDiC have been demonstrated

through calculations from process simulations6-8,10,11 and numerical analysis.9 The energy savings in R-HIDiC ranged between 8 and 27% while economic savings ranged between -33 and 45%. Although the high potential for energy savings in R-HIDiC has been demonstrated, there are not applications at commercial scale. Therefore, heat-integrated reactive distillation columns that avoid using compressors have been proposed. Lee et al.7 studied the heat integration between two reactive distillation columns that operated at different pressure, the proposed configuration attained energy and economic savings of 6 and 15%, respectively. Gao et al.12 proposed an externally heat-integrated reactive distillation system (EHIRDS) with a high-pressure RD and a low-pressure separation column for the separation of a hypothetical quaternary mixture. The


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results showed that an EHIRDS with three heat exchangers was the solution with the minimum cost. This work combines optimization techniques and process simulations to derive compressorfree and heat-integrated RD sequences with further reduction of cost and energy consumption. In addition, the process control schemes with disturbance rejection for the obtained solutions are also studied.

2. KINETIC AND THERMODYNAMIC MODELS The synthesis of DPC from DMC and PA is a two-step reaction that takes place through the formation of the intermediate methyl phenyl carbonate (MPC). Eq. 1 shows the first step: the transesterification of DMC and PA to generate MPC and MA while Eqs. 2 and 3 show the alternatives in the second step: the transesterification of MPC and PA to DPC and MA or the disproportionation of MPC to DPC and DMC. Eq. 4 shows the overall reaction.1  → DMC+ PA ←  MPC+ MA

(1)

 → MPC+ PA ←  DPC+ MA

(2)

 → 2MPC←  DPC+ DMC

(3)

 → DMC+ 2PA←  DPC+ 2MA

(4)

Eqs. 5-7 show the rate expressions for the reversible reactions in Eqs. 1-3.

r1 = k f1 CDMCCPA − kb1 CMPCCMA

(5)

r2 = k f 2 CMPCCPA − kb2 CDPCCMA

(6)

2 r3 = k f 3 C MPC − k b3 C DPC C DMC

(7)


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where ri is the reaction rate of the ith reaction in kmol/m3s. k

fi

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and kbi are the forward and

backward reaction rate coefficients, respectively. Cj is the concentration of each component j in kmol/m3. The Arrhenius equation was adopted to consider the temperature dependence of the reaction rate coefficients. Table 1 shows the pre-exponential factor (k0) and the activation energy (Ea) of each reaction rate coefficient in Eqs. 5-7.1

Table 1. Parameters for each reaction rate coefficient k0 (m3/kmol s)

Ea (kJ/kmol)

k f1

135

5.42x104

kb1

52

5.49x104

k f2

1210

6.15x104

k b2

611

5.62x104

k f3

8.2x104

7.68x104

k b3

1.09x105

7.08x104

Also, azeotropes do not exist, and there are large boiling-point temperature differences between the components of the reaction system. The order of normal boiling point temperature of pure components is: DPC (302 ºC) > MPC (234.7 ºC) > PA (195.7 ºC) > DMC (90.2 ºC) > MA (57.1 ºC). Therefore, the ideal model was used as the property method in this study to represent the thermodynamic vapor-liquid behavior in the simulations of the reaction system.1


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3. DESIGN OF HEAT-INTEGRATED REACTIVE DISTILLATION SYSTEMS Heat integration between columns has been widely used to reduce the energy consumption in distillation processes. This energy conservation method can be extended to a two-column RD sequence in which one of the reactants is fed in excess to the RD column, and then separate it in a separation column (SC) and recycle it to the RD column. Thus, heat integration between the RD column and the SC is possible by adjusting the columns operating pressure. However, when the condenser in the high-pressure column and the reboiler in the low-pressure column are the only places subject to heat integration, the pressure difference between columns is at its maximum value. The previous situation is not advantageous from the reaction and the vaporliquid equilibrium viewpoints because the catalyst can deactivate at high temperatures and the relative volatility between components decreases as pressure increases. Thus, more expensive steam cost and energy consumption are necessary for the reboiler of the high-pressure column. Also, from the reaction viewpoint, the pressure must be adjusted because it significantly affects the reactant conversion and heat of reaction. For reversible endothermic reactions, an increase in pressure improves the conversion and speeds up the reaction rate. For equilibriumlimited exothermic reactions, a decrease in pressure improves the conversion and speeds up the reaction rate. For kinetic-controlled exothermic reactions, low pressure slows down the reaction rate and reduces the conversion while high pressure accelerates the backward reaction rate more than the forward reaction rate, which ends up decreasing the reactant conversion and heat of reaction. Consequently, the operating pressure must be carefully selected so as to maximize reactant conversion and reaction heat.13 Figure 1 shows the heat integration possibilities between an RD column and an SC by using an indirect sequence. The dotted lines denote possible combinations for heat integration while the


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solid lines denote the selected heat integration. In addition, one stage in the rectifying section (filled circle) can supply heat to any stage in the stripping section (dotted and solid lines). Similarly, one stage in the stripping section (filled circle) can accept heat from any stage in the rectifying section (dotted and solid lines). a)

b) C

A

C A

Reactive Column

Separation Column

B

Reactive Column

Separation Column

B

D

D

T C

Design and Control of Reactive Distillation Sequences with Heat

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