Novel Process Design of Synthesizing Propylene ... - ACS Publications

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Novel Process Design of Synthesizing Propylene Carbonate for Dimethyl Carbonate Production by Indirect Alcoholysis of Urea Li Shi,† San-Jang Wang,*,‡ David Shan-Hill Wong,§ and Kejin Huang*,† †

College of Information Science and Technology, Beijing University of Chemical Technology, Beijing 100029, PR China Center for Energy and Environmental Research, National Tsing Hua University, Hsinchu 30013, Taiwan § Department of Chemical Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan ‡

ABSTRACT: Dimethyl carbonate (DMC) is a green compound with a broad variety of application. Recently, CO2-based routes to produce DMC have attracted much attention because of the environment benefits of CO2 utilization. In the study, we investigate the process design of synthesizing propylene carbonate (PC) for the DMC production using CO2 as a raw material by indirect alcoholysis of urea. The indirect alcoholysis route of urea shows many advantages because of cheap raw materials, mild and safe operation conditions, and environmentally friendly chemicals. Some different processes for PC synthesis by this route are proposed, designed, and optimized in this work. These processes can be classified in terms of two operation types: near-neat operation and excess reactant operation. Reactive distillation (RD) and heat integration technologies are used to intensify PC synthesis processes. Two processes are designed under the near-neat operation. Three RD plus conventional distillation (CD) processes with heat integration are designed under the excess reactant operation. Simulation results reveal that the novel intensified process containing a RD column and a CD column with internal vapor compression provides the most economical design by fully utilizing the special azeotrope characteristic of the propylene carbonate and propylene glycol pair.

1. INTRODUCTION Carbon dioxide (CO2) emission has recently become a global issue because of the significant and continuous rise of

raw material for the production of antioxidants, agrochemicals, pharmaceuticals, etc. and as a potential solvent for adhesives, coatings, and electrolytes in lithium ion batteries.8 There are several reaction routes to manufacture DMC. DMC is traditionally produced by oxidative carbonylation of methanol or by phosgenation of methanol. However, these two routes use poisonous and/or corrosive gases. In addition, methanol carbonylation presents the possibility of an explosion hazard. Recently, DMC production utilizing CO2 has attracted much attention because it supplies direct environment benefits when valuable products are manufactured from emitted and undesired CO2. The CO2-based methods to produce DMC include direct synthesis with methanol, transesterification of propylene carbonate (PC) or ethylene carbonate (EC) with methanol, and direct and indirect alcoholysis of urea. The direct synthesis of DMC from CO2 and methanol is regarded as a promising route for DMC production based on environmental and economic issues. However, it is still far from large-scale commercialization because the thermodynamic stability and kinetic inertness of CO2 result in poor conversion and low yield of DMC.9 The development of new catalysts

Figure 1. Concept of the plant-wide process for DMC production by indirect alcoholysis of urea.

atmospheric CO2 concentration under the accelerated growth of petrified carbon-based energy consumption. The reduction of CO2 emission has become a critical issue and requires effective strategies and technologies. CO2 utilization and chemical conversion into high value-added products play a key role in minimizing CO2 emission. Dimethyl carbonate (DMC) is an important organic compound and chemical intermediate and is labeled a “green chemical” because of its fast biodegradability, low toxicity and bioaccumulation, and excellent solubility.1−5 As an environmentally benign building block for the synthesis of various organic chemicals,6,7 DMC has been employed as an alternative for toxic and carcinogenic compounds in carbonylation and alkylation reactions. Furthermore, it is extensively utilized as a © XXXX American Chemical Society

Received: Revised: Accepted: Published: A

June 7, 2017 August 10, 2017 September 14, 2017 September 14, 2017 DOI: 10.1021/acs.iecr.7b02341 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research and/or the use of efficient dehydrating agents is the key to achieve high yield and selectivity in this reaction route. In the transformation reaction route, PC (or EC) synthesis is integrated with DMC synthesis. DMC is synthesized by reacting PC (or EC) and methanol. PC (or EC) is mainly synthesized by cycloaddition of propylene oxide (PO) (or ethylene oxide, EO) with CO2. This PC (or EC) synthesis reaction has many significant advantages such as high atom economy and product yield. However, it presents potential safety problems because this reaction must be conducted under high pressure and PO (or EO) is a dangerous chemical.10−12 In addition, propylene glycol (PG) (or ethylene glycol, EG) produced along with DMC could be a disadvantage for this reaction because the cost of byproduct PG (or EG) is less than that of reactant PO (or EO). In the direct alcoholysis of urea, DMC and ammonia are produced by reacting urea and methanol. Urea is synthesized by the reaction of CO2 and ammonia. Thus, it is possible to synthesize DMC from CO2 by combining the reactions of urea synthesis and urea methanolysis. However, the formation of several side products, which results in poor selectivity and low yield of DMC, is one of the major drawbacks of employing this reaction route.13 Instead of the cycloaddition of PO (or EO) with CO2 to synthesize PC (or EC), the indirect alcoholysis of urea starts from urea and PG (or EG) to synthesize PC (or EC) and ammonia. This reaction route provides many benefits, such as cheap and easily available raw materials, mild reaction conditions, safe operation, environmentally friendly chemicals, etc.10−12,14−16 Figure 1 shows the concept of the plant-wide process for the DMC production by indirect alcoholysis of urea. Urea, PC, and DMC are sequentially synthesized from the plant-wide process. The most important characteristic in this plant-wide process is that PG, a byproduct in the transesterification reaction for DMC synthesis, can be used as the reactant for PC synthesis and the released ammonia from PC synthesis can be recycled back to produce urea by reacting with CO2. The overall process forms a green chemical cycle, which increases the utilization of raw materials for the DMC production. There have been some reports17−21 discussing the process design of urea synthesis and DMC synthesis. However, recently, very few reports investigate the process design for PC synthesis. In the present study, some different processes for PC synthesis by indirect alcoholysis of urea are proposed, designed, and optimized by minimizing total annual cost (TAC). These processes can be classified in terms of two operation types: near-neat operation and excess reactant operation. Reactive distillation (RD) and heat integration technologies are used to intensify PC synthesis processes. Two processes are designed under the near-neat operation. Three RD plus conventional distillation (CD) processes with heat integration are designed under the excess reactant operation. Simulation results reveal that the heat-integrated process containing a RD column and a CD column with internal vapor compression can provide the most economical design.

Table 1. Binary Interaction Parameters of UNIQUAC Model for Urea−Ammonia and PG−PC Pairs i

j

aij

bij (K)

PG

PC

ammonia

urea

1 2 1 2

2 1 2 1

1.49 −1.42 0 0.38

−958.83 674.11 1664.20 −560.18

Figure 2. VLE relationship for PG−PC pair under pressures (a) 0.12 atm and (b) 1 atm.

Urea + PG → PC + 2NH3

(2)

PC + 2MeOH ⇌ DMC + PG

(3)

Urea is produced by reacting CO2 and ammonia in the first step. Next, urea is reacted with PG to coproduce PC and ammonia. The final step is used to synthesize DMC by the transformation reaction of PC and methanol (MeOH). The byproducts, ammonia and PG produced from steps 2 and 3, respectively, are recycled back to steps 1 and 2, respectively, as reactants. There have been some reports about the simulation of the urea synthesis loop.17,18 Process design of DMC synthesis by step 3 can also be found in some literature contributions.19−21 However, there are very few studies about the process design for step 2 to synthesize PC. Therefore, in the

2. CHEMICAL REACTIONS There are three reaction steps given below for DMC production using CO2 as the raw material by indirect alcoholysis of urea. CO2 + 2NH3 = Urea + H 2O

component 2

component 1

(1) B

DOI: 10.1021/acs.iecr.7b02341 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 3. Process flowsheet of CSTR+CD configuration for PC synthesis in conjunction with RD column for DMC synthesis.

Figure 4. Relationship between TAC and design variables in the CSTR+CD configuration for PC synthesis.

UNIQUAC activity coefficient model. In the PC synthesis reaction system, only the thermodynamic data of urea− ammonia and PG−PC pairs are found in the literature.19,23,24 Table 1 lists their binary interaction parameters of the UNIQUAC model. The solubility of ammonia in the reaction mixture is represented by using Henry coefficients. The phaseequilibrium relationships for the other pairs are determined in the study by the ideal model from ChemCad software. Figure 2 shows the VLE relationship for PG−PC pair. It indicates that homogeneous minimum-boiling azeotrope can be found for this pair under low pressure (0.12 atm).23 However, there is no azeotrope under high pressure (1 atm). This important characteristic is fully utilized to reduce energy consumption in the following process design for PC synthesis.

present study, we primarily explore the design of different PC synthesis processes. Some possible configurations are proposed to synthesize PC. Wang22 presented a kinetic model (eq 4) for PC synthesis by using MgO as the heterogeneous catalyst. ⎛ 562.602 ⎞ ⎟·C rPC = 1.5888· exp⎜ − urea · C PG ⎝ T ⎠

). T and mol concentration ( liter ) of

where rPC is the reaction rate of PC synthesis Ci represent temperature (K) and

(4)

(

mol liter·min

component i, respectively. The catalyst concentration is assumed to be 2 wt %. In the study, the process simulation is conducted by a rigorous model provided by ChemCad software. The vapor− liquid equilibrium (VLE) relationship is described by the C


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Figure 5. Relationship between TAC and design variables in the RD column for DMC synthesis.

Table 2. TAC Comparison of Different Configurations for PC Synthesis in Conjunction with DMC Synthesis CSTR+CD+DMC cost CC for column (×103 US$) CC for reboiler (×103 US$) CC for condenser (×103 US$) CC for compressor (×103 US$) CC for reactor (×103 US$) CC for flash (×103 US$) steam cost (×103 US$/year) water cost (×103 US$/year) catalyst cost (×103 US$/year) electricity cost (×103 US$/year) total CC (×103 US$/year) total OC (×103 US$/year) TAC (×103 US$/year)

CSTR +CD 38.0 128.7 109.5 0.00 1381.9 27.2 890.4 0.8 0.4 0.0 210.7 891.6 1102.2 (0.0%)

RD+DMC

DMC

RD

DMC

648.2 729.2 115.8 0.0 0.0 0.0 2538.0 46.1 0.02 0.0 186.7 2584.2 2770.8

309.8 272.0 95.0 0.0 0.0 0.0 888.2 0.5 0.01 0.0 84.6 888.8 973.4 (−11.7%)

653.1 734.4 116.7 0.0 0.0 0.0 2546.3 47.2 0.02 0.0 188.0 2593.5 2781.5

RD+CD_HI2+DMC

RD+CD_HI3+DMC

RD +CD_HI1

RD +CD_HI2

RD +CD_HI3

517.2 1136.3 108.3 0.0 0.0 0.0 834.6 0.7 0.01 0.0 220.2 835.3 1055.5 (−4.2%)

DMC 656.6 733.4 117.7 0.0 0.0 0.0 2545.0 48.4 0.02 0.0 188.5 2593.4 2781.8

421.1 804.9 103.2 244.0 0.0 0.0 621.9 0.7 0.01 51.7 196.6 674.2 870.8 (−21.0%)

DMC 664.9 735.5 115.7 0.0 0.0 0.0 2541.3 45.9 0.02 0.0 189.5 2587.2 2776.7

425.7 759.3 122.0 196.2 0.0 0.0 629.6 0.7 0.01 43.4 187.9 673.7 861.6 (−21.8%)

DMC 664.9 735.4 115.7 0.0 0.0 0.0 2539.9 45.9 0.02 0.0 189.5 2585.9 2775.4

steam (at 41 barg and 254 °C), and cooling water (at 30 °C) are 14.19 US$/GJ, 17.70 US$/GJ, and 0.354 US$/GJ, respectively. The unit cost of electricity is 16.8 US$/GJ. Figure 3 shows a process flowsheet for PC synthesis by using some CSTRs, flash tanks, and one CD column (named as CSTR+CD). Two reactants are fed into the first CSTR, and then the exit mixture from the CSTR is introduced into the first flash tank. Vapor product (mainly containing ammonia) is removed from the flash tank, and liquid product is fed into the second CSTR followed by the second flash tank. CSTRs are used for PC synthesis until the requirement of urea reaction conversion is satisfied. Then the product of the last CSTR is introduced into the CD column for separation. The remaining ammonia, not removed from the flash tanks, is withdrawn from

3. PROCESS DESIGN OF PC SYNTHESIS BY CSTR+CD In the design of the process to synthesize PC, the objective is to minimize the TAC by adjusting design variables. TAC is defined as TAC = operating cost + capital cost/payback year

RD+CD_HI1+DMC

(5)

Here, a payback of eight years is used. The operating cost (OC) includes the costs of steam, cooling water, catalyst, and electricity. The capital cost (CC) comprises the costs of the CSTR (continuous stirred-tank reactor), flash tank, column shell, stage, heat exchanger, and compressor. The formulas of these cost estimations are taken from Turton et al.25 An annual operating time is assumed to be 8322 h. The unit costs of medium-pressure steam (at 10 barg and 184 °C), high-pressure D


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Figure 6. Process flowsheet of RD configuration for PC synthesis in conjunction with RD column for DMC synthesis.

the partial condenser at the column top, and high-purity PC is achieved from the column bottom. In the study, urea is fed with the flow rate of 40 kmol/h, and its reaction conversion is designed to be greater than 99.98 mol % in the following different process configurations. The purity of ammonia removed from the flash tanks and the partial condenser is designed at 99 mol %, while the PC purity of column bottom is set at 99.5 mol %. The CSTR and flash tank are operated at the same temperature (160 °C as suggested by Wang22) and pressure. There are some design and operation variables in this CSTR+CD configuration. The design variables include the feed flow rate of PG, the number and volume of CSTRs, the pressure of CSTR, the pressure of CD column, and the number of total stages and feed stage in the CD column. In the capital cost calculation of CSTR in Turton et al.,25 the volume of CSTR is restricted to be not larger than 35 m3. Therefore, the number and volume of CSTR are adjusted to satisfy the constraint of urea reaction conversion. The pressure of CSTR is changed to meet the purity requirement of ammonia vapor product from the flash tank. The operation variables include the condenser duty and the reboiler duty in the CD column. The condenser duty and reboiler duty are changed to satisfy the overhead product and bottom product specifications, respectively. The steps to minimize TAC for this configuration are given as follows: 1. Guess the feed ratio of PG and urea. Then adjust the number and volume of CSTRs to satisfy the requirement of urea conversion, and change the pressure of CSTR to meet the purity requirement (99 mol %) of ammonia vapor product from the flash tanks. 2. Guess the pressure of the CD column. 3. Guess the number of total stages and feed stage in the CD column. 4. Adjust condenser duty and reboiler duty to satisfy the composition specifications of ammonia (99 mol %) and PC (99.5 mol %) at the CD top and bottom, respectively. 5. Go to step 3 and repeat step 4 until TAC is minimal. 6. Go to step 2 and repeat steps 3 and 4 until TAC is minimal.

7. Go to step 1 and repeat steps 2−4 until TAC is minimal. Because PG, a byproduct in the transesterification reaction for DMC synthesis, is converted into the reactant for the PC synthesis by indirect alcoholysis of urea, the optimal RD column to synthesize DMC by reacting PC and MeOH is also designed by the following procedures. 1. Guess the feed ratio of MeOH and PC. 2. Guess the number of total stages in the RD column. 3. Guess the feed stage of PC. 4. Guess the feed stage of MeOH. 5. Adjust reboiler duty and reflux ratio to satisfy the requirements of PG composition (99.9 mol %) at column bottom and PC reaction conversion (99.98 mol %), respectively. 6. Go to step 4 and repeat step 5 until TAC is minimal. Continue the procedures of going to previous step and repeating next steps used in the optimization for the CSTR+CD configuration until the TAC is minimal. The kinetic model and the thermodynamic model from Holtbruegge et al.26 are adopted in the RD column design for DMC synthesis. DMC and MeOH form a minimum-boiling homogeneous azeotrope. In the DMC synthesis reaction, sodium methoxide is used as a homogeneous catalyst to catalyze the reaction of PC and MeOH. Holtbruegge et al.19 specified that the homogeneous catalyst has a low solubility in the high-boiling reactant PC. In the study of Holtbruegge et al.,20 PC reactant was fed above MeOH reactant into the RD column, and this catalyst was fed along with the MeOH stream. In the DMC synthesis column, there are large amounts of MeOH in the reflux stream. Therefore, in our study, this catalyst is selected to be fed into the reflux stream for mixing with MeOH and then returned to the RD column for catalyzing the reaction of PC and MeOH. In addition, it is noted that the PG along with the homogeneous catalyst is withdrawn from the RD column bottom. The separation of PG and this catalyst is necessary, and PG can then be used as the reactant for PC synthesis. The method for the separation of PG and this catalyst includes the steps of carbonization, filtration, and E


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Figure 7. Relationship between TAC and design variables in the RD configuration for PC synthesis.

and design variables for PC synthesis and DMC synthesis, respectively. The optimal feed ratio of PG and urea is designed at 1.01, very near the neat operation. The requirement of urea reaction conversion cannot be satisfied if the feed ratio of PG and urea is equal to 1. The number of total stages and feed stage in the CD column, operated at 0.05 atm, are 8 and 5, respectively. In the DMC synthesis column, the optimal ratio of

purification. These steps can be found from the research of Zhang et al.27 In the study, the geometry of every column is determined by the criterion that the flood percent on every stage, calculated by using the sizing function provided by ChemCad software, is guaranteed not to exceed 80% at the nominal operating condition. Figures 4 and 5 give the relationship between TAC F


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Figure 8. Process flowsheet of RD+CD_HI1 configuration for PC synthesis in conjunction with RD column for DMC synthesis.

duty and the reboiler duty. Reboiler duty is varied to satisfy the PC product specification (99.5 mol %) at column bottom while condenser duty is changed to maintain the ammonia product purity at 99 mol % from column top. The optimization procedures to minimize TAC are given below for the RD design of PC synthesis.

MeOH and PC is designed at 8.2. The RD column has one stage in the rectifying section, followed by a reactive section with 50 stages (including a partial reboiler). PC and MeOH are fed into the column at stages 3 and 48, respectively. The requirement of PC reaction conversion cannot be satisfied when the PC feed is located above the third stage. Figure 3 also shows the optimal conditions with the minimal TAC. Three CSTRs are provided to satisfy the requirement of urea reaction conversion. Each CSTR with a volume of 27.7 m3 is operated at 160 °C and 9.8 atm. Two flash tanks are used, and ammonia with 99 mol % purity is achieved from vapor product of flask tanks. The vapor ammonia product with 99 mol % purity is also recovered from the partial condenser of the CD column. Highpurity PC with 99.5 mol % is withdrawn from the CD column bottom and sent into the RD column for DMC synthesis. DMC/MeOH mixture is recovered from the column top while high-purity PG with 99.9 mol % is withdrawn from the column bottom and recycled back to the first CSTR. DMC and MeOH can form an azeotrope, and the azeotrope can be separated by extractive distillation or pressure swing distillation according to our previous study28 and some literature reports.29−32 Therefore, in the present study, the emphasis is not put on the separation of DMC and MeOH. Table 2 gives the optimal TAC (1102.2 × 103 US$/year) for the PC synthesis process by the CSTR+CD configuration. The capital cost of three CSTRs accounts for the main part of the capital cost in this configuration. The TAC of DMC synthesis column is also given in Table 2.

1. 2. 3. 4.

Guess the feed ratio of PG and urea. Guess the pressure of the RD column. Guess the number of total stages in the RD column. Guess the stage numbers of the rectifying and reaction sections in the RD column. 5. Guess urea and PG feed stages. 6. Adjust condenser duty and reboiler duty to satisfy the ammonia product and PC product specifications at column top and bottom, respectively. 7. Go to step 5 and repeat step 6 until TAC is minimal. Continue the procedures of going to previous step and repeating next steps used in the optimization for the CSTR+CD configuration until the TAC is minimal. The optimal design method of the RD column to synthesize DMC is the same as that used in the previous section. Figure 7 gives the relationship between TAC and design variables in the RD configuration for PC synthesis. The optimal feed ratio of PG and urea is designed at 1.02, very close to the neat operation. Column pressure is 0.12 atm. The number of total stages in the PC synthesis column is 31, while urea and PG feeds are located at stages 11 and 27, respectively. The stage numbers of rectifying, reaction, and stripping sections are 3, 27, and 1, respectively. Figure 6 also shows the optimal design with the minimal TAC. High-purity (99.5 mol %) PC withdrawn from the bottom of the PC synthesis column is fed into the RD column for DMC synthesis. In the DMC synthesis column, DMC/MeOH mixture is also recovered from the column top while high-purity PG with 99.9 mol % is withdrawn from the column bottom and recycled back to the RD column for PC synthesis. The minimized TAC for the PC synthesis by the RD configuration, also given in Table 2, is 973.4 × 103 US$/year. In comparison with the CSTR+CD configuration, capital cost can be decreased considerably, and TAC is reduced by 11.7% for the RD configuration.

4. PROCESS DESIGN OF PC SYNTHESIS BY RD In this section, RD technology is used to synthesize PC and DMC. The RD configuration for PC synthesis is shown in Figure 6. The RD column for DMC synthesis is also given because the product from the RD column bottom is recycled back to the RD column for PC synthesis. Two reactants are fed into the first RD column for PC synthesis. Ammonia is withdrawn from the partial condenser. PC is achieved from the column bottom and then fed into the second RD column for DMC synthesis. There are some design and operation variables in the PC synthesis column. The design variables include the feed ratio of PG and urea reactants, the pressure of the RD column, the number of total stages, the stage numbers of the rectifying and reaction sections, and the locations of urea and PG feed stage. The operation variables include the condenser G


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Figure 9. Relationship between TAC and design variables in the RD+CD_HI1 configuration.

5. PROCESS DESIGN OF PC SYNTHESIS BY HEAT-INTEGRATED RD+CD

containing PC and excess PG, of the RD column is introduced to a CD column for the separation of PC and PG. Heat integration between the RD and CD columns can then be implemented to reduce the energy consumption of the PC synthesis process by properly adjusting the pressures of RD and CD columns. In this study, three configurations of heatintegrated RD+CD are proposed, and their optimal designs are

In sections 3 and 4, the optimal feed ratio of PG and urea is very close to 1. In this section, we explore the process design with excess PG reactant in the RD column for PC synthesis. In this type of configuration, the bottom product, mainly H


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given below. Figure 8 shows the first configuration of heatintegrated RD+CD (named as RD+CD_HI1). The overhead temperature of CD column is designed to be greater than the bottom temperature of the RD column. A minimum temperature difference of 10 K is ensured between the overhead vapor of the CD column and the bottom liquid of the RD column. Then the latent heat of the overhead vapor from the CD column can be released to the reboiler of the RD column. In the RD column, design variables contain feed ratio of PG and urea, column pressure, number of total stages, stage numbers of the rectifying and reaction sections, and feed stages of urea and PG. Operation variables consist of condenser duty and reboiler duty in the column, which are used to satisfy the requirement of overhead ammonia product purity (99 mol %) and urea reaction conversion, respectively. In the CD column, design variables include column pressure and number of total stages and feed stage. Two operation variables, reflux ratio and reboiler duty, are varied to satisfy the product specifications of PG (98 mol %) and PC (99.5 mol %) at the column top and bottom, respectively. The following is the optimization procedures for the process design of PC synthesis by the RD +CD_HI1 configuration. 1. Guess the feed ratio of PG and urea. 2. Guess the pressure of the RD column. 3. Guess the number of total stages in the RD column. 4. Guess the stage numbers of the rectifying and reaction sections in the RD column. 5. Guess urea and PG feed stages in the RD column. 6. Adjust condenser duty and reboiler duty to satisfy the requirements of overhead ammonia product purity and urea reaction conversion, respectively. 7. Guess the pressure of the CD column. 8. Guess the number of total stages and feed stage in the CD column. 9. Adjust reflux ratio and reboiler duty to satisfy the composition specifications of PG and PC at CD column top and bottom, respectively. Implement heat integration between CD column top and RD column bottom.

10. Go to step 8 and repeat step 9 until TAC is minimal. Continue the procedures of going to previous step and repeating next steps used in the optimization for the previous configurations until the TAC is minimal. The optimization steps used in the previous sections for DMC synthesis by RD are also adopted here. Figure 9 provides the relationship between TAC and design variables in the RD +CD_HI1 configuration for PC synthesis. The optimal feed ratio of PG and urea is 1.25. The stage numbers of rectifying, reaction, and stripping sections in the RD column are 4, 27, and 1, respectively. Urea and PG feeds are located at stages 13 and 26, respectively. The number of total stages and feed stage in the CD column are 34 and 32, respectively. The pressures of RD and CD columns are designed to be 0.04 and 0.9 atm, respectively. Figure 8 also gives the optimal design of RD +CD_HI1 configuration. The overhead temperature of the high-pressure CD column and base temperature of the lowpressure RD column are 184.5 and 161.4 °C, respectively. The large difference between these two temperatures makes heat integration a practical method to reduce energy consumption. The condenser duty of the CD column and the reboiler duty of the RD column are 1741.7 and 5675.5 MJ/h, respectively. The condenser duty in the high-pressure CD column can be utilized to produce vapor in the RD column base, and an auxiliary steam-driven reboiler is needed with a duty of 3933.8 MJ/h. A DMC synthesis column similar to that in Figure 6 is also provided in Figure 8. The minimized TAC of the RD+CD_HI1 configuration for PC synthesis, also given in Table 2, is 1055.5 × 10 3 US$/year. In comparison with the CSTR+CD configuration, the TAC is reduced by 4.2% for the RD +CD_HI1 configuration. However, the TAC is increased from 973.4 × 103 US$/year for the RD configuration with almost neat operation to 1055.5 × 103 US$/year for the RD+CD_HI1 configuration with excess reactant operation. The increased capital cost due to the addition of CD column is greater than the reduced operating cost due to the heat integration in the RD+CD_HI1 configuration. In the CD column shown in Figure 8, column top and bottom are all operated under high temperature. One of the I


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stages or high energy consumption. Nevertheless, the azeotrope vanishes under high pressure. That is, much economical benefit can be achieved by operating the bottom section of the CD column at low pressure and the top section of the CD column at high pressure. This configuration is shown in Figure 10 and is referred to as RD+CD_HI2. The CD column is divided into

methods to reduce energy consumption of the CD column is to operate it under low pressure. However, from the VLE relationship for PG−PC pair shown in Figure 2, PC−PG can form a homogeneous minimum-boiling azeotrope near the pure PG end under low pressure. This indicates that using the CD column to separate these two components would require many J


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top and bottom sections. The overhead vapor of the bottom section is compressed by a compressor and used as a heating medium of the base of the top section. Therefore, the top section of the CD column can be operated under high pressure and temperature. The pressure of the top section can be manipulated such that overhead temperature of the CD column is greater than the base temperature of the CD column or the base temperature of the RD column. Therefore, heat integration can be implemented to save energy consumption by two different ways. The first way is to release the latent heat of overhead vapor to the reboiler in the CD column itself. If the condenser duty is greater than the reboiler duty, the additional latent heat can be released to the reboiler of the RD column. For the RD column in Figure 10, the same design and operation variables as those in the RD+CD_HI1 configuration are used in the RD+CD_HI2 configuration. For the CD column, the pressure and stage number of top and bottom sections, respectively, are new design variables in the RD +CD_HI2 configuration. The design steps for the optimal condition of PC synthesis in the RD+CD_HI2 configuration are outlined next. 1. Guess the feed ratio of PG and urea. 2. Guess the pressure of the RD column. 3. Guess the number of total stages in the RD column. 4. Guess the stage numbers of the rectifying and reaction sections in the RD column. 5. Guess urea and PG feed stages. 6. Adjust condenser duty and reboiler duty to satisfy the requirements of overhead ammonia product purity and urea conversion, respectively. 7. Guess the pressures of top and bottom sections in the CD column. 8. Guess the stage numbers of top and bottom sections in the CD column. 9. Guess the feed stage of the CD column. 10. Adjust reflux ratio and reboiler duty to satisfy the composition requirements of PG and PC at CD column top and bottom, respectively. Implement heat integration between CD column top and bottom.

11. Go to step 9 and repeat step 10 until TAC is minimal. Continue the procedures of going to previous step and repeating next steps used in the optimization for the previous configurations until the TAC is minimal. The optimization steps used previously for DMC synthesis by RD are also adopted here. Figure 11 provides the relationship between TAC and design variables. The optimal feed ratio of PG and urea is 1.16. The pressure of RD is designed to be 0.05 atm. The stage numbers of rectifying, reaction, and stripping sections in the RD column are 3, 23, and 1, respectively. Urea and PG feeds are located at stages 11 and 23, respectively. The pressures of top and bottom sections in the CD column are 1.25 and 0.02 atm, respectively. There are 20 and 8 stages in the top and bottom sections, respectively, in the CD column. The feed is located at stage 4 of the bottom section. The optimal design of the RD+CD_HI2 configuration is also provided in Figure 10. The overhead temperature of the high-pressure top section and base temperature of the lowpressure bottom section in the CD column are 195.0 and 151.2 °C, respectively. The condenser duty of the top section and the reboiler duty of the bottom section are 1100.2 MJ/h and 676.5 MJ/h, respectively. Thus, the latent heat of the top section can not only release 676.5 MJ/h to the reboiler of the bottom section in the CD column but also release 423.7 MJ/h to the reboiler of the RD column. An auxiliary steam-driven reboiler is required with a duty of 5266.1 MJ/h in the RD column. The TAC of the optimal RD+CD_HI2 configuration for PC synthesis, given in Table 2, is 870.8 × 103 US$/year. In comparison with the CSTR+CD configuration, a 21.0% reduction of TAC can be obtained for the RD+CD_HI2 configuration. In comparison with the RD+CD_HI1 configuration, TAC can be reduced by 17.5% for the RD+CD_HI2 configuration because it takes full advantage of the characteristic of the PC−PG VLE relationship by adding an internal compressor between top and bottom sections of the CD column. In addition to the heat integration shown in Figure 10, the overhead vapor of the top section in the CD column can release total latent heat to the reboiler of the RD column. This configuration is named RD+CD_HI3. The optimization K


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of PG and urea is 1.14. The pressure of RD is designed to be 0.03 atm. The stage numbers of rectifying, reaction, and stripping sections in the RD column are 2, 23, and 1, respectively. Urea and PG feeds are located at stages 11 and 23, respectively. The pressures of top and bottom sections in the CD column are 1.36 and 0.02 atm, respectively. The top and

procedures for this configuration are almost the same as those for the RD+CD_HI2 configuration except step 10. Heat integration is implemented between CD top and RD bottom in the RD+CD_HI3 configuration. Figure 12 shows the optimal design of this configuration. Figure 13 gives the relationship between TAC and the design variables. The optimal feed ratio L



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ACKNOWLEDGMENTS This work was supported by the Ministry of Science and Technology of Taiwan under Grant No. MOST 105-2221-E007-128-MY2 and the Ministry of Economic Affairs of Taiwan under Grant No. 106-EC-17-D-11-1466.

bottom sections in the CD column have 20 and 9 stages, respectively. The feed location is at stage 4 of the bottom section. The overhead temperature of the high-pressure top section and base temperature of the low-pressure bottom section in the CD column are 187.8 and 152.3 °C, respectively. The condenser duty of the top section and the reboiler duty of the bottom section are 994.9 and 631.3 MJ/h, respectively. The reboiler duty and bottom temperature of the RD column are 5695.5 MJ/h and 161.9 °C, respectively. Therefore, the latent heat of the top section in the CD column can all be released to the reboiler of the RD column. An auxiliary steam-driven reboiler is necessary with a duty of 4700.6 MJ/h for the RD column. The TAC of this configuration, given in Table 2, is 861.6 × 103 US$/year. In comparison with the CSTR+CD configuration, TAC can be reduced by 21.8% for the RD +CD_HI3 configuration. This configuration provides the most economic design for the PC synthesis in the plant-wide process to produce the final product DMC. In addition to the TAC of PC synthesis process, Table 2 also gives the TAC of DMC synthesis column under different configurations. There are only small variations in the TAC of this column for the five designed configurations.

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REFERENCES

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6. CONCLUSION Process design of PC synthesis for DMC production by indirect alcoholysis of urea is investigated in the study. There are five different configurations proposed. CSTR+CD and RD configurations are designed under near-neat operation. Capital cost of the CSTR+CD configuration can be decreased considerably, and TAC is reduced by the RD configuration. In addition, three heat-integrated RD+CD configurations are designed under excess reactant operation. Heat integration is implemented between CD column top and RD column bottom by adjusting the pressures of RD and CD columns, respectively, in the RD+CD_HI1 configuration. However, this configuration cannot provide more economic design than the RD configuration because the increased capital cost is greater than the reduced operating cost in this configuration. On the other hand, much economical benefit can be achieved by operating the bottom section of the CD column at low pressure and the top section of the CD column at high pressure in RD +CD_HI2 and RD+CD_HI3 configurations. These two configurations take full advantage of the azeotrope characteristic of the PC−PG pair by adding an internal compressor between top and bottom sections of the CD column. Simulation results indicate that RD+CD_HI3, with heat integration between the top section in the CD column and the bototm of the RD column, provides the most economic novel design of PC synthesis for the DMC production.

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +886-3-5715131, ext. 33624 . Fax: +886-3-5721694. Email: [email protected]. *Tel.: +86-10-64437805. Fax: +86-10-64437805. E-mail: [email protected]. ORCID

San-Jang Wang: 0000-0002-6683-5006 Kejin Huang: 0000-0003-2649-0223 Notes

The authors declare no competing financial interest. M


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