Design and Optimization of a Process for the Production of Methyl Methacrylate Via Direct Methylation

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Article Design and Optimization of a Process for the Production of Methyl Methacrylate via Direct Methylation

Taotao Liang 1,2, Xiaogang Guo 2,3,*, Abdulmoseen Segun Giwa 1,4, Jianwei Shi 3, Yujin Li 3, Yan Wei 3, Xiaojuan Wang 5, Xuansong Cao 3, Xiaofeng Tang 3 and Jialun Du 3

1 Green Intelligence Environmental School, Yangtze Normal University, Chongqing 408003, China; [email protected] (T.L.); [email protected] (A.S.G.) 2 Faculty of Materials and Energy, Southwest University, Chongqing 400715, China 3 College of Chemistry and Chemical Engineering, Yangtze Normal University, Chongqing 408003, China; [email protected] (J.S.); [email protected] (Y.L.); [email protected] (Y.W.); [email protected] (X.C.); [email protected] (X.T.); [email protected] (J.D.) 4 State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, China 5 College of chemistry and bioengineering, Guilin University of Technology, Guilin 541004, China; [email protected] * Correspondence: [email protected]; Tel.: +86-023-72782170

Received: 25 April 2019; Accepted: 6 June 2019; Published: 18 June 2019

Abstract: Methyl methacrylate (MMA) plays a vital role in national productions with broad application. Herein, the production of MMA is realized by the improved eco-friendly direct methylation method using Aspen Plus software. Three novel kinds of energy-saving measures were proposed in this study, including the recycle streams of an aqueous solution, methacrolein (MAL), and methanol, the deployment of double-eﬀect distillation instead of a normal one, and the design of a promising heat-exchange network. Moreover, MMA with a purity of 99.9% is obtained via the design of a MAL absorber column with an optimal stage number of 11 and a facile chloroform recovery process by using the RadFrac model. Thus, the proposed green process with energy-conservation superiority is the vital clue for developing MMA, and provides a reference for the production of MMA-ramiﬁcations with excellent prospects.

Keywords: MMA; double-eﬀect distillation; energy conservation; MAL; purity

1. Introduction Methyl methacrylate (MMA), an essential chemical raw material, has drawn growing interest in the fields of poly(methyl methacrylate) (PMMA), functional coatings, lubricant additives, ion exchange resins, construction materials, lighting equipment, etc., due to excellent transparency and weather resistance [1–6]. It is worth mentioning that the production of MMA needs to grow quickly and steadily due to the increasing demand for MMA in the world [7–10]. Thus, it is necessary to develop a green process for preparing MMA efficiently. In fact, several processes for obtaining MMA have been developed up to now, including the C2-method (ethylene carbonylation, etc.) [11], C3-method (acetone cyanohydrin (ACH), etc.) [12], and C4-method (isobutylene direct methylation method (IDMM), methacrylonitrile process, etc.) [13,14], which are clearly seen in Figure1. Based on the mentioned methods, lots of researchers have devoted efforts to exploring MMA production processes, notably including a separate process of a mixture of methyl methacrylate, methanol and water. For example, Wu et al. studied the optimization

Processes 2019, 7, 377; doi:10.3390/pr7060377 www.mdpi.com/journal/processes Processes 2019, 7, x FOR PEER REVIEW 2 of 13 Processes 2019, 7, 377 2 of 12 mixture of methyl methacrylate, methanol and water. For example, Wu et al. studied the optimization design and dynamic control of the simplified separation process for MMA by using a design and dynamic control of the simplified separation process for MMA by using a one-column one-column flowsheet with a middle decanter, which can cut operating costs by 35.9% compared to flowsheet with a middle decanter, which can cut operating costs by 35.9% compared to the industrial the industrial three-column design. Large disturbances of the feed composition can be effectively three-column design. Large disturbances of the feed composition can be effectively controlled [15]. controlled [15]. Chang et al. studied an alternative design for separating mixtures (including methyl Chang et al. studied an alternative design for separating mixtures (including methyl methacrylate, methacrylate, methanol, and water) using a distillation column, a stripper and a decanter. methanol, and water) using a distillation column, a stripper and a decanter. Compared with the Compared with the traditional design process, the operating cost of during this improved design traditional design process, the operating cost of during this improved design significantly reduced significantly reduced to 25.9%. In addition, based on open/closed-loop sensitivity testing, the overall to 25.9%. In addition, based on open/closed-loop sensitivity testing, the overall design turns out to design turns out to ensures the product of MMA with a high degree of purification [16]. The azeotropicensures the distillation product of and MMA ionic with liquid a high eco-envi degreeronmental of purification separation [16]. The of azeotropicmethacrolein distillation (MAL) was and investigatedionic liquid eco-environmentalby Yan et al. to obtain separation MMA, of methacroleinand results indicated (MAL) was that investigated used green by process Yan et al. had to obtain MMA, and results indicated that used green process had significant economic and green-degree significant economic and green-degree advantages [14]. Compared with related reports, C4-IDMM hasadvantages gradually [14 been]. Compared regarded with as relateda promising reports, green C4-IDMM technique, has gradually due to the been low-cost regarded equipment, as a promising short reactiongreen technique, path, and due sufficient to the low-cost raw materials, equipment, and so short on [12,13,17], reaction path, though and ther suffiecient are no raw large-scale materials, uses and inso industry. on [12,13 ,17However,], though it thereis still are a nochallenge large-scale to develop uses in industry.the production However, process it is of still MMA a challenge based on to develop the production process of MMA based on C4-IDMM economically. The method used in our C4-IDMM economically. The method used in our study is based on the improved direct methylation ofstudy isobutylene is based onmethod the improved proposed direct by the methylation Asahi Kasei of isobutyleneCorporation. method There proposedis no MAA by step the Asahiduring Kasei this process,Corporation. which There effectively is no MAA avoids step side during reactions this process, such as whichMAA epolymerization,ffectively avoids simplifies side reactions the process such as flow,MAA and polymerization, reduces energy simplifies consumption, the process thereby flow, gr andeatly reduces reducing energy operating consumption, costs and thereby investment greatly costsreducing and operatingmaking it costsmore andcompetitive investment costs and making it more competitive.

FigureFigure 1. ComparisonComparison diagram diagram of of different diﬀerent production processes of methyl methacrylate (MMA).

Thus, the focus of this report is on the facile design of an MMA synthesis route by using a direct isobutylene methylation method method to to reduce the energy consumption.consumption. Moreover, ASPEN simulation software, an an effective eﬀective and and common common tool, tool, is used is used for forprocess process simulation simulation and anddata dataanalysis analysis at the atsame the time.same time.

2. Simulation Simulation and and Experimental Experimental Methods

2.1. Main Main Reaction Reaction Equation

The production process-C 44-IDMM-IDMM of of MMA MMA in in this this study study is is based based on on the the Asahi Asahi direct direct method, method, also called direct methylation as follows. Firstly, Firstly, the oxidation of isobutylene (Aladdin Industrial Corporation, Shanghai,Shanghai, China)China) produces produces methylacrolein, methylacrolein, which which acts acts with with air inair the in liquid the liquid phase. phase. Then, Then,the product the product of the previousof the previous step undergoes step undergoes the one-step the one-step esteriﬁcation esterification oxidation oxidation reaction reaction with excess with excessmethanol methanol (Aladdin (Aladdin Industrial Industrial Corporation, Corporation, Shanghai, Shanghai, China) toChina) obtain to MMA, obtain where MMA, the where catalyst the is catalystPd (Aladdin is Pd Industrial(Aladdin Corporation,Industrial Corporation, Shanghai, China),Shanghai, and China), the carrier and the is CaCO carrier3 (Aladdin is CaCO3 Industrial (Aladdin IndustrialCorporation, Corporation, Shanghai, China)Shanghai, or ZnO China) (Aladdin or ZnO Industrial (Aladdin Corporation, Industrial Shanghai, Corporation, China), Shanghai, and the Processes 2019, 7, x FOR PEER REVIEW 3 of 13 ProcessesProcesses2019 2019, 7, 377, 7, x FOR PEER REVIEW 3 of3 12of 13 China), and the activity and stability of the whole reaction is improved by adding metal elements of China),Pb, Bi andor Zn the, etc. activity [18,19 and]. The stability synthetic of the route whole is shown reaction clearly is improved in the following by adding Equation metal elements (1): of activityPb, Bi and or Zn, stability etc. [18,19]. of the The whole synthetic reaction route is improved is shown byclearly adding in the metal following elements Equation of Pb, (1): Bi or Zn, etc. [18,19]. The synthetic route is shown clearly in the following Equation (1):

2.2. Whole Process Design 2.2.2.2. Whole Whole Process Process Design Design The process route for MMA based on direct methylation mainly includes an isobutylene TheoxidationThe process process unit, route MMAroute for MMA synthesisfor basedMMA and onbased directrefining on methylation directunit, and me mainlymethanolthylation includes recyclingmainly an includes isobutylene unit. The an main oxidationisobutylene process is unit,oxidation MMAshown synthesis inunit, Figure MMA and 2. synthesis refiningPractically unit,and speaking, refining and methanol unit,using and recyclingisobutylene methanol unit. asrecycling The raw main material unit. process The, MAL main is shown is process produced in is shown in Figure 2. Practically speaking, using isobutylene as raw material, MAL is produced Figurethrough2. Practically a MAL speaking, synthesis using reactor isobutylene (RMAL) with as H raw2O material,and O2. Mixed MAL gases is produced pass through through the a MALquenching synthesisthroughspray reactor towera MAL (R(1) synthesisMAL, the) withdehydration reactor H2O and (R towerMAL O2) .with Mixed(2), Hand2O gases theand MAL passO2. Mixed throughabsorption gases the towerpass quenching through (3) to sprayobtain the quenching towerMAL with (1),spray thehigh dehydration tower purity (1),. Then, the tower dehydrationgas (2), phases and thefrom tower MAL the (2), top absorption and of thtowere MAL tower (3) absorptionundergo (3) to obtain tail tower-gas MAL treatment(3) withto obtain high in separatorMAL purity. with (4) Then,highafter gas purity. phasesmethanol Then, from recovery gas the topphases, ofand tower from a degasification (3)the undergo top of towe tail-gasprocessr (3) treatment undergoin separator tail-gas in separators (5) treatment and (4)(6). after The in methanolseparator liquid phases (4) after methanol recovery, and a degasification process in separators (5) and (6). The liquid phases recovery,from and the abottom degasification top of tower process (3) go in into separators RMMA to (5) produce and (6). an The MMA liquid solution phases wit fromh relative the bottom low purity. topfrom ofThe tower the methanol bottom (3) go intoand top RMALofMMA tower recoveredto (3) produce go into from an R MMAobtained to solutionproduce MMA withan solution MMA relative solutionin separation low purity.with towerrelative Thes methanol (7) low and purity. (8) are andThe MALrecirculated methanol recovered andto fromtower MAL obtaineds recovered(1), (2) MMA, and from solution(3). obtained Then, in separationafterMMA separation solution towers in of separation (7) MMA and (8) and aretowers water recirculated (7) in and the (8) to MMA are towersrecirculatedextraction (1), (2), and tankto (3).towers (9), Then, the (1), MMA after (2), separation andorganic (3). phaseThen, of MMA goafteres and intoseparation water a double in theof-effect MMAMMA rectification extractionand water tank processin (9),the theMMAwith an MMAextractionMMA organic low tank phase pressure (9), goes the intotower MMA a double-eT -organic1 (10),ff heatphaseect rectification-exchanger goes into (11), processa double-effect separating with an MMAtankrectifications (12) low and pressure process (13), towerand with MMA an T-1MMA (10),high heat-exchanger lowpressure pressure tower (11),tower T-2 separating (14)T-1 in(10), the tanksheat-exchanger scrubbing (12) and towers (13), (11),, andand separating MMAthen the high tanksMMA pressure (12) with andtower high (13), purity T-2 and (14) isMMA in finally thehigh scrubbingobtained. pressure towers, tower and T-2 then (14) the in MMAthe scrubbing with high towe purityrs, and is finally then the obtained. MMA with high purity is finally obtained.

Figure 2. The process design sketch of synthetic process of MMA by isobutylene direct methylation method. Figure 2. The process design sketch of synthetic process of MMA by isobutylene direct methylation Figuremethod 2. The. process design sketch of synthetic process of MMA by isobutylene direct methylation method. Processes 2019, 7, 377 4 of 12

2.3. Process Simulation

2.3.1. Thermodynamic Method Determination Aspen Plus software (Version 8.4, Aspen Technology, Inc., Bedford, MA, USA) contains lots of different physical methods selectively applied to various conditions and materials with diverse characteristics [20–25]. In this report, the raw materials are mainly isobutylene, methanol, and air, where the gas phase conforms to Henry’s Law of the ideal gas and the gas law, and the liquid phase belongs to a non-ideal system. Therefore, the universal functional activity coefficient (UNIFAC) thermodynamic calculation model is selected for the MAL refined synthesis process, which is used for tower (1) and (2) (with the dashed red boxes, in Figure2). The non-random two-liquid Redlich-Kwong (NRTL-RK) thermodynamic calculation model is used for other processes, such as the double-effect rectification process. It is worth mentioning that the application of the two models for accurately simulating and analyzing the vapor-liquid equilibria (VLE) and liquid-liquid equilibria (LLE) properties of non-ideal solutions is feasible.

2.3.2. Process Design The whole process of obtaining MMA includes two steps of MAL and MMA production and a synthetic refining section. During the oxidation reaction of isobutylene, the gas phase components in the reaction system mainly contain isobutylene and oxygen. The reaction temperature and operating pressure are 380 ◦C and 1 bar (1 bar = 0.1 MPa), respectively. Due to the oxidation reaction of isobutylene with Mo-Bi as the catalyst, which is an exothermic reaction, the volume ratio of water vapor:isobutylene:air = 6:6:88 [26], and the reactor is selected as a stoichiometry reactor (RStoic) with an isobutylene conversion and MAL selectivity of 97.31% and 92.34%, respectively [26]. In the MAL oxidative esterification reaction, the system contains MAL, methanol, air, and other components. The reaction temperature and operating pressure are controlled at 50 ◦C and 1 bar, respectively, and the reactor is an RStoic with MAL conversion and MMA selectivity of 98.8% and 85.9%, respectively [27]. A strict-distillation module can calculate the rectification, gas-liquid separation, absorption, extraction, and countercurrent double-effect rectification for all separation units.

2.3.3. Data Acquisition During the simulation process, the substances contained in the physical property system are ordinary and can be directly retrieved from the Aspen Plus database. The relevant operating parameters in the chemical engineering process and the actual operating conditions data play important roles in determining the initial value of the simulation process.

3. Results and Discussion

3.1. Process Recycle Design Based on the need to save energy-saving, three kinds of recycle streams are used in the production process of MMA. Speciﬁcally, one is the water stream, as shown in Figure2, where the water used in the quench spray tower goes into the dehydrating tower, and the extruded water goes back to the quench spray tower by a centrifugal pump, realizing water recycling. In addition, the energy-saving of one speciﬁc process largely depends on the full use of raw materials or a key intermediate product in this study. We focus on MAL and methanol. Clearly, the two streams from two separation towers for MAL and methanol (7 and 8, in Figure2) are re-used back in the dehydrating tower (methanol, MAL, 1’), MAL absorption tower (methanol, MAL, 2’), and following mixer (methanol, MAL, 3’), contributing to increasing their utilization and reducing the consumption and the loss of intermediate products, further increasing the yield. Processes 2019, 7, 377 5 of 12

Processes 2019, 7, x FOR PEER REVIEW 5 of 13 3.2. Design of a Highly Efficient MAL Absorption Column How to absorb the MAL obtained from isobutylene is a key to preparing the MMA. Methanol How to absorb the MAL obtained from isobutylene is a key to preparing the MMA. Methanol regarded as an excellent absorbent for MMA, not only realizes the separate effect, but also does not regarded as an excellent absorbent for MMA, not only realizes the separate effect, but also does not introduce novel impurities, due to methanol being a reactant and absorbent, respectively. Thus, introduce novel impurities, due to methanol being a reactant and absorbent, respectively. Thus, there there is no need for subsequent steps such as rectification and purification for MAL, shortening the is no need for subsequent steps such as rectification and purification for MAL, shortening the process process and reducing the energy and economic investment simultaneously. Additionally, the and reducing the energy and economic investment simultaneously. Additionally, the desired MAL can desired MAL can be separated from a liquid by countercurrent contact with methanol in a MAL be separated from a liquid by countercurrent contact with methanol in a MAL absorber column with a absorber column with a pressure of 0.2 MPa and an operating temperature of 34.2 °C. The absorption pressure of 0.2 MPa and an operating temperature of 34.2 C. The absorption rate of MAL by the MAL rate of MAL by the MAL absorption tower is 99.32% in this◦ study. absorption tower is 99.32% in this study.

3.2.1.3.2.1. MALMAL AbsorberAbsorber ColumnColumn StageStage TheThe MALMAL absorber absorber column column stage stage largely largely depends depends on the on absorptive the absorptive amount ofamount MAL. Theof MAL. sensitivity The ofsensitivity the absorption of the absorption is analyzed is using analyzed Aspen using Plus’ As RadFracpen Plus’ model. RadFrac The model. stage The of the stage absorber of the absorber column column ranged from 5 to 20. Figure 3 shows the flow rate of MAL (fMAL) at the bottom of the column ranged from 5 to 20. Figure3 shows the ﬂow rate of MAL ( fMAL) at the bottom of the column as a as a function of stage number. Clearly, the “fMAL” increases sharply as the stage increasing from 5 to function of stage number. Clearly, the “fMAL” increases sharply as the stage increasing from 5 to 11, 11, where it is close to the peak value. But, the “fMAL” almost remains stable as the stage number where it is close to the peak value. But, the “fMAL” almost remains stable as the stage number continues tocontinues rise. As to is rise. well As known, is well the known, higher the the higher stage number, the stage the number, higher the columnhigher the height, column indicating height, theindicating higher pressurethe higher drop pressure [14]. Therefore, drop [14]. the Theref columnore, stage the column number stage of 11 shouldnumber be of preferred 11 should after be preferred after considering the equipment fee and high “fMAL” comprehensively. considering the equipment fee and high “fMAL” comprehensively.

FigureFigure 3.3. EEffectﬀect ofof stagestage numbernumber onon thethe ﬂowflow rate rate of of MAL MAL ( f(MALfMAL) in the bottom ofof thethe absorberabsorber column.column.

3.2.2.3.2.2. MethanolMethanol AbsorbentAbsorbent FlowFlow RateRate TheThe AspenAspen Plus’s Plus’s RadFrac RadFrac model model is also is used also to used determine to determine the methanol the flowmethanol rate. Theflow relationship rate. The f f betweenrelationship of the between “ MAL” of and the methanol “fMAL” and flow methanol rate is shown flow inrate Figure is sh4own. Clearly, in Figure “ MAL 4.” increasesClearly, “ withfMAL” f theincreases flow rate with of the the flow methanol rate of solvent. the methanol However, solvent. the growth However, rate the of “ growthMAL” decreases rate of “f graduallyMAL” decreases and nearlygradually reaches and to nearly 0 when reaches the methanol to 0 when flow the rate methanol is larger thanflow 3000rate kmolis larger/h, which than indicates3000 kmol/h, that excesswhich absorbentsindicates that are excess not necessary, absorbents in addition are not tonecessary, reasons of in higher addition cost to and reasons energy of consumption.higher cost and Thus, energy the preferredconsumption. methanol Thus, flow the ratepreferred is ca. 3000methanol kmol/ hflow for therate MAL is ca. absorption 3000 kmol/h process. for the MAL absorption process. Processes 2019, 7, x FOR PEER REVIEW 6 of 13

Processes 2019, 7, 377 6 of 12 Processes 2019, 7, x FOR PEER REVIEW 6 of 13

Figure 4. The flow rate of MAL (fMAL) in the bottom of the absorber column as a function of absorbent flow rate.

3.3. Controllable Design for MMA with High Purity

3.3.1. Double-Effect Distillation Design

The purity of MMA is the key factor for evaluating the practicability of the product and Figure 4. TheThe flow flow rate of MAL ( fMAL) )in in the the bottom bottom of of the the absorber absorber co columnlumn as as a a function function of of absorbent absorbent efficiency of the process. Here, MMA with a purity of 99.9% is produced by distilling the chloroform flowflow rate. in the mixed liquid of MMA and chloroform in the extraction tank (that is, a chloroform recovery 3.3.tower) Controllable for purifying Design MMA. for MMA The with computational High Purity model used is a RadFrac model. Normal and 3.3. Controllable Design for MMA with High Purity double-effect distillations are used in this study. Clearly seen in Figure 5, though the product (99.9%) 3.3.1. Double-Effect Distillation Design 3.3.1.is also Double-Effect obtained by Distillation normal distillation Design (Figure S1 in the Supporting Information) the energy consumptionThe purity of of normal MMA isrectification the key factor is much for evaluating higher than the practicability that of double-effect of the product distillation, and effi ciencywhich The purity of MMA is the key factor for evaluating the practicability of the product and ofrealizes the process. great energy Here, MMA saving with efficiency a purity of of 39% 99.9% and is 74% produced for the by heat-exchange distilling the chloroform equipment in namely the mixed the efficiency of the process. Here, MMA with a purity of 99.9% is produced by distilling the chloroform liquidreboiler of MMAand condenser, and chloroform respectively, in the extraction showing tank a gr (thateat is,advantage a chloroform of double-effect recovery tower) distillation for purifying for in the mixed liquid of MMA and chloroform in the extraction tank (that is, a chloroform recovery MMA.saving Theenergy. computational model used is a RadFrac model. Normal and double-effect distillations are tower) for purifying MMA. The computational model used is a RadFrac model. Normal and usedFigure in this 6 study. displays Clearly the process-sheet seen in Figure simulation5, though the of productthe chloroform (99.9%) recovery is also obtained tower. Clearly, by normal the double-effect distillations are used in this study. Clearly seen in Figure 5, though the product (99.9%) distillationspecific detailed (Figure parameters S1 in the Supporting are marked Information) for each st theream. energy It is consumptionworth mentioning of normal that rectificationthe purity of is is also obtained by normal distillation (Figure S1 in the Supporting Information) the energy muchMMA higherincreases than to that 99.9% of double-e (Kmol)ffect from distillation, 52.2% which(Kmol) realizes in the great double-effect energy saving distillation efficiency process, of 39% consumption of normal rectification is much higher than that of double-effect distillation, which andindicating 74% for the the high heat-exchange purity of the equipment product and namely supe theriority reboiler of the and designed condenser, chloroform respectively, recovery showing tower a realizes great energy saving efficiency of 39% and 74% for the heat-exchange equipment namely the greatbased advantage on the double-effect of double-e distillationffect distillation design. for saving energy. reboiler and condenser, respectively, showing a great advantage of double-effect distillation for saving energy. Figure 6 displays the process-sheet simulation of the chloroform recovery tower. Clearly, the specific detailed parameters are marked for each stream. It is worth mentioning that the purity of MMA increases to 99.9% (Kmol) from 52.2% (Kmol) in the double-effect distillation process, indicating the high purity of the product and superiority of the designed chloroform recovery tower based on the double-effect distillation design.

Figure 5. Energy consumption comparison of normal and double-eﬀect distillation.

Figure6 displays the process-sheet simulation of the chloroform recovery tower. Clearly, the speciﬁc detailed parameters are marked for each stream. It is worth mentioning that the purity of MMA increases to 99.9% (Kmol) from 52.2% (Kmol) in the double-eﬀect distillation process, indicating

Processes 2019, 7, x FOR PEER REVIEW 7 of 13

Figure 5. Energy consumption comparison of normal and double-effect distillation.

3.3.2. Design of a Promising Chloroform Recovery Process The proposed chloroform recovery process includes two kinds of chloroform recovery towers with low and high pressure, respectively. The operating pressure in the high-pressure column T-2 is 3 bar, three times larger than that in the low-pressure column T-1 tower. The operating temperature Processes 2019 7 is 60.7 and, ,98.9 377 °C for T-1 and T-2, respectively (seen in Figure 6). For the T-1 tower,7 ofthe 12 condensed-gas phase materials are discharged continuously at the top of tower, and there is no thereboiler high purityat the ofbottom the product of the and tower, superiority while ofa thekettle designed-type reboiler chloroform is used recovery in tower tower T based-2, with on theno double-econdenserﬀect at distillationthe top of T design.-2 for accurate simulation and calculation.

Figure 6. Chloroform recovery process simulation based on double-eﬀect distillation by the Aspen Plus software. Figure 6. Chloroform recovery process simulation based on double-effect distillation by the Aspen 3.3.2.Plus Design software. of aPromising Chloroform Recovery Process

TheThe proposednext analysis chloroform focuses on recovery the determination process includes of essential two kinds parameters of chloroform for towers recovery T-1 and towers T-2, withsuch lowas the and feed high position, pressure, the respectively. number of Thethe operatingplate, and pressurethe top output, in the high-pressure using the RadFrac column model T-2 is in 3 bar,Aspen three Plus. times larger than that in the low-pressure column T-1 tower. The operating temperature is 60.7 andFigure 98.9 7 ◦shCows for T-1 the and flow T-2, rate respectively for MMA and (seen chloroform in Figure6 as). Fora function the T-1 of tower, the number the condensed-gas of the plate. phaseAs for materials tower T-1 are in dischargedFigure 7a, continuouslythe flux of MMA at the at topthe oftop tower, of the and tower there decreases is no reboiler rapidly, at the being bottom the ofexact the opposite tower, while of that a kettle-type of chloroform, reboiler when is used the number in tower of T-2, the with plate no is condenser relatively atlow the (<10). topof Both T-2 the for accurateflux of MMA simulation and chloroform and calculation. remain approximately stable as the number of plates continues to increasThee. nextSimilar analysisly, as shown focuses in on Figure the determination 7b, the flow rate of essential of MMA parameters at the bottom for towersof the tower T-1 and T- T-2,2 at suchfirst increases as the feed quickly position, as the the number number of of plate the plate,s increases and the from top 2 output, to 15, and using ris thees more RadFrac slowly model as the in Aspennumber Plus. of plates changes from 15 to 27. It is almost unchanged when the number of plates is greater than Figure27, resulting7 shows into the flowoptimal rate number for MMAs of and plate chloroforms of 10 and as a27 function for towers of the T- number1 and T of-2, the after plate. the Asconsideration for tower T-1 of inthe Figure maximal7a, the flux flux of of chloroform MMA at the and top the ofthe minimal tower decreasesflux of MMA rapidly,, respectively being the . exactMoreover, opposite the ofrelationship that of chloroform, between when the thefeed number position of theand plate component is relatively is studied low (< 10).in Figure Both the 8. fluxFor oftower MMA T-1 and in Figure chloroform 8a, the remain constituent approximately content of stablechloroform as the in number the top of of plates the tower continues increases to increase. sharply Similarly,with the number as shown of in plate Figures growing7b, the flow up, rateand ofthe MMA corresponding at the bottom transformation of the tower T-2law atfor first MMA increases is the quicklyopposite. as However, the number they of platesall reach increases their stable from value 2 to 15, at andthe same rises moretime slowlywhen the as thenumber number of the of platesplates changesis ca. 8. Interestingly from 15 to 27., the It is variation almost unchanged tendency whenof the thecon numberstituent ofcontent plates of is greaterMMA is than like 27, a parabolic resulting intocurve optimal as the numbersnumber of of plate platess increases of 10 and (as 27 clearly for towers seen T-1in Figure and T-2, 8b) after, and the the consideration flux of MMA ofis thethe maximalmaximum flux as ofthe chloroform number of and plate thes minimal is ca. 9, flux where of MMA, the constituent respectively. content Moreover, of chl theoroform relationship is the between the feed position and component is studied in Figure8. For tower T-1 in Figure8a, the constituent content of chloroform in the top of the tower increases sharply with the number of plates growing up, and the corresponding transformation law for MMA is the opposite. However, they all reach their stable value at the same time when the number of the plates is ca. 8. Interestingly, the variation tendency of the constituent content of MMA is like a parabolic curve as the number of plates increases (as clearly seen in Figure8b), and the flux of MMA is the maximum as the number of plates Processes 2019, 7, 377 8 of 12 Processes 2019, 7, x FOR PEER REVIEW 8 of 13

minimum simultaneously, which provides theoretical evidence for determining the optimal position is ca.of 9, the where towers the T-1 constituent and T-2. content of chloroform is the minimum simultaneously, which provides theoretical evidence for determining the optimal position of the towers T-1 and T-2.

ProcessesFigureFigure 2019 7. The ,7. 7, xTheﬂow FOR flow PEER rate rate forREVIEW for MMA MMA and and chloroform chloroform at at the the top top of of tower tower T-1 T-1 ( a()a) and and at at the the bottom bottom ofof9 towerof 13 T-2 (towerb) as aT-2 function (b) as a offunction the number of the number of plates of inplates the in respective the respective towers. towers.

FigureFigure 8. The 8. The relationship relationship between between thethe feedfeed position and and the the rate rate of offlow ﬂow for forMMA MMA and chloroform and chloroform at at the topthe oftop tower of tower T-1 T-1 (a )(a and) and the the rate rate of of ﬂowflow for MMA MMA and and chloroform chloroform at the at thebottom bottom of tower of tower T-2 as T-2 a as a functionfunction of feed of feed position position (b ().b).

The influence of output in towers T-1 and T-2 on the separation effect is investigated by sensitivity analysis in Figure 9. The initial value is 20 Kmol/h. For tower T-1 in Figure 9a, there is no significant change in the amount of loss of MMA with the top output smaller than 28 Kmol/h and then increasing at a high rate of speed. The recovery amount of chloroform decreases slowly at the first stage (top output from 10 to 30 Kmol/h), and reduces sharply with a top output greater than 30 Kmol/h. The top output in tower T-1 is controlled at ca. 22 Kmol/h. Clearly, the optimal bottom output in tower T-2 is ca. 65 Kmol/h because of the lower flow of chloroform and the higher flower of MMA (seen in Figure 9b). Processes 2019, 7, 377 9 of 12

The influence of output in towers T-1 and T-2 on the separation effect is investigated by sensitivity analysis in Figure9. The initial value is 20 Kmol /h. For tower T-1 in Figure9a, there is no significant change in the amount of loss of MMA with the top output smaller than 28 Kmol/h and then increasing at a high rate of speed. The recovery amount of chloroform decreases slowly at the first stage (top output from 10 to 30 Kmol/h), and reduces sharply with a top output greater than 30 Kmol/h. The top output in tower T-1 is controlled at ca. 22 Kmol/h. Clearly, the optimal bottom output in tower T-2 is ca. Processes 2019, 7, x FOR PEER REVIEW 10 of 13 65 Kmol/h because of the lower flow of chloroform and the higher flower of MMA (seen in Figure9b).

Figure 9. (a) The recovery amount of chloroform and loss amount of MMA as a function of the output Figure 9. (a) The recovery amount of chloroform and loss amount of MMA as a function of the at the top of tower T-1, and (b) the rate of ﬂow for MMA and chloroform at the bottom of tower T-1 output at the top of tower T-1, and (b) the rate of flow for MMA and chloroform at the bottom of versus the output at the bottom of tower T-2. tower T-1 versus the output at the bottom of tower T-2. 3.4. Energy-Saving Optimization 3.4. Energy-Saving Optimization Energy-saving or cost-saving is essential for material-preparation, process design, etc. in almost all areasEnergy-saving [28–31], an or especially cost-saving in theis essential including for industrial material-preparation, production ofprocess MMA. design, The heat etc. exchanger in almost allnetwork areas [28–31], design isan used especially to reduce in the energy including consumption industrial in thisproduction working of by MMA. using The the software-Aspenheat exchanger networkEnergy Analyzerdesign is (Versionused to reduce 8.4, Aspen energy Technology, consumption Inc., in MA, this USA),working based by using on loop the andsoftware-Aspen pinch-point Energydesign principles,Analyzer (Version which includes 8.4, Aspen several Technology conditions, Inc., of MA, no utility USA), cooler based above on loop the and pinch-point, pinch-point no designutility heaterprinciples, under which the pinch-point, includes several and no conditions heat exchange of no across utility the cooler pinch-point, above etc.the pinch-point, Figure 10 shows no utilitythe energy heater consumption under the pi changench-point, before and and no afterheat optimizingexchange acro thess heat-exchange the pinch-point, network, etc. Figure with the10 showscorresponding the energy ﬁtting consumption diagram, displayed change before in Figure and S2after and optimizing Table S1 in the the heat-exchange Supporting Information. network, with It is theworth corresponding mentioning thatfitting before diagram, the heat-exchange displayed (H-E)in Figure network, S2 and there Table is no heatS1 in exchange the Supporting between Information.one process hotIt is stream worth andmentioni a processng that cold before stream the (i.e., heat-exchange without using (H-E) utilities). network, Clearly, there through is no heat the exchangeintegrated between heat exchange one process network hot design, stream heatingand a proc steamess consumptioncold stream (i.e., is reduced without by using 12.6%, utilities). and the Clearly,cooling utilitythrough is reduced the integrated by 11.2% heat that is,exchange the thermal network and cooling design, loads heating after steam heat-exchange consumption network is reduceddesign are by much 12.6%, lower and thanthe cooling before energy-savingutility is reduced optimization, by 11.2% that with is, the the heat-exchange thermal and areacooling falling loads by after54 percent, heat-exchange indicating network the great design advantage are much of heat-exchange lower than before network energy-saving research. optimization, with the heat-exchange area falling by 54 percent, indicating the great advantage of heat-exchange network research. Processes 2019, 7, 377 10 of 12 Processes 2019, 7, x FOR PEER REVIEW 11 of 13

FigureFigure 10. 10. TheThe changechange inin heatheat consumptionconsumption forfor thermalthermal loadload (a()a) and and cooling cooling load load ( b(),b), and and the the heat-exchangeheat-exchange area area (c ()c before) before and and after after heat-exchange heat-exchange (H-E) (H-E) network network optimal optimal design. design. 4. Conclusions 4. Conclusions In summary, a novel preparation process of MMA with extensive applications has been successfully In summary, a novel preparation process of MMA with extensive applications has been designed via an improved green direct methylation method using Aspen Plus simulation software. successfully designed via an improved green direct methylation method using Aspen Plus Methanol is used to efficiently adsorb MAL, and a double-effect distillation design with T-1 and T-2 simulation software. Methanol is used to efficiently adsorb MAL, and a double-effect distillation towers helps to separate MAL from the mixture of MAL and chloroform, which is a critical step for design with T-1 and T-2 towers helps to separate MAL from the mixture of MAL and chloroform, the production of MMA. The purity of the MMA product can reach 99.9% following recycle streams, which is a critical step for the production of MMA. The purity of the MMA product can reach 99.9% double-effect distillation process controllability, and heat-exchange network optimization. Thus, this following recycle streams, double-effect distillation process controllability, and heat-exchange purposed green process for producing MMA exhibits productivity and energy-saving superiority, network optimization. Thus, this purposed green process for producing MMA exhibits productivity providing valuable theoretical guidance for the practical production of MMA or its derivatives in and energy-saving superiority, providing valuable theoretical guidance for the practical production many domains. of MMA or its derivatives in many domains.

SupplementarySupplementary Materials: Materials:The Figure following S1. The are purification available online process at ofhttp: MMA//www.mdpi.com using a normal/2227-9717 distillation/7/6/ 377design./s1. Figure S1. The purification process of MMA using a normal distillation design. Figure S2. The fitting result Figure S2. The fitting result diagram of processes for MMA before and after optimizing the heat-exchange diagram of processes for MMA before and after optimizing the heat-exchange network. The red and blue lines representnetwork. the The heat red increase and blue and lines heat represent decrease ofthe united heat facilitiesincrease (suchand heat as condenser, decrease reboiler,of united etc.). facilities Table S1.(such The as coolcondenser, and hot reboiler, stream information etc.). Table afterS1. The the cool heat-exchange and hot stream process, information based on after Figure the S2. heat-exchange process, based on Figure S2. Author Contributions: T.L. and X.G. designed the MMA production process, wrote the first draft of the manuscript andAuthor conducted Contributions: the research. T.T. Y.W., and J.S.,X.G. X.C., designed Y.L., X.T. the and MMA J.D. completedproduction theprocess, optimization wrote the of the first process draft usingof the Aspen Plus. A.S.G. and X.W. revised and polished the manuscript. All authors contributed to rechecking the manuscript and conducted the research. Y.W., J.W., X.S., Y.J., X.F. and J.L. completed the optimization of the manuscript and approved the final version. process using Aspen Plus. A.S.G. and X.J. revised and polished the manuscript. All authors contributed to Funding:recheckingThis the research manuscript was fundedand approved by the National the final Natural version. Science Foundation of China (21805014), the scientific and technological research program of the Chongqing Municipal Education Commission (KJQN201801424), theFunds Young：This Scientist research Growth was Support funded Projectby the ofNational Yangtze Na Normaltural UniversityScience Foundation (No. 2018QNRC10), of China (21805014), the Research the Projectscientific of Yangzteand technological Normal University research (No. program 2015XJXM02), of the and Chongqing the Guangxi Municipal Natural ScienceEducation Foundation Commission (No. 2016GXNSFBA380216). (KJQN201801424), the Young Scientist Growth Support Project of Yangtze Normal University (No. Acknowledgments:2018QNRC10), theThe Research authors Project acknowledge of Yangzte the financial Norm supportal University from the (No. National 2015XJXM02), Natural Science and the Foundation Guangxi ofNatural China (researchScience Foundati core fundingon (No. No. 2016GXNSFBA380216). 21805014), Chongqing Municipal Education Commission (research core funding No. KJQN201801424), Yangtze Normal University (No. 2018QNRC10 and No. 2015XJXM02), and the GuangxiAcknowledgments: Natural Science The Foundation authors acknowledge (research core the funding financial No. 2016GXNSFBA380216).support from the National Natural Science ConflictsFoundation of Interest: of ChinaThe (research authors declarecore funding no conflict No. of21805014), interest. Chongqing Municipal Education Commission (research core funding No. KJQN201801424), Yangtze Normal University (No. 2018QNRC10 and No. 2015XJXM02), and the Guangxi Natural Science Foundation (research core funding No. 2016GXNSFBA380216).

Conflicts of Interest: The authors declare no conflict of interest. Processes 2019, 7, 377 11 of 12

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