TABLE 1 Value of Parameters n Langmur Adsorpton Isotherm [1] Speces m 3 /kmol kmol/kg Glycerol TABLE 3 Bolng Pont and Azeotropc Temperature

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1 Desgn and Control of a Reactve-Dstllaton Process for Glycerol Utlzaton to Produce Tracetn Chung-Cheng Lee, Hao-Yeh Lee, Shh-a Hung and I-Lung Chen Abstract Due to ncreasng of bodesel producton n recent years, the utlzaton of ts byproduct, glycerol, nto hgh-value chemcals becomes more mportant. The esterfcaton reacton of glycerol wth acetc acd to produce tracetn s one such applcaton. Tracetn s used manly as a plastczer and a gelatnzng agent n polymers, explosves and also as an addtve n tobacco, pharmaceutcal compounds, and cosmetcs. In ths study, reactve-dstllaton process s proposed to produce tracetn wth hgh converson and selectvty. Total annual cost (TAC) s mnmzed to obtan the optmal desgn flowsheet. Fnally, the control strategy for ths system s devsed to properly reject dsturbances from two feed streams T I. INTRODUCTION he demand for renewable energy has led to an ntensve research to develop ndustral applcaton of bodesel n recent years. The output of ts byproduct glycerol s also ncreasng. Hence, convertng glycerol to hgh-value products has become an mportant ssue. Tracetn s one of such products derved from glycerol. Gelosa et al. [1] used Amberlyst-15 as the catalyst n the esterfcaton system of glycerol and the catalyst has an operatng lmtaton under 120 ºC. He proposed the reacton mechansm as a Langmur model and verfed the parameters n model by experment data. The conventonal process suffers from lmted selectvty and converson because all the three reactons are reversble. The use of reactve dstllaton (RD) column can be explored to obtan desre product wth hgh selectvty and converson. Hasabns and Mahajan [2] used a RD column wth Amberlyst-15 as catalyst and 1,2-dchloroethane (EDC) as an entraner to remove water. Chen et al. [3] demonstrated that sobutyl acetate (IBA) s an effectve entraner to remove water n a heterogeneous azetropc column. In ths paper, optmal desgn of a RD process s obtaned by usng IBA as an entraner. Control strategy wll also be developed to reject feed dsturbances. II. INETIC AND THERMODYNAMIC MODELS A. netc Model The three-step esterfcaton reacton to produce tracetn s Ths work was supported by the Natonal Scence Councl of the R. O. C. under Grant No NSc E CC1 Second author s wth Department of Chemcal Engneerng, Natonal Tawan Unversty of Scence & Technology, Tape 106, Tawan. Other authors are wth Department of Chemcal Engneerng, Natonal Tawan Unversty, Tape 106, Tawan (correspondng author: I-Lung Chen, phone: ; fax: ; e-mal: lungchen@ntu.edu.tw). shown as eq. (1)-(3). There are two ntermedates n the system, monoacetn (Mono) and dacetn (Dac). Both of the ntermedates have two somers. Base on the batch experment conducted by Lao et al. [4], 1-Monoacetn and 1,3-Dacetn are chosen to represent the two ntermedates respectvely. Glycerol + Acetc Acd Monoacetn + H 2 O Monoacetn + Acetc Acd Dacetn + H 2 O Dacetn + Acetc Acd Tracetn + H 2 O The knetc expresson of Langmur-Hnshelwood form by usng Amberlyst-15 as heterogeneous catalyst can be seen n eq. (4)-(6): r1 k1 G ly H Ac 1 r k M ono H Ac N v 1 EQ,1,1 N v 1 EQ,2,2 N v 1 EQ,3,3 r3 k3 D ac H Ac 1 (6) The concentraton of adsorpton phase was used to calculate the reactor rate. The rates constants, concentratons, and equlbrum constants were calculated by eq. (7)-(9). k m (1) (2) (3) (4) (5) E A,m k exp( ) (7) m,0 RT N 1 L C k (8) m,0 L 1 C EQ, m k k m m where r m s reacton rate (kmol/(kg-s)), k m s forward reacton rate constant (kg/(kmol-s)), k -m s backward reacton rate constant (kg/(kmol-s)), k m,0 s pre-exponental factor, E A,m s actvaton energy of the reactons (kj/kmol), Γ s the concentraton of adsorpton phase (kmol/kg),ν,m s stochometry coeffcent,, s equlbrum constant, T s EQ m temperature (), R s deal gas constant (kj/(kmol-)), (9) L C s 147

2 TABLE 1 Value of Parameters n Langmur Adsorpton Isotherm [1] Speces m 3 /kmol kmol/kg Glycerol TABLE 3 Bolng Pont and Azeotropc Temperature Rankng at 0.15Bar Computed Data speces Temp. Temp. Composton speces ( o C) ( o C) (Mole bass) H 2O Dacetn Acetc Acd Water Monoacetn Dacetn Tracetn Acetc Acd Tracetn Glycerol Monoacetn Glycerol/ Monoacetn Glycerol/ Dacetn Glycerol/ Tracetn Monoacetn/ Dacetn / / / / 527 TABLE 2 Values of Exponental Factor and Actvaton Energy Reactons (m) E A.m k m,0 kj/kmol kg/kmol-s 1. Gly + HAc Mono + H 2O 1.46х х Mono + HAC Dac + H 2O 4.07х х Dac + Hac Tr + H 2O 4.24х х Mono + H 2O Gly + HAc 1.48х х Dac + H 2O Mono + HAc 2.92х х Tr + H 2O Dac + HAc 9.46х х10 4 concentraton of component n lqud phase (kmol/m 3 ), s adsorpton constant for component (m 3 /kmol), s standard constant of component (kmol/kg). Subscrpt m refers to the number reacton. B. Thermodynamc Model There are sx components n the esterfcaton reacton wth two addtonal entraners (IBA and EDC) n the whole process. Glycerol and acetc acd wth 95mol. % purty are fed nto the system. Hasabns and Mahajan [2] chose UNIQUAC thermodynamc model to descrbe the vapor-lqud and vapor-lqud-lqud equlbra. In ths study, we further use Hayden-O Connell method to more properly descrbe the vapor behavor of acetc acd. The bnary parameters of Gly-H 2 O, HAc-H 2 O, HAc-IBA, H 2 O-IBA use Aspen Plus bult-n data wth the remanng ones estmated by UNIFAC method. Table 3 shows the bolng-pont and azeotropc temperature rankng of system wthout entraner. Because the deactvaton temperature of the catalyst s 120 o C, Fgure 1. RCM and LLE at 40 ºC of the both systems azeotropc temperature rankng of system wthout entraner. Because the deactvaton temperature of the catalyst s 120 o C, the operaton pressure was desgned for all reacton zones under ths lmt. The heavest component s dacetn whle the lghtest component s water. However, water and acetc acd are not easy to separate. Hence, the dea of addng an entraner to carry out water was consdered. Fg. 1 shows the resdue curve map (RCM) and LLE for IBA-water-tracetn system and EDC-water-tracetn system at 1 bar. The heavest component s tracetn, and the lghtest component s the azeotropes formed by water-edc and water-iba wth temperature of ºC and ºC respectvely. Both the azeotropes can be separated nto two phase naturally after coolng down to 40 ºC n decanter. The organc phase stream s fed back to RD column and the aqueous phase stream s drawn out of the system. III. OPTIMAL DESIGN FLOWSHEET In ths part, two knds of confguratons wll be nvestgated for ths process. One s for a RD column wthout entraner. The other desgn confguraton s wth entraner whch has an addtonal decanter at the RD top (Fg. 3). A. Reactve-Dstllaton Process wthout Entraner The RD system s composed manly by three parts, rectfyng, reactve, and strppng sectons. The strppng 148

3 Fx 1.N ref 2.N rxn 3.N str 4.N Gly Least TAC by verfyng NF HAc Least TAC by verfyng NF Gly If not, vary 1~4 4 vapor-lqud-lqud equlbrum s allowed n each tray. The catalyst s assumed to occupy half of the holdup at each tray n the reactve secton. The glycerol and acetc acd feed flow rate are set as 5 kmol/h and 15 kmol/h, respectvely. Both feeds nclude 5 mol% of water mpurty. The optmal desgn flowsheet wll be determned by comparng the TAC of each case. Glycerol Acetc acd Least TAC by verfyng NF str Least TAC by verfyng NF Rxn Least TAC by verfyng NF ref Fgure 2. The optmal desgn flowsheet RDC Optmal Result Organc Tracetn Water Makeup Fgure 3. Proposed flowsheet of RD column wth entraner secton prevents the reacton temperature over the deactvaton temperature and enhances the purty of bottom product. The rectfyng secton enhances the purty of top product. Top vapor of the RD column, after condensaton, s partally fed back to the system. There are two desgn specfcatons for the RD column. The purty of top product s at very small acetc acd loss (1 mol. %) and the purty of bottom product s set to be 99 mol. % tracetn. The desgn varables ncludes: numbers of stages of the rectfyng secton (N ref ), the reactve secton (N rxn ), and the strppng secton (N str ); the feed locatons of glycerol (NF gly ) and acetc acd feed (NF HAc ). All these varables wll be decded by TAC (total annual cost). The TAC calculaton s based on Douglas [5]. We vared all the desgn varables step by step to get the least TAC value whch s shown by Fg.2. TAC s the sum of the operatng cost and the annual captal cost. The operatng cost ncludes steam for the reboler, coolng water for the condenser, and catalyst cost. The captal cost ncludes the column shell, nternal trays, reboler, and condenser. The payback perod s assumed to be 3 years n the calculaton. TAC captal TAC TAC (10) operatng 3 In the rgorous Aspen Plus smulaton of the RD column, B. Entraner-based RD The use of entraner decreases the bol pont and keeps the reactve zone temperature below the thermal stablty lmt of the catalyst. Moreover, the use of entraner also helps n ncreasng the effcency of water removal. The major dfference between these two confguratons s addtonal decanter used at the top of RD column. The top vapor s not drectly partally reflux to the RD column after t s cooled. It s frst condensed at 40 ºC and fed nto a decanter to separate nto two lqud phases. The organc phase s completely recycled back to top of the RD column, and the aqueous phase s drawn out of the system. Makeup flow s fed nto decanter to balance the mnor loss of entraner through two outlet streams. There are two desgn degree-of-freedom (DOF) for ths confguraton. One s reboler duty and the other one s the makeup flowrate. Reboler duty s set to keep tracetn purty at 99 mol. % and makeup flow s set to keep top acetc acd loss at 1 mol. %. The case of usng IBA as entraner s consdered frst. The optmal desgn varables of ths flowsheet were obtaned by the same way as n the prevous subsecton. Notce that the feed rates and the purty of the products are set to be exactly the sameas n the prevous subsecton. Another case, whch uses EDC as entraner, s conducted under the same desgn varables as n the IBA case. C. Comparson of the desgn flowsheets The lqud flow data s shown n Table 4. As we can see n the table, the yeld of the top and bottom products are almost the same. However, the organc reflux for EDC case turn out to be 33kmol/h. It s much larger than the value of IBA case. Although the bolng pont of water-edc azeotrope s qute low, the azeotropc composton s more water than water-iba (see Fg. 1). Ths means that IBA s much more capable of carryng water to the top of the RD column. Smulaton results showed that both the energy and the equpment costs of the EDC case are much more than the case of usng IBA. Consequently, IBA s a better entraner for water removal n ths process. The comparsons of the optmal desgn flowsheet wthout entraner wth that of usng IBA as entraners are shown n Table 5. Notce that the trays are countng from top to bottom wth the 1 st tray as the condenser (or the decanter) and the last tray as the reboler. The total number of stages of the RD wthout entraner s much larger, and the reacton secton s also much larger than the case wth IBA as entraner. The entraner-based RD desgn can save about 19% energy requrement, but the 149

4 TABLE 4 Lqud Flow of the Two Dfferent Entraner Entraner IBA EDC Lqud Flow Flowrate (kmol/h) Flowrate (kmol/h) 0.2 Water Tracetn Organc Makeup TABLE 5 Comparson of the Two Desgn Flowsheet Wthout Column Confguraton IBA entraner Total No. of trays No. of trays n rectfynge secton (N r) 9 1 No. of trays n reacton secton (N rxn) 35 7 No. of trays n strppng secton (N s) 1 1 Reactve trays Glycerol feed tray 2 3 Acetc acd feed tray 27 4 Column dameter (m) Reboler duty (kw) Total captal cost ($1000/yr) Column Column trays Heat exchangers Decanter Total operatng cost ($1000/yr) catalyst Energy Makeup Total annual cost ($1000/yr) overall operatng cost s slghtly hgher due to the makeup cost. The total annual cost of the entraner-based RD desgns s 33% less than the RD wthout entraner. From the vew of the operatng cost, the use of entraner does not gve an obvous beneft. However, sgnfcant total annual cost can be saved by usng ths entraner-based desgn. IV. CONTROL STRATEGY DEVELOPMENT The overall control strategy of ths system s developed to hold the product purty specfcatons under feed flow rate and feed composton changes. Pressure-drven smulaton n Aspen Plus Dynamcs TM s used n the control strategy development. Twenty mnutes of resdence tme wth 50% lqud level s used to calculate the volume of RD column base. The resdence tme of the decanter s assumed at thrty mnutes n order to settle the two lqud phases Fgure 4. Open-loop senstvty tests Fgure 5. Closed-loop senstvty tests A. Inventory Control Loops The nventory and some smple regulatory control loops are determned frst. Bottom sump level s controlled by manpulatng the tracetn product flow. The organc phase level n the decanter s controlled by the organc reflux flow. The aqueous phase level s controlled by the aqueous outlet. Top pressure of the RD column s assumed to be controlled by the top overhead vapor flow rate. In real ndustral stuaton, the operatng pressure whch s less than atmospherc pressure can be mantaned by a vacuum system. The decanter temperature s controlled at 40 ºC by the cooler duty. A rato scheme for determnng the acetc acd feed flow s mplemented to mantan a constant feed rato, whle the value of ths constant can be manpulated by a tray temperature control loop. After the nventory control loops are determned, there are three manpulated varables left. They are: feed rato, reboler duty, and the makeup flow rate. These three manpulated varables can be used to hold the product purtes n spte of feed dsturbances. B. Tray Temperature Control Loop(s) It s assumed that the onlne composton measurement s ether unavalable or may mantan troublesome, thus tray temperature(s) wll be used to ndrectly hold the product specfcatons. The smplest overall control strategy s to consder sngle tray temperature control loop. In ths case, f 150

5 Fgure 7. Relaton between setpont and throughput changes Fgure 6. Overall control strategy of the proposed desgn. only one of the remanng varables s selected as the manpulated varable for the temperature control loop, the feed rato should be selected. The reason s that mantanng of the correct stochometrc rato of the two reactants s crucal for a RD column, otherwse the un-reacted reactant would be a waste. Because the feed composton dsturbances are assumed to be unmeasured, the acetc acd/glycerol feed rato needs to be adjusted va the tray temperature loop n order to hold at the correct stochometrc rato. For sngle temperature control strategy, the other two desgn varables are often mantaned at some rato to other varables n the system. For ths desgn, t s not possble to use the smplest sngle-loop control strategy. The problem was manly due to the unmeasured feed composton varatons nevtably occurred n the system. It terns out that besdes the feed rato, at least one of the remanng desgn varables needs to be adjusted by another tray temperature control loop. One mportant queston s whch one should we select as the second manpulated varable? The answer of ths queston can easly be obtaned by open-loop and closed-loop senstvty of the system. Wth the nformaton of both senstvty tests (Fg. 4 and 6), the upper stages shows sgnfcant senstvty to the feed rato and the bottom stages are senstve to makeup flowrate. Hence, the makeup flowrate s selected as the second manpulated varable nstead of reboler duty. From the two fgures of senstvty tests, stage 3 and stage 10 were chosen for the temperature control. Obvously, stage 10 temperature has the most devaton when makeup flowrate changes and least devaton for closed-loop senstvty test. For another controlled varable, there s a trade-off for the determnaton of control pont. Although stage 5 temperature has the most devaton n the open-loop senstvty test, t also has the same stuaton n the closed-loop senstvty test. As a result, we choose stage 3 for ts best response of the dsturbance. The remanng manpulated varable need to be determned s the reboler duty. From closed-loop senstvty tests, all operatng varables of the system at desred values can be determned to perfectly control two product purtes under unmeasured feed composton dsturbances. By observng the canddate ratos wth the reboler duty, fxng the rato of reboler duty/acetc acd feed at a constant value s selected. The proposed overall control strategy s summarzed n Fg. 6. All level loops were tuned usng P-only controller wth c=2.0 except for the organc level loop usng c=1 to speed up the response of the recycle loop. Tght PI tunng constants was chosen for the pressure control loop. The remanng two crucal tray temperature loops were tuned usng teratve relay-feedback tests wth Tyreus-Luyben [6] tunng rules untl the tunng constants were converged. C. Closed-Loop Dsturbance Rejecton Tests Two dsturbance rejecton tests were ntroduced to observe the controllablty and operablty of the control strategy. The frst test s to allow for throughput changes. These can be done by ncrease/decrease the setpont of the glycerol feed flow loop. Because the throughput changes are consdered as a known dsturbance, we could change both the setponts of tray temperature and Q/F rato n order to mantan the purty of the products. The tray temperatures manly effect the extent of reacton and the reboler duty nfluences the separaton of HAc and water, and there are lnear relatons between the percentage of throughput changes and setpont value (Shown n Fg. 7). The setpont of stage 10 temperature control loop needs not to be changed under throughput changes. Fg. 8 dsplays the closed-loop results wth up to +10% and down to -10% throughput changes for the proposed control strategy. The two controlled tray temperatures are all returned back to ther setponts (See two mddle column plots n Fg. 8.). By observng the manpulated varable of the stage 3 temperature loop, the feed rato also returned back to the correct stochometrc rato after some transent responses. The changes of the makeup flow rate s dctated by the temperature control loop of stage 10, whle the reboler duty s correspondngly ncreased /decreased by a rato scheme n the control strategy n Fg. 7. The major test s the purty of the two man products, glycerol and water. It s observed that the two purtes can be controlled back to the orgnal specfcatons (see two top plots n Fg. 8) after varyng the setpont of one temperature control loop and the Q/F rato. The next dsturbance test s the feed composton changes. Ths s a very realstc stuaton because the nert (water) n ths glycerol feed often vares. Fg. 9 dsplays the closed-loop responses wth up to +50% and down to -50% changes of the water content n ths feed stream. The control strategy needs to adjust the feed rato downward when there s more nert n 151

6 two entraner-based RD processes. The best process s determned base on TAC savngs. By comparson, t s shown that entraner-based RD usng IBA as entraner s the best desgn flowsheet wth sgnfcant least TAC than the other two desgns. The overall control strategy of the proposed desgn flowsheet s also determned. Two tray temperatures (stages 3 and 10) n the RD column are controlled by manpulatng feed rato and makeup flow rate. The remanng manpulated varable of reboler duty s mantaned at a constant rato to HAc feed. From the dynamc dsturbance rejecton tests, the proposed desgn s capable of holdng product specfcatons despte throughput and feed composton changes. Fgure 8. Response of throughput changes REFERENCES [1] D. Gelosa, M. Ramaol, G. Valente, and M. Morbdell, Chromato-graphc reactors: Esterfcaton of glycerol wth acetc acd usng acdc polymerc resns, Ind. Eng. Chem. Res., vol. 42, pp , [2] A. Hasabns and S. Mahajan, Entraner-based reactve dstllaton for esterfcaton of glycerol wth acetc acd, Ind. Eng. Chem. Res., vol. 49, pp , [3] I. L. Chen,. L. Zeng, H. Y. Chao, and J. H. Lu, Desgn and control of acetc acd dehydraton system va heterogeneous azeotropc dstllaton, Chem. Eng. Sc., vol. 58, pp , [4] X. Lao, Y. Zhu, S. G. Wang, H. Chen, and Y. L, Theoretcal elucdaton of acetylatng glycerol wth acetc acd and acetc anhydrde, Appl. Catal. B., vol. 94, pp , [5] J. M. Douglas, Conceptual Desgn of Chemcal Processes. New York: McGraw-Hll, [6] W. L. Luyben and M. L. Luyben, Essentals of Process Control, New York: McGraw-Hll, 1997, pp Fgure 9. Response of feed composton changes ths feed stream so that the stochometrc rato can stll be mantaned between glycerol and acetc acd. From the closed-loop results, t s observed that the control strategy fulfll ts goal n correctly adjustng the feed rato durng feed composton changes. The purty of the tracetn product has only a small devaton from the desgn values. The rejecton of feed composton dsturbance n acetc acd s also smulated wth up to +50% and down to -50% changes of the water content. The purty of tracetn s about and mol% under the two dsturbances. Due to page lmt, dynamc responses wth HAc feed composton changes are not shown. V. CONCLUSION Ths study presents a feasble desgn for the esterfcaton of glycerol wth acetc acd. Three desgn flowsheets are explored n ths work: one RD process wthout entraner and 152