Please study the process described in the attached file. Your task is to: 1. Specify instrumentation at key process locations and 2. Draw feedback control loops to control the process./n 112

DESIGN AND CONTROL OF THE BUTYL ACETATE PROCESS which has a reboiler temperature of about 344 K). The use of heat integration is not included in this study but would obviously reduce energy consumption. The butanol/butyl acetate separation is somewhat difficult and therefore requires a distil- lation column with a modest reflux ratio (RR = 1.92) and a modest number of stages (47). 8.3 PROCESS FLOWSHEET Figure 8.4 gives the flowsheet of the process after economic optimization, to be discussed in Section 8.4. Flowsheet stream conditions, equipment sizes, and heat exchanger heat duties are provided. 8.3.1 Reactor Fresh feed with composition 60 mol% methyl acetate and 40 mol% methanol and flowrate 100 kmol/h is fed into a 4 m³ reactor, which operates at 350 K and 5 atm. The residence time in the reactor is about 6 min since the kinetics are fast and chemical equilibrium is quickly attained. Fresh butanol (59.4 kmol/h) is added to a butanol recycle stream (120.6 kmol/h, 90 mol% butanol, 10 mol % butyl acetate) to give a total butanol stream (Bo) of 180 kmol/h, which is fed into the reactor. A methyl acetate/methanol recycle (171.2 kmol/h, 64 mol% methyl acetate, 36 mol % methanol) is combined with the feed stream (100 kmol/h), giving a total methyl acetate/methanol stream (Mo) of 271.2 kmol/h, which is fed to the reactor. These two total streams (Btot and Mot) will be the major design optimization variables. The Feed 100 kmol/h 0.60 MeAc 0.40 MeOH 305 K Reactor 4 m³ 350 K 5 atm -89 kw Bot 180.0 kmol/h 0.067 BuAc 0.933 BUOH 399 K 15 atm Mtot 271.2 kmol/h 0.6254 MeAc 0.3746 MeOH 399 K 15 atm RR-0.317 ID=1.72 m 3.64 MW BuOH Feed 59.4 kmol/h 305 K; 15 atm 401 K MeAc Recycle; 171.2 kmol/h; 0.64 MeAc; 0.36 MeOH C1 20 BuOH Recycle; 120.6 kmol/h; 0.90 BuOH; 0.10 BuAc 36 333 K 1.2 atm 3.75 MW D1 271.27 kmol/h 0.4075 MeAc 0.5915 MeOH 10-6 BUOH 0.0010 BuAc A B1 179.9 kmol/h 0.0010 MeAc 10-11 MeOH 0.6066 BUOH 0.3924 BuAc C2 18 26 329 K 1.1 alm 3.02 MW AR=0.996 ID=1.38 m 3.01 MW 344 K MeOH Product 100.1 kmol/h 0.0100 MeAc 0.9872 MeOH 0.0027 BuAc Figure 8.4. Proposed flowsheet. 459 K C3 27 46 437 K 4 atm 3.50 MW RR=1.92 ID=1.78 m 4.05 MW BuAc Product 59.32 kmol/h 0.003 MeAc 0.010 BUOH 0.987 BuAc icluded a distil- es (47). ussed in at duties flowrate nce time ; quickly kmol/h, kmol/h, 54 mol% 1), giving e reactor. bles. The 7K tm 0 MW BuAc Product 59.32 kmol/h 0.003 MeAc 0.010 BUOH 0.987 BuAc 8.3 PROCESS FLOWSHEET 113 per-pass conversion of methyl acetate is about 34%. A small amount of heat (89 kW) must be removed from the reactor. 8.3.2 Column C1 Reactor effluent is fed to column C1, which splits the two light components from the two heavy components. Methyl acetate and methanol are taken overhead while butanol and butyl acetate leave in the bottoms. The column has 37 stages and is fed on Stage 20. We use the Aspen tray-numbering convention of counting trays from the top with the condenser as Stage 1. The specifications for this column are somewhat unusual. The key components are not adjacent in terms of boiling points. The normal boiling points of methyl acetate, methanol, butanol, and butyl acetate are 330.1, 337.8, 390.8, and 399.3 K, respectively. So the separ- ation in this column should be to keep butanol from going overhead and methanol from going out the bottom. But as the stream data given in Figure 8.4 show, the concentration of butanol in the distillate is much lower than the concentration of butyl acetate. In the bot- toms, the concentration of methanol is much lower than the concentration of methyl acetate. This odd behavior may be the result of the azeotropes. The specification for the distillate is 0.1 mol % butyl acetate. The specification for the bottoms is that the sum of the methanol and the methyl acetate is 0.1 mol %. For some values of parameters, the dominant impurity in the bottoms is methanol, but for other values of parameters, the dominant impurity in the bottoms is methyl acetate. The sum of the compositions is obtained in the Radfrac model in Aspen Plus by selecting both components as the Selected component in the Design spec feature. The required RR in column C1 is 0.317 and reboiler heat duty is 3.64 MW. Low-pressure steam is used in the reboiler (433 K at 6 atm) since the base temperature is 401K, The column operates with a condenser pressure of 1.2 atm, which gives a reflux-drum tempera- ture of 333 K and permits the use of cooling water in the condenser. 8.3.3 Column C2 The distillate stream from column C1 is fed to column C2, which produces high-purity (98.72 mol %) methanol out the bottom and a distillate stream with a composition (64 mol % methyl acetate) near the azeotrope. The distillate is recycled back to the reactor at a rate of 171.2 kmol/h. The column has 27 stages and is fed on Stage 18. The specifications are 64 mol % methyl acetate in the distillate and 0.1 mol % methyl acet- ate in the bottoms. To achieve these specifications, the RR is 0.996 and reboiler heat duty is 3.01 MW. Low-pressure steam is used in the reboiler since the base temperature is 344 K. The column operates with a condenser pressure of 1.1 atm, which gives a reflux-drum temperature of 329 K and permits the use of cooling water in the condenser. 8.3.4 Column C3 The bottoms from C1 is fed to column C3, which produces high-purity (98.7 mol %) butyl acetate out the bottom and a butanol-rich distillate (90 mol %) that is recycled back to the reactor at a rate of 120.6 kmol/h. The column has 47 stages and is fed on Stage 27. The specifications are 0.1 mol % butanol in the bottoms and 10 mol % butyl acetate in the distillate. To achieve the specifications, the RR is 1.92 and reboiler heat duty is 4.05 MW. 114 DESIGN AND CONTROL OF THE BUTYL ACETATE PROCESS Since the base temperature is 459 K, high-pressure steam (527 K at 42 atm) must be used in the reboiler. The column operates with a condenser pressure of 4 atm, which gives a reflux-drum temperature of 437 K. As noted earlier, this temperature is high enough to permit heat inte- gration with the low-temperature reboiler (344 K) in column C2. The total energy consumption of the three columns in this flowsheet is 10.7 MW, not con- sidering any heat integration. Using a price of $7.78/GJ for low-pressure steam (columns Cl and C2) and $9.83/GJ for high-pressure steam (column C3), the total energy cost of the process is $2,888,000/yr. Figures 8.5 through 8.7 give temperature and composition profiles for the three distilla- tion columns. The control system developed later in Section 8.5 will use these temperature profiles to select appropriate trays for temperature control. (a) Temperature K 340.0 350.0 360.0 370.0 380.0 390.0 400.0 410.0 (b) 1.0 0.8 0.7 0.6 0.5 X (mole frac) 0.4 0.3 0.2 0.1 6.0 1.0 11.0 6.0 11.0 Temperature K 16.0 16.0 21.0 Stage 26.0 26.0 31.0 MEAC MEOH BUAC BUOH 31.0 36.0 36.0 21.0 Stage Figure 8.5. Column C1 (a) temperature profile, and (b) composition profiles. 41.0 41.0 e used in lux-drum heat inte- ', not con- lumns Cl ost of the e distilla- mperature -1.0 41.0 1. (b) Temperature K X (mole frac) 345.0 340.0 335.0 330.0 0.75 0.5 0.25 1.0 1.0 6.0 1.0 11.0 6.0 16.0 Stage 11.0 8.3 PROCESS FLOWSHEET Temperature K 21.0 16.0 Stage Figure 8.6. Column C2 (a) temperature profile, and (b) composition profiles. 21.0 26.0 MEAC MEOH 26.0 31.0 31.0 115 8.3.5 Flowsheet Convergence Convergence of steady-state simulators when recycle streams are present can be very diffi- cult. The butyl acetate process has two recycle streams, which can present problems. However, successful and robust convergence was achieved by using the strategy of fixing the total butanol flowrate Bot entering the reactor (see Figure 8.4) by varying the fresh feed of butanol. A Flowsheet design spec in Aspen Plus is used for this objective. The fresh feed stream, which is a mixture of methyl acetate and methanol, is fixed at 100 kmol/h. A value of Bot is specified, and the flowsheet is converged. There is a