Removal and Recovery of Organic Solvents from Aqueous Waste

Article pubs.acs.org/IECR

Removal and Recovery of Organic Solvents from Aqueous Waste Mixtures by Extractive and Pressure Swing Distillation G. Modla*,† and P. Lang† †

Department of Building Services and Process Engineering, Budapest University of Technology and Economics, H-1521 Budapest, Muegyetem rkp. 3-5, Hungary S Supporting Information *

ABSTRACT: Different scenarios for handling wastewaters of different concentrations containing acetone and methanol (in equimolar quantities) in order to remove and/or recover organic components are investigated by rigorous simulation. The extractive and pressure swing distillation processes are optimized by genetic algorithm (GA). The different alternatives including incineration of the whole amount of waste are compared on the basis of the capital, operation, and total annual costs. The influence of the water content of the waste mixture is also investigated. The best results are obtained with the separation of the whole quantity of the waste mixture into specified purity products by extractive distillation. The results obtained for pressure swing distillation with heat integration remained only slightly under those of the extractive distillation.

1. INTRODUCTION Several processes of the pharmaceutical, textile, and paint-making industries produce wastewaters highly polluted by solvents which are mainly low molecular weight alcohols and ketons. The high solvent concentrations impede feeding these liquids directly to the treatment systems. Many governments impose strict regulations for the maximum chemical oxygen demand (COD) of wastewaters before they can be returned to the environment and/or they can leave the chemical plant. (In Hungary the maximum chemical oxygen demand is 1000 mg/L.) Nowadays these wastewaters are usually incinerated. However the distillation can be an efficient method for their purification making possible to drain the purified water into the sewer and to recover the valuable organic components and to recycle them to the original technology. In this study we investigate the purification of a wastewater containing a different amount of water and equimolar acetone and methanol and the recovery of the organic components by distillation. For separating the azeotrope acetone-methanol a special distillation method must be applied such as extractive (ED) or pressure swing distillation (PSD). By the extractive distillation a third component (entrainer/ solvent) is added to the original mixture in order to enhance the separation. The entrainer is fed at a different location than the main feed. This process in a continuous system was studied among others by Yeh et al.,1 Knapp and Doherty,2,3 Laroche et al.,4 Kossack et al.,5 Gil et al.,6 and Zhang et al.7,8 Lang et al.9 suggested the extractive distillation in batch for separating the mixture acetone-methanol, using water as solvent. This process was also presented for an industrial application.10 The different aspects of the batch extractive distillation were studied, too.11−13 A great number of articles have been published on pressure swing distillation in continuous14−21 and in batch22−27 systems. Modla and Lang28 suggested two new double column configurations for batch pressure swing distillation. The double column batch stripper was studied also for the separation of the mixture acetone-methanol.29 © 2012 American Chemical Society

Since in some cases the extractive and pressure swing distillations are competitive ways for separating the azeotropic mixture, they are compared in several studies by different aspects.17,18,20,29 Luyben18 compared the steady-state design and the dynamic control of extractive and pressure swing distillation for the separation of the mixture acetone-methanol. The methanol column of the extractive distillation system was operated at 5 bar making possible the application of heat integration. He found that the extractive distillation system has a 15% lower total annual cost. However he emphasized that the third component (water) introduced appears as trace impurities in both the acetone and methanol products. Moreover it is much more difficult to attain higher purities in the extractive distillation system than in the pressure swing one because of ternary vapor− liquid equilibrium constraints (e.g., tangent azeotrope acetone− water). The determination of the columns size (number of stages) and operational parameters (e.g., reflux ratio) was heuristically optimized by running the rigorous simulation with an increasing number of stages until the reflux ratio stopped decreasing, which gives an approximation of the minimum reflux ratio. These runs were made with the distillate and bottom product compositions held at their specified values. In this study the extractive and pressure swing distillation are compared on the basis of Total Annual Cost. Recently the number of the applications of the Genetic Algorithm optimization method in the distillation process design increased.30−34 Barakat and Sørensen31 applied the Genetic Algorithm to optimize equipment size and operation conditions of distillation and hybrid processes. It is advisible to do some research for weighing the pros and cons of the different scenarios before the top management of a company decides about a new investment. The management will Received: Revised: Accepted: Published: 11473

February 7, 2012 May 26, 2012 August 9, 2012 August 9, 2012 dx.doi.org/10.1021/ie300331d | Ind. Eng. Chem. Res. 2012, 51, 11473−11481

Industrial & Engineering Chemistry Research

Article

d. Total Annual Cost. The total cost contains the capital and all operation costs, and it is reduced by the annual income originating from the products (AIP, methanol: 0.34 $/kg, acetone: 1.2 $/kg)

choose that scenario which is the most suitable for the long-range plan of the company. In this paper different scenarios are presented for processing waste mixtures containing acetone and methanol in considerable concentration. The goals of this paper are as follows: (1) to investigate different scenarios for handling waste mixture to remove and/or recover organic components, (2) to optimize the processes by genetic algorithm (GA), (3) to compare them on the basis of the (capital, operation, total annual) costs, and (4) to study the influence of water content of the waste mixture.

TAC = ACC + (AOCc + AOCi + AOCs) − AIP

We suppose that the organic solvents purified can be recycled to the plant; therefore, the price of the amount of new solvents saved can be considered as income. e. Optimization. In this work steady state genetic algorithm optimization framework is used to determine the geometrical (size) and operational parameters of the columns. The genomes are number of plates, feed plate location(s), reflux ratio, or reboiler heat duty. The initial population is created randomly. The applied GA parameters are as follows: Mutation 5%, Number of individuals 30, Crossover 70%, Number of populations 100. At first a wider region is permitted for the genomes. In the next this region is tightened. When there is no solution of the genomes generated (the simulation of the distillate column does not converge), the genomes are regenerated automatically, and this individual is not counted into the population. This individual is named “not viable”. The program of GA is written in Visual Basic under EXCEL. It calls the CHEMCAD 6.336 professional flow-sheet simulator for the rigorous calculations. The flowchart of the optimization is shown in Figure 1.

2. COST ANALYSIS METHOD The geometrical (sizes) and operational parameters of the columns are determined by genetic algorithm (GA) optimization. The Fitness Function (FF) contains the annual capital (ACC), operation (AOCc) costs of the columns and the additional operational costs (incineration (AOCi) and draining of sewage costs (AOCs), and a Penalty function (Pf) cost if the required composition of the distillate or bottom is not reached Fitness = ACC + AOCc + AOCi + AOCs + Pf

In some cases the additional operational (AOCi and AOCs) costs are ignored in the Fitness Function since they can be considered constant at the specified conditions of the column. (The quantity and composition of the products do not change during the optimization of the column.) a. Capital Costs. ACC is calculated by the formula taken from the book of Douglas35 ACC =

3. SCENARIOS − SEPARATION METHODS The aqueous waste solvent mixtures (Feed (F1)) contain water in different concentrations. The mole fraction of water (xF1,w) varies between 0.9 and 0.1. The mole fractions of acetone and methanol are equal in all cases studied (xF1,A = xF1,M). The total quantity of the waste mixture is 18000 m3/Y (6000 h/Y operation). 3.1. Incineration (I). The most simple and frequent method for the disposal of aqueous waste solvent mixtures (wastewaters) is incineration which is performed in special plants. In this case there is no capital cost (ACC = 0), but the fee of incineration means an additional operation cost (AOCi > 0). If the whole quantity of wastewater is incinerated, besides AOCi there is no other cost or income. This scenario is considered the basic one with which the other scenarios are compared. In our case the annual fee of incineration of the whole amount of waste mixture (AOCi in Table 1) varies between 10241 (for the most dilute waste) and 2257 k$/Y (for the most concentrated waste). 3.2. Stripping (ST). In the first process step the organic components are removed from the waste mixture (Figure 2) in a stripping column. The bottom product is treated water which can be drained to the sewer (COD ≪ 1000, xw = 0.9999). The distillate is an acetone-methanol mixture which can be incinerated or separated by extractive or pressure swing distillation in further distillation columns. The water concentration of the bottom product is specified (0.9999). The optimized parameters are the number of stages (N), the feed stage location (Nf), and the reflux ratio (R). First we investigated the case, when the distillate is not further processed (ST (1)). In this case the Fitness Function contains only the costs (FF = ACC + AOCc + AOCi + AOCs). We found that in the optimal cases almost the whole amount of water is withdrawn in the bottom product W1 (Table 1). The distillate contains only 0.004−0.008 mol/mol water. It must be still noted that if a significant part of the water was not separated

⎛ M&S ⎞ 0.8 ⎜ ⎟ ∗ 120 ∗ (d ∗ 3.28) ∗ (H ∗ 3.28) ⎝ 280 ⎠ ∗(2.18 + Fc)/LCT

where M&S is the Marshall-Swift index (1483), H is the height of the column (H = N*0.8 [m]), where N is the number of theoretical stages, d is the diameter of the column [m], LCT is the life cycle time (LCT = 3 year), and Fc is the design consideration (at the high pressure column Fc = 3.6, otherwise Fc = 1.6) d=

4(R + 1)V πφ vg

where R is the reflux ratio, V is the volumetric flow rate of distillate in vapor phase [m3/s], ϕ is the ratio of the free cross section area of the column for the vapor flow (ϕ = 0.7), and vg is the linear velocity of vapor [m/s] (vg = 0.833 m/s). b. Operation Costs. The operation costs of the columns (AOCc) are calculated on the basis of the reboiler heat duty. The price of the heating steam is 30 $/GJ. The fee of draining of the sewage whose COD must not exceed 1000 mg/L: 0.0023 $/kg. The fee of incineration depends on water content: at 0 mol % 0.1 $/kg; at 90% 0.6 $/kg. (These data have been obtained from a pharmaceutical plant.) Pf =

⎞ ⎛ 1 ⎟ ⎜ − − 1 abs(K Z) ⎠∗K ⎝ 1 100000

2

c. Penalty Function. where K1 is the purity required [mol/mol], Z is the actual purity [mol/mol], and K2 is the proportionality factor (its value is selected so that the condition Pf > 2(ACC + AOCc) be satisfied). 11474

dx.doi.org/10.1021/ie300331d | Ind. Eng. Chem. Res. 2012, 51, 11473−11481


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Figure 1. The flowchart of the optimization.

reduce the fixed (ACCc) and operation costs (AOCc) of the column, the incineration cost (AOCi) would be much higher

from the organic components removed in the distillate, the TAC would be higher. Although the less sharp separation would 11475



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Table 1. Costs of the Incineration and Stripping Process Steps I

ST (1)

ST (2)

xF1,w [mol/mol]

TAC [k$/Y]

ACCc [k$/Y]

AOCc [k$/Y]

AOCi [k$/Y]

AOCs [k$/Y]

TAC [k$/Y]

xD,w [mol/mol]

ACCc [k$/Y]

AOCc [k$/Y]

AOCi [k$/Y]

AOCs [k$/Y]

TAC [k$/Y]

xD,w [mol/mol]

0.9 0.8 0.6 0.4 0.2 0.1

10241 8937 6742 4841 3094 2257

60 68 80 89 88 75

259 308 396 457 493 481

382 647 1000 1216 1371 1453

31 23 13 7 3 1

732 1046 1489 1770 1956 2010

0.005 0.004 0.005 0.004 0.005 0.008

66 69 67 71 70 68

255 313 452 531 590 613

371 632 974 1187 1334 1391

31 23 13 7 3 1

723 1037 1506 1797 1997 2073

0.001 0.002 0.003 0.003 0.003 0.002

special distillation method must be applied such as extractive or pressure swing distillation. By extractive distillation water can be applied as an efficient solvent. For the extractive distillation two different alternatives were studied: (1) extractive distillation after stripping (three column system) and (2) extractive distillation without separate stripping (two column system). 3.3.1. Extractive Distillation after Stripping (Indirect Extractive Distillation). By this method the distillate of the stripping column C1 is processed by extractive distillation (Figure 3). If the water content of the wastewater is high enough (by our computational experiences if xF1,w > 0.8) one part (W1f) of the treated water W1 is not withdrawn and drained but it is fed to the extractive column (C2) as solvent. In the extractive column the distillate (D2) is acetone of specified purity (99 mol %). (The water does not reverse the volatility order, the univolatility line arrives at the acetone−water edge.4) If the water content of the wastewater is not high enough, an additional amount of water must be fed which can be recycled water (W3r) from the last column bottom product (W3). In the last column (C3) the distillate (D3) is methanol (of 99 mol %) and the bottom product (W3) is treated water which can be drained into the sewer (W3s) or recycled to Column C2 (W2r). 3.3.1.1. Acetone Production Step (ST-iEXA). If the bottom product (W2) would not be further processed, besides the capital and operation costs of columns C1 and C2, the fee of draining of W1s (the part withdrawn of the bottom product of C1) and that of incineration of the bottom product (W2) of C2 should be taken into consideration as costs. However the costs are reduced by the value of the acetone product (purchase price of acetone). At the determination of the sizes and operation parameters of the acetone production column it is necessary to take into

Figure 2. Removal of the organic components from the waste mixture by stripping.

since due to the presence of water (1) the amount of mixture to be incinerated would be higher and (2) the specific incineration cost would be also higher because of its higher water content. In the second case (ST (2)), we supposed that the distillate will be further processed so we enforced the minimal quantity of water in the distillate by applying the Penalty Function (FF = ACC + AOCc + AOCi + AOCs + Pf(xD,w)). In this case the distillate contains only 0.001−0.004 mol/mol water (Table 1). The total annual costs of the two versions of stripping step (ST(1) and ST(2)) differ only slightly. We can state that by applying stripping instead of incineration (I) considerable reduction of TAC can be realized for all wastewater compositions studied. The more dilute is the wastewater the greater is the reduction of TAC. 3.3. Recovery of the Organic Components by Extractive Distillation. As the mixture acetone-methanol forms a minimum boiling homoazeotrope for its separation a

Figure 3. Flow-sheet of the recovery of organic components by indirect extractive distillation. 11476



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Table 2. Costs of the Indirect Extractive Distillation Scenarios I

ST-iEXA

ST-iEXA-M

xF1,w [mol/mol]

TAC [k$/Y]

ACCc [k$/Y]

AOCc [k$/Y]

AOCi [k$/Y]

AOCs [k$/Y]

AIP [k$/Y]

TAC [k$/Y]

ACCc [k$/Y]

AOCc [k$/Y]

AOCi [k$/Y]

AOCs [k$/Y]

AIP [k$/Y]

TAC [k$/Y]

0.9 0.8 0.6 0.4 0.2 0.1

10241 8937 6742 4841 3094 2257

129 153 170 195 205 210

426 549 766 944 1090 1126

6861 9636 7873 6627 5548 5203

9 0 0 0 0 0

2878 4897 7557 9211 10364 10815

4548 5445 1256 −1441 −3516 −4270

173 206 225 258 268 276

596 807 1051 1258 1419 1461

0 0 0 0 0 0

22 23 14 7 3 1

3321 5640 8696 10647 11974 12495

−2521 −4603 −7406 −9123 −10284 −10757

Figure 4. Flow-sheet of the recovery of the organic components by direct extractive.

Table 3. Costs of the Direct Extractive Distillation Scenarios I

dEXA

dEXA-M

xF1,w [mol/mol]

TAC [k$/Y]

ACCc [k$/Y]

AOCc [k$/Y]

AOCi [k$/Y]

AOCs [k$/Y]

AIP [k$/Y]

TAC [k$/Y]

ACCc [k$/Y]

AOCc [k$/Y]

AOCi [k$/Y]

AOCs [k$/Y]

AIP [k$/Y]

TAC [k$/Y]

0.9 0.8 0.6 0.4 0.2 0.1

10241 8937 6742 4841 3094 2257

65 80 112 125 133 139

213 313 377 439 475 482

15208 13495 12723 12756 12918 13959

0 0 0 0 0 0

2869 4855 7519 9168 10295 10739

12617 9033 5693 4152 3232 3841

99 140 161 175 183 192

396 622 707 776 809 823

0 0 0 0 0 0

53 45 43 42 43 47

3136 5566 8538 10335 11519 11976

−2588 −4759 −7628 −9343 −10485 −10915

consideration that the acetone remained in the bottom (W2) disturbs or even prevents the methanol production. Hence in the Fitness Function a Penalty Function (Pf(xW2,A)) is applied in order to minimize the quantity of acetone in W2. It must be still noted that we found that the number of the not viable individuals is much higher than usually, since it is much more difficult to attain a high purity in the extractive distillation column distillate. The different costs obtained for this scenario (ST-iEXA) are shown in Table 2. Comparing the costs of the scenarios ST and ST-iEXA (Tables 1 and 2) we can state that the acetone production step itself decreases the TAC only at moderate water contents (xF1,w ≤ 0.6). The incineration costs of this scenario are even higher for xF1,w ≤ 0.8 than the cost of the incineration of the whole waste since the high amount and water content of the bottom product W2 to be incinerated (water makeup is necessary for the extractive distillation). At low water contents (xF1,w ≤ 0.4) this scenario already provides profit due to the value of acetone. 3.3.1.2. Methanol Production Step (ST-iEXA-M). If methanol is also produced in prescribed purity and the whole three component mixture is separated into products of specified purity,

then the following items must be considered: capital and operation costs of all columns (C1, C2, C3), fee of draining of all bottom product withdrawn (W1s,W3s), and the value of acetone and methanol produced (D2, D3). (In this case there is no incineration cost.) By the application of these steps the total annual cost can be further reduced. Moreover considerable profit can be reached with the products acetone and methanol (Table 2). 3.3.2. Extractive Distillation without Separate Stripping (Direct Extractive Distillation). Production of acetone and methanol and purification of water can be also performed in a two column system by direct extractive distillation of the wastewater (Figure 4). 3.3.2.1. Acetone Production Step (dEXA). In the first (extractive) column (C1) acetone is separated from methanol and water as distillate (D1) by the aid of water recycled from Column 2 (W2r). If the bottom product (W1) was not further processed, but it would be incinerated, the fee of its incineration should be taken into consideration as costs, besides the capital and operation costs of Column C1. The total annual cost of this process would be higher than that of the incineration in spite of the income originating from the acetone production (Table 3), 11477



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Figure 5. Flow-sheet of the recovery of the organic components by pressure swing distillation without recycle.

Figure 6. Flow-sheet of the recovery of the organic components by pressure swing distillation with recycle.

3.4.1. Pressure Swing Distillation without Recycling. If pressure swing distillation is performed without recycling (Figure 5) the stripping process step remains unchanged (ST(2)). 3.4.1.1. Methanol Production Step (ST-PSM). If the distillate would not be further processed, besides the capital and operation costs of columns C1 and C2, the fee of draining of the bottom product of C1 (W1) and that of the incineration of the distillate of C2 (D2) should be taken into consideration as costs. (In this case recycling could not be applied.) However the costs are reduced by the value of the methanol product, but profit cannot be realized (Table 4). 3.4.1.2. Acetone Production Step (ST-PSM-A). If acetone is also produced we have three products of specified purity, but there is also a byproduct (the distillate of the last column (D3) whose composition is near to that of the azeotrope A-M at 10 bar), and the following items must be considered: capital and operation costs of all columns (C1, C2, C3), fee of draining of whole bottom product (W1), fee of incineration of D3, and the value of acetone and methanol produced (W2, W3). Though the recovery of both organic components is moderate, with the acetone production step itself some profit can be already realized (Table 4). 3.4.2. Pressure Swing Distillation with Recycling. In order to avoid considerable loss of organic components with the distillate of high pressure acetone column (D3) this stream must be recycled (Figure 6). D3 cannot be recycled to Column 2, as expected, because of its water content even if it is low. Therefore D3 is fed back into Column C1 (onto the third plate). Since in this case there is no incineration cost and the income is higher than without recycling (PSD-NR), much more profit can be realized (Table 5).

because the extra water fed as solvent appears in the bottom product making its incineration very expensive. Hence this scenario (acetone production without that of methanol) is not recommended at all. 3.3.2.2. Methanol Production Step (dEXA-M). By applying a second column (C2) methanol (as distillate, D2) and treated water (as bottom product, W2) can be produced. One part of the treated water (W2r) is recycled to the first column as solvent. By this second step the total annual cost drastically decreases (Table 3). The fee of incineration disappears which largely compensates the increase of capital cost (ACC) and operational costs of columns (AOCc). Moreover considerable profit can be reached with the products acetone and methanol. Comparing indirect and direct extractive distillation scenarios with methanol production (iEXA-M in Table 2 and dEXA-M in Table 3) we can conclude that the difference in the profit is very small to the favor of the direct method. (The costs ACCc and AOCc and the income AIP are lower, while AOCs is higher for the direct method.) 3.4. Recovery of the Organic Components by Pressure Swing Distillation. By pressure swing distillation (Figures 5 and 6) methanol is produced at atmospheric pressure in Column C2 and acetone at 10 bar in Column C3, respectively. (The column C2 cannot be operated at the higher pressure since in this case only acetone−water azeotrope could be produced as distillate and acetone-methanol azeotrope as bottom product, respectively. The residue curve maps are shown in the Supporting Information Appendix A1 (Figures A1.2a-b).) Contrary to the extractive distillation by the pressure swing distillation both organic components are produced as bottom product. This method is studied in two versions: (1) pressure swing distillation without recycling and (2) pressure swing distillation with recycling. 11478



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Table 4. Costs of the Pressure Swing Distillation Scenarios without Recycling I

ST-PSM

ST-PSM-A

xF1,w [mol/mol]

TAC [k$/Y]

ACCc [k$/Y]

AOCc [k$/Y]

AOCi [k$/Y]

AOCs [k$/Y]

AIP [k$/Y]

TAC [k$/Y]

ACCc [k$/Y]

AOCc [k$/Y]

AOCi [k$/Y]

AOCs [k$/Y]

AIP [k$/Y]

TAC [k$/Y]

0.9 0.8 0.6 0.4 0.2 0.1

10241 8937 6742 4841 3094 2257

133 160 184 201 208 209

440 625 931 1106 1237 1285

282 481 741 903 1015 1058

31 23 13 7 3 1

303 514 789 965 1083 1131

583 774 1080 1253 1381 1422

200 250 297 328 344 348

535 779 1169 1392 1554 1615

185 316 487 594 667 695

31 23 13 7 3 1

1463 2491 3839 4680 5259 5484

−512 −1122 −1872 −2358 −2690 −2825

Table 5. Costs of the Pressure Swing Distillation I

PSD-NR

xF1,w [mol/mol]

TAC [k$/Y]

TAC [k$/Y]

ACCc [k$/Y]

AOCc [k$/Y]

pressure swing distillation with recycle AOCi [k$/Y]

AOCs [k$/Y]

AIP [k$/Y]

TAC [k$/Y]

0.9 0.8 0.6 0.4 0.2 0.1

10241 8937 6742 4841 3094 2257

−512 −1122 −1872 −2358 −2690 −2825

209 254 296 327 344 349

652 (530) 1014 (820) 1501 (1184) 1772 (1372) 1980 (1528) (2081) 1638

0 0 0 0 0 0

31 23 13 7 3 1

3316 5655 8701 10631 11943 12408

−2424 (−2546) −4363 (−4556) −6891 (−7208) −8525 (−8924) −9615 (−10067) −9977 (−10420)

Table 6. Comparison of the Different Scenarios by Total Annual Cost xF1,w

I [k$/Y]

ST (1) [k$/Y]

ST (2) [k$/Y]

ST-iEXA [k$/Y]

ST-iEXA-M [k$/Y]

dEXA [k$/Y]

dEXA-M [k$/Y]

ST-PSM [k$/Y]

ST-PSM-A [k$/Y]

ST-PSM-A-R [k$/Y]

0.9 0.8 0.6 0.4 0.2 0.1

10241 8937 6742 4841 3094 2257

732 1046 1489 1770 1956 2010

723 1037 1506 1797 1997 2073

4548 5445 1256 −1441 −3516 −4270

−2521 −4603 −7406 −9123 −10284 −10757

12617 9033 5693 4152 3232 3841

-2588 -4759 -7628 -9343 -10485 -10915

583 774 1080 1253 1381 1422

−512 −1122 −1872 −2358 −2690 −2825

−2424 (−2546) −4363 (−4556) −6891 (−7208) −8525 (−8924) −9615 (−10067) −9977 (−10420)

Table 7. Comparison of the Costs of the Best ED and PSD Processes (k$/Y) direct extractive distillation (dEXA-M)

PSD with heat integration (ST-PSM-A-R)

xF1,w

ACCc

AOCc

AOCs

AIP

TAC

ACCc

AOCc

AOCs

AIP

TAC

0.9 0.8 0.6 0.4 0.2 0.1

99 140 161 175 183 192

396 622 707 776 809 823

53 45 43 42 43 47

3136 5566 8538 10335 11519 11976

−2588 −4759 −7628 −9343 −10485 −10915

209 254 296 327 344 349

530 820 1184 1372 1528 1638

31 23 13 7 3 1

3316 5655 8701 10631 11943 12408

−2546 −4556 −7208 −8924 −10067 −10420

However the profit can be still increased by the application of heat integration. The boiling point of azeotrope (top stream temperature of the high-pressure Column C3 (133.9 °C at 10 bar)) is higher than that of the methanol product (bottom stream temperature of the low-pressure Column C2 (near to 64 °C at 1.01 bar)). A heatexchanger (economizer) is used to recover energy from the top vapor of Column C3 by (partial) vaporization the reboil stream of Column C2. The results obtained with heat integration are presented in Table 5 (in brackets). It must be still noted that the number of the not viable individuals was much higher than in other cases, so the time demand of these calculations was also higher. The detailed calculation results (sizes of the columns and the mass flow rates) are shown in Supporting Information Appendix A2 and A3, respectively.

The incineration (I) is rather expensive mainly for high water contents. The worst scenario with the highest Total Annual Cost is when acetone is recovered in one extractive distillation column (stripping column and extractive acetone column is merged) and the remaining bottom product is incinerated (dEXA). In this case the TAC is higher than that of incineration of the whole wastewater (I), since extra water must be used for the acetone production which leaves in the bottom product (W1) increasing its incineration cost. Hence this process is highly not recommended. The best results are obtained by direct extractive distillation with the methanol production (dEXA-M) at all water content although the results obtained for indirect extractive distillation (iEXA-M) and pressure swing distillation with recycling and with heat integration (ST-PSM-A-R) remained slightly under those of extractive distillation. It must be noted that the difference is less than 6% which is lower than the accuracy of the cost estimation. Annual capital and operation costs of the best ED and PSD processes are shown in Table 7.

4. COMPARISON OF THE DIFFERENT SCENARIOS In this section first the different scenarios are compared by Total Annual Costs (Table 6). 11479



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AIP = Annual Income of the Products [$/Y; k$/Y] TAC = Total Annual Cost [$/Y; k$/Y] d = diameter of the column [m] D = distillate flow rate [kg/h] F = feed flow rate [kg/h] N = number of stages Nf = feed plate location Qr = reboiler heat duty [MJ/h] R = reboil ratio W = bottom product flow rate [kg/h]

By both ED and PSD considerable profit (negative TAC) can be realized due to the high income from the products (AIP). On the decrease of the water content (increase of content of organic components), though the main costs (ACCc and AOCc) increase, the profit considerably increases due to the increase of AIP. The capital cost of the PSD is always higher because of the higher number of columns and the application of higher pressure. The profit is always slightly higher for the ED than for the PSD.

5. CONCLUSION Different scenarios for handling waste mixtures of different concentrations containing acetone and methanol (in equimolar quantities) in order to or recover and/or remove organic components were investigated by rigorous simulation with the professional flow-sheet simulator CHEMCAD 6.3. The extractive (ED) and pressure swing distillation processes are optimized with genetic algorithm by minimizing of the total cost. (The program of GA calling the flow-sheet simulator is written in Visual Basic under EXCEL.) Capital, operation, and total annual costs of the following scenarios are compared: (1) incineration of the whole amount of waste, (2) stripping, (3) indirect (three-column system, ED after stripping) and direct (two-column system, ED without separate stripping) extractive distillation, and (4) pressure swing distillation without and with heat integration. The influence of the water content of the waste mixture was also investigated. We stated that by the best scenarios the recovery of organic solvents can be performed economically with considerable profit. The lower was the water content of the waste mixture the higher profit was realized. The highest profit was obtained by the scenarios where the whole quantity of the waste mixture is separated into specified purity products (acetone, methanol, and water with COD < 1000). The best results were obtained by direct extractive distillation. The results obtained for indirect extractive and pressure swing distillation (with heat integration) remained only slightly under those of the direct extractive distillation.

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Index

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Appendix A1 (figures and tables). This material is available free of charge via the Internet at http://pubs.acs.org.

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1..3 = column index a = acetone m = methanol w = water

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Hungarian Scientific Research Fund (OTKA) (No. K-82070) and by the Janos Bolyai Research Scholarship of the HAS.

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NOTATION ACC = Annual Capital Cost [$/Y; k$/Y] AOCc = Annual Operation Cost of the column [$/Y; k$/Y] AOCi = Annual Operation Cost of the incineration [$/Y; k$/Y] AOCs = Annual Operation Cost of the draining of sewage [$/Y; k$/Y] 11480



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