Vapor Compression for Batch Distillation - American Chemical Society

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Vapor Compression for Batch Distillation: Comparison of Different Working Fluids G. Modla* and P. Lang Department of Building Services and Process Engineering, Faculty of Mechanical Engineering, Budapest University of Technology and Economics, Muegyetem rkp. 3-5, H-1521 Budapest, Hungary ABSTRACT: Application of a heat pump system is a possibility for decreasing the high energy demand of distillation. The use of different working fluids for a vapor compression system for batch distillation is investigated. In this study working fluids are classified as “wet” and “dry” based on their behavior during the compression. The separation of a low relative volatility mixture (n-heptane−toluene) is investigated by rigorous simulation performed with a professional flow-sheet simulator. Standard reactorreboiler is applied and simulated in a rigorous way (including the conditions of heat transfer). The basic criteria for the selection of the possible working fluids are determined. Based on these criteria n-hexane, n-pentane, ethanol, and isopropyl alcohol are studied as working fluids. The effectiveness of different working fluids is compared. The effect of the main operational parameters on the length of the payback period is examined. The minimal payback time is determined for the different working fluids. The best results are obtained for n-hexane.

1. INTRODUCTION Reducing energy consumption and eliminating waste is one of the main goals of the European Union. The EU climate and energy package was published in June 2009.1 The climate and energy package is a set of binding legislation which aims to ensure the European Union meets its ambitious climate and energy targets for 2020. These targets, known as the ″20−20−20″ targets, set three key objectives for 2020: • A 20% reduction in EU greenhouse gas emissions from 1990 levels; • Raising the share of EU energy consumption produced from renewable resources to 20%; • A 20% improvement in the EU’s energy efficiency. Distillation is one of the most widely used separation methods in the chemical industry though its energy requirement is extremely high. The separation of the components of liquid mixtures is based on the volatility differences. The two highest heat duties are caused by creating the vapor flow in the reboiler (heating) and condensing the top vapor in the condenser (cooling). A great number of attempts have been made to reduce the energy requirement of distillation processes. Different energy saving methods achieved by means of internal and external heat integrations were widely studied for the continuous distillation process.2−25 A comprehensive review of different heat pump (HP) systems for the continuous distillation was given by Bruinsma and Spoelstra.24 First these authors described the conventional heat pump configurations. In the vapor compression (VC) system a working fluid (WF), which is different from the components to be separated, is evaporated at the condenser, compressed to a higher (saturation) temperature, condensed at the utility side of the reboiler, and then cooled down by expansion over a throttle valve to a (saturation) temperature below that of the condenser. In the vapor recompression (VRC) system the vapor leaving the top of the column is used as heating medium in the reboiler. The advantages of VRC over VC are that the condenser is much smaller and that the temperature lift is lower because heat is exchanged only once. Due to these advantages VRC has become © XXXX American Chemical Society

the standard technology, except for the cases where the top vapor cannot be compressed. Bruinsma and Spoelstra24 also described the novel heat pump methods for distillation (thermoacoustic, compression-resorption, adsorption heat pumps, and heat integrated distillation columns). Kiss et al.23 summarized all HP technologies (including the thermal VRC, cyclic distillation, Kaibel and dividing wall columns). They also proposed a practical selection scheme of energy efficient distillation technologies. A new approach to predict the economic performance of different types of heat pumps was introduced by van de Bor and Infante Ferreira26 who proposed a heat pump selection map. A new selection scheme of energy efficient distillation technologies is proposed by Kiss et al.,27 with a special focus on heat pumps. These authors mentioned among the most promising technologies the vapor compression, as well. The advantages of batch distillation (BD) over the continuous one are well-known if the quantity and composition of the mixture to be separated vary frequently. Distillation is usually performed in batch among others in the pharmaceutical industry and in the production of fine chemicals. The energy saving methods for batch distillation have been much less studied than for the continuous one. Different closed operation modes of batch distillation were investigated in several studies28−31 in order to reduce the energy demand of the process. Takamatsu et al.32 proposed the first thermally coupled batch distillation configuration, where the rectifying tower is surrounded by a jacketed reboiler. Later this internally heat integrated batch distillation with a jacketed still (IHIBDJS) configuration was studied by Jana et al.33 as well. Received: October 11, 2014 Revised: December 15, 2014 Accepted: December 23, 2014

A

DOI: 10.1021/ie504023q Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

column is considered adiabatic. During the whole process the temperature of the reboiler increases, while the temperature of the top vapor increases or it is nearly constant depending on the operational policy (constant reflux ratio or constant distillate composition) applied. In Figure 1 the conventional batch distillation column is shown (BD) without any heat pump system.

Another energy efficient structure (“variable speed vapor recompression system”) was introduced and studied by Jana and his co-workers.34−39 In this system the compression ratio (CR) was manipulated. They pointed out the great economical potential of the vapor recompression systems for batch distillation. Jana and Maiti40 studied the VRC for BD applying separate constant CR values for the start-up and for the production period. They distinguished two cases depending on the relation of the energy (QCV) achievable from the internal heat source (compressed top vapor) and the heat required for liquid reboiling (QR). When QCV > QR the vapor inflow rate to the compressor, in the opposite case the external heat input is manipulated. The vapor formed during the expansion is condensed in a condenser installed in the overhead vapor line so its latent heat is not utilized. They showed the advantages of the application of VRC for the separation of a nonreactive binary and a reactive quaternary system. In order to reduce further the external heat consumption of the IHIBDJS Jana and Maiti41 proposed an additional thermal arrangement into this configuration that couples the overhead vapor with the reboiler liquid. The ideal IHIBDS system proposed required an auxiliary heat supply from external source only for the first relatively short period of the process (for less than 10% of the total batch time). Recently Jana42 gave a comprehensive review of the advances in HP assisted continuous and batch distillation. They emphasized that economic benefits from the HP system come to the expense of difficulties of operation and the degree of heat integration and controllability have an inverse relationship. Modla and Lang43 studied the BD separation of a close boiling hydrocarbon mixture (n-heptane−toluene) by rigorous dynamic simulation and cost calculations. The following HP methods were investigated: vapor recompression, vapor recompression with the application of an external heat exchanger (BD-VRC-E) and vapor compression (BD-VC, with n-pentane as working fluid). The most favorable results (shortest payback time of the additional investment) were obtained for the BD-VRC-E and BD-VC systems. (For the BD-VRC the pay-back times were slightly (by ca. 10%) lower than for the BD-VC.) In the first part of this paper different potential working fluids (n-pentane, n-hexane, methanol, ethanol, and isopropyl alcohol) are studied for the BD-VC separation of the mixture n-heptane−toluene. The criteria for the selection of working fluids are described. The working fluids judged suitable are divided into two groups based on their behavior during the compression. In the second part of this paper dynamic simulation calculations are made with different modules of the ChemCad44 professional flow-sheet simulator. A standard reactor-reboiler is applied, which is simulated in a rigorous way (including the conditions of heat transfer). The influence of the main operational parameters (e.g., compression ratio) on the effectiveness (cost saving, payback time) of the process is investigated, and the best operational parameters are determined based on the minimization of the payback period of heat pump system. Impact of vapor compression in batch distillation on CO2 emission is presented.

Figure 1. Conventional batch distillation configuration (BD).

By the conventional BD process processing of one batch consists of the following steps (parts): filling of the feed (charge) into the reboiler, start-up (warming-up of the charge to its boiling point, heating-up of the column (creation of plate hold-ups, purification of the distillate) without product withdrawal, production (of forecut(s), main cut(s), after cut(s)), shutdown (cooling and drainage of the residue). In the start-up step (Step 1), the column runs in total reflux mode, while the production step (Step 2) involves the batch processing under partial reflux conditions. From the beginning of the warming-up until the end of production the temperature of the reboiler continuously increases (at the end of heating-up it begins to stabilize). During the heating-up the concentration of the lightest component increases on the upper trays of the column and the concentrations of the other components increase in the reboiler (still pot). After a certain time the vapor already reaches the condenser, and the flow rate of the top vapor becomes considerable. Near to the end of the heating-up the temperature of the top vapor increases to a smaller and smaller extent. By the constant reflux ratio operational policy when the concentration of the lightest component in the distillate reaches its prescribed purity the reflux ratio is set to a prespecified (finite) value and the distillate product withdrawal is started. In the present study the column runs in start-up mode until the top vapor concentration reaches a predefined value. The product withdrawal continues until the average distillate composition falls onto its specified value. During the whole production the reboiler and condenser duties are nearly equal. The temperature of the top vapor increases. (By the constant distillate composition policy it remains constant.)

1. THE BATCH DISTILLATION PROCESS In batch distillation heat is added through the jacket of the reactor-reboiler and extracted in the condenser, while the B


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2. BATCH DISTILLATION WITH VAPOR COMPRESSION In batch distillation with vapor compression (VC, Figure 2) cooling and heating of the column is ensured with the working

The compressor (the heat pump system) can be already operated during the heating-up of the column. When the flow rate of the top vapor is not sufficient yet (in our case in the first half an hour), an additional heat exchanger (spare heater) must be applied in order to vaporize the working fluid. Comparing the vapor compression and vapor recompression (VRC) methods we can state that VRC, which is applied more frequently in the industry for continuous distillation, has the following disadvantages against VC in the case of batch distillation: - The heat pump can be started only at the end of the heating-up. - The composition of the heating medium (top vapor) and therefore the conditions of heat transfer (heat transfer coefficient on the WF side, latent heat of vaporization, heat capacity of WF). This variation is drastic when separating a multicomponent mixture.

3. PROPERTIES OF WORKING FLUIDS The thermodynamic and physical properties, stability, environmental impacts, safety and compatibility, and availability and cost are among the important considerations when selecting a working fluid.45 3.1. Criteria for Selecting Working Fluids. The basic criteria for the selection were the following ones: 1. In order to ensure a sufficient driving force for the heat transfer the bubble point of the working fluid at 1.01 bar must be less at least by 15 °C than the lowest temperature of the top vapor (in our case the bubble point of the light component (n-heptane) at 1.01 bar, 98.4 − 15 = 83.4 °C). 2. The working fluid must condense in the reboiler; therefore, its critical temperature must be higher than the maximal temperature at the utility side of the reboiler (in our case the bubble point of the heavy component (toluene) at 1.11 bar plus the temperature difference for the heat transfer: 111 + 15 = 126 °C or the saturation temperature of the heating steam of 4 bar: 143.7 °C). (If this criterion is satisfied the maximal pressure at the utility side of the reboiler remains under the critical pressure of the working fluid, as well.) Vapor pressure-temperature and pressure-enthalpy curves of the components to be separated and of some potential working fluids are shown in Figures 4a and 4b. (The vapor pressures are calculated by the LIBRARY equation, and enthalpies are calculated by the LATENT HEAT of ChemCad.) The relevant thermodynamic data of these substances are given in Table A1. On the basis of the above criteria the following substances are suitable for working fluids: n-pentane, n-hexane, methanol,

Figure 2. Scheme of batch distillation with vapor compression.

fluid (WF) is independent of the mixture to be separated. The basic parts of a VC cycle are as follows (Figure 3). The working fluid is evaporated at the condenser (between 1 and 2), compressed to a higher pressure with higher saturation temperature (3 → 4), condensed in the reboiler (5 → 6), and cooled down by expansion over a throttle valve (7 → 1) to a (saturation) temperature below the condenser temperature. The optional parts of the cycle depending among others on the thermodynamic properties of the working fluid are superheating of the working fluid (2 → 3, if necessary in order to prevent the (partial) condensation of working fluid in the compressor) and in the reboiler cooling down of working fluid to its dew point (4 → 5, if it leaves the compressor as superheated vapor), cooling down (subcooling) of the condensed working fluid from its bubble point before expansion (6 → 7).

Figure 3. Thermodynamic cycles of VC for different types of WFs a. Wet WF b. Dry WF. C


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fluids are n-pentane and n-hexane, while the worst ones are isopropyl alcohol and ethanol (Table1). - The latent heat of vaporization (λ) at the operating pressures is high. The higher the value of λ the lower the flow rate of working fluid, the smaller size compressor is needed resulting in the decrease of both investment and operating costs (best working fluids: isopropyl alcohol, ethanol, and worst: n-pentane). Moreover it is favorable if - Pout is the lowest possible (smaller wall thickness, simpler gaskets) decreasing the investment costs. - The decrease of the latent heat to the given increase of the boiling point (ΔT) is low. The smaller the decrease of λ for the given ΔT, the higher is the ratio of the heat of condensation in the heat rate furnished in the reboiler, which is advantageous from the point of view of the utility side heat transfer conditions. Table 1 shows the relevant thermodynamic data of substances studied. For the different working fluids for the temperatures of utility side of the reboiler (126.0 and 143.7 °C) the vapor pressures (p0rel) and latent heats (λrel) are divided with their values at the temperature of utility side of the condenser (83.4 °C). (E.g. the vapor pressure of n-pentane at 126.0 °C is 2.57 times greater than at 83.4 °C.) 3.2. Behavior of Different Working Fluids during the Compression. Chen et al.45 studied the organic Rankine and supercritical Rankine cycles for the conversion of low-grade heat into electrical power, as well as selection criteria of potential working fluids. They classified the working fluids on the basis of their behavior during expansion. The working fluid can be “dry”, “isentropic”, or “wet” depending on the slope of the saturation vapor curve in a T−s diagram (dT/ds). Since the value of dT/ds leads to infinity for isentropic fluids, the above authors used the inverse of the slope (ds/dT) to express how “‘dry’” or “‘wet’” a fluid is from the point of view of expansion. In this study the behavior of the different working fluids during compression is investigated. From this point of view the working fluids can be divided into two different types. If the saturated vapor working fluid partially condenses during the compression, then it is called “wet” otherwise “dry” fluid. For the “wet” fluids (n-pentane and n-hexane, that is, the two paraffin hydrocarbons) the saturated vapor side of the pressureenthalpy curve strongly inclines to the right (Figure 3a). If saturated vapor (point 2) was compressed it would partially condense in the compressor without preliminary superheating (to point 3). For the “dry” fluids (alcohols (and water)) the right branch of the logP-h curve is almost vertical (Figure 3b). On compressing the saturated vapor gets superheated, which cools down to its dew point in the reboiler (point 5) before

Figure 4. a. Vapor pressure−temperature curves. b. Pressure-enthalpy curves.

ethanol, and isopropyl alcohol. (Water, which would be advantageous from practical point of view, does not fulfill Criterion 1.) From the point of view of compression costs it is favorable if - The increase of the vapor pressure (p0) to the given increase of the boiling point (ΔT) is low. The lower the increase the lower is the compression ratio and so the lower are the operating costs. From this point of view the best working Table 1. Thermodynamic Data of the Substances Studieda n-pentane n-hexane methanol ethanol isopropyl alcohol water n-heptane toluene a

NBP [°C]

p° [bar] 83.4 °C

p°rel 126.0 °C

p°rel 143.7 °C

λ [kJ/mol] 83.4 °C

λrel 126.0 °C

λrel 143.7 °C

Tcr [°C]

Pcr [bar]

36.07 68.73 64.70 78.29 82.26 100.0 98.43 110.56

3.99 1.53 2.01 1.21 1.03 0.53 NR NR

2.57 2.92 3.75 4.21 4.32 4.51 NR NR

3.57 4.23 5.96 6.86 7.07 7.55 NR NR

22.65 28.07 33.85 38.27 39.79 41.58 NR NR

0.827 0.874 0.888 0.899 0.871 0.949 NR NR

0.737 0.814 0.835 0.849 0.809 0.927 NR NR

196.5 234.2 239.5 240.8 235.2 374.2 NR NR

33.7 29.7 80.9 61.5 30.0 221.2 NR NR

NR: not relevant. D


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Industrial & Engineering Chemistry Research condensing. These working fluids do not need superheating before compression. The minimal extent of the preliminary superheating of wet working fluids (ΔTs,in) and the extent of superheating of dry working fluids (ΔTs,out) during compression are determined for two different condensation temperatures: - the minimal condensation temperature of the working fluid: boiling point of toluene at the bottom pressure +15 °C (minimal ΔT for the heat transfer in the reboiler) = 126 °C (temperature of saturated water-steam of 2.4 bar, Table 2)) and Table 2. Superheating of Different WFs before and during Compression (Tout,min = 126 °C) working fluid

Pin [bar]

ΔTs,in [°C]

Pout [bar]

Tout [°C]

ΔTs,out [°C]

n-pentane n-hexane methanol ethanol isopropyl alcohol

4 1.53 2.01 1.21 1.03

14 17 0 0 0

10.24 4.46 7.54 5.09 4.46

126.4 126.1 203.6 153.4 131.6

0.4 0.1 77.6 27.4 5.6

Figure 5. ChemCad model of the BD-VC system.

is applied. (The production step is finished when the concentration of n-hexane in the product tank decreases to its specified value.) 4.2. Cost Calculations. To analyze the economic feasibility of a process, the payback period (PBP) or total annualized cost is usually used as the indicator for determining the cost savings. The details of the cost calculation, which is based on Douglas’s book,46 can be found in Appendix 2. The payback period is estimated by combining investment (IC), depreciation costs (DC), and the yearly savings (YS). In our case the yearly saving is based on the difference of the operation costs of BD and BD-VC. For the sake of simplicity, for the BD the operating costs are taken from the cost of steam (30 $/GJ) and for the BD-VC the cost of the electricity (108 $/MWh → 30 $/GJ). We supposed that the yearly operation time of the process can be 4000 or 6000 working hours, and hence the compressor lifetime (CLT) is 15 or 12.5 years, respectively. The utility costs are directly calculated on unit time basis. The annual capital investment is estimated by dividing the investment cost of the compressor by its lifetime. 4.3. Investigation of the BD Process. In this section the BD process is investigated. The constant reflux ratio operational policy is applied, since it is the simplest one and widespread in the industry. The effect of the pressure of the water-steam heating medium and the influence of the operational policy are studied. In the first case the pressure of steam is 2.4 bar (saturation temperature is 126 °C), and in the second case it is 4 bar (saturation temperature is 143.7 °C). The influence of the duration of start-up on the recovery of n-heptane (η) and on the heat demand is investigated for different reflux ratios. The limits of the start-up duration are as follows: - minimum: the top vapor reaches the product purity prescribed (98 mol %) - maximum: the purity of top vapor practically reaches its maximum (99.78 mol %) which can be produced with the given column under infinite reflux ratio. In Case 1, when the steam pressure is 2.4 bar the minimum start-up duration is 120 min and the maximum is 410 min. In case 2, when the steam pressure is 4 bar, the minimum start-up duration is 93 min and the maximum is 278 min. The reflux ratios (R) investigated, 10, 12, and 14, are relatively high, since the separated mixture is a low relative

Table 3. Superheating of Different WFs before and during Compression (Tout,min = 143.7 °C) working fluid

Pin [bar]

ΔTs,in [°C]

Pout [bar]

Tout [°C]

ΔTs,out [°C]

n-pentane n-hexane methanol ethanol isopropyl alcohol

4 1.53 2.01 1.21 1.03

19 24.5 0 0 0

14.23 6.47 11.9 8.3 7.28

143.7 143.8 250.1 178.3 149.6

0.0 0.1 106.4 34.6 5.9

- a higher condensation temperature: 143.7 °C (condensation temperature of water-steam of 4 bar, Table 3). Results show that the methanol is very sensible for the compression. The outlet temperature is extremely high in both cases, which could cause several problems (e.g., heat shock). (In the second case Tout even exceeds the critical temperature.) Hence methanol is excluded from the potential WFs, and in the second part of this paper optimal operational parameters will be determined by rigorous calculations for n-pentane, n-hexane, ethanol, and isopropyl alcohol.

4. SIMULATION RESULTS In this section first the BD then the BD-VC process with different types of working fluids is investigated. The BD-VC processes are compared with the basic, conventional, optimized batch distillation process (without any heat pump system). For the calculation we used different dynamic modules of the ChemCad professional flow-sheet simulator. The ChemCad model of the BD-VC system is shown in Figure 5. (The controller is applied for varying the heat pump load during the production step as described in Section 4.4.) 4.1. Basic Data. The mixture is n-heptane−toluene. The charge (800 dm3) contains 50 mol % n-heptane. The specified product purity is 98 mol % n-heptane. The number of theoretical stages (N) for each case is 50 (excluding the total condenser and the reboiler). The pressure drop of the column is 0.1 bar. The reactor-reboiler is DIN AE-1000 type. At the beginning of the process the feed mixture (charge) is filled in the reboiler, and the column is empty (“dry start-up”). During the start-up step (heating-up of the column) total reflux E


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Figure 6. Influence of start-up duration on the yield and the heat demand of the whole process (steam of 2.4 bar).

Figure 7. Influence of start-up duration on the yield and the heat demand of the whole process (steam of 4 bar).

working fluids. The effect of the main operational parameters is studied. First the operational variables of the process are investigated. We simulated the same batch distillation process so the start-up duration (Δtst,2.4 bar = 280 min, Δtst,4 bar = 160 min), the reflux ratio in the production step (R = 12), and batch operation time (BOTBD‑VC = BOTBD) remain unchanged. The variables for the heat pump are the following ones: - Compressor inlet pressure (Pin): It is an independent variable, but it has both a lower (atmospheric pressure) and an upper bound. In our former work43 we stated that Pin has small effect on the payback period. In this work always its maximal value (upper limit) is specified (determined from the bubble point of the more volatile component at the condenser pressure - temperature difference of the heat transfer in the condenser). - Presuperheating of the working f luid (ΔTs): It is an independent variable. It has a lower limit (ΔTs,min) for the wet working fluids (in order to prevent the condensation in the compressor). It has also an upper limit (ΔTs,max), which depends on the temperature of working fluid leaving the reboiler, since the heat for superheating is ensured by the working fluid leaving the reboiler. Between these streams a minimal temperature difference is needed. - Compressor outlet pressure (Pout): It is an independent variable, but it has a lower limit (Pout,min), which is determined from the minimal condensation temperature at the utility side of the reboiler (bubble point of the less volatile component at the reboiler pressure + temperature difference of the heat transfer in the reboiler, in our case 111 +15 = 126 °C). - Flow rate of the working f luid (Vwf): Since Δtst is fixed, Pout and ΔTs determine its value in the start-up (Vwf).

volatility mixture. For R = 10 the recovery of n-heptane (η) is too low (66%), so these results are not presented. For all cases we chose a start-up duration (tst) until which the recovery sharply increases, and after which it increases only slightly, it remains almost constant (Figures 6 and 7). The results for the durations chosen are presented in Table 4. It can be seen that on the increase of steam pressure Table 4. Results for the Optimal Start-up Duration Psteam [bar]

R

tst [min]

BOT [min]

SQ [MJ]

η [%]

2.4 2.4 4.0 4.0

12 14 12 14

280 270 160 150

1367 1621 748 869

1820 2050 1770 1980

92.9 96.6 92.9 96.2

(for the same reflux ratio) the value of SQ only slightly decreases. However, BOT considerably decreases, that is, the yearly capacity becomes much higher. The specific heat demand increases (nearly) linearly with the start-up duration (Figure 8). As it is expected for the higher reflux ratio (R = 14) the specific heat demand is higher. On the basis of the above results in order to reach high recovery with low specific heat demand the following set of parameters has been selected: steam pressure of 4 bar, R = 12, tst = 160 min. At t = 160 min the purity of top vapor is 99.60 mol %. In this case the heat demand of the start-up is 522 MJ. The batch operation time (BOT) is 748 min, the recovery of n-heptane is 92.9%, and the total heat demand is 1770 MJ. 4.4. Investigation of the BD-VC Process. In this section the BD-VC process is investigated applying different F


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Figure 8. Influence of the start-up duration on the specific heat demand.

Hence the independent variables are ΔTs and Pout. The value of VWF is calculated for the given values of Pout and ΔTs. Case 1 −Minimal Condensation Temperature of the Working Fluids. The compressor outlet pressure is minimal (it equals to the vapor pressure of the working fluid leaving the reboiler at 126 °C). The working fluids are superheated to a minimal extent. The duration of the heating up is the same as in the BD process (280 min), when the water steam pressure was 2.4 bar. BOT is 1367 min in all cases. In Table 5 the main operational parameters (inlet and outlet pressures, presuperheating temperature, flow-rate of working

Table 7. Values of Main Operational Parameters (Case 2) WF

Pin [bar]

ΔTs [°C]

Pout [bar]

Tout [°C]

Vwf [kg/h]

SMP [MJ]

n-pentane n-hexane ethanol isopropyl alcohol

4.00 1.53 1.21 1.03

19 24.4 0 0

14.23 6.47 8.3 7.28

143.7 143.7 178.3 149.6

775 687 250 346

573 505 482 500

Table 8. Costs and PBP for Two Different Working Times (Case 2) 4000 h/Y


Pin [bar]

ΔTs [°C]

Pout [bar]

Tout [°C]

Vwf [kg/h]

SMP [MJ]


4.00 1.53 1.21 1.03

14 17 0 0

10.24 4.46 5.09 4.46

126.4 126.1 153.4 131.6

570 545 220 280

555 528 560 541

6000 h/Y

WF

IC [$]

DC [$/Y]

YS [$/Y]

PBP [Y]

DC [$/Y]

YS [$/Y]

PBP [Y]


51675 49671 52126 50780

3445 3311 3475 3385

3110 3391 3052 3273

16.6 14.7 17.0 15.5

4134 3974 4170 4062

5698 6080 5621 5926

9.0 8.2 9.3 8.6

IC [$]


87326 78872 75838 78236

5822 5258 5056 5215

6000 h/Y

YS [$/Y]

PBP [Y]

DC [$/Y]

YS [$/Y]

PBP [Y]

5602 6814 7303 6967

15.6 11.6 10.4 11.2

6986 6310 6067 6259

10149 11799 12472 12015

8.6 6.7 6.1 6.5

In batch distillation during the production step the temperature of the reboiler liquid is increasing since the concentration of the more volatile component decreases in it. The heat transfer rate is dropping (Figure 9) since both the active surface area and the driving force for the heat transfer are diminishing.


WF

DC [$/Y]

fluid flow, and total electric energy demand of the compressor (SMP)) and in Table 6 the calculated costs are presented. The payback periods (PBP) are extremely high for the operation of 4000 h/Y, and they are higher than the compressor lifetime (15 year). Case 2 − Higher Condensation Temperature of the Working Fluids. The dew point of working fluids leaving the reboiler is equal to that of the heating steam of 4 bar (143.7 °C). The duration of the heating up is 160 min. The working fluids are superheated to a minimal extent. BOT is 748 min in all cases. In Table 7 the main operational parameters and in Table 8 the calculated costs are presented. The payback periods are lower than in Case 1, but they are still too high. Case 3 - Control System for BD-VC. In this case the heat pump load is varied by a controller during the production step.

Figure 9. Evolution of the heat transfer rate (BD).

The heat pump must be operated with a higher load at the start-up period and with a lower, continuously decreasing load during the production step. (The load of the compressor is proportional to the flow rate of the working fluid). During the production step the increase of the vapor fraction (above 0.05) in the working fluid leaving the reboiler (Figure 10a) is prevented with a PI controller modifying the flow rate of the working fluid (Figure 10b). G


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Figure 10. Evolution of the vapor fraction in the WF leaving the reboiler and of the flow rate of WF (WF: n-pentane).

Figure 11. Actual thermodynamic cycles of n-pentane for the end of start-up (a. t = 160 min) and for the production (t = 590 min) without (b) and with control (c).

The actual thermodynamic cycles of the working fluid n-pentane are shown in Figures 11a-c for the end of the start-up (Figure 11a) and for the production (t = 590 min) without and with control. Comparison of Figures 11b and 11c also shows that with the application of the control system the cycle is more efficient (requires less working fluid) than without it. In Table 9 the main operational parameters and in Table 10 the calculated costs are presented. As it is expected, by varying the compressor load the electrical energy demand can be considerably decreased (by 20−25% for the total process).


Pin [bar]

ΔTs [°C]

Pout [bar]

Tout [°C]

Vwf,max [kg/h]

SMP [MJ]


4.00 1.53 1.21 1.03

19 24.4 0 0

14.23 6.47 8.3 7.28

143.7 143.7 178.3 149.6

775 687 250 346

453 397 363 382

WF

IC [$]

DC [$/Y]

YS [$/Y]

PBP [Y]

DC [$/Y]

YS [$/Y]

PBP [Y]


87476 78969 78948 78304

5832 5264 5063 5220

6739 7862 8429 8097

12.9 10.0 9.1 9.7

6999 6317 6075 6264

11585 13373 14163 13712

7.4 5.9 5.4 5.7

Thanks to the energy saving the payback periods become shorter by 11−17%. Case 4 − Optimization of the BD-VC Process. In this case the PID controller is applied (based on the results of Case 3), and the two main operational parameters, compressor outlet pressure and the extent of presuperheating, are determined by optimization for each working fluids. The optimization method based on two-dimension sensitivity analysis. The objective function is the minimal payback period of the compressor (heat pump system) investment.


6000 h/Y

H


Article


Figure 12. a. Effect of presuperheating on the electrical energy demand (n-hexane, Pout = 6.4 bar). b. Effect of Pout on the electrical energy demand (n-hexane, ΔTs = 40 °C).

Moreover the results show that for the wet fluids the total electrical energy demand (SMP) considerably decreased (by 2−20%), as well. The outlet temperature is much higher for dry working fluids, which can cause several problems (application of special construction materials, distillation of heat sensitive materials). Comparing the payback periods (Figure 13) calculated for two different durations of operation we stated that due to the optimization for the wet working fluids PBP considerably (by 20−40%), for the dry working fluids only slightly decreased (by 3−8%). The best results (lowest PBP-s) were obtained for n-hexane. On the increase of the duration of yearly operation, as it was expected, PBP considerably decreased (by 40%). It must be still noted that if the distillation column with the heat pump system can be also used for the separation of another mixture where the light component is more volatile than the n-heptane, then n-pentane can be preferred to n-hexane, because it can be also applied for lower top temperatures by decreasing the inlet pressure. (If the inlet pressure of n-pentane working fluid is decreased to 1.0 atm the minimum boiling point of the light component is 51 °C).

In order to find the minimal payback period the following principle is applied. The optimum is at the minimal electrical energy demand of the start-up, since the investment and operational costs of the compressor are proportional to the electrical energy demand and the operational cost of the production step is proportional to that of the start-up step. The seeking ranges of the optimization are for - compressor outlet pressure: between P out,min and (p0(143.7 °C) + 3 bar), - extent of presuperheating: between 0 and ΔTs,max. In the searching space for each Pout-ΔTs combination the electrical demand of the start-up (SMPst) is determined. For all working fluids we found that on the increase of ΔTs SMPst decreases in a monotone way (Figure 12a), and there is a Pout value where SMPst has a minimum (Figure 12b). The pressure with the minimal value of SMP is close to the condensation pressure for 143.7 °C, and it is close to the optimal value of the outlet pressure. In Table 11 values of main operational parameters and in Table 12 the costs are shown for the minimum payback period. Table 11. Values of Main Operational Parameters with Optimization WF

Pin [bar]

ΔTs [°C]

Pout [bar]

Tout [°C]

Vwf,max [kg/h]

SMP [MJ]


4.00 1.53 1.21 1.03

50 50 50 50

14.0 6.425 8.3 7.2

174 169 231 199

612.5 575.0 219.5 298.0

375 346 358 363

5. ENVIRONMENT IMPACT OF THE HEAT PUMP The application of the heat pump has not only economic benefits, but it can be also favorable from environmental point of view since significant CO2 (GHG) emission reduction can be achieved. At the carbon-dioxide (CO2) emission calculation we suppose that the heating demand is supplied by a gas boiler. The CO2 emission factor of the gas boiler is 50.35g CO2/MJ. Moreover we suppose that the cooling demand is supplied by a free cooler (the heat is added to the ambient air) whose electric energy demand can be neglected. The electric energy demand of compressor can be transformed to CO2 emission. The electricity emission factor depends on the country47 (Appendix 3). In this study we calculated with a factor of 317 g CO2/kWh. (This data set does not contain the electricity emission factor for the United States.) At the emission calculation the basic yearly working hour is 4000 h, which means 320 batches. At the original, conventional batch column the yearly CO2 emission is 28.5 ton. In Table 13 the CO2 emission reductions are listed for different working fluids. It can be seen that significant emission reduction can be reached for all working fluids.

Table 12. Costs and PBP for Two Different Working Times with Optimization 4000 h/Y

6000 h/Y

WF

IC [$]

DC [$/Y]

YS [$/Y]

PBP [Y]

DC [$/Y]

YS [$/Y]

PBP [Y]


76793 71833 75180 76536

5120 4789 5011 5102

8325 8937 8499 8460

9.2 8.0 8.9 9.0

6143 5747 6014 6123

14023 14842 14255 14221

5.5 4.8 5.3 5.4

Comparing the results without (Case 3, Table 9) and with optimization (Case 4, Table 11) it can be seen that the value of Pout only slightly (by 1−2%) varied, while that of Vwf,max (by 13−17%) considerably decreased due to the optimization. I


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Figure 13. Payback periods for different working fluids.

■

Table 13. CO2 Emission Savings for Different Working Fluids WF

ECO2 [t/Y]

ECO2 [%]


17.93 18.75 18.41 18.27

62.92 65.79 64.60 64.11

APPENDIX

Table A1. The Value of the Parameters Used for the Phase Equilibrium Calculations ij AB

Bij

Bji

alpha

−80.5304 213.964 0.3022 NRTL parameters for N-heptane (A) − Toluene (B)

Cost Calculation

6. SUMMARY The purpose of the current study was to investigate different working fluids for a heat-pump system integrated to a batch distillation column equipped with a reboiler of DIN standard AE-1000 type. The selected test mixture was a low relative volatility one (n-heptane−toluene). The first part of this study presents the relevant physicochemical parameters of the possible working fluids and discusses the types of working fluids, as well as selection criteria for potential working fluids. Based on these criteria the working fluids selected are n-pentane, n-hexane, ethanol, and isopropyl alcohol. On the basis of their behavior during the compression the working fluids were classified as “wet” (partial condensation: n-pentane and n-hexane) and “dry” (superheating: ethanol and isopropyl alcohol) In the second part of study for batch distillation the effect of the reflux ratio, start-up duration, and the pressure of heating medium are studied on the total energy demand and on the recovery, and then the values of these parameters were selected. In the last part of this study influence of the main operational parameters of the heat pump system was studied, and the effectiveness of different working fluids is compared. For all working fluids the optimal operational parameters are determined, minimizing the payback period time of the heat pump system. Due to the optimization the payback time for the wet working fluids considerably diminished (by 20−40%) while for the dry working fluids only slightly decreased (by 3−8%). We concluded that if the yearly working time reaches 6000 h, then the investment of thte heat pump system for batch distillation process returns within less than 6 years. The lowest payback period (4.8 year) was obtained for n-hexane. For the representative binary system significant savings in energy consumption and CO2 (GHG) emission was achieved by the vapor compression in comparison with the conventional standalone batch column.

Investment Cost of the Compressor [$]. The investment cost is estimated by the formulas of Douglas46 ⎛ M&S ⎞ 0.82 ⎟ · 517.5 · (1.34·MP) ·(2.11 + Fd) IC = ⎜ ⎝ 280 ⎠

(1)

where M&S is the Marshall-Swift index (1483 for the year 2012), MP is the motor power [kW], and Fd = 1. Depreciation Costs [$/Y]. Compressor manufacturers recommend for a compressor in continuous operation a general maintenance (complete renewal of the compressor) after 10 years of operation, which is rather costly. In our cases, the compressor works 4000 hours/year or 6000 hours/year, so the general maintenance can be extended to over 15 or 12.5 years. Accordingly, the compressor lifetime (CLT) is 15 or 12.5 years.

⎛ IC ⎞ ⎟ DC = ⎜ ⎝ CLT ⎠

(2)

Yearly Saving [$/Y]. ⎛ YOT ⎞ ⎡ ⎟ · SQ YS = ⎜ ·UP − SPcomp ·UPel ⎤⎦ − DC ⎝ BOT ⎠ ⎣ BD steam

(3)

where YOT is the yearly operation time (4000 h/Y or 6000 h/Y), BOT is the batch operation time (h), SQ BD is the total heat demand of the BD process [GJ] for one batch, UPsteam is the unit price of heating (steam, 30 $/GJ), SPcomp is the electricity demand of the compressor for the BD-VC process [GJ], and UPel is the unit price of electricity (30 $/GJ). Payback Period [Y]. PBP = J

IC YS

(4) DOI: 10.1021/ie504023q Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article


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Table A2. Electricity Emission Factor for Different Countries47 country

■

electricity emission factor (g CO2/kWh)

France

79

Hungary

317

Italy

406

Japan

416

Spain

238

U.K.

457

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS This work was supported by the Hungarian Research Funds (OTKA, project No.: K-106268). NOTATION BOT = batch operation time [min] DC = depreciation cost [$/Y] h = molar enthalpy [J/mol] IC = investment cost [$] N = number of theoretical stages p0 = pure component vapor P = pressure [bar] PBP = payback period [Y] Q = heat duty [kJ/h] SMP = total electric energy demand of the compressor [MJ] SPr = total product [mol] SQ = heat demand [MJ] T = temperature [°C] V = vapor mass flow rate [kg/h] x = liquid mole fraction [-] y = vapor mole fraction [-] YS = yearly savings [$/Y] ΔP = pressure drop [bar] ΔTs = presuperheating [C] η = recovery of n-heptane [%]

Abbreviations

BD batch distillation VC vapor compression Subscripts

bp i in out r st rel WF

■

bubble point component inlet outlet reboiler start-up process step relative working fluid

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Vapor Compression for Batch Distillation - American Chemical Society

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