Energy Efficiency Limitations of the Conventional Heat Integrated

Ind. Eng. Chem. Res. 2011, 50, 119–130

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Energy Efficiency Limitations of the Conventional Heat Integrated Distillation Column (HIDiC) Configuration for Binary Distillation† Anirudh A. Shenvi,‡ D. Michael Herron,§ and Rakesh Agrawal*,‡ School of Chemical Engineering, Purdue UniVersity, West Lafayette, Indiana 47907, United States, and Air Products and Chemicals Inc., Allentown, PennsylVania 18195, United States

In the recent literature, a typical heat integrated distillation column (HIDiC), with heat integration at multiple locations between the rectifying and stripping sections, is often claimed to be more energy efficient than a single binary distillation column. However, we find that there are many binary feed separations where HIDiC is less energy efficient than simple heat pump schemes using only one or two heat transfer locations. Furthermore, we show that the energy efficiency of HIDiC cannot be solely decided based on the feed composition or product purities. A better performance indicator is the temperature profile along the height of the rectifying section relative to the corresponding temperature profile in the stripping section. On the basis of the analysis of these temperature profiles, we suggest a novel method to reduce heat transfer locations between the two distillation sections of HIDiC with negligible, if any, negative impact on the overall energy efficiency. 1. Introduction Distillation is the most widely used separation technique, accounting for 95% of all separations in chemical process industries.1 In a conventional distillation column (Figure 1) with a feed, a top product, and a bottom product, heat is added at the bottom of the stripping section, which corresponds to the highest temperature point in the column. Heat is rejected at the top of the rectifying section, and this corresponds to the lowest temperature point in the distillation column. Thus, distillation involves the degradation of heat from a higher temperature level to a lower temperature level in order to perform the work of separation. If heat rejected in the rectifying section of the distillation column is not reutilized, then it reduces the efficiency of distillation. One method to reutilize the heat rejected in the rectifying section of a distillation column is to upgrade and transfer it to the stripping section of the same column. A “totally reversible distillation” is an extreme case of improving the efficiency of distillation. It can be realized by upgrading and transferring heat from an infinitely large number of locations in the rectifying section to an infinitely large number of locations in the stripping section of the column. Such a totally reversible scheme requires an infinitely large number of separation stages as well as compressors and other associated equipment. For practical considerations, it is necessary to have finite separation stages and also limit the number of compressors in a heat integration scheme. The objective of this work is to identify, for a given binary separation, the most energy efficient heat integration scheme using one or at most two compressors and a finite number of separation stages. One of the alternative schemes for upgrading and transferring heat from the rectifying section to the stripping section is the heat pump scheme. The concept of using heat pumps in distillation has been known for a long time.2 The direct vapor † This article is dedicated to Professor Stanley I. Sandler, who has been a great teacher and a mentor to R.A. * To whom correspondence should be addressed. E-mail: agrawalr@ purdue.edu. ‡ Purdue University. § Air Products and Chemicals Inc.

recompression scheme3-5 is the simplest application of heat pumps in distillation. This scheme, as shown in Figure 2, employs a one-point heat transfer from the top of the rectifying section to the bottom of the stripping section. In this scheme, vapor from the top of the distillation column is compressed (in compressor COMP 1) to the desired pressure (hot stream) and condensed against liquid from the bottom of the distillation column (cold stream) in heat exchanger HEX 1. The heat necessary for vaporization of the cold stream is provided by adjusting the condensing temperature and flow rate of the hot stream entering the heat exchanger. Figure 2a shows an arrangement of the heat pump scheme wherein the temperature of vapor from the top of the distillation column is increased in heat exchanger HEX 2 prior to compression. For the same pressure of the hot stream exiting from the compressor, this leads to higher temperature and, hence, a higher enthalpy of the hot stream. The hot stream then transfers heat to the cold stream from the bottom of the distillation column in heat exchanger HEX 1. The heat transfer in HEX 1 condenses the hot stream. Heat exchanger HEX 3 subcools the hot (liquid) stream, leaving HEX 2 by using an external coolant to avoid flashing of the liquid after throttling. Figure 2b shows an alternate arrangement of the heat pump scheme in which the flow rate of vapor from the top of the distillation column is

Figure 1. Conventional distillation column.

10.1021/ie101698f  2011 American Chemical Society Published on Web 11/08/2010

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Ind. Eng. Chem. Res., Vol. 50, No. 1, 2011

Figure 2. Heat pump scheme (without intermediate exchangers): (a) increasing enthalpy; (b) increasing flow rate.

increased. The hot stream leaving heat exchanger HEX 1 is not subcooled and is allowed to flash after passing through a throttle valve, thus generating some vapor flow at the top of the distillation column. The vapor needed to vaporize liquid from the bottom of the distillation column is compressed, while the remaining vapor is condensed by using an external coolant in heat exchanger HEX 2. The heat pump scheme (Figure 2) requires heat to be pumped from the lowest temperature point to the highest temperature point in the distillation column. However, the introduction of intermediate reboilers and (or) intermediates condensers into the heat pump scheme results in some portion of the total heat being pumped over a temperature range lower than the maximum temperature range existing in the distillation column. Consequently, this has a potential to increase the thermodynamic efficiency compared to the heat pump scheme without intermediate exchangers. Such schemes with intermediate heat exchangers have been described and analyzed by several researchers.6-11 On the basis of thermodynamic efficiency analysis, Agrawal and Herron10,11 have provided guidelines for the incorporation of intermediate exchangers into a binary

distillation column. It must be noted that the irreversibilities in distillation can be significantly reduced by using just one intermediate reboiler or one intermediate condenser.10-13 A subsequent increase in the number of intermediate exchangers has diminishing operating benefits but increases the capital cost significantly. Hence, it is generally sufficient to limit the heat pump schemes to having just one intermediate reboiler (IR) and (or) one intermediate condenser (IC). Figure 3 shows a heat pump scheme with an intermediate reboiler in the stripping section, and Figure 4 shows a heat pump scheme with an additional intermediate condenser in the rectifying section. In Figure 3, vapor from the top of the column is compressed in two stages. After the first stage of compression (through compressor COMP 1), a portion of the compressed vapor is condensed in the intermediate reboiler, shown as heat exchanger HEX 5. The heat exchange in the intermediate reboiler consequently vaporizes some liquid at an intermediate location in the stripping section, thus providing boilup at that location in the column. The remaining vapor goes through the second stage of compression (in compressor COMP 2) and is condensed against vaporizing liquid from the bottom of the

Figure 3. Heat pump scheme with an intermediate reboiler (IR) in the stripping section.

Figure 4. Heat pump scheme with an intermediate reboiler (IR) in the stripping section and an intermediate condenser (IC) in the rectifying section.


Figure 5. HIDiC.

column in heat exchanger HEX 1. Heat exchanger HEX 2 preheats vapor from the top of the column and the two heat exchangers, HEX 3 and HEX 4, are subcoolers that use external coolant. The schematic of a heat pump scheme with both an intermediate reboiler and an intermediate condenser (Figure 4) is similar to that with just an intermediate reboiler (Figure 3). However, in Figure 4, there is an additional heat exchange in the intermediate condenser shown as heat exchanger HEX 6, which condenses some vapor at an intermediate location in the rectifying section. An internally heat integrated distillation column scheme, referred to as HIDiC or iHIDiC, provides another alternative approach for heat integration of the rectifying and stripping sections.14,15 Figure 5 shows a typical schematic representation of the HIDiC scheme. In the HIDiC setup, the rectifying section operates at a higher pressure than the stripping section. Vapor leaving the stripping section is compressed (in compressor COMP 1) and introduced at the bottom of the rectifying section. Liquid from the bottom of the rectifying section is fed back to the stripping section after pressure equalization using a throttle valve. A sufficient pressure ratio between the rectifying and stripping sections enables the temperature of a portion of the rectifying section to be higher than that of a corresponding portion of the stripping section, thus creating a temperature driving force for heat transfer between the two sections. The heat transfer in the HIDiC extends throughout the length of each section, resulting in continuous condensation in the rectifying section and continuous evaporation in the stripping section. As the amount of heat transfer along the length of the two sections is increased, the bottom reboiler and top condenser duties are consequently reduced. Distillation without external reboil or (and) external reflux can be theoretically achieved in the HIDiC scheme by appropriately adjusting the feed state, the pressure ratio between columns, and the amount of heat transfer between the rectifying and stripping sections. The early concept of the HIDiC scheme was proposed by Haselden14,16 for gas separation, thus introducing the use of distributed heating and cooling sources over extended zones of the distillation column. He suggested that using such distributed heating and cooling sources would be most effective for separations with less than 98% purity requirements of the product streams.14 In a U.S. patent,16 Haselden also described an apparatus that is assembled with a material called overflow packing to transfer heat from the rectifying section to the stripping section. Later on, other such devices for heat integration of the rectifying and stripping sections have been disclosed in patents.17-23 Herron et al.,20 for example, used a multiplepassage plate-fin heat exchanger with two adjacent passages functioning as the rectifying and stripping sections. Their setup

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additionally comprises a condensing section, located spatially above the rectifying section, in thermal contact with the stripping section. This invention provides a low operating cost method for producing oxygen from air. The HIDiC scheme differs from the heat pump schemes in the way heat transfer is set up between the rectifying and stripping sections. A simple heat pump scheme without intermediate exchangers has the top of the rectifying section thermally connected with the bottom of the stripping section. The intermediate exchangers in a heat pump scheme can be placed along the length of the distillation column at locations that provide the most energy benefits. On the other hand, a typical HIDiC scheme (Figure 5) is restricted in its heat transfer arrangement, with heat being transferred from the top of the rectifying section to the top of the stripping section, between intermediate locations all along the length of each section, and finally from the bottom of the rectifying section to the bottom of the stripping section. HIDiC and heat pump schemes clearly differ in the number of locations that are heat integrated between the rectifying and stripping sections. A heat pump scheme with even a couple of intermediate exchangers has heat integration at much fewer locations that the HIDiC scheme. Further, because of the larger number of heat integration locations, the HIDiC scheme could become more difficult to operate and control while retaining its energy efficiency compared to the heat pump schemes. The simpler column design and ease of operation and control make the heat pump schemes attractive if they can be nearly as energy efficient as HIDiC. If the heat pump schemes are more efficient than the HIDiC scheme, the choice of options becomes obvious. Mah et al.24 and Fitzmorris and Mah25 referred to the HIDiC scheme as secondary reflux and vaporization (SRV) distillation and indicated its potential for reduction in the utility requirement over the conventional distillation column (Figure 1). Several studies based on user-written models,26-28 commercial process simulators,29-31 such as Aspen Plus, and pilot-plant32 and benchscale experiments33 have claimed energy savings of up to 60% from the HIDiC scheme compared to the conventional distillation column. Nakaiwa et al.15 presented an overview of research that demonstrates the benefits of the HIDiC scheme over the conventional distillation column. While the HIDiC scheme has been extensively compared in the literature with the conventional distillation column, there have been few prior attempts to compare the energy efficiency of the HIDiC and heat pump schemes. These attempts29,31,34 have limited comparisons of the HIDiC scheme solely with the heat pump schemes without intermediate exchangers and for specific feed and process conditions. There is clearly a need to systematically compare the HIDiC scheme and the heat pump schemes, both with and without intermediate exchangers, and provide a general approach to identify the most energy efficient heat integration strategy. In this work, we compare the energy efficiency of the different heat integration alternatives between the rectifying and stripping sections for nonazeotropic binary distillations. We systematically determine if the large number of heat integration locations in a typical HIDiC always translates into increased benefits compared with the simple heat pump schemes. We also demonstrate how the temperature profiles in the HIDiC scheme can be used to determine the most favorable match of locations to transfer heat from the rectifying section to the stripping section. This strategy of matching the temperature profiles is generally applicable for any separation, and it also helps to identify energy efficient heat integration approaches.

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Figure 6. Illustrating symmetric composition change in the rectifying and stripping sections: a liquid-vapor composition diagram.

2. HIDiC versus Heat Pump for Symmetrical Temperature Profiles The shape of the temperature profile in the rectifying section relative to that in the stripping section depends on the composition of the feed mixture, its thermodynamic properties, and the purity of the product streams. As will be demonstrated in this and subsequent sections, the degree to which the shapes of the temperature profiles “match-up” determines how effective the HIDiC schemes are in reducing energy consumption. In this section, we analyze the separation of nonazeotropic binary mixtures with feed and product specifications that result in a symmetrical composition change in the rectifying and stripping sections (Figure 6). This situation is most favorable for the HIDiC scheme. The focus of this section is to compare the energy efficiency of the simple heat pump schemes with that of the HIDiC scheme. The common bases chosen to compare all of the heat integration schemes in this article are as follows: (a) Same feed and product conditions: The feed as well as the top and bottom products in all of the heat integration schemes have the same flow rate, composition, pressure, and vapor fraction. This ensures that we are comparing schemes performing the same degree of separation. (b) Same temperature approach: All of the heat integration schemes have heat transfer from the rectifying section to the stripping section with the same minimum temperature difference (∆Tmin) between the hot and cold streams. (c) Zero external reboiler heat duty: The amount of heat transfer from the rectifying section to the stripping section in all of the schemes is appropriately adjusted so as to provide the entire boilup required to effect the separation. We use Aspen Plus software to simulate the different heat integration schemes. All simulations are carried out using the stage-by-stage distillation model RADFRAC. In the simulations, we assume no pressure drop in the distillation columns. Also, the cooling utility requirement in the heat integration schemes is assumed to be satisfied by cooling water. Because the basis of “zero external reboiler heat duty” ensures that there is no heating utility requirement for separation, the energy efficiency of a heat integration scheme is thus determined only by the compressor power requirement. 2.1. Profile 1. Profile 1 (Figure 7) shows a set of temperature profiles in the rectifying and stripping sections of the HIDiC scheme. The stage temperatures in the respective sections are plotted against the normalized stage location in Figure 7. The HIDiC scheme with profile 1 has an equal number of stages in the rectifying and stripping sections, and all of the stages are thermally integrated. Each stage of the rectifying section

Figure 7. Profile 1 (T ) top end; B ) bottom end).

transfers heat to a corresponding stage of the stripping section. For instance, the top stage 1 of the rectifying section transfers heat to the top stage 1 of the stripping section, stage 2 of the rectifying section transfers heat to stage 2 of the stripping section, and so on. In simulation terms, heat transfer from the rectifying section to the stripping section is done using heat streams. A set of heat streams withdraw heat from the rectifying section, and an equivalent number of heat streams add the same amount of heat to the stripping section. It must be noted that heat integration between the rectifying and stripping sections in the HIDiC scheme does not necessarily imply heat exchange across all of the stages. If the rectifying and stripping sections have an unequal number of stages, then part of the two sections is heat integrated whereas the remaining part performs as a conventional column.34-36 Recently, Chen et al.37 have approximated the HIDiC design with three heat integrated stages between the rectifying and stripping sections. They showed through a case study that, instead of heat integration throughout each section, the same benefit of the HIDiC scheme can be obtained by having heat integration at just three locations between the two sections. However, for all HIDiC simulations in this paper, we have chosen the rectifying and stripping sections to have an equal number of stages with heat integration across all of the stages. The temperature profiles given by profile 1 are observed in the HIDiC scheme for the separation of a benzene-toluene feed mixture. The feed for such a case consists of an equimolar mixture of benzene and toluene. The feed flow rate, feed pressure, and feed quality are 100 kmol/h, 1 bar, and 0.5, respectively. The top product is drawn as a vapor, and the bottom product is a liquid. Profile 1 is observed for the top product having 95% benzene and the bottom product having 95% toluene. Such feed and product specifications result in a symmetrical composition change in the rectifying and stripping sections (Figure 6). The symmetrical composition change in the two sections is reflected in the symmetric nature of the temperature change in the rectifying and stripping section temperature profiles (Figure 7). In the HIDiC scheme for the benzene-toluene feed mixture giving profile 1, the stripping section operates at 1 bar pressure. The pressure ratio between the rectifying and stripping sections and the amount of heat transfer at each stage are iteratively adjusted so as to reduce the external reboiler heat duty to zero and also to ensure that the heat exchange network operates at the minimum temperature approach. Here, the chosen minimum temperature approach between the rectifying section (hot) and the stripping section (cold) profiles is 3 °C (that is, ∆Tmin ) 3 °C). In the HIDiC scheme, an equal amount of heat can be


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a

Table 1. Simulation Results for Profile 1

total number of stages compressor: inlet/outlet pressure (bar); flow rate (kmol/h) COMP 1 COMP 2 power consumption (kW) to upgrade heat (COMP 1 + COMP 2) to pressurize distillate to 3 bar total relative a

heat pump without IR or IC

heat pump with IR

heat pump with IR and IC

HIDiC

100

100

100

200

1/2.33; 80.28

1/1.96; 80.27 1.96/2.33; 47.83

1/1.93; 50.72 1.93/2.33; 50.72

1/1.64; 135.65

79.67 62.79 142.46 1.00

72.78 62.79 135.57 0.95

67.93 62.79 130.72 0.92

77.59 35.40 112.99 0.79

100 kmol/h of an equimolar benzene-toluene feed mixture with feed quality ) 0.5, feed pressure ) 1 bar, and each product at 95% purity.

transferred across all of the heat integrated stages or the amount of heat transfer can be proportional to the temperature difference between the stages.36,38 The latter approach is a more practical one because it is equivalent to having equal heat transfer areas across all of the heat integrated stages under the assumption of a constant overall heat transfer coefficient. In this article, the latter approach of heat transfer is used in all of the HIDiC simulations. Gadalla39 and Suphanit36 have presented detailed descriptions of the simulation procedure for the HIDiC scheme. Our procedure, although developed independently, follows the same algorithm as that described in the literature, and hence we have not described it in detail in this work. The heat pump schemes (with and without intermediate exchangers) are simulated for the benzene-toluene separation example with the same feed and product specifications as those used for the HIDiC simulation. The rectifying and stripping sections in the heat pump schemes operate at a pressure of 1 bar. In all of the heat pump simulations in this paper, the rectifying and stripping sections have an equal number of stages. The heat pump schemes are also simulated such that the external reboiler heat duty is reduced to zero and the minimum temperature difference between the hot and cold streams in the heat exchangers is maintained at 3 °C. In the heat pump schemes with intermediate exchangers, the location and amount of heat transfer in the intermediate exchangers are optimized to minimize the power consumption. In order to have enough stages to account for the reversibility in the different heat integration approaches, a sufficiently large number of stages is chosen for the simulations. Also, compressors in all of the simulations are adiabatic and operate at 75% isentropic efficiency. The vapor-liquid equilibrium data are obtained using the NRTL thermodynamic model. In the simulations, the rectifying section in the HIDiC scheme operates at a higher pressure than the rectifying section in the heat pump scheme. Thus, the top product from the HIDiC scheme will be produced at a higher pressure. To ensure that the top product is at the same pressure, the top product in all of the heat integration schemes is pressurized to a common pressure of 3 bar. Table 1 summarizes the simulation results obtained for the benzene-toluene separation example with profile 1 in the HIDiC scheme. The total number of stages is the sum of the number of stages in the rectifying and stripping sections of the heat integration schemes. In the heat pump scheme without intermediate exchangers (Figure 2) and the HIDiC scheme (Figure 5), a single compressor (COMP 1) upgrades heat from the rectifying section to transfer to the stripping section. Heat pump schemes with intermediate exchangers (Figures 3 and 4) require two compression stages (COMP 1 and COMP 2). Additionally, to obtain the top product at a pressure of 3 bar, another compressor is required in all of the heat integration schemes (this additional compressor is not shown in Figures 2-5). The total power consumption for a heat integration scheme is the


summation of all of the compressor powers. Also, it was observed that the arrangement of the heat pump scheme without intermediate exchangers shown in Figure 2a consumes less power than the arrangement shown in Figure 2b. Hence, the results reported in Table 1 are for the heat pump scheme shown in Figure 2a. For the benzene-toluene separation example with profile 1 in the HIDiC scheme, HIDiC consumes around 13-21% lower power compared to the heat pump schemes and thus provides considerable savings (Table 1). In profile 1, the temperature profiles in the rectifying and stripping sections of the HIDiC scheme are almost parallel to each other. The nearly parallel temperature profiles in the two sections ensure that heat is reversibly transferred at all locations in the HIDiC scheme across temperature differences that are close to the minimum allowed (3 °C). This efficient stagewise heat transfer in the HIDiC scheme makes it the preferred scheme for heat integration. It must be noted that benzene-toluene feed separation is just a representative example to obtain parallel temperature profiles of the kind shown in Figure 7. However, profile 1 is generally observed for the separation of other binary mixtures with a symmetrical composition change in the stripping and rectifying sections and a low product purity requirement. The low product purity requirement is defined as the product purity that results in reasonable stage-to-stage composition changes and hence the corresponding temperature changes in the product zones of each of the distillation sections. Also, for any such separation with parallel HIDiC temperature profiles, the HIDiC scheme will be the most favorable heat integration scheme. The observation that HIDiC is efficient for low purity products is in agreement with the original suggestion by Haselden.14 2.2. Profile 2. Profile 2 (Figure 8) shows a symmetric temperature change in the rectifying and stripping sections. Similar to profile 1 (Figure 7), profile 2 is also observed in the separation of binary mixtures with a symmetrical composition change in the rectifying and stripping sections. However, profile

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Table 2. Simulation Results for Profile 2a

total number of stages compressor: inlet/outlet pressure (bar); flow rate (kmol/h) COMP 1 COMP 2 power consumption (kW) to upgrade heat (COMP 1 + COMP 2) to pressurize distillate to 3 bar total relative a


heat pump with IR

heat pump with IR + IC

HIDiC

alternate strategy

100

100

100

200

200

1/2.52; 92.52

1/2; 92.61 2/2.52; 63.01

1/2.02; 62.31 2.02/2.52; 62.31

1/1.92; 151.83

1/1.94; 148.20

99.70 62.45 162.15 1.00

91.55 62.45 154.00 0.95

84.29 62.45 146.74 0.91

115.11 26.27 141.38 0.87

113.91 25.76 139.67 0.86

100 kmol/h of an equimolar benzene-toluene feed mixture with feed quality ) 0.5, feed pressure ) 1 bar, and each product at 99.5% purity.

2 shows the presence of large temperature differences between the hot and cold profiles at the two ends and the heat transfer pinch is observed close to the center of the profile. Because of these large temperature differences, stagewise heat transfer in the HIDiC scheme results in some of the heat being transferred over a temperature difference greater than the chosen minimum temperature approach (∆Tmin ) 3 °C) for the heat exchange. This reduces the efficiency of heat transfer in the HIDiC scheme compared to profile 1 (Figure 7), which has all the heat transfer across nearly constant temperature driving force. In profile 2, the large temperature differences at each of the ends are due to the constant temperature (flat) portions at the top end of the rectifying section and at the bottom end of the stripping section, that is, near the product withdrawal locations in the rectifying and stripping sections, respectively. A flat portion results when the corresponding product purity is relatively high such that at the product end stage-to-stage composition changes are small and a significant number of stages at the end are dedicated to increasing the product purity. Because of small composition changes over a substantial height of the distillation section, the corresponding temperature change is also small. The benzene-toluene feed used for profile 1 was also used to generate profile 2, with the only difference that the top and bottom products contained 99.5% benzene and 99.5% toluene, respectively. The simulation results for various heat integration schemes are provided in Table 2. For this case, the heat pump scheme with intermediate reboiler consumes 5% less power compared to the heat pump scheme without intermediate exchangers. An additional 4% power benefit is obtained by introducing an intermediate condenser in the heat pump scheme along with the intermediate reboiler. The HIDiC scheme consumes 13% less power than the heat pump scheme without intermediate exchangers. Although the HIDiC scheme lowers the power requirement by a further 4% compared to the heat pump scheme with intermediate reboiler and intermediate condenser, the number of heat integration locations required to obtain this benefit is significantly large. A major fraction of the power benefits of the HIDiC scheme is obtained by introducing a couple of intermediate exchangers into the heat pump scheme. Also, note that, in order to compare the heat integration schemes on the same basis, we chose to pressurize the distillate to a common pressure of 3 bar. In the heat pump schemes, the vapor distillate was compressed from 1 to 3 bar, whereas in the HIDiC scheme, the vapor distillate was compressed from 1.92 to 3 bar. However, if we were not to compress the distillate and instead obtain the distillate from the heat pump and HIDiC schemes at the corresponding rectifying section pressure, then the power in the heat integration schemes would be consumed to only upgrade the heat from the rectifying section (first row under the power consumption section in Table 2). The simula-

Figure 9. Effect of the total number of stages on the power consumption of the HIDiC scheme and heat pump scheme without intermediate exchangers.

tion results, for such a scenario, show that the heat pump schemes consume less power than the HIDiC scheme. Thus, depending on the pressure at which the distillate is required for downstream processing, heat pump schemes could be the better heat integration alternative. Thus, from the above analysis for profile 2, we can conclude that capital and operability cost considerations as well as downstream requirements are likely to make the simpler heat pump scheme worth considering as an attractive heat integration option. In the HIDiC scheme, reduction in the heat transfer efficiency due to heat transfer across large temperature differences translates into reduced power benefits for this case. Although simulations were done only for the benzene-toluene feed example, the results should be applicable to any other separation example with profile 2 in the HIDiC scheme. As an additional study, for the same benzene-toluene separation example giving profile 2 in the HIDiC scheme, the power consumption of the HIDiC scheme and the heat pump scheme without intermediate exchangers was compared as a function of the total number of stages in the two distillation sections. Such a study shows that HIDiC benefits are observed only with a large number of separation stages in each section (Figure 9). The HIDiC scheme with less than 60 total stages (roughly) consumes more power than the heat pump scheme without intermediate exchangers with the same total number of stages. HIDiC performs better in comparison to the heat pump scheme without intermediate exchangers when it has more than 60 total stages. This indicates that, for HIDiC temperature


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3. Matching the Temperature Profiles for Improved Efficiency

Figure 10. Profile 3 (T ) top end; B ) bottom end). Table 3. Simulation Results for Profile 3a

total number of stages compressor: inlet/outlet pressure (bar); flow rate (kmol/h) COMP 1 power consumption (kW) to upgrade heat (COMP 1) to pressurize distillate to 3 bar total relative


HIDiC

100

200

1/2.54; 94.00

1/2.54; 161.79

102.16 62.42 164.58 1.00

176.66 10.11 186.77 1.13

a 100 kmol/h of an equimolar benzene-toluene feed mixture with feed quality ) 0.5, feed pressure ) 1 bar, and each product at 99.9999% purity.

profiles given by profile 2, a heat pump becomes a more efficient heat integration option than the HIDiC scheme when fewer separation stages are used. 2.3. Profile 3. The HIDiC temperature profile given by profile 3 (Figure 10) has a symmetric temperature change in the rectifying and stripping sections and also has flat portions in the profile that are similar to those of profile 2 (Figure 8). However, in profile 3, the temperature does not change in nearly half of the rectifying section and in more than half of the stripping section. The separation of mixtures into products with ultrahigh purity requirements lead to such extended flat portions in the temperature profile. An example providing the shape of the temperature profiles in the HIDiC scheme shown in profile 3 is the separation of an equimolar benzene-toluene feed mixture used for profiles 1 and 2, with top and bottom products now containing 99.9999% benzene and 99.9999% toluene, respectively. For this example, simulations show that the HIDiC scheme consumes 13% more power than the heat pump scheme without any intermediate exchangers (Table 3). Thus, here we see that a simple heat pump scheme is able to outperform the HIDiC scheme. For separations with HIDiC temperature profiles given by profile 3, stagewise heat transfer of the HIDiC scheme across very large temperature differences between the hot and cold profiles results in a more pronounced heat transfer inefficiency compared to that of separations with profile 1 (Figure 7) or profile 2 (Figure 8). Therefore, for such separations with profile 3, the HIDiC scheme is not suitable and one should consider the simple heat pump schemes for better energy efficiencies.

Profile 1 (Figure 7) is best suited to stagewise heat transfer because heat is reversibly transferred at all of the locations across temperature differences that are close to the minimum allowable temperature difference for heat transfer. On the other hand, in profile 2 (Figure 8) and profile 3 (Figure 10), stagewise heat transfer is inefficient because of the irreversibility associated with a sizable fraction of the heat transfer across large (greater than the minimum allowable) temperature differences between the hot and cold regions. In this section, we analyze profiles 2 and 3 to determine the most favorable way to heat integrate the rectifying and stripping sections for such profiles. This analysis is directed toward matching the temperature profiles in the rectifying and stripping sections to ensure efficient heat transfer. 3.1. Analysis of Profile 2. In profile 2 (Figure 8), points 1-4 are located on the rectifying section temperature profile and points 5-8 are located on the stripping section temperature profile. Note that, because of the higher pressure of the rectifying section, its temperature profile is “hot” and that of the stripping section is “cold”. There are three distinct heat integration zones between the rectifying and stripping sections: zones 1-2-5-6, 2-3-6-7, and 3-4-7-8. In the HIDiC scheme, stagewise heat transfer in zone 2-3-6-7 between portion 2-3 along the rectifying section and portion 6-7 along the stripping section is across temperature differences that are close to the chosen minimum temperature difference of 3 °C. However, stagewise heat transfer is across large temperature differences in zones 1-2-5-6 and 3-4-7-8. We propose an alternate strategy to transfer heat efficiently between the rectifying and stripping sections of profile 2. In this strategy, heat is transferred from a hot location in the rectifying section to a corresponding cold location in the stripping section following two criteria: (i) heat is always transferred across a temperature difference that is close to the minimum allowable temperature difference; (ii) heat is exchanged between the highest point feasible in the rectifying section and the lowest point allowed in the stripping section. The use of the second criterion generates more reflux at the highest location in the rectifying section and more boilup closer to the bottom of the stripping section. This alternate strategy is indicated by arrows in Figure 8. The efficient stagewise heat transfer in zone 2-3-6-7 of profile 2 is retained in this alternate strategy. However, in zones 1-2-5-6 and 3-4-7-8, stagewise heat transfer of the HIDiC scheme is replaced by single-point heat transfer. For instance, in zone 3-4-7-8 of profile 2, the temperature difference between location 3 in the rectifying section and location 7 in the stripping section is nearly the chosen minimum temperature difference (∆Tmin ) 3 °C). Also, because portion 7-8 of the stripping section temperature profile is nearly flat (constant temperature), the temperature difference between location 3 and all of the locations along portion 7-8 is also about 3 °C. Furthermore, because the temperature rises from location 3 to location 4, all of the heat that needs to be transferred from the rectifying section 3-4 to the stripping section 7-8 can be transferred from location 3 to location 8. In other words, additional vapors are allowed to rise in the rectifying section 3-4, collected at a higher location 3, and then condensed with the liquid at the bottom of the stripping section at location 8. This leads to higher reflux in portion 3-4 of the rectifying section and more boilup near the bottom of the stripping section 7-8. Similarly, the single-point heat transfer in zone 1-2-5-6 between locations 1 and 6 is also across the minimum temperature approach. This leads to a

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Figure 11. Schematic of the alternate heat integration strategy for profile 2.

Figure 12. Schematic of the alternate heat integration strategy for profile 3.

higher liquid to vapor (L/V) flow rate ratio in the rectifying section 1-2 and is helpful in producing a relatively pure distillate. Single-point heat transfer offers a dual advantage compared to the stagewise heat transfer. Instead of the distribution and transfer of heat stagewise across large temperature differences, heat is now reversibly transferred between the rectifying and stripping sections with single-point heat transfer. Also, singlepoint heat transfer generates vapor flow at the bottommost location of that particular zone in the stripping section and liquid flow at the corresponding feasible topmost location in the rectifying section. This maximizes the potential of the vapor and liquid generated in the respective sections to do work of separation. To illustrate the efficacy of the proposed heat integration strategy for profile 2, this alternate strategy was simulated for the same benzene-toluene separation example as that used in the simulation of the HIDiC scheme. The schematic of the alternate heat integration strategy is shown in Figure 11. In the alternate strategy, the stripping and rectifying sections have 100 stages each. The alternate strategy has stagewise heat transfer from stages 9 to 80 of the rectifying section to the corresponding stages of the stripping section, and the amount of heat transfer is proportional to the temperature difference. These stages fall roughly within zone 2-3-6-7 (the parallel portions) of profile 2. Additionally, heat is transferred from stage 1 (location 1 in profile 2) of the rectifying section to stage 8 (location 6 in profile 2) of the stripping section and from stage 81 (location 3 in profile 2) of the rectifying section to stage 100 (location 8 in profile 2) of the stripping section. The simulation of the alternate strategy conforms to the common bases chosen for comparison of the heat integration schemes. The results obtained for the alternate strategy are provided in the rightmost column of Table 2. The alternate strategy, with fewer heat integration locations than the HIDiC scheme, consumes 1% less power compared to the HIDiC scheme. The increased stage-to-stage irreversibility limits the energy savings obtained from the alternate strategy, as will be discussed in the subsequent section (section 3.2). Although the power benefit obtained using the alternate strategy is not significantly large, this example clearly demonstrates the potential benefits due to appropriate matching of the temperature profiles in the rectifying and stripping sections. 3.2. Analysis of Profile 3. The temperature profiles in the rectifying and stripping sections of profile 3 (Figure 10) can also be matched to transfer heat efficiently. Similar to the alternate heat integration strategy employed for profile 2, singlepoint heat transfer between locations 3 and 8 in zone 3-4-7-8

and between locations 1 and 6 in zone 1-2-5-6 avoids irreversibility because of heat transfer across large temperature differences. However, in profile 3, portion 2-3 is nearly flat and this portion in the rectifying section has temperatures that are similar to the temperature at the top of the rectifying section at location 1. Thus, from our second criterion, all of the heat that needs to be transferred from any location in portion 1-3 should be transferred from location 1. This leads to the conclusion that the transfer of heat from the top of the stripping section is equivalent to the gradual transfer of heat from various locations of the rectifying section to the stripping section. Similarly, in the stripping section, the temperature levels in portion 6-7 are similar to the temperature at the bottom location 8. Thus, all of the heat that needed to be transferred to portion 6-7 can be directly transferred to the bottom of the stripping zone. This means that for profile 3 if we were to build a distillation column system similar to that of HIDiC, with the stripper at a lower pressure than the rectifying section, then rather than exchanging heat at multiple places between the two sections, one could transfer all of the heat in one direct heat exchange from the top of the rectifying section to the bottom of the stripping section. This will greatly simplify the equipment arrangement and is shown in Figure 12. The only transfer of heat in heat exchanger HEX 1 (Figure 12) from the top of the high pressure rectifying section to the bottom of the low pressure stripping section has other ramifications besides equipment simplification. Because all of the heat is transferred across the minimum acceptable temperature difference, the total exergy loss with the heat transfer is minimized. However, the losses appear now in terms of increased vapor and liquid flow rates in the majority of the zones in the two distillation columns. The increased vapor and liquid traffic will lead to a reduced number of stages and greater stageto-stage irreversibility. If this HIDiC arrangement is left intact, the energy savings may not be significant, as was observed with the alternate strategy for profile 2 (Table 2). However, the fact that the heat in Figure 12 is to be transferred from the top of the rectifying section, one is no longer constrained to operate the entire rectifying column at high pressure. The two sections may be operated at the same pressure, and only the amount of vapor needed for reflux and boilup needs to be compressed. This avoids compression of the entire vapor that flows through the rectifying section. Clearly, one would expect power savings from the simple heat pump scheme compared to the HIDiC-equivalent scheme in Figure 12. The simulations of the heat pump and HIDiC schemes for the example problem depicting profile 3 have already shown that the simple heat pump scheme is the more preferred heat


Figure 13. Illustration of an unsymmetrical composition change in the rectifying and stripping sections: a liquid-vapor composition diagram.


integration alternative (Table 3). Additionally, on the basis of the guidelines provided by Agrawal and Herron,10,11 intermediate exchangers can be incorporated in the heat pump scheme to further provide potential benefits. 4. Unsymmetrical Temperature Profiles and HIDiC Performance In section 2, we compared the HIDiC scheme with the heat pump schemes for symmetric HIDiC temperature profiles. In this section, we compare the HIDiC scheme with the heat pump schemes for separating binary mixtures having unsymmetrical HIDiC temperature profiles. The basis for comparison of the heat integration schemes is the same as that chosen earlier for the examples with symmetric HIDiC temperature profiles. An unsymmetrical temperature change in the rectifying and stripping sections is observed in the HIDiC scheme for the separation of binary mixtures with an unsymmetrical composition change (Figure 13) in the two sections. 4.1. Profiles 4 and 5. HIDiC temperature profiles, profile 4 (Figure 14) and profile 5 (Figure 15), have more temperature change in the stripping section than in the rectifying section. This indicates that there is more separation or more composition change in the stripping section than in the rectifying section. These unsymmetrical temperature profiles also have a large temperature difference between the hot and cold profiles at the top ends and a relatively small temperature difference at the bottom ends of the two distillation sections. The heat transfer efficiency in the HIDiC scheme is reduced because of the irreversibility associated with stagewise heat transfer across such unsymmetrical profiles. Further, profile 4 has flat portions near the top and bottom product withdrawal locations, whereas such flat portions are not seen in profile 5.

127


A benzene-toluene feed mixture is chosen as a representative example to simulate the HIDiC scheme with profiles 4 and 5. The benzene-toluene feed consists of 75% benzene, and the feed flow rate, feed pressure, and feed quality are 100 kmol/h, 1 bar, and 0.25, respectively. The top product is drawn as a vapor and the bottom product as a liquid. The product compositions giving profile 4 are 99.5% in both the top and bottom products, respectively, and the product compositions giving profile 5 are 95% in both of the products. The product purity for obtaining profile 4 is high enough to ensure flat portions in the HIDiC profile at the product withdrawal locations. The heat pump schemes are also simulated with the specified feed and product specifications for the benzene-toluene mixtures. The Aspen Plus simulation details for all of the heat integration schemes are the same as those described for the benzene-toluene feed mixture giving profiles 1-3. The results for the case with profile 4 show that the heat pump scheme with one intermediate reboiler provides more power savings compared with the HIDiC scheme (Table 4). The heat pump scheme with one intermediate reboiler consumes around 5% less power than the HIDiC scheme. Even for the case with profile 5, the HIDiC scheme is only slightly more energy efficient than the simpler heat pump scheme with just one intermediate reboiler (Table 5). The use of one more intermediate reboiler should bring the power consumption for the heat pump scheme in par with that of HIDiC. Furthermore, the number of stages used in the HIDiC scheme are far in excess of those used for the heat pump case. Thus, irrespective of the product purity requirement, the heat pump scheme with one or more intermediate reboilers is the favored heat integration scheme for separating mixtures with HIDiC temperature profiles 4 and 5. This result is consistent with the guidelines provided by Agrawal and Herron10,11 for introducing intermediate reboilers in a distillation column. They suggested the use of intermediate reboilers for separations with more temperature change in the stripping section and intermediate condensers for separations with more temperature change in the rectifying section. It should be noted that for HIDiC temperature profile 1 (Figure 7), where the product purity was low at 95%, HIDiC was found to be energy efficient. However, for the same low purities with HIDiC temperature profile 5 (Figure 15), heat pump schemes are found to be at par with HIDiC in energy consumption. Clearly, the applicability of HIDiC cannot be solely determined on the basis of the low purity products but are in conjunction with the shape of the temperature profiles in the rectifying and stripping sections. For unsymmetrical HIDiC temperature profiles such as profiles 4 and 5, it is possible to design a HIDiC system where

128



total number of stages compressor: inlet/outlet pressure (bar); flow rate (kmol/h) COMP 1 COMP 2 power consumption (kW): to upgrade heat (COMP 1 + COMP 2) to pressurize distillate to 3 bar total relative a


heat pump with IR

HIDiC

100

100

200

1/2.52; 83.56

1/1.74; 83.56 1.74/2.52; 43.74

1/2.08; 168.31

90.06 93.99 184.05 1.00

72.05 93.99 166.04 0.90

141.51 32.61 174.12 0.95

100 kmol/h of a 75% benzene-25% toluene feed mixture with feed quality ) 0.25, feed pressure ) 1 bar, and each product at 99.5% purity.


total number of stages compressor: inlet/outlet pressure (bar); flow rate (kmol/h) COMP 1 COMP 2 power consumption (kW): to upgrade heat (COMP 1 + COMP 2) to pressurize distillate to 3 bar total relative a


heat pump with IR

HIDiC

100

100

200

1/2.33; 65.59

1/1.68; 65.59 1.68/2.33; 33.16

1/1.86; 150.22

66.96 97.67 164.63 1.00

53.63 97.67 151.30 0.92

106.13 44.09 150.22 0.91

100 kmol/h of a 75% benzene-25% toluene feed mixture with feed quality ) 0.25, feed pressure ) 1 bar, and each product at 95% purity.

heat is transferred between fewer locations of the two distillation sections without much significant impact on the total energy consumption. For illustration, first consider profile 4 in Figure 14. A horizontal line P1 at a distance of the minimum temperature approach (∆Tmin) from the top end 1 of the rectifying section is drawn. This line intersects the stripping section profile at point 6. Now all of the heat transfers between portions 1-2 and 5-6 can be reduced to the heat transfer from location 1 to location 6. Similarly, a horizontal line P2 at a distance of ∆Tmin from the bottom end 8 of the stripping section is drawn. This line intersects the rectifying section profile at point 3. All of the heat transfer between portions 3-4 and 7-8 can be now transferred between locations 3 and 8. Such fewer heat transfer locations can potentially simplify the HIDiC equipment design as well as reduce the number of stages in the distillation columns. An exercise similar to the one for profile 4 can be done for profile 5 to reduce the number of heat transfer locations between the two columns. Line P1 in Figure 15 gives portions 1-2 and 5-6 that are similar to those in Figure 14. However, heat transfer from location 2 to location 6 is now across a much bigger temperature difference and provides an opportunity to further decrease the number of heat transfer locations. A horizontal line P3 at a distance of ∆Tmin from location 2 on the rectifying section temperature profiles is drawn. This line intersects the stripping section profile at location 7. Now all of the HIDiC heat transfer from portion 2-3 to portion 6-7 can be converted to heat transfer from location 2 to location 7. If needed, this process can be further repeated. Once again the reduced heat transfer locations will make heat transfer between the two distillation columns more efficient but will increase the irreversibility in the distillation columns. It is possible that, for some shape of the HIDiC temperature profile in the two distillation sections, the exercise of matching the temperature profiles might result in a system with just a couple of heat integration locations. For such a case, it would be beneficial to consider heat pump schemes with or without a few intermediate exchangers instead of the HIDiC system with the entire rectifying section operating at a high pressure. In the


heat pump schemes, only the amount of vapor required for reflux and boilup at appropriate locations in the two sections will need to be compressed. This would avoid compression of the entire vapor leaving the stripping section and might make the simple heat pump schemes an attractive option for heat integration by providing savings in energy consumption and capital cost. Finally, it should be mentioned that HIDiC temperature profiles corresponding to profiles 4 and 5 also exist for the case when feed is rich in the heavy component. In such cases, compared to the stripping section, there will be more temperature change in the rectifying section. Now heat pump schemes with intermediate condenser(s) would be the preferred choice compared to the HIDiC scheme.10,11 4.2. Profile 6. Profile 6 (Figure 16) represents an extreme case of an unsymmetrical temperature change in the rectifying and stripping sections of the HIDiC scheme. It shows a relatively constant temperature profile in the rectifying section, while the entire temperature change takes place in the stripping section. In the HIDiC scheme, most of the heat is transferred across a very large temperature difference between the hot and cold curves and a minimum temperature difference is observed at the bottom end of the HIDiC profile.

Ind. Eng. Chem. Res., Vol. 50, No. 1, 2011 a

consideration for heat integration and have not been included in this work.

Table 6. Simulation Results for Profile 6

total number of stages compressor: inlet/outlet pressure (bar); flow rate (kmol/h) COMP 1 power consumption (kW): to upgrade heat (COMP 1) to pressurize distillate to 20 bar total relative

129


HIDiC

232

244

13/17.96; 1159.17

13/17.83; 1582.49

274.68 0.63 275.31 1.00

366.24 0.67 366.91 1.33

a 100 kmol/h of a 93% propylene-7% propane feed mixture with feed quality ) 1, feed pressure ) 13 bar, and products of 99.5% propylene and 96% propane.

Profile 6 is observed in HIDiC schemes for producing polymer-grade propylene from a propylene-propane feed that is rich in propylene. Separation of this binary mixture has been studied by Olujic et al.29,34 Using the data provided by Olujic et al.,29 the propylene-propane feed is a saturated liquid with 93% propylene and a flow rate of 100 kmol/h. The compositions of the top and bottom products are 99.5% and 4% propylene, respectively, and both the products are drawn as liquids. The pressure of the feed and the bottom product is 13 bar, whereas the top product is drawn at 20 bar. The stripping section in the HIDiC scheme is at 13 bar. The minimum temperature difference between the stage temperatures in the rectifying and stripping sections of the HIDiC scheme is 6.55 °C. This same minimum temperature approach is used to design the reboiler/ condenser heat exchangers in the heat pump simulations. The arrangement of the heat pump scheme without intermediate exchangers shown in Figure 2b is used in the simulation. The distillation column pressure for heat pump simulations is 13 bar. The vapor-liquid equilibrium data are obtained using the Peng-Robinson thermodynamic model. The compressors are adiabatic and operate at 75% isentropic efficiency. The pressure of the top product is increased using a pump operating at 75% efficiency. For this extreme case of unsymmetrical temperature profiles, simulations show that the heat pump scheme without intermediate exchangers performs better than the HIDiC scheme and provides 33% power savings (Table 6). The amount of vapor compressed is significantly more in the HIDiC scheme than in the heat pump scheme. As a result, HIDiC consumes more power compared to the heat pump scheme without intermediate exchangers even though the discharge pressure from COMP 1 is lower in the HIDiC scheme for the same inlet pressure of the vapor. For profile 6, we could have reached the same conclusion by merely matching the temperature profiles in the rectifying and stripping sections of the HIDiC scheme. Because there is very little separation in the rectifying section, it is favorable to transfer all of the heat from the top of the rectifying section to the bottom of the stripping section across the minimum allowable temperature difference (indicated by an arrow in Figure 16). This situation is similar to the one discussed for profile 3 in Figure 10. Finally, it should also be noted that, although the examples considered in this paper are ideal mixtures, the approach of matching the temperature profiles between the rectifying and stripping section can be extended to the separation of azeotropic mixtures until the azeotropic composition is reached. However, for separations of mixtures with tangent pinches, sometimes it may be more beneficial to have intermediate reboilers in the rectifying section or intermediate condensers in the stripping section.40 Such cases of tangent pinch would require separate

5. Conclusions While previous attempts have predominantly compared the HIDiC scheme with the conventional distillation column, we systematically compare the energy efficiency of the HIDiC scheme to that of the simple heat pump schemes with and without intermediate exchangers. In our analysis, different shapes of the temperature profiles in the rectifying and stripping sections of the HIDiC scheme are considered to characterize different nonazeotropic binary separations. We have successfully shown that no single heat integration scheme is always the most energy efficient. We have also demonstrated that heat integration at a large number of locations in the HIDiC setup does not always translate into increased power benefits. The efficacy of HIDiC cannot be solely decided based on feed composition or product purities. A better indicator is the relative variation in the temperature as a function of the height in the rectifying and stripping sections of the HIDiC scheme. The hot temperatures from the rectifying section when plotted along with the cold temperatures from the stripping section as a function of the relative locations in the two distillation sections provide a visualization of the efficiency by which heat is transferred from the rectifying section to the stripping section. The HIDiC scheme is more efficient for separations in which the temperature profiles of the rectifying and stripping sections are nonflat and parallel to each other. For shapes of the HIDiC temperature profiles having zones with large temperature differences between the hot and cold curves, we found that the majority, if not all, of the power benefits of the HIDiC scheme can be obtained using simple one- or twostage heat pumps between the rectifying and stripping sections. Also, for such cases, we observed that the HIDiC power benefits, if any, are realized only with a large number of separation stages in the distillation sections. Thus, for such cases of temperature profiles, simple heat pump schemes with heat integration at just a couple of locations are the preferred heat integration options. We also show that, for the cases with zones of large temperature differences between the hot and cold curves in the temperature profiles of the two distillation sections, heat transfer inefficiencies can be decreased by reducing the number of heat transfer locations. In this case, from the topmost location of the inefficient zone in the rectifying section, all of the heat transfer associated with that zone is now transferred to the bottommost location of the corresponding inefficient zone in the stripping section. This reduces the heat transfer locations and decreases exergy losses associated with heat transfer. However, the increased vapor and liquid flow rates in the corresponding rectifying and stripping sections contribute to increased irreversibility in the distillation columns. As a result, the efficiency improvement in the heat transfer is not expected to lead to much improvement in the overall process efficiency. However, the number of stages in the distillation columns could decrease substantially. Therefore, the proposed strategy of reducing the heat transfer locations in a HIDiC scheme should be considered during the design phase of a HIDiC process. Acknowledgment The authors thank the Department of Energy (DOE Grant DE-FG36-06GO16104) for providing financial support.

130


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ReceiVed for reView August 10, 2010 ReVised manuscript receiVed October 1, 2010 Accepted October 5, 2010 IE101698F

Energy Efficiency Limitations of the Conventional Heat Integrated

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