A Novel Divided-Wall Heat Integrated Distillation Column

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Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

A Novel Divided-Wall Heat Integrated Distillation Column: Thermodynamic and Economic Feasibility Amiya K. Jana* Energy and Process Engineering Laboratory, Department of Chemical Engineering, Indian Institute of Technology, Kharagpur 721 302, India

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S Supporting Information *

ABSTRACT: This work introduces a new divided-wall thermally integrated column that aims to enhance the thermodynamic reversibility of distillation operation. Dividing the cylindrical shell into two equal semicylinders by a vertical wall, they are proposed to operate under a diabatic condition under the framework of traditional heat integrated distillation column (HIDiC) scheme. This leads to transfer of the heat from the high- to low-pressure side through that wall, thereby reducing the utility consumption. In this way, this novel dividedwall HIDiC configuration attempts to utilize the internal energy source and thus improves its thermodynamic efficiency and economic performance. Simulating a binary wide-boiling system, it is observed that the proposed scheme secures a 42.30% savings in energy consumption with a payback period of 3.45 yr. Although the traditional HIDiC shows a better performance in terms of energy savings (50.80%) and payback time (3.31 yr), the proposed HIDiC is more practically relevant from the design and operational perspective. As far as HIDiC is concerned, it first appeared in late 1970s11 with a general process configuration. This scheme introduces thermal coupling between the trays of two divided sections, namely rectifying and stripping sections, through a couple of internal heat exchangers that can provide the necessary heat transfer area. Subsequently, Seader12 proposed a major modification in the HIDiC configuration in which the stripping and rectifying sections are configurated as separate, closed vessels having heat pipes for transferring thermal energy from the high pressure (HP) rectifying section to the low pressure (LP) stripping section. Glenchur and Govind13 introduced a concentric HIDiC in which the annular stripping section is configured around the rectifying section. In addition to the wall surface, the heat transfer panels are proposed14 as an additional heat transfer area to enable the desired operation of the concentric column. The design methodology for this type of column is devised by Olujic et al.15 and Gadalla et al.16 with the application of pinch technology. On the other side, Aso et al.17 developed a fractionating heat exchanger (i.e., shell and tube column) adopting multiple tubes in a shell. It is worth noting that all these HIDiC configurations involve extensive complexity in design and operation. However, among them, the HIDiC with internal heat exchangers has emerged as a potential thermal integration scheme and thus here it will be called traditional HIDiC.

1. INTRODUCTION Energy demand is growing steadily because of rapid urbanization and pace of industrialization, particularly in developing nations. It is a fact that the hydrocarbon-based fuels are the principal source of energy to meet the world’s energy needs. However, these natural resources are of significant concern because of their rapid depletion and high availability in politically unstable regions. Furthermore, they emit CO2 gas through combustion into the atmosphere. It is worth mentioning that contributions of CO2 to global warming and climate change are not disputed in scientific society. Taking these crucial issues into account, research has started in finding alternative energy sources of hydrocarbon fuels. At the same time, significant effort has been put forward in enhancing the energy efficiency of old and useful process technologies. In this respect, distillation has emerged as a potential candidate because of its large share in separationbased industries and low thermodynamic efficiency. It is reported1 that the distillation alone accounts for an estimated 95% of all fluid separations, and its thermal efficiency is in the range of 5−20%. For reducing the utility consumption of the distillation column, thermal integration has been identified as a promising route. In this light, there are a couple of popular configurations proposed. They include the vapor recompression column (VRC),2−4 dividing wall column (DWC),5−7 and internally heat integrated distillation column (HIDiC).8−10 The first two process units have been successfully commercialized and are being used worldwide.5 © XXXX American Chemical Society

Received: Revised: Accepted: Published: A

November 16, 2017 July 27, 2018 August 24, 2018 August 24, 2018 DOI: 10.1021/acs.iecr.7b04748 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

Figure 1. Schematic representation of the (A) traditional HIDiC and (B) proposed divided-wall HIDiC column.

Although the HIDiC scheme was proposed a long time ago11 and several research groups8−10 are involved in further advancing this technology, it is still not ready for commercialization mainly because of its design complexity and implementation problems. With this research gap, an alternative HIDiC configuration is conceptualized18 with vertical partitioning of the entire tower. Subsequently, the HIDiC with an internal dividing wall has appeared in the literature19 for a feasibility study of the batch processing that inherently occurs under unsteady state conditions. There is no such configuration investigated for continuous operation of a distillation column, detailing tray configuration and the heat

transfer mechanism, cost analysis, and a systematic comparison with the traditional HIDiC. It is with this intention that the present work has been undertaken. In this contribution, a new divided-wall HIDiC column is introduced. For the development of two diabatic sections for thermal integration, the conventional column shell is vertically divided by a wall into two equal parts. Applying the HIDiC technique, one divided part is proposed to operate at an elevated pressure, keeping the other part unaltered. In this way, the proposed divided-wall column retains a simple structure and better potential for commercialization over those of traditional HIDiC,8 which involves additional complexity in B

DOI: 10.1021/acs.iecr.7b04748 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article


3. PROPOSING DIVIDED-WALL HIDIC CONFIGURATION AND ITS OPERATING PRINCIPLE Figure 1 shows the schematic representation of a novel divided-wall HIDiC column. In this configuration, the distillation shell is proposed to divide vertically by a wall into two equal semicylinders. The left part receives the feed stream and includes a reboiler at the bottom. The right part of the divided column accompanies an overhead condenser. Accordingly, the distillate stream leaves from the right semicylinder, and the bottom product leaves from the left one. For a thermal driving force to be created between these two semicylinders, the concept of HIDiC technique is proposed to be used for elevating the pressure of the right semicylinder. For this, a compressor is installed over the left semicylinder to increase the pressure of its top vapor before entering the same at the bottom of the right semicylinder. In the same line, a throttling valve is used to depressurize the bottom liquid of the high pressure (HP) right semicylinder before recycling it to a tray of the low pressure (LP) left semicylinder. With this, the heat transfer can take place from the HP (heat source) to the LP (heat sink) column, thereby aiming to reduce the flow of external reflux and boil-up vapor, respectively. This in turn leads to the decrease of hot and cold utility requirements in the reboiler of the LP column and the condenser of the HP column, respectively. It should be noted that, in the proposed scheme, a divided wall is introduced to use as a thermal medium in the HIDiC shell, and thus, this configuration is referred to as the divided-wall HIDiC. This qualitative analysis clearly indicates that the proposed divided-wall HIDiC has the potential of reducing energy consumption and thus the operating cost. However, this configuration additionally includes a compressor and a dividedwall, and thus, it is not so straightforward to comment on the superiority of this structure over its conventional analogues.

column internals and in staging a couple internal heat exchangers vertically. Simulating a binary wide-boiling system, the proposed scheme is evaluated in terms of energy consumption and cost. The novelty of this work lies in the energy-efficient cost-effective configuration of a HIDiC column that does not involve any internal heat exchangers.

2. MOTIVATION In a conventional distillation column, heat is introduced in the reboiler (highest temperature end), and the same is removed from the top condenser (lowest temperature end). It is a fact that this temperature differential existing between the reboiler and condenser is responsible for the degradation of energy even when heat leaks and other losses are excluded. This leads to a highly irreversible operation of distillation with a surprisingly low thermal efficiency. For this column to be energy efficient, the rejected heat needs to be recovered and reutilized for both the separation and entropy generation so that the supply of energy from an external source can be minimized. This goal can be efficiently achieved through the thermal integration route. One of the popular configurations of thermal coupling is an internally heat integrated distillation column (HIDiC), which was proposed a couple of decades ago by Mah and his team members11 under the name secondary reflux and vaporization (SRV). In this scheme, the column shell is horizontally divided into two parts with reference to feed tray, namely rectifying and stripping sections. As usual, the rectifier includes an overhead condenser, and the stripper has a reboiler. Compressing the top vapor of the stripping section, it is introduced at the bottom of the rectifying column so that it leads to generate a thermal driving force between them. Then, the trays of these two diabatic columns are thermally paired through the internal heat exchangers (HEs). This traditional HIDiC column additionally accompanies a compressor and a throttling valve for pressure adjustment. As indicated above, the HIDiC technique makes the column internal configuration very complicated. This is mainly because of the arrangement required for connecting the trays of two diabatic sections through a couple of internal HEs. Moreover, these heat exchangers are vertically staged along the height of the column, making the distillation structure more complicated. The complexity of HIDiC is further increased if there is any space constraint existing between two divided sections for installation of the heat exchangers. Because of these potential problems, the HIDiC technology is still not in a position for commercialization. Keeping these crucial issues in mind, a new divided-wall HIDiC column configuration is introduced here. Unlike the traditional HIDiC that divides the distillation shell horizontally, the proposed scheme primarily splits the cylindrical column along the central line by a vertical wall that acts as a heat transferring medium between two divided semicylinders. As a consequence, there is no requirement of internal HEs that make the proposed HIDiC unconstrained in terms of the space requirement between those two diabatic sections. More importantly, there is no such complexity associated in column internals.

4. MATHEMATICAL MODELING For investigating the techno-economic feasibility of the proposed divided-wall HIDiC column, a mathematical model is formulated. This configuration mainly consists of a column equipped with a divided-tray shell in conjunction with a reboiler, condenser, and compressor. In the following, these two major components are modeled separately. 4.1. Divided-Tray Column. Assumptions. For this divided-tray separator, we adopt the following assumptions: A1. Perfect mixing and equilibrium on all stages. A2. No heat loss to the surroundings. A3. No vapor holdup and a variable liquid holdup on each tray. A4. Nonideality in liquid phase. A5. Trays are not ideal (i.e., tray efficiency < 100%). A6. Heat transfer occurred through the divided wall between its adjacent layers of liquids on two sides. A7. Tray spacing is set at twice the liquid height in the downcomer.20,21 Modeling Equations. For developing the model, we consider a typical nth tray shown in Figure 2. On the basis of the stated assumptions, the modeling equations can be derived as follows. Total Mole Balance. dmn = Ln − 1 + Vn + 1 − Ln − Vn dt C

(1)


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Figure 2. A typical nth plate.

Component Mole Balance. d(mnxn , j) dt

= Ln − 1xn − 1, j + Vn + 1yn + 1, j − Lnxn , j − Vnyn , j

(2)

Energy Balance. d(mnHLn) dt

= Ln − 1HLn ‐ 1 + Vn + 1HVn + 1 − LnHLn − VnHVn − Q In (3)

Equilibrium. yn , j = kn , jxn , j =

Figure 3. Tray configuration (cross-flow) in the proposed column.

γn , jP0n , j PT

xn , j

distillation column (CDiC) shell to handle almost the same vapor traffic existing between them. At this stage, we adopt this condition mainly to fulfill our target of maintaining the same heat duty between the HIDiC and its conventional analogous. For the time being, let us consider the same shell height (h) for both of them. Accordingly, we have

(4)

Summation. C

∑ xn,j = 1 j=1

(5a)

dHIDiC =

C

∑ yn,j = 1 j=1

(6)

in which A n = dHIDiC × he

(8)

At this point, it should be pointed out that this increased diameter of the HIDiC may lead to a decrease in the total number of trays (i.e., height, hHIDiC) to meet a certain criterion concerning product purity and volume. This is due to the occurrence of phase separation in the two consecutive semicylinders. It is worth noting that, here, the selection of the total number of trays for the HIDiC column is made by performing a sensitivity test such that the proposed scheme can achieve the desired product quality and productivity. This is shown later with an illustrative example. Referring to Figure 3, the height of liquid in contact with the divided wall can be estimated for eq 7 on the basis of a pair of two consecutive trays. This height apparently includes the tray spacing between the concerned pair of trays and weir height corresponding to the top tray of that pair. It should be highlighted that the values of the weir height and tray spacing are typically adopted as 0.0508 m (2 in.) and 0.6096 m (2 ft),14 respectively. Now, based on assumption A7, we adopt the effective height as 50% of 0.6604 m (= 0.0508 + 0.6096) for a pair of stages, yielding he = 0.1651 m/stage. Now the total heat transfer from the HP to LP column can accordingly be computed as

(5b)

where x represents the liquid phase mole fraction, y the vapor phase mole fraction, L the liquid flow rate, V the vapor flow rate, H the enthalpy, C the total number of components, PT the total pressure, P0 the vapor pressure, QIn the heat exchange rate from a tray of HP column to that of LP column through the wall, k the vapor−liquid equilibrium constant, and γ the activity coefficient. The subscript/superscript n indicates the tray index, j the component index, L the liquid stream, and V the vapor stream. Heat Transfer. As indicated in Figure 3, both semicylinders have independent trays with the same mechanical design. Accordingly, in the proposed HIDiC column, the heat transfer from the tray of the HP column to that of the LP column takes place through the divided wall. Accordingly, we have Q In = UnA n(TnHP − TnLP)

2 dCDiC

(7)

where Un denotes the overall heat transfer coefficient, An the heat transfer area, Tn the temperature, dHIDiC the diameter of HIDiC column, and he the effective liquid height existing on both sides of the wall that takes part in heat transfer. As we are dividing the cylindrical shell into two semicylinders, it is logical to make the divided-wall HIDiC shell volume double compared to the volume of the conventional

nt

QI =

∑ Q In n=1

(9)

where nt denotes the total number of trays. Internal Flow Rates. The internal heat exchanged through the divided wall leads to generate the internal liquid and vapor D


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Industrial & Engineering Chemistry Research flows for the trays of HP and LP columns, respectively. For a typical nth tray, these flow rates can be estimated as Vn =

Ln =

that is proposed to use as a buffer mainly to obtain the close, if not same, product quality and productivity between the HIDiC and CDiC. As indicated earlier, this equality condition is a prerequisite for quantifying the performance improvement to be achieved through the heat integration. In eq 14, f acts as a multiplication factor to convert the electrical energy to thermal energy. In the present work, we adopt f = 3.23 For the CDiC, eq 14 has the form

Q

In C ∑i = 1 λiyi

(10a)

Q

In C ∑i = 1 λixi

(10b)

where λ is the latent heat and is considered to be temperature dependent.22 4.2. Compressor. To formulate the compressor model, we further assume the following: A8. Isentropic compression. A9. No subcooling/superheating occurred in the compressor. A10. Temperature of exiting compressed vapor considered as the saturation temperature. For estimating the compressor duty Qcomp (in hp), we use the following form of expression20 μ Q comp = 3.03 × 10−5 VPin[(CR)μ − 1/ μ − 1] μ−1 (11) where CR =

ij T yz Pout = jjj out zzz jT z Pin k in {

j=1

μ (μ− 1)

μj − 1

(12)

(13)

5. PERFORMANCE INVESTIGATING INDICES 5.1. Energy Consumption. To evaluate the performance of the proposed divided-wall HIDiC column, we need to determine the energy consumed in both the conventional column and its heat integrated counterpart. As indicated earlier, the HIDiC scheme involves two energy components, namely compressor and reboiler, both of which are operated with the use of external energy sources. On the other hand, the heat transferred from HP tray-to-LP tray through the divided wall falls under the internal energy. Accordingly, the total heat requirement for the proposed HIDiC configuration can be calculated from cons Q HIDiC = Q E + fQ comp

(14)

nt

∑ Q In ± Q B n=1

(17)

6. A CASE STUDY 6.1. Conventional Distillation Column (CDiC). To evaluate the divided-wall HIDiC, we simulate the distillation model formulated earlier for a binary methanol/water system. For fractionating this wide-boiling mixture, the representative column includes a cylindrical tower having a reasonably large number of trays (24 trays),a reboiler, and a total condenser. Here, the trays are counted from top down, indicating topmost tray as Stage 1 and bottom-most tray as Stage 24. The column operates at a top pressure of 101.325 kPa (1 atm) with a stage pressure drop of 0.3 kPa. Along with the assumptions and considerations stated before (see section 4), we additionally used the Wilson model to predict the vapor liquid equilibrium (VLE), nonlinear Francis weir formula for tray hydraulics, and algebraic equations for phase enthalpies.25 The mathematical model, consisting of MESH equations, is simulated using a computer-assisted algorithm given in Jana.25 The operating conditions and simulation results are documented in Table 1. It is evident that the column setup achieves a methanol purity of 98.89 mol % in the distillate and a water composition of 98.28 mol % in the bottoms. 6.2. The Proposed Divided-Wall HIDiC. As the configuration of the proposed HIDiC detailed previously, the

in which the compressor duty (Qcomp) can be obtained from eq 11. Here, QE represents the reboiler duty of the HIDiC column involved with external heat source, and it can be expressed as QE = QR −

l = QR QE o o o m o o =0 Q o n comp

Now, it is quite straightforward to calculate the energy savings achieved through the proposed heat integration. 5.2. Economics. In addition to energy savings, the dividedwall HIDiC scheme is also proposed to evaluate mainly in terms of total annualized cost (TAC) that combines the operating cost (OC) and capital investment (CI), both in yearly basis. It should be pointed out that the OC is traditionally calculated for each year on the basis of yearly working hours (here, 8000 h/yr), whereas the same is followed for CI on the basis of a payback period (here, 5 yr). The installation costs of all the integral components of the concerned distillation columns are given in the Appendix20 (see Supporting Information). The cost inflation in terms of Marshall and Swift (M&S) index24 of 1569 is used to update the equipment costs. Operating the HIDiC column requires a coolant (here, cooling water), heating medium (here, steam), and electricity for the overhead condenser, bottom reboiler, and compressor, respectively. The costs of these three components, respectively, are taken9 as $0.03/t, $13/t, and $0.1/kW.hr. The operating cost of the compressor is determined based on the brake horsepower (bhp) (= hp/ 0.8).20

yj

C

∑

(16)

with

where CR refers to the compression ratio and Tin and Tout are the inlet and outlet temperatures, respectively, with respect to the compressor. Note that, in eq 11, the pressure (inlet pressure, Pin and outlet pressure, Pout) is in lbf/ft2, and the overhead vapor leaving the left semicylinder (V) is in ft3/min. The polytropic coefficient (μ) can be obtained from 1 = μ−1

cons Q CDiC = QR

(15)

For this, the reboiler duty of the conventional distillation column (CDiC) QR is known. Again, one can calculate QI based on eqs 6−9. Here, QB is an additional energy component E


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Industrial & Engineering Chemistry Research Table 1. Operating Conditions and Column Specifications divided-wall HIDiC

items

CDiC

feed flow rate, kmol/h feed composition (methanol/water), mol % methanol composition in distillate, mol % water composition in bottom product, mol % distillate rate, kmol/h bottoms rate, kmol/h reflux ratio reboiler duty, kW tray efficiency, % total number of trays (excluding total condenser and reboiler) feed stage stage pressure drop, kPa top stage pressure, kPa

100 60/40 98.89 98.28 60 40 1.5 504.65 80 24

100 60/40 98.91 98.30 60 40 1.5 210.95 80 16 + 161

12 0.3 101.325

162 0.3 101.325 (left semicylinder)

1

Each semicylinder has 16 trays. 2Left semicylinder.

distillation shell is centrally divided into two semicylinders by a wall. As shown in Figure 1, the left-hand division includes an overhead compressor and a reboiler, and the condenser is equipped at the top of the right-hand division. In the following, this new heat integrated configuration is systematically developed through various testing and feasibility studies. 6.2.1. Selecting Process Parameters. The conceptual design of the HIDiC column involves a couple of sensitivity tests for selecting three parameters. They include the total number of stages (nt), feeding location in the LP column for the depressurized bottom liquid of the HP column and compression ratio (CR). Here, the values of the following parameters are adopted from the CDiC as reflux ratio (RR) = 1.5 and stage pressure drop (ΔP) = 0.3 kPa. Our aim is to select the said parameter set on the basis of a minimum TAC along with a close matching of product purity and productivity with their desired values. In this regard, we first select the CR followed by nt and then tray location all based on the stated criteria. We follow this sequence to conduct the sensitivity tests in a couple of cycles and finalized them as nt of 16 in each semicylinder, tray location at 12th stage, and a CR of 2.45. Figure 4 depicts a sample plot showing the tuning of CR. The optimal parameter set leads to a TAC of 20.25 × 104 $/yr and a close to maximum energy savings. It is also evident in Table 1 that the selected parameter set of the divided-wall HIDiC leads to yield the top and bottom products with almost the same purity and productivity with its conventional analogous. 6.2.2. Thermodynamic Feasibility. Running the HIDiC simulator with the optimal CR of 2.45, we obtain a comparative temperature profile between the HP and LP sides in Figure 5. In this simulation experiment, no heat transfer through the divided-wall is taken into account. It is evident that there is a consistent thermal driving force (ΔP) existing along the entire shell length between the HP and LP sides, indicating a possibility of heat exchange between them. Thus, it is fairly true to say that the example distillation column is a suitable case for heat integration under the proposed scheme. 6.2.3. Energetic and Economic Potential of the Proposed Configuration. The divided wall in the proposed HIDiC column acts as a heat exchanging medium through which the

Figure 4. Selecting CR based on (A) TAC and (B) energy savings.

Figure 5. Comparative temperature profile between the two divided sections.

heat is transferred from the trays of the HP column to that of LP column. As stated before, along with the tray liquid, the clear liquid of the downcomer participates in thermal exchange. For the sample column, the diameter of CDiC (dCDiC) is ∼1 m, yielding the HIDiC diameter (dHIDiC) of F


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Industrial & Engineering Chemistry Research 1.414 m [eq 8]. Adopting the value of overall heat transfer coefficient, U as 500 Btu h−1 ft−2 °F−1 (= 2839.15 W m−2 K−1)26 and calculating the heat transfer area A from eq 7 as 0.23345 m2, we get the UA with respect to each tray as 662.80 W K−1 stage−1. Energetic Potential. For the proposed divided-wall column, the optimal CR is selected above as 2.45. With this, running the HIDiC simulator, one can have the information on temperature driving force (ΔT) between the two thermally coupled trays through the divided wall. Accordingly, it is straightforward to calculate the total heat transfer from the HP to the LP column through the stainless steel (SS) wall as ∼293.70 kW using the expression nt

QI =

nt

∑ Q In = UA ∑ ΔTn n=1

n=1

(18) Figure 6. Influence of the overall heat transfer coefficient on energy savings of the proposed HIDiC column.

As a consequence, the external energy needed to be supplied to the reboiler is reduced to 210.95 kW, which can be calculated from eq 15 with QB = 0. It is true to say that this zero QB and optimal CR of 2.45 used in operating the proposed column have ensured a very close output in terms of purity and productivity with the conventional column. Using eq 14, we finally obtain the total heat consumed by the HIDiC scheme (QHIDiC cons ) as 291.25 kW. Similarly, for the CDiC column, we have QCDiC cons as 504.65 kW, which is actually equal to the reboiler duty (QR). Now, one can easily estimate the energy savings secured by the HIDiC column as ∼42.30%. Heat Transfer Coefficient and Its Effect. For the proposed thermal coupling (Figure 3), the estimation of overall heat transfer coefficient (U) should involve the divided wall and the tray liquid in both sides of that wall. For such thermal arrangements, the U values are reported for various systems;26 using them, one can perform the feasibility test for a sample system, particularly when the exact U value is not there. It is recommended by Kern26 that the U value would be typically in the range of 250−700 Btu h−1 ft−2 °F−1 (i.e., 1419.58−3974.81 W m−2 K−1) when one side of the divided-wall contains either water or steam and in other side there is water, methanol, ammonia, or aqueous solution. The above range of U is also applicable when both sides of the wall are occupied by the aqueous solutions. For other liquids, particularly for organic compounds, the U value is reasonably low,26 and therefore, the proposed HIDiC is not recommended to use for fractionating them. Further, a sensitivity test is conducted to investigate the influence of overall heat transfer coefficient on energy savings of the proposed HIDiC column. For this, the U value has been varied from 250 to 700 Btu h−1 ft−2 °F−1 (i.e., 1419.58− 3974.81 W m−2 K−1) in Figure 6. It is evident that the energy savings linearly increases with the increase of U. Economic Potential. In addition to estimating the energy savings, we further carry out the cost analysis with respect to two indices, namely, total annualized cost (TAC) and payback time. In this regard, Table 2 shows a detailed component-wise cost evaluation. It is evident that the proposed divided-wall HIDiC scheme involves a 24.39% increase in capital investment, whereas it reduces the operating cost by 32.52%. Overall, this novel configuration secures a 5.24% savings in TAC. The attractiveness of the proposed column can also be measured by its payback time of excess capital, which is 3.45 yr.

Table 2. Comparative Economic Evaluation CDiC

divided-wall HIDiC

Capital Costs × 10−4, $ 28.44 29.20 2.27 2.53 10.92 7.20 3.61 13.30 11.60 5.84a

column shell column tray compressor reboiler condenser wall internal heat exchanger total 51.21 63.70 Operating Costs ×10−4, $/yr steam 9.46 3.81 cooling water 1.67 1.58 electricity 0.00 2.12 total 11.13 7.51 TACb × 10−4, $/yr 21.37 20.25 TAC savings, % 5.24 payback period, yr 3.45

traditional HIDiC 28.44 2.27 11.93 2.87 12.31 6.52 64.34 3.09 1.62 2.45 7.16 20.03 6.27 3.31

a

Twenty percent of the column shell cost. bFor a payback period of 5 yr.

6.3. A Comparison between the Proposed HIDiC and Traditional HIDiC. To evaluate the performance of the proposed divided-wall HIDiC further, we compare it with the traditional HIDiC column.27 For this traditional scheme, a total of 12 internal heat exchangers are used for heat transfer from the HP rectifier to the LP stripper. The stage pressure drop (0.3 kPa) and reflux ratio (1.5) values are adopted from the CDiC. The compression ratio (2.32) and UA (1.51 kW K−1 stage−1) are selected using the same procedure followed earlier for the divided-wall HIDiC. Here, the liquid stream leaving the bottom of the HP column is flashed back to the topmost tray of the LP column. With this, it is observed that the traditional HIDiC secures ∼50.80% energy savings, whereas the proposed HIDiC, as stated before, provides 42.30% savings. Moreover, a comparative cost analysis is performed in Table 2. It is obvious that there is not much difference in economic performance between them. As shown, the traditional HIDiC secures 25.64% and 35.67% savings in capital investment and operating cost, G



■

respectively, whereas the respective savings are 24.39% and 32.52% for the proposed HIDiC. The payback time that reflects the overall performance is obtained as 3.31 yr for the traditional HIDiC and 3.45 yr for the divided-wall HIDiC. For the traditional HIDiC column, there is a scope of enhancing UA (overall heat transfer coefficient × heat transfer area) through the internal heat exchangers arranged vertically, which leads to an increase in the energy savings. On the other hand, there is no such scope of increasing UA in the dividedwall HIDiC because of its fixed wall. This is the main reason for having a little weaker performance of the proposed HIDiC than that of the traditional HIDiC. Although the traditional HIDiC secures a little better performance compared to that of the proposed HIDiC, the divided-wall column offers relatively simple design and operation. This is because the traditional arrangement needs to connect the trays of two diabatic sections through a couple of internal heat exchangers. Moreover, these heat exchangers are vertically staged along the height of the column, making the distillation structure more complicated. The complexity of traditional HIDiC is further compounded when a space constraint exists between two sections for installation of the heat exchangers. Therefore, the proposed divided-wall HIDiC has more practical relevance.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b04748.

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Appendix (cost estimating formula and parameter value) (PDF)

AUTHOR INFORMATION

Corresponding Author

*Tel.: +91-3222-283918; fax: +91-3222-282250. E-mail: [email protected]. ORCID

Amiya K. Jana: 0000-0003-1367-5480 Notes

The author declares no competing financial interest.

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REFERENCES

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7. CONCLUSIONS In this contribution, a new divided-wall HIDiC column is introduced for a feasibility study. Dividing the cylindrical shell vertically by a wall, one of the semicylinders is proposed to operate at an elevated pressure, running the other one at the same pressure with the CDiC. This pressure difference (i.e., temperature difference) leads to transfer heat from the trays of the HP side to that of the LP side through the wall, thereby reducing the flows of external reflux and boil-up vapor. In this way, the proposed configuration can reduce the consumption of both cold and hold utilities in the condenser and reboiler, respectively. However, this divided-wall scheme involves the additional capital investment for a compressor and a wall. Thus, we need to perform a quantitative analysis to reach a concrete conclusion regarding the superiority of the heat integrated column over its conventional counterpart. Simulating a binary wide-boiling methanol/water system, it is investigated that the proposed configuration secures a promising performance with 42.30% savings in energy consumption and a payback period of 3.45 yr. At this point, it should be noted that the failure rate of the compressor in HIDiC is much higher than that of the reboiler in the conventional scheme, and this concern should be taken into account whenever one attempts to validate the superiority of the proposed distillation column. Unlike the traditional HIDiC scheme that is not yet commercialized because of its design and operational complexity, the proposed configuration does not involve any internal heat exchangers (HEs) and thus requires no vertical staging of HEs between two diabatic sections and involves no design complexity in column internals. Therefore, although the traditional HIDiC provides a better performance in the aspects of energy savings (50.80%) and payback time (3.31 yr), the divided-wall HIDiC has more practical relevance. To extend the applicability of this proposed HIDiC, we are currently involved in developing the column configuration for the use of multicomponent separations. H


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A Novel Divided-Wall Heat Integrated Distillation Column

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