Pressure-Swing Dividing Wall Column with Multiple Binary Azeotropes

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Pressure-swing dividing wall column with multiple binary azeotropes: Improving energy efficiency and cost savings through vapor recompression Md Aurangzeb, and Amiya K. Jana Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03586 • Publication Date (Web): 28 Feb 2018 Downloaded from http://pubs.acs.org on March 2, 2018

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Industrial & Engineering Chemistry Research

Pressure-swing dividing wall column with multiple binary azeotropes: Improving energy efficiency and cost savings through vapor recompression Md Aurangzeb and Amiya K. Jana

*

Energy and Process Engineering Laboratory, Department of Chemical Engineering, Indian Institute of Technology−Kharagpur, India−721 302

Abstract This work introduces a novel thermal integration scheme for a pressure-swing distillation train used to separate a mixture of methanol/benzene/acetonitrile. This mixture typically forms three azeotropes as shown through developing its residue curve map with two distillation boundaries at atmospheric pressure. This ternary mixture is traditionally separated via a triple column pressureswing distillation (TCPSD) scheme, in which, all columns operate at different pressures. Here, the first two columns of the TCPSD are proposed to be replaced by a single column having a dividing wall that allows heat transfer through it. Thus, the resulting scheme is called pressureswing dividing wall column (PSDWC). Aiming to improve its performance, further advancement is subsequently made by developing the heat intensified PSDWC (HiPSDWC) and vapor recompressed PSDWC (VRPSDWC). The performance of these proposed schemes is evaluated in the context of energy saving and total annual cost (TAC). Among these configurations, it is investigated that the VRPSDWC secures the best performance, providing a 75.67% savings in external energy consumption, which is 2.45 times that of the HiPSDWC. The attractiveness of the best performing VRPSDWC is further quantified by a 13.25% TAC savings and a reasonably short payback time of 2.97 yr (3.91 yr with considering penalty of 10%).

Keywords: Pressure-swing distillation train; dividing wall column; vapor recompression; energetic and economic potential

*

Corresponding author. Tel.: +91-3222-283918; fax: +91-3222-282250. E-mail address: [email protected] (A. K. Jana).

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1. Introduction Fossil fuels have major contribution in meeting our primary energy demand, with coal as the fastest growing fuel in the last 25 years.1 Again, 90% of the energy requirement in transportation sector is sourced from petroleum-based liquid fuels.2 However, there are many limitations associated with these natural resources, such as finite reserves, detrimental effect to environment because of emitting carbon dioxide, a greenhouse gas, among others. With this, a research attention has been paid to make an improvement in energy efficiency of several conventional processes to lower their energy consumption and carbon discharge. In this light, the thermal intensification approach is an efficient way to achieve the target. Distillation is widely used to separate various hydrocarbon mixtures into pure products. Interestingly, distillation alone shares 60% of the total energy consumed in chemical industries.3,4 It is also fairly true that it operates with a thermodynamic efficiency of 5-10%.5 For this, distillation is one of the natural entities for thermal integration. In this respect, dividing wall column (DWC) has already attracted attention from industry. Along with its relatively simple design and operation compared to other thermally coupled distillation scheme (e.g., heat integrated distillation column (HIDiC)), DWC ensures significant savings in both energy and capital investment.6-8 It is observed that DWC is a feasible option to carry out numerous separation problems, including bioethanol dehydration9 and extractive distillation of methylal-methanol mixture10. In a distillation process, the separation of components greatly depends upon their distribution between the contacting liquid and vapor phase. However, there is no such difference between these two phases in an azeotropic mixture. As a result, the simple fractional distillation column fails to serve the purpose and it requires a special treatment. This work is concerned with the separation and purification of an azeotropic mixture in an efficient way. In the last few decades, several attempts have been made to separate azeotropic mixtures. A few of the advanced techniques employed to separate them include membrane separation, pressure-swing distillation, and azeotropic and extractive distillation. These methods are commonly used in industry and well explained in numerous textbooks.11,12 The membrane fabricated from polymer material is used to separate several binary and ternary constant boiling mixtures. Few of them are alcohol-alkane13, dimethyl carbonate-methanol14 and ethyl acetate-ethanol-water15. The membrane separation involves thermal, chemical and mechanical instability, and low permeation rate.16 The azeotropic (and extractive) distillation 2 ACS Paragon Plus Environment

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uses an entrainer to break the azeotrope formed and obtain the required component of high purity. On the other hand, a pressure-swing distillation column is beneficial in terms of economy and in high purity.17,18 Additionally, it avoids the inevitable problem that may arise from an entrainer used in azeotropic distillation. So far, there are several studies reported on separating wide boiling and binary azeotropic mixtures (using a third component) in dividing wall column.19-21,12 As far as ternary mixture forming azeotropes is concerned, Knap and Doherty22 have developed a process named as triple column pressure-swing distillation (TCPSD). Recently, Zhu and co-workers23 have formulated

an

economically

optimized

three

column

configuration

to

separate

methanol/benzene/acetonitrile (M-B-A) mixture, which forms more than one azeotrope. This work introduces a dividing wall column in that configuration to explore the performance improvement from energetic and economic perspective making further advancement in thermal integration. In this contribution, a conventional triple column pressure-swing distillation (TCPSD) is developed for the separation of a ternary mixture (methanol/benzene/acetonitrile) that forms multiple azeotropes into pure products (i.e., desired purity of 99.9%). The first two columns of that TCPSD is merged in one tower with a dividing wall that allows heat transfer through it, and starts from bottom and ends somewhere at an intermediate stage. Then this pressureswing dividing wall column (PSDWC) is coupled with the last column of the distillation train. Since the PSDWC operates at lower pressure than that coupled last column, the vapor stream of the latter column can be utilized as a heat source in liquid reboiling of the DWC. For this, the vapor recompression concept is proposed to use and thus, the resulting scheme is called vapor recompressed PSDWC (VRPSDWC). When the minimum thermal driving force is existed over the operational time, there is no need of any compressor. In such a case, the thermal coupling made between the hot vapor and cold reboiler liquid of the PSDWC is referred here as heat intensified PSDWC (HiPSDWC). To evaluate the best configuration for this complicated azeotropic system, two performance indices, namely energy consumption and cost, have been used. For a fair comparison in terms of these indices between the developed heat integrated schemes with reference to the conventional TCPSD, it is required to operate all of them at a close, if not same, dynamics. For this, along with developing a proper column configuration, an open-loop control scheme is formulated, which is particularly useful at transient state. This work is organised as follows. A brief introduction of the pressure-swing distillation and its further advancement made is presented in Section 2. In Section 3, the proposed 3 ACS Paragon Plus Environment


schemes and there working principle are elaborated. The model equations are provided in Section 4. The two performance indices required for quantitative performance analysis are detailed in Section 5. Subsequently, the performance of the proposed heat integrated configurations is demonstrated in Section 6. The last Section 7 of this article concludes the work. 2. Triple Column Pressure-Swing Distillation (TCPSD): A Conventional Scheme Figure 1 shows a triple column pressure-swing distillation reported in literature23 that is employed to separate an azeotropic mixture consisting of methanol/benzene/acetonitrile. The first column that operates at high pressure (= 607.95 kPa) produces acetonitrile as bottom product (Pdt1). The distillate stream ( D1 ) from this tower is fed to the second column operated at relatively low pressure (= 101.33 kPa) in order to recover methanol as Pdt2. Subsequently, its distillate D2 enters the third column, which is again at high pressure (= 607.95 kPa) where benzene is obtained as the bottom product (Pdt3). The distillate D 3 of the last column contains a reasonable amount of methanol and benzene, and thus, it is recycled back to the first tower. It should be noted that each of the triple column system is equipped with a reboiler and a condenser. In this way, a pressure sensitive azeotropes of three components swing through these three columns that operates at different pressures. This method leads to break the azeotrope formed and make easy to separate them into pure fractions.

3. Proposed Heat Integration in TCPSD and Working Principle 3.1. Pressure-swing DWC (PSDWC) Now, we would like to modify the triple column configuration detailed above by introducing a dividing wall column (DWC). The working principle of this pressure-swing DWC (PSDWC) is demonstrated in Figure 2. As shown, this scheme consists of a DWC in conjunction with a conventional tower. Actually, the first two columns are merged to a dividing wall column with a partition wall that starts at the base and advances to a certain tray of the DWC at the intermediate level. This wall allows the heat transmission from the high temperature side to low temperature side. The DWC consists of three sections; the top section includes a total condenser to convert the vapor into liquid, and left and right bottom sections have reboiler for each of them to vaporize the bottom liquid.


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The feed is introduced to the left side of the divided wall. The right side receives the distillate, which is produced in the second column, as a feed that consists of an azeotrope too. Likewise, the azeotropic distillate of the first column enters as a feed to the second tower. This variation in pressure leads to change the azeotropic composition and distillation boundaries, which result in producing pure components. As shown in Figure 2, the acetonitrile and methanol are taken out from the DWC as Pdt1 and Pdt2, respectively, whereas the benzene is obtained from the second tower as Pdt3. A liquid dispenser is used in the top section just above the dividing wall to split the liquid that comes down the column to the two sides of the wall. Similar to the TCPSD (Figure 1), here all the products are obtained from the bottom of the towers. However, compared to the TCPSD, the proposed PSDWC scheme reduces the number of columns and condenser by one, and it also suppresses the dilution of light key with the fresh feed. Because of the reduced equipment size, this PSDWC leads to improve the energetic and economic potential.

Residue Curve Map: To Analyse the TCPSD and PSDWC It is interesting to note that there is an interaction among the molecules of the constituents, degree of that interaction may exceed or lag in comparison to the interaction between the same type of molecules. This, in turn, leads to a deviation from Raoult’s law. If the deviation is great enough to cause a peak or valley in vapor pressure versus composition profile then at that point, the vapor has same composition as the liquid. Therefore, the deviation of multicomponent system from Raoult’s law leads to azeotrope. Figure 3 shows the residue curve maps (RCMs) of methanol/benzene/acetonitrile mixture at three different pressures (i.e., 101.33, 607.95 and 193.32 kPa). These RCMs are generated in Aspen Plus simulator. Figure 3 indicates that there are three azeotropes existed, namely methanol-acetonitrile, methanol-benzene and acetonitrile-benzene. It is also observed that three vertices of RCMs that represent pure component are stable node because each residue curve ends at its respective vertex. Whereas, methanol-benzene point is unstable node as each residue curve originates from here, and the acetonitrile-benzene and methanol-acetonitrile are saddles. Figure 4 indicates that there are two distillation boundaries moving from the methanolbenzene azeotrope to the acetonitrile-benzene and methanol-acetonitrile azeotropes. These distillation boundaries divide the RCM into three distillation regions, and each region encloses pure product. Moreover, the change in pressure results in overlap of the boundaries that gives the overview of principle of separation. For TCPSD, the actual feed mixture is 5 ACS Paragon Plus Environment


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separated into overhead (D1) and bottom (Pdt1) with the help of D3 (the recycling stream) in column C1 operated at 607.95 kPa. Thus in tower C1, the pure acetonitrile is recovered at the bottom and methanol-benzene-acetonitrile moves to the top. The distillate D1 that crosses Region 1 to Region 2 enters the second column (C2) operated at 101.33 kPa as feed and further splits into two streams (D2 and Pdt2) in order to get pure methanol in stream Pdt2. The overhead product D2 in Region 3 is separated into pure benzene (that comes out from the bottom product Pdt3) and distillate D3 in column C3 operated at 607.95 kPa. Likewise, the proposed pressure-swing dividing wall column can also be analysed.

3.2. Vapor recompressed PSDWC (VRPSDWC) Figure 5 depicts the vapor recompressed pressure-swing dividing wall column (VRPSDWC) that operates with connecting the heat source (condenser) and heat sink (reboiler). Since the bottom of a distillation column is at higher temperature than its top, the said heat source (at Tn T ) is colder than the heat sink (at TB ). Aiming to extract the latent heat of the overhead

vapor leaving the top of the first and second towers of the PSDWC, it is required to employ a compression system to develop a thermal driving force, ∆TT ( = Tn T − TB ) of 15 K. For this, the compression ratio (CR) can be estimated from: µ / ( µ −1)

 T + ∆TT   = B (1)  Tn  T   Here, Pi and Po designate the inlet and outlet pressure of the top vapor, respectively, with P CR = o Pi

reference to the compressor. The polytropic coefficient of any species, j ( = µ j = C p , j / Cv , j ) is a function of temperature and µ is determined from:

1 = µ −1

C

yj

j =1

j

∑µ

(2)

−1

In the above equation, C p and C v denote the heat capacity at constant pressure and volume, respectively, and y j is the mole fraction of component j in vapor phase.

An Open-Loop Control Policy The proposed VRPSDWC scheme has two operating criteria, namely compression ratio (CR) and heat consumption ( Qcons ). The first term (i.e., CR) is associated with a compression


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system and it can be regulated using eq 1. However, in most of the cases, it is set at a fixed value because of the operation of a continuous distillation at steady state. As far as the second term (i.e., Qcons ) is concerned, it should be kept same between the PSDWC and its vapor recompressed counterpart (i.e., VRPSDWC). This is required for a fair comparison between them. For this, an open-loop control policy needs to be devised for adjusting the manipulated variable. Accordingly, one should estimate the thermal energy produced from an internal source ( QCV ). It is basically the total amount of latent heat ( λ ) liberated by compressed overhead vapor. Now, there exist two possibilities (Scenario 1 and 2) as discussed below: Scenario 1 ( QCV > Qcons ): In this case, the total quantity of heat obtained from the internal source (i.e., QCV ) is more than the required heat (i.e., Qcons ). Accordingly, two options are open: either use the required heat or the available heat that includes the excess amount of energy (i.e., QCV − Qcons ). The latter option may lead to undesired reboiling of the heavier fraction, causing degradation in product purity. Thus, this scenario involves the splitting of overhead vapor load ( Vn T ) into two segments. The first part of Vn T , namely Vn T C is elevated to a high pressure in the compressor so that the thermal driving force attains its desired value (i.e., 15 K). This leads to release the latent heat of overhead vapor and boil the bottom liquid. The excess amount of Vn T , namely Vn T r is sent to the top condenser. Now, the flow rate of these vapor streams is calculated from: Vn T C =

Qcons

λT

(3)

nTC

Vn T r = Vn T − Vn T C

(4)

Here, Tn T C ( = ∆TT + TB ) indicates the temperature of the overhead vapor that comes out of the compressor. As indicated, Qcons is the actual amount of heat duty required to run the pressure-swing dividing wall column. Scenario 2 ( QCV < Qcons ): This scenario prevails when the entire overhead vapor ( Vn T ) cannot meet the reboiler heat demand. In this circumstance, it is necessary to provide supplementary heat from an outside source ( QE ) as:



QE = Qcons − QCV

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(5)

In the above equation, QE is the manipulated variable. In this study, steam is used as an external heating medium, and its flow rate ( mS ) can be regulated as: mSλS = QE

(6)

which yields,

mS =

Qcons

λS T

−

Vn T λ T

S

n TC

(7)

λS T

S

In eqs 6 and 7, λS is the latent heat of steam and TS is its corresponding temperature. It is observed that the VRPSDWC under this scenario does not need any overhead condenser.

3.3. Heat intensified PSDWC (HiPSDWC) Figure 6 depicts a heat intensified pressure-swing dividing wall column (HiPSDWC). As mentioned earlier in Section 3.1, the first DWC column operates at lower pressure than the second column. This difference in pressure (i.e., temperature) can be exploited thermodynamically for heat intensification further. Accordingly, the overhead vapor coming from the second tower can be employed as a heat source against the reboiler liquid of first column as heat sink. This configuration is named here as HiPSDWC. It is worth noticing that instead of using heat pump to compress the overhead vapor and reuse its heat to boil-up the bottom liquid as done in VRPSDWC (Figure 5), that high pressure (HP) vapor is directly coupled with the low pressure (LP) reboiler content. By this way, one can avoid the use of compressor, thereby reduce the equipment and electricity cost. Accordingly, as shown in Figure 6, attempt is made to thermally pair the HP (T3) and LP (T1 or T2) streams. It should be noted that the minimum temperature difference of 15 K is required to maintain between the heat source and sink. With this, there exists a scope in HiPSDWC configuration that the amount of internal heat generated may not be equal to the heat required. In such a situation, the inflow rate of HP top vapor to the reboiler of two sides in the dividing wall needs to be adjusted. Like VRPSDWC, this can be accomplished in the HiPSDWC with two scenarios. For Scenario 1 ( QCV > Qcons ), one should split the vapor (

Vn T Hi ) into three parts. The first two parts are sent to the left ( Vn T L ) and right reboiler ( Vn T R )


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of the LP column, and the remaining fraction ( Vn T r ) goes to the condenser of the HP column. Accordingly, they are estimated from: Vn T L =

Q1, LP

(8)

λT

3

Vn T R =

Q2, LP

(9)

λT

3

Vn T r = Vn T Hi − Vn T L − Vn T R

(10)

Here, Q1, LP and Q2, LP denote the actual heat duty (steam heated) of the reboilers associated with the LP column (i.e., first column of the HiPSDWC). As far as Scenario 2 ( QCV < Qcons ) is concerned, it is same as formulated earlier for the VRPSDWC. However, in both the cases, there is a possibility of energy and cost savings. It is interesting to note that for the example azeotropic system, the HiPSDWC involves Scenario 2. Therefore, unlike PSDWC, the HiPSDWC configuration includes a single unit of condenser which is mounted over the first tower.

4. Model Representation This section formulates the model for the TCPSD and the proposed schemes (PSDWC, VRPSDWC and HiPSDWC). To construct the dynamic model, the following assumptions are considered:

A1. Liquid composition is uniform (well-mixed) on each stage A2. Liquid and vapor streams leaving each tray are not in phase equilibrium, and the Murphree vapor-phase efficiency of 70% is introduced to describe the departure from equilibrium A3. Tray is mostly occupied by liquid with some vapor bubbles. Since liquid density is much larger than the vapor density, vapor holdup is neglected A4. Tray pressure drop of 0.3 kPa is adopted A5. Height of liquid in the downcomer is assumed at half of the tray spacing24 A6. Amount of vapor produced (if less than 5%) in throttling valve is neglected A7. Isentropic compression system



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With these presumptions, the model is built for a sample nth plate, shown in Figure 7, based on the fundamental principle, namely conservation of mass (or mole) and energy.

Total mole balance

dmn = Ln+1 + Vn−1 − Ln − Vn + F dt

(11)

Element ( j ) mole balance

dmn xn, j

dt

= Ln+1 xn+1, j + Vn−1 yn−1, j − Ln xn , j − Vn yn, j + FxF , j

(12)

Energy balance dmn H L , n

= Ln +1 H L , n +1 + Vn −1 H V , n −1 − Ln H L , n − Vn H V , n + F H F ± Qw , n

dt

(13)

Phase equilibrium yn , j =

γ n , j P sat n, j P

x n , j = K n , j xn , j

(14)

Summation of vapor and liquid mole fractions Nc

∑ xn , j = 1

(15)

j =1

Nc

∑ yn , j = 1

(16)

j =1

The variables that appeared in the above equations are defined in a separate Nomenclature section latter. It is true that the dynamic transition in internal energies on each tray is much faster than the changes in liquid holdup or compositions.25 Therefore, energy balance (eq 13) is transformed from differential to algebraic form to determine the vapor flow rate in the column. Whereas, the flow rate of liquid moving down a tray is computed using the Francis weir correlation.25 The enthalpy ( H n ) of vapor and liquid streams is calculated from the empirical correlations available in literature26 and reported in Table S1. Further, the nonideal nature of liquid defined by activity coefficients ( γ n , j ) or vapor-liquid equilibrium constant ( 27 K n , j ) is determined using the Hildebrand activity model .

As mentioned above, the triple column pressure-swing distillation is the basic configuration to separate the ternary mixture of methanol/benzene/acetonitrile that forms multiple azeotropes into its pure constituents. Their thermodynamic properties are listed in 10 ACS Paragon Plus Environment

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Table S1, which are used to calculate the variables that are involved in the mathematical model. These variables include saturation pressure, enthalpy of vapor and liquid, latent heat of vaporization, liquid activity coefficient and tray hydraulics (i.e., liquid flowrate). The formulas for these entities are also highlighted in Table S1. At this point it should be noted that we have developed here the dynamic model of the distillation column and its simulation algorithm keeping its future use in mind for control and optimization. However, for the current study, its static version is used.

Heat transfer through the dividing wall

The term, Qw ,n in eq 13 represents the heat transfer through the dividing wall of the pressureswing DWC and its heat integrated schemes. Figure 7 shows the heat transfer mechanism through the wall. The heat exchange occurs from high to low temperature liquid on the trays and in the downcomer regions by two modes; conduction through the wall and convection in the liquid of both sides of that wall. As the heat resistance network shown in Figure 7, the governing equation is expressed as: Qw , n = UAHT (TH − T L )

(17)

where, AHT = hliq dc

(18)

Here, U denotes the overall heat transfer coefficient, which basically corresponds to the reciprocal of sum of convective and conductive resistances. The area of heat transfer, AHT is the product of the column diameter ( d c ) and effective height ( hliq ) of liquid on the tray (eq 18). To determine hliq , a set of two trays in each side of the wall is considered. Basically, the sum of the plate spacing ( hps ) and the weir height ( hw ) of the top plate of that set determines the total height of liquid. Using assumption A6, we write for a single plate as: 11  hliq =  (hps + hw ) 22 

(19)

Compressor

The model for a compressor is developed based on assumption A7. Accordingly, the compressor duty28, Qcomp in hp is computed from:



Qcomp = 3.03 × 10− 5

µ

µ −1 Vn T C Pi  (CR ) µ − 1 µ −1  

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(20)

The variables that are used in eq 20 are defined earlier in Section 3.2.

5. Relative Performance Indices Here, we attempt to use two performance indices, namely energy savings and total annual cost (TAC). Our main purpose is to evaluate the performance of the pressure-swing DWC, VRPSDWC and HiPSDWC over its conventional scheme (i.e., TCPSD). The said indices are presented below in brief.

5.1. Energy savings To estimate the energy savings offered by the proposed schemes with reference to the standard process (Figure 1), the information regarding their heat consumption is required. For s1 s2 the TCPSD, PSDWC and HiPSDWC, the total energy usage denoted by Qcons , Qcons and s3 , respectively, includes solely the external steam duty ( Qex ). Whereas, for vapor Qcons s4 recompressed pressure-swing dividing wall column ( Qcons ), it combines the external steam

duty and the electrical energy required for compressor. It yields: s4 Qcons = Qex + f et Qcomp

(21)

In eq 21, a multiplication factor ( f et ) of 3 is adopted to convert the compression work into the thermal energy required to produce an equivalent amount of electrical power. It is estimated empirically, taking into account the cost involved with fuel, production technology s3 s1 s2 s4 and electricity with respect to Japan.29 Once we know Qcons , Qcons , Qcons and Qcons , the

percent of energy savings can be easily calculated as: s

Q s1 − Q p ( =2 , 3or 4 ) Energy saving (%) = cons s1cons × 100 Qcons

(22)

5.2. Economic analysis In this section, a detailed economic analysis of the different schemes are presented in terms of total annual cost (TAC) ($/yr). It combines the operating cost (OC) and capital investment (CI) according to the following equation:


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TAC = OC +

CI PP

(23)

Here, PP denotes the payback period in year. Moreover, we assume yearly working time of 8000 hours. For estimating the capital investment, the installed cost of column, tray, reboiler, condenser and compressor are added. On the other hand, the OC is the total cost of steam, coolant and electricity, and these are adopted as $17 per ton, $0.06 per ton and $0.084 per kW.h, respectively.30 It is important to mention here that the compressor and motor efficiency are chosen as 0.8 and 0.6 respectively, in order to calculate its operating cost. The TAC is estimated based on the empirical relation as reported in Douglas28. Here, we consider a cost inflation index (M&S) of 1672.0 (CEPCI, 2nd Quarter, 2011).31 The material used to construct all the distillation towers is stainless steel. Along with TAC, one can also find the payback period, which is estimated by dividing the difference in capital investment (between the proposed and conventional process) with the difference in operating cost (between the conventional and proposed scheme). The formulas to get the installed cost of tower, tray, heat exchanger (i.e., condenser and reboiler) and compressor are presented below: Distillation tower  M &S 1.066 0.802 Installed cost ($) =  (cin + cm cp ) ∂  937 .636 d c hc 280  

(24)

The coefficients, cin = 2.18, cm = 3.67 and cp = 1.0 (up to 3.4 atm pressure). Here, the factor

∂ (= 1.1) includes the cost of dividing wall32 in PSDWC as well as in its heat integrated schemes. To measure the height of the column (hc), the tray spacing of 0.6 m and an additional 10% allowance28 are taken into account. Accordingly, the height of the column with a factor f (= 1.1) is calculated from: hc = 0.6 (N stage − 1) f

(25)

Distillation tray  M &S 1.55 (26) Installed cost ($) =   97.243 d c hT (cs + ct + cm ) 280   The constant, cs = 1.0, ct = 0.0 (for sieve tray type) and cm = 1.7. The stack height of the tray ( hT ) is obtained from eq 25 by making the factor f equal to 1.



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Heat exchanger  M &S 0.65 Installed cost ($) =   474 .668 A (cin + (cd + cp )cm )  280 

(27)

The coefficients, cin = 2.29, cm = 3.75, cp = 0.0 (up to 10.2 atm pressure), and cd = 0.8 for condenser and 1.35 for reboiler. The heat transfer area for condenser and reboiler is obtained from: A=

Q U HE ∆TLMTD

(28)

Here, U HE denotes the overall heat transfer coefficient and is adopted as 852 W/(m2.K) for condenser and 1420 W/(m2.K) for reboiler.33 Compressor  M &S 0.82 Installed cost ($) =  517 .5(BHP ) (cin + ct ) 280  

(29)

Here, the constants, cin = 2.11 and ct = 1.0.

6. Results and Discussion The performance of heat integration in TCPSD (PSDWC, VRPSDWC and HiPSDWC) over the triple column pressure-swing distillation is evaluated in the light of energy efficiency, and economy. Recalling, these schemes are used to separate a ternary mixture, consisting of methanol, benzene and acetonitrile. Their performances are discussed in detail in the subsequent sections.

6.1. Performance evaluation of TCPSD Based on the input data provided in Table 1, the TCPSD is simulated using the simulation algorithm presented in the Supporting Information file. The first column (C1) with pressure 607.95 kPa requires 1072.4 kW of heat duty and 46 trays to produce pure acetonitrile (99.9 mol%) as Pdt1. The distillate (D1) which consists of 85.09 mol% and 14.84 mol% of methanol and benzene, respectively with negligible amount of acetonitrile is further separated in the second column (C2) operated at 101.33 kPa to recover 99.9 mol% of pure methanol with 49 stages and reboiler duty of 658.3 kW. The distillate of C2 is further sent to a next high pressure (= 607.95 kW) column (C3) that consists of 14 stages. Here, we obtain pure benzene by supplying 253.3 kW of energy. 14 ACS Paragon Plus Environment

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Therefore, it is observed that the conventional process, TCPSD involves 1984 kW of heat to separate the mixture into its pure constituents. Figure 8a shows the composition dynamics of methanol, benzene and acetonitrile in the respective reboilers of TCPSD configuration, and it ensures that the system reaches steady state.

6.2. Performance improvement by PSDWC, VRPSDWC and HiPSDWC In this section, we evaluate the performance of the proposed heat integration schemes with respect to the conventional triple column pressure-swing distillation. The input (feed composition, temperature and flow rate) and output (product purity and flow rate) conditions are kept close, if not same, in order to make a fair comparison between them.

6.2.1. Pressure-Swing DWC (PSDWC) Table 1 reports the relevant information of the proposed pressure-swing dividing wall column and its thermally integrated scheme, namely VRPSDWC and HiPSDWC. Performing a sensitivity test in Figure 9a, the pressure of the first column (CD1) of PSDWC is selected as 193.32 kPa. In this simulation experiment, the pressure of the second column (CD2) is kept same (i.e., 607.95 kPa) with that of C3 of the TCPSD, since CD2 and C3 represent the same column. The pressure of CD1 is chosen so that one can achieve the close products purity to that of the base configuration. While developing the PSDWC and its modified forms, the heat flow characteristic associates with the dividing wall is also incorporated. Figure 9b reveals that there is significant difference existed in temperature between the trays on each side of that wall. The quantity of heat exchange is calculated from eq 17 with the help of eqs 18 and 19. The overall heat transfer coefficient, U for the present organic liquid mixture is adopted as 305.4 W/(m2.K).34 The tray spacing, hps is adopted as 0.6 m (= 23.22 inch) and the weir height, hw of 0.127 m (= 5 inch). These are used in eq 19 to estimate an effective height of liquid ( hliq ), which is equal to 0.182 m and this value is further multiplied with the diameter of column that gives 0.12 m2 of heat transfer area with reference to the dividing wall. So, substituting these sets of information in eq 17, one can easily compute Qw , n . It is important to mention here that 99.9 mol% of the above mentioned chemicals are obtained from the bottom liquid that leaves the columns of PSDWC with total external heat consumption of 1788.3 kW which is less by 195.7 kW to that of TCPSD. Therefore, using eq 15 ACS Paragon Plus Environment


22, it is estimated and reported in Table 2 that the pressure-swing dividing wall column secures a 9.86% savings in energy consumption. It is also evident from Table 3 that the PSDWC is economically sound as it reduces the operating cost by 9.88% with the total annual cost savings of 13.60%.

6.2.2. Vapor Recompressed PSDWC There is a scope of improving energy efficiency of pressure-swing dividing wall column, which may curtail the cost. The energy profile documented in Table 2 shows that vapor leaving the columns CD1 and CD2 releases 1773.13 kW of energy in the respective condensers. Therefore, if this heat is reused to reboil the liquid in those reboilers then there is an opportunity of reducing the external heat requirement. As discussed earlier, utilizing this heat requires an increase of temperature of the vapor than the bottom liquid by 15 K. Figure 5 shows that the temperature of the vapor (from CD1) is 349.12 K and that of the bottom liquid at both side of the dividing wall are 378.60 K (Reboiler 1) and 356.95 K (Reboiler 2). Hence, a compressor (Compressor 1) is required, which increases the vapor temperature to 393.6 K (= 378.6 + 15 K). Now, this compressed vapor releases 1319.3 kW of energy with a CR of 2.36 and compressor duty of 100.23 kW. The Reboiler 1 actually requires 871.67 kW of duty and this leads to Scenario 1 (see Section 3.2). As a result, the compressed vapor is split in the ratio of 0.66 and 0.34; the former is used to replace the external source of heat (steam), which is required to vaporize the liquid in Reboiler 1. The latter fraction can only provide 447.63 kW of heat to Reboiler 2, which makes a deficit of 19.04 kW. So, the flow rate of steam needs to be manipulated to meet the exact energy demand. Likewise, Compressor 2 is employed in the second tower (CD2) having compressor ratio equals 2.65. By applying 39.71 kW of work (i.e., compressor duty), this unit produces 406.22 kW of heat as an internal source, which is used in Reboiler 3. Table 2 gives the details of different types of heat associated to VRPSDWC. Using eq 22 it is estimated that VRPSDWC secures 75.67% savings in energy consumption with respect to triple column pressure-swing distillation. As indicated, the compressed vapor gets liquefied after releasing latent heat in a reboiler. Thereafter, its pressure is recovered by the use of a throttling valve. Accordingly, as shown in Figure 5, this operation is carried out in TV1 and TV2. As a result, 25.4% liquid gets vaporized in TV1 of the DWC and it is less than 5% in case of TV2 of the right hand column.


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For this, an overhead condenser (Condenser 1) is used for the DWC in VRPSDWC, whereas the vapor formation is neglected in TV2. Unlike PSDWC, the vapor recompressed pressure-swing dividing wall column accompanies two compressors. Hence, there is an increase in capital cost with a significant drop in operating cost. Table 3 highlights the cost associated to several components of the VRPSDWC. It is investigated that this process gives better savings in operating cost (= 55.85%) and TAC (= 13.25%) with a payback period of 2.97 yr. Adding a 10% penaltya to the capital cost, the payback period has increased to 3.91 yr.

6.2.3. Heat Intensified PSDWC Figure 6 shows that the column CD2 is operating at high pressure (= 607.95 kPa) so the temperature of vapor, T3 (= 386.89 K) leaving the tower is higher than the temperature of liquid in Reboiler 1 (= 378.60 K) and Reboiler 2 (= 356.95 K). Clearly, the thermal driving force of more than 15 K exists between the vapor at T3 and the Reboiler 2 liquid, thus these two are thermally coupled. This vapor can only give 416.92 kW of latent heat and the additional amount of energy (= 1371.4 kW) is obtained externally from steam to separate the ternary mixture into its pure constituent. The data listed in Table 2 shows that the heat intensified PSDWC reduces energy consumption by 30.88%. Interestingly, HiPSDWC eliminates the application of heat pump as done in VRPSDWC, and it uses only one condenser unlike its parent configuration (PSDWC). Table 3 reports the cost associated with the different components of HiPSDWC. With reference to its conventional counterpart, it is observed that this proposed scheme offers 28.02% savings in TAC. From this comparative study it is evident that HiPSDWC performs better than TCPSD and PSDWC in terms of cost and energy consumptions. Furthermore, it is also a better option than VRPSDWC when priority is only given on the economic performance. Compared to heat integrated distillation column (HIDiC), PSDWC is simpler in terms of design and operation because it does not require any additional thermal arrangements along the column height. Actually, the implementation of several vertically placed heat exchangers is itself a challenging task and perhaps for this, so far there is no industrial application of

a

It is rational to include extra penalties to the cost estimation of PSDWC and its derived form because the plant layout, labor cost, etc. are involved there.



HIDiC noticed35. At the same time, unlike the HIDiC, a single DWC column can separate mixtures consisting of three or more components into pure products. On the other hand, the application of heat pump assisted distillation is well known.36 According to Fony and Benko37, when there are several restrictions on the integrability of the distillation column, the realization of heat pump assisted distillation is the promising energy saving technique. Since the heat pump systems are easy to introduce and the operation of plant generally becomes simple, the vapor recompression system is recommended to couple with PSDWC with the aim of an improved performance.

6.3. Uncertainty in cost estimation There are a couple of parameters considered that may create uncertainty in cost estimation of the proposed heat integrated schemes, namely TCPSD, PSDWC, VRPSDWC and HiPSDWC. They include Marshall and Swift (M&S) inflation index, and overall heat transfer coefficient of reboiler ( U Reb ) and condenser ( U Con ). It is reported31 that the M&S index was 100 in 1926 and its latest value available is 1672 in 2011. We are sure that in the current quarter, this index is greater than 1672 and thus, it is typically varied from 1338 to 2006. On the other hand, both U Reb and U Con differ considerably based on their respective medium used. Like M&S index, these coefficients are also widely varied as shown in Figure 10. The impact of these parameters is shown on the TAC. As mentioned before, U Reb and U Con are adopted33 as 1420 W/(m2.K) and 852 W/(m2.K), respectively. It is observed that the

change in TAC savings of PSDWC is insignificant with the change of reboiler and condenser heat transfer coefficient, and Marshall and Swift inflation index. On the other hand, for a fixed M&S index and U Con , the increase in U Reb increases the TAC savings of HiPSDWC marginally and a bit more in case of VRPSDWC, whereas it is reverse for U Con when M&S index and U Reb remain unchanged. Again, for a fixed U Reb and U Con , the change in Marshall and Swift index significantly affects the TAC savings of VRPSDWC mainly because of its involvement with two compressors units that leads to a large change in capital investment. Unlike VRPSDWC, the HiPSDWC has no compressor and thus, the effect is not so significant on TAC savings.


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7. Conclusions This article introduces a novel thermal integration scheme to separate a ternary mixture of methanol/benzene/acetonitrile that forms multiple azeotropes. Conventionally, this mixture is fractionated into pure components by means of a triple column pressure-swing distillation, each column of which works at different pressure. Here, we propose to combine the two towers of TCPSD into one column having a dividing wall. This scheme is named as pressureswing dividing wall column. The performance of PSDWC is further enhanced by the use of heat pump. For a meaningful comparison between the proposed schemes over the conventional TCPSD, an open loop control policy is formulated. A comparative evaluation is made among these candidates in terms of two performance indicators, namely energy savings and TAC. The proposed VRPSDWC has shown appealing performance over the other schemes. It secures 75.67% savings in energy consumption. The best performance by VRPSDWC is also accomplished by 13.25% TAC saving with a payback period of 2.97 yr (3.91 yr with considering 10% penalty).

Supporting Information This information is available free of charge via the Internet at http://pubs.acs.org/. The Supporting Information includes the simulation algorithm (that describes the computational steps involved in developing the process simulator of TCPSD and PSDWC), and Table S1 (that reports the thermodynamic correlations and property data for the methanol/benzene/acetonitrile system).

Nomenclature Abbreviation BHP

Brake horsepower

CI

Capital investment

CR

Compression ratio

DWC

Dividing wall column

HIDiC

Heat integrated distillation column

HiPSDWC

Heat intensified pressure-swing dividing wall column

HP

High pressure

LP

Low pressure



M&S

Marshall and Swift inflation index

M-B-A

Methanol/benzene/acetonitrile

OC

Operating cost

Pdt

Product

PP

Payback period

PSDWC

Pressure-swing dividing wall column

VRPSDWC Vapor recompressed pressure-swing dividing wall column RCM

Residue curve map

TAC

Total annual cost

TCPSD

Triple column pressure-swing distillation

Symbol A

Area of heat exchanger (m2)

AHT

Heat transfer area of dividing wall (m2)

C1 , C 2 , C3

Column index of TCPSD

CD1 , C D2

Column index of PSDWC

CP

Heat capacity at constant pressure (J/(mol.K))

CV

Heat capacity at constant volume (J/(mol.K))

D

Distillate flow rate (kmol/hr)

dc

Column diameter (m)

F

Feed flow rate (kmol/hr)

f et

Energy factor

f

Cost factor

H

Enthalpy (J/mol)

hc

Tower height (m)

hliq

Liquid height (m)

hps

Tray spacing (m)

hT

Stack height of trays (m)

hw

Weir height (m)

K

Vapor-liquid equilibrium constant

L

Liquid flow rate (mol/min)


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mn

Plate liquid holdup (mol)

mS

Steam flow rate (mol/min)

P

Pressure (kPa)

Qcons

Heat consumption (kW)

Qcomp

Heat duty of compressor (kW)

QCV

Heat released from compressed vapor (kW)

QE

Heat obtained from steam (kW)

Qw , n

Heat transferred through the dividing wall (kW)

R

Reflux

TB

Bottom liquid temperature (K)

TH

Hot zone temperature (K)

TL

Cold zone temperature (K)

Tn T C

Vapor temperature at the outlet of compressor (K)

Tn T

Vapor temperature at the overhead of tower (K)

TS

Steam temperature (K)

U

Overall heat transfer coefficient (W/(m2.K))

V

Vapor flow rate inside the column (mol/min)

Vn T

Vapor flow rate from the top of column (mol/min)

Vn T C

Amount of vapor flow into the compressor (mol/min)

x

Liquid composition (mol fract)

y

Vapor composition (mol fract)

∂

Cost factor for dividing wall

γ

Liquid activity coefficient

λ

Latent heat (J/mol)

µ

Polytropic coefficient

∆T

Thermal driving force (K)

Subscript/superscript 1,2,3

Column index

Hi

Heat integration



i

Inlet

j

Component index

LMTD

Logarithmic temperature difference

n

Tray index

o

outlet

s1 , s 2 , s3 , s 4

Configuration index

sat

Saturation

References (1) International Energy Outlook 2016. https://www.eia.gov/outlooks/ieo/pdf/0484(2016).pdf (accessed May 2016). (2) Beach, F. Leveraging Natural Gas to Reduce Greenhouse Gas Emissions. https://www.c2es.org/site/assets/uploads/2013/06/leveraging-natural-gas-reduce-ghgemissions.pdf (accessed June 2013). (3) Mix, T. W.; Dweck, J. S.; Weinberg, M; Armstrong, R. C. Energy Conservation in Distillation. Chem. Eng. Prog. 1978, 74, 49-55. (4) Díez, E.; Langston, P.; Ovejero, G.; Romero, M. D. Economic Feasibility of Heat Pumps in Distillation to Reduce Energy Use. Appl. Therm. Eng. 2009, 29, 1216-1223. (5) Bruinsma, D.; Spoelstra, S. Heat Pumps in Distillation. Distillation & Absorption Conference, Eindhoven, The Netherland, Sept 12-15, 2010. (6) Kaibel, G. Distillation Columns with Vertical Partitions. Chem. Eng. Technol. 1987, 10, 92-98. (7) Schultz, M. A.; Stewart, D. G.; Harris, J. M.; Rosenblum, S. P.; Shakur, M. S.; O’Brien, D. E. Reduce Costs with Dividing-Wall Columns. Chem. Eng. Prog. 2002, 98, 64-71. (8) Yildirim, Ö.; Kiss, A. A.; Kenig, E. Y. Dividing Wall Columns in Chemical Process Industry: A Review on Current Activities. Sep. Purif. Technol. 2011, 80, 403-417. (9) Kiss, A. A.; David, J.; Suszwalak, P. C. Enhanced Bioethanol Dehydration by Extractive and Azeotropic Distillation in Dividing-Wall Columns. Sep. Purif. Technol. 2012, 86, 70-78. (10) Xia, M.; Yu, B.; Wang, Q.; Jiao, H.; Xu, C. Design and Control of Extractive DividingWall Column for Separating Methylal-Methanol Mixture. Ind. Eng. Chem. Res. 2012, 51, 16016-16033.


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(11) Petlyuk, F. B. Distillation Theory and Its Application to Optimal Design of Separation Units; Cambridge University Press: New York, 2004. (12) Luyben, W. L.; Chien, I. L. Design and Control of Distillation Systems for Separating Azeotropes; Wiley: New York, 2010. (13) Aptel, P.; Challard, N.; Cuny, J.; Neel, J. Application of the Pervaporation Process to Separate Azeotropic Mixtures. J. Membr. Sci. 1976, 1, 271-287. (14) Wang, L.; Li, J.; Lin, Y.; Chen, C. Separation of Dimethyl Carbonate/Methanol Mixtures by Pervaporation with Ploy(Acrylic Acid)/Poly(Vinyl Alcohol) Blend Membranes. J. Membr. Sci. 2007, 305, 238-246. (15) Xia, S.; Wei, W.; Liu, G.; Dong, X.; Jin, W. Pervaporation Properties of Polyninyl Alcohol/Ceramic Composite Membrane for Separation of Ethylacetate/Ethanol/Water Ternary Mixtures. Korean J. Chem. Eng. 2012, 29, 228-234. (16) Alkhudhiri, A.; Darwish, N.; Hilal, N. Membrane Distillation: A Comprehensive Review. Desalination 2012, 287, 2-18. (17) Zhu, Z.; Wang, L.; Ma, Y.; Wang, W.; Wang, Y. Separating An Azeotropic Mixture of Toluene and Ethanol via Heat Integration Pressure Swing Distillation. Comput. Chem. Eng. 2015, 76, 137-149. (18) Luyben, W. L. Comparison of Extractive Distillation and Pressure-Swing Distillation for Acetone-Methanol Separation. Ind. Eng. Chem. Res. 2008, 47, 2696-2707. (19) Asprion, N.; Kaibel, G. Dividing Wall Columns: Fundamental and Recent Advances. Chem. Eng. Process. 2010, 49, 139-146. (20) Kiss, A. A.; Rewagad, R. R. Energy Efficient Control of A BTX Dividing-Wall Column. Comput. Chem. Eng. 2011, 35, 2896-2904. (21) Li, Y.; Xia, M.; Li, W.; Luo, J.; Zhong, L.; Huang, S.; Ma, J.; Xu, C. Process Assessment of Heterogeneous Azeotropic Dividing-Wall Column for Ethanol Dehydration with Cyclohexane as An Entrainer: Design and Control. Ind. Eng. Chem. Res. 2016, 55, 8784-8801. (22) Knapp, J. P.; Doherty, M. F. A New Pressure-Swing-Distillation Process for Separating Homogeneous Azotropic Mixtures. Ind. Eng. Chem. Res. 1992, 31, 346-357. (23)

Zhu,

Z.;

Xu,

D.;

Liu,

X.;

Zhang,

Z.;

Wang,

Y.

Separation

of

Acetonitrile/Methanol/Benzene Ternary Azeotrope via Triple Column Pressure-Swing Distillation. Sep. Purif. Technol. 2016, 169, 66-77. (24) Jana, A. K. A New Divided-Wall Heat Integrated Distillation Column (HIDiC) for Batch Processing: Feasibility and Analysis. Appl. Energy 2016, 172, 199-206. 23 ACS Paragon Plus Environment


(25) Luyben, W. L. Process Modeling, Simulation and Control for Chemical Engineers, 2nd ed.; McGraw-Hill: New York, 1989. (26) Reid, R. C.; Sherwood, T. K.; Prausnitz, J. M. The Properties of Gases and Liquids; McGraw-Hill: New York, 1987. (27) Hildebrand, J. H.; Scott, R. L. The Solubility of Non-Electrolyte, 3rd ed.; Reinhold: New York, 1950. (28) Douglas, J. M. Conceptual Design of Chemical Processes, 1st ed.; McGraw-Hill: New York, 1988. (29) Iwakabe, K.; Nakaiwa, M.; Huang, K.; Nakanishi, T.; Røsjorde, A.; Ohmori, T.; et al. Energy Savings in Multicomponent Separation using An Internally Heat-Integrated Distillation Column (HIDiC). Appl. Therm. Eng. 2006, 26, 1362-1368. (30) Huang, K.; Shan, L.; Zhu, Q.; Qian, J. Adding Rectifying/Stripping Section Type Heat Integration to A Pressure-Swing Distillation (PSD) Process. Appl. Therm. Eng. 2008, 8, 923-932. (31) Economic Indicator 2012. https://www.scribd.com/document/168895829/CEPCI-2012 (accessed Jan 25, 2016). (32) Sun, L. Y.; Chang, X. W.; Qi, C. X.; Li, Q. S. Implementation of Ethanol Dehydration Using Dividing-Wall Heterogeneous Azeotropic Distillation Column. Sep. Sci. Technol. 2011, 46, 1365-1375. (33) Shi, L.; Huang, K.; Wang, S. J.; Yu, J.; Yuan, Y.; Chen, H.; Wong, D. S. H. Application of Vapor Recompression to Heterogeneous Azeotropic Dividing-Wall Distillation Columns. Ind. Eng. Chem. Res. 2015, 54, 11592-11609. (34) Kern, D. Q. Process Heat Transfer, 1st ed.; McGraw-Hill: New York, 2001. (35) Harwardt, A.; Marquardt, W.

Heat-Integrated Distillation Columns: Vapor

Recompression or Internal Heat Integration?. AIChE J. 2012, 58, 3740–3750. (36) Jana, A. K. Advances in Heat Pump Assisted Distillation Column: A Review. Energy Convers. Manag. 2014, 77, 287-297. (37) Fonyo, Z.; Benkö, N. Comparison of Various Heat Pump Assisted Distillation Configurations. Trans. Inst. Chem. Engrs. 1998, 76, 348–360.


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Figure 1. A triple column pressure swing distillation (TCPSD) for separation of a ternary mixture of methanol/benzene/acetonitrile.



Figure 2. The pressure-swing dividing wall column (PSDWC) for separation of a ternary mixture of methanol/benzene/acetonitrile.


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Figure 3. Residue curve maps of methanol/benzene/acetonitrile: a) 101.33 kPa; b) 193.32 kPa; c) 607.95 kPa.



Figure 4. Distillation boundaries position: a) TCPSD; b) PSDWC.


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Figure 5. The vapor recompressed pressure-swing dividing wall column (VRPSDWC) for separation of a ternary mixture of methanol/benzene/acetonitrile.



Figure 6. Heat intensified pressure-swing dividing wall column (HiPSDWC) for separation of a ternary mixture of methanol/benzene/acetonitrile.


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Figure 7. Mechanism of heat transfer, input and output streams associated with an equilibrium stage of the PSDWC.



Figure 8. Dynamics of bottom liquid composition: a) TCPSD; b) PSDWC.


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Figure 9. Pressure-swing dividing wall column: a) Effect of pressure on product purity; b) Temperature profile.



Figure 10. TAC savings influenced by the uncertainty involved with: a) Marshall and Swift inflation index; b) reboiler heat transfer coefficient; c) condenser heat transfer coefficient.


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Table 1. Input and steady state data: TCPSD and the proposed configuration. Terms

TCPSD

PSDWC

System: methanol/benzene /acetonitrile

Input information (feed condition) Composition (mol fract)

0.78/0.05/0.17

Temperature (K)

0.78/0.05/0.17

341.41

Flow rate (kmol/hr)

341.41

28

28

Input information (column) C1

C2

C3

CD1

CD2

46

49

14

42/4

14

Feed location

10/18

31

11

20/25

11

Column pressure (kPa)

607.95

101.33

607.95

193.32

607.95

Column diameter (m)

0.66

0.72

0.32

0.66

0.32

2.87

0.46

Tray

Weir height (m) Reflux ratio

0.127 (= 5 inch) 1.32

2

0.01

Liquid distributionI (%)

61

Steady state information Pdt1 flow rate (kmol/hr)

4.80

4.84

Pdt2 flow rate (kmol/hr)

21.93

21.88

Pdt3 flow rate (kmol/hr)

1.28

1.28

I

percent of flowing liquid directed to left side of the wall.



Page 36 of 38

Table 2. Energy profile of conventional and proposed schemes. Entities

TCPSD

PSDWC

VRPSDWC

µ

1.16/1.15

CR

2.36/2.65

Condenser duty (kW)

1969.91

1773.13

Energy generated internally (kW) Energy supplied externally (kW)

1984

1788.3

Compressor duty (kW) Energy consumed (kW)

HiPSDWC

344.48

1356.21

1725.5

416.92

62.81

1371.4

139.94 1984

Energy savings (%)

1788.3

482.62

1371.4

9.86

75.67

30.88


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Table 3. Economic analysis of the conventional and proposed schemes. Component cost

TCPSD

PSDWC

VRPSDWC

HiPSDWC

7.43

6.50

6.50

6.50

0.515

0.451

0.451

0.451

4.28

2.98

0.402

2.59

4.59

0.919

1.28

3.11

Capital investment Column (105$) 5

Tray (10 $) I

5

Reboiler (10 $) II

5

Bottom reboiler (10 $) 5

Condenser (10 $)

5.61

4.55

Compressor (105$)

12.83

Total CI (105$)

17.83

14.48

26.05

13.57

Steam (104ton/yr)

2.60

2.34

0.082

1.79

Steam cost (105$/yr)

4.42

3.98

0.139

3.05

Coolant amount (105 ton/yr)

9.04

8.14

1.58

6.23

Coolant cost (104$/yr)

5.43

4.88

0.95

3.74

Operating cost Reboiler

Condenser

Compressor Electricity cost (105$/yr)

1.96

Total OC (105$/yr)

4.96

4.47

2.19

3.43

9.88

55.85

30.85

7.37

7.40

6.14

TAC savings (%)

13.60

13.25

28.02

Payback period (yr)

-6.84

2.97

-2.78

-3.88III

3.91

-1.89III

OC savings (%) TAC (5 yr) (105$/yr)

8.53

Payback period (with a penalty of 10% added to the CI) (yr) I

Steam heated reboiler;

II

Compressed vapor heated reboiler;

III

The negative payback period is obtained since

the concerned scheme provides savings in both capital and operating cost.



Table of Contents (TOC) Graphic

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Pressure-Swing Dividing Wall Column with Multiple Binary Azeotropes

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