An Adaptive Vapor Recompression Scheme for a Ternary Batch

Feb 24, 2012 - In an effort to reduce energy consumption, a batch distillation with a side withdrawal (BDS) was proposed recently. In this contributio...
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An Adaptive Vapor Recompression Scheme for a Ternary Batch Distillation with a Side Withdrawal G. Uday Bhaskar Babu, Etendra K. Pal, and Amiya K. Jana* Department of Chemical Engineering, Indian Institute of Technology-Kharagpur, West Bengal-721 302, India ABSTRACT: Batch distillation is known to be less energy efficient than its continuous counterpart. However, it has received renewed interest last years owing to the flexibility it offers. In an effort to reduce energy consumption, a batch distillation with a side withdrawal (BDS) was proposed recently. In this contribution, we first construct the fundamental model for an unsteady state BDS process and simulate the column to derive an operational policy. Although the representative process was proved as an energy efficient scheme, in order to improve further the thermodynamic efficiency of the BDS structure, we propose an adaptive vapor recompression mechanism that considers the top vapor recompression with a variable speed. Apart from the compression ratio (CR), the other two manipulated variables involved are the external heat input to the reboiler through steam and the flow rate of overhead vapor entering the compressor. It is investigated that the two manipulated variables are required to adapt simultaneously throughout the batch operation; one is the CR and another one is either the heat input to the still or the overhead vapor rate. Finally, the influence of the adaptive vapor recompression scheme on the energetic and economic aspects is evaluated through intensive comparison against the conventional stand alone column. The proposed adaptive vapor recompression strategy appears overwhelmingly superior to its conventional counterpart in terms of thermal efficiency and economic performance.

1. INTRODUCTION Batch distillation is a very efficient unit for the separation of multicomponent mixtures into pure components. It is widely used in the fine chemical and pharmaceutical industries that work with a high variety of products with high added value that are changing continuously following market fluctuations and have a small lifetime. Because of the rapid growth of the fine chemical, biochemical, polymer, and pharmaceutical industries, the demand of batch distillation is increasing exponentially. Although it offers a number of appealing advantages over continuous distillation, several intrinsic drawbacks,1 such as long batch time, high temperature in the still pot, and complex operation, are associated with the regular/conventional batch distillation. To overcome these shortcomings, various alternative batch column configurations have been explored.1−4 Among them, the probably most popular and efficient schemes include the middle vessel batch distillation and batch distillation with a side withdrawal (BDS). Conventional batch distillation is known to be thermodynamically less efficient compared to its continuous counterpart.3 It is shown2,3,5 that the multivessel batch columns, including the double-vessel BDS structure, can exhibit advantages in terms of separation time or energy requirements. Therefore, in this work, the relatively new BDS column is selected as a prospective candidate for further investigation. By enhancing the thermal efficiency of a process, along with the energy and cost savings, the flue gas emissions, usually associated with energy consumption, to the environment can also be reduced. Keeping these alarming issues in mind, we explore the possibility of heat integration of the BDS structure aiming to improve further the energy efficiency. Various heat integration methods have been proposed in literature for reducing the external energy input by effectively utilizing the internal heat source. A number of research groups6−15 are © 2012 American Chemical Society

devoted in developing the thermally integrated distillation technology. Fewer than a half-dozen papers dealing with the heat integration of batch distillation have appeared so far, so this area is only beginning to be explored. Takamatsu et al.16 first introduced an internally heat integrated batch distillation and subsequently, Maity et al.17 adopted their approach for further investigation and development. Most recently, Johri et al.18 proposed a novel external energy integration approach, which is based on the concept of direct vapor recompression column (VRC), for a regular batch rectifier. In this contribution, their work is extended to explore an adaptive VRC scheme for a relatively new ternary batch column with a side withdrawal and two charges. On the basis of our knowledge, this is the first work that deals with the heat integration of a multicomponent batch distillation with a side withdrawal (BDS).

2. THE PROCESS Figure 1 represents a novel process for the separation of ternary mixtures via cyclic operation of a batch distillation provided with a side withdrawal.1,4 The process consists of a distillation tower equipped with still pot and reflux drum (two vessels), to which the charge is loaded at the beginning of the batch operation. This column can be visualized as a batch stripper placed above the batch rectifier. Since the representative column has both a rectifying and a stripping section, it is not difficult to obtain a light and a heavy fraction simultaneously from the top and the bottom of the column, while an Received: Revised: Accepted: Published: 4990

July 2, 2011 February 7, 2012 February 24, 2012 February 24, 2012 dx.doi.org/10.1021/ie201413p | Ind. Eng. Chem. Res. 2012, 51, 4990−4997

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Table 1. Operating Conditions and Specifications of BDS system

cyclohexane/n-heptane/ toluene

feed composition (cyclohexane/n-heptane/ 0.2/0.6/0.2 toluene) fresh feed charged (reflux drum/trays/reboiler), 15000/43 × 500/15000 mol reboiler duty (startup phase), cal/min 1.6 × 108 intermediate product flow rate (production 300 phase), mol/min column pressure, mm Hg 760 tray efficiency 80% tray volume, mL 0.064 × 106 Composition (Cyclohexane/n-Heptane/Toluene) at the Steady State (at 35th min) reflux drum composition 0.6263/0.3667/0.0070 31st stage composition 0.0035/0.9503/0.0462 still composition 0.0/0.5080/0.4920 Composition (Cyclohexane/n-Heptane/Toluene) at the End of Batch Operation (at 135th min) [Constant Reflux Ratio (=1.00655) and Reboil Ratio (=1.00355)] reflux drum composition 0.8959/0.1028/0.0013 intermediate product composition 0.0535/0.850/0.0965 still composition 0.0/0.0063/0.9937

Operational Policy. At the beginning, the reflux drum, column trays, and still are all initially filled with the liquid feed mixture. Heat is added to the still for producing vapor that is fed into the tower. Like the batch rectifier, the CBDS can be operated in two consecutive phases/modes, namely the startup phase or the close operation mode and the production phase or the open operation mode. Both the mechanisms are detailed below. Startup Phase. In this mode of operation, the column is run at total reflux condition with no withdrawal of product. Naturally, the light (cyclohexane) and heavy boiler (toluene) accumulate in the top and bottom vessels, respectively. During the same time period, the concentration of the intermediate component (n-heptane) gradually increases in a region of intermediate stages. Finally the column reaches steady state, taking about 35 min (startup period). Table 1 includes the steady-state composition and Figure 3 represents the steadystate tray composition profile. Simulation results clearly indicate that a maximum purity of intermediate component is

Figure 1. Schematic representation of the BDS column.

intermediate fraction can also be recovered from an intermediate stage. The distillation tower has a total 43 trays, excluding the total condenser and still. The numbering has started from bottom up; tray 1 is the bottommost tray and tray 43 is the topmost one. The model of the ternary system is derived for a typical nth tray (Figure 2) in the Appendix. The model and column specifications are presented in Table 1.

Figure 2. Quantities associated with a typical nth tray.

The batch distillation model consists of a set of ordinary differential equations (ODEs) coupled with algebraic equations/correlations. The ODEs are obtained from mass and energy balances around each plate of the distillation column. The algebraic equations are used to predict the thermodynamic and physical properties, plate hydraulics, and actual vapor-phase compositions. To simulate the modeling equations, the computer codes are developed in the MatLab environment. Next, we formulate the operational policy of the example process. In the sequel, we denote this column as the conventional batch distillation with a side withdrawal (CBDS).

Figure 3. Concentration profile at steady state. 4991

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achieved in tray 31. Therefore, this tray 31 is selected as a withdrawal stage for the side product. Production Phase. Steady-state results (Table 1) reveal that the maximum achievable n-heptane purity in tray 31 is about 0.95. After 35 min of startup operation, the production phase starts, and the intermediate component is taken out with a flow rate of 300 mol/min until the composition remains above 0.85. Unlike the regular batch rectifier, the CBDS has two refluxes: one is the top reflux and other one is the side reflux associated with tray 31. In open operation mode, the top reflux and bottom reboil ratios are kept constant and they have been determined based on the sensitivity analysis. Table 1 provides this information along with the composition at the end of batch cycle. The transient behavior of the representative CBDS system is demonstrated in Figure 4 in terms of composition dynamics. Furthermore, Figure 5 depicts the temperature profile along the column length at steady state condition. For the example system, we obtain the production period of nearly 100 min (actually 100.07 min) with average compositions of about 0.896 for top, 0.90 for intermediate, and 0.861 for bottom products. Apart from this typical mode of batch operation, one may achieve higher product purities by following either of the two operating policies suggested below for the CBDS column during the production period. (i) After reaching the average intermediate product purity at its specified value (here ∼90%), we can stop the batch operation and thereafter, collect the reflux drum liquid (∼89.6% pure) and reboiler content (∼99.4% pure) as the top and bottom products, respectively, with treating the tray liquid in the subsequent batch run. (ii) Alternatively, beyond the specified time period (i.e., 100 min), a off-cut can be collected in order to further purify the light and heavy boiler from the remaining intermediate boiler. In this latter approach (not followed here), the slop-cut can either be recycled to the column or be treated in the subsequent batch cycle. In the next phase of our study, we aim to develop a novel adaptive VRC strategy that can be implemented in both startup and production period.

3. A NOVEL ADAPTIVE VRC:AVRBDS SCHEME It is true that the CBDS column is more flexible and thermodynamically efficient scheme compared to the regular batch rectifier. The key objective of this work is to reduce further the energy consumption of the CBDS. For this, we develop an adaptive vapor recompression BDS (AVRBDS) configuration shown in Figure 6. Aiming to utilize the internal heat source, the overhead vapor is compressed so that there exists a reasonable temperature difference (ΔT0) between the compressed overhead vapor (T43,0) and bottom liquid (TB). Here we assume that the total condensation of overhead vapor occurs in the still only if ΔT0 ≥ 20 K. For adjusting the pressure, the condensate formed at higher pressure is returned to the reflux drum through a throttling valve. The following equation has been used to estimate the theoretical horsepower (hp) for a centrifugal gas compressor:20 ⎤ ⎡ (μ− 1)/ μ (3.03 × 10−5)μ ⎥ ⎢⎛ Po ⎞ − 1⎥ V43Pi⎢⎜ ⎟ hp = μ−1 ⎝P ⎠ ⎦ ⎣ i

Figure 4. Composition profile of (A) reflux drum, (B) intermediate product, and (C) still at startup (first 35 min) and production phase (until 135 min).

rate (V43) is in ft3/min. The polytropic coefficient (μ) is assumed here as 1.3. 3.1. Estimation of Manipulated Variables. Like the CBDS, the adaptive vapor recompression structure operates at both startup and production phases. To present a meaningful comparison between the CBDS and AVRBDS, here the process dynamics is kept unchanged as much as possible throughout the batch operation. Recall that the startup phase runs under complete reflux condition with constant reboiler duty. On the

(1)

In the above equation, the pressure (inlet pressure, Pi, and outlet pressure, Po) is in lbf/ft2, and the overhead vapor flow 4992

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Here, T43 denotes the temperature of overhead vapor. Note that the CR should vary throughout the batch operation due to its dynamic behavior. Overhead vapor inflow rate (when QCV > QR). We aim to properly utilize the latent heat of compressed vapor (QCV) in order to reduce or avoid the heat input from external source to the still. The reboiler duty involved in CBDS is represented by QR. Note that when QCV > QR, we need to split the overhead vapor flow (V43) as V43 = V43i + V43c

(3)

where V43i is an input stream to the compressor and V43C is left for condensation. Obviously, the AVRBDS requires a trimcondenser. In addition, there is no need of any external heat source, and V43i is manipulated as

Figure 5. Tray temperature profile of CBDS column at steady state.

V43i λ(T43o) = Q R

(4)

where λ denotes the latent heat. External heat input to the still (when QR > QCV). First we attempt to utilize the internal heat source in the still. Interestingly, when QR > QCV, we need an external heat source (i.e., steam) to provide the remainder of the heat (Qr) to the still pot. Obviously, here the condenser is not required and the auxiliary heat inflow rate Qr is adjusted as Q r = Q R − Q CV

(5)

To present a fair comparison between the CBDS and AVRBDS, we aim to conduct a comparative study with fixed dynamical performance. Therefore, the total amount of heat input from internal and/or external source to the reboiler should match the value of QR. 3.2. Energy Savings. The heat load involved in operating the thermally integrated column Q is estimated by the sum of the still duty Qr plus three times the compressor duty Qcomp obtained using eq 1. Accordingly, Q = Q r + 3Q comp

The factor of 3 for the compression duty is supposed to convert the compressor work into the thermal energy needed to produce an equivalent amount of electrical power, and is determined empirically, taking into account the energy costs of electricity in Japan.21 For the example batch system, eq 6 is used for determining the energy consumption in both startup phase (QSP) as well as production phase (QPP). The total energy consumption of the AVRBDS (QAVRBDS) is then calculated as

Figure 6. Schematic representation of the adaptive vapor recompression BDS structure.

other hand, the top reflux and bottom reboil ratios are remained fixed in the production period. Therefore, the heat input to the reboiler varies throughout the production phase. In the AVRBDS scheme, three manipulated variables, namely the compression ratio (CR), the inflow rate of overhead vapor to the compressor, and the heat supplied to the reboiler by an external agency, are proposed. Actually, the CR is considered as an operating variable, whereas the other two manipulated variables are the process variables. In the following, a mechanism is devised for the adjustment of these variables. Compression ratio (CR). It is required to adapt aiming to ensure ΔT0 ≥ 20 K that leads to the complete condensation of compressed vapor in the still. It can be calculated as18 μ /(μ− 1) Po ⎛ T43o ⎞ CR = ⎟ =⎜ Pi ⎝ T43 ⎠

(6)

Q AVRBDS = Q SP + Q PP

(7)

It is supposed that in every year, 3556 batch cycles (≡ 8001 h) are operated. We calculate the percent energy savings using energy savings (%) =

Q CBDS − Q AVRBDS × 100 Q CBDS

(8)

3.3. Economic Evaluation. Of course, when assessing the relative economics of different arrangements, one must take the capital cost of the system into account as well as the operating costs. In this work, an economic comparison is made in terms of total annual cost (TAC), which includes both the yearly

(2) 4993

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gradual accumulation of low and high boilers in the top and bottom section, respectively. We should note an interesting behavior in Figure 7 that immediately after the production phase is started, there is an oscillatory nature of ΔTT that continues for sometimes with decreasing amplitude. We obtain this type of response mainly because of two reasons:24,25 the sudden shifting of total reflux operation to partial reflux mode, and (ii) the withdrawal of side product with a reasonably large flow rate. Anyway, the simulation results show that the maximum ΔTT is obtained at the end point (at 135th min (say at t1-th time)). It implies that the maximum CR is required at t1-th time. Table 3 reports the effect of CR on ΔT0 with respect to t1-th time step. The latent heat released by the compressed vapor

operating cost and 33.33% of capital cost, according to assumed plant lifetime of 3 years. It is expressed as17 TAC ($/yr) = yearly operating cost +

capital investment payback period (9)

The costs of equipments (distillation column, heat exchanger, and compressor) are included within the capital investment, and the operating cost combines the cost of utilities (heating steam, cooling water, and electricity). The cost estimating formulas used in this paper are given in Table 2. The operating Table 2. Cost Estimating Formula and Parameter Value20,22 column shell

((M & S)/280)101.9Dc1.066Lc0.802(cin + cmcp) where Dc is the column diameter, Lc is the column height, M & S = 1569, and the coefficients cin = 2.18, cm = 3.67, and cp = 1.2

column tray

((M & S)/280)4.7Dc1.55Lc(cs + ct + cm) where the coefficients cs = 1, ct = 0, and cm = 1.7

heat exchanger

((M & S)/280)101.3A0.65(cin + cm(ct + cp))

Table 3. Effect of CR on ΔT0 (= T43, 0 − TB) at t1-th Time CR 1 1.1 1.2 1.5 1.7 1.74 1.75 1.8 2

where the coefficients cin = 2.29, cm = 3.75, ct = 0.1, and cp = 1.35 compressor

((M & S)/280)517.5(bhp)0.82(2.11 + Fd) where Fd = 1.0. This expression is valid in the range of 30 < bhp