Parameter analysis and optimization of ideal heat integrated distillation columns (HIDiC)

European Symposium on Computer Aided Process Engineering - 10 S. Pierucci (Editor) 9 2000 Elsevier Science B.V. All rights reserved.

661

Parameter Analysis and Optimization of Ideal Heat Integrated Distillation Columns (HIDiC) Masaru Nakaiwa a, Kejin Huang a, Kiyoshi Naito a, Akira Endo a, Masaru Owa a, Takaji Akiya a, Takashi Nakane a and Takeichiro Takamatsu b aNational Institute of Materials and Chemical Research, Tsukuba 305-8565, Japan [email protected] p blnstitute of Industrial Technology, Kansai University, Suita 564-8680, Japan [email protected] Parametric analysis is performed for ideal heat integrated distillation columns (HIDiC) and heuristics are provided for the effective process design. A better process configuration is suggested, which is demonstrated to have both higher energy efficiency and higher flexibilities than its original configuration. Simulation results confirm the conclusion. 1. INTRODUCTION Ideal heat integrated distillation column (HIDiC) was created by chasing reduction of energy consumption in distillation columns. The way to pursue energy reduction is to adopt heat integration between rectifying and stripping sections and this results in a seemingly far different counterpart of conventional distillation columns. Figure 1 shows a representation of the process and Table 1 lists a representative operating condition. The ideal HIDiC is such a process that its stripping section and rectifying section are separated into two columns, while connected through a number of internal heat exchangers. To accomplish internal heat transfer from the rectifying section to the stripping section, the rectifying section is operated at a higher pressure and a higher temperature than those of the stripping section. For adjusting the pressures a compressor and a throttling valve have to be installed between the two sections. Owing to the heat integration, a certain amount of heat is transferred from the rectifying section to the stripping section and generates the reflux flow for the rectifying section and vapor flow for the stripping section. Thus the condenser and reboiler are, in principle, not needed, as a result, both fixed and operating cost could be reduced. The synthesis and analysis of the ideal HIDiC were already discussed thoroughly by Takamatsu and his coworkers [1]. The examination of process dynamics and operation were conducted by Nakaiwa et al. [2, 3] recently. Although these studies indicated the ideal HIDiC is certainly more energy efficient than conventional distillation columns and operation feasible, one remained question to be answered is the process flexibility. Does the ideal HIDiC hold its superiority in a large operation region over conventional distillation columns? If not, what measures should be taken to enhance the process flexibilities? These questions are very essential to the applicability of the process to the chemical process industry. In spite of sharp difference in process configurations and operation, there exist similarities in concept between the ideal HIDiC and conventional distillation colullms. Clarifying these similarities is very useful to understanding the principle of the ideal HIDiC and this constitutes another purpose of this work. In this work we will focus on investigating the static characteristics and synthesizing the optimum process configuration for the ideal HIDiC. Special emphasis will be paid to the enhancement of process flexibility to operating condition changes.

662 Table 1 Steady-state operating conditions for three processes Items Conventional 22 No. of stages 12 Feed stage Vapor feed Liquid feed 12 1.0 Stage holdup (kmol) 0.1013 Rectifying section pressure (MPa) 0.1013 Stripping section pressure (MPa) 100 Feed flow rate (kmol/h) Feed composition (Benzene) 0.5 (Toluene) 0.5 Feed thermal condition 0.5 2.4 Relative volatility Vaporization heat (kJ/kmol) 30001.1 9803 Heat transfer rate (W/K) 0.995 Overhead product composition 0.005 Bottom product composition

Ideal HIDiC 20 11 11 1.0 0.2963 0.1013 100 0.5 0.5 0.5 2.4 30001.1 9803 0.995 0.005

5.0

Better HIDiC 22 11 12 1.0 0.2026 0.1013 100 0.5 0.5 0.5 2.4 30001.1 9803 0.995 0.005

4.0 3.5

,..-., 4 0

3.0

Compressor

3.0

>.i

2.5

2,O 099

-

I 0994

I 0,996

I 0998

50

(a) 3.0

5.0

9 2.9

4.5

35 2.7

~ 3.0

26

2,5

2.5

20 0.I

Throttling valve

0,2 0.3 0.4 0,5 0 6 zf[-]

I 100 F [kmol/h]

I 125

150

(b)

i 4.0

Fig. 1. Schematic representation of an ideal HIDiC

i 75

2.O

yl [-]

F, zf

\---)

I 0,992

0.7 0.8

0,9

L.. 6

18

;

'/2

I 24

I 26

218

30

nl-] (c) (d) Fig.2. Functions of pressure difference, Pr-Ps

2. P A R A M E T R I C S T U D I E S F O R T H E I D E A L H I D i C 2.1 Functions of pressure difference, Pr'Ps The pressure difference, Pr-Ps, plays an important role in heat integration between the rectifying and the stripping sections. When process design has been finished, the purer the specifications for the overhead and bottom products become, the higher the pressure difference, Pr-Ps, will be needed (Fig. 2a). Up to a certain high product specifications, the advantage of the ideal HIDiC is expected to be totally lost, because electricity is generally several times more expensive than heating steams. It is therefore extremely necessary to assess the flexibility of the ideal HIDiC. 2.2 Functions of feed thermal condition, q The mass balance equation for the ideal HIDiC yields: zf : (1-q)yl+qxn

(1)

663 It is readily to understand that the feed thermal condition q influences only the material balance of the process. It is therefore reasonable to call it a variable for material balance control, although it is an energy term that reflects the thermodynamic state of the feed. 2.3 Influences of feed flow rates, F

Figure 2b illustrates the relations between feed flow rate and the pressure difference between the rectifying and the stripping sections, when the both end products have been kept on their specifications, respectively. The larger the feed flow rate is, the higher the pressure difference, pr-ps, will be. As the pressure difference is enhanced, the superiority of the ideal HIDiC will diminish because electricity is usually several times more expensive than heating steam. It is anticipated that up to a certain flow rate the potential of energy saving will be totally lost. On the other hand, when the feed flow rate becomes too small, the necessary pressure difference will go down drastically, and so does the process energy efficiency. Thus, after an ideal HIDiC has been constructed, only within a certain range of the feed flow rate, can the energy efficiency of the process be justified. It is, therefore, imperative to guarantee high process flexibility by intensive process design. 2.4 Influences of feed composition, zf

Figure 2c illustrates the necessary value of the pressure difference, Pr-Ps, between the rectifying and the stripping sections in order to keep both top and bottom products on their specifications, when the ideal HIDiC is fed with mixtures of different compositions. Around the region of feed composition equaling 0.5, the pressure difference, Pr-Ps, reaches its maximum value. Away from 0.5 it gradually decreases. As the variations of pressure difference is quite limited in magnitude (in this case, around 0.4 atm), it is reasonable to consider that feed composition will not influence the process energy efficiency substantially, and thus it will not impose strict requirements to the process flexibility, either. 2.5 Effect of feed location

As the feed is a vapor/liquid mixture (0
Figure 2d shows the relation between the total number of stages and the pressure difference, Pr-Ps, between the rectifying and the stripping sections. The illustration was obtained when the both end products are kept on their specifications, respectively. As the number of stages has been increased, the necessary pressure difference, Pr-Ps, becomes lower. In other words, the operating cost is reduced with the expense of fixed investment. When the total number of stages reaches a certain value, the direction of heat transfer from the rectifying to the stripping sections becomes difficult to be maintained, especially when extemal disturbances occur. Inverse heat transfer is detrimental to process energy efficiency and adds difficulties to process operation, thus it should be avoided. As the number of stages is further increased, the pressure difference reaches the minimum value and involves almost no changes. Under this circumstance, inverse heat transfer occurs and the net heat transfer remains the same as before, namely, Q = Q r s - Qsr = c o n s t

(2)

664

3. SUMMARY Pressure difference is the most important variable that influences the design of the ideal HIDiC. It is the way of energy input to the process. It is imperative to assess the economical operating region because electricity is generally several times more expensive than heating steams. Production specifications and feed flow rate are the main factors that can substantially affect the process energy efficiency. For pursuing consistent process design, not only should pressure difference be carefully chosen, but also effective measures have to be taken to guarantee the process with high flexibility, as drastic changes in operation conditions might occur due to market requirements and process retrofit. 3.1 A Better Process Configuration A better configuration of the ideal HIDiC is shown in Fig. 3, which is created with the aid of the above parametric studies. The vapor and liquid portions of the feed are divided and fed into the column at different locations. The vapor portion of the feed is introduced at a lower location than n/2+ 1 and liquid portion of the feed is introduced at a higher location than n/2+1. The addition of an overhead partial condenser and a bottom partial reboiler provides two extra degrees of freedom for process optimization. It is also extremely effective in improving flexibilities of the ideal HIDiC, because they can effectively overcome the influences introduced by feed flow rates and product specifications. For example, when the pressure difference is too high for reaching a separation, the condenser and reboiler can generate certain external reflux and reboil flows and move the process to an operating condition that is suitable for heat integration between the rectifying and stripping sections. In the sequel we will refer the new configuration as better HIDiC. Comer

... .......V' .....Yl .

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~ F(1-~ F, zf ;::

HeatT

,a~ ._'_'2....

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Ln,x n _ Throttlingvalve Fig.3. A suggested configuration for ideal HIDiC

3.2 Flexibility Comparisons After conceptual design of the ideal HIDiC has been completed, it is necessary to examine its optimal operating region. In this work it is undertaken by comparisons of profits among different process configurations, namely, a conventional distillation column, an ideal HIDiC and the better HIDiC. For the ideal HIDiC, the decision variables are the feed thermal condition q and the pressure difference, Pr-Ps, between the rectifying and the stripping sections.

J1(pr-ps, q): V1Cd+L,,Cb+Q1C1-FCf-QjCq-EpCp

(3)

The term Ep is the electric power of compressor, which is dependent closely on the pressure difference between the rectifying and the stripping sections.

665 For the better HIDiC, it is assumed to have a partial condenser, a partial reboiler and the same number of stages as the ideal HIDiC. The decision variables are the feed thermal condition q, the pressure difference, pr-ps, between the rectifying and the stripping sections, reflux and reboil ratios, R and S. Here, we fix, Pr-Ps, at 1 atm, and q at 0.5 arbitrarily for simplification. (4) Jg(P,.-Ps,q,R, S) = V1Cd+LnCb +QIC1-FCf-'Qj~q-QcCc-QbCb-EpCp For the conventional distillation column, it is assumed to have a partial condenser, a partial reboiler and the same number of stages as the ideal HIDiC, but without heat integration between its rectifying and stripping sections. The decision variables are the reflux ratio, R, and reboil ratio, S.

(5)

J~(R, S) = VICd+ LnCb-FCf-QjCq-QcCc-QaC6

First, the influences of feed flow rate are examined. Figure 4 shows comparisons of profits: J1, J2, and J3 with feed flow rate as a varying parameter, when the both end products are kept on their specifications, respectively. It is clearly shown that the ideal HIDiC is, only within some region, namely, F<230kmol/h, more economical than its conventional counterparts. Beyond this region the ideal HIDiC will lose its advantages in energy utilization. The general HIDiC is always more energy efficient than the conventional distillation column and this demonstrates its higher flexibility than the ideal HIDiC. In fact, with the addition of a bottom trim-reboiler and overhead trim-condenser, the general HIDiC can be designed to be more energy efficient than the ideal HIDiC through optimization of the extemal reflux and reboil flows, irrespective to any changes in operating conditions. For example, when F<150 kmol/h, it is not necessary to employ extemal reflux and reboil flows. However, when F>150kmol/h, it is suggested to employ them. These strategies can guarantee the general HIDiC to be more flexible and energy efficient than the ideal HIDiC. Second, the influences of product specifications are examined. Figure 5 shows comparisons of the profits: J1, J2, and J3 with the top product specification as a varying parameter, when the bottom product specification has been kept as Xn=l-yl. The higher the specifications for top and bottom products become, the less the advantages of the ideal HIDiC will be. Beyond certain higher product specifications (here, 0.998), the ideal HIDiC will be more energy intensive than its conventional counterpart. This is because higher product specifications have to be achieved by higher pressure difference and this inevitably introduces higher electricity cost. For the general HIDiC, this problem has been alleviated substantially. As can be seen, the general HIDiC is, in a wider region, more energy efficient than the conventional distillation column, although we arbitrarily fixed, the pr-ps, at 1 atm in the simulation. It is the extemal reflux and reboil flows that provide degrees of freedom for avoiding the unnecessary higher electricity cost. Through process optimization the optimal region of the better HIDiC could be even larger than that shown on the figure. Regarding the influences of feed composition, although not shown here, the ideal HIDiC appears to be always more energy efficient than its conventional counterparts, and so does the better HIDiC.

~

4O

4O

2O

20

0

-20

-40 0

-20

100

200

300

F [kmol/h]

Fig.4. Comparisons ofprofits J1, J2 and J3

-40

0.99

0.9925

0.995 Yl [ - ]

0.9975

Fig.5. Comparisons ofprofits Jl, J2 and J3

666 4. CONCLUSION A better process configuration is synthesized through parametric studies. The process configuration is demonstrated to have higher operation flexibilities and higher energy efficiency than its original configuration. Simulation results justify the conclusion. 5. A C K N O W L E D G M E N T This work is supported by New-Energy and Industry Technology Development Organization (NEDO) through Energy Conservation Center of Japan and hereby is acknowledged.

6. N O M E N C L A T U R E A : area [m 2] C = cost or price [$/kmol] E = electric power of compressor [kW] F = feed flow rate [kmol/s] J = operating cost [S/s] L = liquid flow rate [kmol/s] n = number of total stages [-] Pr-Ps = pressure difference between rectifying and stripping sections [kPa] [-] q = thermal condition of feed [kJ/s] Q = heat duty [-] R = reflux ratio S = reboil ratio [-] [kJ/m 2K] U = overall heat transfer coefficient [kmol/s] V = vapor flow rate [-] x = mole fraction of liquid [-] y = mole fraction of vapor [-] z~ = feed composition c = condenser f = feed b = bottom d = distillate p = compressor rs = from rectifying to stripping sections sr = from stripping to rectifying sections REFERENCES [ 1] T. Takamatsu, M. Nakaiwa, K. Huang, T. Akiya, and K. Aso, Comput. Chem. Eng., $21 (1997) $243. [2] M, Nakaiwa, K. Huang, M. Owa, T. Akiya, T. Nakane, and T. Takamatsu, Comput. Chem. Eng., $22 (1998) $389. [3] M. Nakaiwa, K. Huang, A. Endo, K. Naito, M. Owa, T. Akiya, T. Nakane, and T. Takamatsu, Comput. Chem. Eng., $23 (1999) $851. [4] P.C. Wankat and D.P. Kessler, Ind. Eng. Chem. Res., 32 (1993) 3061.