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서울시립대학교 화학공학과

Exergy Analysis of Distillation Column with Multiple Reflux Flows

Il-Hoon Hwang, Yonsoo Chung

Department of Chemical Engineering, University of Seoul

Introduction

Distillation is the most widely used separation process and the largest energy user of all separation processes in the chemical industry. Much work has been already done to improve the energy efficiency of distillation column. Exergy analysis has been used successfully to assess the thermodynamic efficiency of distillation columns.

Exergy analysis allows us to identify the possible modifications to increase the thermal efficiency of the distillation column. Those possible modifications allow a better heat integration during the distillation process. Many researchers have studied beneficial changes in operating conditions and layout settings, such as changes in the feed condition and the use of side-condensers and/or side-reboiler, while others have examined different types of distillation, such as reactive distillation and extractive distillation.

In this work, a new distillation column with multiple reflux flows is proposed. It introduces the effects of intercooling and interheating through the multiple reflux flows. The proposed distillation column has two major changes in operating condition; One is the intermediate cooling, and the other is a different composition of each stage. We showed the energy and exergy balances in the column given by the temperature and composition. The performance of the new distillation column has been evaluated by the binary mixture of ethanol and acetone that is easy to separate.

Multiple Reflux Flow

The conventional distillation column has only one reflux flow. The reflux is a condensed liquid stream which has the lowest temperature in the column. With the distribution of this coldest stream through the whole rectifying section, the intercooling effect can be obtained. Le Goff et al.[1] and Zemp et al.[2] showed that intercooling can improve the thermodynamic efficiency of the distillation column. Le Goff et al.[1] showed that intermediate heating and cooling is available to decrease the exergy loss and also revealed that a side-condenser increased the installation cost. The reflux flows are the substance of intermediate cooling to reduce the cost because a condensed liquid is a cold utility itself. The composition is changed by the reflux flow as well as temperature. If it is not changed, the effect of temperature could be the same of side-condensers. It leads us to propose a distillation column with multiple reflux flows. Fig. 1 shows the schematic diagram of the distillation column with multiple reflux flows.

Operating Lines

First, operating lines of the distillation with multiple reflux flows were studied and the exergy loss of the system due to the change of the composition was checked later. As an example, l00 mol/hr of acetone-ethanol was selected. The operating condition was chose as follows: 61.5℃ with a mole fraction of acetone of Xf = 0.5 to obtain a top product with Xd=0.9. The McCabe-Thiele diagram is a beneficial tool to know the minimum stage, feed stage and minimum reflux ratio. With the aid of the McCabe-Thiele diagram, 7 stage-column with the feed entering the 5th stage was chosen.

Operating lines can only lie in the region between the equilibrium line and the diagonal line of the y-x diagram. The end points of the operating lines are fixed on the diagonal because the reflux and the reboil flows are generated by condensing or reboiling part of the top and bottom products, respectively. A mathematical model for the column with multiple reflux flows can be established from material balance under the assumption of the constant liquid and vapor flowrates. The reflux flow entering each stage of the rectifying section is assumed as an equimolar and saturated liquid flow. Eqs. (1)∼(3) show the operating lines at each stage

[eqn: y_2 ~=~ {L_1 } over {V_2 } x_1 ~+~ [ D ``+`` (``1- {1} over {m}``)`` L_0 `` ]``{x_D} over {V_2 }]	(1)	[eqn: y_3 ~=~ {L_2 } over {V_3 } x_2 ~+~ [ D ``+`` (``1~-~ {2} over {m}``)`` L_0 `` ]``{x_D } over { V_3 }]	(2)

...

[eqn: y_m+1 ~=~ {L_m } over {V_m+1 } x_m ~+~ [ D ``+`` (``1~-~ {m} over {m}``)`` L_0 `` ]``{ x_D } over { V_m+1 }]	(3)

where L, V and m are liquid and vapor flowrates and the number of reflux flows, respectively. Eq. (3) is rearranged to Eq. (4) with the reflux ratio R.

[eqn: y_m+1 ~=~ {L_m } over {V_m+1 } x_m~+~ D ``{ x_D } over { V_m+1 }]

[eqn: =~ {R } over {R+1 } x_m~+~ ``{ 1 } over { R+1 } x_D]	(4)

Eq. (4) is exactly the same as the operation line of the conventional distillation columns. The intermediate reflux flows cause changes of internal vapor and liquid flows. In consequence, the liquid-to-gas ratio L/G is changed and the operating line breaks in the proposed distillation column. Depending on the details of heat supply or removal, the position of the operating line might also be shifted. The second operating line starts from the same point because of the same composition of reflux flows but it has varying slope. As seen in Fig. 2, the operating lines are moved closer to the equilibrium line with intermediate reflux flows. The increase of the number of stages with the improvement of the thermodynamic performance was also reported in previous studies[1, 2].

Exergy Analysis of Distillation Column

Exergy analysis has been described extensively in the books of Kotas[3] and Szargut et al[4]. Exergy is defined as the minimal work necessary to move produce a material from a specified state to the equilibrium state with the environment.

The exergy is a sum of the physical and chemical exergies. Physical exergy is the work obtainable by taking the substance through reversible processes form its initial state temperature T and pressure P, to the stage determined by the temperature To and pressure Po of the environment. Chemical exergy is equal to the maximum amount of work obtainable when the substance under consideration is brought from the environment state.

The exergy loss of the conventional column was calculated. The resulting exergy loss profile shows large peaks the feed and first stages. It is clearly caused by the temperature/composition mismatch at the feed and first stages due to subcooled nature of the feed and reflux flows[2].

Fig. 3 shows the exergy loss of the acetone-ethanol system with single reflux flow and with two reflux flows. The exergy analysis showed that the overall exergy loss of the conventional distillation column was five times as large as that of the proposed distillation column. This analysis showed that the composition of the reflux has effected to reduce the peak of exergy loss at the first stage and the overall exergy loss.

Conclusions

A new distillation column with multiple reflux flows was proposed and evaluated. Compared to the conventional column, it showed the significant decrease of the exergy loss with the increase of the number of theoretical trays. The improvement of the thermodynamic efficiency comes from the effects of intercooling. The new column can be accomplished in a relatively economic way because it does not require expensive equipments like side-reboilers and side-condensers.

Literature Cited

1. Pierre Le Goff, Thiery Cachot, and Ricardo Rivero: □Exergy analysis of        Distillation Processes", Chem. Eng. Technol., 19, 478-485 (1996).

2. Roger J. Zemp, Sergio H.B. de Faria and Maria de L. Oliveira Maia:          "Driving Force Distribution and Exergy Loss in the Thermodynamic Analysis    of Distillation Column", Computers Chem. Eng., 21, s523-s528 (1997).

3. Tadeusz J. Kotas: "The Exergy Method of Thermal Plant Analysis", 1sted.,      Butterwirth  (1985).

4. Jan Szargut, David R. Morris and Frank R. Steward: "Exergy Analysis of      Thermal, Chemical and Metallurgical Processes", 1sted., Hemisphere (1988).