엑서지를 이용한 Sulfolane Extraction Unit의 열역학적 해석

조효은, 정연수

서울시립대학교 화학공학과

Thermodynamic Analysis of Sulfolane Extraction Unit Based on Exergy

Hyo-Eun Cho, Yonsoo Chung

Department of Chemical Engineering, University of Seoul

Introduction

BTX products are very important among the basic aromatic products. Benzene is used as raw material of synthetic resin, rubber, surfactant. Toluene is used for solvent of paint, rubber, additive. And xylene is utilized as raw material for synthesis. The most popular extraction technology for BTX products is the sulfolane extraction process. Sulfolane has good selectivity to aromatics: the recovery of benzen, toluene and xylene exceeds 99.9 wt-%, 99.8 wt-% and 98 wt-%, respectively. Saving of energy and improving of efficiency are important problems as the demand for BTX is increasing. The exergy analysis based on the first and second laws of thermodynamic is for more efficient energy use. It enables us to identify the location, the reason and the magnitudes of energy losses in the systems, and also suggests the methods or the directions to improve efficiencies.

Exergy analysis for the Extraction Unit of BTX process using the simulation and real data has been performed in this paper. Driving and material exergy losses[1] have been quantified. Location and the reason of major exergy losses have been pinpointed and improvement strategies have been suggested.

Theory

1. Sulfolane Extraction Unit

The BTX plant is comprised of four major units as shown in Fig. 1: hydrotreating, extraction, fractionation and hydrodealkylation units. The main feed of BTX plant, pyrolysis gasoline, is fed into the hydrotreating unit. The separated C6∼C8 stream called the heart cut is one of the major products of the hydrotreating units. The heart cut is fed to the extraction unit where aromatic components are separated from non-aromatics by the sulfolane extraction process. The sulfolane extraction process or sulfolane process takes its name from the used solvent: sulfolane, or tetrahydrothiophene 1-1 dioxide. Sulfolane has been used widely in the chemical industry for the extraction because it has good selectivity to aromatics, good solubility to water and a higher boiling point compared with that of the involved hydrocarbons[2]. Fig.  2 shows that extractor, raffinate wash column, stripper column, solvent regenerator, water stripper and recovery column are the major devices in the sulfolane extraction unit. The heart cut and recovery solvent are fed into the extractor and are countercurrent to each other in it. The top is lean solvent stream having high aromatics and the bottom is rich solvent stream having high nonaromatics. Raffinate, non-aromatics, is removed in the raffinate wash column. Residual non-aromatics are removed in the stripper. Dissolved hydrocarbons in the water are removed in the water stripper. Heavy polymers and tar are removed in the solvent regenerator. And the solvent is separated from aromatics in the recovery column.

The extracted aromatics are separated into benzene, toluene and xylene in the fractionation unit which is another major part of BTX plant. According to the market condition, the C9∼C10 components from the hydrotreating unit and toluene and xylene from the fractionation unit are further converted into benzene through the hydrodealkylation unit.

2. Process Simulation

A normal operating condition of a commercial BTX plant located in Korea was selected and used in the simulation as a base case.  Pyrolysis gasoline is a complex mixture of hydrocarbons of which exact composition is not well defined. Hydrotreating unit was simulated in ASPEN PLUS using 62 components for the pyrolysis gasoline. In the field of industry, this method was adopted to represent pyrolysis gasoline. The heart cut, from the hydrotreating unit, was used as the feed of sulfolane extraction unit. Basic physical properties of chemical components were obtained from Reid[3] and the database of ASPEN PLUS. Sulfolane and water are strong polar compounds. So the UNIFAC equation of state was used in calculation of physical properties. The UNIFAC and modified UNIFAC models can handle any combination of polar and nonpolar compounds[4].

3. Exergy

The exergy is defined as the maximum  available work from a reversible process that moves a unit mass of materials at a specified condition to the state of equilibrium with the environment[5]. Exergy value was calculated as the sum of physical and chemial exergies because chemical reactions were envolved in BTX plant. Widely accepted Szagut's standard chemical exergy[6] has been used in this study.

Exergy Anlaysis

1. Driving Exergy Loss

From the calculated exergy value of each stream and utility, the driving  exergy loss of a system was quantified. The system boundary can be a unit, a group of interrelated units, or a whole plant. At first the exergy loss of each unit was quantified. Then the total driving exergy loss was quantified adding all of driving exergy losses of each unit. The total driving exergy loss occured at all of units was 97 GJ/hr. Fig. 3 shows distribution of driving exergy losses of units. It reveals that only 4 out of 19 units are responsible for 94% of the total driving exergy losses. And it reveals that the distillation columns are the major units for the driving exergy losses. Especially, the stripper column is the most responsible unit for the driving exergy loss. Fig. 3 shows that the most important point in improving system efficiency is the stripper column.

2. Material Exergy Loss

To figure out material exergy losses, system boundary must be expanded up to its maximum, that is, the whole extraction unit in this study. It was carried out with merging adjacent units and adding driving exergy losses.  Fig. 4 shows the result. It apparently reveals exergy inventories. With the system boundary as shown in Fig. 1, useful outputs of the extraction unit are the extract stream. Fig. 4 shows that 61% of total exergy input was converted into that of useful product streams. All other outputs must be losses. The material exergy loss from the extraction unit was summed up to 402 GJ/hr, 4 times bigger than the driving exergy loss.

Conclusions

The extraction unit has been analyzed using the notion of exergy. The material exergy loss accompanied with useless components has been identified and quantified. Material exergy loss is directly related with material flow[1]. The decreasing of material exergy loss is related with the decreasing of pollutant impact on environment. The driving exergy loss has been relatively small compared with the material exergy loss. The stripper has been noted as the weakest unit from thermodynamic point of view. The distillation colums are the major locations where driving exergy losses occur in extraction unit. The efficient usage of the raffinate seems the most important issue in improving system efficiency.

Acknowledgement

The authors deeply appreciate the financial support of the Korea Science and Engineering Foundation.

Reference

[1] Chung, Y., Cho, H. E., "Exergy Analysis of Hydrotrating Unit", ECOS' 2000, University Twente, Nederland, 1461 (2000).

[2] Bailes, P. J., Hnason, C., Hughes, M. A., "Liquid-Liquid Extraction: Nonmetallic Materials", Chem. Eng., 115 (1976).

[3] Reid, R. C., Prausnitz, J. M., Poling, B. E., "The Properties of Gases and Liquids", McGraw-Hill, 1986.

[4] AspenTeck, "Physical Property Methods", Steady State Simulation ASPEN PLUS 10, 2-23 (1999).

[5] Sussman, M. V., "Availability (Exergy) Analysis : A Self Instruduction Manual", Mulliken House, Lexington, MA, 1980.

[6] Szargut, J., Morris, D. R., Steward, F. R., "Exergy Analysis of Thermal, Chemical, and Metallurgical Processes", Hemisphere, 1988.

Fig. 1. Schematic diagram of BTX plant.

Fig. 2. Schematic diagram of extraction unit.

Fig. 4. Exergy inputs/outputs(GJ/hr)

of extraction unit.

Fig. 3. Distribution of driving exergy losses.