엑서지를 이용한 Fractionation Unit의 열역학적 해석

조효은, 정연수

서울시립대학교 화학공학과

Thermodynamic Analysis of Fractionation 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 product. Benzene is used as raw material of synthetic resin, rubber, surfactant: toluene  as solvent of paint, rubber, additive: xylene as raw material for synthesis. Benzene is the second largest volume petrochemical next to ethylene. And energy saving and efficiency improvement become important problems as the demand of BTX is increasing. These factors made the needs of exergy analysis for the BTX process. The exergy analysis based on the first and second laws of thermodynamics has been adopted to improve energy efficiency. The notion of exergy is interpreted as a measure of energy quality. It enables us to identify the location, the reason and the magnitudes of energy losses in the systems, and also suggests the method or the direction to improve energy efficiencies.

Exergy analysis for the fractionation unit as well as hydrodealkylation 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. Fractionation and Hydrodealkylation (HDA) Units

There are two kinds of sources for BTX. One is a pyrolysis gasoline, which is the by-product of naphtha cracking to yield C2, C3 olefins. And the other is a reformate from the reformer of refinery streams[2]. In this study, BTX process where the pyrolysis gasoline entered was selected. The BTX plant is comprised of four major units as shown in Fig. 1: hydrotreating, extraction, fractionation and HDA 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 extract is fed into the fractionation unit and separated into benzene, toluene and xylene through the fractionation unit which is another major part of BTX plant. HDA where toluene reacting with hydrogen is converted into benzene and methane has been used as swing capacity according to the market condition. Biphenyl is produced in the toluene HDA as a by-product. Fig. 2 shows that fractionation unit comprises clay tower, benzene, toluene, xylene and solvent columns, and HDA unit comprises HDA reactor and stabilizer.

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. Hydrotreating unit was simulated in ASPEN PLUS using 62 components for the pyrolysis gasoline[1]. And the sulfolane extraction unit was simulated using the product of the hydrotreating unit for the feed[3]. In this study, the extract from the sulfolane extraction unit was used as the feed of fractionation unit. And  C9-C10, one of the major product of hydrotreating unit[1], was used as one of the feed of HDA unit. Basic physical properties of chemical components were obtained from Reid[4] and the database of ASPEN PLUS. The Peng-Robinson equation of state was used in calculation of physical properties. It is recommended for gas processing, refinery, and petrochemical applications[5]. The process simulation provided values of flowrate, composition, temperature, pressure, enthalpy and entropy of each process stream. It also estimated heat duties of utility streams.

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[6]. Exergy value was calculated as the sum of physical and chemical exergies because chemical reactions were involved in BTX plant. Widely accepted Szargut's standard chemical exergy[7] has been used in this study.

Exergy Analysis

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 occurred at all of units was 129 GJ/hr. Fig. 3 shows distribution of driving exergy losses of units. It reveals that only 5 out of 42 units are responsible for 60% of the total driving exergy losses. And it reveals that the distillation columns are the major units for the driving exergy losses. Especially, the benzene 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 benzene column.

2. Material Exergy Loss

To figure out material exergy losses, system boundary must be expanded up to its maximum, that is, the whole fractionation and HDA units 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 fractionation and HDA units are the benzene and xylene streams. Fig. 4 shows that 56% of total exergy input was converted into that of useful product streams. All other outputs must be losses. The material exergy loss from the fractionation and HDA unit was summed up to 419 GJ/hr, 3.3 times bigger than the driving exergy loss.

Conclusions

The fractionation and HDA units have 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 flows[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 benzene column has been noted as the weakest unit from thermodynamic point of view. The distillation columns are the major locations where driving exergy losses occur in fractionation and HDA units. The efficient usage of the material losses, especially to-ethylene stream, 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. and Cho, H.-E.; □Exergy Analysis of Hydrotreating Unit,□ ECOS'2000, Process Integration, p. 1461-1468, University of Twente, the Netherlands (2000)

[2] Wittcoff, H.A.; Industrial Organic Chemicals in Perspective, Part One: Raw Materials and Manufacture, p. 92-95, John Wiley & Sons, New York, U.S.A. (1979)

[3] Cho, H.-E. and Chung, Y.; "Exergy Analysis of Sulfolane Extraction Unit", PSE Asia 2000 Proceedings, p. 429-434, Kyoto, Japan (2000)

[4] Reid, R.C., Prausnitz, J.M. and Poling, B.E.; The Properties of Gases and Liquids, 4th ed., p. 656-732, McGraw-Hill, New York, U.S.A. (1986)

[5] AspenTeck; Physical Property Methods, p. 2.15-2.17, Steady State Simulation ASPEN PLUS 10, Elan, California, U.S.A. (1999)

[6] Sussman, M.V.; Availability (Exergy) Analysis : A Self Instruction Manual, 2nd ed., p. I.3-I.27, Mulliken House, Lexington, U.S.A. (1981)

[7] Szargut, J., Morris, D.R. and Steward, F.R.; Exergy Analysis of Thermal, Chemical, and Metallurgical Processes, 1st ed., p. 297-313, Hemisphere, New York, U.S.A. (1988)

Fig. 1. Schematic Diagram of BTX Plant.

Fig. 2. Schematic Diagram of Fractionation and Hydrodealkylation Units.

Fig. 4. Exergy Inputs/Outputs (GJ/hr)

of Fractionation and Hydrodealkylation Units.

Fig. 3. Distribution of Driving Exergy Losses.