China Petroleum Processing and Petrochemical Technology Review
December 30, 2010
Progress in the Technology for Desulfurization of Crude Oil Liu Lin; Lü Hong; Qian Jianhua; Xing Jinjuan (Liaoning Province Key Laboratory for Synthesis and Application of Functional Compounds, Bohai University, Jinzhou 121013)
Abstract: The poor quality of crude oil obviously leads to high sulfur contents of oil products, and the technology for desulfurization of crude oil is urgently needed so that the sulfur contents in petroleum product could be reduced from the root. This paper describes the progress in technology for desulfurization of crude oil. The present technologies for desulfurization of crude oil include caustic washing, dry gas desulfurization, hydrodesulfurization (HDS), etc. The new combined technologies for desulfurization of crude oil being studied are: biodesulfurization (BDS), hydrogenationbacterial catalysis, the microwave-catalytic hydrogenation, the BDS-OD-RA desulfurization and oxidative desulfurization in electrostatic fields, and the ultrasonic/microwave-catalytic oxidation applied in our lab, with their development trends being also discussed. Key words: crude oil; desulfurization; development progress
1 Introduction
2 Current Technologies for Desulfurization of Crude Oil
The poor quality of crude oil currently can obviously result in the high sulfur contents of oil products, which can
2.1 Caustic washing method for oil desulfurization Caustic washing method is mainly aimed at removing the sulfides from the crude oil by using the caustic solution. At
lead to corrosion, catalyst poisoning, environmental pollution and other negative consequences[1-4] (Figure 1)[5].
present, this method is widely used in the oil industry. The advantage of this method is that the process is simple and
The treatment of high-sulfur crude oil is becoming the focus of research in China’s oil refining industry[6]. The con-
the cost is lower. But this method contains many defects such as the poor quality of oil products and lower effi-
ventional equipment cannot deal with the high-sulfur crude oil during petroleum refining processes. The increasing
ciency of desulfurization. However, this method can’t remove all sulfur compounds from the crude oil, especially
sulfur content of crude oil also results in an increase in sulfur content in automotive gasoline, diesel fuel, and jet
the organic sulfides. At the same time, it brings too much sulfide-containing wastewater, which greatly pollutes the
fuel. To meet the needs for producing clean fuels, decreasing the sulfur content of crude oil becomes an urgent task.
environment. So, it is urgent to improve this method or find a better way to reduce the sulfur contents in petroleum
Studying new desulfurization technology and raising the efficiency of desulfurization processes are the keys to bring-
products.
ing more profits to the oil refining companies[7].
2.2 Dry gas desulfurization method According to the report[8], the first equipment in China for treating heavy crude oil through dry gas desulfurization method has been used in the No.3 joint oil gathering station at Tahe Oilfield of the Northwest Oilfield Branch Company. The cost of this equipment is about 500 million RMB, and it is the first time in China to use this dry gas treatment method at the crude oil desulfurization system in reality. This dry gas desulfurization system works as follows. Figure 1 Global trend on crude oil quality[5] —API gravity; —Sulfur content, m%
Corresponding author: Dr. Liu Lin, Telephone: +86-4163400328; E-mail:
[email protected]
1
December 30, 2010
Liu Lin, et al. Progress in the Technology for Desulfurization of Crude Oil
Firstly, the sulfur compounds are removed from the crude
There are two disadvantages associated with this method.
oil; and secondly, after settling treatment and dehydration
Firstly, this process is not very efficient. Secondly, one kind of bacteria can only remove one or few sulfides[13]. We are
of crude oil, the sulfur compounds are removed again so
now focusing on developing new bacterium, preparing new biocatalysts, improving the capability of the desulfuriza-
that corrosion of the process equipment and the pipelines will be minimized to secure better working conditions and
tion technology. The efficiency of biocatalysts and environmental adaptability are the important factors in com-
avoid accidents. Gas stripping process can reduce significantly the H2S con-
mercialization of the desulfurization technology[14-16]. Other factors to be assessed include the location of refineries and
tent in crude oil. But this method has its disadvantage because the efficiency of desulfurization will change with the
the biological treatment costs.
water content in the feedstock. Sulfate-reducing bacteria
According to a report, Iran’s Sharif University of Technology has used the bacterial desulfurization process to treat
(SRB) can convert a part of organic sulfides into inorganic sulfides, causing the H2S content to increase in the next sulfur-containing compounds is very low.
heavy and sour crude oils in place of the existing refinery desulfurization process. The bacteria can remove 65%—
2.3 Hydrodesulfurization (HDS)
76% of total sulfur from heavy crude oil extracted from two different oilfields, while this desulfurization process is car-
phase of desulfurization[9]. And the rate for removal of other
ried out at a temperature below 30
Hydrodesulfurization is one of the catalytic desulfuriza-
[17-18]
.
tion processes, which aims at turning organic sulfur com-
The HDS process and the BDS process have different
pounds into H2S using H2 as the reactant in the presence of
characteristics, they can complement with each other. Now some researchers are trying to use both HDS and BDS to-
metal catalysts operating at high temperature and pressure. The resultant hydrogen sulfide is then removed from the
gether for processing the high-sulfur crude oils under high pressure and at low space velocity during hydro-upgrading
system[10]. This method is widely used in oil industry in China since 1955. However, the HDS process features a
process. On the other hand, researchers are making strenuous efforts to manufacture cheaper H2 product, and when
complicated procedure, high production cost and high materials consumption. With an increasing ratio of heavy
the technology continues its development, it is possible to combine the two technologies in the near future[19-21].
crude oil supply to the oil refinery, the sulfur content in crude slate is growing, which leads to a shortened catalyst
3 New Combined Technologies for Desulfurization of
life at the refinery. Furthermore, the HDS process needs
Crude Oil
more H2, so the production cost will increase a lot[11].
The currently available desulfurization technologies are not perfect, and their disadvantages loom more clearly.
2.4 Biodesulfurization (BDS) BDS is a new bio-catalytic desulfurization method, which
The current technologies for desulfurization of crude oil cannot be suited to the market needs obviously. All
has been applied since the 1980s. According to the principle of enzyme catalysis for implementing the specific
domestic and foreign researchers have been doing a lot of effective work to reduce or eliminate the pollution
reaction for C—S bonds cleavage performed by micro bio-
resulted from processing of sour crude oils. Many new combined technologies for desulfurization of crude oil
logical flora, sulfides in crude oil can be turned into elemental sulfur that can be removed. In the process of
are being studied, such as the biodesulfurization (BDS) process, the hydrogenation-bacterial catalysis method,
desulfurization, the sulfur containing pollutants are transformed into sulfides and H2S by biological reduction, and
the microwave-catalytic hydrogenation method, the BDS-OD-RA desulfurization process, the oxidative des-
the elemental sulfur can be removed via the process of biological oxidation. Normally we use photosynthetic bactecrude oil should stay a few days in the system to be treated
ulfurization in electrostatic fields, and the ultrasonic/ microwave-catalytic oxidation method that has been
by the BDS method[12].
applied in our lab.
ria and colorless sulfur bacteria in the BDS process, and
2
China Petroleum Processing and Petrochemical Technology
3.1 Hydrogenation-bacterial catalytic desulfurization
respectively[24].
Leshchev S. M.[22] invented the hydrogenation-bacterial catalytic process. This method involves two steps: firstly, hydrogenation of the crude oil is carried out in order to remove unstable organic sulfides contained in the crude oil, then an artificial culture of organic sulfur compounds with selective bacteria named Rhodococcus rhodocrous is used in the desulfurization process. During the biological oxidation process, organic sulfides (such as dibenzothiophene series) are subjected to C—S bond cleavage reaction and turned into inorganic sulfides. Hence this method can remove sulfur compounds from oil products efficiently.
3.3 BDS-OD-RA three-step integrated desulfurization process The BDS-OD-RA method is a three-step integrated process. Desulfurization of high-sulfur crude oil by BDS needs anaerobic conditions, combined with oxidation and adsorptive desulfurization to make this process viable. Desulfurization of low-sulfur crude oil by BDS needs aerobic conditions, combined with oxidation and adsorptive desulfurization (as shown in Table 2)[25]. So this combined technology for crude oil desulfurization is an important development trend.
3.2 Microwave- catalytic hydrogenation process for desulfurization
3.4 Oxidative desulfurization in electrostatic fields The process of oxidative desulfurization of oil products in
Microwave energy can be used as the power for regeneration of the HDS catalyst. Integrating microwave,catalysis and hydrogenation together in the desulfurization process is more efficient than the traditional technology of desulfurization as shown in Table 1[23]. And microwave inducement can improve the effect of chemical desulfurization and better desulfurizing results were gained. The desulfurizing efficiencies increased from 18.6%, 21.8%, 28.5% and 24.3% to 34.7%, 33.3%, 34.5% and 43.3%,
electrostatic fields applied in our lab is carried out as follows. During the pretreatment process, the desulfurizer is loaded into the desalter drum, and the feed oil in an electric field is heated under intensive agitation and mixed with the desulfurizing agent to achieve a complete desulfurization of the oil. Then the sulfides that are removed after desulfurization reaction are discharged to the wastewater treatment system[26-27].
Table 1 Microwave-promoted HDS using iron powder as the catalyst[23-24] Test No. 1 2 3 4 5 6 7
Feed oil Dibenzothiophene Crude oil
Reaction conditions 50 psi H2, at 200 for 20 min 50 psi H2, at 200 for 20 min
Level of desulfurization, % 8 25
Crude oil Crude oil Crude oil Crude oil
20 psi H2, at 200 for 20 min No H2, at 200 for 20 min 50 psi H2, at 250 for 20 min 50 psi H2, at 200 for 30 min
10 No reaction detected 24 27
Crude oil
50 psi H2, at 200
7
for 110 s
Table 2 Desulfurization of crude oils achieved through three-step integrated process[25] HCO LCO HCO Sample to be treated 1.88 0.378 1.88 S content, % First-step treatment S content after the first step, % Desulfurization rate, % Second-step treatment S content after the second step, % Desulfurization rate, % Third-step treatment
LCO 0.378
BDS (AN) 0.58 69.14 OD
BDS (AN) 0.138 63.40 OD
BDS (AO) 0.69 63.29 OD
BDS (AO) 0.15 61.4 OD
0.11 94.15 RA 0.09
0.07 81.48 RA 0.053
0.23 87.76 RA 0.10
0.08 78.84 RA 0.022
S content after the third step, % 94.3 94.6 85.9 95.21 Desulfurization rate, % Notes: HCO: heavy crude oil, LCO: light crude oil, BDS: biodesulfurization, OD: oxy-desulfurization, RA: reactive adsorption, AO: aerobic, AN: anaerobic.
3
Liu Lin, et al. Progress in the Technology for Desulfurization of Crude Oil
December 30, 2010
This method is mainly aimed at removing most of mercap-
may cause the formation of hydrogen peroxide in the mix-
tans and few thioethers contained in the feed oil. But with increasing boiling points of petroleum fractions, their mer-
ture to react upon sulfides that are then turned into some sulfates, sulfoxides and few sulfones. The solvent can be
captans and disulfides contents drop down immediately, and the thioethers content grows first, and then falls down,
reused after regeneration, while sulfoxides and sulfates can be used to make other products[30-31]. The ultrasonic-oxida-
while the content of thiophenes keeps rising. Most of sulfur in crude oil exists in the form of thioethers and
tive desulfurization process has a lot of advantages such as simple operation, lower cost, low reaction temperature, low
thiophenes in higher boiling petroleum fractions, these inactive sulfur compounds are much more difficult to be re-
operating cost and high efficiency. This technology has a bright future[32].
moved than active sulfides[27].
Using microwave in combination with the catalytic oxida-
Moreover, some researches are also studying the methods for preparation of Sb/Sn intermetallic compound with Al addi-
tive desulfurization is more efficient than a single catalytic oxidative desulfurization of crude oil as evidenced by the
tion and its behavior for desulfurization of crude oil emulsions. The desulfurization mechanism was achieved by the electro-
results presented in Table 3. Ultrasonic wave can become a very useful tool in the oil
chemical reaction between sulfides and the Sb/Sn surface under electric current induction in combination with the physi-
desulfurization industry. Ultrasonic wave combined with other technologies can reduce the temperature, the con-
cal adsorption on active sites like vacancies formed by removing Al atoms on the alloy surface[28].
sumption of power and solvent, the amount of waste discharge and the production cost, which can also increase the
3.5 Desulfurization process involving ultrasonic/ microwave - catalytic oxidation
efficiency of recycling and product purity. On the other hand, this process does not cause environmental pollution to promise a rosy future. This method can fully apply the principle of cavatition resulted from ultrasonic wave vibrations
The SulphCo Inc. uses the ultrasonic wave to remove the sulfur compounds from oil, and large-scale ultrasound tests are being carried out in the EU. The company’s technology
to realize the function of extraction and mechanical agitation, and the microwave power is applied to improve the pre-des-
can turn the sour heavy oil into light oil with little sulfur content to produce more useful oil products[29].
ulfurization effect that is applied in our lab currently.
This technology is realized by mixing the feed oil and the
4 Prospects on Development of Crude Oil Desulfurization
water together with a small amount of oxidizing agent and catalyst in a container, in which the mixture will react upon
At present, the commercial methods for desulfurization of petroleum products have their own deficiencies. It is neces-
each other under cavatition effects. In the course of reaction the feed oil receives the energy from ultrasonic wave
sary to improve the desulfurization process to maximize the sulfur removal efficiency and turn the waste into trea-
to mix intensively with water, while the temperature will raise to more than several thousand degrees of centigrade
sure that should be reused. The desulfurization efficiency and economic benefits should be compatible, and the ex-
with the pressure increasing to more than 1013.2 MPa, that
isting facilities should be fully utilized to provide quick
Table 3 Effects of different desulfurizers and microwave inducement on desulphurization[33]
DCP BPO
Dosage of desulfurizer, % 0.01 0.01
Mass fraction of sulfur content after desulfurization, % 0.961 (12.6%) 0.860 (21.8%)
Mass fraction of sulfur content after desulfurization coupled with microwave induction, % 0.856 (22.2%) 0.857 (22.1%)
BBPV BPMC H 2O 2 Peroxy acetic acid
0.01 0.01 0.05 0.05
0.787 (28.5%) 0.833 (24.3%) 0.962 (12.5%) 0.860 (21.8%)
0.720 (34.5%) 0.624 (43.3%) 0.914 (16.9%) 0.734 (33.3%)
Formylhydroperoxide
0.05
0.895 (18.6%)
0.718 (34.7%)
Desulfurizer
Note: The reaction was conducted at a temperature of 60
for a reaction time of 30 min.
4
China Petroleum Processing and Petrochemical Technology
returns. And it is also necessary to support the develop-
References
ment of appropriate technologies that can reduce low-value products, increase high-value products, improve economic
[1] Liu Lin, Qian Jianhua, Xing Jinjuan, et al. Cause analysis of jet contaminated by oil tank truck[J]. Petroleum Processing and
efficiency, and enhance the competitive edge of the enterprise[34].
Petrochemicals, 2010, 41(2):45-48 (in Chinese). [2] Liu Lin, Zhai Yuchun, Qian Jianhua, et al. Research on the inhibitor for silver strip corrosion of jet fuel[J]. Petroleum Processing and Petrochemicals, 2004, 35(4):55-57 (in Chinese).
The difficulties in crude oil desulfurization are aggravated by the existence of polycyclic aromatic hydrocarbons (PAHs), nitrogenous compounds and H2S. Global crude oil
[3] Liu Lin, Qian Jianhua, Zhang Baoyan. Research on the inhibitor of the silver strip corrosion in jet fuel[J]. Journal of Chinese Society for Corrosion and Protection, 2004, 24(6):376-378 (in Chinese). [4] Liu Lin, Zhai Yuchun, Qian Jianhua, et al. Influence of sulfide
production has been nearly reaching the limit that technology can hardy break through. By building new devices or optimizing the related technologies, we can improve the product yield and reduce the materials consumption dur-
on silver strip corrosion of jet fuel[J]. Petroleum Refinery Engineering, 2002, 32(5):35-36 (in Chinese). [5] Yuan Qingtang. Progress and outlook on technologies for processing inferior crude oil in China[J]. China Petroleum Processing
ing oil refining, which will bring about significant economic returns. In the near future, the key to “zero sulfur”
and Petrochemical Technology, 2008, 3(1):1-7. [6] Shi Yahua. Desulphurization in Petroleum Processing Process [M]. Beijing: China Petroleum and Chemistry Press, 2009:13 (in Chinese).
emissions and minimum consumption of oil that is used to produce H2 applied in the fuel cells is how to produce cleaner fuels by a most effective, environmentally friendly, and economically viable way[35-36].
[7] Liu Haiyan, Yu Jianning, Bao Xiaoju. The State-of-the-art and future perspectives of world petroleum refining technology[J]. The Chinese Journal of Process Engineering, 2007, 7(1): 176 -185 (in Chinese).
Removing the sulfur compounds during the pretreatment of crude oil should be based on the forms and natures of sulfur compounds existing in crude oil. Upon desulfurization and desalting of crude oil, a ternary system of hydrated
[8] Tong Kai, Zhan Zheng, Dong Gang, et al. First equipment for heavy crude oil dry gas desulfurization is put into operation successfully raised in China[N]. Sinopec News, 2009-3-19 (2 issue) (in Chinese).
liquid is injected into crude oil, which is separated from the water by gravity under the action of a demulsifier, and after
[9] Xiang Tingsheng, Wan Jiayun, Cai Chunfang. Treatment of crude oil using sulphate-reducing bacteria in H2S formations[J]. Natural Gas Geoscience, 2004, 15(2): 171-173 (in Chinese). [10] Xiong Jian. Elementary study on biodesulfurization of petroleum
treatment with a desulfurization agent coupled with the ultrasonic/microwave treating in an electric field, the lowsulfur oil can be obtained. This is the key to tackling the problem for desulfurization of oil that can allow for reduc-
[D]. Nanjing: Nanjing University of Technology, 2003 (in Chinese). [11] Khan Rashid M, Nero Vincent P, Brugger Lee Ann, et al. Sweetening of Sour Crudes[P]. WO: 2004007645, 2004-01-22. [12] Qian Bozhang, Wu Hong. Petroleum biodesulfurization and
tion of the cost for processing crude oil and for the production of clean fuels. The advanced desulfurization technologies need less investment, and are more efficient for treating the crude oils that can solve the problems of pollution,
its application prospect[J]. Petroleum Refinery Engineering, 1999, 29(8): 26-31 (in Chinese). [13] Ohshiro T, Izumi Y. Microbial desulfurization of organic sulfur compounds in petroleum[J]. Biosci., Biotechnol., Biochem.,
equipment corrosion, equipment overload, and catalyst poisoning during catalytic cracking process to ensure a long-cycle, full-load and safe operation of the equipment. Pretreatment of crude oil by means of desulfurization pro-
1999, 63 (1): 1. [14] Shi Deqing, Zhao Jinsheng, Yang Jinrong, et al. Degradation of dibenzothiophene by Pseudomonas stutzeri UP1[J]. China Environmental Science, 2004, 24(6): 730-733 (in Chinese).
cess is the direction for development of oil refinery industry to enhance its economic benefits.
[15] Konishi J. Maruhashi. Residue of dibenzothiophene (DBT) sulfone monooxygenase is involved in C2S bond cleavage specificity of alkylated DBT sulfones[J]. Biotechnology Letters, 2003, 25: 1199-1202.
Acknowledgement: The authors are grateful to financial supports from the Science and Technology Office of Liaoning Province (Project Number: 2008403001) and the Liaoning Provincial Office of Education for Innovation
[16] Tanaka Y, Matsui, Konishi T J, et al. Biodesulfurization of benzothiophene and dibenzothiophene by a newly isolated
Team (Project Number: 2009T002).
5
December 30, 2010
Liu Lin, et al. Progress in the Technology for Desulfurization of Crude Oil
Rhodococcus strain[J]. Appl Microbiol Biotechnol, 2002, 59: 325328. [17] Zhang Wen. Iran developed bacteria to process heavy crude oil
[27] Liu Guiling. Research on application of fine chemicals of petroleum in pretreatment process of crude oil[D]. Shanghai: East China University of Science and Technology, 2004 (in Chinese).
desulfurization[J]. Petroleum Processing and Petrochemicals, 2008, 29(12):54 (in Chinese). [18] Sarah Torkamani, Jalal Shayegan, Soheila Yaghmaei, et al. Study of the first isolated fungus capable of heavy crude oil
[28] Wang Yong, Yun Zhi. Preparation of SbSn intermetallic compound with Al addition and desulfurization behavior for emulsion of crude oil[J]. Journal of Chemical Industry and Engineering (China), 2008, 59(2):467-471 (in Chinese).
biodesulfurization[J]. Ind. Eng. Chem. Res., 2008, 47 (19): 74767482. [19] Yu Guoqing, Gao Jinsen, Xu Chunming. Process technology of the sour crude oil with catalytic cracking[J]. Petrochemical In-
[29] Zhang Wen. SulphCo Inc. doing large-scale desulfurization of crude oil by ultrasonic testing in Europe[J]. Petroleum Processing and Petrochemicals, 2009, 40(4): 56 (in Chinese). [30] Amos Avidan, Bechtel Coporation, Mark Cullen. Sulphco Inc.
dustry Technology, 2004, 11(1):58-68 (in Chinese). [20] Liu Tao, Dai Lishun, Niu Chuanfeng, et al. Study on hydroupgrading of highly acidic crude oils[J]. Petroleum Processing and Petrochemicals, 2009, 40(2):1-4 (in Chinese).
Desulfurization via selective oxidation-pilot plant results and commercialization plans[C ]. National Petroleum Refiners Association Annual Meeting, 2001, AM-01-55. [31] Li Jingjing, Tang Xiaodong. Application progress of ultra-
[21] Cao Xianghong. Technology strategy of refining and petrochemical industry in China facing the oil-shortage era[J]. Petroleum Processing and Petrochemicals, 2006, 37(6):1-8 (in Chinese). [22] Leshchev S.M. Method of Removing Organic Sulfur Com-
sonic technique in petrochemical industry[J]. Technology & Development of Chemical Industry, 2005, 34(2): 22-25 (in Chinese). [32] Qian Jianhua, Zhou Yuenan, Liu Lin,et al. Study on kinetics for desulfurization of model diesel. China Petroleum Processing
pounds from Petroleum Products[P]. RU: 1505960,1989-09-07 [23] Nicholas E. Leadbeater, M. Rashid Khan. Microwave-promoted desulfurization of heavy and sulfur-containing crude oil[J]. Energ & Fuels, 2008, 22(3):1836-1839
and Petrochemical Technology, 2009, 6(2):43-47. [33] Lu R, Yang J, Xu X. Microwave-chemical desulfurization of sulfurous crude oil[J]. Pet. Sci. Technol, 2009, 27(16): 1789-1799. [34] Yang Hongyun, Zhao Dezhi, Shen Yaoya. The current situa-
[24] Lu R, Yang J, Xu J, et al. Microwave-chemical desulphurization of sulfurous crude oil [J]. Petroleum Science and Technology, 27 (16): 1789-1799. [25] Prachi Agarwal, D. K. Sharma. Comparative studies on the
tion and development trend of desulfurization process of oil product [J]. Journal of Petrochemical Universities, 2001, 14(3): 26-30 (in Chinese). [35] Ge Hui, Li Xuekuan, Qin Zhangfeng, et al. Recent progress of
bio-desulfurization of crude oil with other desulfurization techniques and deep desulfurization through integrated processes[J]. Energy & Fuels, 2010, 24(1): 518-524. [26] Lin Zhiping, Yin Shuhua, Jiang Tianyan, et al. Desulphurization
catalyst development for deep hydrodesulfurization of oil products [J]. Chemical Industry and Engineering Progress, 2008, 27 (10): 1490-1497 (in Chinese). [36] Long Jun, Zu Deguang. Suggestions for developing heavy
process for high-sulfur crude oil and the desulfurization agent[P]. CN:1215149, 2005-08-17 (in Chinese).
crude processing technologies in China[J]. Petroleum Processing and Petrochemicals, 2004, 35(11): 1-4 (in Chinese).
Cost-Effective and Environmentally Benign Recovery of PTA Mother Liquor at Chemical Fiber Plant of Urumqi Petrochemical Company The Chemical Fiber Plant of Urumqi Petrochemical Com-
used as the solvent with the reminder routed to the wastewater treatment system. The PTA mother liquor recovery and utilization project was put on stream in 2008, and its utilization rate was raised in 2009. Currently the solid recovered from the mother liquor of the oxidation unit contains mainly PTA and PT acid amounting to 234.88 t/a, which is equivalent to an incremental 230 tons of PTA. A total of 40000 tons of filtrate can be recovered annually, which means a reduction of wastewater discharge of 40 kt/a. The materials recovered can bring about an incremental income of 1.8625 million RMB to the enterprise.
pany has implemented the project for recovering the PTA mother liquor. This project aims to recover the feedstock and desalter water and to reduce wastewater discharge. The PTA mother liquor from the oxidation unit after being cooled down in the spiral-plate heat exchangers is subjected to filtering in two program-controlled filters, in which the PT acid/PTA particles are removed and pulped prior to being circulated as the reaction materials. The filtrate after filtering is partly
6
