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Several PTA plant process routes and device air compressor technology
Classification: Technical Data
Time：2017-09-26 20:12
PTA is the abbreviation of purified terephthalic acid (PurifiedTerephthalicAcid). PTA is the basic material for the production of polyester fiber and packaging industry. PTA industry is the main raw material for chemical fiber industry. PTA industry has the characteristics of capital-intensive and technology-intensive. The process packages and key equipment of all PTA units in China have long been complete sets of imported foreign technologies and equipment, all of which are monopolized by foreign investors. Due to the complex process route and high technical content of this production unit, for decades, it has been monopolized by large foreign companies. PTA market production situation is global.
PTA is the abbreviation of purified terephthalic acid (PurifiedTerephthalicAcid). PTA is the basic material for the production of polyester fiber and packaging industry, and is the main raw material for chemical fiber industry ｡
PTA industrial characteristics
PTA industry has the characteristics of capital-intensive and technology-intensive. The process packages and key equipment of all domestic PTA units have been imported foreign technologies and equipment for a long time, all of which are monopolized by foreign businessmen. Due to the complex process route and high technical content of this production unit, it has been monopolized by large foreign companies for decades ｡
PTA Market Production Situation
Global PTA production is mainly concentrated in Asia. By the end of 2003, the PTA production capacity of mainland China was 3.98 million tons, only lower than that of Taiwan and South Korea, ranking third in the world. Although China's PTA production capacity has been expanding in recent years, it is still far from meeting the market demand. The market gap is still huge. The import was about 4.3 million tons in 2002, 4.54 million tons in 2003, and the international market dependence exceeded 60% ｡
With the continuous development of China's polyester industry, the market demand for PTA products has increased rapidly in recent years. Expanding PTA production capacity and vigorously developing PTA production technology has become an important part of promoting the development of polyester industry ｡
PTA basic chemical reaction
Use cobalt acetate as catalyst, acetic acid as solvent and air as oxygen source for liquid phase oxidation of p-xylene (PX) to produce crude terephthalic acid (CTA)｡
Process flow
In actual production, air is used as oxygen source, and the temperature in the reactor (about 188 ℃) is maintained by the heat released in the reaction. The heat energy of the tail gas (temperature is 180 ℃) generated in the reaction is recovered by heat exchanger (waste heat boiler), and the pressure energy is recovered by tail gas turbine ｡
After heat exchange, the tail gas (temperature: 40 ℃) is reheated to 130~170 ℃ after passing through the high-pressure absorption tower, then enters the tail penetration, and 15% ~ 20% of the tail gas is extracted from the tail penetration when it expands to 3 ~ 5MPa for TA transportation. The tail gas discharged from the tail penetration enters the incinerator for incineration and is discharged into the atmosphere to ensure that the emission quality of the tail gas can meet the requirements of environmental protection ｡
Unit Configuration
Generally, PTA units are composed of compressors, turboexpanders and steam turbines, which can be divided into single-shaft units composed of single-shaft compressors and multi-shaft units composed of gear compressors ｡
Single shaft unit: high pressure air compressor, low pressure air compressor, turbo expander and steam turbine ｡
Multi-shaft unit: multi-shaft compressor, turbo-expander and steam turbine ｡
At present, the typical configuration scheme of air compressor unit of PTA plant is: tail gas expander of air compressor unit of steam turbine (one group of three units) or tail gas expander of air compressor generator of steam turbine (one group of four units) ｡
Several process characteristics of the device
The differences between the process routes of different PTA devices are mostly concentrated in the oxidation reaction mode and oxidation reactor. The existing patent protection is also basically concentrated in PTA devices with a capacity of less than 500000 tons. The oxidation reactor can be simply classified into the following three types:(1) low temperature and low pressure oxidation reactor;(2) medium temperature and medium pressure oxidation reactor;(3) High pressure oxidation reactor. Different oxidation reaction methods used in PTA unit will lead to great difference in the configuration of air compressor unit ｡
1. Low temperature and low pressure oxidation reaction
In low temperature and low pressure oxidation reaction mode, the outlet pressure of compressor is not more than 10bar, the flow rate of waste heat steam and tail gas generated in the process is small, and the temperature is low; during normal operation of the unit, the energy provided by waste heat steam and tail gas is not enough to fully supply the consumption of compressor unit, so it is difficult to realize the energy balance of the whole unit ｡
Therefore, the general air compressor is driven by a motor. After normal operation, the tail gas expander will provide some energy to reduce the consumption of motor power. This device has a relatively simple structure and low investment cost. However, due to the insufficient utilization of a large part of waste heat steam in the process, the later operation cost of the device is too high, and the overall economy of the scheme is poor. Low temperature and low pressure oxidation reaction has strong competitiveness in small PTA devices, for large PTA device has obvious defects ｡
2. High pressure oxidation reaction
The application of high temperature oxidation reaction in large PTA plant has only started in recent years. Due to the increase of pressure and temperature in the oxidation reactor, the waste heat steam and tail gas generated in the process have large flow rate and high temperature, which can be converted into more energy for compressor consumption, even slightly surplus. In order to effectively utilize energy, the patented technology represented by Dupont Company
The exhaust gas is subjected to high-temperature oxidation treatment to increase the output power of the exhaust gas expander as much as possible. Therefore, a set of generator set or a combined device of starter motor and generator set will be added to the equipment list of the air compressor unit. If good conditions are available, such as the excess power in the unit is allowed to be connected to the grid for external sales, then the high-pressure reaction mode will undoubtedly have good economy, high-pressure reaction mode requires high one-time investment, as well as excessive use and maintenance costs. In order to ensure the normal operation of the generator set, the rotating speed of all moving equipment of the air compressor set must be constant at 1500rpm, which reduces the adjustment performance of the compressor set and affects the operation efficiency of the device and the flexibility of variable working condition adjustment ｡
3. Medium temperature and medium pressure oxidation reaction
Medium temperature and medium pressure oxidation reaction method is the process flow commonly used in large PTA plants at present. As the medium temperature and medium pressure oxidation process can make full use of the energy of the whole plant, it has high economy. At the same time, due to the reduction of one-time investment cost, the unit configuration scheme is simple, the operation and maintenance are simple, and it has strong competitiveness in large PTA plants ｡
The oxidation reaction at medium temperature and medium pressure can fully utilize all the waste heat steam and tail gas generated in the process, and establish a complete energy balance. Since the compressor unit is driven by variable speed machinery, the device has high operation flexibility, especially for the adjustment of variable working conditions. The compressor unit can realize perfect coordination with the process flow, and maintain high efficiency operation at any possible operating point. Compared with the other two oxidation reaction modes, medium temperature and medium pressure oxidation process technology is mature, stable operation, and low operation and maintenance costs ｡
Core equipment in PTA plant
Air compressor is one of the core equipment in the whole PTA plant. Due to the particularity of technological process, the forms of compressors in PTA plant are generally fixed in the following categories: single-shaft centrifugal compressor and multi-shaft centrifugal compressor. From the principle analysis, axial flow compressor can also meet the process requirements to a certain extent. However, due to high air pressure in PTA plant, the large water content in the atmosphere and the corrosion effect of the metal caused by the high acetic acid content in the factory environment put the machinery and equipment at great risk. Therefore, there is no axial flow compressor in all PTA devices at present. This is determined by the particularity of the process flow and the characteristics of the compressor itself ｡
Single-shaft and multi-shaft centrifugal compressors are widely used in petroleum and chemical processes. As can be seen from statistical data, the application of single-shaft centrifugal compressors is much larger than that of multi-shaft centrifugal compressors in terms of quantity and field of use. This is determined by the super stability of single-shaft centrifugal compressors in operation, and this is also an important reason why many petrochemical enterprises favor single-shaft compressors, it is an undeniable fact that the number of multi-shaft centrifugal compressors used in PTA plants is much more than that of single-shaft compressors, and they have good operating performance ｡
Production of domestic compressor manufacturers
For the air compressor of PTA plant, the existing compressor technology of Shenyang blower (group) co., ltd. in China can meet the needs and belongs to mature technology. compared with other air compressors, SRR series and MCL series compressors of Shen drum can meet the needs of air compressors of PTA plant. SRR series compressors are single-shaft and single-cylinder compressors and adopt semi-open and closed high-efficiency ternary impellers, make the compression process close to isothermal compression, greatly reducing the power consumption of the compressor; The compressor flow rate is convenient to adjust, and the compressor can be adjusted within the range of 70% ~ 105 flow rate. The compressor rotor is light in weight and relatively easy to start. Moreover, MCL series compressors have small floor area and are easy to maintain and overhaul. MCL series compressors adopt double-cylinder, high-efficiency closed ternary impellers with reasonable layout, small axial thrust, mature technology and high reliability ｡