PTA is the abbreviation of Purified Terephthalic Acid. PTA is the basic material for the production of polyester fiber and the packaging industry, and the main raw material for the chemical fiber industry.

PTA industrial features

The PTA industry is capital-intensive and technology-intensive. The process packages and key equipment of all PTA plants in China have long been complete sets of imported technology and equipment, all of which are monopolized by foreign businessmen. Due to the complex process route and high technology content of this production plant, It has been monopolized by large foreign companies for decades.

PTA market production

The 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 Taiwan and South Korea, ranking third in the world. Although my country's PTA production capacity has been expanding in recent years, it still It is far from meeting the market demand, and the market gap is still huge. In 2002, it imported about 4.3 million tons, and in 2003, it imported about 4.54 million tons, and its dependence on the international market exceeded 60%.

With the continuous development of my country's polyester industry, the market demand for PTA products has grown rapidly in recent years. Expanding PTA production capacity and vigorously developing PTA production technology have become an important part of promoting the development of polyester industry.

The basic chemical reaction of PTA

Using cobalt acetate as a catalyst, using acetic acid as a solvent, and using air as an oxygen source to carry out a liquid-phase oxidation reaction of paraxylene (PX), crude terephthalic acid (CTA) is generated.

Process flow

In the actual production, air is used as the oxygen source, the heat released during the reaction process is used to maintain the temperature in the reactor (about 188 °C), and the exhaust gas (temperature is 180 °C) generated in the reaction is passed through a heat exchanger (waste heat boiler) for heat treatment. Heat energy recovery, pressure energy recovery through the exhaust gas turbine.

After heat exchange, the tail gas (temperature is 40℃) is heated to 130~170℃ after passing through the high pressure absorption tower, and then enters the tail pass, and when it expands to 3~5MPa from the tail pass, 15%~20% of the tail gas is extracted as TA The exhaust gas discharged from the tail gas enters the incinerator and is incinerated and then discharged into the atmosphere to ensure that the emission quality of the exhaust gas can meet the requirements of environmental protection.

Unit configuration

Usually PTA units are composed of compressors, turboexpanders, and steam turbines, which can be further 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, steam turbine.

Multi-shaft units: multi-shaft compressors, turboexpanders, steam turbines.

At present, the typical configuration scheme of the air compressor unit of the PTA unit is: steam turbine + air compressor unit + tail gas expander (one set of three units) or steam turbine + air compressor + generator + tail gas expander (one set of four units).

Several technological features of the device

The differences between the process routes of different PTA units are mostly concentrated in the oxidation reaction mode and oxidation reactor, and the existing patent protection is basically concentrated here. For PTA units with a capacity of less than 500,000 tons, the oxidation reactor can generally be simple It is classified into the following three types: (1) low temperature, low pressure oxidation reactor; (2) medium temperature, medium pressure oxidation reactor; (3) high pressure oxidation reactor. Different oxidation reaction methods used in PTA units will lead to air compressor units There is a big difference in the configuration.

1. Low temperature, low pressure oxidation reaction

In the low-temperature and low-pressure oxidation reaction method, the outlet pressure of the compressor does not exceed 10bar, the flow rate of waste heat steam and exhaust gas generated in the process is small, and the temperature is low; the energy provided by the waste heat steam and exhaust gas during the normal operation of the device is not enough to fully supply the consumption of the compressor unit, and it is difficult to The energy balance of the whole device is achieved.

Therefore, the general air compressor is driven by a motor. After normal operation, the exhaust gas expander will provide a part of the energy to reduce the power consumption of the motor. The structure of this device is relatively simple and the investment cost is low; however, because a large part of the waste heat steam in the process is not It is fully utilized, resulting in high operating costs in the later stage of the device and poor overall economics of the scheme. The low-temperature and low-pressure oxidation reaction has strong competitiveness in small-scale PTA plants, but it has obvious defects for large-scale PTA plants.

2. High pressure oxidation reaction

The application of high-temperature oxidation reaction in large-scale PTA plants has only begun in recent years. Due to the increased pressure and temperature in the oxidation reactor, the waste heat steam and exhaust gas generated in the process have a large flow rate and high temperature, which can be converted into more The energy is supplied to the consumption of the compressor, and even has a slight surplus; in order to effectively utilize the energy, the patented technology represented by Dupont Company has

The exhaust gas has undergone high-temperature oxidation treatment to maximize the output power of the exhaust gas expander. Therefore, a generator set or a combination of starter motor and generator set will be added to the equipment list of its air compressor set configuration plan. If equipped Under very good conditions, such as: the excess electricity in the device is allowed to be connected to the grid for external sales, then the high-pressure reaction method is undoubtedly very economical. The disadvantage is that the high-pressure reaction method requires too much one-time investment, and there are too large Use and maintenance costs; in order to ensure the normal operation of the generator set, the rotational speed of all the dynamic equipment of the air compressor unit must be constant at 1500rpm, which reduces the adjustment performance of the compressor unit, affects the operating efficiency of the device and the flexibility of adjustment under variable working conditions.

3. Medium temperature, medium pressure oxidation reaction

The medium-temperature and medium-pressure oxidation reaction method is currently a commonly used process in large-scale PTA plants. Since the medium-temperature and medium-pressure oxidation process can fully utilize the energy of the entire plant, it is highly economical; at the same time, due to the reduction of the primary investment cost , The unit configuration scheme is simple, operation and maintenance are simple, and it has strong competitiveness in large-scale PTA devices.

The oxidation reaction at medium temperature and pressure can make full use of all the waste heat steam and exhaust gas generated in the process, and establish a complete energy balance; because the compressor unit is driven by a variable speed machine, the device has a high operating efficiency. Flexibility, especially for the adjustment of variable operating conditions. The compressor unit can be perfectly coordinated with the process flow and maintain high efficiency at any possible operating point. Compared with the other two oxidation reaction methods, medium temperature, medium temperature The high-pressure oxidation process technology is mature, the operation is stable, and the operation and maintenance costs are low.

The core equipment in the PTA device

The air compressor is one of the core equipment in the entire PTA device. Due to the particularity of the process flow, the compressors in the PTA device are generally fixed in the following categories: single-shaft centrifugal compressors and multi-shaft centrifugal compressors In principle, the axial flow compressor can also meet the requirements of the process to a certain extent. However, in the PTA device, due to the high air pressure, the large water content in the atmosphere and the high acetic acid content in the factory environment, the The corrosion effect brought by metal puts machinery and equipment at great risk. Therefore, no axial compressor has been used in all PTA units 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. From the statistics, it can be seen that the application of single-shaft centrifugal compressors is more than that of multi-shaft centrifugal compressors in terms of quantity and field of use. Centrifugal compressors are much larger. This is determined by the superior stability of single-shaft centrifugal compressors in operation. And this is also an important reason why many petrochemical companies favor single-shaft compressors. However, the undeniable fact is: The number of multi-shaft centrifugal compressors used in PTA units is much more than that of single-shaft compressors, and it has a good operating performance.

Production situation of domestic compressor manufacturers

For the air compressor of the PTA device, the existing compressor technology of the domestic Shenyang Blower (Group) Co., Ltd. can meet the needs and is a mature technology. Compared with other air compressors, it is only a change of process parameters. SRR series and MCL series compressors can meet the needs of air compressors in PTA installations. Among them, SRR series compressors are single-shaft, single-cylinder compressors, using semi-open and closed high-efficiency three-way impellers, making the compression process close to isothermal compression, The power consumption of the compressor is greatly reduced; the flow of the compressor is easy to adjust, and the compressor can be adjusted within the range of 70% to 105% of the flow; the rotor of the compressor is light in weight, and it is relatively easy to start; and the footprint is small, easy to maintain and overhaul .MCL series compressor adopts double-cylinder, high-efficiency closed ternary impeller, reasonable arrangement, small axial thrust, relatively mature technology and high reliability.