Several PTA equipment process routes and equipment air compressor technology

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

   PTA industrial characteristics

The PTA industry has the characteristics of capital-intensive and technology-intensive. The process packages and key equipment of all domestic PTA devices have long been a complete set of imported foreign technology and equipment, all monopolized by foreign investors. Due to the complex process route of the production device, the high technical content, It has been monopolized by large foreign companies for decades.

   PTA market production situation

Global PTA production is mainly concentrated in Asia. As of the end of 2003, China's PTA production capacity was 3.98 million tons, which was only lower than Taiwan and South Korea and ranked third in the world. Although my country's PTA production capacity has been expanding in recent years, it is still Far from meeting market demand, the market gap is still huge. About 4.3 million tons were imported in 2002, and about 4.54 million tons were imported in 2003, and the 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 has become an important content to promote the development of the polyester industry.

  PTA basic chemical reaction

  Use cobalt acetate as a catalyst, use acetic acid as a solvent, and use air as an oxygen source to perform liquid phase oxidation of xylene (PX) to produce crude terephthalic acid (CTA).

   process flow

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

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

  Unit configuration

   Usually PTA unit is composed of compressor, turbo expander and steam turbine, which can be divided into single shaft unit composed of single shaft compressor and multi-shaft unit composed of gear compressor.

  Single shaft unit: high pressure air compressor, low pressure air compressor, turbo expander, steam turbine.

  Multi-shaft unit: multi-shaft compressor, turbo expander, steam turbine.

   The typical configuration scheme of the air compressor unit of the current PTA device is: steam turbine + air compressor unit + exhaust gas expander (three engines set) or steam turbine + air compressor + generator + exhaust gas expander (four engines set).

   Several technological characteristics of the device

The differences between the process routes of different PTA devices are mostly concentrated on the oxidation reaction method and the oxidation reactor, and the existing patent protection is basically concentrated here. For PTA devices 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 and low pressure oxidation reactor; (2) Medium temperature and medium pressure oxidation reactor; (3) High pressure oxidation reactor. Different oxidation reaction methods used in the PTA device will lead to air compressor units There is a big difference in the configuration.

  1. Low temperature and low pressure oxidation reaction

In the low-temperature and low-pressure oxidation reaction method, the compressor outlet pressure does not exceed 10 bar, and the waste heat steam generated in the process, the exhaust gas flow 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 provide the consumption of the compressor unit, which is difficult Realize the energy balance of the entire device.

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. This device has a relatively simple structure and low investment cost; but because a large part of the waste heat steam in the process is not It is fully utilized, resulting in excessively high operating costs in the later stage of the device, and poor overall economic efficiency of the program. Low-temperature and low-pressure oxidation reaction has strong competitiveness in small PTA devices, but it has obvious defects for large PTA devices.

   2. High pressure oxidation reaction

The application of high-temperature oxidation reactions in large-scale PTA devices has only begun in recent years. Due to the increased pressure and temperature in the oxidation reactor, the waste heat steam generated in the process, the exhaust gas flow is large, and the temperature is high, which can be converted into more Energy is consumed by the compressor, even slightly surplus; in order to effectively use energy, the patented technology represented by Dupont company

The exhaust gas has been subjected to 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 unit configuration plan. If available 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 downside is that the high-pressure reaction method requires too much one-time investment, and there is too much Use and maintenance costs; in order to ensure the normal operation of the generator set, the speed of all moving equipment of the air compressor unit must be constant at 1500rpm, which reduces the adjustment performance of the compressor unit, and affects the operating efficiency of the device and the flexibility of variable-condition adjustment.

  3. Medium temperature and medium pressure oxidation reaction

The medium temperature and medium pressure oxidation reaction method is a process commonly used in large-scale PTA devices at present. Because the medium temperature and medium pressure oxidation process can make full use of the energy of the entire device, it has high economic efficiency; at the same time, due to the reduction of one-time investment cost , The unit configuration scheme is simple, operation and maintenance are simple, and it has strong competitiveness in large-scale PTA devices.

The medium temperature and medium pressure oxidation reaction can make full use of all the waste heat steam and tail 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 operation Flexibility, especially for the adjustment of variable working conditions. The compressor unit can achieve perfect coordination with the process flow and maintain high-efficiency operation at any possible operating point. Compared with the other two oxidation reaction methods, medium temperature and medium temperature The pressure oxidation process technology is mature, stable operation, and low operation and maintenance costs.

   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, the form of the compressor in the PTA device is generally fixed in the following categories: single-shaft centrifugal compressor and multi-shaft centrifugal compressor From the principle analysis, the use of axial compressors can also meet the requirements of the process to a certain extent. However, due to the high air pressure in the PTA device, the large water content in the atmosphere and the high acetic acid content in the factory environment give The corrosion effect of metal puts machinery and equipment at a huge risk. Therefore, there is no performance in the use of axial compressors in all current PTA devices. This is determined by the particularity of the process and the characteristics of the compressor itself.

Both single-shaft and multi-shaft centrifugal compressors are widely used in petroleum and chemical processes. From the statistical data, it can be seen that the application of single-shaft centrifugal compressors is better than that of multi-shaft compressors in terms of quantity and application fields. Centrifugal compressors are much larger. This is due to the super 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 devices is far more than that of single-shaft compressors, and they have good operating performance.

   Production status of domestic compressor manufacturers

For the air compressor of the PTA device, the existing compressor technology of China Shenyang Blower (Group) Co., Ltd. can meet the needs and is a mature technology. Compared with other air compressors, it is only a change in process parameters. SRR series and MCL series compressors can meet the needs of air compressors for PTA devices. Among them, SRR series compressors are single-shaft and single-cylinder compressors, using semi-open and closed high-efficiency ternary impellers, making the compression process close to isothermal compression. Greatly reduce the power consumption of the compressor; the compressor flow rate adjustment is convenient, the compressor can be adjusted within the range of 70% to 105% flow rate; the compressor rotor is light in weight and relatively easy to start; and it has a small footprint and is easy to maintain and repair 。MCL series compressor adopts double-cylinder, high-efficiency closed ternary impeller, reasonable layout, small axial thrust, relatively mature technology and high reliability.