Ammonia recovery technology for coking coal gas

2018-08-21 19:59

Summary

The main sources of coal gasification wastewater are washing water, crude gas washing and purification, wastewater generated during the recovery of phenolic ammonia rectification, etc. The ammonia recovery is carried out by a single tower pressurized line extraction stripping process. The ammoniated system designed by our company can be energy-saving and environmentally friendly, and the operation automation is high, and it is unattended. The distilled ammonia water fully meets the requirements of industrial production. The wastewater after steaming can directly enter the biochemical treatment system. The system has low investment, short recycling period and short construction period. It does not require cement frame equipment and total capital investment to save more than 30% compared with traditional technology.

First, the technical background

As the basic chemical raw material of China, coal's clean conversion is related to the dual interests of China's economy and environment. Coal gasification is widely used by coal chemical companies in its own way. In the process of coal gasification, a large amount of wastewater is produced, which contains many substances that can be reused and needs to be recycled. The ammonia recovery unit recovers ammonia pollutants from coal gasification wastewater. In the phenol ammonia recovery process, the ammonia in the wastewater is mainly removed by a stripping process.

Second, the technical brief

Coal gasification is a thermochemical process in which solid fuels (mainly coal or coke) and gasifying agents (mainly air) are converted into small molecules of combustible other raw materials by pyrolysis reaction under high temperature conditions. the process of. Small molecules combustible other raw materials mainly include H2, CO, CH4 and other gases. The gas after coal gasification cracking needs to be washed. Therefore, the main sources of coal gasification wastewater are washing water, coarse gas washing and purification, and wastewater generated during phenolic ammonia rectification and recovery. These wastewaters mainly contain ammonia and phenols. , water-soluble pollutants such as cyanide, high molecular organic substances, lipids, and sulfides.

Ammonia recovery adopts single tower pressurized line extraction and stripping process. Single tower purification has the advantages of simple process, low investment, low energy consumption, simultaneous recovery of ammonia and vulcanization, smooth and flexible operation.

The single-column pressure line extraction steam stripping process includes two parts, a stripper and a three-stage segregation system, as shown in Figure 1. Among them, the stripper adopts a pressurized stripping side line ammonia extraction technology. After the pretreated raw material sewage pump is pressurized, it is divided into two strips into the stripping tower, and one way is cooled by the cooler to 30~40 °C as the cold feed to the first tray of the stripper; the other road and the tower The heat output from the outlet and the side line is 140~150 °C as the hot feed into the middle of the stripper, and the ratio of cold and hot feed is 0.25~0.40. The tower pressure is 0.5~0.6Mpa, and the tower is heated by reboiler or direct steam at a temperature of 160~165°C. After separation, acid gas such as hydrogen sulfide and carbon dioxide is obtained at the top of the tower, and after cooling, it enters the acid gas separation tank, the tower kettle is purified water, and the crude ammonia gas extracted from the side line enters the three-stage segregation system, and the extraction ratio is 8% to 14%. (based on total feed).

The three-stage partial condensation system is mainly composed of three partial condensers. The stepwise variable temperature and pressure transformation method is used to thicken the crude ammonia gas to obtain ammonia gas with a purity of more than 99%, and enter the subsequent ammonia refining process. The temperature of the three-stage condenser is approximately 115~125°C, 85~95°C, 40~50°C.

Flow chart

Zhongtian Technology

Figure 1 Flow chart of single tower pressurized side line extraction and stripping process

Third, the principle of the process

The sewage stripping process is a complex system in which chemical equilibrium, ionization equilibrium and phase equilibrium coexist. The single tower pressurized side line extraction stripping process is a task of separating acid gas, ammonia and purified water for sewage treatment by using a tower. The stripping tower can be divided into three sections from top to bottom according to the function: the top of the tower to the hot inlet is the acid gas rectifying section, the hot inlet to the side line is the acid gas stripping section, and the side line is pumped to the bottom of the tower for ammonia stripping. segment.

Under the condition of maintaining a certain pressure at the top of the tower and controlling the temperature of the tower body, the raw sewage is cooled and the hot feed enters the stripping tower from the upper part of the tower. At higher temperatures, both carbon dioxide and hydrogen sulfide are present in the liquid phase in a free molecular state. Under the hot feed decompression flashing and the bottom strip steam stripping, the ions are converted into carbon dioxide, hydrogen sulfide, ammonia molecules in the liquid phase, and the liquid phase is transferred into the gas phase to realize the gas. In the acid gas rectification section, under the low temperature absorption of the cold feed at the top of the column, since the solubility of ammonia is much larger than that of the acid gas, water vapor and ammonia are transferred into the liquid phase to flow downward with the liquid flow. In the acid gas stripping section, the cooled absorption water that absorbs water vapor, ammonia, and a small amount of acid gas is mixed with the hot feed wastewater flashed with carbon dioxide, hydrogen sulfide, and ammonia, and then moved to the middle of the column. Here, the acid gas and ammonia are repeatedly subjected to the stripping action of the high-temperature steam rising from the lower portion of the tower and the absorption of the flowing liquid from the upper portion, and the acid gas and ammonia are continuously absorbed and continuously stripped. Due to the high temperature in the middle, most of the acid gas is finally stripped to the top of the tower, and the ammonia is absorbed by the liquid stream to the middle of the tower. The ammonia accumulated in the middle of the tower reaches the gas-liquid equilibrium, and the liquid concentration is close to its solubility. At the same time, it is necessary to continue moving to the bottom of the tower with the liquid flow.

In the ammonia stripping section, due to the stripping action of the bottom steam and the extraction of the side line, ammonia is transferred from the liquid phase to the vapor phase and extracted from the side line. The ammonia in the stripping tower gathers in the middle of the tower, and the concentration distribution forms a peak. This is the common result of the low temperature absorption at the top of the tower, the high temperature stripping of the tower and the side line extraction. The basic factors affecting the height and position of the ammonia peak are: the temperature at the bottom of the tower rises, the peak of the ammonia peak becomes steep, the peak of ammonia rises, the position of the ammonia peak increases; the concentration of raw water increases, the ammonia load in the stripper increases, ammonia As the peak increases, the ammonia circulation increases and the ammonia peak increases. The crude ammonia extracted from the side line accounts for about 50% of the stripping load of the stripper, accounting for 50%~60% of the total heat supply. It is the key factor affecting the stripping strength and steam unit consumption in the lower part of the tower. As the extraction ratio is gradually increased from the small, the water quality at the bottom of the tower is obviously improved, and the steam consumption and sideline condensation are also increased, and the ammonia circulation amount generated by the circulation is also increased. When the extraction ratio is greater than a certain value, the ammonia peak in the tower is widened due to the large amount of ammonia circulation. Although the stripping intensity in the tower is large, the purified water quality is worse, and the steam consumption is also significantly increased. The general control side line extraction ratio is 8% to 14%.

The ammonia concentration in the crude ammonia extracted from the side line is generally 15% to 20%, and the other components are mainly water vapor, and a small amount of hydrogen sulfide and carbon dioxide. The three-stage segregation system is characterized by variable temperature and pressure, which fully utilizes the characteristics of high temperature water separation, low temperature sulfur fixation and stepwise concentration to reduce the content of water and hydrogen sulfide in the crude ammonia gas. The sideline crude ammonia gas temperature is about 155 ° C, and through the first-stage segregation, about 70% of the crude ammonia gas is condensed, and the ammonia concentration can reach 40% or more. The second-stage segregation only plays a role of transition, further increasing the concentration of ammonia in the gas phase and condensing a certain amount of water, and the ammonia concentration after centrifugation can reach 70%-80%. The gas phase concentration of ammonia in the third-stage segregation is higher, and the solubility of hydrogen sulfide in the low temperature condition is increased. The ammonia in the liquid phase reacts with hydrogen sulfide and carbon dioxide to form ammonium hydrogensulfate and ammonium hydrogencarbonate. As the reaction progresses, the hydrogen sulfide in the gas phase is continuously dissolved in the water, and the purpose of removing hydrogen sulfide and bringing the ammonia concentration in the gas phase to 99% or more is achieved. If the by-product is ammonia water, the third-stage partial condensation can be directly arranged into ammonia water. If the by-product is liquid ammonia, since the ammonia gas still contains about 200 ppm of hydrogen sulfide, this part of hydrogen sulfide will be directly compressed into liquid ammonia without being treated, which will seriously affect the product quality and cause corrosion of the ammonia compressor.

Fourth, technical advantages

Shandong Zhongtian Technology Engineering Co., Ltd. is a mature and energy-saving process for wastewater ammonia evaporation. It has unique process design and is the leader in the industry. The ammonia concentration can be controlled by adjusting the temperature of the divider, using an advanced process to achieve one-tower distillation, and adopting advanced technology to greatly reduce the amount of steam, so as to be energy-saving and environmentally friendly, and at the same time, the operation is highly automated and unattended. The distilled ammonia water fully meets the requirements of industrial production. The wastewater after steaming can directly enter the biochemical treatment system. The system has low investment, short recycling period and short construction period. It does not require cement frame equipment and total capital investment to save more than 30% compared with traditional technology.