Methyldiethanolamine desulfurization process
1 The absorption tower in which the crude material gas is washed by the second-stage solution at 2.8MPa, and the lower part is absorbed by the decompression flash desorption solution. In order to improve the purification degree of the gas, the upper part is regenerated by steam heating wash. The rich liquid coming out of the absorption tower passes through two flash tanks successively to depressurize, and the energy for the first depressurization of the solution is recovered by the turbine. The recovered energy is used to drive the semi-lean liquid circulation pump. The rich liquid contains more hydrogen and ammonia in the steam released from the high-pressure flash tank, which can be compressed and sent back to the decarburization tower. CO2. Most of the obtained semi-poor liquid is pumped into the lower section of the absorption tower with a circulating pump, a small part is sent to the steam-heated regeneration tower for regeneration, and the obtained lean liquid is sent to the upper section of the absorption tower for use. The CO2 gas containing water vapor obtained from the top of the regeneration tower is sent to the low-pressure flash tank for use as a degassing medium.
2 The key to technical operation
(1) The share of barren solution and semi-barren solution
The proportion of poor solution/semi-poor solution is usually 1/3~1/6, which depends on the partial pressure of CO2 in the raw material. If the partial pressure of CO2 is high, the selected proportion can be higher (such as 1/6), so that the heat energy consumption will be reduced, and the temperature of the lean liquid is usually 55 ~ 70 ℃.
(2) Temperature of barren solution and semi-poor solution
The temperature of semi-poor liquid is usually 70~80°C. If the liquid inlet temperature is high, the heat energy consumption will be low, but if it is too high, it will affect the temperature at the bottom of the absorption tower, making the absorption capacity of the solution smaller, but it will increase the heat energy consumption. There is an appropriate solution temperature ratio for each feed gas working condition. It can not only ensure the degree of purification but also make full use of its physical properties, so that its heat energy consumption can be reduced to a minimum.
(3) CO2 removal and consumption When the absorption pressure is 2.7 MPa, CO2 can be removed below 0.005, and the CO2 purification degree is within 0.1%. The heat energy it consumes depends on the partial pressure of CO2 in the raw material gas. High partial pressure and low thermal energy consumption, usually in a one-stage adiabatic CO2 removal process. In principle, there is no need to consume heat energy, but to maintain a stable absorption and desorption temperature, it depends on the heat balance between raw gas, purified gas and regeneration gas. Usually, because the regeneration gas takes away a lot of heat, it is necessary to supplement the heat (such as using low-level energy such as hot water) to maintain the temperature.
(4) High pressure flash evaporation and the purity of recovered CO2
The solubility of non-polar gases such as hydrogen, nitrogen, methanol, CH and other high-grade hydrocarbons in MDEA solution is low, so the loss of the purified gas is very small, but when the absorption pressure is high, the CO2 in the regeneration gas is less than 98% , if the absorption pressure is 2.7 MPa, there is high-pressure flash steam in the process to improve the purity of CO2, the flash pressure is selected according to the purity requirements, usually about 96% of CO2 can be recovered, and its purity can reach 99.5, when the absorption pressure < 1.8 MPa, the process CO2 with a purity greater than 98.5% can be obtained without using high-pressure flash distillation.
(5) Solvent loss: Since MDEA reacts with CO2 to form bicarbonate instead of nitrogen formate, it will not degrade. In addition, the vapor partial pressure of MDEA itself is low (less than 0.01 mmHg at 25°C), so the loss of MDEA is very small,
3. Technical features
(1) MDEA solution has good stability, is not easy to degrade, and is not corrosive to carbon steel. (2) MDEA itself has low vapor partial pressure and low volatility.
(3) MDEA decarbonization technology can also remove hydrogen sulfide and organic sulfur while absorbing CO2.
(4) It has relatively low solubility for non-polar gases H2 and N2 during the absorption process, so the loss of purified gas is also small. These characteristics constitute its bright future as a decarburization solvent.
