Crude liquid ammonia and purified CO2 gas enter the urea synthesis tower after being compressed, and unreacted ammonia and CO2 in the synthesis reaction liquid are decomposed under reduced pressure once, and the tail gas after ammonia removal is sent to the ammonia processing workshop to generate ammonium salt. Unreacted materials do not return to the synthesis system, so it is called non-circulating method. If the unreacted materials are decomposed under reduced pressure and partially returned to the synthesis system after condensation, it is called partial circulation method. This method has high cost and backward technology, and has long been eliminated.
Ⅱ. Total solution circulation method
After urea synthesis reaction, unconverted reactants ammonia and CO2 are separated from urea solution through multistage decompression and thermal decomposition, and then all of them return to the synthesis system, thus improving the utilization ratio of raw materials ammonia and CO2. This method is called full circulation method. According to the different recovery methods of unreacted ammonia and CO2, it can be divided into solution total circulation method, gas separation method (selective adsorption), slurry circulation method and hot gas circulation method.
① Gas separation method (i.e. selective absorption method). In this method, urea-nitric acid aqueous solution is used as absorbent to selectively absorb ammonia in decomposed gas. After the absorption liquid is regenerated, ammonia is recovered and returned to the synthesis tower after compression and condensation. Or the mixture of ammonia, CO2 and water vapor decomposed by heating under reduced pressure is absorbed by MEA solution, and the remaining ammonia is condensed and returned to the synthesis tower.
② Total circulation of aqueous solution. After urea synthesis, unreacted ammonia and CO2 are decomposed and separated, absorbed into ammonium carbamate solution with water, and then circulated to the synthesis system, which is called total circulation method of aqueous solution. Since 1960s, it has been popularized rapidly, occupying a great advantage in urea production, and it is still being improved. Typical examples are the total circulation method of aqueous solution of Stamikaben in the Netherlands, the total circulation method of aqueous solution of Kemi branch in the United States, and the improved C method and D method of Mitsui Dongdong in Japan.
See table 6- 13 for the process conditions of the three processes of total circulation of aqueous solution.
Table 1
Process conditions of total circulation of aqueous solution
way
Pressure //MPa
Temperature/℃
NH3/CO2
H2O/ CO2
exchange rate
Stamikaben method
Monte Edison method
Modified c method of mitsui east pressure
19.62~2 1.58
19.62
22.56~24.53
185~ 188
195~200
190~200
4.0
3.5
4.0
0.60
0.55
0.37
62%
63%
72%
Three. Gas stripping
The total circulation method of aqueous solution does not consume expensive solvents, saves investment, and has been widely used, making positive contributions to the development of urea industry. However, there are many problems in the total circulation method of aqueous solution, such as insufficient utilization of energy, cooling of reaction heat by a large number of circulating liquids, and insufficient utilization; The primary ammonium carbamate pump is seriously corroded, and its manufacture, operation and maintenance are very troublesome. In order to recover trace CO2 and ammonia, the process becomes too complicated. Stripping is a new method developed on the basis of the transformation of total circulation of aqueous solution.
The so-called stripping method is to use stripping agents such as CO2, ammonia, shift gas or other inert gases to heat under a certain pressure to promote the decomposition of ammonium carbamate that has not been converted into urea and the gasification of liquid ammonia. The efficiency of stripping decomposition is affected by pressure, temperature, liquid-gas ratio and residence time. Too high temperature will accelerate the hydrolysis of ammonia and the increase of biuret, while too low pressure will reduce the condensation absorption rate of decomposition products. The shorter the stripping time, the better. Hydrolysis and condensation reactions can be prevented. Therefore, stripping is based on the principle of two-stage synthesis, that is, liquid ammonia and gas CO2 react in a high-pressure condenser to generate methylammonium, and the dehydration reaction of methylammonium is carried out in a urea synthesis tower. In fact, in order to maintain the reaction temperature of urea synthesis tower, a part of methylamine is left in the synthesis tower, not all of it is completed in the high-pressure condenser. The reaction in the stripper is as follows
This is a reversible reaction of endothermic and volume increase. As long as there is enough heat and the partial pressure of any component in the reaction product can be reduced, the decomposition reaction of ammonium carbamate can always proceed to the right. Gas stripping is based on this principle. When CO2 gas is introduced, the partial pressure of CO2 is 1, while the partial pressure of ammonia tends to zero, which leads to the continuous reaction. Similarly, ammonia extraction has the same result.
Gas stripping process is an important technical improvement in urea synthesis and production at present. Compared with the total circulation method of aqueous solution, it has the advantages of simplified process, low energy consumption, reduced production cost, large scale of single series, balanced and safe operation, long operation period and so on. Gas stripping mainly includes Stamikaben CO2 stripping, SNAM ammonia stripping, IDR (equal pressure double stripping) method and ACES method.
Fig. 6- 12 is a schematic diagram of CO2 stripping process in Stamikaben, the Netherlands. This method was successfully studied in 1964 and was widely used in the early 1970s. Now it has become the production process with the largest number of factories, and the single series can reach1756 ~ 2100 t/d.
After CO2, the feed gas for urea synthesis, is pressurized (its pressure is the same as that of the synthesis tower), it first enters the CO2 stripper, and most of the ammonium carbamate that has not been converted into urea is decomposed and escapes with CO2. The heat required for stripping decomposition is provided by 2.45MPa steam, and the outlet of CO2 stripper flows into the high-pressure ammonium carbamate condenser. The materials flowing into the condenser include pressurized raw liquid ammonia and ammonium carbamate solution after high-pressure washing. The liquid is condensed in the high-pressure ammonium carbamate condenser through a high-pressure liquid ammonia ejector, and CO2 is absorbed for ammonium carbamate reaction. The heat released in the absorption process and the reaction process produces low-pressure steam of 0.294MPa. Most of the reactions to produce ammonium carbamate are carried out in the high-pressure condenser, and some of the reactions to produce ammonium carbamate are carried out in the urea synthesis tower. In order to maintain the heat required for methylamine dehydration in urea synthesis tower, the operating temperature of urea synthesis tower can be controlled by controlling the condensation amount of high pressure condenser. The reaction mixture flowing from the bottom of urea synthesis tower enters CO2 stripping tower, and the reaction mixture flowing from the bottom of CO2 stripping tower is decompressed to 0.2533MPa and then enters into low-pressure decomposer, where it is further decomposed by heating and the residual methylamine and ammonia are escaped. After flashing, the urea solution at the bottom of the tower is sent to two-stage vacuum evaporation. Concentrated to 99.7% (mass fraction) of molten urea, and finally sent to the granulation tower to make granular products. The mixed gas from the top of the decomposition tower is condensed and absorbed at low pressure, and the generated ammonium carbamate solution is pumped into the high-pressure scrubber. The mixed gas at the top of urea synthesis tower also enters the high-pressure scrubber for recovery.