Process Optimization Of Braun Urea Production Based On Mass And Energy Balances

Synthetic ammonia industry plays a decisive role in the status of basic chemical industry. Energy consumption is very high in ammonia synthesis, accounted for one-tenth of total energy consumption in chemical production in the world. It has become the problem of modern industry that is addressed seriously to reduce the energy consumption of synthetic ammonia process. Braun techno logy uses natural gas as feedstock. Its key lies in the cryogenic refined of syngas, and gasification section is relatively independent of ammonia synthesis section, besides, the flow of synthetic relief gas is very small and the utilizaon rate of the syngas is high.This paper is based on mass balance, energy balance and material/energy coordinated matching, the key factors influencing the urea consumption of Braun is analyzed, it is found: 1) In gasification section, the material and energy balance can’t be matched when the air enter the secondary reformer to elevate the furnace temperature and to provide nitrogen,in another way, when it meets the requirements of furnace temperature (800℃), nitrogen is excess, that causes the waste of the utilities; 2) In urea synthesis section, the CO2 provided by gasification section can’t matched with ammonia production in theory of ammonia synthesis process. And part of the syngas is used as fuel gas, which not only wastes the utilities, but also the productivity is reduced.For the problem material/energy cannot achieve reasonable index in the gasification process at the same time, this paper proposes the method of using oxygen-enriched air instead of normal air, in order to bring down energy consumption of nitrogen surplus in gasification section. Through the material balance, there is excess 462 kmol/h N2 in gasification section of Braun technology which can produce 300000 tons synthetic ammonia. This article adopts the membrane separation to producing the oxygen-enriched air,whose concentration is from 21% to 30%. So the balance of material and energy is improved. as a result, the concentration of methane is increased from 16.5% to 37.22% in vent gas of purifier. The stock logistics can be recycled back to a furnace, so as to improve the utilization rate of raw materials.Aiming at the question that the carbon dioxide provided by gasification section can’t meet the need in urea synthesis, the CO2 is recycled from flue gases as supplementary in this paper. Through the material balance, there is excess 72.2 kmol/h synthesis gas in gasification section of Braun technology which can produce 300000 tons synthetic ammonia, when it is used to produce urea. It means that lacks of CO218.05 kmol/h. The chemical solvent method MSA is used for recovery of carbon dioxide from flue gases. As a result the utilization ratio of synthesis gases are improved, at the same time, the emission of CO2 is reduced.This paper made a simulation of 300,000 tons ammonia production a year using Brown technology. In gasification section, when we choose PI membrane oxygen enrichment membrane, the pressure in the side of feed gas is 415 kPa, and the pressure in the side of permeate is 102 kPa, the cost of oxygen enrichment is lowest. When purifying exhaust is cycled and the rich oxygen concentration changes to 29.09%, the process tends to be stable. The unit cost of the syngas is from 34.49 yuan/kmol dropping to 30.91 yuan/kmol and the syngas produced 5767 kmol/h; When purifying exhaust isn’t cycled and the rich oxygen concentration is 26.5%, The unit cost of the syngas is minimum and the value is 32.51 yuan/kmol. The method of MSA recycling CO2 as a supplement is used to the synthesis of urea. When the production of the syngas is 5767 kmol/h, the production of CO2 is 86.1 kmol/h. At the time, the equipment investment is 260000 yuan/year, the utility is 5.745 million yuan/year. After the optimization of the two methods, the final economic benefits is 5.09 million yuan/year.