Optimization Of Hydrazine Hydrate Evaporation Technology By Urea Method

At present, there are four hydrazine hydrate production. technologies:Raschig process,urea method,ketazine method and hydrogen peroxide method.:Urea method is still a dominant technology in China, because the product has almost no organic impurities, and can feed the need in field of special purpose. The bottleneck of the urea method is a high energy consumption, in the main, at the evaporation-distillation unit. In this paper, a novel technical solution was proposed. The key idea is the reuse of the low grade energy of overhead steam at distillation column by use of steam jet heat pump technology. By this way the secondary steam condensate treatment of hydrazine-water mixture can be omitted on one hand, and the energy cost of distillation reboiler can be saved on the other hand. It seems no report in literature for the recovery of energy by use of heat pump technology in the hydrazine hydrate production process, and this is the invention of this paper.Firstly, a mathematical model of evaporation distillation unit was established. The key model parameters, such as activity coefficients, enthalpy, equation of state, are estimated by use of various methods. Hydrazine hydrate mixtue is an inorganic salt electrolyte solution. The NRTL model with Redlich-Kwong equation, ENRTL-RK, was adopted for hydrazine-water system thermodynamics estimation. The results show that a boiling point of pure hydrazine with 113.55℃, an azeotrope of 52.37%(mol) and 121.57℃ calculated by Aspen database are close to the literature data.Secondly, the existing evaporation-distillation process was simulated by use of Aspen v8.0 with ENRTL-RK thermodynamics. Energy consumption at each steps was calculated and analysis. The results show that for the existing process urea method process only sensible heat was recovered by preheating evaporation feed with steam condensate. The energy recovery is as low as 2.93%. while 88.23% of the total energy for evaporator and condenser effulents have not yet been utilized. This is the bottleneck of the energy recovery. Based on the analysis a single stage jet heat pump project design was put forward firstly. Simulation shows that, enthalpy difference under different pressure steam has little effect on energy recovery rate, which is mainly affected by the ejector capacity. Energy recovery is increased with motive steam pressure. The optimal driving steam pressure is 20bar, at this condition the mixed steam can be used both for evaporator and reboiler. The amount of injection steam was 836.88 kg/hr, and the steam consumption is reduced from 6452.98 kg/h to 5579.20 kg/h. The overhead steam recovery rate was 15.45% under optimum conditions.Futhermore, multi-stage injection and single stage injection with power booster scheme were compared by simulation, the results show that:overhead steam recovery rate of two-stage injection is 17.04% with the first stage pressure of 19.21 bar and second stage of 19.74 bar, which indicates that the multistage injection scheme has less effect on the improvement of energy recovery.Finally, a single stage injection with booster compression of ejector steam was developed. MZ 100-2000 centrifugal blower with 75kW motor power was selected as booster compressor at the condition of 3867.82 m3/h and 200 kPa. The net energy recovery (offsetting electricity consumption) is 968.58 kW and the overhead steam energy recovery rate increses remarkablly to 42.13%.Single stage injection with booster compression was selected as the optimal scheme, with easy implementation, low investment cost and maximum energy recovery. For the scale of 10,000 t/a 40% hydrazine hydrate production process,-2510 tce, Energy Save of Productions is-28.80%.