STABILITY ANALYSIS AND OPTIMIZATION OF A MULTIBED QUENCH REACTOR FOR AMMONIA SYNTHESIS

Abstract

The stability analysis and optimization of an axial flow three bed quench type ammonia synthesis reactor was carried out to optimize its performance. The reactor operation at optimal cold shot fractions for a given set of the operating and design parameter values will result in the maximization of the rate of production of ammonia and stable operation. This will result in low bed temperatures and reduced total pressure drop. The low bed temperatures will result in increase in catalyst life whereas reduced pressure drop will reduce the operating cost. Modern large capacity reactors are used for production of ammonia used as a feedstock in the production of urea. Urea is essential to boost agricultural production in India. A realistic and accurate mathematical model of a large capacity multibed autothermic quench-type ammonia synthesis reactor was formulated and solved by modified Milne-Predictor-Corrector numerical integration technique using an appropriate convergence strategy. The optimization of the cold shot distribution was achieved by taking maximization of the rate of ammonia production as an /objective function. The Box complex direct search optimization technique was used for sixteen set of conditions over a wide range of values of six operating and design parameters. These parameters were feed gas flow rate, H /N ratio in feed, inerts 2 2 concentration in feed, catalyst activity factor, total volume of catalyst and operating pressure of the reactor. (ii) In order to estimate the model parameters for an industrial reactor for simulation study, data from plant were extracted for the period of steady-state operation over several months. The data had a serious limitation that no measured value of cold shot fractions were available except for the first bed inlet where its value was always kept at zero. Validation of simulation model from the1 plant data was carried out by obtaining best values of model parameters and cold shot fractions. The estimated model parameters are: frequency factor and activation energy in the reverse reaction rate constant correlation for the catalyst used; correction for fugacity coefficient term; and heat exchange capacity of external heat exchanger. Their best values are found 16 3 to be 4.11482 * 10 mol NH /s/m ; 97622.4 kJ/kmol; 1.379; and 3 6 3 316000 W/K at feed flow rate of 0.74*10 Nm /h., (where N indicates N.T.P. conditions), respectively. The simulated cold shot values as fractions of total feed gas for the average plant conditions (base case) are found to be 0.245 and 0.100 for the second and the third bed inlet, respectively. Cold shot to the first bed was taken to be zero as per plant practice. The optimization computations for one set of conditions required generally 5 to 8 minutes of CPU time on DEC 20 computer system. The optimization results indicate that the conversion and the bed temperatures are quite sensitive to the values of the operating and design parameters. Cold shot fractions at optimal conditions are strongly dependent on these parameters. An indiscriminate use of cold shot fractions resulted in either quenching of the reactor or a non optimal performance resulting (iii) in significant loss of production, higher bed temperatures and increased pressure drops. The use of optimal cold shot fractions increased the rate of production of ammonia by 20 to 110 t/d (where 1 t = 1000 kg and 1 d = 86.4 ks) compared to actual plant production of 1286.9 t/d, even if the operating and design parameters changed in adverse direction by about 10 to 30 percent from the base value. The rate of ammonia production shows an increase with an increase in flow rate , catalyst activity, operating pressure or total catalyst volume; or and a decrease in inerts concentration. It was found that the region near optimal is not sharp w,ith constraints on upper values of cold shot fractions resulting in the extinction of the reactor. It is further observed that optimal cold shot fractions do show a trend, to an extent linear with repect to change in parameters, namely, feed gas flow rate, catalyst activity factor, total catalyst volume and the reactor operating pressure. An increase in the rate of ammonia production of 10.3 percent (132 t/d) is observed if the operation is carried out at optimal cold shot fractions to first, second and third bed of 0.110,0.233 and 0.232, respectively, for the base case. It is observed that the effect of change in H /N ratio in the feed 2 2 gas from 3.0 is not significant on reactor performance and rate of ammonia production. It is observed that the reactor stability near it3 optimal operation is quite sensitive to increase in cold shot fractions and an increase beyond a critical value may result in its extinction or blow-out. The use of simulation model is, therefore, highly desirable to operate the reactor near (iv) optimal values of cold shot fractions for any set of parameter values in order to achieve maximum ammonia production rate. Simulation model can also be used for developing a suitable control strategy for cold shot distribution for ensuring optimal reactor operation.