Title: Mathematical Model for a Radial Flow Ammonia Synthesis Reactor.
Author : Mohamed Tarek Mohamed El-Tohfa
Collection : Ph.D. Chemical
Abstract:
Modeling of fixed bed catalytic reactors is of great industrial importance. An industrial Ammonia synthesis reactor is considered as a case study to develop and verify the model that best represents the reactor behavior. The one-dimensional pseudo-homogeneous model has been implemented in MA TLAB R2013a to solve the governing equations and compare the solution with measured conversion and temperature profiles taken from ammonia plant III at the Abu-Qir Fertilizers Company. For design conditions both temperature and conversion profiles for the three beds were presented with very good agreement with the new catalyst design temperatures where a maximum deviation is 1.4 between measured and calculated value. For the operating conditions after the plant start-up the catalyst activity coefficient (/1) was assumed as a fresh catalyst to be, /3 = 1. The results obtained in the present study indicate that the maximum deviation from the actual operating conditions was 3.1 . The model developed in the present study indicates that the calculated catalyst activity, /3, shows a very good agreement with the literature. A value of 0.65 can be assigned to the activity coefficient at the end of a catalyst life of 15 years, where the model results found the maximum deviation from the actual operating conditions was 0.91 . The model developed in the present study has the ability to predict the first bed outlet temperature. This is considered to be the highest temperature in the reactor where the maximum allowable catalyst temperature, according to the catalyst manufacturer, is 530°C. Such a temperature cannot be practically measured. The Effect of variation of operating conditions on ammonia production was studied and was found that the ammonia production rate increases by increasing the circulation rate through the ammonia converter, decreasing inert content, increasing pressure, decreasing inlet ammonia concentration and vice versa.
The model developed and reported herein could be used to determine the optimum bed inlet temperatures by varying the first bed inlet temperature and second bed inlet temperature to obtain higher ammonia production rates. The addition of a new catalytic fourth bed (booster converter) in series downstream from the existing converter was studied in order to increase the conversion per pass of the ammonia synthesis reaction, ammonia production rate and energy saving. The developed and reported herein was utilized to determine the length of the fourth bed with the ammonia conversion and ammonia production. The fourth bed with 12 m length produces 1420 TPD ammonia compared with 1206.8 TPD without a fourth bed and with ammonia conversion 17.5 compared with 15.2 NH3 before fourth bed (booster converter) and by decreasing the fourth bed inlet temperature to 370°C, 12 m length produce 1650 TPD (more than 20) ammonia while 1500 TPD was produced by only 6 m length.
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