Modeling and Optimization of An Ammonia Reactor Using Multiple-Shooting Method

Optimization is concerned with selecting the best among the entire set by efficient
quantitative methods. It is much very often required to solve dynamic optimization
problems in the design and operation of complex chemical processes. The objective of
this final year project is to model and find anoptimal design for anammonia reactor. For
the past few years, ammonia has been widely used in the manufacturing of fertilizers,
explosives and other chemical products. Hence modeling and optimizing ammonia
synthesis has received a lot of attention among process industries. Apart from
determining the optimal reactor length, the comparison ofresults obtained from different
methods is presented.
The production of ammonia depends ontemperature of feed gas at the top of the reactor
(top temperature), the partial pressures of the reactants and the reactor length. The
optimal design problem requires obtaining the optimal reactor length with maximum
economicreturns corresponding to various top temperatures.
This paper presents an alternative approach in solving the boundary value problem and at
the same time determines the optimal solution. This method is called multiple-shooting.
The software used for this modeling is MATLAB version 6.1. The ODE integration
routine technique used is 'ode45' and the optimization routine of 'FMINCON' is
selected. Apart from determining the optimal length ofreactor, the comparison of results
reported in earlier literature is analyzed and presented.
In this project, the values profiles of top temperature (Tf), reacting gas temperature (Tg)
and mole flow rate of nitrogen per area catalyst (Nn2> at a top temperature of 694K were
generated. From the results obtained, a top temperature of 694K yields an objective
function value of $5.0155 x 106 at an optimum reactor length of 6.6953m. These values
agree with the latest literature work onthe same case study that uses different method. By
analyzing the results, multiple shooting method is found to be a robust, simple and fast
computation technique for optimization problems in chemical processes.