Modeling and s imu lation of ammon ia synthesis reactor

Petroleum & Coal ISSN 1337-7027

Available online at www.vurup.sk/pc

Petroleum & Coal 48 (2), 15-23, 2006

MODELING AND SIMULATION OF AMMONIA SYNTHESIS

REACTOR

Ali Dashti, Kayvan Khorsand, Mehdi Ahmadi Marvast, Madjid Kakavand 1

Research Institute of Petroleum Industry (RIPI), Pazhouheshgah Blvd.,

Khairabad, Qom Road, Tehran, Iran.

1corresponding author,

Received May 1, 2006; accepted June 24, 2006

Abstract

In this paper an industrial ammonia synthesis reactor has been modeled. The reactor under study is of horizontal type. This reactor which is under the license of Kellogg Company is equipped with three axial flow catalyst beds and an internal heat exchanger in accompany with a cooling flow. The achieved modeling is one dimensional and non-homogenous. Considering the sever effect of internal heat exchanger on reactor operation, it has been simulated by calculation of film heat transfer coefficients in its tube and shell and then, taking into account the shell thermal resistance and fouling coefficient, obtaining the overall heat transfer coefficient. So in the developed software, the heat transfer coefficient is first calculated using the conditions of the input flow to the exchanger and then the input flows to the first and second beds are calculated. The differential equations have been solved using Rung Kutta 4 method and the results have been compared with the available industrial data. Finally the capability of the developed software for industrial application has been investigated by changing the reactor operation conditions and studying their effects on reactor output.

Key words: ammonia, reactor, modeling, simulation

1. Introduction

Ammonia Synthesis is a very important process in chemical complexes. Ammonia is the initial chemical material for a variety of industries. It is used in production of chemical fertilizers, explosive materials, polymers, acids and even coolers. Simulation can play an important role to give an insight of the industrial units and hence simulation of Ammonia unit is very important to help us investigate the different operation modes of this unit and optimize that. Ammonia synthesis loop is the most important part of this unit which better understanding of its bottleneck can led us to make the operation yield higher than before.

2. Ammonia Synthesis Loop

In ammonia production unit, the synthesis loop is located after the syngas production and purification units. Ammonia synthesis process takes place in high pressure and hence high power multi cycle compressors are used to supply the required pressure. The Kellogg synthesis reactor incorporated in this loop is of horizontal type with three beds. To control the temperature between first and second beds, an internal heat exchanger has been used in which the input feed to the first bed and the output gas from the same bed have thermal exchange. In addition to the mentioned heat exchanger, the quench gas flow is also used for control of temperature. The input feed to the reactor, is first divided into two parts, before entrance. One part is considered as feed and the other part as quench gas. The feed, after entering the reactor, passes through the empty spaces of the beds as well as the reactor walls and gets slightly heated. When reaching to the end of the reactor, it passes through the shell of the internal heat exchanger and its temperature reaches 400 oC. Tubes of the heat exchanger contain the output gas from the first bed. Quench gas is used to control the inlet temperature of the first bed. The required temperature for inlet of the first bed is 371 oC. Output gas,

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