Simulation of the Sieve Plate Absorption Column for Nitric Oxide Absorption Process Using Neural Networks

We present the modeling of the absorption column performance using feed-forward and radial basis function type neural networks. The input and output data for training of the neural networks are obtained from a rigorous model of the absorption column. The results obtained form the neural network models are compared with the results obtained mainly from the simulation calculations. The results show that relatively simple neural network models can be used to model the steady state behavior of the column. There are no essential preferences for the type of neural network applied for simulation purposes. Radial basis function based architecture is better in case of lower training data are available. INTRODUCTION Absorption column is one of the most important and expensive apparatus in nitric acid plant. Modern industrial nitric acid plants use mainly sieve plate absorption columns with weirs, Fig.1. Absorption columns built about presently are over 6 m diameter and heights over 80 m. From the point of view of high capital costs and environmental protection regulation requirements the exact mathematical model of processes taking place in the column is indispensable. Ability of prediction of result of mass flow rate changes as well as temperature and pressure variations is useful in operational conditions for absorption column. In initial period of development of technology of nitric acid production the absorption columns were designed using experimental data collected from commercial plants. During growth of knowledge of basic theory of the process some calculations were made using graphic methods. Application of computers has allowed to develop more precise mathematical models of the absorption process and to perform fast and exact calculations of the columns. Problem of mathematical modeling of absorption columns was object of interest of many researchers and practitioners. The mathematical models for NOx absorption can be divided into two types: models applying an idea of plate efficiency used in mass transfer calculations, models applying mass transfer coefficients for mass transfer calculations on each plate. Fig. 1 Diagram of a nitric acid plant One of the first model of the first type was Koukolik’s and Marek’s model [5] and Roudier’s, Longeat’s and Enjalbert’s model. The models are simplified and temperature and pressure distribution is assumed at each sieve plate. The plate’s efficiency was calculated using also simplified relationships. Other models belonged to the same type were developed later by Sobotka [7], Holma and Sohlo [8], Carta and Pigford [9] and Guiterrez-Canasa [10]. They had similar limitations relying on lack of pressure drop calculations for gas passing over the plates and heat balance calculations for NOx oxidation in area among the plates. In the second type models the way of calculation of NOx oxidation was similar to the first type but mass transfer rate calculations were performed in a different way. Emig et al [11] assumed constant mass transfer coefficients for the whole column. Equlibrium partial pressure of gas phase was calculated using Henry’s constant. However, this model did not involved pressure losses and heat balance between the plates. A model of similar degree of accuracy was presented by Counce and Perona [12]. A novel approach has been used in models developed by Wiegand, Scheibler and Thiemann [13] from Uhde firm and Pradhan and Suchak [14]. The authors have used commonly known equations cited in literature or elaborated by themselves to calculate reaction rate and equlibrium data in gas phase between the plate area and in liquid phase as well. Pressure drop and heat balance models were also incorporated into their calculations. In INS Pulawy a model of sieve plate column has been developed basing on plate efficiency strategy. Comparing another models, the developed model is significantly complex. Using the model a simulation calculations of heat and mass transfer can be done for each plate of the column. Of course, the model can be applied to any sieve plate column. The model has been tested using data collected from different nitric acid industrial plants. Positive results allowed to use the model to design a column for a new commercial plant [15,16]. Furthermore, simulation results obtained with the model allowed to design a new type of absorption column. Moreover, to speed up calculations a neural model approach has been applied. The new approach will assure to apply the model for optimization of a nitric acid plant performance. The neural network based model can be easily adopted to the existing environment using available data collected from the computer based data acquisition systems. MATHEMATICAL MODEL In the nitric acid plant the mixture of ammonia and air is passed over catalyst gauze. Among many reactions the a major one: O 6H 4NO 5O 4NH 2 2 3 + = + (I) The gas stream leaving the ammonia oxidation reactor is cooled in a heat exchanger. If its temperature is below a determined limit than NO is transformed into NO2, and N2O4 and a condensate appears as well. Some part of nitric oxides react with condensing water forming a solution of weak nitric acid which concentration depends on time of contact and amount of the condensed water. The condensate is then fed with a pump on a proper plate of the absorption column. In the absorption column the different reactions can occur in gas and liquid phase from which the most important are: • Oxidizing of nitric oxide to nitric dioxide and dimerization to N2O4 2 2 2NO O 2NO → + (II) 4 O N 2NO 2 2 ↔ (III) • Reaction of dioxide and tetroxide with water 3 2 2 2 HNO HNO O H 2NO + → + (IV) 3 2 2 4 2 HNO HNO O H O N + → + (V) O H 2NO HNO 3HNO 2 3 2 + + → (VI) Besides, gas is bubbled through a pool of liquid where water vapor condenses. The cool gas stream consisting of (NO, NO2, N2O4), O2, H2O and small amount of N2O, CO2 and Ar is introduced at the bottom of the column. Besides, one of the sieve plate is supplied with acid condensate formed in the condenser. The last plate is fed with a water allowing to obtain a stream of nitric acid of desired concentration. Heat transfer coils filled with water are placed in the pool of liquid over the plates. They are designed to remove the heat liberated during oxidation of NO and formation of NO2, N2O4 and HNO3. Model for the absorption column has been developed using the following assumptions: • gas and liquid phases are ideally mixed in the pool of liquid over the plate, • there are not concentration and temperature gradients in the pool of liquid, • HNO2 decomposes to HNO3, NO and H2O in the pool of liquid, • gas and liquid phases flow in a plug flow type, • heat losses to environment are negligible, • reaction heat is exchanged by the heat transfer coils, • temperature of gases leaving the pool of liquid and the liquid temperature are the same, • chemical reactions proceed in the empty sections without heat exchange with environment. Fig. 2. A commercial nitric acid column Model for sieve plate The following reaction occurs in the liquid phase: NO 2HNO O H 3NO 3 2 2 + → + (VII) The extent of the reaction depends on plate construction features, composition of gas mixture entering the sieve plate and nitric acid concentration formed on the plate. Efficiency of the plate is defined a distance of NOx partial pressure and its equilibrium pressure. The equilibrium pressures of NO and NO2 have been calculated using Zhidkov and Skorcov relationship [17].