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Published: Kem. Ind. 52 (10) (2003) 483–493
Paper reference number: KUI-11/2003
Paper type: Conference paper
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Gasoline Quality Improvement Using Isomerization Proces Modeling

I. Lukec, K. Sertić Bionda and D. Lukec


The isomerization proces has an important position in the production of motor gasoline due to its role in the quality improvement of the light gasoline fraction. The proces causes a conversion of n-paraffins (butane, pentane, hexane) into iso-paraffins, thus increasing the octane number of the light gasoline. The basic goal of this paper was to show posibilities of mathematical modeling in proces advancement and quality improvement. It is performed on the example of real isomerization proces included in Rijeka Refinery. For building the simulation model of the whole isomerization proces, proces variables and data are included. The most important ones are shown in Table 1 and Table 2. Before building the model of the whole proces, mathematical model of the isomerisation reactor must be solved. Mathematical reactor model is presented with material balance of the reactor, posible chemical reactions are shown in Table 3. Beside the reactions of isomerisation, reactions of cracking are also included. These reactions were presumed on basis of parameters of chemical equilibrium, mechanism and kinetics (Table 4) Also, for model to be solved, the optimization criteria must be defined, because model contains more variables then equations. For this purpose, new variables Delta 1 to Delta 5 are included in equations shown in Table 5. The modified material balance together with optimization criteria makes the mathematical model of the isomerization reactor. The model was solved using Lingo, software for solving systems of linear and non-linear equations. After solving the reactor model, obtained results are used in creating the simulation model of the whole proces, using the proces simulation software ChemCad. Simulation model consists of models of all main proces equipment: reactor, columns, vesels, heat-exchangers, pumps, compresors, as shown in Figure 1. To show posibilities of proces advancement and product quality improvement, four different simulation models of isomerization proces are performed, including the model of the present real proces, part of Rijeka Refinery. Beside the simulation model of the proces without improvement, other three proces models were build and simulated: 1. Proces advancement with feed prefractionation in the deisopentanizer column, Figure 2. 2. Proces advancement with molecular sieves, Figure 3. 3. Proces advancement with deisohexanizer column, Figure 4. Results are shown in Tables 6 to 11. Conversions of chemical reactions obtained by reactor model are shown in Table 6. From results in Table 6 can be seen that from 20 presumed chemical reactions, only 11 of them are taking place in reactor. Results of reactor model are compared to experimental data, and as shown in Table 7, they show a good match with the proces data. That enables the conversion results to be included in the model of the whole proces. The results from simulations of the whole model are presented in Tables 8 and 9 From chemical compositions of isomerate given in Table 9, octane numbers are calculated. From calculated octane numbers, shown in Table 10, it is obvious that the proces with molecular sieves shows a huge increase of octane number in comparison to the proces without improvement. On the other hand, Table 11 shows that proces with deisopentanizer column results in the highest isomerate flow-rates increase. In choosing the best improvement of the isomerization proces, the biggest influence has the Refinery's strategy.

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mathematical modelling, isomerization process, product quality improvement