ANALYSIS OF OPTIMAL PROCESS FLOW DIAGRAMS OF LIGHT NAPHTHA ISOMERIZATION PROCESS BY MATHEMATIC MODELLING METHOD

An approach to simulation of hydrocarbons refining processes catalytic reactors. The kinetic and thermodynamic research of light naphtha isomerization process was conducted. The kinetic parameters of hydrocarbon feedstock chemical conversion on different types of platinumcontent catalysts was established. The estimation of efficiency of including different types of isomerization technologies in oil refinery flow diagram was performed.


Introduction
The increase of straight run normal alkanes C 5 -C 6 fraction research octane rating to 92 points is an effective decision of important applied problem of changing the structure of native gasoline pool.The high octane flow of isomerate does not content a sulfur, aromatic and olefin hydrocarbon that is why it is an excellent component for finished gasoline compounding.The selection of optimal process flow diagram (PFD) of light naphtha isomerization is an important research and practice task which can be solved more efficiently with a help of mathematical modelling method.

Research Object
In this paper the research object is catalytic isomerization of light gasoline fraction unit on sulfated catalyst with different recycle flows direction (figure 1) and isomerization unit with recycle of unconverted hexanes on chlorinated Pt-content catalyst (figure .2).Isomerization catalysts based on sulfated metal oxides combines high activity and resistance to poisons and have ability to regeneration [1][2][3]."One pass" isomerization technology is the simplest, has minimal capital and operation costs and provides the production of isomerate with 80-82 RON.Including in "one pass" isomerization recycles of unconverted C 5 -C 6 hydrocarbons takes more difficult conditions of its operation but helps to reach the increase in RON about 10-12 points.

Fig. 2. Light naphtha isomerization PFD on Pt/Cl -/Al 2 O 3 catalyst with recycle of unconverted hexanes
Catalysts based on chlorinated alumina oxide due to longtime operation lose chlorine.In result its activity is decreased.In addition it requires a caustic treatment unit to utilize organic chlorine.Chlorinated catalyst can't be regenerated and its operation cycle is 3-5 years.

Methods
In this paper for description of unsteady catalytic processes of hydrocarbon feedstock isomerization the method of mathematical modelling is used.According to characterized methodology [4][5][6][7][8][9][10][11][12][13][14][15] the construction of mathematical model of complicated chemicalengineering process consist in sequential implementation of following stages: conducting of thermodynamic analysis, estimation of kinetic parameters of thermodynamically possible reactions, construction of reactor model and chemical-engineering system at all.The full mathematical description of hydrocarbon feedstock catalytic isomerization process is formatted on components material and heat balances: , where r=0, C=C 0 , if Z=0, Т=Т 0 , where r=0, T=Tin; where G is a feedstock flow rate, m 3 /h; z = G*t (t is a overall time of catalyst work starting from the new catalyst load, h); Ci is a concentration of ith component, mol/m 3 ; V is a volume of the catalyst bed,m 3 ; a is a catalyst activity; ρ is density of hydrocarbon mixture, kg/m 3 ; Cmp is a specific heat capacity of hydrocarbon mixture, J/(kg*K); Q j is jth reaction heat, J/mol, T is temperature, K; rj is jth reaction rate, mol/(m 3 h), a -activity of catalyst.
The program realization of this model is implemented in Delphi 7. The initial data for calculation are feedstock composition and operation parameters of the process.The adequacy of the model is estimated by comparison of experimental data from industrial isomerization plant with calculated values [10].The activity of catalyst is decreased due to processes of coke formation, poisoning and ageing.

Results and Discussion
The research of kinetic parameters of purposeful reactions of light naphtha isomerization process for chlorinated and sulfated catalysts is presented in table 1.

Table 2 .
Rate constants of major reactions for sulfated catalyst are taken as 1, rate constants for chlorinated catalyst are performed as a part of 1.In such a way major reactions of isomerization process pass faster on sulfated catalyst.The isomerate composition for different light straight run cuts isomerization technologies is presented in table 2. Calculated isomerate composition Application of catalyst based on sulfated zirconia in industrial isomerization process with recycles of unconverted hydrocarbons C5-C6 allows to produce isomerate with RON 10-12 points higher than traditional "one pass" isomerization technology and with Fig. 3. Isomerate RON vs different types of catalyst and PFD