Material Balance And Reaction Kinetics Modeling For Penex Isomerization Process In Daura Refinery

Penex Deisohexanizer isomerization of light straight run naphtha is a significant process for petroleum refining and proved to be effective technology to produce gasoline components with a high octane number. Modeling of the chemical kinetic reactions is an important tool because it is a better tool for optimization of the experimental data into parameters used for industrial reactors. The present study deals on the isomerization process in Daura refinery. Material balance calculations were done mathematically on the unit for the kinetics prediction purpose. A kinetic mathematical model was derived for the prediction rate constants K1 and K2 and activation energy Ea at operating temperatures range 120-180C. According to the model, the results show that with increasing of temperature leads to increased K1 directly, where the K2 values proportional inversely. The activation energy results show that Ea1 (nC6)< Ea1 (C5) < Ea1 (CH), and for Ea2 (iC5)< Ea2 (2,2-DMB) < Ea2 (2,3-DMB)< Ea2 (MCP)..


Introduction
The Penex Deisohexanizer (DIH) isomerization is a process operated using the catalytic reaction of npentanes, n-hexanes, and mixtures thereof to produce isomerate hydrocarbons. The reactions take place in a hydrocatalytic fixed bed reactor to promote conversion and minimize hydrocracking [1]. The process of light naphtha can produce isomerate hydrocarbons with higher octane number, in addition, that the typical Penex unit product can blend into gasoline pool, and the nonconverted low octane components (nC 6 , CH and MP's) from the deisohexanizer column (DIH) can be recycled to the reactor section for further upgrading [2].
The major elements of Penex (DIH) isomerization processes are reactors operated using chlorinatedalumina (Pt/Al 2 O 3 -Cl) as catalyst [3,4]. This catalyst is proven to be active at lower temperatures (120-180 o C) in which equilibrium favorable to produce iso-paraffins. [5]. The scheme of Penex (DIH) process is illustrated in Fig. 1. [6] The most important process variables in Penex isomerization unit are the reactor temperatures. The higher temperatures than equilibrium lead to increase the amount of hydrocracking and increase the carbon formation on the catalyst.
A typical UOP Penex unit is provided with two reactors in series. All of the benzene rings in the LSRN feed are hydrogenated in the first reactor and some conversion of cyclohexane (CH) and methyl cyclopentane (MCP) to hexanes also occurs, as does some hydrocracking of C 7 components to C 3 and C 4 .
The hydrogenation of benzene rings, naphthene conversion to hexane, and C 7 hydrocracking are exothermic reactions and, for a typical feedstock, contribute more to the temperature rise in the reactor than exothermic reaction of paraffin isomerization.
The concentrations and outlet temperature will be influenced by the catalyst and by the mass of C 6 cyclic and C 7 components in the feed.

nC 5 Reaction Mechanism [7]
The Isomerization reaction of normal paraffins to isoparaffins is slightly exothermic and thermodynamically favorable at a temperature range 120-180 o C. The kinetics of the reaction proceed in series through an olefin as an intermediate product formed by dehydrogenation of n-paraffins through an adsorption mechanism on the surface of metal site catalyst.
݊ ‫݁݊ܽݐ݊݁‬ ௧ ሱ ሮ ݊ ‫݁݊݁ݐ݊݁‬ Because of low equilibrium conversion of paraffin isomerization, n-olefins can be converted to carbonium ion with the aid of injection of strong chloride acid as initiator ion.
This initiator ion allows equilibrium forward to removes n-olefin from the first hydrogenation reaction, and then to rearrange the molecules.
‫݉ݑܾ݅݊ݎܽܿ‬ ‫݊݅‬ ՜ ‫ݏ݅‬ ‫݁݊ܽݐ݊݁‬ ‫݉ݑܾ݅݊ݎܽܿ‬ ‫݊݅‬ The high catalytic acidity causes a hydrogenation reaction to proceed at a higher reaction rate. Then, the carbonium ion converted iso-paraffin to iso-olefin by dehydrogenation step.

Other Reactions [9]
There are several reactions occurring inside and outside of the reactors.

Inside Reactors:
a) Naphthene ring of methyl cyclopentane (MCP) and cyclohexane (CH) present in the Penex feed will hydrogenate to n-paraffins, and then to iso-paraffins. The increasing in reactor temperature leads to increase Naphthene ring opening reactions. b) As the temperature increased, the naphthenes shift forwards to (MCP) production.
Hydrocracking of Penex feed according to types of feed quality. For example, C 7 molecules tend to hydrocrack easily than smaller ones, paraffins of C 5 and C 6 hydrocrack to some extent. The severity of hydrocracking tends to reduce yield and increase product temperature.

Outside Reactors :
a) Hydrogen chloride is injected annually at period of maintenance time, to limit corrosion problems in units, the reaction run until removed any rust present in the unit.
6 HCl + Fe ଶ O ଷ ՜ 2FeCl ଷ + 3 H ଶ O b) Perchloroethylene (C 2 Cl 4 ) is injected before reactors at approximately the temperature of (110°C) or higher. The present hydrogen will react with this promoter in the presence of a catalyst (Pt/Al 2 O 3 -Cl) to produce hydrogen chloride.
Neutralization of hydrogen chloride formed in above reaction using caustic soda (NaOH) to form salt and water in the scrubber.

Material Balance
As shown by Fig. 1, there are three streams entering to the rector of Penex unit, the feed (LSRN), makeup gas (H 2 ) and recycle (from Deisohexanizer) and outlet stream exit as a product from the reactor. The product stream (F3) exits from the reactor, enters to stabilizer column. In this process, overhead light gases (CH 4 ,C 2 H 6 , C 3 H 8 , and i-C 4 H 10 ) and some of nC 5 and iC 5 which is removed and exit from the top of column, in addition to H 2 and HCl , which then enter to scrubber.
The outlet product from the top of stabilizer column enters to the scrubber column where NaOH solution injected into the scrubber. From the bottom of the stabilizer, the product stream contains (nC 5 , iC 5 , nC 6 , 2-MP, 3-MP, 2,2-DMB, 2,3-DMB ,MCP and CH) which goes to Deisohexanizer (DIH) column. The outlet product from the bottom of stabilizer enters to the DIH column. There are two streams exit from DIH column, top stream (D) and bottom stream (W).
The mass balance calculations on the unit assuming feed rate of Penex unit =10, 000 BPD is presented in Table (1) and (2) respectively.  The concentration of reactants in the isomerization reactions (nC 5 , nC 6 and CH) in the inlet stream to the reactor is calculated from the mass balance, these concentrations are used to calculate K 1 (rate constant for the formation of intermediate olefin). The results are shown in Table (3).
Whereas the mole fraction of the produced isomers outlet from reactor (iC 5 , 2,2-DMB 2,3-DMB and MCP) are calculated from the mass balance ,these mole fractions are used to calculate K 2 (rate constant for the formation of isomers). T

Development Model
In the typical isomerization process, several reactions are taken place: paraffin isomerization; naphthene hydrogenation; naphthene isomerization, benzene saturation, hydrocracking; and naphthene alkylation. Table (4) list the reactions and equilibrium conversion suggest for isomerization of normal paraffinic components in the feed of Penex process. [10] ݊ ‫݂݂݊݅ܽݎܽ‬ The model developed in this study are taken following assumptions: 1 st order reactions at steady state, isothermal conditions, gas phase reaction, constant physical properties, and neglected gradient in pressure drop, temperature and concentration respectively. Mass balance has been developed over the cross section of the segment of the catalyst bed, as illustrated in Fig. 3.
The design equation for volume of tubular reactor: And reactant concentration of C A in gas phase: :KHUH İ LV UHIHU WR YRLGDJH LQ UHDFWRU EHG Putting equation (13) into (12) and re-arrange, to get the reaction rate constant K1 as shown in equation (14). From Arrhenius equation (15) ,the activation energy (Ea 1 ) and frequency factor (K o ) can be calculated by plot (ln K 1 ) vs. (1/T), as illustrated in Fig. 4.

Results And Discussion
The model shown by equation (14) can be used to calculate isomerization reaction rate constant K 1 , using data given from Penex unit in the Daura Refinery: Volume of catalyst in each reactor =35.66 m 3 ,Hold up = 75 %, ‫ܭ‬ DQG 7 o = 50°C.
Table (5) represents the results of K 1 at temperatures range of 120-180 o C. It was shown that calculated reaction constants of (K 1 ) values increased with increasing temperature and reach maximum value at a temperature (180°C).
Applying equation (15) and plot (lnK 1 ) vs (1/T) for components listed in Table (6) are shown in Fig. 5. The activation energy (Ea 1 ) and Frequency factor (Ko 1 ) are calculated from these plots and tabulated in Table (7). The results show that Ea 1(nC6*) < Ea 1(nC6**) <Ea 1(C5) < Ea 1(CH).  Where nC 6 * for nC 6 ļ -DMB at x=0.35,and nC 6 ** for nC 6 ļ -DMB at x=0.1 Equation (11) can be used to calculate the rate constant K 2 for reaction on olefin to iso-paraffin. The required data were taken from Daura Refinery; as LHSV =1.5, residence time t =1/ LHSV = 0.667 h. The values of K 2 are shown in Table ( 8), which was shown that K 2 values decreasing with increasing temperature, which means that product isomerate concentrations were increased as a result of temperature sensitivity of isomerization reactions.