Available online at ScienceDirect. Procedia Engineering 152 (2016 ) 81 86

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1 Available online at ScienceDirect Procedia Engineering 152 (2016 ) International Conference on Oil and Gas Engineering, OGE-2016 Mathematical modeling sulfuric acid catalyzed alkylation of isobutane with olefins Ivashkina Е.N. a *, Ivanchina E.D. a, Nurmakanova А.Е. a, Boychenko S.S. a, Ushakov A.S. a, Dolganova I.O. a a National Research Tomsk Polytechnic University, 30 Lenin Avenue, Tomsk , Russian Federation Abstract The article provides the results of applied mathematical model of sulfuric acid catalyzed alkylation of isobutane with olefins. It was shown that increasing the content of propylene in the feedstock to 2% leads to decrease in octane number of alkylate to points. Alkylates produced from feedstock with less isobutane mass fraction have a smaller value of octane number. Using mathematic model it was determined that the main parameter influencing the acid activity in the alkylation process is the fresh acid concentration The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license 2016 The Authors. Published by Elsevier Ltd. ( Peer-review Peer-review under under responsibility responsibility of the of the Omsk Omsk State State Technical Technical University University. Keywords: mathematical modeling; alkylate; optimization; octane number; sulfuric acid alkylation. 1. Introduction Alkylation of isobutane with C3-C5 olefins results in the production of alkylate, which is a valuable blending component for the gasoline. It has a low vapor pressure and a high octane number. New, stricter environmental laws and regulations can significantly increase the alkylate demand. For example, legislative requirements are being introduced to lower the content of olefins, aromatics and such impurities as sulfur and nitrogen compounds in order to make the fuel more environmentally friendly. Even MTBE * Corresponding author. Tel.: address: ivashkinaen@tpu.ru The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license ( Peer-review under responsibility of the Omsk State Technical University doi: /j.proeng

2 82 Е.N. Ivashkina et al. / Procedia Engineering 152 ( 2016 ) as a main high-octane blending components are now under pressure to be removed from the gasoline pool for the very same reason in some states [1]. In Russia, under the Customs Union Technical Regulation "On Requirements for Automobile and Aviation Gasoline, Diesel and Marine Fuel, Jet Fuel and Mazut Heating Oil" (CU TR 013/2011) the sulfur, olefin, benzene, aromatic and oxygen content requirements were tightened, performing gradual refuse to use a monomethylaniline(mma), manganese and iron compounds. As a result, according to EIA estimates, the proportion of alkylate in the USA market is expected to reach the value of about 25 30% in the nearest future [2]. Isobutane alkylation was discovered in 1932 when V.Ipattieff and G.Pines disclosed that, at low temperatures, olefins react with isoalkanes in solutions of strong acids, such as AlCl3/HCl or BF3/HF, producing highly branched heavier alkanes [3]. By the 1940s, alkylate was in commercial production using a sulfuric acid or anhydrous hydrogen fluoride as a catalysts [4]. Nowadays, liquid Brønsted acid catalysts (HF и H2SO4) are used in established alkylation technologies [5, 6, 7]. Mathematical modeling petrochemical processes is an important task of modern science and technology. Whereas, the deep understanding of thermodynamic and kinetic laws of reactor processes is necessary for successful modeling such complicated processes [8,9]. The aim of this work is to determine the influence of operating variables and feed composition on the yield and hydrocarbon composition of alkylate with the method of mathematical modeling. 2. Study subject The subject of the research is sulfuric acid alkylation unit developed by the STRATCO for producing of highoctane alkylate. The raw material is butane-butylene fraction (BBF) from the FCC unit, containing about 50% olefins and isobutane fraction from the gas fractionation unit. Usually, the BBF used in alkylation contains several impurities, such as propane, n-butane, n-pentane, diene, mercaptan and so on, which have a negative effect on acid consumption and act as a diluent reducing the effective concentration of isobutene [10]. The alkylate has research octane numbers (RONs) of about and a low vapor pressure. It also does not contain olefins or aromatics, but consists of isoalkanes only. The octane fraction make up the bulk of alkylate. Depending on the feedstock composition, the mass content of octane fraction varies from 75 to 85%. It generally consists of isooctanes: 2,4,4-; 2,3,4-;2,3,3-trimethylpentanes and 2,4-; 2,3-dimethylhexanes. The heptane fraction is the product of alkylation with propylene and it ranges from 2 to 5 wt.% of alkylate. This fraction slightly reduces the octane number of alkylate. Alkylation of isobutane by butylene is characterized by formation of small amounts (up to 15 wt.%) of high molecular weight hydrocarbons (C9 and above) [11]. The principal scheme of alkylation process implemented in the industry is presented in Figure 1. Fig. 1. The principal scheme of alkylation process.

3 Е.N. Ivashkina et al. / Procedia Engineering 152 ( 2016 ) There are several important reaction variables that can impact the product quality: Isobutane Concentration (Isobutane/Olefin Ratio) It is necessary to maintain the high concentration of isobutane in the reaction zone to promote the desired alkylation reactions and inhibit the undesired reactions, such as polymerization and self-alkylation. Typically, the insobutae /olefin molar ratio in the reactor feed flow ranges from 7/1 to 10/1. However, at high isobutane /olefin ratios, more isobutane has to be separated and recycled to the reaction section, that leads to the increase in separation costs. Reaction Temperature The reaction temperature in alkylation process is usually about С. At higher temperatures, polymerization reactions rates become significant and acid consumption increases. At lower temperatures, viscosity of sulfuric acid increases, and this leads to significant effect on refrigeration costs. Sulfuric Acid Concentration In H2S04 alkylation plants, acid concentration should be maintained at 90 98% values by adding the fresh acid of % concentration. High concentration of sulfuric acid affects alkylates quality positively, primarily improving its octane number. Olefins space velocity Olefins space velocity is the volume of olefins charged per hour divided by the average volume of sulfuric acid in the reactor. With optimal values of other parameters, values of olefins space velocity most often ranges from 0.25/hr to 0.50/hr. Volume ratio of acid to hydrocarbons Volume ratio of acid to hydrocarbons acid concentration should be from 1:1 to 1:2. 3. Methods While compiling the mathematical model of such complicated chemical process as alkylation, there is no objective need and technical possibilities consider all transformations of the individual components. A list of possible reactions and formalized reaction network were compiled and composed on the base of the analysis of experimental data and results of thermodynamic calculations of the alkylation process (Fig. 2). Fig. 2. The formalized reaction network of process of sulfuric acid alkylation of isobutane with olefins: ki is a constant rate of respective reaction.

4 84 Е.N. Ivashkina et al. / Procedia Engineering 152 ( 2016 ) Chemical conversions of hydrocarbon feedstock that lead to formation of such individual components as 2,2,4- trimethylpentane, 2,3,4-trimethylpentane, and 2,3,3-trimethylpentane, 2,4-; 2,3-dimethylhexanes were also considered. At the same time, hydrocarbons, which contents in the product mixture of the alkylation reactor is insignificant and their contribution to the alkylate octane number is negligible, were grouped (I-9, I-10, I-11). For calculations of thermodynamic and kinetic reaction characteristics, we used Density functional theory (DFT) at the B3LYP level. This method was chosen due to its high accuracy in relation to empirical methods such as PM3. The basis selected for these calculations is the G(d,p) basis. The polarization functions were necessary since each step in the reaction pathway involves at least one polar molecule [12]. In order to consider physico-chemical properties of sulfuric acid, it is nessesary to set parameters, such as the static (eps) and optical (epsinf) dielectric constants; for sulfuric acid they are respectively 101 and Results and discussions The calculations showed that alkylate, produced from feedstock with less isobutane, has a smaller value of octane number. Wherein the difference in composition of the feedstock contributes to antiknock index by the amount of points. The model calculations showed that octane number of alkylate does not dependent on the concentration of propane and n-butane in the feedstock, because they are not involved in the alkylation reaction. However, high amounts of these components reduces the yield of alkylate (Fig.3, 4). Fig. 3. Dependence of the propane concentration on the alkylate yield. Fig. 4. Dependence of the n-butane concentration on the alkylate yield.

5 Е.N. Ivashkina et al. / Procedia Engineering 152 ( 2016 ) Propylene reacts with isobutane forming products (2,4-dimethylpentane, 2,3-dimethylpentane and 2,2- dimethylpentan) that have a lower octane number than isooctane (Fig. 5). Fig. 5. Dependence of the propylene concentration on the RON(research octane number) of alkylate. Calculations show that the presence of n-pentane in the feedstock results in a low-quality of alkylate. The increase in C5 and C5+ content is also affects the product quality adversely. Analyzing industrial data, we can be deduce that the content of C5 and C5+ components changes slightly and influences the octane number of alkylate negligibly. The fresh acid concentration is the main parameter influencing the acid activity. The higher is concentration of water and acid soluble oil in the fresh acid, the less is concentration of the spent acid. The concentration of isobutane in the reaction zone is the important operating variable that impacts the product quality. Using the mathematical model of isobutane alkylation with olefins we found out that high concentration of isobutene in the reaction zone decreases the probability of olefin-olefin polymerization that will result in low concentration of acid soluble oils (ASO) in sulfuric acid (Fig. 6). Fig. 6. Dependence of the concentration of the ASO on the concentration of the isobutane in the reactor.

6 86 Е.N. Ivashkina et al. / Procedia Engineering 152 ( 2016 ) Conclusion During the carried out research we developed the mathematical model of isobutane alkylation with olefins and determined the influence of feed composition on yield and octane number of alkylate. As a result of studies, we found out that increasing the propylene concentration in the feedstock to 2% leads to reducing in the fraction octane number. Thus, normal butane, propane reduces the alkylate yield. Alkylates produced from feedstock with less isobutene mass fraction have a smaller value of octane number. It was determined that the main parameters influencing the acid activity in the alkylation process are fresh acid concentration and the isobutane concentration in the feedstock. References [1] Sven Ivar Hommeltoft, Isobutane alkylation Recent developments and future perspectives//applied Catalysis A: General 221 (2001) [2] Energy Information Administration, Alkylation is an important source for octane in gasoline, February 13, [3] Jorge M. Martinis and Gilbert F. Froment, Alkylation on Solid Acids. Part 1. Experimental Investigation of Catalyst Deactivation// Ind. Eng. Chem. Res. 2006, 45, [4] A Dissertation by Jorge Maximiliano Martinis Coll, Single event kinetic modeling of solid acid alkilation of isobutane with butenes over proton-exchanged Y-zeolites// Texas A&M University, Desember [5] A.A. Chudinova, A.A. Salishcheva, E.N. Ivashkina, O.A. Reutova, K.S. Gulyaev, A.M. Demin, A.A. Syskina, Estimation of Sulfocationites Application Expediency as Catalysts of Benzene Alkylation Process with Propylene, Procedia Chemistry, 2014, Vol. 10, pp [6] A.E. Nurmakanova, A.A. Salishcheva, A.A. Chudinova., A.A. Syskina, E.N. Ivashkina, Comparison between Alkylation and Transalkylation Reactions using ab Initio Approach, Procedia Chemistry, 2014, Vol. 10, pp [7] I.O. Dolganova, I.M. Dolganov, E.D. Ivanchina, E.N. Ivashkina, N.S. Belinskaya, V.V. Platonov, Reactor-regenerator system joint work optimization in benzene alkylation with higher olefins unit, Procedia Chemistry, 2014,Vol. 10, pp [8] E.D. Ivanchina, E.N. Ivashkina, I.O. Dolganova, V.V. Platonov, Effect of Thermodynamic Stability of Higher Aromatic Hydrocarbons on the Activity of the HF Catalyst for Benzene Alkylation with C9 C14 Olefins, Petroleum Chemistry, 2014, Vol. 54, No. 6, pp [9] E. Khlebnikova, A. Bekker, E. Ivashkina, I. Dolganova, E. Yurev, Thermodynamic Analysis of Benzene Alkylation with Ethylene, Procedia Chemistry, 2015, Vol.15, pp [10] A. Chudinova, A. Salischeva, E. Ivashkina, O. Moizes, A. Gavrikov, Application of cumene technology mathematical model, Application of cumene technology mathematical model, Procedia Chemistry, 2015, Vol.15, pp [11] A.E. Nurmakanova, E.N. Ivashkina, E.D. Ivanchina, I.A. Dolganov, S.S. Boychenko, Procedia Chemistry, 2015,Volume 15, pp [12] Frisch E. Gaussian 09 User s Reference, Frisch E., Frisch M.J., Clemente F.R., Trucks G.W., Gaussian Inc., 2009.