Chapter 11 Gasoline Production

Petroleum Refining Chapter 11: Gasoline Production Chapter 11 Gasoline Production INTRODUCTION Convert SR naphtha to motor gasoline stocks through 1. Reforming 2. Isomerization Production of motor gasoline stocks though 1. Alkylation Light Naphtha Crude Coker Hydro/Cat Cracker Isomerization Unit High Octane Isomerate by transforming straight-chain paraffins (low octane) isomers (high octane) Heavy Naphtha Crude Coker Hydro/Cat Cracker Catalytic Reformer High Octane Reformate Sold as regular/ premium gasoline Olefins i-c 4 Alkylation Unit Catalyst: Sulfuric or hydrofloric acid Alkylate (high octane) iso-paraffins boiling in the gasoline range. Blended into premium motor gasoline & aviation gasoline 11-1

Prof. Tareq A. Albahri 2018 Kuwait University Chemical Engineering Introduction 1. CATALYTIC REFORMING Objective To increase the octane number of heavy straight run (HSR) naphtha (for motor gasoline blending) using a catalytic process. Catalytic reforming does not change in the IBP and FBP range of naphtha feed. Produces large amount of H2 gas that can supplement the refinery H2 system. Reforming units can use Platinum (Pt) and Rhenium (Rh) catalysts. MAA CCR Platformer uses Platinum (Pt) catalyst. Capacity Table 11-1: Catalytic Reforming Units Capacity in Kuwait. Feed Capacity Refinery Unit (BPD Reformate Produced (BPD) Octane Number (Clear) MAA CCR Platforming 2X 90 ZOR CCR Platforming Feed & Product Properties Table 11-2: Rheniformer Feed and Product Properties. Fractionator Feed Reformate (total naphtha) API Sulfur, ppm N 2, ppm Octane Number (clear) RVP, psi Distillation (ºF) IBP 10% 30 50 70 90 EP Recovery 65.6 101 4 - - 126 163 196 221 246 284 316 98 48.6 - - 95.4 6.2 122 162 217 248 275 313 349 98 Figure 11-1: Catalytic Reforming Unit Catalytic Reforming Process Description (Figure 11-3) The system consists of three reactors, fired heaters, hydrogen recycle system, and product debutanizing facilities. Hydrotreated HSR Naphtha is preheated then vaporized in a fired heater. It is mixed with H2 then passed over a catalyst at about 900 ºF and 350 psig in the first reactor. Intermediate heat is provided after first and second reactors because of the endothermic reactions that reduce the temperature in the reactor. 11-2

Petroleum Refining Chapter 11: Gasoline Production The effluent from the third reactor is cooled, by heating other streams in the unit (for efficient heat recover) then separated into a liquid product and a H2-rich gas that is recycled. The excess hydrogen which is used as make-up to the refinery hydrotreating system is bled-off as needed to maintain system pressure. A stabilizer is used to remove light gases (C4 - ) from the reformate product, thus, control its RVP. Catalyst & Reactions Octane improvement is mainly achieved through 1. Dehydrogenation of naphthenes (low octane) to aromatics (higher octane). (endothermic) Dehydrogenation + 3H 2 isopropylcyclohexane ON = 61.1 (clear) isopropylbenzene ON = 99.3 (clear) 2. Dehydrocyclization of paraffins (low octane) to aromatics (higher octane). (slightly endothermic) Dehydrocylization + 4H 2 n-heptane ON = 0 (clear) Toluene ON = 96.5 (clear) 3. Isomerization of Paraffins (low octane) to isoparaffins (higher octane). isomerization n-octane ON = -15 (clear) iso-octane (2,2,4-trimethypentane ) ON = 100 (clear) 4. Hydrocracking of heavy paraffins (low octane) to light paraffins (higher octane). n-nonane ON = -20 (clear) Hydrocracking + H 2 + n-butane ON = 89.6 (clear) n-pentane ON = 62.6 (clear) 11-3

Prof. Tareq A. Albahri 2018 Kuwait University Chemical Engineering Continuous Catalytic Reforming (CCR) Platforming. (Figure 11-4) The CCR process unit can consist of either 1. Reactors stacked on top of each other (UOP) MAA refinery. 2. Side-by-side reactors (IFP). In both cases the sequence of flow of the reactants is like that shown for the semiregenerative system. In the (stacked design) CCR Platforming Unit, freshly regenerated catalyst is introduced in the top of the upper reactor between two concentric perforated cylinders (made from Johnson screens) and flows by gravity from top to bottom. The reactants are introduced on the outside of the outer cylinder and flow radially through the catalyst to the center of the inner cylinder. Partially aged catalyst is removed from the bottom of the lowest reactor and sent to an external regenerator where the carbon is burned from the catalyst, and the catalyst is reduced and acidified before being returned to the upper reactor. Reforming Catalyst The reforming catalyst contains platinum supported on chlorinated alumina base. In most cases rhenium is combined with platinum to form a more stable catalyst which permits operation at lower pressures. Platinum is thought to serve as a catalytic site for hydrogenation and dehydrogenation reactions and chlorinated alumina provides an acid site for isomerization, cyclization, and hydrocracking reactions. Reforming catalyst activity is a function of surface area, pore volume, and active platinum and chlorine content. Catalyst activity is reduced during operation by coke deposition and chloride loss. In a high-pressure process, up to 200 barrels of charge can be processed per pound of catalyst before regeneration is needed. In semi-regenerative reforming process unit, the activity of the catalyst can be restored by high temperature oxidation of the carbon followed by chlorination and is able to operate for 6 to 24-month periods between regenerations. The activity of the catalyst decreases during the on-stream period and the reaction temperature is increased as the catalyst ages to maintain the desired operating severity. Normally the catalyst can be regenerated in situ at least three times before it must be replaced and returned to the manufacturer for reclamation. Catalyst for fixed-bed reactors is extruded into cylinders 1/32 to 1/16 in. (0.8 to 1.6 mm) diameter with lengths about 3/16 in. (5 mm). In continuous catalytic reforming (CCR) process unit the catalyst is regenerated continuously online. The catalyst for continuous units is spherical with diameters approximately 1/32 to 1/16 in. (0.8 to 1.6 mm). Figure 11-2: Catalyst for semi-regenerative and continuous regeneration platforming process. 11-4

Petroleum Refining Chapter 11: Gasoline Production Figure 11-3: Catalytic Reforming Semi-Regenerative Process simplified 11-5

Prof. Tareq A. Albahri 2018 Kuwait University Chemical Engineering Figure 11-4: UOP CCR (continuous catalyst regeneration) Platforming Process (MAA Refinery) 11-6

Petroleum Refining Chapter 11: Gasoline Production Figure 11-5: Schematic Representation of CCR (continuous catalyst regeneration) Platforming Process & Reactor 11-7

Prof. Tareq A. Albahri 2018 Kuwait University Chemical Engineering 2. ISOMERIZATION Introduction The octane numbers of the (light straight run) LSR naphtha [C5-180⁰F (C5-82⁰C)] can be improved using isomerization process. Isomerization convert normal paraffins to their isomers. This results in significant octane increases. In once-through isomerization the clear RON of LSR naphtha can be increased from 70 to about 82 84. If the normal components are recycled, the resulting RON will be about 87 93 clear. Reaction temperatures of about 200 400⁰F (95 205⁰C) are preferred to higher temperatures because the equilibrium conversion to isomers is enhanced at the lower temperatures. At these relatively low temperatures a very active catalyst is necessary to provide a reasonable reaction rate. The composition of the reactor products can closely approach chemical equilibrium. Following is a simplified conversion summary for a typical LSR Naphtha cut. 1 LSR component Feed weight Product weight RONC (unleaded) Isopentane 22 41 92 Normal pentane 33 12 62 2,2-Dimethybutane 1 15 96 2,3-Dimethybutane 2 5 84 2-Methylpentane 12 15 74 3-Methylpentane 10 7 74 Normal hexane 20 5 26 Total 100 100 If the normal pentane in the reactor product is separated (by fractionation or by vapor phase adsorption on a molecular sieve bed) and recycled the product RON can be increased Some hydrocracking occurs during the reactions resulting in a loss of gasoline and the production of light gas. The light gas produced is typically in the range of 1.0 to 4.0 wt% of the hydrocarbon feed to the reactor. A representative flow scheme for an isomerization unit is shown in Figure 11-6 1 The values are on a relative weight basis and do not account for the weight loss resulting from hydrocracking to molecules lighter than pentane. 11-8

Petroleum Refining Chapter 11: Gasoline Production Figure 11-6: UOP H-O-T Penex isomerization unit. Isomerization Yields Isomerization yields vary with feedstock properties and operating severity. Typical operating conditions given in Table 11-3. Isomerization yield is increased by: 1. High temperature (which increases reaction rate) 2. Low space velocity 3. Low pressure 4. High hydrogen-to-hydrocarbon ratios reduce the hydrocarbon partial pressure and thus favor the formation of isomers. A typical product yield is given in Table 11-4 for 12 psi RVP C5+ isomerate product with 13 RONC and MONC improvement. Table 11-3: Isomerization unit typical Operating conditions Reactor temperature 200 400⁰F 95 205⁰C Pressure Hydrogen/HC mole ratio Single-pass LHSV Liquid product yield 250 500 psig 0.05 : 1 1 2 v/hr/v >98 wt% 1725 3450 kp Table 11-4: Isomerization Yields Component Vol% on feed C3 0.5 ic4 1.5 nc4 1.0 C5-C7 (Isomerate) 102.0 11-9

Prof. Tareq A. Albahri 2018 Kuwait University Chemical Engineering Isomerization Reactions Isomerization of paraffins and cyclopentanes usually results in a lower octane product than does conversion to aromatics. However, there is a substantial increase over that of the un-isomerized materials. These are rapid reactions with small heat effects. 1. Isomerization of normal paraffins to isoparaffins: n-pentane (61.7 ON) isopentane (93.5 ON) & n-hexane (31 ON) 2-MP (74 ON) & 3-MP (76 ON) & & 2,2-DMB (94 ON) 2,3-DMB (105 ON) Figure 11-7: Isomerization of paraffins in LSR naphtha Isomerization Catalysts The available catalysts used for isomerization contain platinum on various bases. Some types of catalysts require the continuous addition of very small amounts of organic chlorides to maintain high catalyst activities. This is converted to hydrogen chloride in the reactor, and consequently the feed to these units must be free of water and other oxygen sources to avoid catalyst deactivation and potential corrosion problems. A second type of catalyst uses a molecular sieve base and is reported to tolerate feeds saturated with water at ambient temperature. A third type of catalyst contains platinum supported on a novel metal oxide base. This catalyst has 150⁰F (83⁰C) higher activity than conventional zeolitic isomerization catalysts and can be regenerated. Catalyst life is usually three years or more with all these catalysts. An atmosphere of hydrogen is used to minimize carbon deposits on the catalyst, but hydrogen consumption is negligible. 11-10

Petroleum Refining Chapter 11: Gasoline Production 3. ALKYLATION Introduction Objectives To produce high octane alkylate (96.4 RON min.) suitable for gasoline blending through the reaction of isobutane with light olefins. Location: Only at MAA Capacity: The design capacity is 4,879 BPSD of C4 produced from the MTBE unit. Types: Two types of Alkylation units exist depending on catalyst type. 1. Sulfuric Acid Alkylation 2. HF acid Alkylation Reactions Olefins (mainly isobutene) react with isobutene in the presence of sulfuric acid catalyst to form isoparaffins in the gasoline boiling range (mainly 2,2,4- trimethylpentane) with octane numbers ranging from (93 98 RON). + H2SO4 + Heat isobutylene (gas) isobutane (gas) 2,2,4-trimethylpentane (isooctane) (liquid) + H 2 SO 4 + Heat propylene (gas) isobutane (gas) 2,2-dimethylpentane (isoheptane) (liquid) The reaction is instantaneous and highly exothermic Feed & Product Properties The Alkylation unit olefin feed stream is the C4 raffinate produced at the MTBE unit. It represents a suitable Alkylation feed due to its isobutane/isobutene ratio (47 and 39 v% respectively). During MTBE shutdown an external source of isobutene is used to maintain the desired ratio in the feed. Isobutane is separated from field butane (containing both n- and iso-butane) using a deisobutanizer (DIB). The top product is isobutane which is stored in a drum. 11-11

Prof. Tareq A. Albahri 2018 Kuwait University Chemical Engineering Table 11-5: Alkylation Unit Feedstock properties. A. MTBE Raffinate B. Field Butane C. Sulfuric Acid Total Sulfur, ppmw Total oxygenates, ppmw (sum of DME Methanol and MTBE) Water, ppmw 20 60 800 11-60 - - 20 60 800 Component wise Break-up Kg.mole/hr mole% Ethane Propane Isobutene n-butane i-pentanes n-pentanes - 1 25 72 2 Trace C6 ethylene propylene avg. C4= avg. C5= 1,3 butadiene D. Caustic Solution 0.0 0.325 189.315 47.216 5.087 0.085 0.0036 0.0 0.041 174.48 2.622 1.011 Strength 98.5 W% Sp.gr. 1.85 Table 11-6: Alkylation Unit Product Specifications. n-butane from field butane deisobutanizer Max isobutene content Fuel gas (butane ex-isostripper) C5 + hydrocarbon H2SO4 content Alkali Content Spent sulfuric acid Strength as H2SO4 (titration) Alkylate product RON (clear) RVP @100ºF Total sulfur ASTM Distillation IBP 50% 90% EBP 5 wt% 2 wt% max 0 0 90 wt% 96.4 min 9.0 max 100 ppmw max 102 223 258 401 Process Description The unit consists of four sections (Figure 11-9) 1. Reaction section. 2. Refregeration section. 3. Reactor effluent treating section. 4. Iso-stripping section. 11-12

Petroleum Refining Chapter 11: Gasoline Production Reaction Section: The olefin feed, iso-butane feed, and recycled iso-butane stream are first mixed then cooled to 57 ºF. Any condensed water is removed from the cooled hydrocarbon in a coalescer. The HC goes to a contactor where the reaction occurs. The contactor consists of an impeller for mixing (Figure 11-10), a circulation tube, and a tube bundle used to remove the heat of reaction. Using the impeller to circulate the feed and the acid, creates an emulsion. Part of the emulsion is withdrawn and the other is sent to an acid settler where the acid settles at the bottom by gravity and flows back to the contactor. Side reactions in the reaction zone consume the acid, therefore, fresh acid is charged to the contactor continuously to maintain 90 wt% H2SO4 and equivalent amount of spent acid is withdrawn. The hydrocarbon phase separated from the acid in the settler, flows into the tube side of the contactor through a pressure control valve which creates a pressure drop of 2 5 psig, flashing (partial vaporizing) the light hydrocarbons. This cools the stream temperature to 30 ºF, absorbing the heat generated by the alkylation reactions. Refrigeration Section The hydrocarbon stream leaving the tube bundle flows to a suction trap/flash drum which separates liquid from the vapor. The vessel consists of two liquid compartments and a common vapor space. - The contactor hydrocarbon stream accumulating on one side is pumped to the effluent treating section. - The cold condensate mainly isobutene (from the refrigeration section) accumulating on the other side is sent to the Contactor. The common vapor is condensed by cooling in a seawater condenser after compression to 46 psig. Part of the condensate is caustic treated to remove the acidic components and sent to a C3/C4 splitter. The remaining condensate enters an economizer drum where it is flashed. The vapor flows to the flash drum side of the suction trap/flash drum. Effluent Treating Section The liquid phase of the contactor tube is washed with fresh sulfuric acid (in the acid wash vessel) and alkaline water (in the alkaline water wash vessel) to remove the corrosive compounds (esters and acid traces) formed by the reaction of sulfuric acid with olefins. Isostripper The treated contactor effluent is sent to the isostripper column. The overhead iso-butane vapor is recycled to the reaction section. The side product (n-butane 67.6 M% purity) is sent either to Mogas (motor gasoline) or fuel gas blending or both. The bottom liquid is the Alkylate product. 11-13

Prof. Tareq A. Albahri 2018 Kuwait University Chemical Engineering Figure 11-8: Auto-refrigeration Sulfuric Acid Alkylation Unit. 11-14

Petroleum Refining Chapter 11: Gasoline Production Figure 11-9: Detailed Sulfuric Acid Alkylation Unit in MAB refinery. 11-15

Prof. Tareq A. Albahri 2018 Kuwait University Chemical Engineering Figure 11-10: Stratco Sulfuric Acid Contactor in MAB refiner. 11-16