Penex Process de isomerizacion PDF

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PENEX PROCESSESIn worldwide production of automotive gasoline permanent tendency to the toughening of not only its operating but also its ecological characteristics is observed. So, international and domestic regulations to automotive gasoline considerably limit the content of benzene, aromatic hydr...

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PENEX PROCESSES In worldwide production of automotive gasoline permanent tendency to the toughening of not only its operating but also its ecological characteristics is observed. So, international and domestic regulations to automotive gasoline considerably limit the content of benzene, aromatic hydrocarbons, olefin hydrocarbons and sulfur. In 1970s the variants of hydrogenation of the benzene, contained in the reformate, proceeding without the decrease of product octane number have been offered. However for decrease of the total aromatics content the dilution of reformate with high-octane nonaromatic components is required. This situation is complicated by refusal from tetraethyl lead (TEL) and deficit of butane-butylene fraction (because of the lack of FCC duty), which is used for the production of high-octane alkylate in the world practice. Thereby the development of isomerization process is one of the effective methods for solution of this problem. It allows the producing of commercial gasoline which corresponds to the current and perspective requirements to the fuels and provides necessary flexibility of processing. 1. TYPES OF ISOMERIZATION PROCESSES

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Three types of industrial isomerization processes are worked out currently: high-temperature isomerization process (360-440 °С) on fluorinated-alumina catalysts; medium-temperature isomerization process (250-300 °С) on zeolite catalysts; low-temperature isomerization process on chlorinated-alumina catalysts (120-180 °С) and sulfated metal oxides (180-210 °С ).

2. THERMODYNAMIC AND KINETIC LAWS OF ISOMERIZATION PROCESS The schemes of proposing processes are analogous generally. The differences are defines by performances of usable catalysts due to their type. Main parameter which is the octane number of produced isomerizate depends on process temperature. That‘s why we will dwell on the issue of thermodynamic of isomerization reaction. First of all hydrocarbons isomerization reaction is balanced reaction, and equilibrium yield of isoparaffins increases with temperature reducing, but it can be reached only after an ―infinite residence time‖ of the feed in reaction zone or an equivalent very small value for LHSV. On the other hand an increase in temperature always corresponds to an increase in reaction velocity. So that at low temperature the actual yield will be far below the equilibrium yield, because of low reaction velocity. On the contrary, at higher temperature, the equilibrium yield will be more easily reached, due to a high reaction rate. Consequently, at higher temperature the yield of isoparaffins is limited by the thermodynamic equilibrium, and at lower temperature it is limited by low reaction rate (kinetic limitation) (Figure3.1). The comparative estimation of isopentanes content in sum of pentanes for different types of isomerization catalysts is represented below (Figure 3.2).

Figure 3.1. Dependence of n-paraffins conversion on reaction temperature

Figure 3.2. Comparative estimation of isomerization catalysts

The conversion level of n-paraffins on zeolite catalysts is low, as it is limited by thermodynamic equilibrium. In the case of chlorinated-alumina catalysts and sulfated metal oxides conversion of n-paraffins is higher because of high equilibrium content of isocomponents in product.

3. TECHNOLOGIES OF ISOMERIZATION PROCESS ON DIFFERENT CATALYSTS 

ZEOLITE CATALYSTS

Zeolite catalysts are less active and used at higher operating temperature compared to another types of catalysts, and consequently the octane number of isomerizate is low. However they possess high resistance to impurities in the feed and capability for total regeneration in the reactor of the unit. The technological scheme of this process is provided with fire-heaters for heating hydrogen and feed mixture up to reaction temperature. It is necessary high ratio of hydrogen to hydrocarbon feed (along with isomerization, hydrogen is spent for hydrotreating and dearomatization of the feed); that‘s why compressor for supplying of recycle hydrogen-rich gas and separator for separation of hydrogen-rich gas are necessary . Hysopar catalyst should be marked out among zeolite catalysts; it is the most progressive in the world catalyst market, because it considerably exceeds all another catalysts by resistance to impurities in the feed (available sulfur content is 100 ppm permanently and 200 ppm during short periods of time) 

CHLORINATED-ALUMINA

Chlorinated-alumina based catalysts are the most active and supply the highest isomerizate yield and isomerizate octane. It should be noted that during isomerization catalysts loose chlorine, consequently the activity is reduced. That‘s why chlorine compound injection to the feed (usually ССl4) is provided for keeping of high activity. As a result, caustic soda washing from organic chloride in special scrubbers is necessary. Considerable drawback is that this type of catalyst is very sensible to poisonous impurities (to the oxygen compounds including water, to nitrogen) and requires pretreatment and drying of the feed. In addition the problems occur at regeneration The first generation catalyst of UOP is I-8, which was improved later in more active I-80 type catalyst. The latest developments of UOP Company are high-performance I-8 Plus, I-82, I84 catalysts for Penex process and I-122, I-124 catalysts, which are used in Butamer process (nbutane isomerization process with purpose to produce isobutane, which is the feed for alkylation unit). In development of new catalysts UOP has the target to decrease its platinum content without losing the activity, thereby to reduce significantly its operating costs. It is not of small importance for present-day refinery. 

SULFATED METAL OXIDES BASED CATALYSTS

Sulfated metal oxides based catalysts get heightened interest last years as they combine main advantages of medium-temperature and low-temperature catalysts. They are active, resistant to poisonous impurities and able for regeneration. The only drawback, as for zeolite catalysts, is necessity in compressor for recycling of hydrogen-rich gas.

The CИ-2 catalyst has an activity, which is higher than activity of PI-242 [5] and characterized with unique sulfur resistance. If necessary, the process can be carried out without pretreatment of the feed. In this case the octane number of isomerizate is reduced by 2 points, but total lifetime (8-10 years) doesn‘t changes and service cycle is no less than 12 months. The feed may contain considerable quantity of benzene which is hydrogenated efficiently on the catalyst. The Pt/WO3-ZrO2 catalyst shows higher activity and selectivity in isomerization reaction of n-alkanes compared to sulfated-zirconia catalysts. The advantage of this type of catalyst is explained by rapid surface diffusion of hydrogen atoms, which are converted into protons and hydrides on the Lewes acid sites, thereby increasing catalyst activity and selectivity.

TECHNOLOGIES SCHEMES OF ISOMERIZATION PROCESS Penex Process The Penex process has served as the primary isomerization technology for upgrading C5/C6 light straight-run naphtha feeds since UOP introduced it in 1958. This process has a wide range of operating configurations for optimum design flexibility and feedstock processing capabilities. The Penex process is a fixed-bed procedure that uses high activity chloridepromoted catalysts to isomerize C5/C6 paraffins to higher octane branched components. The reaction is conducted in the presence of a minor amount of hydrogen. Even though the chloride is converted to hydrogen chloride, carbon steel construction is used successfully because of the dry environment. For typical C5/C6 feeds, equilibrium will limit the product to 83 to 86 RON (Research Octane Number) on a single hydrocarbon pass basis. To achieve higher octane, UOP offers several schemes in which lower octane components are separated and recycled back to the reactors. These recycle modes of operation can lead to product octane as high as 93 RON. Hydrocarbon Once-Through Penex Process Hydrogen Once-Through Penex process flow scheme results in a substantial saving in capital equipment and utility costs by eliminating product separator and recycle gas compressor. The stabilizer separates the light gas from the reactor effluent (Fig3.3). Typically, two reactors in series are used to achieve high on-stream efficiency. The catalyst can be replaced in one reactor while operation continues in the other. One characteristic of the process is that catalyst deactivation begins at the inlet of the first reactor and proceeds slowly as a rather sharp front downward through the bed. The adverse effect that such deactivation can have on unit on-stream efficiency is avoided by installing two reactors in series. Each reactor contains 50% of the total required catalyst. Piping and valving are arranged to permit isolation of the reactor containing the spent catalyst while the second reactor remains in operation. After the spent catalyst has been replaced, the relative processing positions of the two reactors are reversed. During the short time when one reactor is off-line for catalyst replacement, the second reactor is fully capable of maintaining continuous operation at design throughput, yield, and conversion. Several factors are considered when choosing a process flow scheme. One of the most important aspects is desired product octane. The hydrocarbon once-through flow scheme is the most widely used isomerization process for producing moderate octane upgrades of light naphtha. Economically efficient ―onethrough‖ scheme without any recycle can be used with minimum investment in realization of isomerization process Figure (3.3).

Figure (3.3) Block diagram of “one- through” process

TABLE 3.2 Typical Estimated Yields for Once-through Processing

Penex Process With Recycle And Fractionation Separation and recycle of unconverted normal C5 and C6 paraffins and low octane C6 isoparaffins back to the reactor, produce a higher octane product. The most common flow scheme uses a deisohexanizer (DIH) column to recycle methylpentanes, n-hexane, and some C6 cyclics. It is the lowest capital cost option of the recycle flow schemes and provides a higher octane isomerate product, especially on C6 rich feeds. In the Penex/DIH process the stabilized isomerate is charged to a DIH column producing an overhead product containing all the C5 and dimethylbutanes. Normal hexane and some of the ethylpentanes are taken as a side-cut and recycled back to the reactors. The small amount of bottoms (C7+ and some C6 cyclics) can be sent to gasoline blending or to a reformer. The addition of a deisopentanizer (DIP) or a super DIH will achieve the highest octane from a fractionation hydrocarbon recycle flow scheme. In this scheme, both low octane C5 and normal and isoparaffin C6 are recycled to the Penex reactors . The scheme with deisopentanizer (DIP) before the reactor section allows the producing of isomerizate with high octane number, increasing of conversion level of n-pentanes and reducing the reactor duty simultaneously. The technology is reasonable in the case of isopentanes content in the feed more than 13-15 % Figure (3.4) Figure (3.4) Block diagram of process with DIP

The scheme with deisohexanizer (DIH) after the isomerization reactor is the simplest way to produce the isomerizate with higher octane number. In this case non-converted low-octane components (methylcyclopentane and n-hexane) are recycled into reactor. However the given scheme allows only increasing of hexanes conversion, but doesn‘t raise the content of isopentanes in the product (Figure 3.5). The scheme of the process may include both deisopentanizer and deisohexanizer (with DIP and DIH)

Figure (3.5) Block diagram of process with DIH

TABLE 3.3 Typical Estimated Yields for Deisohexanizer Processing

Scheme with recycle of n-pentane (with DIP and DP) requires providing with depentanizer of isomerizate after the reaction section and deisopentanizer before the reactor. Schemes with recycle of n-pentane and n-hexane. For total conversion of all linear paraffins (not

only n-С6 but also n-С5) into isomers, their total recycle is necessary which can be realized by set of distillation columns (with DIP, DIH and DP) or by adsorption on molecular sieves. The method of adsorption on molecular sieves (in liquid or vapor phase) is based on capability of pores with definite size to adsorb selectively the molecules of n-paraffins. The next stage is desorption of n-paraffins from pores and its recycle to the feed stock. Stages of adsorption and desorption are repeated in cycles or pseudo-continuously. Penex / Molex Process This flow scheme uses Molex technology for the economic separation and recycle of nparaffin from the reactor effluent. The Molex process is an adsorptive separation method that utilizes molecular sieves for the separation of n-paraffins from branched and cyclic hydrocarbons. The separation is effected in the liquid phase under isothermal conditions according to the principles of the UOP Sorbex separations technology. Because the separation takes place in the liquid phase, heating, cooling and power requirements are remarkably low. Sorbex is the name applied to a particular technique developed by UOP for separating a component or group of components from a mixture in the liquid phase by selective adsorption on a solid adsorbent. In broad outline, Sorbex is a simulated moving bed adsorption process operating with all process streams in the liquid phase and at constant temperature within the adsorbent bed. Feed is introduced and components are adsorbed and separated from each other within the bed. A separate liquid of different boiling point referred to as ‗desorbent‘ is used to displace the feed components from the pores of the adsorbent. Twoliquid streams emerge from the bed – an extract and a raffinate stream, both diluted with desorbent. The desorbent is removed from both product streams by fractionation and is recycled to the system. A simplified schematic flow diagram of a gasoline Molex unit is shown in Fig. 3.6. The adsorbent is fixed while the liquid streams flow down through the bed. A shift in the positions of liquid feed and withdrawal, in the direction of fluid flow through the bed, simulates the movement of solid in the opposite direction. It is, of course, impossible to move the liquid feed and withdrawal points continuously. However, approximately the same effect can be produced by providing multiple liquid access lines to the bed, and periodically switching each net stream to the next adjacent line. A liquid circulating pump is provided to pump liquid from the bottom outlet to the top inlet of the adsorbent chamber. A fluid-directing device, known as a ‗rotary valve‘, is also provided. The rotary valve functions on the same principle as a multiport stopcock.

Figure (3.6) Block diagram of Penex/Molex process

TABLE 3.4 Typical Estimated Yields for Molex Processing

UOP offers the processes with adsorption systems on the molecular sieves in vapour phase (Penex/Iso Siv) and liquid phase (Penex/Molex (Figure 3.9)), and process, which combines adsorptive separation of unconverted n-paraffins from isomers and deisohexanizing Penex/DIH/PSA.

Penex-Plus technology, which is for processing of the feed with high benzene content (from 7 up to 30 % vol. in the case of light straight-run gasoline fraction and light reformate blend), includes feed treatment section which is hydrogenation of benzene. TABLE 3.5 TYPICAL PENEX ESTIMATED INVESTMENT COST

TABLE 3.6 TYPICAL PENEX ESTIMATED UTILITY REQUIRMENT

TABLE 3.7 TYPICAL PENEX ESTIMATED OPERATING REQUIRMENTS

PENEX FLOW DIAGRAM DISCRIPTION FEEDSTOCK REQUIREMENTS To maintain the high activity of the Penex catalyst, the feedstock must be hydrotreated. However, costly pre-fractionation to sharply limit the levels of C6 cyclic and C7 compounds is not required. In fact, the Penex process affords the refiner with remarkably good flexibility in the choice of feedstocks, both at the time of design and even after the unit has been constructed. The latter is important because changes in the overall refinery processing scheme may occur in response to changing market situations. These changes could require that the composition of the isomerization feed be modified to achieve optimal results for the entire refinery. The Penex system can be applied to the processing of feeds containing up to 15 percent C7 with minimal or no effect on design requirements or operating performance. Generally, the best choice is to operate with lower levels of C7+ material because these compounds are better suited for upgrading in a reforming process. Charge containing about 5.0 percent or even higher amounts of benzene is completely acceptable in the Penex chargestock and will not produce carbon on the catalyst. When the feed has extremely high levels of benzene, a Penex-Plus unit is recommended. (The ―Plus‖ section can be retrofitted to an existing Penex unit should the refiner want to process high-benzene feedstock in an existing Penex unit.) The low-octane C6 cut recovered from raffinate derived from aromatic extraction operations typically contains a few percent of olefins and is completely acceptable as Penex feed without pre-hydrogenation. Sulfur is an undesirable constituent of the Penex feed. However, it is easily removed by conventional hydrotreating. Sulfur reduces the rate of isomerization and, therefore, the product octane number. Its effect is only temporary, however, and once it has been removed from the plant, the catalyst regains its normal activity. Water, other oxygen-containing compounds, and nitrogen compounds are the only impurities normally found in the feedstock that will irreversibly poison the Penex catalyst and shorten its life. Fresh feed and makeup hydrogen are dried by a simple, commercially proven desiccant system. PROCESS FLOW DIAGRAM The UOP Penex Unit can be divided into ten sections. A. Sulfur Guard Bed B. Liquid Feed Driers C. Makeup Hydrogen Driers D. Feed Surge Drum E. Exchanger Circuit F. lsomerization Reactors G. Stabilizer H. Stabilizer Gas Scrubber I. Separator and Compressor Section (Recycle Gas Units Only)

FIGURE 4.1 PENEX FLOW DIAGRAM

A. SULFUR GUARD BED The purpose of the sulfur guard bed is to protect the Penex catalyst from sulfur in the liquid feed. The hydrotreater will remove most of the sulfur in the Penex feed. The guard bed reduces the sulfur to a safe level for H.O.T. Penex operation and serves as insurance against upsets in the NHT which could result in higher than normal levels of sulfur in the feed. The guard bed is loaded with UCP ADS-11 adsorbent, a nickel- containing extrudate designed to chemisorb sulfur from the liquid feed. The feedstock is heated to the required temperature for sulfur removal, usually 250°- (120°C) and passed down flow over the adsorbent. Once sulfur breakthrough occurs, normally after one year or so of operation, the guard bed is taken off line and reloaded with fresh adsorbent. The Penex Unit need not be shut down during the short period of time required to reload the guard bed so long as the NHT is performing properly. B. LIQUID FEED DRIERS The liquid feed driers are used to dry the Penex liquid feed to less than 0.1 ppm H20. The piping is designed so that either drier can be in the lead or the lag position in series flow operation. Either drier can be operated individually while the other is being regenerated. The driers are designed for a 48 hour cycle which includes 24 hours in the lead position, 7 hours regenerating, and 3 hours cooling and 14 hours in the lag position. Proper drier operations are essential in the Penex process since the catalyst is water intolerant. Typically, type 4A molecular sieves are employed within the driers.

Charge to the liquid feed driers is hydrotreated LSR naphtha from the Naphtha hydrotreater stripper bottoms. Additional recycle feed from a Molex unit or deisohexanizer may ...