JPH0243526B2 - - Google Patents

Description

[Detailed description of the invention]

FIELD OF INDUSTRIAL APPLICATION This invention relates generally to the separation of normal and non-normal paraffins. More particularly, the present invention relates to reducing the amount of undesirable residues in the circulating purge gas used in such separations. BACKGROUND OF THE INVENTION Separating a mixture of chemical compounds into two or more fractions by selective adsorption based on molecular sieves is a conventional method that takes advantage of the uniform diameter of the pores of a given zeolite molecular sieve adsorbent. be.
The separation of normal and isoparaffins has been found to be particularly suitable for this technique, and a number of processes have been proposed for this purpose. Most of these processes involve contacting a gas phase mixed hydrocarbon feed with a 5 angstrom molecular sieve to adsorb linear hydrocarbon compounds, followed by lower pressure or higher temperature;
It was usually based on the desorption of linear compounds with the aid of a purge gas. In some cases, this has been done with little or no change in temperature or pressure, using a purge that adsorbs the adsorbed linear compounds strongly enough to have a displacement effect. One process of particular interest is one that operates under essentially isobaric and isothermal conditions, even though the desorption is performed using a non-adsorbent purge gas instead of a highly sorbent purge material. The process started in 1972
No. 3,700,589, issued October 24, 2005, the disclosure of which is incorporated herein by reference. An improvement to the aforementioned patent is described in US Pat. No. 4,176,053, issued November 27, 1979, the disclosure of which is also incorporated herein by reference. The present invention is an improvement to the process and makes it possible to reduce undesirable residues in the circulating purge gas used in the process. The present invention is also covered by a US patent issued on July 1, 1980.
This is an improvement on the TIP process disclosed and referred to in No. 4210771. According to a particular embodiment of the process for separating normal paraffins from a mixture with non-normal paraffins, such as that of U.S. Pat. sequentially through each of at least four fixed beds of the system containing a zeolite molecular sieve adsorbent having an effective pore diameter of substantially 5 angstroms, each of which periodically states (a) to (d) of: (a) the vapor in the bed void space consists primarily of non-sorbing purge gas and the incoming feedstock consists of said non-sorbing purge gas from the bed void space; (b) passing the feed in co-current flow through the bed and discharging the normal components of the feed; (c) adsorption in which the unadsorbed portion of the feedstock is selectively adsorbed into the internal cavities of a crystalline zeolite adsorbent and removed from the bed as an effluent with a very reduced content of normal components; (c) in the mass transfer zone; Stoichiometric point is 85-97 of floor length
A bed loaded with normal adsorbate to the extent that % passes through and containing a mixture of normal and non-normal in the void space of the bed in essentially the proportion of the feedstock is placed countercurrently with respect to the direction of the adsorption step (b). A flow of non-sorbing purge gas into the bed is sufficient to remove the void space feedstock vapor but approximately in the bed effluent.
void space purging by passing the non-sorbing purge gas in countercurrent with respect to the adsorption step (b); selectively adsorbed feedstock normals are recovered as a product stream by passing them through the bed to desorb most of the adsorbed normals and ensuring that the bed void space vapor consists primarily of non-sorbing purge gas. Purge and remove. The above process further incorporates into the feedstock in the gas phase a mixture of normal and non-normal which during step (c) is purged from each bed of the system to another bed of the system to undergo adsorption in step (b). This includes direct circulation. In conventional practice, the effluent from the bed during step (c), void space purging, is cooled to condense high boiling paraffinic components and flash to remove and recover all. The molecular sieve adsorbent used in the above process and the present invention is a naturally occurring or synthetic three-dimensional crystalline zeolite aluminosilicate, which is capable of removing water of hydration from the aluminosilicate without disrupting the crystal lattice. and which selectively adsorbs normal paraffins based on their molecular size from a mixture with branched and/or cyclic paraffins that constitute the feed stream. Since normal paraffin has a nominal cross-sectional diameter of about 5 angstroms, molecular sieves having pore diameters of about 5 angstroms are suitable for the practice of this invention. Particularly suitable is the cationic form of zeolite A with a pore diameter of about 5 angstroms. Zeolite A is well known in the art as a synthetic zeolite with a very large adsorption capacity and exhibits an apparent pore diameter ranging from about 3 to about 5 angstroms depending on the cationic species present. When made with sodium cations, zeolite A has a pore diameter of about 4 angstroms. If 25%, preferably at least 40%, of the sodium cations are replaced with calcium and/or magnesium cations, the preferred pore diameter increases to about 5 angstroms. Zeolite A, as used herein and in the claims, refers to zeolite as described and defined in US Pat. No. 2,882,243. Other zeolite molecular sieves of suitable cationic form, having pore diameters of about 5 angstroms, and those suitable for use even though they have a lower adsorption capacity than Zeolite A, are disclosed in U.S. Pat.
No. 2950952 Zeolite T, including the minerals chabazite and erionite. The hydrocarbon stream treated according to the invention consists essentially of a mixture of branched chain paraffins and normal paraffins with boiling points in the gasoline and kerosene range. Such mixtures occur as both light and heavy petroleum naphtha, natural gasoline, natural gas condensate, but can be products of processing and refining industries other than petroleum production. Typically, the hydrocarbons of these streams contain from about 3 to about 13 carbon atoms and are preferably substantially free of olefinic and acetylenically unsaturated species. In addition, impurities in sulfur compounds are
Advantageously, if present, it is present in a concentration of less than about 400 parts per million, and the impurity level in the water is below saturation. Although the process can be operated without regard to the relative molar proportions of normal and non-normal present in the feedstock, the process is suitable for bulk separations in which both non-normal and normal paraffins each constitute at least 10 mole percent of the feedstock. The greatest advantage is provided when operating as one of the The entire process is approximately 350°~750〓(177°~399℃)
Choose from a range of temperatures that operate at substantially uniform temperatures.
At temperatures below about 350° C. (177° C.), the efficiency of the non-sorbing purge gas is reduced to the point that undesirably large quantities are required to adequately purge normals from the bed. At temperatures above about 750°C (399°C), the rate of coke deposition increases rapidly, requiring more frequent oxidative regeneration of the adsorbent. It should be understood that the process is referred to as "isothermal" because the temperatures of the feedstock and purge gas streams are essentially the same upon entering the bed, i.e. within about 30°C (17°C). be. In this, as in all adsorption-desorption cycles, thermal gradients can develop within the bed due to the heat of adsorption and desorption. The pressure range suitable for this method is about 50 to about 400 psia.
(3.5-28Kg/ ^cm2A ). Typically, the suitable pressure will depend on the particular feedstock involved, with higher pressures being used for more volatile feedstocks to enhance the resulting separation and facilitate condensation of the product effluent. It is important that none of the feedstock components condense within the void space of the bed, as such liquid phase materials may be affected by the limited amount of non-sorbing purge gas available for this purpose. This is because it cannot be removed. Therefore, the pressure should be maintained at less than 80% of the critical pressure of the highest boiling key component of the feedstock or less than about 60% of the dew point pressure of the feedstock at the process temperature, whichever is lower. It is. Similarly, as in the case of the term "isothermal" above, the process is "isobaric" because the pressures of the adsorber feed and purge gas streams are the same within customary limits at their respective inlet ends of the bed. It is called. Thus, the term "isobaric" is used in its art-recognized meaning to indicate that the process does not employ pressure swing desorption. The term "key component" as used herein with the description of pressure requirements means any paraffinic component of the feed mixture that is present in significant amounts. As is well understood in the art, what constitutes a significant amount of a particular component of a mixture depends, in part, on the other components present and the nature of the processing the raw materials have undergone. However, in this method, the key component is typically present in an amount of about 10 mole percent or more. When pressure conditions are dictated by dew point standards, the dew point of the hydrocarbon mixture involved is
"Process Heat Transfer", Donald K. (Q), Cologne, McGraw-Hill Books, New York, NY, pp. 319-325 (inclusive)
It can be determined by the method described in .
Other methods for making these calculations are well known in the art. It goes without saying that routine experiments can be relied upon instead of calculating the dew point. In this process, the non-sorbing purge gas used to flush the bed void space vapor and carry the desorbed normal paraffin out of the bed is small enough in molecular size to enter the intracrystalline cavities of the molecular sieve, but not per se. Any permanent gas or mixture of gases that is not adsorbed strongly enough to displace the adsorbed normal hydrocarbons to a significant extent. Nitrogen, hydrogen, helium, methane are such materials and are preferred in the practice of this invention. Other permanent gases are known in the art, but although operable, they are not commercially available at a reasonable cost, making them impractical. Bed void space, for purposes of this invention, is intended to mean any space within the bed that is not occupied by solid material, except for the intracrystalline cavities of the zeolite crystals. Agglomerates of zeolite crystals (agglomerates)
Pores in any binder that can be used to form a bed are also considered bed void spaces. As described earlier in this specification, the adsorption process in which normal paraffin is selectively adsorbed onto the bed is carried out so that the stoichiometric point of the normal hydrocarbon mass transfer region is 85 mm above the bed length.
Continue for a period like ~97% moved. The term mass transfer zone, as used herein, has the same meaning as generally accepted in the art, i.e., the adsorbate loading of the adsorbent bed and the concentration of adsorbed fractions in the fluid stream are both Represents the section of the adsorbent bed that increases over time. A "stoichiometric point" is a point within the mass transfer zone where the depleted volume in the front of the mass transfer zone is equal to the undepleted volume in the transfer zone. A time-controlled sequence that integrates at least four adsorption beds of essentially equal capacity to optimize the four-stage cycle of the process and provide substantially constant flow rates of both normal and non-normal hydrocarbon product streams. It is preferable to use it in This system provides optimal conditions for heat exchange and purge gas recovery, as well as advantageous mass transfer and pressure drop characteristics during adsorption step (b). All process valves can be automatically controlled from a timer control system. The following designation is given in connection with FIG. 1 of the figure to illustrate a typical flow for adsorber circulation with feedstocks as described in the table. FIG. 1 is a flow diagram of such a four-stage cycle using a seven-bed adsorbent system. This invention is applicable to the naphtha range, n-paraffin/isoparaffin separation process of the invention regardless of the amount or composition of the mixed pentane/hexane feed used in the invention, examples of which are provided hereinafter. give. For purposes of illustration, assume that the system has already achieved steady state. The temperature of all first adsorption beds, the total feed stream to the bed, and the effluent from the bed is 700°C (371°C). The floor is approx.
It is under a pressure of 250 pisa (18 Kg/cm ² A). The composition of the first feedstock is tabulated below:

[Table] Referring to FIG. 1 in the figure, a raw material stream is supplied from a pipe line 10 to an accumulator tank 12, and is extracted from the tank 12 by a pump 14 to a supply rate regulator 16, and then to a heat exchanger. 18, where it is heated to about 450 DEG -500 DEG C. (232 DEG -260 DEG C.) by heat exchange with the effluent from the adsorption bed undergoing a second stage of adsorption. The partially heated raw material stream is heated to a temperature of 700° C. (371° C.) using a gas-fired heater 20. The feed stream flow rate from heater 20 to line 22 is 356,578 pounds per hour (161,744 Kg) and has a normal paraffin content of about 200% by weight.
Flow from line 22 is directed partly to line 24 by pressure regulator 23 (the use of which is described below) and partly by flow regulator 28 in line 26. and direct it toward conduit 26. From line 26, a small portion of the raw material from line 22, i.e. every hour
99,026 pounds (44,918 Kg) is directed through manifold 30 and valve 32 to adsorption bed 34. Seven adsorption beds in the system, namely beds 34, 36, 37, 38, 4
Each of 0, 41, and 42 is 1/16 inch (1.6 mm)
Cylindrical pellet-shaped calcium zeolite A
Accommodates 93,000 pounds (42,000Kg). Each floor is length
It is 17.5 feet (5.34 m) and 12.5 feet (3.81 m) in diameter. Bed 34 contains residual hydrogen purge gas from the previous desorption step at the time the feed enters through valve 32. The flow rate of the feed through line 26, manifold 30, and valve 32 is adjusted to uniformly flush residual hydrogen through bed 34 over a period of approximately 1 minute, i.e., the effluent from bed 34 is approximately 3845 pounds per hour. 1744Kg). During this first stage of the adsorption process in bed 34, the hydrogen effluent flows from the bed via valve 45 to manifold 10.
Passes 4. During the one minute period that hydrogen was being flushed from bed 34, the feedstock passed from valve 23 through line 24, manifold 62, and valves 64, 65, and 66 to beds 36 and 66, respectively, at a rate of 79,184 pounds per hour (35,918 Kg).
Passes 37 and 38. Normal paraffin in the raw material is adsorbed in each of the beds 36, 37, and 38, and non-normal and residual hydrogen purge gas that is not adsorbed is transferred to a pipe 48, a heat exchanger 18, a pipe 50, a water cooler 52,
It flows through separator 54 and collects the condensed product in acumulator 56. Residual hydrogen purge gas in the non-normal effluent exits separator 54 via line 57 and ultimately passes through line 203 to purge circulation compressor 60. Since the quasi-cyclic purge gas separated from the non-normal effluent necessarily contains at least a small amount of non-normals, direct introduction of the circulating purge gas into the circulating compressor 60 will eliminate the non-normals during the second stage desorption process. Together with the desorbed normal, it passes from the manifold 76 to the normal product accumulator 114, which is undesirable for the purity of the normal product. influence According to the invention, circulating purge gas in line 57 containing residual non-normals is introduced into cleaning vessel 200. The wash vessel 200 can be a trayed or packed column in which a small portion of the normal product is washed, which is introduced into the wash vessel 200 via a circulating purge gas line 204. In the cleaning vessel 200, residual non-normals in the circulating purge gas are replaced with essentially normal products and the circulating purge gas with reduced non-normal content is passed through line 20.
3 and then returned to the circulating compressor 60. The stream exiting the wash vessel 200 via line 206 contains the wash normals and non-normals removed from the circulating purge gas, and this stream is recycled to the feedstock. Flushing residual hydrogen from bed 341
During the minute, ie, stage (a), the bed 40 is undergoing a first stage of purging with hydrogen and the hydrocarbons in the void space of the bed are being flushed from the bed, ie, undergoing stage (c). During the same one minute interval, beds 41 and 42 undergo a second stage of desorption, step (d), in which normal hydrocarbons are desorbed from the molecular sieve adsorbent using hydrogen and removed from the beds. . The hydrogen gas stream is passed from the compressor 60 to a line 72 and a heat exchanger 74 where the hydrogen gas stream is reduced by heat from the hot desorption normal flowing through the manifold 76 from either of the adsorption beds. Heated to approximately 550° to 6000° (288° to 316°C). The hydrogen gas stream passes through a heat exchanger 74 and through a conduit 78 to a gas-fired heater 80 where it is
〓 (371°C) and passes through the pipe 82 from there. A flow regulator 84 controls the flow of gas from the pipe 82.
and 85 into two streams, and the smaller stream is routed through line 8 at a rate of 7709 pounds per hour (3497 Kg).
6, countercurrently (with respect to the previous adsorption stroke) into bed 40 via manifold 88 and valve 90. The low regulated flow rate used for the 1 minute first stage desorption is to flush unadsorbed hydrocarbons from the bed voids without excessively desorbing normals from the adsorbent. The effluent from bed 40 was 124,693 lb/hr (56,561 Kg) of hydrocarbons and 2,038 lb/hr (924 Kg)
hydrogen and the manifold 6 via the valve 92.
Passes 2. Intercooling, phase separation, and reheating the effluent from the first stage of desorption, which contains all desorption normals in addition to the void space loading from the previous adsorption step, into the feedstock used during the second stage of adsorption. circulate directly. In fact, valve 92 is used as a second stage adsorbent feed valve when bed 40 is at that stage in the cycle. The majority of the hydrogen flow from line 82;
That is, 37,659 pounds (17,082 Kg) per hour is applied to the control valve 8.
5. Pass via line 102 to manifold 104 where it mixes with the pre-operation first stage adsorption effluent from valve 45 and then passes in equal amounts via valves 105 and 106 to beds 41 and 42. During this period, the selectively adsorbed normal paraffin is desorbed from the zeolite molecular sieve and flashed from the bed. The effluent from beds 41 and 42 was 85,000 lb/hr (39,000 Kg/hr) of normal paraffin and 20,654 lb/hr (9369 Kg/hr) of hydrogen.
This is supplied to the manifold 76 through valves 107 and 108 and then passed through the heat exchanger 74. The cooled normal paraffin and hydrogen exiting the heat exchanger 74 are passed through a pipe 112 to a separator 110
There, the normal is supplied to the normal accumulator 114, and the hydrogen is circulated through the pipe 116 to the purge circulation compressor. The above description refers to only one minute of the seven minute cycle of the system. For the next minute, floor 34 is
Starting the staged adsorption process, beds 37 and 38 are still in second stage adsorption, bed 36 begins first stage desorption, bed 40 enters second stage desorption, bed 41
is still in desorption and the appropriate valves are operated so that bed 42 begins the first stage adsorption process. Similarly, after each 1 minute and at the end of 7 minutes a new cycle was started and the bed had completed all stages of adsorption and desorption. The chart below shows the action of each of the seven beds for each minute. In the diagram, A-1 represents step (a) adsorption in which a low feed rate feed stream is used to flush residual hydrogen from the bed. A-2
represents a conventional adsorption process in which the feed rate of the hydrocarbon mixture is of the same order as the effective use of the bed, ie, stage (b) adsorption herein. D-1 represents step (c) desorption, in which purge gas is used in an amount sufficient to remove hydrocarbon vapors from the bed void space, and D-2
represents step (d), hydrogen purge with a flow rate sufficient to desorb normals from the bed.

TABLE The purpose of valve 23 in A-1 supply line 24 is best illustrated by the following example. Assuming that the supply pressure in the pipe line 22 is 250 psia (17.6 Kg/cm ² A),
and also for this example flow control valves 28, 84,
85 and all conduits are assumed to be negligible. Therefore, A-1 supply lines 26 and 30 are
250 psia (17.6 Kg/cm ² A). The effluent pipe 104 of A-1 is designed in consideration of the pressure drop caused by the adsorber container.
It is 249 psia (17.5 Kg/cm ² A). Therefore, this includes lines 102, 82, 86 and D-1 supply line 88.
The pressure must be on the government. D-1 effluent line 62 is rated at 246 psia (17.3
Kg/cm ² A). The pipe line 62 and the pipe line 24 are A-
Since there are also two supply pipes, their pressure is approximately
246 psia (17.3 Kg/cm ² A). Therefore, in this example, the valve 23 makes the conduit 24 more open than the conduit 22.
Must be controlled as low as 4 psi (0.3 Kg/cm ² A) to ensure mixing of the D-1 effluent with the A-2 feedstock. Since the bed is circulated at an operating temperature of 700°C (371°C), carbonaceous deposits gradually accumulate. This fouling reduces the capacity of the adsorber and causes normal paraffin to break through into the isomeric product stream, reducing recovery of normal paraffin. The rate at which this deposit builds up depends on factors such as temperature, impurities in the feedstock, nature of the feedstock, cycle time, and residual paraffin loading. Deactivation of this type of adsorbent is temporary, since under certain conditions the original bed capacity can be restored by burning off this deposit. Oxidative regeneration is a block operation that sequentially burns out five adsorbent beds and is necessary to maintain the working capacity of the molecular sieve. The basics of this procedure are 3
A day outage consists of the following stages: Stage: Time, time (1) Preparation of the system for regeneration 2 (2) Preparation of the system for oxidative regeneration 68 (3) Preparation of the system for the adsorption step 2 Total time 72 Beds When cycling reaches a point requiring oxidative regeneration, the regular process cycle is stopped and
The bed then undergoes an additional long desorption purge to remove as much residual normal paraffin as possible. Countercurrent circulation of nitrogen is performed by purge gas compressor 60.
Established at 100 psia (7 Kg/cm ² A) and 750ⓓ (399°C). Thermal nitrogen circulation has two purposes: to flush the purge gas from the bed if it is flammable (i.e., fuel gas, hydrogen, etc.), and to flush the purge gas from the bed prior to introducing oxygen into the system. This is to raise the temperature of coke to a temperature higher than the ignition point of coke. The effluent gas from the bed manifold 76 is cooled to condense the desorbed hydrocarbons and water. Once the bed has reached temperature, air is introduced into the circulation system at such a rate that the oxygen content of the gas entering the bed is between 0 and 1% by volume. The oxygen in the gas is combusted by the coke at the top of the bed. The heat released by combustion is carried out of the combustion zone as a preheating front that moves ahead of the combustion front. This preheating front end keeps the floor temperature at about 950〓
(510℃). This temperature is controlled by adjusting the amount of oxygen in the incoming gas. Internal pellet temperature
If the temperature exceeds 1300℃ (704℃), the molecular sieve crystal will be permanently destroyed. Therefore, the gas phase temperature is maintained at a maximum of 1000㎓ (538℃). As the combustion front passes through the bed, the temperature decreases to the gas inlet temperature of 750
The temperature drops to 〓(399℃) again. Since coke deposits contain hydrogen, they produce water in addition to carbon oxides during combustion. Molecular sieves can be permanently destroyed if repeatedly exposed to high temperature water.
This water must be removed from the system. A cooling unit is used to remove most of the water to minimize this destruction. Figure 2 shows a simplified process diagram of the prior art.
A primarily hexane feedstock is processed to produce a high purity normal hexane product and the product is recovered from the cycle gas using conventional techniques to cool the stream in lieu of the "washing" of the present invention. Table A shows the material balance around the normal product recovery section of the unit. In the conventional prior art unit associated with Figure 2,
The purity of the normal product is controlled in two ways.
First, the level of cooling is maintained for the recycled vent gas (Stream 1, Figure 2) to minimize the concentration of non-normal hydrocarbons before the stream enters the normal product recovery section. The second method is to purge the adsorber that has just completed the adsorption process countercurrently with hydrogen (Step D-1, Stream 6, Figure 2) to remove non-normal feedstock from the void space within the vessel. . The non-normals remaining in the void space after this step enter into the normal product during the desorption step. All hydrogen leaving the bottom of the adsorber during step D-1 enters the adsorber of the hydrocarbon feed step (A-2) and ends up as recycled vent gas (Stream 1, Figure 2). The material balance in Table A shows that for the conventional unit of FIG. 2, the C ₅ ⁺ normal hexane product purity is 96.3 mole percent. Of the 2.00 mol/hr non-normal in the normal product, 1.41 mol/hr (47%) of these originates from the last void space of the D-1 step, and 1.56 mol/hr (52%) comes from the recycled vent gas. It can be calculated from the mass balance that 0.02 mol/h (1%) comes from hydrogen make-up. Increasing the purity of the normal product beyond 96.3 mole percent for the prior art process diagram of FIG. 2 is expensive and impractical. Reducing the temperature of the product recovery section is expensive from an investment, utility and maintenance standpoint and requires drying of the feedstock as a pre-treatment step. Increasing the hydrogen feed during the D-1 purge step does not work.
Near the beginning of the D-1 step and as the step continues, most of the non-normal is removed from the vessel and the amount of non-normal removed decreases. The more this D-1 purge rate is increased, the more purge gas will exit the bottom of the vessel in the D-1 step and ultimately carry the non-normal through the circulating vent gas and into the normal product recovery section. If the amount of non-normal removed per mole of purge gas during step D-1 is less than the amount of non-normal carried back to the normal product recovery section by the same mole of gas, the amount of non-normal removed per mole of purge gas is Increasing the flow rate reduces the purity of the normal product instead of increasing it. However, in accordance with the present invention, the purity of the normal product is increased by adding normal product washing to the prior art unit process diagram, increasing the adsorbent inventory by only a small amount (2%) and compaction. The horsepower of the machine can be increased by about 5%. FIG. 3 shows the same prior art processing unit as in FIG. 2 with the addition of a normal product wash according to the present invention, which unit is used to increase the purity of the C ₅ ⁺ normal hexane product from 96.3 mol % to 97.6 mol %. increase to In FIG. 3, according to the present invention, a small amount (approximately
5.5%) of normal product. The scrubbed recycle vent gas leaving the top of the scrubber column carries significantly less non-normals back to the normal product recovery section than in the prior art system (FIG. 2). The circulating wash normals (stream 3, Figure 3) return to the feed point and carry with them the recovered non-normals. The wash vessel in this configuration was simulated with one theoretical plate. By increasing the number of theoretical plates to 2 or 3,
The amount of wash normal required to obtain the same increase in product purity is reduced. Table B shows the material balance for the normal product wash and normal product recovery section of FIG. 3 in accordance with the present invention. C ₅ ⁺ normal product (stream 8, Figure 3)
Of the non-normal 1.72 mol/hour, these
1.28 mol/hr (74%) comes from the void space at the end of step D-1, 0.42 mol/hr (25%) comes from the scrubbed recycled vent gas, and 0.02 mol/hr (1%) comes from the hydrogen make-up. This can be calculated from the material balance. The mass balance data described above showed that the contribution to the amount of total _C5 ⁺ impurities in the recycled vent gas non-normal product was reduced to 27% of the amount found in the product without normal product washing. Show that. The added benefit of normal product cleaning is that
The D-1 step feed rate can be increased to higher amounts without negatively impacting normal product purity. Also, instead of providing higher normal product purity, the product cleaning embodiment of the present invention can be used to reduce unit investment and utility with the same product purity (96.3 mole %). You can also do it. By reducing the concentration of recycled vent gas non-normals, the amount of non-normals remaining in the vessel at the end of the D-1 step is approximately doubled, thereby requiring less D-1 step purge. Reducing the D-1 purge rate reduces the amount of normal recycled to the adsorption feed point (Stream 6, Figure 2) and thus reduces the amount of adsorbent required. in this case,
The reduction in adsorbent amount amounts to approximately 10%. The invention can also be applied to what is known as the TIP process described in US Pat. No. 4,210,771. The TIP process is essentially a process that improves the octane rating of a given petroleum fraction by substantially completely isomerizing normal paraffin hydrocarbons contained in a mixed normal and non-normal hydrocarbon feedstream. is a process in which normal pentane and normal hexane contained in a feed stream containing normal pentane, normal hexane, and non-normal hydrocarbons are substantially completely isomerized to produce branched chain iso/pentane and iso/hexane. be. Essentially, the TIP process involves passing a stream containing a mixture of normal and non-normal hydrocarbons through an isomerization reactor, in which a catalyst composition, preferably a zeolite molecular sieve with a hydrogenation component, is combined in the presence of hydrogen in the reactor. It consists of contacting and catalytically isomerizing at least a part of the normal. Other catalyst compositions may be used as well, such as alumina-based catalysts. The temperature of the reactor depends in part on the specific catalyst used, but is preferably between 200° and
within the range of 300℃, and the pressure inside the reactor is 175~175℃ when using a molecular sieve catalyst.
It ranges from 600 psia (12 to 42 Kg/cm ² A). The effluent from the reactor still contains approximately 20-30% normal by weight. The hydrocarbon fraction of the reactor effluent stream is recycled to a zeolite molecular sieve adsorbent bed where the normals are selectively adsorbed and the non-normals are transferred to the adsorbent effluent and ultimately to the isomerate product. Let the adsorbent come out as. Normal is then desorbed from the bed using a hydrogen purge stream. FIG. 4 shows an arrangement for implementing the TIP process with cleaning of the present invention. With reference to FIG. 4, an isomerization reactor, e.g.
The item described in No. 4210771 was given to 300 and pipe 11
Normal paraffin and recycled hydrogen gas are received via step 2 along with raw materials such as mixed pentane and hexane. The isomerized product and the cycle gas, which in this case contains residual normals, are transferred to separator 110, from where the isomerized product is transferred via line 301 to the adsorption bed, as referred to herein in FIG. and processed as described above. The product in this case is non-normal and is recovered at 56. Separator 11
The recycled purge gas from 0 contains residual normals which, if not removed, will reduce the purity of the non-normals recovered at 56. Therefore, in accordance with the present invention, the circulating purge gas from the separator 110 is supplied via line 302 to the washing vessel 200, such as a trayed or packed column, and the circulating purge gas is passed through line 30.
4, a small portion of the non-normal product is introduced into the washing container 200 for washing. In the wash vessel 200, the residual normal in the circulating purge gas is replaced with essentially non-normal products, and the circulating purge gas with reduced normal content is returned to the circulating compressor 60 via line 303. Cleaning container 20 from pipe line 306
The stream exiting 0 contains the "wash" non-normals and the normals removed from the recycle purge gas, and this stream is recycled to the adsorption bed along with the normals from the accumulator 114. Figure 5 is designed to process a pentane/hexane feedstock to produce one high purity non-normal product and recover hydrocarbons from circulating hydrogen using conventional refrigeration in place of the washing of the present invention. A simplified process diagram of the prior art total isomerization process (TIP) is shown. Normal paraffin in the circulating hydrogen in the TIP process contributes to the ineffectiveness and product impurities in the TIP process where the product is non-normal paraffin. Normal paraffin is adsorbed/desorbed from a molecular sieve adsorbent by swinging the partial pressure applied to the normal paraffin adsorbent. Circulating hydrogen (stream 2, 5th
The higher the concentration of normal paraffin in Figure 1, the lower the swing in this partial pressure, thereby allowing a greater amount of adsorbent to be used to separate normal from non-normal. During the desorption process, normal paraffin is loaded onto the top of the adsorbent bed in equilibrium with the concentration of normal paraffin in the recycle gas. During the next adsorption step, this normal residual concentration is
Set a limit to how much the normal concentration of raw materials can be reduced. For this purpose, adsorption (A-
2) The normal paraffin concentration of the adsorption effluent (fluid 3, FIG. 5) leaving the top of the vessel during the process is always higher than the concentration of circulating hydrogen. Valve leakage and normal front end leakage also contribute some impurities to the adsorbent effluent, in this case representing about 50% of the design impurities. It can therefore be seen that in order to maintain a high purity of the adsorption effluent (or ultimately of the non-normal product), the normal paraffin content of the recycled hydrogen must be maintained at a low level. A conventional method to maintain low levels of normal in the circulating hydrogen is to drain the isomerization reactor effluent (Stream 1, Figure 5).
60〓 (16℃).
The normal paraffin content of the recycled hydrogen and C ₅ ⁺ non-normal products obtained after cooling to 60° C. (16° C.) can be found in columns 1 and 2 of Table C. Cooling severity during product recovery to 60〓 (16℃)
By mitigating from 85°C to 29°C, the cooling package associated with conventional TIP units can be eliminated, thus saving cooling package investment and operating utilities. However, there are disadvantages to doing this that offset the savings mentioned above. As can be seen in columns 2 and 3 of Table C, the nC ₅ + nC ₆ content of the circulating hydrogen increases from 1.0 to 1.5 mol% compared to the case of 60ⓓ (16°C), and the C ₅ ⁺ product Normal content from 2.4 mol% to 3.4
increased to mol%. In addition, the higher the normal concentration in the circulating hydrogen, the lower the delta load of normal paraffin in the adsorbent, which reduces the adsorbent to about 10
% more required. This increase in adsorbent, in turn, also requires an approximately 10% higher circulating hydrogen rate. In addition to the above disadvantages, the quality of the product was reduced. This can be seen in the decrease in product RON-0 from 88.5 to 88.1. The product recovery temperature was changed from 60〓(16℃) to 85〓(29℃).
The disadvantages of increasing the temperature to 0.degree. C.) can be reduced by using non-normal product washing in accordance with the present invention as illustrated in FIG. FIG. 6 shows a TIP unit that uses non-normal product scrubbing in accordance with the present invention to improve normal recovery from recycle gas. In the arrangement of FIG. 6, the recycled hydrogen leaving the isomerization product recovery section enters the bottom of the trayed or packed column. In the column, the recycled hydrogen is washed countercurrently with approximately 11% non-normal product. The circulating washed non-normal containing the normal removed from the circulating hydrogen is circulated to the adsorber raw material and reprocessed in the adsorption section. The washed recycled hydrogen exits the top of the vessel and is recycled to the adsorption section. Table D shows the material balance for a non-normal product wash also using 85° (29°C) cooling in the product recovery section of the unit. This mass balance was simulated using 5 theoretical plates in the wash column. From the washed cycle hydrogen composition and the C ₅ ⁺ non-normal product composition (streams 5 and 6) in Table C, it can be seen that the efficiency of the washing process according to the invention in the product recovery section is (60
It can be seen that the efficiency approaches that of chilling (to 16℃). Non-normal washing of the circulating hydrogen reduced the normal content of both circulating hydrogen and non-normal products to low levels previously experienced with chillers. Circulating wash non-normal flow increases the amount of adsorbent and increases the circulating hydrogen demand by about 10%, and also increases fuel costs proportionately. For this reason, care must be taken to optimize the cleaning non-normal speed during design. However, by eliminating the cooling package, the total investment cost of the unit is reduced by about 6% compared to conventional arrangements, and the compressor utility is reduced by about 30%.
% reduced.

【table】

【table】

【table】

【table】

【table】

【table】 [Brief explanation of drawings]

FIG. 1 is a process diagram illustrating the present invention. Second
The figure is a simplified process diagram showing the prior art. FIG. 3 is a simplified process diagram in which the cleaning device according to the present invention is added to the prior art. FIG. 4 is a process diagram showing the TIP process including cleaning according to the present invention. FIG. 5 is a simplified process diagram showing a prior art TIP unit.
FIG. 6 is a simplified process diagram showing a TIP unit using cleaning according to the present invention. 12,56,114...Accumulator, 1
8,74...heat exchanger, 20,80...heater,
34,36-42...floor, 54,110...separator.

Claims (1)

[Scope of Claims] 1. When separating normal paraffin and non-normal paraffin from a gaseous mixture containing normal paraffin and non-normal paraffin, (i) the mixture is separated by a zeolite that selectively adsorbs normal paraffin; (ii) using a non-sorbing purge gas; (a) desorbing the normals from each bed and using the desorbed normals as the purge gas; (b) transferring unadsorbed non-normal paraffins from each bed to produce a separate effluent containing non-normal paraffins in admixture with the purge gas; and (iii) above (ii)(a). ) condensing the normal from the effluent of (ii)(b) to obtain the normal product and circulating a purge gas containing the normal residue; (iv) separating the non-normal from the effluent of (ii)(b) above to obtain the normal product; In a method of obtaining product and circulating a purge gas containing non-normal residues, not selecting a purge gas containing residual gas from the selected effluent of (ii)(a) or (ii)(b). A process as described above, characterized in that a portion of the product from the effluent is recycled after washing to reduce the residue content of the recycled purge gas.