MXPA97005507A - Separation process of isoalcanos / n-alcanos poradsorcion in gaseous phase, using a pressure and four adsorbedoamodulation - Google Patents

Abstract

The present invention relates to an adsorption process and more particularly a vapor phase adsorption process, which puts into operation the separation mechanisms based on the phenomena of exclusion or difference of adsorption.

Description

-IN GASEOUS PHASE, USING A PRESSURE MODULATION AND FOUR ANNOUNCERS " The present invention relates to an adsorption process and more particularly to a vapor phase adsorption process that brings into play the separation mechanisms based on the phenomena of exclusion or difference of adsorption. This refers more particularly to a separation process that uses judiciously the sequence linkages in a PSA, in order to diminish the harmful role of the area of matter transfer. The present invention is applied to the separation by adsorption on zeolitic sieve of a mixture of linear paraffins, a mixture of non-linear paraffins more particularly the separation of isoparaffins and normal paraffins, from a charge of hydrocarbons containing them. More particularly still, the hydrocarbon charge to be treated comes from an isomerization operation of a hydrocarbon cut of 5 carbon atoms / 6 carbon atoms (C5 / C6). The PSA (abbreviation for "Pressure Swing Adsorption") is an adsorption process in which a gas mixture is put in contact with a fixed bed of adsorbent at a high pressure, in order to from REF: 25016 remove certain constituents called "adsorbable" from the mixture. Likewise, if the desorption can be carried out by different means, the common characteristic of the PSA family is to effect the regeneration of the bed by means of depressurization and, in certain cases, by low pressure sweeping. These PSAs have reported numerous successes in the domain of natural gas, in the separation of air compounds, in the production of solvents and in the different sectors of refining. A review of the stages of important PSA development was given in 1984 in an article of the "AIChE Symposium Series" entitled "Twenty Five Years Progress in Adiabatic Adsorption Process" by R. T. Cassidy and E. S. Holmes. The PSA separation processes provide an economical and effective means for separating the constituents of a gas containing at least two compounds having different adsorption characteristics. The adsorbable constituent can be an impurity to be removed from the less adsorbable constituent, the latter being then recovered as a product. The product is the most adsorbable that is desired and must be separated from the less adsorbable constituent. For example, it may be desirable to remove carbon monoxide and light compounds from a hydrogen-rich flow to produce hydrogen at a purity greater than 99%, usable in hydrocracking (hydrodisintegration) or any other catalytic process, in the which catalyst is sensitive to impurities. On the other hand, this may be the most adsorbable product that is useful to recover, such as, for example, the ethylene from a filler to obtain a product rich in ethylene. In the more classical cases of PSA a gaseous charge of multiple components is introduced into at least one of the adsorbing beds at a high effective pressure, to adsorb at least one of the constituents (the adsorbed fraction), while the other constituents (the non-adsorbed fraction) pass through the bed without being retained. At a predefined time, the flow of charge to this adsorber is interrupted and the bed is depressurized by putting into operation one or more stages of co-current depressurization (for the direction of the spill or exit, it will always be a reference in the present description to that of the adsorption), until reaching a predefined pressure level. This allows the less adsorbable compounds or those remaining in the adsorption zone (interstitial volumes) to be evacuated without being mixed to the more adsorbable constituent. The gas recovered during these depressurization steps is generally used to perform the lower pressure equalization or scavenging steps. In the prior art and in the present description the term "pressure equalization" is used to describe the connection of a high pressure adsorber with a low pressure adsorber, carried out until the same pressure level is reached in the two adsorbers. The bed is then depressurized counter-current, and then swept to desorb the compound of the most strongly adsorbed charge, and to evacuate this gas from the bed before the pressurization steps after adsorption. Such processes are described, for example, in US-A-3 430 418 or in the more general work of RT Yang "Gas Separation by Adsorption Processes", Boston, Butterworth (1987), in which the cycles based on the use of several beds are described in detail. In general, and as described in the mentioned works, the processes of the PSA type are put into operation sequentially and through the alternative use of all the adsorption beds. The selectivities that are used in PSA processes are based on three basic mechanisms: diffusional selectivity, shape selectivity (or exclusion) and energy selectivity. In the case of difunsional selectivity, the driving force for the separation is the difference in speed of adsorption, desorption and diffusion of the different compounds in the gas mixture to be separated. In the case of shape (or geometric) selectivity, the separation is made based on the size of the different molecules to be separated, in relation to those of the micropores of the adsorbent. The thicker molecules do not penetrate the porosity of the adsorbent, while the smaller ones adsorb: it is said that the sieving effect and the pore size of the adsorber determine which compounds are adsorbed and those that do not adsorb. For what concerns the energy selectivity, it is the affinity of the adsorbent or the relative adsorption forces for one or other of the compounds, which control the separation. The less strongly adsorbed compound becomes the non-adsorbable fraction and the stronger adsorbed becomes the adsorbable fraction. In the case of processes based on diffusional or geometric selectivity, the speed of transfer of matter has an importance on the efficiency and size of the beds. Taking into account this phenomenon, there is a profile of concentration of the material adsorbed in the bed as the adsorption front progresses in it.

The term "the area of matter transfer" (abbreviated to English "MTZ") is known to the person skilled in the art and refers to the region containing this profile. In this region, the concentration of adsorbed material is zero at one end (towards the outlet of the bed) and equal to that at the load on the other (toward the entrance of the load). In order to produce a flow of high pressure and high purity, the adsorption cycle of PSA is determined in such a way that this material transfer zone does not leave the bed. It turns out that the adsorbent in the MTZ is not saturated up to its equilibrium capacity, and thus has a loss of efficiency of the system. Numerous works have dealt with the reduction of the matter transfer zone through the action on the hydrodynamic parameters of the system. From the reduction of this area of transfer of matter and thus of the unused part of the bed, a reduction of the inventory on the adsorbent, and thus of the size of the beds, results. An object of the invention is to provide a means to decrease the size of the bed, or conversely to increase the dynamic capacity of the adsorbent, in the case of classical PSA processes that use a geometric type selectivity. This increase in capacity is realized without (or at least) an increase in investment or operating costs. It has been discovered in effect that the amount of sieve to be put into operation, for a given separation, may be diminished by a judicious arrangement of the sequence. This increase in operations can be analyzed in different ways: either in terms of the increase in dynamic capacity, or in terms of the increase in WH (volume velocity per hour) for fixed operation. This is due to the fact that, taking into account the arrangement of the sequences suffered by the different adsorbers, which will be described below, the bed at the end of the adsorption is completely saturated by the adsorbable compound (s) ( s), that is to say that the area of matter transfer is the output of the adsorber before it is regenerated. The process of the present invention can be applied to obtain a mixture rich in isoparaffins, which leads to a product with a high octane number, from a mixture that also contains normal paraffins. The process of the invention is described below in relation to Table 1 below, and Figures 1 to 12, which illustrate respectively the sequences from 1 to 12. The invention thus provides a process for separating n-paraffins and of isoparaffins from a mixture that contains them, by gas phase adsorption of the n-paraffins, using this process uses four adsorbers, which will be numbered in the order of 1 to 4, which have substantially the same size and functioning according to at least one cycle of stages which will be described below with reference to the adsorber 2 of figures 1 to 12. The treated mixture comes more particularly from an isomerization of C5 / C6 hydrocarbons.

TABLE 1 In the following description, the terms "bed" or "adsorber" will be used interchangeably to designate the adsorption beds, even when the bed under consideration is not in the adsorption phase. The characteristic cycle of the process of the invention can be defined as follows. a) Sequence 1: In an adsorption stage (F), a charge made up of the effluent of the adsorber preceding it (adsorber 1) is circulated in upstream flow over the adsorber 2. For this, the output of the adsorber 1, in the adsorption phase, is connected in series with the lower part of the adsorber 2, by line 1. The fluid circulates in upward current in the two adsorbers. The product rich in isoparaffins is recovered in the upper part of adsorber 2 by line 2. b) Sequence 2: In an adsorption stage (ADS), the load is directly injected upstream to the lower part of adsorber 2 by line 3. The product rich in isoparaffins is recovered in the upper part of adsorber 2 by the line 2 c) Sequence 3: The adsorption operation (ADS) is continued and, on line 4, a portion of the isoparaffin-rich exit fluid is taken on line 2 to carry out the second pressurization stage RP2 of the adsorber. next 3 in updraft. d) Sequence 4: The adsorption stage (A / F) is continued and connected, by line 4, the high end of adsorber 2 to the lower part of the following adsorber (adsorber 3), previously in the second pressurization phase . In the course of this step, the adsorber 2 receiving the charge on line 3 is saturated with n-paraffin and the material transfer zone moves towards the next adsorber 3, connected in series. e) Sequence 5: A first depressurization (DPI) is carried out by connecting at the bottom, via line 5, the adsorber 2 operating at high pressure, with the adsorber 4 that has finished its sweep stage at a weaker pressure. In this way, an equilibrium of the pressure in the adsorbers 2 and 4 is obtained. The adsorbent in the adsorber 2 is thus totally saturated to the equilibrium of the charge. f) Sequence 6: A second depressurization is carried out (DP2), the upper end of the adsorber 2 being closed; If the charge treated by the process comes from an isomerization unit, for example from C5 / C6 hydrocarbons, the lower part of the adsorber 2 can be connected by line 6, with the recycling circuit towards isomerization, maintained at low pressure . g) Sequence 7: In a first scanning phase (SI), the lower part of the preceding adsorber 1 is connected to the upper part of the adsorber 2 by line 8, the desorbent is injected through line 7 in the head of the adsorber 1 , at the end of the sweep (S2); the effluent that comes out is then relatively poor in the adsorbable fraction. The sweep phase (SI) generally takes place at a pressure of less than 5 bars and preferably less than 3 bars absolute, and is carried out against the current in relation to the adsorption lid. The product exiting on line 6 of adsorber 2 essentially consists of a fraction rich in n-paraffins. If the charge treated by the process comes from an isomerization unit, for example from C5 / C6 hydrocarbons, this effluent can be recycled to the isomerization unit. h) Sequences 8 and 9: In the main sweep stage (STR), only the adsorber 2 is fed by the desorbent in spill or descending outlet, by line 9. In this case also, the effluent that exits through the outlet can be recycled. line 6 to the isomerization unit. i) Sequence 10: In the step of finishing the sweep (S2), the desorbent continues to be fed to the adsorber 2, by line 9, but the output of the adsorber 2 is connected to the following adsorber 3, which initiates the desorption phase ( SI). j) Sequence 11: Pressurization (RPl) of the low pressure is performed at an intermediate pressure, connecting the adsorbers 2 and 4 at the bottom, by line 11. k) Sequence 12: Finally, a second pressurization (RP2) is carried out up to the adsorption pressure by means of a flow that comes from line 12 to part of the tributary of the preceding adsorber 1, which operates in an adsorption stage.

(ADS). The fluid is introduced in the lower part of the adsorber 2 to avoid contaminating the bed if this fluid is not perfectly free of adsorbable products. In summary, the n-paraffins are adsorbed at a high pressure (stages F and ADS) and de-pressured at a weaker pressure, putting into play a pressure abatement effect, combined with a "sweep" elimination effect with the help of a gaseous flow rich in isoparaffins. The desorbent may contain a proportion of n-paraffins comprised between 0 and 20%. The adsorbent bed is generally made up of a molecular sieve based on zeolite capable of selectively adsorbing n-paraffins and having an apparent pore diameter close to 5 A (Angstroms). Zeolite 5A is suitable for this use: its pore diameter is close to 5 Á (Angstroms), and its adsorption capacity for n-paraffins is important. However, two adsorbents, such as cabacita or erionite, can be used. Preferred operating conditions are temperatures of 100 to 400 ° C and more preferably 200 to 300 ° C, and an adsorption pressure of 5 to 40 bar and more preferably 15 to 25 bar. The adsorption cycle generally lasts from 2 to 15 minutes, and preferably from 4 to 6 minutes. The following example 1, not limiting, illustrates the invention. Example 2 is given comparatively.

Example 1 The process of the invention is put into operation in a unit that includes four identical adsorbers that perform the cycle described above. The adsorbers are cylinders of 0.053 m in internal diameter and 4.77 m in height, each containing 8.05 kg of 5 A sieve. The charge and the desorbent are introduced at a temperature maintained at 215 ° C.

The charge is constituted by a light naphtha and comes from the isomerization of an oil cut of 5 to 6 carbon atoms (C5 / C6) having the following mass composition: Constituent% mass Isobutane (iC4) 1.39 Normal butane (nC4) 1.02 Isopentane (iC5) 27.99 Normal pentane (nC5) 11.2 2,2-dimethylbutane (22 DMB) 11.3 2, 3-dimethylbutane (23 DMB) 4.8 2-methylpentane (2 MC5) 14.6 3-methylpentane (3 MC5) 8.7 Normal hexane (nC6) 6.1 Cyclopentane (CC5) 2.0 Methylcyclopentane (MC5) 5.7 Cyclohexane (CC6) 5.2 TOTAL 100.00 The charge and the desorbent feed the separation unit under the expenditure control and the effluents are collected under pressure control.

The charge charge is 19.65 kg / h, its octane number (RON) is 81.6.

The cost of the desorbent is equal to 6.74 kg / h; its mass composition is shown immediately: Constituent% mass Isobutane (iC4) 4.25 Normal butane (nC4) 3.39 Isopentane (iC5) 92.26 Normal pentane (nC5) 0.1 TOTAL 100.00 The adsorption pressure is between 17.7 and 17.1 absolute bars, while the adsorption pressure is 2.6 bar absolute. The duration in seconds of the different sequences of the cycle is indicated by a quarter of the cycle of the following table: Sequence 1 2 3 bed 1 A / F DPI DP2 bed 2 F ADS ADS bed 3 S2 RPl RP2 bed 4 IF STR SRT duration (in seconds) 60 40 100 The overall duration of the adsorption phase for the same bed is equal to sum of the F and ADS sequences, or here 200 seconds. This sum also corresponds to the total duration of the desorption for a given adsorber, sum of the STR and SI sequences.

It can be noted that, due to the fact that the desorbent always feeds an adsorber, it is not necessary to provide a "deviation" gate of the unit (as in the case of Comparative Example 2 presented below).

The balances of the unit are carried out in the cyclical state of convergence, that is to say that the two successive balances lead to the same result.

The effluent rich in isoparaffins is designated by the "IPSORBAT". This has an average mass composition accumulated in a given hour of operation as described below: Constituent% mass Isobutane (iC4) 1.32 Normal butane (nC4) 1.81 Isopentane (iC5) 36.49 Normal pentane (nC5) 1.13 2,2-dimethylbutane (22 DMB) 12.67 2, 3-dimethylbutane (23 DMB) 5.38 2-methylpentane (2 MC5) 16.38 3-methylpentane (3 MC5) 9.77 Normal hexane (nC6) 0.59 Cyclopentane (CC5) 2.24 Methylcyclopentane (MC5) 6.39 Cyclohexane (CC6) 5.83 TOTAL 100.00 The "IPSORBAT" expense is 14.17 kg / h; This product has an octane number (RON) of 88.0.

Example 2 (comparative) The difference with the preceding example lies in the use of 3 adsorbers instead of 4, and .. a unit operates according to a known cycle, described later in relation to table 2 and figures 13 to 21, which illustrate respectively the sequences from 1 to 9.

TABLE 2 NJ The next cycle for each adsorber is made according to nine sequences. These sequences are identical for the three adsorbers, but these are out of phase for a duration equal to one third of the overall duration of the cycle. For example, bed 1 will make the following sequences: - a sequence 1 in which an adsorption stage (ADS) is carried out at high pressure, introducing the load in the lower part of bed 1 by line 20, and passing it upstream through the bed, leaving the product in the upper part by line 21; - two sequences 2 and 3 in which the adsorption stage (ADS) is continued in updraft, and is recovered on the effluent leaving the adsorber 1 by line 21, a part that is sent by line 22 to the adsorber next 2, to effect the second pressurization (RP2); - a sequence 4 in which a pressure equilibrium stage (DPI) is carried out connecting by its upper ends through line 23, the adsorber 1 with the adsorber 3, at the end of the sweep (STR) to initiate the repressurization; - a sequence 5 in which a second depressurization (DP2) is performed, connected to the lower part of the adsorber 1 with the recycling circuit towards the isomerization via the line 24; - a sequence 6 in which a sweep stage (STR) is carried out by introducing the desorber upstream on bed 1 through line 25, the effluent containing the adsorbed n-paraffins being recycled via line 24 to the isomerization unit; - a sequence 7 in which a pressurization stage (RPl) is carried out by means of the connection of the bed 1 with the bed 2 by the line 26; Y - two sequences 8 and 9 in which a second pressurization stage (RP2) is carried out joining the bed 1 to the bed 3 with the bed 3, which then functions in an adsorption stage (ADS).

Each adsorber has an internal diameter of 0.053 m and a height of 6.36 m and contains 10.7 kg of 5A sieve. The same total sieve mass is therefore used. The compositions as well as the loading and desorbent charges are the same as in Example 1 according to the invention. This is the same for your entry temperatures.

The adsorption pressures are between 17.7 and 17.4 absolute bars and the desorption pressure is 2.6 bar absolute.

The duration of the different sequences of the cycle are indicated in Table 2, with a cycle in 600 seconds. It will be noted that the times of the adsorption and pressure equilibrium sequences (DP1 / RP1) are the same as in Example 1 according to the invention. However, it is necessary to foresee a "deviation" of the unit during the sequence where the desorbent is not used (DP1 / DP2).

The effluent collected in the process, rich in isoparaffins, has a composition in average mass accumulated over one hour of operation, which occurs immediately: Constituent% mass Isobutane (iC4) 1.78 Normal butane (nC4) 1.31 Isopentane (iC5) 36.08 Normal pentane (nC5) 2.45 2,2-dimethylbutane (22 DMB) 12.33 2, 3-dimethylbutane (23 DMB) 5 24 2-methylpentane ( 2 MC5) 15 93 3-methylpentane (3 MC5) 9 50 Normal hexane (nC6) 1.31 Cyclopentane (CC5) 2.18 Methylcyclopentane (MC5) 6.22 Cyclohexane (CC6) 5.67 TOTAL 100.00 The "IPSORBAT" expense is 13.77 kg / h; and it has an octane number (RON) of 87.3.

The comparison of the results of example 1 according to the invention in comparative example 2, shows that the octane gain is more important if one proceeds according to the invention, then, in the latter case, an octane number is obtained of 88.0.

It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention. Having described the invention as above, property is claimed as contained in the following:

Claims (11)

1. A process for the separation by gas-phase adsorption of n-paraffins and disoparaffins from a hydrocarbon charge that comes from an isomerization stage, the process is characterized in that it uses 4 adsorbers, numbered from 1 to 4, which has substantially each operating according to at least one cycle of steps which, described with reference to the second adsorber, comprises: a) a sequence in which an adsorption stage (F) is carried out, circulating in updraft over the second adsorber a charge consisting of effluent from the first adsorber that precedes it, connecting in series the output of the first adsorber, in the adsorption phase, with the lower part of the second adsorber; b) a second sequence in which an adsorption stage (ADS) is carried out, the load being directly injected upstream of the inside of the second adsorber and a product rich in isoparaffins is recovered in the upper part of the second adsorber; c) a third sequence in which the adsorption operation (ADS) is continued and a part of the isoparaffin-rich exit fluid is taken to perform the second pressurization stage RP2 of the next, following upstream adsorber; d) a fourth sequence in which the adsorption stage (A / F) is continued, with the second end of the second adsorber being connected to the lower part of the next adsorber, previously in the second pressurization phase, with the second adsorber receiving the charge that it is then saturated in n-paraffins and the area of material transfer that moves towards the third adsorber next, connected in series; e) a fifth sequence in which a first depressurization (DPI) is made by connecting the lower adsorber, operating at high pressure, with the fourth adsorber that has completed its sweep stage at a weaker pressure, thus obtaining a balance of the pressure in the second and fourth adsorbers; f) a sixth sequence in which a second depressurization is performed (DP2), the upper end of the second adsorber is closed; g) a seventh sequence in which a first scanning phase (SI) is carried out by connecting in series in the lower part of the first adsorber preceding the upper part of the second adsorber, injecting the desorbent in the upper part of the first adsorber at the end of swept (S2), with the effluent at the exit relatively depleted in the adsorbable fraction, the output product of the second adsorber consists essentially of a fraction rich in n-parafams; h) two eighth or ninth sequences in which the main sweep stage (STR) is carried out by feeding only the second adsorber by the desorbent in downward spillage; i) a tenth sequence in which a step of finishing the scan (S2) is carried out, continuing to feed the second adsorber with desorbent, and connecting the output of said second adsorber to the next third adsorber, which initiates the desorption phase (SI); j) an eleventh sequence in which a pressurization (RPl) of the low pressure is performed, at an intermediate pressure connecting the second and fourth decorators at the bottom; and k) a twelfth sequence in which a second pressurization (RP2) is carried out up to the adsorption pressure by means of a flow coming from part of the effluent of the first adsorber, which operates in an adsorption stage (ADS), introducing the fluid in the lower part of the second adsorber.

2. The process according to claim 1, characterized in that in sequences 7 to 10, the sweep is carried out with the help of a gaseous flow rich in isoparaffins.

3. The process according to claim 2, characterized in that the desorbent contains a proportion of n-paraffins comprised between 0 and 20%.

4. The process according to any of claims 1 to 3, characterized in that the adsorbent bed is constituted by a molecular sieve based on zeolite, capable of selectively attaching the n-paraffins and having an apparent pore diameter close to 5 Á (Angstroms).

5. The process according to claim 4, characterized in that the adsorbent bed is constituted by 5A zeolite, cabacita or erionite.

6. The process according to any of claims 1 to 5, characterized in that the adsorption cycle is operated for 2 to 15 minutes at temperatures of 100 to 400 ° C and at an adsorption pressure of 5 to 40 bar.

7. The process according to claim 6, characterized in that at the temperatures of 200 to 300 ° C and at an adsorption pressure of 15 to 25 bar, the adsorption cycle is operated for 4 to 6 minutes.

8. The process according to any of claims 1 to 7, characterized in that in step (g) the sweeping phase is carried out a pressure lower than 5 bar absolute.

9. The process according to any of claims 1 to 7, characterized in that step (g) the sweeping phase is carried out at a pressure lower than 3 bar absolute.

10. The process according to any of claims 1 to 9, characterized in that the treated filler comes from an isomerization unit of 5 to 6 carbon atoms (C5 / C6).

11. The process according to claim 10, characterized in that the desorption effluent of the second adsorber is recycled to the isomerization unit of C5 / C6, by connecting the base of the second adsorber with the recycle circuit to the isomerization, maintained at low pressure, in the sixth, seventh, eighth and ninth sequences.