Patent Application
20060115421
The invention relates to a process for adsorption separation using, as the adsorbent mass, a solid containing a zeolite with a particular structure, analogous to that of IM-12.
Adsorption separation currently constitutes the technology of choice when technologies based on liquid-vapour equilibrium such as distillation cannot separate different species of a mixture.
Adsorption separation is widely used to separate and purify gas and liquid in many fields, from the petroleum, petrochemicals and chemicals industries to environmental and pharmaceutical applications.
Typical industrial applications for adsorption separation are the production of industrial gas (oxygen, nitrogen, hydrogen), separation of hydrocarbons (linear and branched paraffins, xylenes, for example), air, water and effluent treatments to eliminate pollutants (sulphur-containing compounds, volatile organic compounds, etc), drying, separating chiral isomers, etc.
Adsorption separation processes are well known in the prior art.
A summary of the characteristics of that type of process can, for example, be found in volume B3 of Ullmann's Encyclopedia (p9-37 to 9-50) or in volume 4 of the “Handbook of porous solids”, Wiley & Sons.
Of all of the processes for adsorption separation, we may cite the process known as simulated counter current (SCC) described, for example, in United States patent U.S. Pat. No. 2,985,589 and French patent FR-A-2 681 066, the process known as pressure swing adsorption (PSA) described, for example, in U.S. Pat. No. 6,641,664, FR-A-2 655 980, FR-A-2 837 722 or FR-A-2 774 386 and the process known as thermal swing adsorption (TSA) described, for example, in U.S. Pat. No. 6,432,171 and European patent EP-A-1 226 860.
The principle of a process for adsorption separation resides in selective adsorption of one or more constituents on a microporous solid.
The adsorption solids may be of a number of types, for example zeolites or molecular sieves, silica gels, aluminas, activated charcoal.
All of those solids are characterized by a large specific surface area, of the order of 300 to 1200 m2/g. The zeolites are differentiated from other types of solid adsorbents in that they are microporous crystalline solids and adsorption takes place within the crystals. The term “microporous” means a pore size of less than 20 Å.
A large number of natural or synthetic zeolites exist and are recorded in the “Atlas of Zeolite Structure Types” by Ch Baerlocher, W M Meier and D H Olson, 5th edition, review, 2001, Elsevier, published by the International Zeolite Association (IZA).
They are distinguished by their composition and crystalline structure.
The crystalline structure describes a two-dimensional or three-dimensional network of channels and/or pores of a defined size which constitutes the microporous volume.
The size of the openings which control access to said pores is also an important parameter in adsorption separation.
Of the zeolites which have been synthesized over about the past forty years, some solids have resulted in significant advances in the adsorption fields. These include Y zeolite (U.S. Pat. No. 3,130,007) and ZSM-5 zeolite (U.S. Pat. No. 3,702,886).
Of recently synthesized zeolites, IM-12, which is described in the Applicant's patent application 03/11333, may be mentioned. In addition to a novel crystalline structure, solid crystalline IM-12 has a chemical composition, expressed as the anhydrous base, in terms of moles of oxides, defined by the following general formula: XO2:mYO2:pZ2O3:qR2/nO, in which R represents one or more cations with valency n, X represents one or more tetravalent elements other than germanium, Y represents germanium, and Z represents at least one trivalent element.
The letters m, p, q respectively represent the number of moles of YO2, Z2O3 and R2/nO, m being in the range 0 to 1, p being in the range 0 to 0.5 and q being in the range 0 to 0.7.
Said crystalline solid IM-12 has a novel topology with a two-dimensional system of interconnected channels comprising two types of straight channels defined by openings with 14 and 12 X and/or Y and/or Z atoms respectively, said atoms being 4-coordinate, i.e. surrounded by four oxygen atoms.
The term “pore diameter” is used as a functional definition of the size of a pore in terms of the size of molecule which can enter that pore. It does not define the actual dimension of the pore as it is usually difficult to determine since it is often irregular in shape (i.e. usually non-circular).
D W Breck provides a discussion of the effective pore diameter in his book entitled “Zeolite Molecular Sieves” (John Wiley & Sons, New York, 1974) on pages 633 to 641.
Since the cross sections of zeolite channels are rings of oxygen atoms, the pore size in zeolites may also be defined by the number of oxygen atoms forming the annular section of the rings, designated by the term “member rings”, MR.
As an example, the “Atlas of Zeolite Structure Types” by Ch Baerlocher, W M Meier and D H Olson, 5th edition, review, 2001, Elsevier, indicates that zeolites with structure type FAU have a network of 12 MR crystalline channels, i.e. with a section constituted by 12 oxygen atoms. The crystalline solid IM-12 has a two-dimensional network of interconnected channels comprising two types of straight channels defined by 14 and 12 MR openings. This definition is well known to the skilled person and will be used below.
Adsorption separation is based on selective adsorption (either thermodynamic or kinetic) of the various gaseous or liquid constituents constituting the feed due to specific interactions between the surface of the adsorbent solid and the adsorbed molecules.
For simplification, we shall henceforth use the term “adsorbent” to designate the solid adsorbent and “adsorbate molecule” or “adsorbate” to designate the adsorbed species.
Adsorption separations may be based on steric, kinetic or thermodynamic equilibrium effects.
When a steric effect is involved, only molecules with a critical diameter less than the pore diameter are adsorbed in the adsorbent.
The various species contained in the mixture are thus separated as a function of the molecular size of those species.
A typical example of that type of separation is the separation of linear and branched alkanes using 5A zeolite as illustrated in the Applicant's patents EP-A-0 820 972 and U.S. Pat. No. 6,353,144.
In addition to the steric effect, the mixtures of molecular species may be separated by a kinetic effect if one of the species is adsorbed much faster or more slowly than the other species contained in the mixture.
Whether the steric or the kinetic effect dominates depends on the size and distribution of the micropores.
If the critical diameter of a molecule of adsorbate is comparable with that of the pores of the adsorbent, a steric and kinetic effect may be produced as the smallest adsorbate molecules may adsorb more rapidly. Such an effect occurs, for example, when separating multibranched paraffins on a zeolitic adsorbent with a mixed structure with principal channels having a 10 MR opening and secondary channels having an opening with at least 12 MR, as illustrated in the Applicant's patent FR-A-2 813 310.
That patent describes a process for separating multibranched paraffins from a hydrocarbon feed containing hydrocarbons containing 5 to 8 carbon atoms per molecule, in particular linear, monobranched and multibranched paraffins, using a zeolite with structure type NES (for example NU-85 or NU-86 zeolite).
Adsorption separations based on thermodynamic equilibrium effects are based on preferential adsorption of one of the compounds with respect to other compounds contained in the mixture to be separated. In the case of said separations termed “thermodynamic” separations, the adsorbent has a pore diameter that is larger than the critical diameter of the molecules to be separated, in fact as large as possible, to facilitate macroporous diffusion of molecules. One example of that type of separation is the separation of para-xylene from a feed containing xylenes and ethylbenzene on faujasite type zeolites the prior art of which is illustrated in the Applicant's patent EP-A-0 531 191.
One essential characteristic of adsorption technology is its transitory and generally cyclic function since, after an adsorption phase, the adsorbent solid must be partially or completely regenerated for subsequent use, i.e. it must be freed of adsorbed species, generally using a desorption solvent or by reducing the pressure (PSA processes) or by a temperature effect (TSA processes).
This dynamic function results in a certain complexity of adsorption processes as regards equipment, process control, dimensions and optimization of the adsorption and desorption cycles.
Separation performances depend not only on thermodynamic properties, but also on kinetic and hydrodynamic properties. In particular, the adsorbent should have as large a pore volume as possible and a pre size that is suitable for the desired separation type.
The invention concerns a group of processes for adsorption separation using an adsorbent characterized in that it contains a solid with a crystalline structure analogous to that of solid IM-12 and having a chemical composition, expressed as the anhydrous base in terms of moles of oxide, by the formula XO2:mYO2:pZ2O3:qR2/nO, in which R represents one or more cations with valency n, X represents one or more tetravalent elements other than germanium, Y represents germanium and Z represents at least one trivalent element.
The mixture containing the molecular species to be separated may be any mixture of hydrocarbons, meaning that each species forming the mixture may contain any number of carbon atoms.
The molecular species to be separated from the hydrocarbon mixture does not have to be a hydrocarbon.
The invention is applicable in many and varied fields, from the petroleum, petrochemicals and chemical industries to environmental and pharmaceutical applications.
The process of the invention may be carried out in both the liquid and in the gas phases. The operating conditions for the separation unit depend on the yield and degree of purity of each of the desired streams. As an example, a cyclic PSA or TSA type process functions at temperatures and pressures which allow adsorption and desorption of the desired species. In general, the temperature is fixed at between about 0° C. and 400° C., and preferably between 50° C. and 250° C.
The pressure may be between about 0.01 MPa and about 15 MPa, preferably between about 0.05 MPa and about 5 MPa. Desorption is carried out in a number of manners, for example by reducing the pressure (PSA) or by increasing the temperature (TSA processes).
In the same manner, a simulated counter current process functions at a temperature which is usually fixed at between about 20° C. and 250° C., preferably between 60° C. and 210° C. The pressure is higher than the bubble pressure of the species to be separated, to maintain a liquid phase throughout the system. The volume ratio of the desorbant to the feed is generally in the range 0.5 to 30.
If one of the molecular species is an impurity, i.e. typically in a concentration of less than 1% by weight, and more particularly in the range 0.1% by weight to a few ppm by weight, the process of the invention may be reduced to passing the stream to be treated through one or more beds of adsorbents in a temperature range in the range −50° C. to 300° C., preferably in the range −50° C. to 100° C. The bed or beds may be regenerated using a purge gas which traverses the bed or beds in a temperature in the range from −50° C. to 300° C., preferably in the range −50° C. to 150° C. to desorb the impurity from the adsorbent.
The adsorbent will be adapted to the envisaged application. Thus, several parameters such as the ratio X/Ge, the ratio (X+Ge)/Z, the nature of the cation(s) R, will be adjusted to ensure optimal performance of the process. In the same manner, the form in which the adsorbent will be used (extrudates, powder, beads) will depend on the type of process used.
The invention provides a group of adsorption separation processes which will generically be termed a separation process, using an adsorbent characterized in that it contains a solid with a crystalline structure analogous to that of solid IM-12 and has a chemical composition, expressed as the anhydrous base in terms of moles of oxides, by the formula XO2:mYO2:pZ2O3:qR2/nO, in which R represents one or more cations with valency n, X represents one or more tetravalent elements other than germanium, Y represents germanium, and Z represents at least one trivalent element. French patent application no 03/11333 describes the zeolite IM-12 and its separation process.
Compared with the prior art, the process of the invention has the following advantages:
The crystalline structure of the crystalline solid IM-12 is a three-dimensional structure formed by tetrahedra. In particular, it comprises units of the double ring to four tetrahedral type. The peak of each tetrahedron is occupied by an oxygen atom. Solid crystalline IM-12 has a novel topology with a two-dimensional system of interconnected channels comprising two types of straight channels defined by openings with 14 and 12 X and/or Y and/or Z atoms respectively, said atoms being 4-coordinate, i.e. surrounded by four oxygen atoms.
The dimensions of said channels are respectively 9.5×7.1 Å for 14 MR channels and 8.5×5.1 Å for 12 MR channels.
The nitrogen adsorption isotherm at 77 K of the silicogermanate IM-12 shown in
By way of comparison, faujasite type zeolites, which have among the highest microporous volumes and the largest pore openings, have a microporous volume of approximately 0.35 cm3/g measured by nitrogen adsorption at 77 K, and window diameters of 7.4×7.4 Å (“Atlas of Zeolite Structure Types” by Ch Baerlocher, W M Meier and D H Olson, 5th edition, review, 2001, Elsevier, cited above. It should be noted that in the case of faujasites, part of the pore volume (the sodalite cages) is not accessible to molecules other than water and nitrogen. Thus, for example, the pore volume which is accessible to a multibranched alkane such as 2,2,4-trimethylpentane is 0.27 cm3/g.
The group of adsorption separation process aimed at separating a product or a group of products from a feed containing them form the subject matter of the present invention.
Thus, the invention is applicable in many and varied fields, from the petroleum, petrochemical and chemical industries to environmental and pharmaceutical applications.
More particularly, envisaged applications are the production of industrial gas (oxygen, nitrogen, hydrogen), the separation of hydrocarbons and elimination of pollutants (sulphur-containing compounds, volatile organic compounds, etc).
Preferably, the separations which concern the present invention are:
The adsorbent is adapted to the envisaged application. Thus, several parameters such as the ratio X/Ge, the ratio (X+Ge)/Z, the nature of the cation or cations R, are adjusted to ensure optimum performance of the process. In the same manner, the form in which the adsorbent is used (extrudates, powder, beads) will depend on the type of process employed.
The invention will be better understood from the following examples which illustrate the invention without, however, limiting its scope.
Example 1 illustrates adsorption separation based on a steric and kinetic effect.
Example 2 concerns the separation of xylenes.
Example 3 illustrates a process for separating ortho-xylene from a mixture of xylenes and ethylbenzene.
The hydrocracking reaction produces undesirable heavy aromatic compounds (HPNA, heavy polynuclear aromatics) which clog equipment and reduce catalyst service life. Their formation increases with conversion and the mean molecular weight of the feed.
In general, the unconverted fraction has to be recycled at the outlet from the reactor. During the operation, heavy aromatic compounds accumulate in this recycle. Said accumulation results in even more clogging of the reactor. Only the heaviest compounds, however, generate such problems. It is thus important to remove them from the recycle stream using a separation process. IM-12, with its very large pores, is an adsorbent of choice for said separation.
A IM-12 silicogermanate was produced in accordance with Example 1 of the Applicant's patent application no 03/11333. It consists of mixing, in a beaker:
After homogenizing, the gel obtained was placed in an autoclave and heated for 6 days at 170° C., with stirring. After filtering, the product was washed with distilled water and dried at 70° C. The sample was then calcined in a muffle furnace in a constant stream of air at a maximum temperature of 550° C.
The silicogermanate IM-12 was obtained in its calcined form, and had the formula SiO2: 0.23 GeO2.
Table 1 below shows the kinetic diameters of various molecules containing one or more aromatic rings as calculated by Henry W Haynes Jr, Jon F Parcher and Norman E Heimer (Ind Eng Chem Process Des Dev, 22, 401-409 (1983)).
The dimensions of the IM-12 channels were respectively 9.5×7.1 Å for 14 MR channels and 8.5×5.1 Å for 12 MR channels. Adsorption separation based on a steric and kinetic effect can thus isolate products with a molecular weight that is greater than or less than that of coronene, such as ovalene (8 aromatic rings), their alkylated derivatives, dimers of coronene, and more generally any molecule with a molecular diameter greater than that of coronene.
For said separation, the adsorbent was placed in several fixed beds disposed in parallel. The temperature during the adsorption phase was in the range 50° C. to 250° C. and the pressure was in the range from atmospheric pressure to 4 MPa.
For this example, we carried out a drilling test (test 1) (frontal chromatography) to determine the ability of IM-12 to separate ortho-xylene from other xylenes.
IM-12 was synthesized using the method described Example 1.
The adsorbent was then placed in a column. The quantity used for each test was 2.63 g. For each test, the temperature of the column was kept at 150° C. and the pressure was sufficient to ensure that the phase was liquid, i.e. about 1 MPa. The desorbant used was toluene.
The effluent from the column was sampled (30 samples) then analyzed by gas chromatography to determine the composition of the effluent at various time intervals.
In a first test, the composition of the feed was as follows:
Para-xylene: 45% by weight;
Meta-xylene: 45% by weight;
Iso-octane: 10% by weight (used as a tracer to estimate non-selective volumes and not involved in separation).
In a second test (test 2), the composition of the feed was as follows:
Para-xylene: 50% by weight
Ortho-xylene: 50% by weight
The drilling curve obtained corresponding to said feed is shown in
The following operating mode was employed:
The capacity of the sieve and its selectivity were then calculated and are shown in the following table. The selectivity αox/px was calculated from test 2. Test 1 allowed the selectivity αpx/px to be calculated. The selectivity αox/px was calculated as the product of the two preceding selectivities.
* In contrast to frontal chromatography (drilling curves), the pulse experiments carried out here did not allow the capacity of the sieve to be calculated.
Compared with other adsorbents, it can be seen that IM-12 could produce satisfactory results for ortho-xylene separation.
The zeolite with the closest performance was CSZ-1 zeolite exchanged with lead. Clearly, the presence of heavy metals such as lead should be avoided for environmental reasons. Further, in all cases, the IM-12 had a larger pore size than the other adsorbents, which allowed better diffusion of molecules into the pores and thus a reduced matter transfer resistance.
Ortho-xylene was produced from a feed comprising a mixture of xylenes and ethylbenzene with the following composition by weight:
Para-xylene: 1.0% by weight
Meta-xylene: 63.8% by weight
Ortho-xylene: 28.0% by weight
Ethylbenzene: 7.2% by weight
using a simulated moving bed, in counter-current mode, the unit being composed of 24 equivalent beds, each bed having a volume of 381 cm3 and containing IM-12 produced using the method described in Example 1 and formed into beads. The solvent used was toluene.
The operating temperature was 150° C., the pressure at the recycle pump intake was kept at 1 MPa. All of the injected or withdrawn streams were under controlled flow rate, with the exception of the raffinate which was under pressure control.
There were 5 beds between the desorbant injection and the extract withdrawal, 9 beds between the extract withdrawal and the feed injection, 7 beds between the feed injection and the raffinate withdrawal, and 3 beds between the raffinate withdrawal and the desorbant injection. The following injection and withdrawal rates were used:
Feed: 25.2 cm3/min
Solvent: 37.8 cm3/min of toluene
Extract: 12.0 cm3/min
Raffinate: 51.0 cm3/min
Recycle flow rate (in zone 1): 134 cm3/min.
The valve permutation time (period) was 140 seconds.
The extract had the following composition:
Para-xylene: 0.01% by weight
Meta-xylene: 0.24% by weight
Ortho-xylene: 55.76% by weight
Ethylbenzene: 0.03% by weight
Toluene: 43.96% by weight
The raffinate had the following composition:
Para-xylene: 0.49% by weight
Meta-xylene: 31.47% by weight
Ortho-xylene: 0.72% by weight
Ethylbenzene: 3.55% by weight
Toluene: 63.77% by weight
After distilling the toluene, the extract obtained delivered 99.5% pure ortho-xylene in a yield of 94.8%.
Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The preceding preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever.
In the foregoing and in the examples, all temperatures are set forth uncorrected in degrees Celsius and, all parts and percentages are by weight, unless otherwise indicated.
The entire disclosure of all applications, patents and publications, cited herein and of corresponding French application No. 04/11.629, filed Oct. 29, 2004, is incorporated by reference herein.
The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples.
From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.
| Number | Date | Country | Kind |
|---|---|---|---|
| 04/11.629 | Oct 2004 | FR | national |
