Patent Application
20130280430
1. Field
Silicoaluminophosphate (SAPO) membranes and aluminophosphate (AlPO) membranes.
2. Background Information
Natural gas is a fuel gas used extensively in the petrochemical and other chemicals businesses. Natural gas is comprised of light hydrocarbons-primarily methane, with smaller amounts of other heavier hydrocarbon gases such as ethane, propane, and butane. Natural gas may also contain some quantities of non-hydrocarbon “contaminant” components such as carbon dioxide and hydrogen sulfide, both of these components are acid gases and can be corrosive to pipelines.
Natural gas is often extracted from natural gas fields that are remote or located off-shore. Conversion of natural gas to a liquid hydrocarbon is often required to produce an economically viable product when the natural gas field from which the natural gas is produced is remotely located with no access to a gas pipeline. One method commonly used to convert natural gas to a liquid hydrocarbon is to cryogenically cool the natural gas to condense the hydrocarbons into a liquid. Another method that may be used to convert natural gas to a liquid hydrocarbon is to convert the natural gas to a synthesis gas by partial oxidation or steam reforming, and subsequently converting the synthesis gas to liquid hydrocarbons, such as that produced by a Fisher-Tropsch reaction. Synthesis gas prepared from natural gas may also be converted to a liquid hydrocarbon oxygenate such as methanol.
In a cryogenic cooling process to liquefy hydrocarbons in natural gas, carbon dioxide may crystallize when cryogenically cooling the natural gas, blocking valves and pipes used in the cooling process. Further, carbon dioxide utilizes volume in a cryogenically cooled liquid hydrocarbon/carbon dioxide mixture that would preferably be utilized only by the liquid hydrocarbon, particularly when the liquid hydrocarbon is to be transported from a remote location.
Carbon dioxide also may impair conversion of natural gas to a liquid hydrocarbon or a liquid hydrocarbon oxygenate. Significant quantities of carbon dioxide may impair conversion of natural gas to synthesis gas by either partial oxidation or by steam reforming.
As a result of the corrosive nature of carbon dioxide and the additional difficulty of processing natural gas contaminated with carbon dioxide, attempts have been made to separate carbon dioxide present in a natural gas from the hydrocarbon components of the natural gas prior to processing the natural gas to a liquid. Separation techniques include scrubbing the natural gas with a liquid chemical, e.g. an amine, to remove carbon dioxide, passing the natural gas through molecular sieves selective to separate carbon dioxide from the natural gas. These methods of separating carbon dioxide from a natural gas are effective for natural gases containing 40 percent by volume of carbon dioxide, more typically less than 15 to 30 percent by volume, but are either ineffective or commercially prohibitive in energy costs to separate carbon dioxide from natural gas when the natural gas is contaminated with larger amounts of carbon dioxide, e.g., at least 40 percent by volume.
Production of natural gas from natural gas fields containing natural gas contaminated with on the order of 50 percent by volume or more carbon dioxide is generally not undertaken due to the difficulty of producing liquid hydrocarbons or liquid hydrocarbon oxygenates from natural gas contaminated with such large quantities of carbon dioxide and the difficultly of removing carbon dioxide from the natural gas when present in such a large quantity. However, some of the largest natural gas fields discovered to date are contaminated with high levels of carbon dioxide. Therefore, there is a need for an energy efficient, effective method to separate carbon dioxide from a natural gas contaminated with carbon dioxide, including a carbon dioxide rich natural gas.
Laboratory studies of silicoaluminophosphate (SAPO) and/or aluminophosphate (AlPO) containing membranes, particularly SAPO-34 containing membranes, have demonstrated utility in separating carbon dioxide from contaminated natural gas. Formation of such membranes involves forming SAPO-34 crystals typically from a synthesis gel in and on a porous support at an elevated temperature and autogenous pressure. Forming larger scale, equivalent membranes present challenges in part because of the nature in which SAPO-34 crystals are formed and the ability to control the formation conditions.
The invention may best be understood by referring to the following description and accompanying drawings that are used to illustrate embodiments of the invention. In the drawings:
In one embodiment, a method is disclosed. The method includes contacting a support with a composition including a silicoaluminophosphate (SAPO) and/or an aluminophosphate (AlPO) gel; heating the support; forming SAPO and/or AlPO crystals on the support; and after forming the crystals, modifying the contact between the support and the gel within a time to inhibit solubilization of a portion of the crystals.
In another embodiment, a method includes seeding a support with an amount of uncalcined silicoaluminophosphate (SAPO) and/or uncalcined aluminophosphate (AlPO) crystals; after seeding the support, contacting the support with a composition comprising a SAPO and/or AlPO gel; and heating the support and the composition to form SAPO and/or AlPO crystals from the SAPO and/or AlPO gel on the support and after forming the crystals, modifying the contact between the support and the gel within a time to inhibit solubilization of a portion of the crystals.
In one embodiment, a commercial scale silicoaluminophosphate (SAPO) and/or aluminophosphate (AlPO) membrane having a layer or layers of SAPO and/or AlPO crystals and a method of making a commercial scale SAPO and/or AlPO membrane is disclosed. Membranes are suitable, in one embodiment, to separate components of a gas stream. Particularly, in one embodiment, a SAPO-34 membrane may be used to remove contaminants such as carbon dioxide from a natural gas stream.
Although a tubular structure is shown in
Referring again to
In one embodiment, support 110 is comprised of an asymmetric porous ceramic material, where the layer onto which the SAPO and/or AlPO molecular sieve crystals are formed has a mean pore diameter greater than about 0.2 microns. Representative acceptable mean pore diameters for commercial application include, but are not limited to, 0.005 microns to 0.6 microns.
A support that is a metal material may be in the form of a fibrous-mesh (woven or non-woven), a combination of fibrous mesh with sintered metal particles, and sintered metal particles. In one embodiment, the metal support is formed of sintered metal particles. In another embodiment, support 110 is a porous ceramic or a porous metal hollow fiber formed from any method known in the art.
Referring to
The SAPO and/or AlPO molecular sieve crystals may embed themselves in the pores of the porous support as well as form on the support thus reducing an inner diameter of support 110. Although shown as a defined layer in
A membrane, such as membrane 100 in
U.S. Pat. No. 7,316,727 describes a process of preparing a SAPO-34 synthesis gel. That process is incorporated herein in its entirety. In one embodiment, the synthesis gel is prepared by mixing sources of aluminum, phosphorus, silicon, and oxygen in the presence of templating agent and water. The composition of the mixture may be expressed in terms of the following molar ratios as: 1.0 Al2O3:aP2O5:bSiO2:cR:dH2O, where R is a templating agent or multiple templating agents. In one embodiment, R is a quaternary ammonium templating agent. In one embodiment, the quaternary ammonium templating agent is selected from the group consisting of tetrapropyl ammonium hydroxide (TPAOH), tetrapropyl ammonium bromide, tetrabutyl ammonium hydroxide, tetrabutyl ammonium bromide, tetraethyl ammonium hydroxide (TEAOH), tetraethyl ammonium bromide, or combinations thereof. In other embodiments, one of the templating agents may be a free amine such as dipropyl amine (DPA). In one embodiment, suitable for crystallization between about 420 K and about 500 K, a is between about 0.1 and about 1.5, b is between about 0.00 and about 1.5, c is between about 0.2 and about 10 and d is between about 10 and about 300. If other elements are to be substituted into the structural framework of the SAPO, the gel composition can also include Li2O, BeO, MgO, CoO, FeO, MnO, ZnO, B2O3, Ga2O3, Fe2O3, GeO, TiO, As2O5 or combinations thereof.
In one embodiment suitable for crystallization of SAPO-34, c is less than about 3. In one embodiment suitable for crystallization of SAPO-34 at about 493 K for about 6 hours, a is about 1, b is about 0.3, c is about 1.2 and d is about 150. In one embodiment, R is a quaternary organic ammonium templating agent selected from the group consisting of tetrapropyl ammonium hydroxide, tetraethyl ammonium hydroxide (TEAOH), or combinations thereof.
In one embodiment, the synthesis gel is prepared by mixing sources of phosphate and alumina with water for several hours before adding the template. The mixture is then stirred before adding the source of silica. In one embodiment, the source of phosphate is phosphoric acid. Suitable phosphate sources also include organic phosphates such as triethyl phosphate, and crystalline or amorphous aluminophosphates. In one embodiment, the source of alumina is an aluminum alkoxide, such as aluminum isopropoxide. Suitable alumina sources also include aluminum hydroxides, pseudoboehmite and crystalline or amorphous aluminophosphates (gibbsite, sodium aluminate, aluminum trichloride). In one embodiment, the source of silica is a silica sol. Suitable silica sources also include fumed silica, reactive solid amorphous precipitated silica, silica gel, alkoxides of silicon (silicic acid or alkali metal silicate).
In one embodiment, the synthesis gel is aged prior to use. As used herein, an “aged” gel is a gel that is held (not used) for a specific period of time at a specific temperature after all the components of the gel are mixed together. In one embodiment, the synthesis gel is sealed and stirred during aging to prevent settling and the formation of a solid cake. Without wishing to be bound by any particular theory, it is believed that aging of the gel affects subsequent crystallization of the gel by generating nucleation sites. In general, it is believed that longer aging times lead to formation of more nucleation sites. The aging time will depend upon the aging temperature selected. Preferably, crystal precipitation is not observed during the aging period. Preferably, the viscosity of the aged gel is such that the gel is capable of penetrating pores of a porous support to which it will be contacted.
After initial mixing of the components of the synthesis gel in a container, material can settle to the bottom of the container. In one embodiment, the synthesis gel is stirred and aged until no settled material is visible at the bottom of the container and the gel appears substantially uniform to the eye. In different embodiments, the aging time at 25 C-50 C is at least about twenty-four hours, greater than about twenty-four hours, at least about forty-eight hours, and at least about seventy-two hours. For SAPO-34 membranes, in different embodiments the aging time at 25 C-50 C can be at least about forty-eight hours, at least about seventy-two hours, and between about one days and about seven days.
In the example of forming a tubular membrane having SAPO and/or AlPO molecular sieve crystals formed on an interior surface of a lumen or channel, a surface or surfaces of the support is contacted with SAPO and/or AlPO molecular sieve crystals (block 310,
Seeding a porous support with SAPO and/or AlPO molecular sieve crystals provides a location for subsequent nucleation of SAPO and/or AlPO material (i.e., further crystal growth). In one embodiment, the SAPO and/or ALPO molecular sieve crystals have been previously subjected to a heating or calcining step. In another embodiment, uncalcined crystals (seeds) of SAPO and/or AlPO (e.g., SAPO-34) may be used. Typically, formation of SAPO-34 crystals involves heating at high temperature to drive off templating agents and provide a porous crystal. Calcination often involves temperatures of 400° C. (673 K) for six hours or more. In the use of SAPO crystals as a seed material, it has been found that such crystals do not need to be calcined to effectively function (e.g., as nucleation sites for further crystalline growth).
After the inner surface of the support has been seeded with crystals, to protect the outer surface or circumference of a tubular support from interaction with the synthesis gel, the tubular support is wrapped with a sacrificial material that is inert to the synthesis gel. One representative material for a sacrificial material is polytetrafluoroethylene or TEFLON®, a registered trademark of E.I. Dupont de Nemours and Company of Wilmington, Del.
Following any protection of a surface of a support, the aged synthesis gel is brought into contact with at least one surface of the support (block 320,
Support 110 and the aged synthesis gel are brought into contact in reaction chamber 400. Support 110 and gel 420 are heated in a SAPO and/or AlPO crystal synthesis operation (block 330,
In one embodiment, following the formation of a desired crystalline layer in/on support 110 to form membrane 100 (support 110 including SAPO and/or AlPO molecular sieve crystals), solubilization of the crystals is inhibited by modifying the contact between the support and the synthesis gel. It has been determined that, at least at a commercial processing scale, SAPO and/or AlPO crystals (e.g., SAPO-34 crystals) tend to be soluble in the depleted synthesis gel at temperatures lower than the crystallization temperature. If exposed to this gel for an extended period of time, the crystals that form the SAPO membrane dissolve which can lead to defects in the membrane.
In one embodiment, SAPO and/or AlPO crystals in/on membrane 100 are inhibited from solubilizing by cooling the membrane as rapidly as possible (block 340,
There are a number of ways to rapidly cool a SAPO and/or AlPO membrane. In one embodiment, membrane 100 and synthesis gel 420 are cooled in reaction vessel 400 as fast as possible (block 350,
An alternative method to cool a membrane including SAPO and/or AlPO crystals is to remove synthesis gel 420 from the reaction vessel immediately following the synthesis (block 360,
As an alternative to the cooling method where the synthesis gel 420 is initially removed from reaction vessel 400, the membrane may be removed from the vessel immediately following a formation of a sufficient SAPO and/or AlPO membrane layer (block 370,
Rather than cooling a membrane including SAPO and/or AlPO crystals to inhibit solubilization of the crystals, in another embodiment, the pH of synthesis gel 420 is modified following the formation of the SAPO and/or AlPO membrane layer (block 345,
In one embodiment, following the formation of a SAPO and/or AlPO membrane having a SAPO and/or AlPO layer in/on a support, additional SAPO and/or AlPO crystals may be added to the membrane. In this embodiment, the process operations illustrated in block 320 through block 340 or block 345 of
After SAPO crystal synthesis is complete and the membrane cooled, the SAPO and/or AlPO membrane is calcined in air or an inert gas such as nitrogen or in a partial vacuum to substantially remove the organic template(s). In different embodiments, the calcination temperature is between about 600 K and about 900 K, and between about 623 K and about 773 K. For membranes made using TEAOH or TPAOH as a templating agent, the calcining temperature can be between about 600 K and about 725 K. In one embodiment, the calcination time is between about 4 hours and about 25 hours. Longer times or higher inert gas flow rates may be required at lower temperatures in order to substantially remove the template material. Use of lower calcining temperatures can reduce the formation of calcining-related defects in the membrane. The heating rate during calcination should be slow enough to limit formation of defects such as cracks. In one embodiment, the heating rate is less than about 5.0 K/min. In a different embodiment, the heating rate is about 0.6 K/min. Similarly, the cooling rate must be sufficiently slow to limit membrane defect formation. In one embodiment, the cooling rate is less than about 2.0 K/min. In a different embodiment, the cooling rate is about 0.9 K/min. After calcination, the membrane becomes a semi-permeable barrier between two phases that is capable of restricting the movement of molecules across it in a very specific manner.
A scaled example of forming a SAPO membrane on six centimeter membranes was performed. An asymmetric alpha alumina support (200 nm average pore size on the internal surface) was placed in a silicoaluminophosphate-forming synthesis solution or gel with the following synthesis gel composition:
1Al2O3:1P2O5:0.3SiO2:1.0TEAOH:1.6DPA:150H2O
The support, gel, and reaction vessel were placed in an oven set at 220° C. for six hours. A continuous SAPO-34 membrane layer was formed on the alpha alumina support. Following the formation of the SAPO-34 membrane layer, the membranes were cooled to room temperature over a period of approximately two hours and then allowed to sit in the gel before removal from the spent synthesis solution. The results show the selectivity of the resulting membranes decreases relative to a membrane's exposure time to the spent synthesis solution. The time listed is the total time exposed including the time to cool down. In the first experiment, the membrane was rapidly cooled using an ice water bath and removed from the gel. As shown in the following table, a decrease in permeance and selectivity is noticed in membranes exposed to the gel for 4 hours. A complete loss in selectivity is observed with membranes exposed to the spent synthesis solution for 12 hours. Additional research indicated similar results with longer membranes.
An example of dissolution or etching of SAPO-34 crystals after extended contact with the spent synthesis gel from hydrothermal synthesis is described.
A spent synthesis gel and free SAPO-34 crystals formed after the synthesis of a SAPO-34 membrane on an asymmetric alpha alumina support (200 nm average pore size on the internal surface) were collected after the synthesis. The composition of the synthesis gel and the conditions under which it was subjected is described in Example 1. The SAPO-34 containing spent synthesis gel was then filtered to yield SAPO-34 crystals in the size range of 2-5 microns as well as a filtrate that is now referred to as the spent synthesis gel. Spent synthesis gel has a pH value typically between 9 to 11. The SAPO-34 crystals collected from the filtration were calcined for 4 hours at 400 C in nitrogen with a heating ramp of 1 C/min. Subsequently, the SAPO-34 crystals were contacted with the spent synthesis gel for a period of 1 hour. The crystals were then rinsed with deionized water and characterized by scanning electron microscopy.
In the description above, for the purposes of explanation, numerous specific details have been set forth in order to provide a thorough understanding of the embodiments. It will be apparent however, to one skilled in the art, that one or more other embodiments may be practiced without some of these specific details. The particular embodiments described are not provided to limit the invention but to illustrate it. The scope of the invention is not to be determined by the specific examples provided above but only by the claims below. In other instances, well-known structures, devices, and operations have been shown in block diagram form or without detail in order to avoid obscuring the understanding of the description. Where considered appropriate, reference numerals or terminal portions of reference numerals have been repeated among the figures to indicate corresponding or analogous elements, which may optionally have similar characteristics.
It should also be appreciated that reference throughout this specification to “one embodiment”, “an embodiment”, “one or more embodiments”, or “different embodiments”, for example, means that a particular feature may be included in the practice of the invention. Similarly, it should be appreciated that in the description various features are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects may lie in less than all features of a single disclosed embodiment. Thus, the claims following the Detailed Description are hereby expressly incorporated into this Detailed Description, with each claim standing on its own as a separate embodiment of the invention.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP12/50282 | 1/10/2012 | WO | 00 | 7/10/2013 |
| Number | Date | Country | |
|---|---|---|---|
| 61431990 | Jan 2011 | US |
