EP1963767A2

EP1963767A2 - The use of mofs in pressure swing adsorption

Info

Publication number: EP1963767A2
Application number: EP06850287A
Authority: EP
Inventors: Mark M. Davis; John J. Low
Original assignee: UOP LLC
Current assignee: Honeywell UOP LLC
Priority date: 2005-12-21
Filing date: 2006-12-13
Publication date: 2008-09-03
Also published as: CA2633676A1; EP1963767A4; CN101346182A; AU2006340775A1; WO2007111739A3; NZ569159A; WO2007111739A2; ZA200805557B; CA2633676C; WO2007111739B1

Abstract

A pressure swing adsorption process using metal organic framework materials is presented for removal of light hydrocarbon compounds from a hydrogen gas stream.

Description

THE USE OF MOFS IN PRESSURE SWING ADSORPTION

BACKGROUND OF THE INVENTION

[0001] The present invention relates to adsorption processes, and more particularly to pressure swing adsorption processes. The process employs metal-organic framework materials having a high porosity and high surface areas, and are useful in the separation of hydrocarbons from hydrogen streams.

[0002] It is often necessary to separate one or more components from a gas mixture to generate a purified gas. This can be done for removing an impurity from a gas stream or for concentrating a component or components within a gas stream. [0003] One technique for separation of one component in a gas from a mixture uses adsorption of one or more components from the mixture onto an adsorbent. This process is further enhanced through pressure swing adsorption (PSA). Pressure swing adsorption entails passing a feedstream over an adsorbent where one, or more, components of the feedstream are selectively adsorbed onto the adsorbent, and where the process of adsorption is performed at a relatively high pressure. The adsorbent is regenerated by reducing the pressure over the adsorbent, and a process of desorption is performed at the relatively low pressure. The desorption process can also be accompanied by the passing of a purge gas having a low concentration of the adsorbate to enhance desorption. [0004] The separation of gases from a gas mixture through adsorption in a pressure swing adsorption process is controlled by the pressures used in the process and the capacity of the adsorbent for one, or more, of the components in the gas mixture. The process usually entails a tradeoff between the range in pressure, and the load capacity of the adsorbent for many of the materials used. It is desirable to be able to use materials that can overcome some of these tradeoffs.

SUMMARY OF THE INVENTION

[0005] The invention is a pressure swing adsorption process for removing hydrocarbons from a hydrogen stream. The process passes the hydrogen stream over a metal organic framework material at a high adsorption pressure, generating an effluent stream with a reduced hydrocarbon content. The process then reduces the pressure over the metal organic framework material and releases the hydrocarbon from the material, and generates a stream having hydrocarbons. The process steps are then repeated. In one embodiment, the process uses multiple adsorption beds comprising the metal organic framework material and cycles the pressures sequentially through the beds to produce a continuous process. [0006] Additional objects, embodiments and details of this invention can be obtained from the following detailed description of the invention. [0007] The Figure is the comparison of CH4 adsorption on carbon and MOF-5.

DETAILED DESCRIPTION OF THE INVENTION

[0008] The separation of gases from a gas mixture through adsorption in a pressure swing adsorption process is controlled by the difference between adsorption and desorption pressures and capacity of one of the components in the gas mixture. The process usually entails a tradeoff between the pressure differences and the capacity for many of the materials used. The capacity is the amount of material adsorbed by the adsorbent. It is desirable to be able to use materials that can overcome some of these tradeoffs. [0009] In pressure swing adsorption, a gas made up of at least two constituents, is separated using the differences in selectivity of one of the constituents. Usually, the gas is purified by selectively removing an undesired constituent of the gas. The gas is typically fed into an adsorption unit at an elevated pressure, where one of the constituents is preferentially adsorbed onto an adsorbent. While one constituent is preferentially adsorbed, other constituents are also adsorbed, and it is desired to use adsorbents that have significant differences in the adsorption of the desired constituents.

[0010] The adsorbent is regenerated through reversing the adsorption process to desorb the constituents. This is done by changing the conditions of the adsorbent environment through reducing the pressure. At a defined time or conditions, the gas feed to the adsorption unit is stopped, and the adsorption unit is depressurized. Preferably, the gas feed is stopped when the adsorption unit is near or at capacity for the adsorbent with the desired constituent. The adsorption unit is depressurized to a specified level where the adsorbed constituents desorb generating a desorbent stream that is relatively rich in the constituent that is more strongly adsorbed onto the adsorbent. The desorption process can use an inert gas, or a non- hydrocarbon gas to facilitate the desorption process. The desorption gas is passed over the adsorbent to remove the adsorbed constituents as they desorb from the adsorbent. Preferably, the desorption gas is passed over the adsorbent in a direction opposite the direction of the feed gas to regenerate the adsorbent.

[0011] An aspect of a pressure swing adsorption system is the isotherm for adsorbing a component in a gas dictates the operating pressures and loading onto the adsorbent. Most materials have an isotherm, wherein the saturation limit is rapidly approached, and then there is a small incremental improvement in adsorption for a relatively large increase in pressure. The working capacity of an adsorbent is defined as difference in the amount of the adsorbed components on the adsorbent between the adsorption pressure and the desorption, or regeneration, pressure. Lowering the regeneration pressure can increase the capacity of the adsorbent for selectively removing a component from a gas, but the effluent stream from the regeneration step may need to be recompressed. However, a lower regeneration pressure increases the recompression costs. [0012] In pressure swing adsorption, there are many classes of adsorbents that are suitable. The selection is dependent upon the feed gas constituents and other factors generally known to those skilled in the art. In general, suitable adsorbents include molecular sieves, silica gels, activated carbons, activated aluminas, and other porous metal oxides. When purifying methane containing streams, the methane is often adsorbed along with the impurities that one wishes to remove. The choice of adsorbent presents problems in selecting an adsorbent that has the greatest differential in adsorption between hydrogen and selected impurities, especially light hydrocarbons such as methane and ethane.

[0013] To overcome the tradeoffs and improve PSA, the search is for a high permeability material that also has a high capacity for use in a pressure swing adsorber. This means a material with a very high surface area and a high porosity. It is desired to increase the loading of the adsorbent, while minimizing recompression requirements. This translates to higher desorption pressures.

[0014] One embodiment of the invention is a process using pressure swing adsorption to remove methane and other light hydrocarbon compounds, such as ethane, from a hydrogen feedstream. The process comprises passing a hydrogen feedstream having hydrocarbons over an adsorbent in an adsorption zone, and at a temperature and pressure sufficient to adsorb a portion of the hydrocarbons. The remaining gases in the feedstream becomes an effluent stream having a reduced hydrocarbon content. The adsorbent in the process is a material known as a metal organic framework (MOF), and has a high surface area and high porosity. The surface area of the material is greater than 1500 m²/gm. The pressure in the adsorption zone is then reduced to a pressure for desorbing the hydrocarbons, and generates a desorption effluent stream having an enriched hydrocarbon content. The effluent stream will have an increased methane content, as methane is the primary light hydrocarbon in the hydrogen feedstream. Other light hydrocarbons include ethane, propane, butanes, and small amounts of other hydrocarbons. The process during desorption can include passing a carbon dioxide lean purge gas over the adsorbent. [0015] The process can be carried out by either passing the adsorbent bed through a high pressure adsorption zone, and then moving the adsorbent bed to a low pressure desorption zone, such as occurs with an adsorbent wheel in a rotating drum adsorber. The process can also be carried out by alternately pressurizing the adsorbent bed and passing the feedstream over the bed, and depressurizing the adsorbent bed and passing a purge gas over the bed. [0016] These processes are improved and made continuous by using a sequence of at least two adsorbent beds, wherein the beds are cycled through the adsorption and desorption steps in a sequential manner to provide a continuous operation. The process of cycling the adsorbent beds comprises pressurizing a first adsorbent bed to an adsorption pressure and flowing the feedstream over the first adsorbent bed, while depressurizing a second adsorbent bed to a desorption pressure and flowing a purge stream over the second adsorbent bed. Switching the feedstream and the purge streams to the second adsorbent bed and first adsorbent bed respectively, and pressurizing the second adsorbent bed to the adsorption pressure and flowing the feedstream over the second adsorbent bed, while depressurizing the first adsorbent bed to the desorption pressure and flowing the purge stream over the first adsorbent bed. The process can be further smoothed with respect to pressure changes by additional beds, wherein intermediate beds are pressurized or depressurized before switching flows.

[0017] In the process for reducing hydrocarbon content, and particularly methane, in a hydrogen feedstream, the feedstream is passed over the adsorbent, in a first adsorbent zone, at the highest pressure of the process, with the hydrocarbons adsorbed, generating a hydrocarbon depleted hydrogen stream. The hydrocarbon depleted hydrogen discharges from the adsorption zone so that hydrocarbon adsorption front is formed in the zone at the hydrogen feedstream inlet end and progressively moves toward the outlet. Preferably, the adsorption zone is sized to produce a hydrogen gas product with a hydrocarbon concentration less than 1% by volume. The feedstream to the adsorbent unit is terminated when either the hydrocarbon adsorption front is at a predetermined point in the adsorption unit, or when there is an increase in the hydrocarbon in the hydrogen stream to above a predetermined value. The feedstream is then terminated to the first adsorption zone, and directed to a second adsorption zone. The first adsorption zone is depressurized and a purge gas is passed through the first adsorption zone to regenerate the adsorbent in the first adsorption zone. The purge gas preferably flows in a counter current direction relative to the flow of the feedstream in the adsorption zones to remove the hydrocarbons in the reverse direction that they were adsorbed. [0018] When the first zone has been regenerated, it is repressurized to the pressure level for the feedstream, the feedstream is switched to the first zone, and the second adsorption zone is depressurized and regenerated with a purge gas at regeneration conditions, and the process cycle is repeated.

[0019] The operating conditions for the pressure swing adsorption process include adsorption pressures from 2 MPa (20 atms.) to 5 MPa (50 aims.). The desorption pressure is in a range from 1 kPa (1 atm) to 1.5 MPa (15 atms.), with a preferred range from 500 kPa (5 atm) to 1 MPa (10 atms.). The desorption step is preferably operated at a pressure sufficient to minimize recompressing the desorption effluent stream. The adsorbent needs to be thermally stable for a range of temperatures, and operation is at temperatures between 0⁰C to

400⁰C.

[0020] The process can further comprise passing a purge stream at desorption conditions over the adsorbent to facilitate the desorption of the hydrocarbons. The desorbent effluent stream can be recompressed and directed to a fuel system. It is preferred to desorb the adsorbate at moderate pressures to minimize repressurization of the desorbent effluent stream. A repressurized desorbed hydrocarbon stream can be used as a fuel gas. [0021] New materials have been found to have good properties for adsorption separation. These materials are MOFs, or metal-organic framework materials. MOFs have very high surface areas per unit volumes, and have very high porosities. MOFs are a new generation of porous materials which have a crystalline structure comprising repeating units having a metal or metal oxide with a positive charge and organic units having a balancing counter charge. MOFs provide for pore sizes that can be controlled with the choice of organic structural unit, where larger organic structural units can provide for larger pore sizes. The capacity and adsorption characteristics for a given gas is dependent on the materials in the MOF, as well as the size of the pores created. Structures and building units for MOFs can be found in US 2005/0192175 published on September 1, 2005 and WO 2002/088148 published on November 7, 2002, both of which are incorporated by reference in their entireties. [0022] The materials of use for this process include MOFs with a plurality of metal, metal oxide, metal cluster or metal oxide cluster building units, hereinafter referred to as metal building units, where the metal is selected from the transition metals in the periodic table, and beryllium. Preferred metals include zinc (Zn), cadmium (Cd), mercury (Hg), and beryllium (Be). The metal building units are linked by organic compounds to form a porous structure, where the organic compounds for linking the adjacent metal building units include 1,3,5- benzenetribenzoate (BTB); 1 ,4-benzenedicarboxylate (BDC); cyclobutyl 1,4- benzenedicarboxylate (CB BDC); 2-amino 1,4 benzenedicarboxylate (H2N BDC); tetrahydropyrene 2,7-dicarboxylate (HPDC); terphenyl dicarboxylate (TPDC); 2,6 naphthalene dicarboxylate (2,6-NDC); pyrene 2,7-dicarboxylate (PDC); biphenyl dicarboxylate (BDC); or any dicarboxylate having phenyl compounds. [0023] Specific materials that show improvement in adsorption properties have a three- dimensional extended porous structure and include: MOF- 177, a material having a general formula of Zn₄O(1, 3, 5-benzenetribenzoate)₂; MOF-5, also known as IRMOF-I, a material having a general formula of Zn₄O(1, 4-benzenedicarboxylate)₃; IRMOF-6, a material having a general formula of Zn₄θ(cyclobutyl 1,4-benzenedicarboxylate); IRMOF-3, a material having a general formula of Zri₄O(2-amino 1,4 benzenedicarboxylate)₃; and IRMOF-11, a material having a general formula of Zn₄O (terphenyl dicarboxylate)₃,or ZrL_tO^etrahydropyrene 2,7- dicarboxylate)₃; and IRMOF-8, a material having a general formula of Zn₄θ(2,6 naphthalene dicarboxylate)₃.

[0024] These materials have high capacities due to the high surface areas, and have favorable isotherms where the adsorbent releases a significant amount of the adsorbate, at moderate pressures of around 5 atm. (0.5 MPa). EXAMPLE

[0025] The use of a metal organic framework improves the removal of methane (CH₄) and other light hydrocarbons from a high pressure stream comprising hydrogen (H₂). In this particular example, this is a high waste pressure application where the waste gas stream is directed to a fuel system. By regenerating the adsorbent at moderate pressures, there is significant savings from the reduced repressurization needed. The fuel systems are typically operated at pressures from 4 atm to 7 atm (400 kPa to 700 kPa). In this example, the primary impurity is methane, and the adsorbent activity of MOF-5 is compared with the activity of activated carbon in a PSA system. The isotherms for methane over the adsorbents are shown in the Figure using the basis of lbs of methane per cubic foot of adsorbent bed. The feed stream has a methane partial pressure of 20 atm which is then desorbed at a pressure of 5 atm. The loadings for the activated carbon and the MOF-5 are 1.05 and 2.15 lbs-CH₄/ft³, respectively. The MOF-5 exhibits a loading capacity of more than double that of carbon. To increase the carbon loading, the desorption pressure can be reduced to 1 atm, with a resulting loading on the carbon of 1.8. The low pressure used for carbon must be accompanied with a significant increase in power usage to recompress the methane released during the desorption stage to return the methane effluent stream to a fuel system pressure.

[0026] One aspect of the invention is to have a material, or combination of materials, that changes the shape of the isotherm, so that the capacity-pressure curve does not taper off as pressure increases, but still retains significant capacity increases as the pressure is increased over the normal operating ranges for a pressure swing adsorber. MOFs provide some of this capability.

[0027] While the invention has been described with what are presently considered the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but it is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims.

Claims

CLAIMS:

1. A pressure swing adsorption process for the removal of hydrocarbons from a hydrogen feedstream comprising:

(a) passing the feedstream comprising hydrogen and at least one hydrocarbon constituent over an adsorbent, wherein the adsorbent comprises a metal organic framework (MOF) material, in an adsorption zone at a temperature and adsorption pressure sufficient to adsorb at least a portion of the hydrocarbon constituent in the feedstream and thereby generating an effluent hydrogen stream having a reduced hydrocarbon content, continuing to pass the feedstream over the adsorbent for a time until the adsorbent has substantially reached its adsorption capacity;

(b) reducing the pressure in the adsorption zone to a desorption pressure and for time sufficient to desorb at least a portion of the hydrocarbon therefrom and withdrawing a desorption effluent stream having an enriched hydrocarbon content; and repressurizing the adsorption zone to the adsorption pressure and repeating the steps (a) and (b).

2. The process of claim 1 further comprising passing a purge stream over the adsorbent during the desorbing step.

3. The process of claims 1 or 2 wherein the adsorption zone comprises a plurality of adsorbent beds comprising the adsorbent, and cycling the adsorbent beds through adsorption pressures, and desorption pressures in a sequential manner.

4. The process of claim 3 wherein the process of cycling the adsorbent beds comprises passing the adsorption beds through an adsorption zone and a desorption zone.

5. The process of claim 3 wherein the process of cycling the adsorbent beds comprises: pressurizing a first bed to the adsorption pressure, while depressurizing a second bed to the desorption pressure; switching flow streams from the first bed to the second bed, and from the second bed to the first bed; and pressurizing the second bed to the adsorption pressure, while depressurizing the first bed to the desorption pressure.

6. The process of any of the claims 1-5 further comprising: passing the effluent stream through a second adsorption zone at a temperature and pressure sufficient to adsorb at least a portion of the hydrocarbon in the effluent stream, wherein the adsorption zone has an adsorbent comprising a metal organic framework (MOF) material, and thereby generating a second effluent stream having a reduced hydrocarbon content; and reducing the pressure in the adsorption zone to a desorption pressure sufficient to desorb at least a portion of the hydrocarbon therefrom and withdrawing a desorption effluent having an enriched hydrocarbon content.

7. The process of any of the claims 1-6 wherein the MOF comprises a systematically formed metal-organic framework having a plurality of metal, metal oxide, metal cluster or metal oxide cluster building units, and an organic compound linking adjacent building units, wherein the linking compound comprises a linear dicarboxylate having at least one substituted phenyl group.

8. The process of any of the claims 1-7 wherein the MOFs are selected from the group consisting of MOF-5, a material having a general formula of Zn₄O(1, 4- benzenedicarboxylate)₃; IRMOF-6, a material having a general formula of ZrL₄θ(cyclobutyl 1,4-benzenedicarboxylate); IRMOF-3, a material having a general formula of ZrL₄θ(2-amino 1 ,4 benzenedicarboxylate)₃; and IRMOF-11 , a material having a general formula of Zn₄O(terphenyl dicarboxylate)₃,or Zn₄O(tetrahydropyrene 2,7-dicarboxylate)₃; IRMOF-8, a material having a general formula of Zn₄θ(2,6 naphthalene dicarboxylate)₃, MOF-177, a material having a general formula of Zn₄O(1, 3, 5-benzenetribenzoate)₃ and mixtures thereof.

9. The process of claim 1 wherein the temperature is operated from 0⁰C to 400⁰C; the adsorption pressure is from 2 MPa (20 atms.) to 5 MPa (50 atms.); and the desorption pressure is from 100 kPa (1 atm) to 1.5 MPa (15 atms.).

10. The process of any of the claims 1-9 further comprising recompressing the desorption effluent stream.