WO2017085461A1

WO2017085461A1 - Spirally wound gas-separation modules

Info

Publication number: WO2017085461A1
Application number: PCT/GB2016/053491
Authority: WO
Inventors: Davide Bocciardo; Yujiro Itami; Nori Miyagishima
Original assignee: Fujifilm Manufacturing Europe Bv; Fujifilm Corporation; Fujifilm Imaging Colorants Limited
Priority date: 2015-11-16
Filing date: 2016-11-08
Publication date: 2017-05-26
Also published as: GB201520175D0

Abstract

A gas-separation module comprising: a) a permeate collection tube comprising a hollow cylinder provided with perforations oriented along the longitudinal axis of the cylinder; and b) at least seven membrane envelopes wound spirally around the permeate collection tube; wherein: (i) the membrane envelopes each comprise a feed spacer and gas- separation membrane(s), wherein the feed spacer is disposed between the gas-separation membrane(s) to define a feed channel and wherein the feed spacer comprises a polymer having a Tg above 0°C; (ii) the module comprises a permeate channel between each of the membrane envelopes in gas-communication with the perforations; and (iii) the permeate collection tube and the membrane envelopes are disposed such that when the module is in use, feed gas entering the feed channel can pass through the perforations only after passing through the gas- separation membranes from the feed channel to the permeate channel. Also claimed are gas separation units and a process for separating gases.

Description

SPIRALLY WOUND GAS-SEPARATION MODULES

This invention relates to gas separation modules, to gas separation units and to their use for separating gases.

Gas separation modules typically comprise a permeate collection tube comprising a hollow cylinder provided with perforations oriented along the longitudinal axis of the cylinder. One or more membrane envelopes are wound spirally around the permeate collection tube. The membrane envelopes typically comprise a feed spacer and gas-separation membrane(s), wherein the feed spacer is disposed between the gas-separation membrane(s) to define a feed channel. The modules comprises a permeate channel between each of the membrane envelopes in gas-communication with the perforations and the permeate collection tube. The membrane envelopes are disposed such that when the module is in use, feed gas entering the feed channel can pass through the perforations and into the permeate collection tube only after passing through the gas-separation membrane(s) from the feed channel to the permeate channel.

Typically one or more of the gas-separation modules are fixed into a housing which can withstand high pressures to create a gas separation unit.

Modern gas-separation units typically comprise up to eight modules, sometimes more, in parallel or series, located within the housing. The housing typically has a single feed inlet, a single outlet for residual flow of the retentate gas, and one or more permeate outlets, the inlets and outlets usually being located on a side wall of the housing near opposite ends of the unit.

WO2012122207 describes spiral-wound liquid-separation modules wherein all components are prepared from plastic for use in the water purification industry (RO/UF). The present invention relates to gas-separation modules of the type which are often referred to as spiral-wound gas-separation modules.

A problem with existing gas-separation modules is that they degrade over time, resulting in a decline in their selectivity and permeance. For example, when separating a gas mixture comprising CO2 and CH₄, the selectivity and permeance of the module after 24 hours use are significantly different from than the selectivity and permeance after only 1 hour use. Furthermore, existing gas-separation modules can also suffer from an unacceptable drop in permeate pressure. There is a need for gas-separation modules whose selectivity and permeance change less when used for extended periods of time and which do no suffer from unacceptably high permeate pressure drops.

According to the present invention there is provided a gas-separation module comprising:

a) a permeate collection tube comprising a hollow cylinder provided with perforations oriented along the longitudinal axis of the cylinder; and b) at least seven membrane envelopes wound spirally around the permeate collection tube;

wherein:

(i) the membrane envelopes each comprise a feed spacer and gas-separation membrane(s), wherein the feed spacer is disposed between the gas- separation membrane(s) to define a feed channel and wherein the feed spacer comprises a polymer having a Tg above 0°C;

(ii) the module comprises a permeate channel between each of the membrane envelopes in gas-communication with the perforations; and

(iii) the permeate collection tube and the membrane envelopes are disposed such that when the module is in use, feed gas entering the feed channel can pass through the perforations only after passing through the gas- separation membrane(s) from the feed channel to the permeate channel. The term "comprising" is to be interpreted as specifying the presence of the stated parts, steps or components, but does not exclude the presence of one or more additional parts, steps or components.

Reference to an element by the indefinite article "a" or "an" does not exclude the possibility that more than one of the element(s) is present, unless the context clearly requires that there be one and only one of the elements. The indefinite article "a" or "an" thus usually means "at least one".

The invention will be described for the purposes of illustration only in connection with certain preferred embodiments; however, it is recognized that various changes, modifications, additions and improvements may be made to the illustrated embodiments by those persons skilled in the art, all falling within the spirit and scope of the invention.

FIG. 1 is a perspective, fragmented view of a partially constructed gas- separation module illustrating the invention.

In Fig. 1 , a permeate carrier (24) is attached to permeate collection tube (12) comprising perforations (14). Membrane envelopes (26) comprising a feed spacer (16) disposed between the gas separation membrane (18) are wound spirally around the permeate collection tube (12). For ease of viewing, Fig. 1 shows only four membrane envelopes (26) but the present invention requires at least seven membrane envelopes (26). The embodiment shown in Fig. 1 comprises further permeate carriers (24) disposed between each membrane envelope (26) in order to keep open a passage for permeate gas to flow to the perforations (14) and into the centre of the hollow permeate collection tube (12). The membrane envelopes (26) comprise a rectangular membrane sheet (18) folded around a feed spacer (16) and the folded edge of the membrane envelope abuts the permeate collection tube (12). The stack of at least seven membrane envelopes (26) and permeate carriers (24) is wound around the permeate collection tube (12) to provide a membrane structure comprising two parallel end faces and a third face of circular cross-section. Adjacent membrane envelopes (26) are adhered together such that feed gas passing from the left to the right in Fig. 1 can pass along the feed carriers (16) but cannot enter the permeate carriers (24) without first passing through the walls of membranes (18)

Feed gas may be prevented from entering the permeate carriers (24) without first passing through the membranes (18) by depositing adhesive (sometimes called a "glue line") along the left and right outside edges of the membrane envelopes (26), thereby forming a gas-tight seal.

The function of the permeate collection tube (or "tube" for short) is to collect the gas which has permeated through the membranes. The tube comprises a hollow cylinder provided with perforations oriented along the longitudinal axis of the cylinder e.g. in a random or regular pattern. The perforations allow permeate gas to flow from the permeate channel to the interior of the tube. Thus the permeate collection tube and the membrane envelopes are disposed such that when the module is in use, feed gas entering the feed channel can pass through the perforations (and into the permeate collection tube) only after passing through the gas-separation membranes from the feed channel to the permeate channel. The membrane envelopes are arranged such that the permeate can flow through the perforations and into the tube and the retentate gas cannot flow through the perforations.

The tube is typically constructed of a rigid material, for example a metal (e.g. stainless steel) or a plastics material. One will usually select a material which is stable to the permeate gas(es).

The cylinder may have any cross-sectional shape (e.g. oval, hexagonal, square etc.) although a circular cross-sectional shape is preferred.

The at least seven membrane envelopes typically comprises one or more outer membrane sheets and the inner feed spacer. The membrane sheets are usually rectangular and have two long edges and two short edges. Rectangular membrane sheets may be folded in two at the centre, and the feed spacer may be located inside the fold, typically against the inside, short edge.

The module preferably comprises 7 to 100 membrane envelopes (e.g. 10 or more, 10 to 100, 1 1 to 100, 12 to 100, 13 to 100 or 14 to 100 membrane envelopes), especially 20 to 50 membrane envelopes..

The gas-separation membranes are preferably composite membranes, e.g. comprising a discriminating layer and a porous support. The function of the discriminating layer is to preferentially discriminate between gases, separating a feed gas mixture into a permeate which has passed through the membrane and a retentate which does not pass through the membrane. The permeate and retentate typically comprise the same gases as the feed gas mixture, but one is enriched in at least one of the gases present in the feed gas and the other is depleted in that same gas.

The porous support is typically open pored, relative to the discriminating layer. The porous support may be, for example, a microporous organic or inorganic membrane, or a woven or non-woven fabric. The porous support may be constructed from any suitable material. Examples of such materials include polysulfones, polyethersulfones, polyimides, polyetherimides, polyamides, polyamideimides, polyacrylonitrile, polycarbonates, polyesters, polyacrylates, cellulose acetate, polyethylene, polypropylene, polyvinylidenefluoride, polytetrafluoroethylene, poly(4-methyl 1 -pentene) and especially polyacrylonitrile.

The porous support preferably has an average pore size of at least about 50% greater than the average pore size of the discriminating layer, more preferably at least about 100% greater, especially at least about 200% greater, particularly at least about 1000% greater than the average pore size of the discriminating layer. The pores passing through the porous support typically have an average diameter of 0.001 to 10pm, preferably 0.01 to 1 pm (i.e. before the porous support has been converted into a composite membrane). The pores at the surface of the porous support will typically have a diameter of 0.001 to 0.1 pm, preferably 0.005 to 0.05pm. The pore diameter may be determined by, for example, viewing the surface of the porous support by scanning electron microscopy ("SEM") or by cutting through the support and measuring the diameter of the pores within the porous support, again by SEM.

The porosity at the surface of the porous support may also be expressed as a % porosity, i.e.

% porosity = 100% x (area of the surface which is missing due to pores)

(total surface area)

The areas required for the above calculation may be determined by inspecting the surface of the porous support using SEM. Thus, in a preferred embodiment, the porous support has a % porosity >1 %, more preferably >3%, especially >10%, more especially >20%.

The porosity of the porous support may also be expressed as a CO2 gas permeance (units are m³(STP)/m².s.kPa). When the composite membrane is intended for use in gas separation the porous support preferably has a CO2 gas permeance of 5 to 150 x 10^"5 m³(STP)/m².s.kPa, more preferably of 5 to 100, most preferably of 7 to 70 x 10^"5 m³(STP)/m².s.kPa.

Alternatively the porosity is characterised by measuring the N₂ gas flow rate through the porous support. Gas flow rate can be determined by any suitable technique, for example using a Porolux™ 1000 device, available from Porometer.com. Typically the Porolux 1000 is set at the maximum pressure (about 34 bar) and one measures the flow rate (L/min) of N₂ gas through the porous support under test. The N₂ flow rate through the porous support at a pressure of about 34 bar for an effective sample area of 2.69 cm² (effective diameter of 18.5 mm) is preferably >1 L/min, more preferably >5 L/min, especially >10 L/min, more especially >25 L/min. The higher of these flow rates are preferred because this reduces the likelihood of the gas flux of the resultant composite membrane being reduced by the porous support.

The abovementioned % porosity and permeance refer to the porous support used to make the composite membrane.

The porous support preferably has an average thickness of 20 to 500pm, preferably 50 to 400pm, especially 100 to 300pm.

One may use an ultrafiltration membrane as the porous support, e.g. a polysulfone ultrafiltration membrane, cellulosic ultrafiltration membrane, polytetrafluoroethylene ultrafiltration membrane, polyvinylidenefluoride ultrafiltration membrane and especially polyacrylonitrile ultrafiltration membrane. Asymmetric ultrafiltration membranes may be used, including those comprising a porous polymer membrane (preferably of thickness 10 to 150pm, more preferably 20 to 100pm) and optionally a woven or non-woven fabric support. The porous support is preferably as thin as possible, provided it retains the desired structural strength.

Typically the discriminating layer is present on one side of the porous support or is partially or wholly within the porous support.

Preferred discriminating layers comprise a polyimide, especially a polyimide having -CF₃ groups and optionally carboxylic acid groups. Polyimides comprising -CF₃ groups may be prepared by, for example, the general methods described in U.S. Pat. Reissue No. 30,351 (based on US 3,899,309) US 4,717,394 and US 5,085,676. Typically one or more aromatic dianhydrides, preferably having -CF₃ groups, are condensed with one or more diamines. The diamine(s) and dianhydride(s) copolymerise to form an AB-type copolymer having alternating groups derived from the diamine(s) and dianhydride(s) respectively.

Preferably the discriminating layer comprises groups of the Formula (1 ) wherein Ar is an aromatic group and R is a carboxylic acid group, a sulphonic acid group, a hydroxyl group, a thiol group, an epoxy group or an oxetane group:

Formula (1 ). Optionally there may be a polymeric layer between the porous support and the discriminating layer, often referred to as a gutter layer. Preferred gutter layers comprise a dialkylsiloxane.

The feed spacers are preferably open, channel-forming grid materials, e.g. polymeric grid, or corrugated or mesh material.

The feed spacers are typically constructed entirely from a polymer or a mixture of polymers such that the feed spacer has a Tg above 0^°C.

Preferably the feed spacers are constructed from a polar polymer. Polar polymers typically contain electron withdrawing and/or electron donating atoms of groups, often as part of the backbone of the polymer chain. Thus a dipolar moment will usually be present along the backbone of the polymer chain. Polar polymers typically comprise a halide atom (e.g. fluorine) and/or a backbone chain comprising a nitrogen, sulphur and/or oxygen atom or a combination of such atoms, e.g. as in sulphone, amide, urea and urethane groups. Thus preferred polar polymers are fluoropolymers, polysulphone, polyurethane, polyether, polyamides and polyurea polymers.

Examples of polar polymers include a polyether, polysulfone, polyester, nylon, polyurethane, polyurea, polyamide, and polyethylene terephthalate polymers and also fluoroploymers (e.g. Teflon^(R)), in each case having a Tg above 0°C. The preferred Tg of the polymer is at least 20°C, e.g. 20 to 200^°C, or preferably 20 to 100^°C, especially 25 to 75^°C (e.g. 30 or 50^°C). The feed spacers do not need to all have the same Tg or be constructed from the same materials but it is preferred that the feed spacers all have the same Tg and are constructed from the same materials.

Tg may be measured using Differential Scanning Calorimetry (DSC), e.g. using a Mettler Toledo DSC 823e.

The feed spacers preferably each independently have a thickness of less than 900 pm, more preferably less than 700 pm and especially from 500 to 200 pm.

The permeate channels between each of the membrane envelopes preferably have a depth of 100 to 1000 pm, more preferably 150 to 800pm, especially 200 to 500 pm, especially 250 to 400 pm. The function of the permeate channels is to create a gap through which permeate gas can flow to the permeate collection tube via the perforations therein.

Preferably one or more of the permeate channels, more preferably all of the permeate channels, further comprise a permeate carrier.

The optional permeate carrier(s) preferably each independently comprise one or more macroporous sheet (e.g. a sheet having pores of average size >30pm). The macroporous sheets typically have very high gas permeability. The macroporous sheets are not included to discriminate between gases but instead to provide a pathway for the permeate gases to flow through. Suitable macroporous sheets include woven fabric, non-woven fabric, especially knitted fabric, more especially a warp knitted fabric or a weft knitted fabric. Suitable weft knitted fabrics can be made from one yarn, although more than one yarn can be used.

The permeate carrier(s) optionally comprise, for example, a natural fibre or a man-made fibre, e.g. polyester, polysulfone, polyester, nylon, teflon, polypropylene, polyphenylenesulfide, etc. The fibres are optionally resin coated, e.g. with a resin such as an epoxy or melamine resin.

Preferred permeate carriers comprises at least two macroporous sheets and a gas-impermeable sheet (sometimes called an interfoil) disposed between the two macroporous sheets. Suitable gas-impermable sheets have a preferred thickness of less than 700 pm.

The permeate carrier(s) preferably each independently have an average thickness of 150 to 800μηι, preferably 200 to 500 μητι, especially 250 to 400 pm.

The preferred thickness ratio of the feed spacers to permeate carriers (including all sheets, e.g. any interfoil) is preferably at least 0.5, more preferably at least 1 .0, e.g. from 0.5 to 5, especially 1 to 3. Such ratios are useful for reducing permeate pressure losses.

In a preferred embodiment the permeate carrier further comprises a gas impermeable sheet. For example, the permeate carrier may comprise two macroporous sheets and a gas-impermeable sheet disposed between the macroporous sheets, or one macroporous sheet folded around a gas-impermeable sheet.

The modules optionally further comprise couplings, for example, an end flange, typically with O-ring seals, that can be used to join or snap-fit modules together in series.

Typically each module further comprises an anti-telescope device ("ATD") to prevent the membrane envelopes from unwinding from the permeate collection tube, e.g. when gas is introduced to the module under pressure. As ATD one may use, for example, restraining bands, outer wraps, or a mechanical device. A preferred method for preventing the membrane envelopes from unwinding from the permeate collection tube is by filament winding, in which a glass fibre filament dipped in an epoxy resin is wound around the wound membrane structure and cured. The wound membrane structure can then be loaded into the housing and optionally connected to further modules.

Typically the modules comprise a curved wall of circular cross-section, in addition to the end faces, which wall meets the two end faces. For example the module may have a generally cylindrical shape comprising the two (circular) end faces and a wall (e.g. of circular cross-section) joining the two end faces together. The flat end faces may comprise some surface texture e.g. caused by the edges of the membranes wound spirally around the permeate collection tube.

According to a second aspect of the present invention there is provided a gas separation unit comprising one or more modules according to the first aspect of the present invention.

Preferably the gas separation unit comprises a rigid housing. The modules may be disposed within said housing.

The housing is preferably made from metal (e.g. stainless steel) and may have any cross-sectional profile. Typically the housing is a cylindrical or tube-like structure. The housing optionally further comprises fixtures which enable the unit to be connected to one or more further units, e.g. in parallel. In this way one may construct large modular installations comprising many banks or rows of gas- separation units, each unit holding many modules.

Such gas-separation units may comprise plug-type closures or end caps that may be removed to provide "full-bore" access to the interior of the housing for installation or replacement of the modules. The modules typically have a cylindrical design with simple end seals to enable long chains or strings of modules to be connected in series. This facilitates the loading and unloading of modules into the unit, and simplifies the construction of large capacity gas- separation plants.

According to a third aspect of the present invention there is provided a process for separating a feed gas comprising polar and non-polar gases into a gas stream rich in polar gas and a gas stream depleted in polar gas comprising passing the feed gas through a module according to the first aspect of the present invention or through a unit according to the second aspect of the present invention.

To illustrate the process according to the third aspect of present invention, a feed gas comprising polar and non-polar gases may be separated into a gas stream rich in polar gases and a gas stream depleted in polar gases. In many cases the membranes have a high permeability to polar gases, e.g. CO2, H₂S, NH3, SOx, and nitrogen oxides, especially NO_x, relative to non-polar gases, e.g. such as hydrocarbons such as alkanes, H₂, N₂, and water vapour.

The target gas may be, for example, a gas which has value to the user of the membrane and which the user wishes to collect. Alternatively the target gas may be an undesirable gas, e.g. a pollutant or 'greenhouse gas', which the user wishes to separate from a gas stream in order to meet product specification or to protect the environment.

The gas-separation modules and units according to the invention are particularly useful for purifying natural gas (a mixture which predominantly comprises methane) by removing polar gases (CO2, H₂S); for purifying synthesis gas; and for removing CO2 from hydrogen and from flue gases. Flue gases typically arise from fireplaces, ovens, furnaces, boilers, combustion engines and power plants. The composition of flue gases depend on what is being burned, but usually they contain mostly nitrogen (typically more than two-thirds) derived from air, carbon dioxide (CO2) derived from combustion and water vapour as well as oxygen. Flue gases also contain a small percentage of pollutants such as particulate matter, carbon monoxide, nitrogen oxides and sulphur oxides. Recently the separation and capture of CO2 has attracted attention in relation to environmental issues (global warming).

The gas-separation modules and units according to the invention are particularly useful for separating the following: a feed gas comprising O2 and N₂ into a gas stream richer in O2 than the feed gas and a gas stream poorer in O2 than the feed gas; a feed gas comprising CO2 and N₂ into a gas stream richer in CO2 than the feed gas and a gas stream poorer in CO2 than the feed gas; a feed gas comprising CO2 and CH₄ into a gas stream richer in CO2 than the feed gas and a gas stream poorer in CO2 than the feed gas; a feed gas comprising CO2 and H₂ into a gas stream richer in CO2 than the feed gas and a gas stream poorer in CO2 than the feed gas, a feed gas comprising H₂S and CH₄ into a gas stream richer in H₂S than the feed gas and a gas stream poorer in H₂S than the feed gas; and a feed gas comprising H₂S and H₂ into a gas stream richer in H₂S than the feed gas and a gas stream poorer in H₂S than the feed gas. In the most preferred the module of the invention is used for separating polar gasses like (CO2 and/or H₂S) from apolar gasses such as hydrocarbons at temperatures with feed gas temperatures 30 and 80^°C.

The invention is further illustrated by the following Examples.

In the Examples "porous PET" is HW 2503 polyester and epoxy resin 75:25% from Hornwood.

Gas Permeance and Selectivity

The Permeance of CH₄ and CO2 through a gas-separation unit comprising one module was measured at 40°C. The pressure of gas at the inlet was 6300 kPa (63 bar) and the feed gas in one set of experiments had the composition 13 v/v % CO2 and 87 v/v % CH₄. Flow, pressure, and gas composition of each feed gas, permeate gas, and retentate gas was measured and flux and selectivity were back-calculated according to the formulation described in "Calculation Methods for Multicomponent Gas-separation by Permeation" (Y. Shindo et al, Separation Science and Technology, Vol. 20, Iss. 5-6, 1985. The unit for Permeance: 1 GPU is 7.5x10^"9Nm³/ m².kPa.s.

The permeance of CH₄ and C02 gas was calculated based on the following equation: Permeance of each gas (m³(STP)/m² kPa s)

Permeate flow (m³(STP)/s)

Volume fraction of gas in the permeate

A = Membrane area (m²)

Feed gas pressure (kPa)

Volume fraction of each gas in the feed

Permeate gas pressure (kPa)

is standard temperature and pressure, which is defined here as

25.0°C and 1 atmosphere (101 .325 kPa).

Selectivity (a_Co2 cH ) was calculated from Q_Co2 and Q_CH calculated above, based on following equation:

Permeate Pressure Drop

The permeate pressure drop of the modules refers to the drop in permate pressure after 24 hrs.

Pressure drops were estimated using in-house measurements combined to a mathematical model set out in the article entitled " Approximate modeling of spiral-wound gas permeators" by Runhong Qi and Michael A. Henson in Journal of Membrane Science, 1996. 121 : p. 1 1 -24. The in-house measurements passed gas at different pressures through the feed spacers having Membrane M1 on each side (dimensions 10 x 15 cm). The friction coefficients were then calculated and inputted to the abovementioned mathematical model from which permeate pressure drops were estimated. Examples 1 to 4 and Comparative Examples CEx1 to CEx5

Stage a) Preparation of Membrane 1

Membrane 1 used in the present Examples was prepared exactly as described for Membrane 1 in WO 2015/049498, page 13, line 16 to page 15, end of Table 4, which is incorporate herein by reference thereto.

Stage b) Preparation of Membrane Envelopes ME1 to ME5

Membrane envelopes ME1 to ME5 were prepared by folding a rectangular sheet of Membrane 1 described above around the feed spacers indicated in Table 1 below.

Each feed spacer (FS-1/FS-2/FS-3 or FS-4)) was positioned at the centre of the short edge of Membrane 1 , inside a fold in Membrane 1 and fixed there using an adhesive to give respectively membrane envelopes ME1 or ME2 or ME3 or ME4 or ME5. Table 1

The Delstar/Naltex feed spacers were obtained from Delstar.

The PETEX^® and PEEKTEX^® feed spacers were obtained from Sefar.

The Tgs descibed in Table 1 were measured by Differential Scanning

Calorimetry (DSC) using a Mettler Toledo DSC 823e.

Stage c) Preparation gas separation modules

(c1 ) Preparation of the Permeate Carrier Tube

A permeate collection tube was prepared as follows: A tube having a circular cross-section, an internal diameter of 47mm and an external diameter of 50mm, made from stainless steel Grade 316, was cut to a length of 1 m. Holes of diameter 4mm were drilled through the tube wall to give an aperture ratio of 15% (i.e. the holes occupied 15% of the surface area of the permeate carrier tube wall). (c2) Preparation of the Permeate Carrier

A rectangular, gas-impermeable sheet of PE interfoil was sandwiched between two rectangular sheets of porous PET. The dimensions of the PE interfoils and the porous rectangular sheets were dependant on the number of membrane leaves present in the module. For modules containing 6, 15 or 22 membrane envelopes the dimensions of the PE interfoil were 875 mm X 3180 mm, 875 mm X 1260 mm or 875 mm X 850 mm respectively. For modules containing 6, 15 or 22 envelopes the dimensions of the porous PET were 950 mm X 3230 mm, 950 mm X 1310 mm or 950 mm X 900 mm respectively.

The gas impermeable sheet was positioned at the centre of the short edge between two of the porous PET sheets and fixed there using an adhesive to give a carrier comprising a PE interfoil disposed between two porous PET sheets. For modules containing six membrane envelopes this process was repeated a further 5 times to give 6 permeate carriers. For modules containing 15 or 22 membrane envelopes this process was repeated 14 or 21 times to give 15 or 22 permeate carriers respectively.

(d) Preparation of Modules M1 to M4 and Comparative Modules CM1 to CM5 In Comparative Examples CEx1 and CEx5 the modules contained six rectangular membrane envelopes of the dimensions 990 mm x 6410 mm and the feed spacers had the dimensions 990 mm X 3180 mm.

In Example 4 and Comparative Example CEx2 the modules contained 15 membrane envelopes of the dimensions 990 mm X 2570 mm and the feed spacers had the dimensions 990 mm X 1250 mm.

In Examples 1 to 3 and Comparative Examples CEx3 and CEx4 the modules contained 22 membrane envelopes of the dimensions 990 mm X 1750 mm and the feed spacers had the dimensions 990 mm X 840 mm.

In all of the modules the membrane envelopes and permeate spacers were positioned as shown in Fig. 1 , glued and wound spirally onto the permeate carrier tube. This provided cylindrical, gas-separation modules comprising alternate membrane envelopes and permeate carriers having two parallel, essentially circular end faces and an overall circular cross-sectional profile. Plastic bands were applied to the resultant module to prevent the wound envelopes from unwinding to give gas separation modules M1 to M4 and Comparative Modules CM1 to CM5.

Preparation of Gas-Separation Units

Gas-separation units were constructed by fitting one type of each of that gas separation Modules M1 to M4 or Comparative Modules CM1 to CM5 in series in a steel housing. The housing comprised a feed gas inlet orientated at an angle of 90^° relative to the axis of the module and gas outlets for permeate gas and retentate gas, located on the side wall of the housing.

A gas mixture comprising CO2 and CH₄ was fed into the housing and the selectivity and flux of the gas separation unit (containing one module) was measured after 1 hour and after 24 hours of gas feeding for each module.

The pressure of feed gas at the gas-separation unit inlet was 6300 kPa (63 bar). The feed gas had the composition described in Table 2:

Table 2 - Feed Gas Composition

Results for gas separation Modules M1 to M4 and Comparative Modules CM1 to CM5 are shown in Table 3 below. As can be seen from Table 3, the modules comprising at least 7 membrane envelopes and a feed spacer of Tg >0^°C showed least change in selectivity and permeance over the time period 1 hr to 24 hours. Furthermore, the modules used in Comparative Examples CEx1 and CEx5 suffered from unacceptably high permeate pressure drops of more than 1500 mbar.

Table 3 - Results

Note: ^* The modules used in Comparative Examples CEx1 and CEx5 suffered from unacceptably high permeate pressure drops of mo 5 than 1500 mbar.

Claims

1 . A gas-separation module comprising:

wherein:

(i) the membrane envelopes each comprise a feed spacer and gas-separation membrane(s), wherein the feed spacer is disposed between the gas- separation membrane(s) to define a feed channel and wherein the feed spacer comprises a polymer having a Tg above 0^°C;

(iii) the permeate collection tube and the membrane envelopes are disposed such that when the module is in use, feed gas entering the feed channel can pass through the perforations only after passing through the gas- separation membranes from the feed channel to the permeate channel.

2. A module according to claim 1 wherein one or more of the permeate channels further comprises a permeate carrier.

3. A module according claim 1 or claim 2 wherein the feed spacers have a thickness of less than 900 μιτη.

4. A module according to any one of the preceding claims which comprises 10 to 100 of the membrane envelopes.

5. A module according to any one of the preceding claims wherein the gas- separation membrane(s) comprise a discriminating layer and a porous support.

8. A module according to claim 5 wherein the discriminating layer comprises a polyimide.

7. A module according to claims 5 or 6 wherein the discriminating layer comprises groups of the Formula (1 ) wherein Ar is an aromatic group and R is a carboxylic acid group, a sulphonic acid group, a hydroxyl group, a thiol group, an epoxy group or an oxetane group:

Formula (1).

8. A module according to any one of the preceding claims wherein the thickness ratio of the feed spacer to the permeate carrier is at least 0.5.

9. A module according to any one of the preceding claims wherein the feed spacers comprises a polymer having a Tg of 20 to 200^CC.

10. A module according to any one of the preceding claims wherein the polymer is a polar polymer.

1 1 . A gas separation unit comprising one or more modules according to any one of the preceding claims.

12. A process for separating a feed gas comprising polar and non-polar gases into a gas stream rich in polar gas and a gas stream depleted in polar gas comprising passing the feed gas through a module according to any one of claims 1 to 10 or a unit according to claim 1 1 .