The membrane separation processes to be considered here include the familiar liquid processes: reverse osmosis, nanofiltration, ultrafiltration and microfiltration, but also include the less familiar fluid separation processes of pervaporation, gas/gas separation (or gas permeation), vapour permeation and electrodialysis. A key feature of the bulk chemical sector is that all these membrane separation processes can be found in it, with, for example, the separation of air into its component gases, or the dehydration of ethanol/water azeotropes, or the recovery of hydrogen from cracker gases being as important to their industrial sub-sectors as ultrafiltration or microfiltration are in theirs.
It is important to note that reverse osmosis (with which the whole history of membrane separations began, just 45 years ago) and nanofiltration are not filtration processes in the normally accepted sense of the word, i.e. a separation of fluid from suspended solid particles or liquid droplets whereby the fluid passes without change of phase through a barrier by means of pores that are continuous from one side of the barrier to the other, and whose size is such as to hold back the particles of droplets.
Reverse osmosis (and nanofiltration) by contrast operate by diffusion from a solution. Under the high differential pressure across the membrane, the solvent from the solution actually dissolves in the material of the membrane, diffuses across it and transfers out into the clean solvent on the other side. It is not a perfect separation, because the dissolved species from the feed solution have a definite ability to diffuse through it as well, but the diffusion coefficient for the solvent is so much higher than that for the solute that the separation is virtually complete.
A similar diffusion mechanism applies for the other membrane processes mentioned above – of a vapour out of a liquid mixture for pervaporation, or of one gas out of a mixture for gas or vapour permeation. These diffusion processes all require the continuity of material represented by a polymeric membrane, in sheet or tubular form, and cannot be undertaken in ceramic materials.
Ultrafiltration is another borderline case as it is used to separate solvents from solutions of very large molecules, as well as the liquid from a suspension of colloidal solids. While it is normal to describe microfiltration membranes in terms of a retained particle size, the range being from 0.05μm (50nm) to 3μm, this is more difficult with ultrafiltration, for which the range is sometimes quoted in diameter terms (3–100nm or 0.03–0.1μm), and sometimes as an approximate molecular weight, in terms of Daltons, the unit of atomic weight – when ultrafiltration covers from about 5000D (5kD) up to about 250kD.
Although not quite so meaningful, nanofiltration can be defined as covering 0.8–8.0nm or 200–14000D, with reverse osmosis running up to 200D.
Membrane processes
Most membrane processes – which is another way of saying most membranes – are characterised by two key process parameters: flux (i.e. the rate of flow of fluid through the membrane) and selectivity (i.e. the ability of the membrane to “reject” – prevent the passage of – one or more species in the feed suspension or solution). The selectivity is governed by the intrinsic nature of the membrane material, built into it by its method of manufacture, and measured by its permeability to the species in question. The flux is determined by the specific resistance of the membrane material (i.e. the volumetric or mass flow of fluid per unit of membrane area) under a given differential pressure across the membrane, so that the flux increases with the operating area of the membrane and with the applied pressure.
The main operating problem of membrane separation processes throughout their whole history has been the ease with which the membrane material plugs, causing the resistance to flow to increase. This behaviour is normally called fouling, from the slimy solids present in the brackish water that was the first membrane feed. These solids deposit on the upstream membrane surface and eventually block it. The plugging process is accentuated by the concentration polarisation that occurs in the relatively quiescent fluid zone close to the membrane surface, as the species separated from previously processed fluid build up in this zone and interfere with fresh material trying to get to the surface.
The problems of fouling and concentration polarisation have found some resolution in the process arrangement that causes the feed liquid to flow parallel to the membrane surface, rather than perpendicular to it, so scouring the surface as it moves across, thinning the surface layer and removing deposited material. The consequent “cross-flow” filtration method has been one of the most important equipment developments in the filtration industry, especially when coupled with the devices that cause the membrane surface to move with respect to a closely positioned stationary surface, in rotating or vibrating filter systems.
Not only does the bulk chemical industry employ all of the membrane separation processes, but partly because of that, the sector gives home also to all of the various physical embodiments of membranes: flat sheets, plate and frame, pleated cartridges, tubular, hollow fibre, capillary module, and spiral wound. More particularly, this end-use sector has considerable demand for membranes able to resist high temperature or highly corrosive fluids, such that metallic membranes and ceramic materials, especially of the monolith type with parallel cylindrical chambers, are widely used.
The bulk chemicals industry
The whole range of chemicals manufacturing activity (including makers of personal care and household consumable products), if taken together, is the largest end-user sector for membrane modules. The full range includes some very different industries, varying widely in nature and size, covering the production of:
- gases, acids, alkalis, fertilisers and other bulk inorganic chemicals;
- bulk organic chemicals;
- man-made fibres;
- other agrichemicals;
- inks, paints and other coatings;
- soap, detergents, toiletries and perfumes;
- explosives, glues, essential oils, etc;
- pharmaceuticals and medicinals; and
- chemicals and other products made by biochemical means (but excluding fermented beverages).
The first three categories in this list are the ones normally classed together as the “bulk chemicals and petrochemicals” sector – the subject of this article. The next four categories are taken together as “fine chemicals” production. The whole range of the chemicals industries, including pharmaceuticals and biotechnical products, represented the largest part of the membrane module market in 2007, with 30.6% of the global total, expected to grow at about 8.5% annually for the next few years.
The key bulk chemicals are the inorganic acids (sulphuric, nitric and hydrochloric), phosphates and fertilisers, chlorine, caustic soda and soda ash, and the organic intermediate materials for petrochemicals (olefins, methanol and aromatic compounds), as well, of course, as the petrochemicals themselves, especially the ever-increasing range of thermoplastic polymers.
During the last couple of decades, the chemicals industry in general, and the inorganic chemicals component in particular, have undergone a major structural change. Prior to the restructuring, all of the large chemical companies were well diversified over the whole range of chemical products. By contrast, the chemicals industry is now very changed, with many of the larger companies concentrating on specialty chemicals or life science products, with the latter steadily being split further into agrichemicals and pharmaceuticals. The bulk, or commodity, chemicals activities have largely been spun off, or sold, or moved to parts of the world where costs are lower. Many famous names have vanished completely, including Albright & Wilson, Hoechst, ICI, and Union Carbide.
A consequence of these changes, and of the underlying economic forces, is that the bulk chemicals sector has been growing less rapidly than the whole industry, and so has its demand for membranes. It is estimated that bulk chemicals and petrochemicals applications represented about 40% of the membrane market in the whole chemicals industry in 2007 (12% of the global membrane market), or about $1.5 billions, growing at just over 5% per annum.
As has already been mentioned, the bulk chemicals sector has a wide range of operating conditions in its separation applications. Most other end-use sectors deal with liquids that are relatively bland as far as operating conditions go – moderate temperatures and pressures, low liquid viscosities, and little, if any, corrosive action. In the bulk chemical industries, on the other hand, membranes frequently have to be specified to withstand highly corrosive liquids, toxic materials, and quite high temperatures. The trends, as far as these operating parameters are concerned, are for increased severity, rather than less. One benefit of this sector in the membrane market, however, is that operators are well accustomed to high pressure working, so that the differential pressures required for some membrane processes will normally present no problems.
Membrane applications
The “standard” membrane processes (RO, NF, UF and MF) are now reasonably commonplace in the bulk chemicals sector, even nanofiltration. This is a consequence of the marked reduction in price of membranes in bulk quantities to match the relatively large process throughputs required in this industry. A marked feature of this succession of membrane processes is the way in which each process is moving upwards in effective cut sizes, i.e. “looser” in membrane structure, so as to take market share from the next process in order. Thus, reverse osmosis, which began life as a device solely required to separate sodium chloride from water, is now being used to reject a wider range of solutes. Nanofiltration in turn is dealing with quite large inorganic ions. Ultrafiltration is becoming the “pre-filter” of choice ahead of reverse osmosis desalination plants, and membranes are being employed with great success in the microfiltration range, where until relatively recently, they could not have been considered. A prime example of the latter situation is the rapid spread of the microfiltration-based MBR (membrane bioreactor), combining a biological process with a membrane separation process.
The expansion of membrane separations into the microfiltration range is being driven by the market's wish for finer degrees of filtration of its products (or waste streams), to match its customers' demands for clearer final products or environmental regulators demands for less contaminating effluents. In whichever part of the bulk chemicals sector a need exists for fine filtration, in the 1μm region, then the chances are that it will now be met by a membrane separation process, assuming that the materials of construction can withstand the process conditions.
In the somewhat more exotic processes, mainly involving gases or vapours, gas permeation is widely used for the separation of gas mixtures in both the chemical and petrochemical components, where gas recovery is an essential process. Higher temperature resistant membranes have an important role to play here, especially in petroleum refinery operations, for example in the recovery of hydrogen from cracker off gases, and for the separation of ethylene and aromatics. The separation and recovery of carbon dioxide, en route to its sequestration, as part of a greenhouse gas emission reduction programme, could well become a large membrane market. At present, the largest application for gas permeation is in the separation of air into its constituent gases, where locally situated air separation plants are most likely to be based on membrane separation.
Pervaporation, the separation of two perfectly mixed liquids (usually where they cannot be separated by distillation, because they form an azeotropic, constant boiling mixture at a particular concentration ratio – most famously the azeotrope of ethanol and water), is easily achieved through a membrane, by virtue of the different diffusion rates of the two vapours. It is also used to treat rinse waters that have become contaminated by VOCs (volatile organic compounds, hazardous to human health), such as solvents, degreasers and petroleum-based mixtures. In either case, 99% contaminant removal can be achieved.
Vapour permeation is used for the separation of a mixture of saturated vapours, and is often combined with distillation, to separate solvents from exhaust air streams, especially in degassing operations such as resin production.
The dialysis processes (especially electrodialysis) are used for the selective extraction of ions from solution in water and/or their concentration in one liquid stream. The most important application of dialysis (blood processing for kidney action support) is not replicated in the chemicals sector, but electrodialysis, using ion exchange membranes, is finding increasing usage.
