MEMBRANE SEPARATION PROCESS FOR DEHYDRATING A GAS OR VAPOUR OR LIQUID MIXTURE BY PERVAPORATION, VAPOUR PERMEATION OR GAS SEPARATION

Abstract: A membrane separation process for dehydrating a gas or vapour or liquid mixture by pervaporation, vapour permeation or gas separation comprises: passing the mixture along the retentate side of a membrane and generating a driving force in order to transport water through the membrane to the permeate side thereof. In order to diminish the permeate volume, to improve the coefficient of performance (COP) and to reduce the acquisition and maintenance costs of the equipment the driving force is generated in that a highly concentrated solution of one or more hygroscopic salts (brine) is situated at the permeate side of the membrane. Preferably the hygroscopic capacity of the brines, expressed as a percentage of water vapour pressure reduction with respect to pure water, has a value of over 50%, preferably of over 80%.

Description: Title: Membrane separation process for dehydrating a gas or vapour or liquid mixture by pervaporation, vapour permeation or gas separation The invention relates to a membrane separation process. In the dehydration of a gas by means of gas separation membranes, the dehydration of organic vapours by vapour permeation and the dehydration of organic liquids by pervaporation, it is known to pass the mixture of gases or vapours or liquids along the retentate side of a membrane and to generate a driving force in order to transport the water from the mixture through the membrane to the permeate side thereof. In the case of the drying of gases by water-selective gas separation membranes, the separation is usually based on dissolution and diffusion behaviour. The gas to be dried should be fed to the retentate side at a high pressure and/or a partial vacuum should be applied to the permeate side in order to obtain sufficient driving force for the transport of water through the membrane. A disadvantage of this procedure is the usually relatively very high volume of the permeate compartment which is necessary to counteract pressure loss and, consequently, loss of driving force. Another important disadvantage is the frequently high loss of gas to be dehydrated, which occurs through permeation together with the water vapour. If gases are dehydrated under increased pressure, the gas separation membranes have to have a high mechanical strength in order to be resistant to this high pressure. In vapour permeation and pervaporation processes, use is also made of selective membranes which are frequently composed of a composite of a thick sieve-type base and a very thin compact membrane top layer. Here again, the driving force is generated by applying a pressure difference across the membrane, which is usually achieved by maintaining a vacuum (for example, of less than 10 kPa) at the permeate side. The permeated water can be removed by inserting a condenser upstream of the vacuum pump. The disadvantage of this method is that, as a consequence of the poor mass transfer in the vacuum and the necessity of counteracting pressure drop, the permeate volume has to be very large. In a membrane module, usually only 1 to 10 m 2 of membrane surface is installed per m 3 of module volume. The proportion taken up by the equipment in the costs of a vapour permeation and pervaporation process is consequently relatively high (compare the compactness of a membrane module in reverse osmosis and gas separation processes: 100 to 500 m 2 of membrane surface per m 3 of module volume) . This disadvantage of an unduly large membrane module exists even if a flushing gas such as nitrogen is passed along the permeate side of the membrane, which nitrogen is fed to a condenser in order to remove the water therefrom. Another important disadvantage of the abovementioned processes is the relatively unfavourable coefficient of performance (COP) , which is a measure of the energy efficiency of the water removal by means of a condenser. This is primarily due to the low energy efficiency in the cooling. Finally, the acquisition and maintenance costs of the equipment are high and the vacuum equipment is in general susceptible to malfunc¬ tion. The object of the invention is to avoid said disadvantages of known gas separation, vapour permeation and pervaporation processes, at least to a significant extent. Starting from a membrane separation process for the dehydration of a gas or vapour or liquid mixture by pervaporation, vapour permeation or gas separation comprising: passing the mixture along the retentate side of a membrane and generating a driving force in order to transport water through the membrane to the permeate side thereof, it is proposed, according to the invention, to generate the driving force in that a highly concentrated solution of one or more hygroscopic salts (brine) is situated at the permeate side of the membrane. In general, the brine will be continuously or discontinuously removed from the permeate side of a membrane and the water taken up by the brine will be evaporated from the brine and worked-up brine will be fed back to the permeate side of the membrane. The salts used in the brines are characterised by combinations of small cations and large anionε or of large cations and small anions or of mixtures thereof, which combinations break up water structure. The anions and cations may be either inorganic or organic in nature. Owing to the stability at high temperatures, however, combinations of inorganic ions will be chosen in most cases. The brines should have an appreciable hygroscopic capacity in order to be able to generate sufficient driving force. The required hygroscopic capacity will depend on the water activity of the gas, vapour or liquid mixture to be dehydrated. If said water activity is low, the hygroscopic capacity of the brine should be high. The hygroscopic capacity may be expressed, for example, as the percentage by which a brine lowers the vapour pressure of water at a particular temperature. The table below gives the hygroscopic capacity of a number of salt solutions expressed as a percentage of water vapour pressure reduction in saturated solution at 25 β C. Brine Hygroscopic capacity (%) Brines which are suitable for use in the present invention will have to have a hygroscopic capacity higher than 0#, preferably higher than 8θ#. This means that brines based on, for example, sodium chloride or potassium dichromate will not exhibit the desired effect. Examples of brines having adequate hygroscopic capacity to be used in the membrane processes according to the invention are, inter alia, LiBr, CsF, KC 2 H 3 0 2 , MgCl 2 and mixtures of, for example, LiBr/ZnBr 2 /CaBr 2 . Making use of a highly concentrated solution of hygroscopic salts at the permeate side of gas separation, vapour permeation and pervaporation membranes achieves an appreciable improvement with respect to equipment design (high membrane surface area per m 3 of module volume) and procedure. The brines reduce the water activities at the permeate side (vapour tensions of less than 1 kPa at a temperature of over 8θ * C achievable) in a manner such that a large driving force can be established across the membrane. It is of importance that, during the dehydration of a gas and the dehydration of a vapour, the brine cannot come into contact with the gas or the vapour, respectively. In the event of direct contact, brine in the gas could be entrained by aerosol formation in the dehydrated gas or vapour, and this is extremely disadvantageous. This is an additional advantage of using a membrane which otherwise primarily has the aim of achieving the desired selectivity (sharpness of separation). In addition, the flexibility of the process increases sharply as a result of this: the flows of gas or vapour or of absorption liquid respectively may be varied independently over a wide range. The invention can be used in the following membrane designs: tubular membranes (diameters of a few millimetres to a few centimetres), plate-type membranes (plate and frame), spirally wound membranes and hollow membrane fibres (hollow fat fibres: internal diameter greater than 500 μm and hollow fine fibres: internal diameter between 50 and 200 μm) . Although the permeate volume: membrane surface ratio is usually more than 0.1 m 3 .m 2 in known membrane dehydration processes, a value of less than 0.005 m 3 .m 2 , and in most cases of less than 0.0005 m 3 .m 2 , can be achieved by application of the invention. The compactness of the equipment can therefore be improved very considerably. As a result of using the brine, a negligible pressure loss occurs at the permeate side, even if the permeate compartment is formed by the lumen of "hollow fine fibres". Apart from a high compactness, this guarantees maintenance of driving force and, consequently, of the permeate flux. This also implies that the permeate pressure (the driving force) can be very easily adjusted by regulating the water content of the brine. As a consequence of this, one particular design of an apparatus can be used in very many different applications. In order to use apparatuses according to the state of the art for various applications, the permeate circuit would have to be altered by intervention in order to guarantee optimum process conditions. In principle, the brine can be brought to the same liquid pressure as the pressure of the gas or the vapour or liquid to be dehydrated at the retentate side of the membrane. As a result of this lower requirements are imposed on the pressure resistance/mechanical strength of the membranes. Especially in regard to the use for dehydrating gases, the use of brine offers the possibility of considerably reducing the loss of gas through permeation; for this purpose, use is made of the often limited solubility of the gases to be dehydrated in, in particular, highly concentrated brines. The evaporator, which is used with the aid of a distillation process for evaporating water out of the brine, may be connected to the apparatus. Evaporating water out of the brine demands less energy than the usual condensation in known membrane separation processes. The equip¬ ment is much simpler (no refrigeration technique, no vacuum pump which is susceptible to malfunction) . Because the volume of the permeate space can be considerably reduced, it has proved possible to conduct said processes satisfactorily using hollow fibres, the highly concentrated solution of one or more hygroscopic salts being situated inside the hollow fibres (permeate side) (diameter of, for example, 300 μm) . The invention will now be explained in more detail with reference to some examples. Example 1 A moistened nitrogen having a relative humidity of 75% is passed along an H 2 0-selective gas separation membrane consisting of a hollow fibre of polyphenylene oxide (PP0) . Situated at the permeate side (inside the hollow fibre) is a saturated NaCl solution. It was not possible to detect any marked water permeation (dehydration) . Example 2 A moistened air having a relative humidity of 752 is dehydrated at a temperature of 25°C with the aid of a gas separation process. In this process, an H 2 0-selective hollow gas separation membrane consisting of polyphenylene oxide (PP0) is used. Saturated CsF brine is situated inside the hollow fibres. At a gas speed of 1 m/sec along the outside of the fibre, virtually all the water is removed after a fibre length of only 25 cm. Example 3 An azeotropic mixture of isopropyl alcohol and water is dehydrated at a temperature of 90 β C with the aid of a vapour permeation system, use being made of PP0 hollow fibre membranes. Situated on the inside of the membranes is CsF brine. The permeate flux is approximately 0.3 kg/m .h. Example ■ *. An azeotropic mixture of isopropyl alcohol and water is dehydrated at a temperature of 0°C with the aid of a vapour permeation system. The equipment comprises PPO hollow fibre membranes, an LiBr/ZnBr 2 /CaBr 2 brine being situated on the inside of the membranes. The vaporisation of the permeated water takes place at approximately lδO'C in an evaporating element. The process is carried out continuously and stably for approximately 100 hours. The permeate flux is approximately 0Λ kg/m 2 .h. Example 5 An azeotropic mixture of butanol and water is dehydrated at a temperature of 60 β C with the aid of a pervaporation system. The equipment comprises a flat polyvinyl alcohol composite membrane. Situated at the permeate side is an LiBr/ZnBr 2 /CaBr 2 brine. Permeation of water takes place with a flux of approximately 0.5 kg/m .h. The permeation of alcohol cannot be detected. It was not possible to detect any leakage of brine.

Claims: Claims 1. Membrane separation process for dehydrating a gas or vapour or liquid mixture by pervaporation, vapour permeation or gas separation comprising: passing the mixture along the retentate side of a membrane and generating a driving force in order to transport water through the membrane to the permeate side thereof, characterised in that the driving force is generated in that a highly concentrated solution of one or more hygroscopic salts (brine) is situated at the permeate side of the membrane. 2. Membrane separation process according to Claim 1, characterised in that the hygroscopic capacity of the brines, expressed as a percentage of water vapour pressure reduction with respect to pure water, has a value of over 50#, preferably of over 80%. 3. Membrane separation process according to Claim 2, characterised in that the brine salts contain combinations of small cations and large anions or of large cations and small anions or of mixtures thereof. 4. Membrane separation process according to one of the preceding claims, characterised in that the brines are inorganic in nature. 5. Membrane separation process according to one of the preceding claims, characterised in that the brine is removed continuously or discontinuously from the permeate side of the membrane, that the water taken up by the brine is removed from the brine and in that the worked-up brine is fed back to the permeate side of the membrane. 6. Membrane separation process according to one of the preceding claims, characterised in that the mixture to be dehydrated is under increased pressure and the brine is at essentially the same pressure. 7. Membrane separation process according to Claim 5 or 6, characterised in that the process is continuous and in that the permeated water is evaporated. 8. Membrane separation process according to one of the preceding claims, characterised in that use is made of hollow fibre membranes and the highly concentrated solution of one or more hygroscopic salts is situated inside or outside the hollow fibre membranes. 9. Membrane separation process according to Claim 8, characterised in that moisture-containing gas is passed on the outside of hollow H 2 0- selective PPO gas separation fibre membranes with a brine on the inside of the hollow membranes. 10. Membrane separation process according to Claim 6, characterised in that a mixture of azeotropic isopropyl alcohol and water is dehydrated with the aid of a vapour permeation process, which mixture is passed on the outside of PPO hollow fibre membranes with a brine on the inside of the hollow membranes. 11. Membrane separation process according to one of the preceding claims, characterised in that the brine is a concentrated solution of LiBr or ZnBr 2 or CaBr 2 or CsF. 12. Membrane separation process according to one of Claims 1 to 4 inclusive, characterised in that a mixture of azeotropic butanol and water is dehydrated with the aid of a pervaporation process, the membrane being a flat polyvinyl alcohol composite membrane between the permeate side and the retentate side.

Inventors: HANEMAAIJER JAN HENDRIK, JANSEN ALBERT EDWARD, VAN ENGELENBURG BERTUS, VERSTEEG WILHELMUS FRANCIES, HANEMAAIJER, JAN, HENDRIK, JANSEN, ALBERT, EDWARD, VAN ENGELENBURG, BERTUS, VERSTEEG, WILHELMUS, FRANCIES
Applicants: TNO, NEDERLANDSE ORGANISATIE VOOR TOEGEPAST-NATUURWETEN
Application year: 1991
Main classification: B01D53
IPC codes: B01D53, B01D61