Pa- l

phenols and nitrogen- and sulphur-containing compounds. When this complex mixture is cooled, the condensate forms two phases, a hydrocarbons layer of tar and oil, and an aqueous liquor containing most of the other impurities. A conventional treatment route for this liquor typically has five stages:

1 - solvent extraction to remove phenols 2 - removing free Rmmonia, H2S and CO. by steam stripping 3 - adding alkali to convert fixed ammonia to free ammonia, then steam stripping 4 - biological treatment to oxidise remaining organic compounds 5 - activated carbon treatment to remove residual compounds, particularly highly odorous organic compounds.

The resulting liquid effluent still contains chlorides, which must be removed before, for example, the liquid is used as cooling water makeup or boiler feed water.

British Gas has recently explored the use of reverse osmosis in the treatment of gas liquor, and has found that stages 3,4 and 5 can be undertaken using reverse- osmosis plant; additionally, the chloride-removal step can be undertaken by reverse osmosis. In stage 3 (the conversion of fixed to free ammonia and subsequent removal) the conversion step is unnecessary because the fixed ammonia, present as ammonium chloride, is removed directly by reverse osmosis.

There are important operating and capital cost consequences in the replacement of the final three stages, and the optional chloride-removal step, by a single reverse-osmosis treatment stage. Additionally, the overall process should be more reliable, since the biological treatment step, which is the least reliable stage in the conventional route, is eliminated.

Component Concentration in liquor (mg/l)

Raw liquor

Ammonia 7687 Chloride 1079 Phenol 5392 Sulphate 287 Thiocyanate 823 Carbon (TOC) 5500 Chemical Oxygen 19776 Demand

Dephenolated free Permeate ammonia stripped from

RO unit 1510 79 1079 98.5

52 0.5 1059 83 1174 150 3960 272

A typical set of test results of concentrations of impurities in the raw gas, in the liquor after phenol and free nmmonia were removed, and in the permeate from the reverse osmosis unit, are shown in the table. Patent number: EP 409423 Date: 23 January 1991 Inventors: J.R. Ebbins, S.F. Lopez, C. Taylor Applicant: British Gas PLC

Diagnostic strip and cell culture support A common problem in using conductive enzymatic test strips to analyse body fluids is overwetting, which can flood the electrode employed to measure the conductivity. In other cases, excessive evaporation of water or a carrier fluid can give a falsely high measure of concentration, in such circumstances a semipermeable membrane can be used to coat the 'application' surface of the test strip so that appropriate quantities of fluid reach the other surface electrode.

The hydrophobic nature of

polytetrafluoroethylene makes it intrinsically unsuitable for some applications, including these diagnostic tests. In addition it is protein-repellant, which leads to premature coagulation and blinding of the membrene.It is possible to circumvent some of these problems by impregnating the PTFE with a hydrophilic, water-soluble polymer such as polyvinyl alcohol, then fixing the PVA by cross-linking to make it water-insoluble. Membranes prepared according to these techniques are hydrophilic and protein- attractive.

in this invention it has been found possible to impregnate the PTFE with polyvinyl alcohol, then using a complexing agent to effect attachment. Suitable complexing agents are dibasic inorganic acids; in this case boric acid and its salts have been found useful. The probable mechanism is that the complexing agent engages in polar bonding with available oxygen or hydroxyl groups of the hydrophilic polymer to produce the complex. With PVA and boric acid a bisdiol complex is formed with the borate anion bonded to two PVA chains.

The material can be used to provide a semipermeable membrane coating for test strips as already described, and it can also be employed as a cell culture support material. In this use it is a good support on which to grow endothelial cells

Membrane Technology 13

Patents

which tend to attach to the complex, coating the membrane structure. The endothelial cells thus grown can be used for chemotactic studies. Patent number: EP 408378 Date: 16 January 1991 Inventor: E.G. Norman Applicant: W.L. Gore & Associates (LrK) Ltd

Multiwell filtration apparatus Test plates for in vitro analysis which contain many individual wells or reaction chambers are standard laboratory and clinical tools. Membranes are common components of these plates,

especially where the material of interest is retRined on or in the membrane. Fig.1 shows a typical arrangement.

Many test plates contains reagents which give a colour change or some other visual indicator under appropriate conditions. In some circumstances, however, it is necessary to remove the sections of the plate which contRin adsorbed or absorbed material so that further processing can be undertaken. There are several possible methods; for example, a die-punch can be used to chop out circular sections from the filter-membrane sandwich, or the filter pack can be selectively delaminated to separate it from the well matrix. These techniques are imperfect, however: the die-punch tends to compress the filter pack and express some retentate, and the filter pack can delaminate rather unselectively.

This invention employs an adhesive sheet of material which sticks firmly to the membrane and permits separation of the membrane areas concentric with the well bases from the rest of the pack and the well matrix. Fig. 2 illustrates the structure of the filter-membrane sandwich and the adhesive sheet.

After the test samples and necessary reagents have been introduced to the test wells, and all the liquid contents have been extracted through the filter pack into a receiving plate and collection tray, the vacuum assembly is taken apart to expose the membrane, which is on the bottom of the filter pack. The adhesive sheet is then pressed into contact with the membrane, and is finally pulled away, taking the desired parts of the membrane with it for further processing. The shape of the perforated sections of the

Fig. 1: Vacuum assembly featured in Millipore's multiwell filtration apparatus

tte support

collection tray

plate

vacuum chamber

uum pump

14 Membrane Technology