102133810 separatiokkn process principles

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Separation Process Principles

J. D. Seader / Ernest J. Henley

TP 156.S45.S438

Membrane Separations

In a membrane separation process, a feed consisting of a mixture of two or more components ispartially separated by means of a semipermeable barrier (the membrane) through which one ormore species move faster than another or other species. In membrane process, the feed mixture is

separated into a retentate (that part of the feed that does not pass through the membrane, i.e., is

retained) and a permeate (that part of the feed that does pass through the membrane).

Separation Processes in the Food and Biotechnology Industries Principles and

Applications

A.S. Grandison & M. J. Lewis

TP 156.S45.S4792

Reverse Osmosis Applications

The main applications of reverse osmosis (RO) are for concentrating fluids by removal of water,

thereby competing with processes such as vacuum evaporation or freeze-concentration. ROpermits the use of lower temperatures even than vacuum evaporation, it avoids a phase change

and complete loss of volatiles and it is very competitive from an energy viewpoint.

RO uses much higher pressures than other membrane processes, in the range 20-80 bar, and will

incur greater energy costs. Suitable high-pressure pumps will be required, which are normally of

the positive displacement type, such as piston pumps. These are expensive and contribute a

significant component of the capital costs.

Areas where evaporation is widely used include the dairy, fruit juice and sugar processing

industries. Rejection characteristics for different RO membranes are provided in terms of saltrejection; typically from 80 to 99% rejection of sodium chloride; rejections of other solutes may

also be cited, for example calcium chloride and glucose.

Reverse osmosis membranes were made for a long time from cellulose acetate. More recently,

thin-film composite membranes, based on combinations of polymers, have been introduced,

which allow higher temperatures (up to 80 C) and greater extremes of pH (3-11) to be used,

thereby facilitating cleaning and disinfection. However, those based on polyamides have a verylow tolerance to chlorine. However, their performance can often be significantly different. For

example, Sheu and Wiley (1983) found that the thin film composite membranes were more

efficient in retaining flavours than cellulose acetate, during apple juice concentration. There were

also differences in salt rejections and organic molecules and these results together withdevelopments in both cellulose acetate and thin-film composite membranes have been covered

by Gutman (1987).

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Therefore the main applications of reverse osmosis are for concentrating liquids, recovering

solids and treatment of water.

The potentialities for processing milk by RO are not as great as those for ultrafiltration

(Grandison and Glover, 1994). It can be used for concentrating full cream milk up to a factor of

2-3 times. Flux decline is similar to that of UF, showing a linear relationship when flux is plottedagainst the log of the concentration factor. Flux rates for skim milk are only marginally higherthan those for full-cream milk. Recorded flux rates at the start of the process are up to 40 l/m

2h.

Factors affecting the flux rate are similar to UF. The product concentration attainable is nowherenear as high as that for evaporation, due to increasing osmotic pressure and fouling, due mainly

to the increase in calcium phosphate, which precipitates out in the pores of the membrane.

Therefore most of the commercial applications have been for increasing the capacity of

evaporation plant.

Other possible applications that have been investigated and discussed include: the concentration

of milk on the farm for reducing transportation costs; for yoghurt production at a concentrationfactor of about 1.5, to avoid addition of skim-milk powder; for ice-cream making, also to reduce

the use of expensive skim-milk powder; for cheese-making to increase the capacity of the cheese

vats, and for recovering rinse water. Whey can also be concentrated, to reduce transportation

costs or prior to drying. Flux values for sweet whey are higher than for acid whey, which in turnare higher than for milk, for all systems tested (Glover, 1985). The main reason for differences

between acid whey and sweet whey is believed to be the much higher levels of calcium in acid

whey, which acts as a foulant. Whey can be concentrated from 6% to 24% solids, at as low as7 C.

Pal and Cheryan (1987) reported some success for using RO concentrated milk (31% TS) for

khoa manufacture, with the potential for large savings in energy. However, the average flux wasreported as only 8.1 l/m2h at 30 C.

Grandison and Glover (1994) reported that for all practical purposes all the components of milkare retained by the membrane and only a small proportion of the smallest ions escape. Rejections

of the whole mineral content of milk greater than 99% are reported with rejections of Na+of

99%, K+of 98% and Cl

-of 94%. From a detailed study (Morales et al., 1990), it was found that

different membranes and membrane configurations can influence both flux and rejection of

components during milk and whey processing. They also found that total solids rejection was

independent of temperature and was higher when milk, rather than whey, was processed. In

general, all the membranes were capable of rejecting 100% of the true protein. Rejection of non-protein nitrogen, lactose and total BOD was affected by change in the operating conditions, type

of feedstock and type of membrane employed, whereas rejection of ash was substantially

insensitive to variations in operating conditions and changes in feedstock.

Milk concentrate is thus not likely to have the same extent of heat damage as that produced by

evaporation. It may also be slightly different in composition, which may affect the texture and

stability of products derived from it.

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Fouling is a major problem and the main component of the fouling layer is usually found to be

protein. The stability of the deposit and the ease at which it is removed by rinsing is dependent

upon the low molecular weight components, particularly calcium (Kulozik ad Kessler, 1988b).

RO has found application in the processing of fruit and vegetable juices, sometime incombination with UF and MF. The osmotic pressure of juices is considerably higher than that formilk. There has been a dramatic increase in fruit juice consumption; most juice needs to be

concentrated prior to freezing and is then transported frozen.

It is advantageous to minimize thermal reactions, such as browning, and to reduce loss of

volatiles. From a practical viewpoint, the flux rate and rejection of volatiles is important. RO

modules can cope with single-strength clear or cloudy juices and also fruit pulp. RO can be used

to produce a final product, as in the case of tomato paste and fruit purees, or to partiallyconcentrate, prior to evaporation.

RO is a well-established process for concentrating tomato juice from about 4.5 Brix, to between8 and 12 Brix. Other fruit juices which have been successfully concentrated are apple, pear,

peach and apricot. Where juices have been clarified, osmotic pressure limits the extent of

concentration and up to 25 Brix can be achieved. Unclarified juices may be susceptible to fouling.

With purees and pulps, the viscosity may be the limiting factor and these can be concentrated toa maximum of 1.5 times.

Citrus juices are also concentrated. For oranges, the high hesperin content of the juice results infouling and rapid flux decline. Vegetable juice processing has received some attention, although

the market is nowhere near as large as that for fruit juices. Koseoglu et al. (1991b) present data

for celery, tomatoes, carrots and cucumbers. The macerated vegetables are pressed and the

screened juice is subject to UF. The clear permeate can then be concentrated by RO and addedback to the retentate from UF.

Thin-film composite membranes have been assessed for sugar cane and beet juice concentration,

up to 80 C and pressures between 40 and 80 bar, Kosikowski (1986). Instant coffee is a very

popular beverage and it is possible to concentrate the coffee extract from about 13% to 36% totalsolids at 70 C, with little loss of solids. Thin-film composite membranes have been found to

give a better retention of aromatics. The concentrate is then evaporated to about 48% solids,

prior to drying. Currently, instant tea is also being heavily marketed and RO has been

investigated for preconcentration.

Poor wine is usually produced from grape juice (must) containing less than 17% sugar.

Production of wine from must concentrated slightly by RO is improved compared to that

produced by adding sugar, although the costs are likely to be higher. RO has been reported toremove some of the compounds responsible for the old flavor of wine. It is usually superior to

wine produced from evaporated must.

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Dealcoholization is an interesting application, using membranes which are permeable to alcohol

and water. In a process akin to diafiltration, water is added back to the concentrated product, to

replace the water and alcohol removed in the permeate. Such technology has been used for theproduction of low or reduced alcohol, beers, ciders and wine. It can be applied either as a single

process, using a feed and bleed system, or as a two-stage process, where the concentrate from the

first stage is rediluted with water and subjected to a second RO process. For these application,cellulose acetate membranes are used rather than the thin-film composites, because theirrejection values for ethanol is lower. Gutman (1987) reported that the removal efficiency

(rejection) of ethanol was 12% for cellulose acetate membranes and 28% for polyamide

membranes.