Factors Of Relevance For The Performance Of Membrane Plants

2.1. Properties of the raw material

To facilitate the choice of membrane, process and process conditions, the composition and characteristics of the raw materials should be determined as extensively as possible. Some factors are general, some are specific for the treatment of biological material. Characteristics which should be considered are: type and content of micro-organisms, viscosity, total solids (TS) content, fat content, protein content, salt content (including iron, magnesium, calcium and potassium salts), type and content of surfactants, polysaccharide content, fibre content, oxidized compounds such as free chlorine, hydrogen peroxide, dissolved oxygen, aroma compounds, traces of solvents, pH and pH change with time, etc.

During RO, NF, UF and also to a large extent during MF, micro-organisms are retained by the membrane and are concentrated in the retentate. This is sometimes an advantage, but may be a disadvantage in other processes. High-quality raw materials (low levels of micro-organisms) are essential in the food industry, especially when the retentate is the main product. The presence of surfactants in the process liquids can cause problems such as reduced capacity and increased retention. This is the case, for example, with antifoams which are sometimes used in the food industry. The various effects are dependent on type, concentration and membrane material.

Biological materials often change very quickly. Factors such as storage, transport etc., can affect properties which are important for the quality of the product as well as the final result of the separation process. The properties and handling of the raw materials should, therefore, be documented and controlled as accurately as possible in order to obtain reproducible results. It is also important to investigate and document the variation in the raw material with time (day-to-day variations as well as seasonal ones) and to carry out tests on samples of the material in order to detect variations.

2.2. Pretreatment

Mechanical pretreatment is used to remove fines, fat and fibre, for example. Heat treatment (pasteurization) is a form of pretreatment often used for biological material to kill micro-organisms, inactivate enzymes, prevent a lowering of pH, etc. In some cases it might be necessary to heat-treat the retentate prior to further treatment. Heat treatment of milk before UF (at 55 °C for 15-30 minutes) prevents the precipitation of calcium phosphates on the membrane surface or in the membrane. As the solubility decreases with temperature, the calcium phosphate precipitates during the heat treatment and no longer causes problems during ultrafiltration. Chemical pretreatment, e.g., adjustment of the pH may also be necessary.

2.3. Membranes

Important membrane properties are: selectivity, flux and surface characteristics, and thermal, chemical and mechanical stability. Selectivity is a measure of the ability of the membrane to fractionate components in the feed stream and thus an important parameter in, e.g., ultrafiltration.

High selectivity is essential to obtain the desired fractionation, as low selectivity leads to multi-stage processes which, in most cases, are uneconomical. A high flux (the volume or mass of permeate, or any component in the permeate, passing through the membrane per unit area per unit time) is desirable in order to minimize the size of the membrane plant and thus the cost. It is, of course, also important that a high flux is retained for long periods of operation.

For ultrafiltration, polysulphone is the most common membrane material due to its high thermal and chemical resistance, an advantage especially for cleaning purposes. This material is, however, quite hydrophobic and encounters, sometimes, fouling problems. More hydrophilic membrane surfaces are being developed. Such polymeric membranes are generally a little less resistant than polysulphone. The development of ceramic membranes with hydrophilic surface properties and excellent thermal and chemical properties, is of great interest for the treatment of liquid food and food waste waters. At present, these membranes have mainly been used for crossflow microfiltration due to the high fluxes obtainable compared with those for polymeric membranes. The number of companies offering ceramic microfiltration membranes is increasing, while development of asymmetric polymeric MF membranes seems to be very limited. Recently, more resistant ceramic ultrafiltration membranes have been developed. For UF, the flux improvements are negligible compared with polymeric membranes, and the matter of fouling prevention will be of major importance.

For reverse osmosis, polyamide is the main membrane material, having reasonably good thermal properties but being extremely sensitive to oxidizing agents such as chlorine. Development of more chlorine-resistant polymeric RO membranes is in progress and some are already in the market [3,4,5], Recently some work aiming at producing inorganic nanofiltration membranes has been initiated. Such membrane materials could have great potential if the separation properties prove to be good enough.

2.4. Modules

For food processing, and especially in the dairy industry, where large volumes of liquids are treated, the use of spiral wound sanitary ultrafiltration modules has increased rapidly due to the decreased cost compared to the plate-and-frame systems or tubular systems. In New Zealand, the use of spiral wound modules has increased enormously in the dairy industry and plants with membrane areas of thousands of square metres are being installed. Dow recently introduced a cassette module for ultrafiltration and microfiltration with short transport distances and thus much lower pressure drops than in traditional spiral modules. This module is being tested for water treatment. For highly viscous process streams, a specially designed module was developed by DDS (now Dow).

For reverse osmosis, membranes and modules withstanding higher pressures than those used traditionally have been and are being developed, making it possible to attain higher concentrations and thus to decrease the need for further concentration steps. This is of special interest for the food industry, where heat-sensitive liquids are often processed. The development of steam-sterilizable modules is of interest for the biotechnology industry but also of benefit to the food industry.

2.5. Process conditions

Sometimes, the conditions under which the process is run are more important than the membrane material itself. Start-up mode, trans-membrane pressure, etc. play a significant role in the separation behaviour and fouling tendency, especially in microfiltration. Residence times and running temperature during membrane filtration are of great importance for the bacteriological status of the final product. To minimize the clogging of the membrane, the process liquid should be preheated to the process temperature before being pumped into the membrane equipment. For the same reason, rinsing prior to cleaning should also take place at the process temperature. The permeate side should also be full during normal running, as well as during cleaning and disinfection, as air pockets may result in the risk of microbial growth.

2.6. Process mode

In the application of crossflow microfiltration of liquid foods, very high fluxes are often encountered in the beginning followed by a rapid flux decrease together with a change in separation properties (increased retention). The importance of the processing mode has gained more and more attention. Starting up, circulation velocity, transmembrane pressure etc. are very important. Low transmembrane pressures and high circulation velocities cannot be combined in traditional module designs. For dairy applications of microfiltration using ceramic membranes from SCT, France, a substantial flux improvement was obtained using the Uniform Transmembrane Pressure (UTP) process patented by Alfa-Laval [6], In this process, a uniform transmembrane pressure is maintained by circulating the permeate at high speed concurrently with the retentate, inside the module, but outside the module element.

0 0

Post a comment