CONDENSED AND DRIED MILKS

Lesson 24. MEMBRANE CONCENTRATION

Module 10. Freeze and membrane concentration

Lesson 24
MEMBRANE CONCENTRATION

24.1 Introduction

Industrial membrane filtration plants were introduced in the dairy industry in the beginning of the 70's. The basis for using membrane filtration in the dairy industry is that the dry matter components in milk and whey particles consist of different sizes as shown in Table 24.1. By selecting filters/membranes of different pore sizes and applying pressure on the product to be filtered, it is possible to divide the milk and whey in different fractions. What passes the filter/membrane is permeate and what does not pass is retentate as shown in .

24.2 Membrane Types

• Spiral Membrane: Due to their compact layout and large amount of membrane area per element, spirals are typically used for high-flow applications with minimal or no suspended solids. Their advantage is low hardware and operating costs.

• Ceramic Membrane: Ideally suited for value-added or sanitary products, as well as applications requiring selective separations from fluid streams containing aggressive components such as solvents.

• Stainless Steel Membrane: Rugged design, especially effective for demanding applications with extreme process conditions or feed streams with elevated particulate solids and/or high viscosity.

• Tubular Membrane: Highly resistant to plugging when processing streams with large amounts of suspended solids or fibrous compounds.

• Hollow Fiber Membrane: Extremely high packing density and open channel design; offers the possibility of backwashing from the permeate side, particularly suited for low solids liquid streams.

• Plate and frame: Open channel design allows it to be used for products with very high viscosity, particularly suited for high solids pharmaceutical and food applications.

24.3 Filtration Types

24.3.1 Reverse osmosis

Reverse Osmosis is used to remove water from a product to increase the solids content; evaporator condensate is often 'polished' by reverse osmosis, so that it can be used elsewhere in the dairy. Reverse osmosis is a high-efficient technique for dewatering process streams, concentrating/separating low-molecular-weight substances in solution, or cleaning wastewater. It has the ability to concentrate all dissolved and suspended solids. The permeate contains a very low concentration of dissolved solids. Reverse Osmosis is typically used for the desalination of seawater.

In order to describe Reverse Osmosis, it is first necessary to explain the phenomenon of osmosis. Osmosis may be described as the physical movement of a solvent through a semi-permeable membrane based on a difference in chemical potential between two solutions separated by that semi-permeable membrane.

The example given in the Figure 24.2 serves to demonstrate and clarify this point. A beaker of water as shown in is divided through the center by a semi-permeable membrane. The black dotted line represents the semi-permeable membrane. We will define this semi-permeable membrane as lacking the capacity to diffuse anything other than the solvent, in this case water molecules.

Now, when a little common table salt (NaCl) is added to the solution on one side of the membrane (Fig. 24.2) the salt water solution has a lower chemical potential than the water solution on the other side of the membrane. In an effort to equilibrate the difference in chemical potential, water begins to diffuse through the membrane from the water side to the salt water side. This movement is osmosis. The pressure exerted by this mass transfer is known as osmotic pressure.

The diffusion of water will continue until one of two constraints is met. One constraint would be that the solutions essentially equilibrate, at least to the extent that the remaining difference in chemical potential is offset by the resistance or pressure loss of diffusion through the membrane. The other constraint is that the rising column of salt water exerts sufficient hydrostatic pressure to limit further diffusion. By observation then, we can measure the osmotic pressure of a solution by noting the point at which the head pressure impedes further diffusion.

Reverse Osmosis : By exerting a hydraulic pressure greater than the sum of the osmotic pressure difference and the pressure loss of diffusion through the membrane, we can cause water to diffuse in the opposite direction (Fig. 24.2), into the less concentrated solution. This is reverse osmosis. The greater the pressure applied, the more rapid the diffusion. Using reverse osmosis we are able to concentrate various solutes, either dissolved or dispersed, in a solution.

24.3.2 Nanofiltration

Nanofiltration is used to remove mainly the monovalent ions from whey. A partly demineralization and water removal is obtained. Nanofiltration is selected when Reverse Osmosis and Ultrafiltration are not the correct choice for separation. Nanofiltration can perform separation applications such as demineralization, color removal, and desalination. In concentration of organic solutes, suspended solids, and polyvalent ions, the permeate contains monovalent ions and low-molecular-weight organic solutions like alcohol.

Like Reverse Osmosis, the mass transfer mechanism in Nanofiltration is diffusion. Though generally quite similar in terms of membrane chemistry, the Nanofiltration membrane allows the diffusion of certain ionic solutes (such as sodium and chloride), predominantly monovalent ions, as well as water. Larger ionic species, including divalent and multivalent ions and more complex molecules are highly retained.

Since monovalent ions are diffusing through the Nanofiltration membrane along with the water, the osmotic pressure difference between the solutions on each side of the membrane is not as great and this typically results in somewhat lower operating pressure with Nanofiltration compared with Reverse Osmosis.

Some typical applications for Nanofiltration are:

• Desalination of food, dairy and beverage products or byproducts

• Partial Desalination of whey, UF permeate or retentate as required

• Desalination of dyes and optical brighteners

• Purification of spent clean-in-place (CIP) chemicals

• Color reduction or manipulation of food products

• Concentration of food, dairy and beverage products or byproducts

• Fermentation byproduct concentration.

24.3.3 Ultrafiltration

Ultrafiltration is a selective fractionation process using pressures up to 145 psi (10 bars). Ultrafiltration is widely used in the fractionation of milk and whey, and in protein fractionation. The whey proteins are separated to form a product with 35, 60 or 80% Whey Protein Concentrate. If ultra filtration is applied to skim milk, then Milk Protein Concentrate is obtained. It concentrates suspended solids and solutes of molecular weight greater than 1,000. The permeate has low-molecular-weight organic solutes and salts. The protein fractions are typically evaporated in multi-effect evaporators with either TVR or MVR recompression to save steam, before spray drying.

24.3.4 Microfiltration

Microfiltration is a low-pressure cross-flow membrane process for separating colloidal and suspended particles in the range of 0.05-10 microns and as such used for bacteria removal, fermentation broth clarification and biomass clarification and recovery.

24.4 Application

Membrane filtration is today used in the Food & Dairy industry and likewise in other process plants. Membrane filtration can be used to meet very distinct liquid separations.

Table 24.1 Milk constituents and their sizes

Cross-flow membrane filtration has opened the doors to a variety of new and innovative dairy products. Nowadays, the mechanical separator is not the only means of harvesting a component of milk. Today, not only the cream can be separated but virtually every major component of milk through membrane filtration can be separated. Membrane filtration technology has rapidly gained prominence in the processing of dairy ingredients. Microfiltration, Ultrafiltration, Nanofiltration and Reverse Osmosis, is making it possible to produce products with very unique properties and functionalities.