Lesson 29. ION EXCHANGE PROCESS FOR DEMINERALISATION
Module 3. Processing and utilization of whey
ION EXCHANGE PROCESS FOR DEMINERALISATION
ION EXCHANGE PROCESS FOR DEMINERALISATION
Ion exchange is now the most mature of the demineralisation technologies. It is a fixed-bed technique involving use of resins which have a discrete capacity for absorption of minerals. When this capacity is used up, the absorbed minerals have to be removed from the resin and the resin regenerated, before it can be used again. It is quite widely used for whey demineralisation. However, the process has a number of disadvantages, including potential denaturation of protein due to low pH in the cation column and loss of protein in the anion column that makes the product less widely accepted for use in infant formula than electrodialysis whey. A typical ion exchange unit is shown in .
29.2 Ion Exchange Resin
Modern ion exchange resins are macro-molecular porous plastic materials formed into beads with diameter in the range of 0.4-0.8 mm having a large number of attached bonds on their surface which can absorb (reversibly) one specific type of ions. Chemically they act as insoluble acids or bases, which when converted into salts, remain insoluble. A wide range of base polymers may be used in ion exchange resins (Table 29.1) depending on the specific physical and chemical requirements of the system. The average pore size within the polymer is normally less than 40A°. Macro porous resins are prepared by re-dissolving of part of the matrix of the polymer to controlled pore sizes up to 1000A°.
Table 29.1 Resin matrix materials
The matrix of the resin contains the charged sites involved in the ion exchange process. These sites may be occupied by anionic or cationic groups (Table 29.2). Table 29.3 shows the affinity of each group of resins for the removal of various ions.
Table 29.2 Classification of ion exchange resins
Table 29.3 Ion affinities for different exchange resin group
A major factor determining the effectiveness of an ion exchange resin is the size, valency and concentration of the ionic species to be exchanged. These factors influence the capacity and selectivity of the resin. Lower porosity (higher degree of cross linking) resins are more selective towards smaller ions, which can more easily diffuse to the reactive sites. Lower porosity resins also have a higher exchange capacity per unit. Exchange capacities of resins typically range between 1 and 4 equivalents per litre of resin. When the capacity of an ion exchange resin is exhausted, the resins may be regenerated with regenerant solutions.
29.3 Processing Variables
Both the degree of demineralisation and the feed condition and type of whey and permeate must be considered during the demineralisation of whey and ultrafiltration permeates.
29.3.1 Degree of demineralisation
Levels of up to 95% demineralisation of whey-based fluids can be achieved by ion exchange. In general, the required level of demineralisation is achieved by operating the ion exchange plant at constant flow rate and a high degree of demineralisation, and by using varying amounts of bypass to achieve the desired level of demineralisation in the end product. Ion exchange is generally non-selective with respect to the ions removed. Bypassing is a particularly useful technique since it does not result in a significant change in the distribution of the various minerals present in the end-product. Bypassing can also reduce the losses of protein and lactose.
29.3.2 Feed condition and type
The physical condition of the whey feedstock is an important factor determining the efficiency and effectiveness of ion exchange processes. Most problems are caused by the inadequate removal of agglomerates prior to ion exchange. These agglomerates may be casein or cheese curd fines, or may be produced by pretreatments of the feedstock such as heat treatment or pasteurization. In general, the manufacturers of ion exchange equipment set specific criteria for the presence of fines in the feedstock.
There are often benefits by increasing the concentration of the feedstock prior to ion exchange. Whilst concentration reduces the volume to be processed, it does not reduce the demineralisation load. As the concentration of the solution increases, the selectivity of ion exchange system generally moves towards monovalents. Furthermore, the increased concentration of the non-ash components may also lead to increased losses of these components through, for example, increased protein absorption by the resin.
In general, the source of whey (sweet or acid) does not influence the mineral profile of the dematerialized product, although the demineralisation load is of course higher for acid whey. Some differences do occur amongst those ions present in a multitude of forms (calcium and phosphate). A major factor in the level of demineralisation of these ions is the pH of the feedstock. Losses of the non-mineral components are mainly non-protein nitrogen components.
29.3.3 Temperature of operation
The temperature at which demineralisation is carried out is often dictated by matters not related to the actual demineralisation process. However, care must be taken, because the use of excessively high temperatures of operation can lead to resin degradation. Maximum temperatures of operation for resins vary from 35-50°C for strong base resins, 60-100°C for weak base, to 120 and 150°C for weak and strong acid resins, respectively. The use of lower temperatures will reduce the extent of losses of protein and lactose by resin absorption, and assist in the prevention of microbial growth in the system during the long loading phase. Temperatures of 10°C are often employed for these reasons. The extent of protein losses, and thus reduced yield, can be very significant as temperature rises.
29.4 Operating Problems
Some of the drawbacks of the ion exchange process outlined above have been overcome by the SMR process, through development of a system for reuse of regenerant. Physical and chemical hindrances in the operation of ion exchange systems are outlined below.
Resin beads are physically fragile, but often are subjected to considerable stress in operation. Bead fracture and attrition is common, and can be caused by the effects of changing of the processing parameters during the various cycles of loading, backwashing, regeneration and rinsing. The change in type of solutions to which the resins are exposed during these cycles also subjects them to osmotic shocks. In addition, resin may be subjected to stress owing to pumping operation and temperature changes. All of these factors can lead to loss of performance of the bead, and unwanted particulate matter in the final product.
Chemical fouling of resins is the most common problem encountered in ion exchange processing. The feedstock, whey or permeate contains many organic substances in low concentrations. These may enter the feedstock through the milk, or from sanitizers used on the farm and in plant operations such as cheese- or casein-making. Foulants can also enter the system from the regenerants, and via the wash or rinse waters. Such foulants include tannic, humic and fulvic acids. Foulants can be either organic or inorganic.
Fouling by organic compounds occurs mostly on anionic resins; fouling by inorganic compounds occurs most commonly on cationic resins. Fouling by organic molecules can be the result of their very slow diffusion through the resin matrix, because of their large size. Irreversible bonding can also occur if there is a high selectivity for sites. Strongly basic anion exchange resins cross-linked with acrylic derivatives rather than divinyl-benzene have been reported to be less susceptible to fouling.
Fouling by inorganic ions cationic resins is generally caused by minor elements such as iron, manganese and copper. Precipitates, such as calcium sulphate, can cause surface fouling and scaling of the resins. Inorganic silica fouling can also occur with anionic resins.
29.4.3 Non-mineral losses
Protein losses, owing to factors other than temperature and pre-treatment history, have been reported to occur on demineralisation by ion exchange. Significant losses of non-protein nitrogen (NPN) can also occur. Losses of 50-70% of NPN have been reported on demineralisation of cheddar cheese whey with strong acidic and weak basic resins.
29.4.4 Structural leakages
The possibility of slow leakage of organic material from the resin structure during its life of operation is a cause for concern. Styrene monomer, which may be present in the bead from incomplete polymerization, will slowly leave the bead during its service life. To address this concern, manufacturers make a range of special grades of ion exchange resins designed for food applications. In such products, particular attention has been paid to the control of the polymerization reaction.
29.5 Process Variations
29.5.1 SMR process
A variant of the traditional ion exchange process is the Swedish SMR (Svenska Mejeriernas Riksferening or Swedish Dairy Association) process. It is designed primary to reduce the consumption of regeneration chemicals, which apart from saving money, also leads to a better waste (reduced salt load) situation from demineralisation plant. Instead of using cation resin of H+ form and anion resin on OH- form, this process uses a cation resin on NH4+ form and an anion resin on HCO3- form. The system has two major innovations; first the order in which ions are exchanged is reversed from that commonly employed – anions are exchanged first, and cations second; secondly, the separate regenerants are replaced by a solution of one salt. Thus, both ions in this single regenerant solution are used, one for each of the anion and cation exchange columns.
Using the weak anion exchange resins for treatment in the first stage means that the feedstock to the first column is acidic, as required. The resultant basic product can then be fed to a weak cation resin exchanger for the second step. This allows the use of bicarbonate and ammonium ions - thus, a solution of ammonium bicarbonate is used as the single renenerant. Ammonium carbonate being a thermolytic salt, it may be removed from the demineralised whey by heat, by passing the whey/ammonium bicarbonate mixture through an evaporator. The ammonia and carbon dioxide so produced are then recombined in an absorption column to effectively regenerate the ion exchange regenerant. The main operations of SMR process are shown in .
The SMR process is claimed to reduce the high consumption of chemicals and water normally associated with ion exchange processes during regeneration. It is also claimed to reduce associated water disposal problems and their cost. The process does not result in such wide changes in the pH of the feedstock during demineralisation as in the case of normal ion exchange. During normal ion exchange, decationised whey reaches at pH of 1.5 and on deanionisation, the pH of the product may increase to 9 or 10. For sweet whey, the SMR process restricts the pH variation between 6.5 and 8.2. This has the very beneficial effect on reducing protein losses.
29.5.2 Thermal regeneration
The use of thermal processing for the regeneration of resins can reduce both chemical and effluent processing costs. A range of such products, known as Sirotherm resins, are now commercially available. Mild heat treatment results in regeneration of these resins.
Delaney, R.A.M. 1976. Demineralization of whey. Aust. J. Dairy Technol., 31: 12.
Last modified: Tuesday, 16 October 2012, 9:02 AM