WO1992013463A1

WO1992013463A1 - Membrane separation process and apparatus for the separation

Info

Publication number: WO1992013463A1
Application number: PCT/GB1992/000151
Authority: WO
Inventors: Graeme Spencer Brewer
Original assignee: Demin-Tech Limited
Priority date: 1991-01-30
Filing date: 1992-01-28
Publication date: 1992-08-20
Also published as: GB9102047D0; GB2267846B; GB9314987D0; AU1178492A; GB2267846A

Abstract

A membrane separation process and apparatus uses two different semi-permeable membranes to remove inorganic ions and low molecular weight species from high molecular weight species. The first semi-permeable membrane has a pore size to allow mainly diffusive transport of species across the membrane and a preferred pore size is 20-40 angstroms. The second semi-permeable membrane has a pore size to allow mainly convective transport of species across the membrane and a suitable pore size is between 60-120 angstroms. A stripping fluid contacts the other side of the semi-permeable membranes to carry away any species which pass across the membranes. The stripping fluid of the second semi-permeable membrane can be further treated by nanofiltration and/or ion exchange to remove ions from the stripping fluid. The method and apparatus is suitable for demineralisation of concentrated whey solutions.

Description

MEMBRANE SEPARATION PROCESS AND APPARATUS FOR THE SEPARATION

TECHNICAL FIELD

This invention relates to a membrane separation process and particularly relates to a process involving the use of two different semi-permeable membranes for removing

inorganic ions and low molecular weight species from higher molecular weight species. The process is particularly suitable for separation of salts and lactose from

concentrated whey solutions.

Background Art

Many liquid foods contain unacceptable high levels of salts and therefore are unacceptable or only have limited use as animal fodders or foods fit for human consumption.

One potential source of liquid food which is currently being under utilised is whey. Whey may be defined broadly as the watery parts of milk remaining after separation of the curd that results from coagulation of milk by acid or protelitic enzymes. Whey is a by-product of cheese and casein manufacture and the main components in whey are water, whey proteins, lactose, non-protein nitrogen, fats and ash. The mineral profile of the ash shows monovalent ions potassium, sodium, and chloride as the major components with divalent ions calcium, magnesium and phosphorous as minor components. The whey is normally initially concentrated for spray drying or to crystallize out as much as possible the lactose fraction which can be separated, recovered and used as a valuable product.

The resultant solution comprises a delactosed whey

(DLW) having a lower lactose proportion but having a corresponding increase in the amount of ash. The higher ash component makes the delactosed whey a low value product even though it is still a potential source of valuable whey protein. Typically, delactosed whey powder has a value currently about AU$600 a ton while demineralised delactosed whey powder ran have a value of about AU$3,200 a ton or even higher depending on the percentage of the protein content. Thus, by demineralising whey, a considerable value-added product can be obtained.

There are currently several techniques used to demineralise whey solutions. One known method comprises the use of ion exchange resins. The ion exchange resins normally comprise a cation exchange resin and an anion exchange resin. Ion exchange resins however suffer from several problems if contacted directly with a whey or delactosed whey solution. Firstly, the whey solution is classified as a fouling liquid and therefore particular attention is required for pre-treatment of the whey solution before contact with ion exchange resins to prevent fouling of the resin. The whey should therefore be properly separatee and clarified before ion exchange. In addition, since whey contacting fresh cation and anion exchange resins can experience pH swings from as low as 1.7 to as high as 10 or more, cumulative protein losses can occur. Protein loss also occurs

depending on the operating temperature of the resins and therefore the ion exchange resin is normally kept at a temperature of 5 to 12°C which requires expensive cooling equipment. The ion exchange resin also extracts most non-protein nitrogen which tends to deactivate the resin. Ion exchange resins are therefore unsuitable for direct contact with whey which is not pre-treated or concentrated whey.

A further known method to demineralise or desalt whey is by use of an electrodialysis process. Electrodialysis membranes consist of thin sheets of cation and anion

exchange resins essentially the same as those used in the ion exchange process. The process uses direct electrical current as the driving force to transfer mobile ions such as salts from the feedstream across the charged membrane into a receiving or stripping stream. The receiving or stripping stream is sent to waste. The demineralisation or desalting efficiency of the whey is determined by ash content of the whey to carry the current and therefore membrane and power requirements increase as higher levels of demineralisation is attained. The process is therefore self-defeating.

Furthermore, direct contact of a whey solution with the membranes also results in fouling in a manner similar to that described for the ion exchange process.

Nanofiltration has been suggested as a process to demineralise whey permeates. Nanofiltration is a pressure- driven membrane process which is particularly suitable for the separation of monovalent ions from small molecules such as lactose. The molecular weight cut-off point is generally about 350. Nanofiltration is limited in use to dilute solutions with a low osmotic pressure and does not function for concentrated solutions of relative high osmotic pressure. Nanofiltration is therefore limited in practice to dilute solutions which can comprise an ultraf iltration permeate of a whey solution but is not suitable for direct contact with a concentrated whey solution.

Counter-diffusion processes have been used to dem i ne ra l i s e or desalt whey solutions. The process comprises contacting the whey solution with one side of a semi-permeable membrane having a pore size of 20 angstroms or less. The pore size allows monovalent ions to pass through the semi-permeable membrane while retaining divalent ions, lactose and the whey protein. The method can be used to separate monovalent ions such as sodium, potassium and chloride from whey solutions but is unsuitable to separate lactose and divalent ions from whey solutions.

Australian patent application 22756/88 to Syrinx Equipment Pty Ltd discloses a process to separate salts from an ultra-filtration permeate of whey. The ultra-filtration permeate is formed by contacting the whey solution against one side of an ultra-filtration membrane. The membrane allows passage of monovalent ions and to a lesser extent divalent ions and lactose which are carried away in a permeate stream. The permeate stream is contacted with a nanofiltration membrane and a counter-diffusion membrane to desalt the permeate stream. The process is unsuitable for direct contact with a concentrated whey solution as the nanofiltration membrane will not function with direct contact against a concentrated whey solution for the reasons discussed above.

It is an object of the invention to provide a membrane separation process which can be used in direct contact with a concentrated feedstream and which may alleviate the abovementioned disadvantages.

Disclosure of the Invention In one form, the invention comprises a method for at least partially removing inorganic ions and low molecular weight sugars from a concentrated feedstream containing inorganic ions and low molecular weight sugars comprising the steps of

(A) Contacting said feedstream with one side of a first semi-permeable membrane allowing mainly diffusive transport of inorganic ions across the membrane and

contacting the other side of said first membrane with a first fluid which at least partially dissolves said ions, (B) Contacting the feedstream subsequent to step (A) with one side of a second semi-permeable membrane allowing mainly convective transport of inorganic ions and low molecular weight sugars across the membrane and contacting the other side of said second membrane with a second fluid which at least partially dissolves said inorganic ions and low molecular weight sugars and maintaining a high

concentration gradient across the membrane,

(C) Recovering said feedstream subsequent to step (B). In another form, the invention comprises an apparatus for carrying out the method described above.

The feedstream may comprise a liquid food stream and suitably comprises a whey solution. The whey solution may comprise from 20% to 40% solids of raw undelactosed whey to 32% solids delactosed whey solution. A sugarbeet feedstream may also be used.

The inorganic ions may include monovalent ions such as sodium, potassium and chloride and divalent or trivalent ions such as calcium, magnesium and phosphorous. The ions are typically present as salts or minerals which are either in the milk from which the whey is derived or is added to the milk during the process of producing cheese or casein.

The first semi-permeable membrane typically has a pore size larger than the average size of monovalent ions

together their sphere of hydration to allow such ions to diffuse through the membrane. Preferably, the pore size is designed to allow such ions to pass through tlie membrane but to inhibit larger ions such as divalent to trivalent ions or low molecular weight organic compounds such as sugars. A suitable pore size of the first semi-permeable membrane comprises between 10 to 50 angstroms and preferably between 20 to 40 angstroms.

The second semi-permeable membrane may have a pore size sufficient to allow monovalent ions, divalent and trivalent ions together with their spheres of hydration to pass through the membrane as well as low molecular weight sugars such as lactose but inhibiting passage of larger molecular weight compounds such as whey proteins. A suitable pore size is between 50 to 150 angstroms and preferably between 60 to 120 angstroms.

Each of the semi-permeable membranes may comprise a hollow fibre provided with pores about its periphery. The semi-permeable membranes may also comprise a spirally wound flat sheet. A multitude of such fibres or sheets may be arranged together and may be provided with a common inlet and outlet to form a module. The feedstream may then be able to pass through the inlet, the multitude of hollow fibres and the outlet. A typical module may comprise several thousand fibres which may provide a membrane surface area of between 0.7 to 5.0 square metres. Several modules may be connected together in series and/or parallel depending on the volume and type of feedstream to be processed. Suitably commercial dialysis membranes include cuprophan and high density cuprophan (trade mark) from ENKA

A.G. Germany, Cuprammonium rayon - ASAHI Japan, Polycarbonate

- C.R. Bard - U.S.A. and Acrylonitrile copol ymer - Rhone Poulenc France.

The first and second fluids which contact the other side of the respective semi-permeable membranes suitably comprise water as the sole component or in a mixture with other components. The temperature of the fluid is not critical but should not be of a value to cause appreciable degradation of the components within the feedsteam.

The first and second fluids may be treated subsequent to contacting the respective semi-permeable membranes to at least partially remove the species such as ions and lactose dissolved therein.

Suitably, the second fluid, subsequently to contacting the other side of the second semi-permeable membrane, is caused to contact an ion exchange resin. The ion exchange resin may comprise a cation exchange resin and an anion exchange resin and the second fluid may contact both resins. The ion exchange resin suitably comprises an open "packed suspended bed" design. The second fluid may pass through a closed loop system extending between the second semi-permeable membrane and the ion exchange resin. As the ion exchange resin does not affect the passage of low molecular weight sugars, the closed loop system can result n an equilibrium level of sugars being formed between both sides of the second semi-permeable membrane.

Alternatively, the second fluid, after contact with the ion exchange resin, may be blended with the feedstream either before or after introduction of the feedstream to the second semi-permeable membrane.

In a further alternative, the second fluid, subsequent to contact with the ion exchange resin, may be further treated to remove any dissolved sugars such as lactose. Typically, the second fluid may be concentrated to allow the lactose to crystallize therefrom.

Although the first fluid which contacts the other side of the first semi-permeable membrane may be treated in the manner described above with reference to the second fluid, it is preferred that the first fluid is not further treated as it contains a relatively high proportion of dissolved salts and some low molecular weight sugars. This fluid may be passed to waste.

In a further alternative, the second fluid may, subsequent to contacting the other side of the second semi-permeable membrane, be contacted with one side of a third semi-permeable membrane having a pore size to allow passage of water and monovalent ions such as sodium, potassium and chloride but inhibiting passage of low molecular weight sugars such as lactose. A suitable semi-permeable membrane comprises a nanofiltration membrane. The nanofiltration membrane suitably has a hypothetical pore size of about 10 angstroms (1 nanometer). The second fluid suitably contacts the third semi-permeable membrane under above atmosphere pressure and preferably under high pressure. The permeate passing through the membrane comprises water and dissolved salts which can be passed to waste or further treated to recover the salts.

The nanofiltration membrane may be used alone with the second fluid, subsequent to contacting the nanofiltration membrane, being further processed to remove the lactose therefrom or may be blended with the main feedstream.

Alternatively, the nanofiltration membrane can be used in association with an ion exchange resin. Suitably, the second fluid subsequent to contact with the other side of the second semi-permeable membrane is contacted with one side of the nanofiltration membrane to remove monovalent ions therefrom. Thereafter, the fluid (which is concentrated due to water loss through the nanofiltration membrane) may be caused to contact an ion exchange resin to further remove ions from the fluid and mainly divalent ions.

Subsequent to contact with the ion exchange resin, the second fluid may be further treated to remove lactose therefrom or may be blended with the main feedsteam.

It should be appreciated that in an alternative method, the second fluid may initially contact the ion exchange resin prior to contacting the nanofiltration membrane. BRIEF DESCRIPTION OF THE FIGURES

In order for the invention to be more fully understood, reference will be made to embodiments thereof as illustrated in the accompanying figures in which:

Figure 1 is a flow diagram of the method according to one embodiment of the invention.

Figure 2 is a flow diagram of the method according to a second embodiment of the invention. BEST MODE OF CARRYING OUT THE INVENTION

Figure 1 shows a flow diagram suitable for use to remove inorganic ions and lactose from a delactosed whey solution.

The delactosed whey is first filtered through a 40 micron screen 10 to remove any suspended matter. The filtered delactosed whey is passed into a first module 11 containing a multiplicity of first semi-permeable membranes having a pore size of between 20 to 40 angstroms.

The delactosed whey is caused to contact one side of the first semi-permeable membranes in the module while the other side of the membranes is contacted with a first fluid 12 comprising water flowing in a counter-current manner to the delactosed whey flow. Fast-diffusing monovalent ions such as potassium, sodium and chloride present in the delactosed whey feedstream pass through the first semi-permeable membranes driven by the high concentration gradients and high diffusion coefficients. The main effect is therefore diffusive transport across the semi-permeable membrane. Larger molecules such as lactose diffuse only very slowly across the membrane if at all. The monovalent ions and other diffusing species are collected by the counter- current flow of water 12 which exits module 11 and passes to drain. Some dilution of the delactosed whey feedsteam occurs but this is minimal if the feedstream is kept at high feed rates.

The delactosed (and partially desalted) whey feedstream passes from module 11 into module 13 which comprises a multiplicity of second semi-permeable membranes having a pore size of between 60 to 120 angstroms. The feedstream is caused to contact one side of the second semi- permeable membranes while the other side of the second semi- permeable membranes is contacted by a second fluid 14 in the form of a counter-current water flow. In module 13, mono- divalent and trivalent ions together with lactose are caused to pass arross the membrane due mainly to convective transport effects and are carried away by second fluid 14 which flows in a counter-current manner to the incoming feedstream. The pressure of the incoming feedstream may be increased to increase the convective transport properties and oppose the osmotic effects that draw water from the counter-current water flow 14 through the membrane into the feedsteam.

The second fluid flow 14 passes from module 13 and comprises a dilute solution of dissolved ions and lactose in water. Fluid 14 in this embodiment is caused to contact an ion exchange resin 15 comprising an anion exchange resin and a cation exchange resin. The ion exchange resin removes the ions from the solution, and the fluid subsequent to contacting the ion exchange resin may be passed into module

13 thereby providing a closed loop system. As lactose is not affected by the ion exchange resin, the closed loop system will effectively result in equilibrium level being achieved of lactose or other like sugars, which pass across the second semi-permeable membrane.

The flow diagram of Figure 1 also allows the second fluid subsequent to passing through the ion exchange resins to pass along 16 to be blended with the main feedstream of delactosed whey.

Alternatively, the second fluid subsequent to contacting ion exchange resin 15 can be removed from the system along 17 to be further treated to remove the lactose content.

Figure 2 discloses a second embodiment of the method according to the invention. In this embodiment the initial feedstream comprising delactosed whey of up to 32% solids is initially passed through a screen 21 to remove to any suspended matter and thereafter is passed through 20 into module 11 which comprises a multiplicity of first semi-permeable membranes identical to that disclosed with reference to Figure 1. As with Figure 1, the other side of the semi-permeable membranes is contacted with a first fluid 12 which flows in a counter-current fashion to feedstream 20 and removes any dissolved ions which pass through the membrane. In this first stage, the fast diffusing monovalent ions such as potassium, sodium and chloride quickly pass through the semi-permeable membrane and are carried away by the first fluid 12.

The partially deionised feedstream is subsequently passed into a second module 13 identical to that disclosed with reference to Figure 1 and the pore size results in ions and sugars such as lactose passing through the membrane primarily by convective transport. These products are removed by a counter-current flow of the second fluid in the form of water 22 which passes from module 13 and in this embodiment contacts one side of a nanofiltration module 23 comprising a multiplicity of nanofiltration membranes under high pressure. The nanofiltration membrane allows water and monovalent ions to pass through the membrane as a permeate 24. The resultant product comprises demineralized water containing lactose and mainly divalent ions which can pass along 25 to be further treated to remove lactose therefrom or alternatively can pass along 26 to contact an ion exchange resin comprising an anion exchange and a cation exchange resin.

Subsequent to contacting the ion exchange resin 27 the fluid may pass along 28 to be blended with feedstream 20 or alternatively may pass along 29 to be further treated to remove lactose. EXAMPLE

A sample of delactosed whey was demineralised by a two stage dialysis process. The stage I dialysis membrane had an average pore size of around 30 A angstroms. The stage II dialysis membrane had an average pore size of around 100 A angstroms. In each stage the dialysate was water. Experimental results in Table 1 demonstrate the effect of diffusive transport in stage I and combined diffusive and convective transport in stage II.

The first group of figures in Table 1 shows the ionic and lactose concentrations in the dialysate reject in stage I and stage II, i.e., the components that have passed through the membrane.

The second group of figures in Table 1 shows the ratio of the concentration of other components to the concentration of the monovalent ion potassium.

Concentrations gms/lit in dialysate reject:

K Ca Po₄ Cl Lact STAGE I 1.3 0.02 0.53 0.75 4.0

STAGE II 1.1 0.06 0.76 0.61 25.0

Ratio of other components concentration to potassium:

Ca/K Po₄/K Cl/K Lact/K

STAGE I 0.018 0.40 0.57 3.07 STAGE I I 0 . 054 0. 69 0 . 55 22 . 7

A study of the ratio of the concentration of other componen ts to the f a s t d i f f u s i ng potassium ion shows the effect of pore size on transport of components through the membranes.

Chloride:

The chloride/potassium ratio showed no significant difference between stage I and stage II, i.e., monovalent chloride has same transport characteristics as potassium.

Calcium:

The calcium/potassium ration showed considerable increase in stage II indicating convective transport exerting an influence on the slow diffusing divalent calcium ions.

Lactose:

There was a dramatic increase in the lactose/potassium ratio in stage II. Lactose has a low diffusion coefficient compared to potassium. Only a high convective component in the transport of a slow diffusing middle sized molecule like lactose could explain the comparative large increase in lactose transported across the large pore stage II membrane. No macromolecules such as proteins were found in the stage II dialysate reject.

Free solution diffusion coefficients: *10 cm sec

Potassium chloride 1.99

Lactose 0.65

Lactalbumin 0.106

The method according to the invention allows mono di and trivalent ions and low molecular weight sugars to be removed from a whey solution by transport across two separate semi-permeable membranes of different sized pores while the protein in the whey remains constant and dilution of the whey feedstream is minimal. If the separated lactose is returned to the feedstream, the method results in the removal only of the mono di and trivalent ions to result in a demineralised whey product having a higher market value.

When the ion exchange resins are exhausted they can be sweetened off and regenerated according to known methods.

The ion exchange resins do not directly contact the whey feedstream and therefore are not contaminated in the same manner as known attempts where ion exchange resins are directly contacted with a whey feedstream.

The method can produce a product which is 50% or greater demineralised whey that can be concentrated and spray dried to a powder according to known techniques. The process is rapid, simple, inexpensive, with low power consumption and low levels of effluent as the only untreated effluent is that from the first semi-permeable membrane. (Although this can be treated if desired.)

If the initial feedstream comprises a delactosed whey having 32% solids, the method according to the invention will result in a demineralised delactosed whey powders having 35% protein which can be sold at higher prices than delactosed whey containing mineral impurities.

At value of 32% solids, the delactosed whey has an osmotic pressure far above the limits of direct nanofiltration and is too concentrated for ion exchange. If the initial feedsteam was diluted, the ash load would be too high and would quickly exhaust the ion exchange resin requiring large amounts of regenerating chemicals and producing high effluent loads.

Delactosed whey at 32% solids is also too concentrated for electrodialysis. The method can be used for a variety of dairy applications including demineralisation of sweet whey, demineralisation and deacidification of acid, casein wheys, demineralisation and deacidification of acid cheese wheys such as cottage cheese, demineralisation of skim milk, demineralisation of delactosed whey, demineralisation of concentrated ultrafiltration permeates of milk and whey, demineralisation of ultrafiltration retentates of

ultrafiltration whey protein concentrates, demineralisation of lactose mother liquors and demineralisation of process streams of hydrolised whey or permeates.

It should be appreciated that there is other changes and modifications may be made to the embodiment described without departing from the spirit and scope of the invention as defined in the appended claims.

Claims

1. A method for at least partially removing inorganic ions and low molecular weight sugars from a concentrated

feedstream containing inorganic ions and low molecular weight sugars comprising the steps of

contacting the other side of said first membrane with a first fluid which at least partially dissolves said ions,

(B) Contacting the feedstream subsequent to step (A) with one side of a second semi-permeable membrane allowing mainly convective transport of inorganic ions and low molecular weight sugars across the membrane and contacting the other side of said second membrane with a second fluid which at least partially dissolves said inorganic ions and low molecular weight sugars and maintaining a high

concentration gradient across the membrane,

(C) Recovering said feedstream subsequent to step (B).

2. The method as claimed in claim 1 characterised in that the first semi-permeable membrane has a pore size of between 10-50 angstroms.

3. The method as claimed in claim 2 characterised in that the pore size is between 20-40 angstroms.

4. The method as claimed in any one of claims 1 to 3 characterised in that the second semi-permeable membrane has a pore size between 50-150 angstroms.

5. The method as claimed in claim A characterized in that the second semi-permeable membrane has a pore size between 60 - 120 angstroms.

6. The method as claimed in any one of claims 1 to 5 characterized in that the first fluid comprises water.

7. The method as claimed in any one of claims 1 to 6 characterized in that the second fluid comprises water.

8. The method as claimed in any one of claims 1 to 7 characterized in that the second fluid subsequent to contacting the other side of the second semi-permeable membrane is caused to contact an ion exchange resin.

9. The method as claimed in claim 8 characterized in that the second fluid passes through a closed loop system extending between the second semi-permeable membrane and the ion exchange resin to result in an equalibrium level of low molecular weight sugars being formed between both sides of the second semi-permeable membrane.

10. The method as claimed in claim 8 or claim 9 character! zed in that the second fluid after contact with the ion exchange resin is blended with the feedstream either before or after introduction of the feedstream to the second semi-permeable membrane.

11. The method as claimed in any one of claims 1 to 10 characterized in that the second fluid subsequent to contacting the other side of the second semi-permeable membrane is contacted with one side of a third semi-permeable membrane having a pore size to allow passage of water and monovalent ions such as sodium, potassium and chloride but inhibiting passage of low molecular weight sugars such as lactose.

12. The method as claimed in claim 11 characterised in that the third semi-permeable membrane is a nanofiltration membrane.

13. The method as claimed in claim 11 or 12 characterised in that the second fluid after contact with one side of the third semi-permeable membrane is contacted with an exchange resin.

14. The method as claimed in claim 13 characterised in that the second fluid after contacting the ion exchange resin or the third semi-permeable membrane is blended with the feedstream.

15. The method as claimed in any one of claims 1 to 14 characterised in that the feedstream comprises a

concentrated whey solution of 20% to 40% solids.

16. The method as claimed in claim 15 characterised in that the whey solution is a delactosed whey (DLW).

17. An apparatus for at least partially removing inorganic ions and low molecular weight sugars from a concentrated feedstream containing inorganic ions and low molecular weight sugars characterised in that the apparatus comprises a first module incorporating one or more first

semi-permeable membranes allowing mainly diffusive transport of inorganic ions across the membrane, feedstream inlet means and outlet means associated with said first module to allow the feedstream to enter into the first module and to contact one side of the first semi-permeable membranes and to exit from the first module, first fluid inlet and outlet means to allow a first fluid which at least partially dissolves said ions to enter the first module and to contact the other side of first semi-permeable membrane and to exit the module,

a second module incorporating one or more second semi-permeable membranes allowing mainly convective transport of inorganic ions and low molecular weight sugars across the membrane, feedstream inlet and outlet means associated with said second module to allow the feedstream to enter into the second module and contact one side of the second semi- permeable membranes and exit the second module, second fluid inlet and outlet means to allow a second fluid to enter the second module and contact the other side of the second semi- permeable membranes and to exit from the second module, the feedstream outlet means of the first module being in fluid communication with the feedstream inlet means of the second module.

18. The apparatus as claimed in claim 17 characterized in that the first semi-permeable membranes have a pore size between 20 - 40 angstroms and the second semi-permeable membranes have a pore size between 60 - 120 angstroms.

19. The apparatus as claimed in claim 17 or 18 characterized in that the first and second modules are connected in series and/or parallel.

20. The apparatus as claimed in any one of claims 17 to 19 characterized in further comprising an ion exchange resin assembly in fluid communication with the second fluid outlet means of said second module such that the second fluid passes from the second module into the ion exchange assembly.

21. The apparatus as claimed in any one of claims 17- 20 characterized in further comprising a third module incorporating one or more third semi-permeable membranes having a pore size to allow passage of water and monovalent ions such as sodium, potassium and chloride but inhibiting passage of low molecular weight sugars such as lactose, fluid inlet and outlet means to allow the second fluid to enter into the third module and contact against one side of the third semi-permeable membranes and exit from said third module, and a permeate outlet to allow the permeate passing through said third semi-permeable membrane to exit the third module.

22. The apparatus as claimed in, claim 21 characterized in that the third semi-permeable membrane comprises a nanofiltration membrane.

23. The apparatus as claimed in claim 21 or 22 characterized in that the fluid inlet means of the third module is in fluid communication with the second fluid outlet means of said second module and that the fluid outlet means of said third module is in fluid communication with the ion exchange assembly.

24. The apparatus as claimed in claim 23 characterized in that the fluid exiting from the ion exchange resin assembly is blended with the feedstream subsequent to the feedstream exiting the second module.

25. A product whenever prepared by the method as claimed in any one of claims 1 to 16.