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Homogenising valve technology for the dairy industry
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Monday, 16 June, 2014, 08 : 00 AM [IST]
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Dr J V Parekh
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fiogf49gjkf0d The heart of the homogeniser is the homogeniser valve. This unit may be of widely different shapes and sizes. Most valves of the poppet type also have a secondary part called a breaker ring which surrounds the main homogenising valve so that the fluid strikes the inner surface of the breaker ring at right angles as it leaves the orifice formed by the conical-shaped valve and seat. The valve parts are subject to extreme abrasion because of the high velocity and pressure of the fluid as it passes through the valve; therefore they must be constructed of extremely tough and wear-resisting metals. Most modern valves of this type are made of stellite and tungsten carbide.
The size of the valve must be properly proportioned for the capacity of the machine for best results, as valves which are too large have a tendency to cause excessive clustering, whereas those that are too small may not give proper breakup and may also cause excessive clustering.
Since that time, many changes and modifications have been made in the homogenising valve and homogeniser apparatus. The literature reveals a minimum 100 patents on homogenising valves and homogenisers. The valve designs include single service valves, tapered valves, grooved (stepped) valves, tapered and grooved valves, highly chamfered or knife-edge valves, and many others.
Special purpose valves include unique, patented Micro-Gap valve that processes dairy products at pressures up to 35% lower than conventional valves. Cell-disruption valves that harvest high-yields of intracellular products with minimum debris. And steam-sterilised sealed valves for aseptic operation.
The reason for all these changes was to make homogenisation more efficient, that is, to produce a smaller average particle size with the expenditure of as little energy (pressure) as possible.
It has been found that homogenisation efficiency can be increased, if certain valve geometry is used and specific fluid flow condition are met. The basic concept involves the transfer of homogenising energy to the fluid in the shortest time and with the greatest energy density possible. The ideal way to do this is to generate a very thin film of fluid and to subject this film to high-intensity energy.
This can be achieved with a knife-edge design, which maintains an extremely small opening between the valve plate and valve seat, thereby, generating a thin film of processing fluid. The valve seat geometry has a very narrow travel distance from the high pressure fluid side to the low pressure fluid side, thereby, producing a large pressure differential in a very short time over a very short distance. This pressure change in the fluid film produces intense cavitation and turbulence in the liquid. The flow condition through the narrow gap is also affected by the backpressure on the downstream side of the valve. By adjusting the amount of this backpressure, the cavitation intensity can be maximised in the fluid film. By combining these elements of a narrow land, a very small gap, and an optimum backpressure, a very high-energy density can be applied to the fluid being homogenised.
Homogenisation Theories
It is most likely that a combination of two theories, turbulence and cavitation, explains the reduction in size of the fat globules during the homogenisation process.
Turbulence
Energy, dissipating in the liquid going through the homogeniser valve, generates intense turbulent eddies of the same size as the average globule diameter. Globules are thus torn apart by these eddie currents reducing their average size.
Cavitation
Considerable pressure drop with charge of velocity of fluid. Liquid cavitates because its vapour pressure is attained. Cavitation generates further eddies that would produce disruption of the fat globules.
The high velocity gives liquid a high kinetic energy which is disrupted in a very short period of time. Increased pressure increases velocity. Dissipation of this energy leads to a high energy density (energy per volume and time). Resulting diameter is a function of energy density.
In summary, the homogenisation variables are - Type of valve; Pressure; Single or two-stage; Fat content; Surfactant type and content; Viscosity; and Temperature.
Also to be considered are the droplet diameter (the smaller, the more difficult to disrupt), and the log diameter which decreases linearly and levels off at high pressures.
It would be beneficial to describe the flow profile through a conventional valve and seat. Figure 1 shows a plug type homogenising valve and the corresponding valve seat. The unhomogenised product enters the valve seat at a relatively low velocity but at a high pressure. This pressure is generated by the positive displacement pump and the restriction to flow caused by the valve being forced against the valve seat by some type of actuating force. The positive displacement pump provided a relatively constant flow and, therefore, will generate the required pressures as the restricted area between the valve and seat is increased or decreased. The liquid then moves out through the area between the valve and seat at high velocity, impinges on the impact or wear ring, and then is discharged as homogenised product.
The conventional homogenising valves usually consist of one operating unit (valve and seat) that forms a small slit or opening through which pressurised fluid flows. As the rate of flow increases, the opening must also increase in order to maintain pressure and accommodate flow. At high flow rates the cross-sectional area between the valve and seat must be large, and this can be accomplished by either increasing the gap between the valve and seat or by increasing the diameter of the valve and seat so that the gap is small but the circumference is large. However, when the gap is large, homogenising efficiency is decreased. When the valve diameter is large, actuation of the assembly is difficult. Also, close tolerance machining of large diameter valves is a problem. This conflict between valve size and large flow rates has made it difficult to get the most efficiency possible out of the homogenising valve when the homogeniser is scaled up.
One approach to this problem is an investigation of valve geometry to make the valve more efficient without changing its size.
The previous work on homogenising valves indicated that a knife-edge seat should be more efficient than a conventional seat with a long travel distance or land. When investigating this geometry at flow rates less than 1,000 litre/hr, increases in efficiency is not significant. However, when flow rate is increased, homogenisation can improve substantially.
Figure 2 shows a knife-edge seat and conventional seat for a homogenising valve with a diameter of 34.925 mm. The travel distance of the conventional valve would be about 5.08 mm, whereas the travel distance of the knife-edge design would be 0.254 mm. A typical flow rate for this size valve would be 5,678 litre/hr and at this flow rate the gap between the valve and seat would be about 0.1905 mm at 1,500 psi.
The efficiency of homogeniser is dependent on the gap between the valve and seat and the length of the travel distance. When a valve with a long travel distance is used at a small gap, the fluid dynamics, possibly because of friction loss, favour less, droplet reduction and more droplet coalescence. This result then indicates that the knife-edge valve can be efficient but only under the right conditions.
Testing validity
To test the validity of the importance of gap, a test was performed with a fixed slit valve.
From the data presented thus far, a rudimentary picture can be drawn of the fluid dynamics in the knife-edge valve. By decreasing the land of the seat, the change in pressure of the fluid across the face of the valve seat occurs in a shorter time and distance than in a conventional valve. The small gap between the valve and seat means that the energy of homogenisation is applied to a thin film of fluid, and, therefore, the energy can act with greater intensity through the thin film than it would over a thick film (large gap) where the possibility of dampening the energy would be greater. Thus, the knife-edge valve produces a large energy density in a small volume of fluid, and even though the total energy input of the valve at a certain operating pressure is the same as a conventional valve, the knife-edge design utilises the available energy more efficiently.
The next logical step in this sequence is to consider how we can take advantage of this efficient valve design for high flow rates. Considering a single unit valve and seat, one finds that scale-up presents a problem. For a flow rate of 20,000 litre/h and 140 bar homogenising pressure, the knife-edge valve and seat would require a diameter of 52.8 cm to have a gap of 0.0254 mm. To close the valve to within 0.0254 mm of the seat and thereby generate 2000 psi, the actuating force on the valve opposing the homogenising pressure would be 313,891 kg. When attempting to scale up the valve, one encounters the unreasonable size of the valve and the actuating force required to operate it.
To overcome these obstacles, a way must be found to reduce the diameter of the valve and yet retain the small gap. An examination of the rectangular slit valve experiment shows that a long slit can be put into a small area by stacking the slit and thereby dividing the flow into equal, parallel parts. For a circular slit this can be done by stacking the homogenising valves in parallel and by allowing the flow to be split into equal parts such that each part of the flow is subjected simultaneously to the ideal conditions of homogenisation.
Homogenising valve
Homogenising valve is the main component of a homogeniser. Valves vary in the design, the flow of the liquid through it, by the valve parts and the number of valves employed in the system.
Figure 4 shows the various types of homogeniser valves. Value plugs are pressed with an adjustable force F on the corresponding valve seat. In this way, the homogenising slit is formed when the incoming liquid pressure has been adjusted. Fig 4 a shows a conical shape, b causes changes in direction due to its profile, c shows a simple plate valve, d a conical shape but with a grooved valve face which forces alternating stresses on the liquid to be homogenised, and e shows a valve with a breaker ring which has a flat valve face and a conical seat. The valve parts are subjected to extreme abrasion because of the high velocity and pressure of fluid as it passes through the valve, therefore it must be constructed of extremely tough and wear resisting metals, e.g. stellite, tungsten carbide or ceramics.
The poppet type of valve has a breaker ring which surrounds the main homogenising valve. The valve is held down by a high tension spring; as the fluid pressure comes against it, the valve rises only a few hundredth of a millimeter to form a very narrow space between the valve and valve seat. When a fluid enters the clearance area, pressure energy is converted to velocity energy. In attaining the velocity and maintaining it across the valve face, followed by an almost instantaneous drop to low velocity on emerging from the clearance area, tremendous velocity and energy gradients, with resultant zones of extreme turbulence are encountered in the fluid. Accompanying these velocity and energy gradients such phenomena as shearing action, cavitation, attenuation and impact are there which causes homogenisation.
Changes in the radial cross can be brought about by means of the conical valve. By altering the internal diameter of the breaker ring, the back pressure and therefore the position of the zone of cavitation can be adjusted. The spring maintains the clearance between the valve and valve seat as well as allows the valve to follow the varying flow from the pump, thus producing uniform homogenising forces.
The size of the valve must be properly proportioned for the capacity of the machine for best results. If the valves are too large, they will have a tendency of excessive clustering, whereas those that are too small may not give proper breakup and may also cause excessive clustering. The main factors affecting the degree of homogenisation other than the valve design are the milk temperature, pressure, valve conditions and air content of the milk.
Special types of valves SEO Homogenising Valve
A flat, conical homogenising valve made of several ceramic materials is used for abrasive products. Also available in stellite and tungsten carbide. The SEO achieves the same homogenising effect as the LW, at slightly higher pressures.
XFD Homogenising Valve
Typically used as a single-stage valve for capacities up to 36,000 litre/hour or as the first-stage valve in a two-stage configuration. The XFD is available in stellite and tungsten carbide.
LW Universal Homogenising Valve
The LW (liquid whirl) whirling chambers deliver highly efficient homogenising effect with low power consumption. The LW is a universal valve that can be used for emulsions, dispersion and suspensions. In some cases, the efficiency of the LW valve will eliminate the need for a second stage.
Plug Valve
Normally used as a single stage valve for capacities up to 9500 l/h and as the first stage valve in two stage assembly. Available in stellite, tungsten carbide or ceramics.
Piloted Valve
Typically used as a single stage valve for capacities above 9500 l/h and as the second stage valve in a two stage assembly. Available in stellite.
Micro-Gap Homogenising Valves
The Micro-Gap valve achieves good results with a knife-edge design, which maintains an extremely small opening between the valve plate and valve seat, thereby, generating a thin film of processing fluid. The valve seat geometry has a very narrow travel distance from the high pressure fluid side to the low pressure fluid side, thereby, producing a large pressure differential in a very short time over a very short distance. This pressure change in the fluid film produces intense turbulence flow in the liquid. The flow condition through the narrow gap is also affected by the backpressure on the downstream side of the valve. By adjusting the amount of this backpressure, the turbulence intensity can be maximised in the fluid film. By combining these elements of a narrow land, a very small gap, and an optimum backpressure, a very high-energy density can be applied to the fluid being homogenised.
Conventional homogenising valves usually consist of one valve and seat, which creates a small opening through which pressurised fluid flows. As the rate of flow increases, the size of the opening must also increase, in order to maintain pressure and accommodate flow. At high flow rates the cross-sectional area must be very large, and this can be accomplished by either increasing the gap between the valve and seat or by increasing the diameter of the valve and seat, so that the gap is small but the circumference is large. However, when the gap is large, homogenising efficiency is decreased. When the valve diameter is large, size and actuation of the assembly are unfavorable.
The new homogenising valve assembly overcomes these limitations by stacking homogenising valves in parallel and by allowing the flow to be split into equal parts, such that each part of the flow is simultaneously subjected to the ideal conditions of homogenisation. Each valve member acts as both a valve plate and valve seat. The valve seat has the knife-edge configuration. Because the number of valve members can be varied, a large flow of liquid can be efficiently homogenised by dividing the total flow into the correct number of parts, so that each part goes through one valve member at the correct gap and flow conditions. With this stacked configuration, the flow can be increased without sacrificing homogenising efficiency by increasing the number of valves so that each valve member always works at the optimum rate of flow.
Nano Valves
The Nano Valve is a new high-efficiency homogenising valve that micro sizes milk and dairy products at a lower pressure than was ever possible before, while achieving the same or better results on emulsion quality of the processed product.
With the Nano Valve you can save 30% of the ordinary power consumption while at the same time improving the quality of your product. The Nano Valve concept also guarantees a longer life of the homogenising valve parts.
The Nano Valve allows, at the same homogenising pressure, reduction of particles size of the product compared to traditional valves. The main direct consequence of working at lower pressures is the reduction of energy consumption, meaning lower production cost. The improvement does not only involve the performance; the wear of parts and the operative noise are also noticeably reduced. Nano Valve is supplied with a simple pneumatic control system that is also easy to use in remote control systems. All these characteristics, together with the reduced maintenance, make the Nano Valve a winning improvement.
Nano Valve can easily be installed both on old and new machines, also in aseptic execution.
The effect of the second stage homogenising valve
As a result of the development of sophisticated particle sizing methods, it is now possible to accurately measure homogenising efficiencies for various valves, valve combinations and pressures.
In evaluating valve combinations, it has been found that with fluid milk and other emulsions at any given homogenising pressure, efficiency is increased by the use of a second-stage valve; whereby, 10% to a maximum of 20% of the total pressure is applied by the second-stage valve.
In homogenisation, as the liquid travels from the high-pressure zone in the cylinder through the area between the valve and seat, there is a large increase in fluid velocity and a corresponding decrease in fluid pressure. The fluid velocity initiates very intense turbulence in the fluid jet exiting from the valve and seat.
This turbulence disrupts the disperse phase and produces the homogenisation effect.
The additional use of a second-stage valve will serve to exert a backpressure on the fluid moving through the first-stage valve and, consequently, will influence the intensity of the turbulence by suppressing cavitation of the liquid. Cavitation of the liquid creates a two-phase flow of gas-in-liquid which makes the liquid “spongy”. Elimination of the cavitation enhances the zone of turbulence. With many emulsions the proper use of a second-stage valve will optimise the homogenisation phenomenon. In some cases certain physical attributes, such as viscosity and appearance, can be better controlled.
The second stage valve should not be operated at pressures greater than 30 bar. Exceeding this pressure will produce excessive wear in the second stage valve body, which, typically, does not have an impact/wear ring.
The correct sequence for setting the two-stage valve is as follows: The second-stage valve is set first, followed by the first-stage valve, to give the total pressure desired. For example, for 140 bar the second-stage would be set to read 15 bar, and then the first-stage valve would be set for the total pressure of 140 bar. The actual pressure drop through the first-stage valve would be 125 bar.
The reason for this sequence is that the two valves are not fixed valves, and the second-stage pressure can influence the forces acting on the first-stage valve. Therefore, if the first-stage valve were set first, followed by the second stage, then the forces acting on the first stage would change. This would cause a shift in the first stage pressure. The total pressure would still be the same, but there would be uncertainty as to how the total pressure was distributed between the two valves. If a pressure gauge were positioned between the two valves, then the first and second stage could be adjusted independently.
Judging the efficiency of homogenisation
1. Creaming Index Method
General: Low creaming index is an indication of good homogenisation. Sterilised milk may be graded as under for the quality of homogenisation:
Apparatus
1. Glass Tubes: Three, with ground glass stoppers outside diameter 24mm., length with stoppers 245mm (suitable for use with a 24 tube Gerber Centrifuge): and graduated from 0 to 50 ml
2. Pipette: Three, fine pointed, connected to a suitable vessel and to a vacuum pump
3. Apparatus for the Determination of Fat: As specified in IS:1223-1958
Procedure: Place 50 ml of the material at 20o + 1oC in each of the three tubes. Centrifuge for 15 minutes at 1000 rev/min.
Using the separate pipette, take 5 ml from the upper part of each of the three tubes, carefully taking the cream that adheres to walls of the tube and transfer into a container (Sample I). Then empty the three tubes into a separate container (sample II). Measure the fat content in the samples I and II by the Gerber method as described in IS:1224- 1958 taking all the precautions required to be taken for homogenised milk that is heating the sample at 65o + 2OC for 5 minutes in a water-bath after each centrifuging before taking the reading till identical readings are obtained. Usually three centrifugings are required each lasting for at least 5 minutes.
2. Turbity Method
At present, homogenised milk samples are often held in a refrigerator to determine if the creaming rate is acceptable. However, because creaming rate is directly related to the fat globule size and distribution, the Emulsion Quality Analyser (EQA) can be used to determine the acceptability of the creaming rate before the milk is packaged. The average diameter measured by this technique incorporates data on both the volume-to-surface particle diameter and the particle-size distribution. One is then assured that this measured fat globule size indicates a product featuring the desired stability. Thus, future samples having a globule size equal to or smaller than this value will have an acceptable creaming rate. Conversely, if the globule size is larger than the known reference value, it will be learned immediately that the milk will cream more rapidly. Proper use of this knowledge can save the cost and embarrassment resulting from shipping a poor quality product.
Experience gained with this technique in the APV Gaulin laboratory has lead to some general conclusion relating specific fat-globule sizes to the emulsion quality of homogenised milk. These conclusions are summarised below:
Product Quality
Note that these are generalisations which are characteristic of the emulsion quality of homogenised milk currently being produced by many dairies. Typically, pasteurised milk is about .850 and UHT long-shelf-life milk (3 to 6 months) is .40 to .55. Each dairy must, of course, make its own judgement as to the emulsion quality it desires.
Selection of Homogeniser
The main points to be considered in the selection of a homogeniser are power requirement, capacity, flexibility for different products, ease of cleaning, maintenance, durability, availability of manufacturers’ service, and ease of assembly and operation.
Homogenisation Significantly Improves the Quality of Product
Homogenisation is the process of emulsifying one liquid into another or dispersing solid particles uniformly throughout a product. The process breaks apart particles and liquid globules, reducing their size and substantially improving a number of important product qualities. The difference before and after homogenisation is clearly visible.
Processors all over the world use high pressure homogenisers to improve their products and gain a distinct competitive advantage. In fact, these homogenisers are used to enhance thousands of products in many diverse industries.
Dairy Products: Extended shelf stability, improved smoothness and body for milk, ice cream, desserts, cheeses and yoghurt.
Foods and Beverages: Enhanced smoothness and body, extended shelf stability and reduced ingredient costs for juices, dressings, sauces, baby foods, fat substitutes and nutritional supplements.
Chemicals: Particle size and viscosity control, enhanced colour, uniformity of application and improved stability for defoamer emulsions, clay dispersions, wax emulsions, silicone emulsions, grease, magnetic tape coatings and water repellant coatings.
Pharmaceuticals: Stability, uniformity, narrow particle size distribution and enhanced texture for antibiotics, ointments, liposomes, intravenous emulsion and tablet coatings.
Biotechnology: Cell disruption for harvesting high yields of intracellular products such as bacteria, proteins, yeast, algae and enzymes.
Cosmetics: Smoother textures, better dispersion of thickeners, enhanced colour, increased gloss and better application for hair products, lotions, creams and lipsticks.
Conclusion
The objective of homogenising valve is to seek the most efficient transfer of the available homogenising energy to the product. This objective is met by three conditions: one, a short land or travel distance; two, a small gap; and three, the optimum back pressure for the total homogenising pressure used. By concentrating the energy into a thin film and small volume of fluid and thereby producing a large energy density, the homogenisation phenomenon can be made more efficient.
(The author is senior dairy consultant)
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