Background Of The Invention Freeze concentration as a technique is widely used in beverage industry, such as coffee, tea, <BR> <BR> wine, milk, beer, orange juice, grape juice etc. , for reducing the volume to be handled during storage, transportation and sale.

Freeze concentration of aqueous solutions is a physical process in which the solution is concentrated by freezing and crystallizing water into ice and separating the ice from mother liquor. This process involves refrigeration of the solution, crystallization of water and separation of the formed ice.

Crystallization can be performed either by suspension crystallisation by creating suspensions of crystals or using layer freezing as crystal films grown on cooled equipment surface. In suspension crystallization, it is possible to isothermaily generate crystals in a range of sizes, preferably of large sizes in the mother liquor. However this requires careful control of the process parameters. The final product quality is limited by the amount of residual solution adhering to the crystals after filtration and washing. In contrast the layer freezing process is easier to manage, as there are usually no moving parts and slurry-handling system is not required. Crystal growth rates can be controlled on cooled surfaces with temperature control.

Crystallizers using indirect heat removal may be internally and externally cooled.

Interanlly cooled crystallisers involves the extraction of heat through the wall surrounding the crystallizing solution where as and in the case of externally cooled crystallisers, heat is externally transferred from the feed to the crystallizer.

Externally cooled crystallisers can be divided in to three types involving (Thijssen, H. A. C. , Current developments in the freeze concentration of liquid foods, in: Freeze drying and advanced food technology, Editated by S. A. Goldsmith, L. Rey and W. W. Rothmayr, Academic press, London, UK 1975). supercooling of the feed stream or re-circulation of the whole suspension from the crystallizer to the heat exchanger or

producing sub-critical crystals in an external heat exchanger.

The desirable features of the freeze concentration system are: a. Energy efficient, once through TOCS using heat pump with high Coefficient of Performance (COP) and ability to scale up to a large capacities. b. Condensation of compressed refrigerant vapor at as low condensing temperature as possible c. Low temperature difference between condenser and evaporator of the heat pump. d. Eliminating the use of Scraped Surface Heat Exchanger (SSHE), wash column and recrystallizer. e. Simple system without any requirements for maintaining vacuum and with minimum pumps, conduits, controls and parts by using layer freezing and not suspension crystallization f. Low parasitic power requirement US Patent 4,557, 741 discloses the Niro Process. It includes the main steps viz. refrigeration of solution in SSHE, crystallization of ice in Recrystallizer and separation of ice crystals using Wash Column. Freezing of the solution and formation of nuclei in the form of fine ice crystals takes place on the surface of the SSHE. Recrystallizer is a ripening tank, used to form large ice crystals. Wash column is used to wash and separate ice crystals from solution. However, Niro process suffers from following drawbacks. a. SSHE is expensive. Its size is limited by the mechanism implemented for ice scraping, e. g. 0.5 ton ice per hour. b. System does not scale well to large volumes because of SSHE. c. Scraping work and refrigeration to remove heat due to friction is estimated two third of total energy (Schwartzzberg, 1988, Potential Improvements in food freeze concentration). d. In recirculation, concentrate is mixed with the fresh feed. The system has to operate at much lower evaporator temperature to refrigerate concentrate continuously rather than fresh feed, which increases energy cost. e. Since condenser of the refrigeration system is air cooled, Coefficient of Performance (COP) is less.

US patent 4,457, 769 discloses a system in which liquid mixture being concentrated is circulated through a freeze exchanger until it reaches the desired concentration. The ice crystals formed are relatively of large size in a low viscosity solution. The large ice crystals are easily handled and washed free of the entrained concentrate. This system requires higher energy consumption. The"unenclosed"system suffers from several shortcomings such as: a. Usage of number of conduits and pumps b. Large interface between liquid and surrounding atmosphere, negatively affecting the quality of the concentrate.

US patent 5,394, 706 discloses a single-stage freeze crystallization process. The freeze crystallization apparatus uses indirect cooling process with a secondary cooling media (brine) cooled. The freeze crystallizer includes scraped surface heat exchanger to produce pump and remove ice slurry. There are several drawbacks of the system. Concentrate is mixed with fresh feed therefore much lower evaporator temperature is required to freeze the concentrate rather than fresh feed. Crystallizer used is a bulky shell and tube type heat exchanger.

US patent 4,666, 484 discloses an apparatus for concentration of aqueous liquids which increases in viscosity with increase in concentration, using three stages and employing screw concentrators in series. The concentrate is not mixed with fresh feed. In this process ripening is not required. However this system suffers from number of deficiencies such as: a. Use of two freeze crystallizers along with four pumps contributing to increase in operating and initial cost of the equipment. b. Fine ice crystals and with loose structure with no option for further crystal growth. c. High water consumption to wash ice crystals and increased load on the freeze exchanger resulting in higher energy consumption. d. Entrainment of solution in fine ice crystals, which is difficult to recover solute.

US patent 6,305, 178 discloses freeze concentration using super cooling. The solution is supercooled without ice formation on the heat transfer surfaces. Instantaneous nucleation is induced to obtain fine ice crystals. Ice scraping and ripening are avoided. This system includes wash column to wash and separate ice crystals from solution. The system and its operations get fairly complex with increased operational costs as elaborate piping and continuous vacuum filter is used to handle multiple steps such as supercooling of the solution, instantaneous ice nucleation, separation of fine ice crystals from mother solution, ice crystal growth, transformation and agglomeration, separation of large ice crystals from mother solution..

US patent 4,036, 619 discloses direct contact freeze concentration in which a cooling medium is injected into aqueous solution to form ice slurry. Large part of the concentration is effected by direct injection of cooling medium e. g. butane or freon and the final concentration is carried out in an indirect heat exchanger. The shortcoming of the process is the direct contact of the refrigerant with the materials being processed thereby limiting the application of this process.

Summary Of The Invention The main object of invention is to provide a novel, simple, compact and rugged freeze concentration system in a process using heat pump obviating the use of scraped surface heat exchanger, recrystallizer and wash column.

Another object of the invention is to achieve high Coefficient of Performance of the heat pump.

Yet another object of the invention is to condense the compressed refrigerant vapor at as low a condensing temperature as possible instead of rejecting the heat to the ambient heat sink.

Yet another object of the invention is to provide a system in which it is possible to maintain low temperature difference between evaporator and condenser.

Another object of the invention is to develop a once through system in which fresh feed is supplied and concentrate is collected, without the need to re-circulate the concentrate in the system.

Yet another object of the invention is to develop simple layer freezing system with minimum number of conduits, pumps and controls.

Yet another object of the invention is melting and freezing of frozen solvent on while in contact with the heat exchange surface simultaneously or in sequence with a switching mechanism involving only two components in solution viz. freezer and melter thereby obviating the need for a scraped surface heat exchanger, recrystallizer and wash column.

Yet another object of the invention is to provide an energy efficient continuous layer freezing process using a heat pump.

Yet another object of the invention is to achieve low capital and operating cost by eliminating the need or additional equipment such as the scraped surface heat exchanger, recrystallizer and wash column.

Thus in accordance with the invention the system comprises of: a. Plurality of heat exchange surfaces b. Heat pump c. Switching means d. Optional use of Valve The solution to be concentrated is contacted with the extracting surface of the heat exchanger. The heat pump provides the necessary cooling effect for heat extraction to freeze the solvent from the solution as well as heat delivery for remelting the frozen solvent. The switching mechanism with a valve is used to alter the functions of heat extraction and heat delivery to achieve either a continuous, batch wise or batch wise continuous process.

Detailed Description Of The Invention The features and advantages of this invention will become apparent in the following detailed description and the preferred embodiments with reference to the accompanying drawings. For purpose of the description contained herein, the definitions of the following terms are relevant : a. Feed Stream: This is the input to the said Freeze Concentration System (FCS) and may be any in the form of a solution or suspension. Examples of these include contaminated

water, sea water, brackish water, industrial waste water, chemical process streams containing salts or other chemicals in suspension or in solution, solutions containing food matter or suspensions such as orange juice, coffee, sugarcane juice, and their like.

Generally, the feed stream is in aqueous medium, but the term (and the invention) is not limited to aqueous systems. Non aqueous FCS may also be treated by the system and processes described in herein. b. Concentrate: This refers to the stream coming out of the FCS which has higher weight percentage of impurities, contaminants, salts, food or other dissolved solutes/suspended matter as compared to that in the feed stream.

Figure. 1 shows a schematic of the Freeze Concentration System (FCS) common to batch wise and batch wise continuous process.

The feed, which is stored in the container 1, is passed through the Heat Transfer Surface (HTS) 2 of cross section 3 in the direction 4. There are a series of such heat transfer surfaces (HTS) inclined in the zigzag fashion mounted on the structure 5 to facilitate flow of solution by gravity; a pump may not be required for circulation of solution while the solution passes from one heat transfer surface to the other. The inclination of the heat transfer surface may be varied as required. The refrigerant passage 6 is bonded to the bottom of the channel with a suitable bonding material 7. The refrigerant flows through the refrigerant passage in the direction 8. The refrigerant enters the refrigerant passage at 9 through the header 10 coming from the Heat Pump (HP) 11. The refrigerant may be circulated either in series, parallel or a combination of these as required. Return of the refrigerant is from the connection 13 to the return header 14. The flow of the refrigerant and the feed are in directions opposite to each other. The solution is passed from one channel to other through the tube 15 attached to the heat exchange surface. The concentrate may be collected from the last channel through tube 16; the system is"once through", and re-circulation of feed is not required. System shown in figure 1 with plurality of HTS mounted on structure 5 along with headers and feed storage facility is common in case of batch wise and batch wise continuous system.

Figure 2 shows the schematic of batch wise freeze concentration system wherein a system as shown in Figure 1 is required in addition to storage device 17. Feed stream passes on a plurality of HTS. Refrigerant perform only heat extraction function, which results in the growth of frozen layer of the solvent on the HTS, this is known as layer freezing. This process continues until the desired frozen solvent thickness is reached on the channel surface (HTS).

The progressive freezing of the solvent results in the concentration of the solution. The frozen solvent is washed with solvent or suitable liquid. Compressor of the HP 11 is then switched off.

While heat extraction function continues, the other HTS with condensing refrigerant in HP 11 delivers the heat to the suitable liquid in 17 passing through 20 to the HP 11. The stored suitable liquid in the container 17 (it may be a solvent) which has picked up heat (and is at

higher temperature than frozen solvent) from the condensing refrigerant in HP 11 is passed from 17 through conduit or suitable connection 18 to the HTS over the frozen solvent to melt the frozen solvent (melting cycle). The liquid stored in 17 may be circulated through another passage other than refrigerant passage for melting the frozen mass. The melted solvent along with the liquid used for melting during melting cycle is collected from the last HTS through the connection 16, a suitable part of which is diverted to storage 17 through conduit or suitable. connection 19.

As shown in Figure 3, batch wise continuous freeze concentration system comprises of plurality of HTS systems 5 connected to one or more heat pumps. This system has a switching device to enable, batch wise continuous layer freezing process. This is made possible by alternating a freeze cycle with a melting cycle in a predetermined sequence.

In case of batch wise continuous, switching mechanism 21 which includes a suitable multi way valve, actuator, signal converter 22 and other suitable components based on the end application facilitates switching of the flow of the primary or secondary refrigerant going to the refrigerant passage of HTS systems 5. High Pressure Compressed Refrigerant Vapour (HPCRV) from compressor 23 of the HP passes through a multi way valve to the refrigerant passage of HTS system. The frozen on the HTS is melted by this refrigerant (melting cycle).

High pressure compressed refrigerant is converted to Low Pressure; Low Temperature Refrigerant (LPLTR) while passing through capillary or suitable device 24 placed between HTS systems. This LPLTR is used in other HTS system to freeze the solvent (freezing cycle). The LPLTR from this HTS system passes through multi way valve back to the compressor of the HP.

Temperature or any other suitable signals 25 from the refrigerant passages of HTS systems are converted by signal converter 22 to electrical signals 26 which are used to energize multi way valve sequentially to enable alternate freezing and melting cycle on number of HTS systems.

Optimized switching time results in increased throughput of the HP based FCS and also results in lower energy consumption. This is because of increased evaporator temperature of the HP.

Figure 4 shows schematic of Continuous freeze Concentration system. In case of continuous layer freezing process solvent from the feed stream passing through 32 is continuously frozen on the HTS 27, which may be moving or stationary. Either primary or secondary refrigerant passing through conduit or suitable connection 30 and 31 is used for heat extraction from HTS 27. Frozen solvent is physically dislodged using suitable means with or without scrapping. Frozen solvent so dislodged is collected in a suitable means 28 through 33 where condensing refrigerant passing through 29 from HP 11 is used to melt it continuously.

Switching mechanism may not be required in this case.

The HTS which perform either heat extraction function, that is freezing of solvent from the solution or heat delivering function, that is melting of frozen solvent may be open/closed channels or a suitable combination of open/c) osed trays with surfaces made of any suitable

material such as stainless steel, plastic, metal coated with plastic aluminum based on the end application. HTS in case of continuous freeze concentration process may be fixed or moving.

The cross section of the HTS may be'C-Shaped, circular, rectangular, square or any suitable shape. HTS in plurality can be arranged in zig-zag manner in which there is means to vary pitch to facilitate flow of feed. Flow of the feed is facilitated by gravity, pump or any other suitable means. Feed is introduced at one end and concentrate is collected at the other end without the need to re-circulate the concentrate in the system. A HTS system comprises plurality of HTS.

Batch wise continuous concentration system comprises of two or more number of such HTS systems. In case of continuous concentration system, a suitable means is provided to physically dislodge frozen solvent from HTS and collect it for melting.

The refrigerant passage is used to circulate either primary or secondary refrigerant which can be used to either freeze or melt the solvent from the solution. Secondary refrigerant may optionally be stored if required. The same refrigerant passage performs the functions of heat extraction or heat delivery intermittently at a predetermined sequence by means of a switching mechanism in case of batch wise continuous process. It performs only heat extraction function in case of batch wise and continuous process. The refrigerant passage may vary from circular, rectangular, square,'D-type, flat oval or any other shape. Configuration of the refrigerant passage bonded to the HTS can either be parallel, counter ; and serpentine with suitable pitch or any suitable configuration. This allows the refrigerant flow in parallel, cross or counters direction. The bonding material may be selected from a number of materials such as copper, zinc, silver or any suitable material based on the HTS material. Thermal bonding may be effected by any suitable means such as brazing. The compressed refrigerant vapor is condensed by means of delivering heat to frozen solvent at as low condensing temperature as possible instead of rejecting heat to ambient sink. The compressor of the Heat pump may be directly driven by an engine, turbine or motor depending on the application.

The Freeze Concentration System (FCS) as described in this present invention when applied to a sugar manufacture process exhibits significant energy conservation.

Freeze Concentration System (FCS) with heat pump can be used in place of an evaporator. Energy consumption in the freeze concentration process is a function of temperature lift, which is the difference between condenser and evaporator temperature of the vapour compression cycle (Thijssen, H. A. C. , Current developments in the freeze concentration of liquid foods, in: Freeze drying and advanced food technology, Editated by S. A. Goldsmith, L. Rey and W. W. Rothmayr, Academic press, London, UK 1975). In FCS using heat pump, it is easy to achieve the equivalent of the temperature difference of 15°C between condenser and evaporator. For a temperature difference of 15°C, the electrical consumption is 8 kWh for the compressor of the heat pump with 418 MJ (100,000 kcal) cooling capacity. Such a heat pump

will produce (100,000 x 4.18) kJ/334 kJ/kg = 1.2 ton of ice, where 334 kJ/kg is the heat of crystallization of ice. The COP of such a system is therefore 14.3. (100,000 x 4. 18) The present invention when applied to a the pre concentrate stage of cane juice to concentrate it from 14°Bx to 50°Bx removes 9 kg of of water per kg of saturated steam at 120°C as compared to the removal of 5 kg of water in a Five Effect Evaporator (FEE) without vapour bleeding. In a 2500 Tonne Crushed Cane per Day (TCD) sugar manufacture process approximately 1800 tonnes water is to be separated from 2500 tonnes juice per day. The use of FCS as compared to FEE results in a saving of about 160 tonnes of steam per day yielding an energy saving of over 60% in terms of steam consumption.