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Freeze concentration beats the heat.


New applications are emerging in food, pharmaceuticals, chemicals, petroleum, and waste disposal as FC demonstrates its potential for substantial energy savings compared to conventional distillation and evaporation processes.

Freeze concentration (FC) saves energy and money in packaging, shipping, and storing food products. More important, FC-in contrast to existing heat-evaporation processes-retains volatile flavor and aroma compounds in food products so that no additives are required to restore the taste and smell of the original product.

In recent tests on orange, grapefruit, and pineapple juices conducted by the Citrus Research and Education Center (Lake Alfred, Fla.), reconstituted FC juices were found to be superior in taste to juices produced by evaporation and similar to the original pasteurized juices.

The dairy industry, which is the largest user of energy for concentration in the food sector, is looking to FC for new products such as frozen concentrated milk as well as better use of the milk by-products of cheese production. Dairy Research Inc. and the Electric Power Research Institute (EPRI) in Palo Alto, Calif., are currently running a program to study the effects of FC on dairy products. One surprising result of blind tests they conducted was that reconstituted FC milk was favored by a test group over fresh milk.

The biggest potential for new FC applications is in those industries that consume large amounts of energy for separation processing, according to a 1987 report prepared for EPRI. In the food industry, this includes milk, vinegar, and beer producers. Potential applications also abound in the pulp and paper, pharmaceutical, chemical, and petroleum industries.

FC separates substances via crystallization substantial energy savings compared to the most common concentration approach, which involves heating the liquid to evaporate part or all of it, so that the solvent is purified, the materials dissolved in it are concentrated, or both. It takes only 143.5 Btu to crystallize water but over 1000 Btu to vaporize it.

The amount of energy needed by processes is the sum of two parts: the reversible thermal energy needed to reach the process temperature and the latent heat needed to cause the phase change. For most applications, the sum is much less for freeze processes than for vaporization processes.

The EPRI report noted that the substitution of FC for evaporation and distillation in all industrial applications could save $5.5 billion in energy costs annually.

Another selling point for FC is the purity inherent in the process. The easiest way to understand the basic phenomenon of how crystallization purifies one component in a solution is to relate the process to making ice cubes in a home refrigerator.

Air bubbles are visible inside an ice cube. If the ice is melted in a glass, a residue of "hardness" salts originally in the water will appear on the bottom of the glass. If the glass sits long enough, the water will redissolve these salts. "The reason these form in the ice is that as the cube freezes the advancing ice wall pushes' the impurities into the liquid in the center of the cube," said James Heist, president of Freeze Technologies Corp. Raleigh, N.C.). "As the air and other impurities in the ice concentrate, they exceed their solubility limits and form air bubbles or a precipitated 'sludge' inside the cube. With no way to escape they are trapped and released when the cube melts. Since not all impurities redissolve as slowly as the hardness salts you don't want to form ice in a way that traps the impurities.

"In commercial FC systems," Heist continued, "the ice is grown from the inside out, the reverse of the way an ice cube freezes. In this way the impurities are concentrated in the bulk liquid surrounding the crystals."

Generally, two major steps are needed for FC processes. First, the crystals must be formed and separated from the remaining liquid. The pure crystals can then be recovered and reclaimed as end product, routed to another operation, or discarded. The concentrated liquid is also recovered as an end product; it can be recycled to recover additional components or routed to another plant operation.

Early Efforts

FC was first used commercially in the 1950s. Research in the 1960s and 70s for desalination, petroleum, and food processing applications provided many technical innovations. By the early 80s, FC equipment was installed for concentration of juice and other food products.

However, compared to evaporation or distillation, FC remains the new kid on the block. "There are a lot of applications for distillation that are relatively inexpensive, so you have to justify the capital costs of FC if you are going to replace it," said Heist. "FC system manufacturers see this as a relatively hard sell, so they are going for markets where either the existing technologies can't fill the need or the conventional processes can't do it in one step."

Several other drawbacks have encumbered the industrial use of FC systems. These include limited capacity, relatively high production costs, and limited maximum production concentration. FC systems that incorporate multistage operation, upgraded components, and new design technology have eliminated or diminished these drawbacks.

FC applications cover all three groups for which concentration processes are generally used. In the first group, the end product is a solvent [liquid], the degree of concentration is not high, and the concentrate can be easily disposed. Seawater desalination fits in this group. Only 50 percent of the seawater is converted into fresh water; the remainder is simply discharged back into the sea. For the past five years, an FC system has been in use in Saudi Arabia in a large solar-powered water desalination application.

Common applications in the second group are the concentration of salt, acid, and alkali solutions in the chemical industry and fruit juice and milk in the food industry. Here, the concentrate is the desired product, a relatively high concentration is required, and the solvent is of little value and can be disposed easily.

In the food industry, most food liquids contain 90 percent or more water. Existing evaporation technology can concentrate solutions such as milk or orange juice to a level of about 60 percent solid. New multistage FC systems are approaching this level; they can now reach 55 percent solid in some applications.

In the third group of concentration applications, the purified solvent is of value and the concentrate is either of value or must be disposed in a highly concentrated form. Both the chemical and petroleum industries use FC in processes that normally demand low cost and high efficiency. In the chemical industry, FC is a more efficient process requiring fewer stages of equipment than distiliation techniques for producing industrial alcohols and caustic soda (sodium hydroxide).

"The main problem with freezing in the chemical industry is that it's a new technology that people aren't familiar with and they already have processes that work to some degree," said Heist.

In the petroleum industry, FC is used along with fractional distillation to separate aromatic isomer mixtures. Direct-clathrate systems under development may further reduce energy costs and increase product yield. FC may also be used in naphtha fractionation and sour water treatment.

Several industries have applications where they create a concentrate from a process stream, which is used for resource recovery. In the pulp and paper industry, for example, the chemical separation of wood fiber in pulping operations produces a spent liquid that is then concentrated. The concentrate is burned to generate electricity and steam and to produce an ash that can be used to reconstitute fresh cooking liquor. Much of the energy used in the pulp and paper industry goes into the evaporators now used to concentrate the liquid. An FC system is now being developed for several applications for acid recovery in pulp and paper mill liquor volume reduction.

Another important application of FC is the concentration of waste liquids for disposal. Environmental restrictions on industrial waste disposal have led to expensive treatments before discharge. In a number of cases, conventional treatments are ineffective for waste streams and lagoons containing highly toxic chemicals, petrochemicals, and hazardous materials used in metal plating. In other cases, FC can purify a waste stream in one step that otherwise requires several conventional processes working in series. For example, if a hazardous waste is saturated with salts and contains volatile organic materials, it has to go into a crystallizing evaporator to crystallize the salts. Then the condensate from that has to be cleaned up because the organics come out with the water, making the water equally hazardous. This isn't the case in FC, where the water comes out in the form of pure ice and the organics stay in the concentrate with the crystallized salts.

Because it is relatively insensitive to the type of wastewater treated, FC handles a wide range of these wastewater contaminants more effectively than other separation processes. It can be used on mine drainage water management and on the concentration of dye stuff plant effluent, nuclear reactor wastes, and cooling tower blow-down streams.

Multiple Processes

For different mixtures under different operating conditions, FC offers four types of processes. The processes are identified by whether there is direct or indirect contact between the refrigerant and the mother liquid the other process components are interchangeable). There are two direct contact and two indirect contact processes.

In the indirect contact systems, a heat exchanger wall separates the mixture in the crystallizer from the refrigerant so the refrigerant never mixes with the liquid being processed.

The primary difference between the different indirect freezers that have been manufactured is how crystal deposits are kept from building up on the heat exchanger surface. In one version. mechanical scrapers remove the crystals that form on the heat exchanger surface. Subsequently, these small crystals are pumped to a recrystallizer, where they are mixed with large crystals. Because of the slightly higher temperature that favors the larger crystals, the small crystals melt and recrystallize on the surface of the larger crystals, which cause them to grow. The slurry mixture of concentrate and crystals is then separated and the crystals are cleaned.

In the second type of indirect FC process, crystals form in the liquid rather than on the heat exchanger surface. Controlled operating parameters and special surface treatment techniques modify the heat exchanger surface-in what is termed a falling-film seeded process-and allow ice crystals to grow in the bulk of the solution. Attempts to employ nonscraped surface-indirect freezers started at least as early as 1945 but didn't produce results until the 1970s.

Direct Processes

Direct FC processes employ a refrigerant as an integral part of the materials being separated. If the mixture components have similar freezing points, a liquid refrigerant is added directly to the mixture; otherwise, one component of the mixture acts as the refrigerant. For refrigerants that are secondary materials, the processes are further classified by whether the crystals formed are pure or solid solutions, called clathrates, that contain more than one material in the crystal.

Direct-primary FC processes, in which the refrigerant is a primary component of the mother liquid, operate near the triple point of a substance. At this temperature and pressure point, the liquid, gas, and solid phases of a substance all occur simultaneously.

A vacuum is formed to vaporize part of the mixture, which then acts as a refrigerant by removing heat until one or more of the components crystallize. The development of vacuum freezers that work near water's triple point has been going on since the 1950s. The concept has been difficult to put into practice, however, because of the high volume of vapor that has to be continuously removed. However, this requires very large compressors. Absorbing the vapor into a liquid and freezing the vapor on a much colder surface are two recently developed alternatives to circumvent the need for large expensive compressors.

Secondary-refrigerant FC freezers, first proposed in the 1950s, overcome many of the early problems of the triple-point freezers. These systems introduce a volatile liquid refrigerant into the mixture. The vapor pressure in the freezer is kept below that of the refrigerant, which evaporates and crystallizes the solution. Butane, propane, and various fluorocarbons are some of the refrigerants used. These materials allow the process to be conducted at a higher pressure than the vacuum conditions used in direct-primary freezers. The refrigerant is later stripped from the concentrate and recovered. In a number of applications, product purity has been a problem due to the buildup of liquid refrigerant in the ice. Because refrigerants are always soluble to some degree in the process fluid, it adds to the process complexity by requiring both the product and the concentrate to be stripped.

The clathrate direct-contact freezer is nearly identical to the secondary-refrigeration freezer, except that it allows the temperature at which the crystals form to be raised well above water's freezing point and sometimes close to ambient temperatures. It docs this by introducing a gas that fits into the ice crystal structure, in place of part of the water, to form the clathrate crystals. When the ice melts, the gas is recovered and recycled. The higher temperatures of crystallization are a benefit to process economics. Historically the barrier to developing this process has been crystal growth, which in part is related to the choice of clathrate refrigerants.

Wash Columns

After the crystals are formed in the crystallizer, the crystal/liquid slurry is separated into concentrated liquid and other crystalline components and impurities are washed from the crystal surface, producing pure crystals. To perform the separation, a wash column is used. There are two types of wash column: pressurized and gravity.

In the pressurized wash column, the crystals rise to the top and hydraulic pressure forces a wash liquid, derived from the melted pure crystals, to flow down. The applied pressure also squeezes the concentrate through a filter at the bottom of the column. As the wash liquid flows down the column it removes impurities from the surface of the crystals. At the interface between washed and unwashed crystals, called the wash front, the wash liquid comes in contact with colder crystals and crystallizes on them. In this way, the wash liquid does if mixed with the concentrated liquid. The separation in the wash column is nearly perfect; impurities in the melted crystals can be counted in parts per million and sometimes parts per billion.

The gravity wash column is simpler in design, but larger than the pressurized wash column. Its greater height creates the pressure needed to compact the ice bed. It works in much the same way as the pressurized column but at lower pressures. An ice pack is still formed and moved hydraulically up the column. The washed crystals are primarily driven upward by the pressure developed in the pump that feeds the column with slurry. The ice is purified by washing it with a small percentage of the melted ice.

The performance of wash columns depends on the crystal size and shape and on the viscosity of the mother liquid. Uniformity of crystal size and shape is important to avoid having the wash water seek the path of least resistance and channel through the crystals unevenly. This would result in the mother liquid and contaminants remaining attached to islands of crystals.

One of the major drawbacks of FC compared to evaporation has been the maximum obtainable product concentration. The limiting factory specially in food applications-is the product viscosity, which increases with concentration. Multistage freezers get around this problem. They form crystals in the higher-concentrated stage, so that crystal washing removes the less concentrated solution rather than the more-concentrated solution.

Multistaging increases the capacity and reduces the energy consumption of FC systems by separating the production stages from the separation stage. This increases the separation efficiency of the wash column. The energy consumption is reduced due to the fact that the ice production is now divided over the system so that more ice is made at higher freezing temperatures and lower viscosities, which improves the energy efficiency.

Multistage operation can be designed for up to six stages. Each stage consists of a crystallization and ripening section. In each individual section, ice is produced in a crystallizer and growth takes place in the ripening tank.

Because of multistage operation and improved component design, capacities have increased. At the higher capacities, up to 45,000 pounds of water an hour, the energy requirement per ton of water is reduced. Compared with single-stage operation, the energy requirements are 50 to 70 percent lower.

R&D Needed

FC is not yet appropriate for all separation applications. While the process works well for low and moderately viscous liquids such as food, there are difficulties handling highly viscous substances such as petroleum at low temperatures. While the competitive technologies of evaporation and distillation are mature, FC has a great deal of potential and room for improvement in equipment and design methods.

As yet there is no off-the-shelf equipment for new applications. FC equipment is custom built for each new application, which requires expensive and extensive laboratory testing. "There is so little experience with how freezing works with different types of wastes and when purifying organic solutions," said Heist. "What we don't know is how to design FC for specific applications. We have to learn what to put into a process so there's not a lot of redundancy, so we have equipment that is optimally sized."

Continued areas of development for FC systems include improved crystal growth, more efficient refrigeration, better design methods, and better heat recovery. FC must also demonstrate that it can match the level of concentration available with other technologies.

Improvement in all of these areas will contribute to less-expensive process equipment and will increase the market for FC.
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Title Annotation:FC may save energy compared to conventional freezing processes
Author:Rosen, Jerome
Publication:Mechanical Engineering-CIME
Date:Dec 1, 1990
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