Introduction

Melt crystallization is a standard chemical industries purification process for chemical products such as naphthalene, para-xylene, dichlorobenzene, acrylic acid, monochloroacetic acid, bisphenol A, and others [1]. It is seen as an extremely selective separation process that can reduce contaminant levels to the ppm range. This provides an effective means to purify some products. Freeze concentration (FC) is usually associated with the process when the crystallizing component is water. In this chapter we focus on the crystallization of water and limit our discussion to FC of aqueous solutions. The purpose of FC is to provide a means to separate pure water from a solution. This will provide either high quality water and reduced volumes of waste or, in the case of food liquids, a high quality concentrate with only water removed from the original feed product.

NIRO (formerly Grenco) process technology has developed an FC system used commercially in the food processing industry (the NFC process). The NFC process is described, and the results from pilot plant tests on a caustic wastewater are presented along with economics for a typical treatment case.

Freeze concentration has been applied in the food industry [2,3] for a variety of products including fruit and citrus juices, coffee and tea extracts, beer and wine, vinegar, and dairy products. It features many product quality advantages over the more conventional concentration processes of evaporation and membrane filtration.

Freeze concentration can produce concentrates up to 45-50% total solids (TS), not as high a concentration as evaporators but significantly higher than that achieved with membranes. The process operates at freezing temperatures. This has obvious benefits for the quality of food products but also reduces corrosion, which can occur at the elevated temperatures used in evaporation. The crystallization method is extremely selective in that only water is included in the crystal. Coupled with an efficient separation unit, FC can remove pure water from the product, eliminating the need for further processing. Evaporation can be limited by the relative volatility

Table 1 Characteristics of the Niro Freeze Concentration Process

Crystallization of water using ripening process Pure spherical ice crystals. No solids incorporated in the crystal itself. Ice crystal separation using a wash column Effective separation; losses usually in the ppm or ppb range. No recycle of washwater.

Efficient separation, reliable operation through mechanically forced bed transport. Low processing temperatures

Always below the freezing point of water. Operation with heat-sensitive materials. Reduction of corrosion. Closed, pressurized, flooded system

No gas-liquid interface; reduces oxidation problems and eliminates loss of volatile materials. Self-stabilizing process Simple control system.

Relatively insensitive to changes in feed characteristics. Low-speed rotating equipment

Low maintenance costs. Modular design

Flexibility—multistage systems can be split for dual product operation (or online maintenance). Easily expandable as capacity demands increase. Clean energy source

Electrically powered, which can provide for a positive environmental impact due to controlled centralized production.

Local utility companies may provide economic incentives for installation of electric equipment.

of the constituents, some of which may be carried over with the water vapor and require another treatment/recovery step.

The advantages for hazardous waste disposal lie in the purity of the discharged water. The NFC process uses separate nucleation and growth zones, which produce pure spherical ice crystals that can be easily separated and discharged as pure water. This water can be discharged directly or reused in the process, reducing water requirements. The system operates continuously and does not require shutdown for periodic cleaning of the components. It is a closed system, which eliminates vapor losses and oxidation through contact with air. It is a relatively stable operation with a simple control process. It provides for a 100% turndown ratio from design capacity, is relatively insensitive to fluctuations in feed rate and composition, and is modular in design, improving expansion capabilities. These process characteristics are summarized in Table 1.

The FC process is dependent on two main characteristics of the concentrated product. The viscosity of the concentrate at its freezing point determines the maximum concentration obtainable by FC. Figure 1 shows the viscosity curves for caustic wastewater, beer, and milk for comparison. The maximum concentration is reached when the viscosity of the liquid prevents the growth of ice crystals. The separation then becomes too difficult to maintain the purity of the discharged water. From the standpoint of economics, the maximum concentration may be reached long before the product reaches this viscosity.

The crystal growth rate decreases as viscosity increases, and the system requires longer residence times (and thus larger equipment) to reach a separable crystal size. The freezing point curves for the same products as in Figure 1 are shown in Figure 2. The freezing point depression due to the solute concentration can be so great that the lower temperature limit for the

90 80

5 10 15 20 25 30 35 40 Concentration

Figure 1 The relationship of viscosity to concentration for a caustic wastewater solution (% total solids), skim milk (% total solids), and beer ("Plato).

Viscosity 7Q (mm2/s)

5 10 15 20 25 30 35 40 Concentration

Figure 1 The relationship of viscosity to concentration for a caustic wastewater solution (% total solids), skim milk (% total solids), and beer ("Plato).

refrigerant may be reached. This can be overcome by using specialty refrigerants and multistage refrigeration systems, but these are usually too expensive to be feasible. Typically the NFC system can operate with up to 40-50% total dissolved solids with a product freezing point of around -10 to - 15°C and viscosities of 150-500 cSt.

Evaporation is a well-developed unit operation and can be used in most instances requiring concentration. Even though water requires more energy to evaporate than to crystallize (2325 kJ/kg heat of vaporization compared to 334 kJ/kg heat of fusion for pure water), commercial evaporators with energy recovery systems use approximately the same amount of energy as FC systems. The evaporator condensate from a wastewater treatment system is usually not suitable for direct discharge because of carryover of volatile organics. The condensate stream then requires further treatment before discharge or reuse. Many wastewaters contain chemicals that are quite corrosive at the elevated temperatures in the evaporators and therefore require expensive materials for equipment construction [4] or limit its lifetime. Precipitates usually build up on the heat transfer surface, leading to additional cleaning cycles, lost production time, and production of an additional waste stream that must be treated.

5 10 15 20 25 30 35 40 Concentration

Figure 2 The relationship between equilibrium temperature and concentration for a caustic wastewater solution (%TS), skim milk (%TS), and beer ("Plato).

Equilibrium Temperature _6

5 10 15 20 25 30 35 40 Concentration

Figure 2 The relationship between equilibrium temperature and concentration for a caustic wastewater solution (%TS), skim milk (%TS), and beer ("Plato).

The treatment of wastewater streams with membranes depends greatly on the type on contaminant found in the wastewater. Membrane systems are effectively used in the treatment of oily wastewaters [5]. The costs are reasonable, and their operation is routine. As with all membrane systems, pretreatment is necessary to prevent damage and reduce fouling of the membrane. Low molecular weight compounds found in many reactor effluents require tighter membranes. Membrane use in these situations is generally more expensive due to the larger surface area necessary and requires additional treatment steps before final discharge of the water. Regular cleaning cycles add to the cost of operation due to lost production time and additional treatment systems for the cleaning water supply.

0 0

Post a comment