Recycling with solid-liquid-separation

Wastewater contains solid particles with a wide variety of shapes, sizes, densities, etc. Specific properties of these particles affect their behavior in liquid phases, and thus their removal capabihties. Many chemical and microbiological contaminants found in wastewater are adsorbed by, or incorporated into, these solid particles. Thus, essential for purification and recycling of both wastewater and industrial effluents, is the removal of these solid particles. Flocculation is a technique where polymers are involved in a solid-liquid separation by an aggregation process of colloidal particles [56]. Both synthetic and natural polymers have been utihzed for flocculation. 

In Figure 1 a simplified process scheme of the antisolvent crystallization of sodium chloride is displayed. The process is divided into three steps the crystallization, the solid-liquid separation and the antisolvent recovery or liquid-liquid separation. In the first step sodium chloride is crystallized by mixing the feed brine with an antisolvent. The crystallization is carried out at temperatures below the liquid-liquid equilibrium line in the single liquid phase area (see Figure 2). In the second step the crystals are separated from their mother liquor, e.g. by filtration or in a centrifuge. In the third and final step the antisolvent is separated from the water phase at a temperature above the liquid-liquid equilibrium line in the two liquid phase area, in which the ternary amine-water-salt system splits up into an amine and an aqueous phase. The recovered antisolvent is recycled within the process and most ideally the water phase is reused for the dissolution of crude sodium chloride. In this paper the crystallization and the liquid-liquid separation steps will be treated. 

Zeitsch, K., Problems with fine particle recycling , in Chapter 16, Solid—Liquid Separation, 3rd edn, L. Svarovsky (Ed.), Butterworths, London, 541 558 (1990)... 

The liquid left after distillation is subjected to centrifugation, where most of the suspended solids are separated. The clear liquid can be recycled by adding it back to the starch conversion stage. The moist cake released by the centrifuges is mixed with the syrup produced by the evaporator to form a homogenous mixture and is dewatered in dryers. The dryers produce a Distillers Dried Grains with Solubles (DDGS) meal, which can be formed into pellets. These are used in many applications, most prominently in animal feed. 

The palm oil exiting the low-pressure screw press has approximately 66% oil, 24% moisture, and 10% solids (18). The solid particles are typically separated from the oil using the traditional method of pumping the oil into a tank with approximately 2 hours of residence time to allow the heavier solid particles to settle and be continuously dredged from the base of the tank. After gravity separation, the oil is then pumped through a liquid cyclone to remove residual solids. Solid particles separated from the clean oil stream are saturated with oil and recycled back into the process. After the palm oil is cleaned of solid particles, it is heated and pumped through a vacuum oil-dryer to remove moisture. 

The flow sheet of the technical process is shown in Figure 13.4. For a comprehensive description of critical steps of this technology, see Refs. 14 and 15 the main points are control of the solid-liquid reaction between the glucose and the fatty alcohol with acidic catalysis avoiding a fast polymerization of glucose, the good water removal, the economic separation and recycle of the excess fatty alcohol, and the treatment of the highly viscous APG melt to get the desired product quality with low polydextrose content. 

---