8.7. EVAPORATION OF BIOLOGICAL MATERIALS

8.7A. Introduction and Properties of Biological Materials

The evaporation of many biological materials frequently differs from the evaporation of inorganic materials such as NaCl and NaOH and organic materials such as ethanol and acetic acid. Biological materials such as pharmaceuticals, milk, citrus juices, and vegetable extracts are usually quite heat-sensitive and often contain fine particles of suspended matter in solution. In addition, because of problems due to bacteria growth, the equipment must be designed for easy cleaning. Many biological materials in solution exhibit only a small boiling-point rise when concentrated. This is because suspended solids in a fine, dispersed form and dissolved solutes of large molecular weight contribute little to this rise.

The amount of degradation of biological materials on evaporation is a function of the temperature and length of time. To keep the temperature low, the evaporation must be done under vacuum, which reduces the boiling point of the solution. To keep the time of contact low, the equipment must provide for a low holdup time (contact time) for the material being evaporated. Typical types of equipment used and some biological materials processed are listed below. Detailed descriptions of the equipment are given in Section 8.2.

8.7B. Fruit Juices

In the evaporation of fruit juices, such as orange juice, problems arise that are quite different from those associated with the evaporation of a typical salt such as NaCl. The fruit juices are heat-sensitive and the viscosity increases greatly as concentration increases. Also, solid suspended matter in fruit juices has a tendency to cling to the heating surface, thus causing overheating which leads to burning and spoilage of the matter (B2).

To reduce this tendency to stick and to reduce residence time, high rates of circulation over the heat-transfer surface are necessary. Since the material is heat-sensitive, low-temperature operation is also necessary. Hence, a fruit juice concentration plant usually employs a single and not a multiple evaporation unit. Vacuum is used to reduce the temperature of evaporation.

A typical fruit juice evaporation system using the heat-pump cycle is shown in the literature (P1, C1); it employs low-temperature ammonia as the heating fluid. A frozen concentrated citrus juice process is described by Charm (C1). The process uses a multistage falling-film evaporator. A major fault of concentrated orange juice is a flat flavor due to the loss of volatile constituents during evaporation. To overcome this, a portion of the fresh pulpy juice bypasses the evaporation cycle and is blended with the evaporated concentrate.

8.7C. Sugar Solutions

Sugar (sucrose) is obtained primarily from sugarcane and sugar beets. Sugar tends to caramelize if kept at high temperatures for long periods (B2). The general tendency is to use short-tube evaporators of the natural circulation type. In the evaporation process for sugar solutions, a clear solution of sugar having a concentration of 10-13° Brix (10-13 wt %) is evaporated to 40-60° Brix (K1, S1).

The feed is first preheated by exhaust steam and then typically enters a six-effect forward-feed evaporator system. The first effect operates at a pressure in the vapor space of the evaporator of about 207 kPa (30 psia) [121.1°C (250°F) saturation temperature] and the last effect under vacuum at about 24 kPa (63.9°C saturation). Examples of the relatively small boiling-point rise of sugar solutions and the heat capacity are given in Example 8.5-1.

8.7D. Paper-Pulp Waste Liquors

In the manufacture of paper pulp in the sulfate process, wood chips are digested or cooked and spent black liquor is obtained after washing the pulp. This solution contains primarily sodium carbonate and organic sulfide compounds. It is concentrated by evaporation in a six-effect system (K1, S1).