Section 4.3 Freeze Concentration

Cost the individual components, wash column from Section 10.1 refrigeration unit. Section 3.12, pumps and compressors. Sections 2.1 and 2.3 and heat exchangers. Section 3.3. 

For a complete process to concentrate fruit juices, or coffee, the TM cost = 7000000 for a processing capacity = 1 kg/s with n = 0.48 for the range 0.03-7 kg/s. 

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Freeze concentration is a very common process it is the main subject of Section 16.2. 

Consider a solution of 8.5 g NaCl and 1.7 g glucose per liter. What would be (a) the freezing point of the solution and (b) the ionic strength of the freeze-concentrated solution at —12°C You may assume that the glucose does not crystallize. Consult also Section 2.3. 

Figure Bl.24.16. An example of the application of the PIXE technique using the NMP in the imaging mode. The figures show images of the cross section through a root of the Phaseolus vulgaris L. plant. In this case the material was sectioned, freeze-dried and mounted in vacuum for analysis. The scales on the right of the figures indicate the concentrations of the elements in ppm by weight. It is clear that the transports of the elements through the root are very different, not only in the cases of the major elements Ca and K, but also in the case of the trace element Zn.

Processes available for the extraction of natural organics from waters include vacuum distillation, freeze-drying, freeze concentration, co-precipitation, ultrafiltration, reverse osmosis (RO), solvent extraction, sorption, anion exchange, and non-ionic macroporous sorbents (Aiken (1985)). Many of these methods include chemical treatment that may alter the HS characteristics. In this section, the most common methods for the processing of large samples will be described. An important issue in NOM extraction is the recovety of organics. Minimising the loss of certain fractions, such as small compounds which are difficult to treat, is also important. 

Make salt solutions with a range of concentrations e.g. 0, 5, 10, 15, 20%) by weighing out appropriate amounts of salt and water (for example a 5% solution can be made from 1 g of salt and 19 g of water). Gentle heating will help to dissolve the salt in the most concentrated solutions. Fill a couple of holes in the ice cube tray with each solution and place it in the freezer. Look at the tray from time to time (approximately every 15 min) and note when ice begins to form in each section. Measure the temperature if you have a suitable thermometer. To get an accurate value for the freezing point you need to measure the temperature soon after ice has started to form. If you measure it before ice has formed, the solution will be supercooled (see Experiment 5 below), and if you measure it when a substantial amount of ice has already formed the freezing point will be further depressed by freeze-concentration. 

High heat transfer coefficients, up to 1 kW m K, and hence high production rates, are obtainable with double-pipe, scraped-surface heat exchangers. Although mainly employed in the crystallization of fats, waxes and other organic melts (section 8.2.2) and in freeze concentration processes (section 8.4.7), scraped-surface chillers have occasionally been employed for crystallization from solution. Because of the high turbulence and surface scraping action, however, the size of crystal produced is extremely small. 

A definite advantage of freeze crystallization, important in many food industry applications, is that volatile flavour components that are normally lost during conventional evaporation can be retained in a freeze-concentrated product. In fact, at present, freeze crystallization finds its main application in the food industry, for the concentration of fruit juices, etc. Indirect-contact freezing processes are normally used, e.g. the liquid feedstock is crystallized in a scraped-surface heat exchanger (section 8.2.2) and the resulting ice slurry passes to a wash column where the crystals are separated and washed to recover valuable product. The wash column is the key item in the process. Figure 8.56 shows an example of the Grenco system of freeze crystallization. 

For shdl and tube heat exchange Numerous related topics including evaporation Section 4.1, distillation. Section 4.2, crystallization Section 4.6, freeze concentration Section 4.3, melt crystallization. Section 4.4, PFTR reactors Sections 6.5-6.12. Approach temperature 5 to 8°C use 0.4 THTU/pass design so that the total pressure drop on the liquid side is about 70 kPa. Allow 4 velocity heads pressure drop for each pass in a multipass system. Put inside the tubes the more corrosive, higher pressure, dirtier, hotter and more viscous fluids. Recommended liquid velocities 1 to 1.5 m/s with maximum velocity increasing as more exotic alloys used. Use triangular pitch for all fixed tube sheet and for steam condensing on the shell side. Try U = 0.5 kW/m °C for water/liquid U = 0.3 kW/m °C for hydrocarbon/hydrocarbon U = 0.03 kW/m °C for gas/ liquid and 0.03 kW/m °C for gas/gas. 

In this chapter we consider the separation of species contained in a homogeneous phase, such as a liquid or gas. The separation is based on exploiting a fundamental difference that exists between the species. Section 4.0 gives some overall guidelines. Methods that exploit differences in vapor pressures are evaporation, in Section 4.1 and distillation, in Section 4.2. Methods that exploit differences in freezing temperature and solubility are freeze concentration. Section 4.3, melt crystallization. Section 4.4 and zone refining. Section 4.5. Methods exploiting solubility are solution crystallization. Section 4.6 precipitation. Section 4.7 absorption, Section 4.8, and desorption. Section 4.9. Solvent extraction. Section 4.10, exploits differences in partition coefficient. 

Add 25 g. of finely-powdered, dry acetanilide to 25 ml. of glacial acetic acid contained in a 500 ml. beaker introduce into the well-stirred mixture 92 g. (50 ml.) of concentrated sulphuric acid. The mixture becomes warm and a clear solution results. Surround the beaker with a freezing mixture of ice and salt, and stir the solution mechanically. Support a separatory funnel, containing a cold mixture of 15 -5 g. (11 ml.) of concentrated nitric acid and 12 -5 g. (7 ml.) of concentrated sulphuric acid, over the beaker. When the temperature of the solution falls to 0-2°, run in the acid mixture gradually while the temperature is maintained below 10°. After all the mixed acid has been added, remove the beaker from the freezing mixture, and allow it to stand at room temperature for 1 hour. Pour the reaction mixture on to 250 g. of crushed ice (or into 500 ml. of cold water), whereby the crude nitroacetanilide is at once precipitated. Allow to stand for 15 minutes, filter with suction on a Buchner funnel, wash it thoroughly with cold water until free from acids (test the wash water), and drain well. Recrystallise the pale yellow product from alcohol or methylated spirit (see Section IV,12 for experimental details), filter at the pump, wash with a httle cold alcohol, and dry in the air upon filter paper. [The yellow o-nitroacetanihde remains in the filtrate.] The yield of p-nitroacetanihde, a colourless crystalline sohd of m.p. 214°, is 20 g. 

Add 101 g. (55 ml.) of concentrated sulphuric acid cautiously to 75 ml. of water contained in a 1 htre beaker, and introduce 35 g. of finely-powdered wi-nitroaniline (Section IV,44). Add 100-150 g. of finely-crushed ice and stir until the m-nitroaniUne has been converted into the sulphate and a homogeneous paste results. Cool to 0-5° by immersion of the beaker in a freezing mixture, stir mechanically, and add a cold solution of 18 g. of sodium nitrite in 40 ml. of water over a period of 10 minutes until a permanent colour is immediately given to potassium iodide - starch paper do not allow the temperature to rise above 5-7° during the diazotisation. Continue the stirring for 5-10 minutes and allow to stand for 5 minutes some m-nitrophenjddiazonium sulphate may separate. Decant the supernatant Uquid from the solid as far as possible. 

The properties of a solution differ considerably from those of the pure solvent Those solution properties that depend primarily on the concentration of solute particles rather than their nature are called colligative properties. Such properties include vapor pressure lowering, osmotic pressure, boiling point elevation, and freezing point depression. This section considers the relations between colligative properties and solute concentration, with nonelectrolytes that exist in solution as molecules. 

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