Evaporation scale

Fig. 1. Lacquer-coated optical readout laser disk master. Plating by (a) electroless silver spray coating and by (b) vacuum evaporation. Scale bar, cm.

The first of these, utilized by Yoder, McCalip and Seibert,34 and by Balch, Broeg and Ambler,37 provides for the extraction of the aconitic acid from the sample being investigated, usually with diethyl ether, and the subsequent isolation of the acid from the solvent. In dealing with solid samples, e.g. alkaline earth aconitates, evaporator scale, etc., the prescribed procedure is to dissolve the material in aqueous mineral acid and to extract the acid solution exhaustively with ether. The ether extract is then evaporated under reduced pressure, the dried residue titrated with standard alkali and the titratable acid calculated as aconitic acid. In dealing with such solid samples it is often necessary to make an additional determination for oxalic acid which otherwise would be assumed to be aconitic acid.37 The aconitic acid in liquid samples is usually precipitated as the insoluble lead salt which is separated and treated as any other solid sample. In some cases this procedure is unnecessary and the liquid samples are merely acidified with a mineral acid and then extracted with ether.37 This method for the determination of aconitic acid, however, requires a considerable amount of time and is further complicated by the interference of ether-soluble waxes and non-volatile acids. 

Walthew, D. C., Aspects of Evaporator Scale Formation and Control in the South African Sugar Industry, Proc. Sugar Processing Research Conf, 22 43, 1996. 

Godshall, M. A., and Whartell, L., Composition of Evaporator Scale in Louisiana Mills, Proc. Sugar Processing Research Conf, 379-385, 2002. 

The Use of Magnesium Oxide to Prevent Evaporator Scaling in Sugar Factories. Premier Chemicals, LLC. King of Prussia, PA. 

Water-soluble scales and calcium carbonate account for many evaporator scaling problems. Calcium scales have a much greater impact on evaporator capacity and pose the more serious problem. The rate of calcium scaling is very dependent on temperature and is also directly proportional to solids content. Calcium scales can be controlled by minimizing calcium inputs and, to a great extent, through operation techniques. Special treatments are also sometimes effective. 

Multiple-Effect Evaporation Scaling can be increased because of maldistribution and concentration gradients. Acid treatment can result in corrosion. Additional pumps are needed. 

Figure 15.1a shows a single-stage evaporator represented on both actual and shifted temperature scales. Note that in shifted temperature scale, the evaporation and condensjftion duties are shown at different temperatures even though they are at the same actual temperature. Figure 15.16 shows a similar plot for a three-stage evaporator. 

Bikerman [179] has argued that the Kelvin equation should not apply to crystals, that is, in terms of increased vapor pressure or solubility of small crystals. The reasoning is that perfect crystals of whatever size will consist of plane facets whose radius of curvature is therefore infinite. On a molecular scale, it is argued that local condensation-evaporation equilibrium on a crystal plane should not be affected by the extent of the plane, that is, the crystal size, since molecular forces are short range. This conclusion is contrary to that in Section VII-2C. Discuss the situation. The derivation of the Kelvin equation in Ref. 180 is helpful. 

A small quantity of mandelonitrild may be obtained by extracting the aqueous layer with 25 ml. of benzene, evaporating the benzene, and adding the residue to the main portion. This extraction is hardly worth while except for large scale preparations. 

Historically, soda ash was produced by extracting the ashes of certain plants, such as Spanish barilla, and evaporating the resultant Hquor. The first large scale, commercial synthetic plant employed the LeBlanc (Nicolas LeBlanc (1742—1806)) process (5). In this process, salt (NaCl) reacts with sulfuric acid to produce sodium sulfate and hydrochloric acid. The sodium sulfate is then roasted with limestone and coal and the resulting sodium carbonate—calcium sulfide mixture (black ash) is leached with water to extract the sodium carbonate. The LeBlanc process was last used in 1916—1917 it was expensive and caused significant pollution. 

Consider short-path evaporator (SPE) (internal condenser) when pressure must be in 0.013—0.00013 kPa (0.1—0.001 torr) range for larger-scale production, 0.013—0.0007 kPa (0.01 to 0.005 torr) is practical lower pressure limit. 

Raw juice is heated, treated sequentially with lime (CaO) and carbon dioxide, and filtered. This accomplishes three objectives (/) microbial activity is terminated (2) the thin juice produced is clear and only lightly colored and (J) the juice is chemically stabilized so that subsequent processing steps of evaporation and crystalliza tion do not result in uncontrolled hydrolysis of sucrose, scaling of heating surfaces, or coprecipitation of material other than sucrose. 

A number of options for controlling scale formation are used in plant operations around the world. One approach is use of mechanical means, including thermal shock. Although rare today (ca 1997), this practice can be found ia use with the few obsolete submerged tube evaporators. 

The clay-cataly2ed iatermolecular condensation of oleic and/or linoleic acid mixtures on a commercial scale produces approximately a 60 40 mixture of dimer acids and higher polycarboxyUc acids) and monomer acids (C g isomerized fatty acids). The polycarboxyUc acid and monomer fractions are usually separated by wiped-film evaporation. The monomer fraction, after hydrogenation, can be fed to a solvent separative process that produces commercial isostearic acid, a complex mixture of saturated fatty acids that is Hquid at 10°C. Dimer acids can be further separated, also by wiped-film evaporation, iato distilled dimer acids and trimer acids. A review of dimerization gives a comprehensive discussion of the subject (10). 

---