Normal solubility salts

Fig. 5 Cooling a normal solubility salt solution. (From RefPf)...

Two forms of solubility are possible. Some salts have greater solubilities as the temperature is raised these s ts are called normal solubility salts and examples include NaCl and NaNOy Other salts usually termed inverse solubility salts have lower solubilities as the temperature is rmsed. Examples of inverse solubility salts include CaCO and CaSO. Table 8.4 gives a more extensive list of inverse solubility salts. The relevance of these salts to the components present in river water listed in Table 8.1 is clear. The use of these waters for cooling purposes is very likely therefore, to cause fouling or scaling problems due to the presence of the inverse solubility salts. 

These different characteristics of temperature and solubility can have a pronounced effect on scaling depending on whether or not the saturated solution is being heated or cooled. Inverse solubility salts are likely to form a deposit if the saturated solution is being heated the reverse is true for normal solubility salts. Problems are likely only to be encountered for sparingly soluble salts where relatively small changes of temperature have a significant effect on solubility. 

As already described it is posible for supersaturation to occur if a normal solubility salt is cooled or an inverse solubility salt is heated. Apart from temperature there are other conditions that could lead to supersaturation and hence crystallisation, and may include ... 

The process of crystallisation from waxy hydrocarbons may be regarded as similar in some respects to precipitation from a normal solubility salt. The requirement for supersaturation to exist before crystallisation can begin, is less certain than for inorganic solutions, but it is likely to be related to the concentration of the material in solution. 

A flash evaporator system having no heating surfaces has been developed for separating salts with normal solubility from salts having inverse solubility. Steam is injected directly into the feed slurry to dissolve the normal-solubility salt by increasing the temperature and dilution of the slurry. The other salt remains in suspension and is separated. The hot dilute solution is then flashed to a lower temperature where the normal-solubility salt crystallizes and is separated. The brine stream is then mixed with more mixed salts and recycled through the system. This system can be operated as a multiple effect by flashing down to... 

Precipitation Fouling the crystallization from solution of dissolved substances onto the heat transfer surface, sometimes called scaling. Normal solubility salts precipitate on subcooled surfaces, while the more troublesome inverse solubility salts precipitate on superheated surfaces. 

The former case is usually associated with the use of evaporators for crystallization objectives. Crystal growth on the heat transfer surface competes with the process of crystal growth on the greater deposition area of the suspended crystals. Supersaturation with respect to the heat transfer surface which has e higher temperature than the bulk solution is lower for normal solubility salts and higher for inverse solubility salts. 

This system is the only osmotic system developed commercially at this time that is suitable for the oral administration of insoluble drugs to humans. It has been utilized in the development of several other drugs including isradipine, doxazosin, diltiazem, contraceptive steroids, glipizide, and verapamil [48-53], The system has also been utilized to codeliver the free bases of compounds normally administered as water-soluble salts such as pseudoephedrine and bromo-pheniramine [54], The latter system includes both a loading dose and a controlled release dose and is intended for applications in the over-the-counter market. 

Salts containing the following ions are normally soluble ... 

Reference Electrodes By definition, the normal hydrogen electrode (N H E) is the reference for electrode potentials (see Sect. 2.3.2.1), but practically it is scarcely usable. A reference electrode (RE) has to provide a well-defined potential between the electrolyte and its electric connector, joined with the input of the measuring instrument. Usually, a metal and a slightly soluble salt of this metal is applied (secondary electrode) [76, 77]. The electrolyte in the RE is connected to the electrolyte in the electrochemical cell via a diaphragm, which has to separate both electrolytes, as far as possible without a potential difference (see below). 

A central concept necessary to understanding the mechanisms of CD is that of the solubility product (Ksp). The solubility product gives the solubility of a sparingly soluble ionic salt (this includes salts normally termed insoluble ). Consider a very sparingly soluble salt (say, CdS) in equilibrium with its saturated aqueous solution ... 

Aspirin (acetylsalicylic acid, Figure 7.9) is a derivative of salicyclic acid, which was first used in 1875 as an antipyretic and antirheumatic. The usual dose for mild pain is 300-600 mg orally. In the treatment of rheumatic diseases, larger doses, 5-8 g daily, are often required. Aspirin is rapidly hydrolysed in the plasma, liver and eiythrocytes to salicylate, which is responsible for some, but not all, of the analgesic activity. Both aspirin and salicylate are excreted in the urine. Excretion is facilitated by alkalinisation of the urine. Metabolism is normally very rapid, but the liver enzymes responsible for metabolism are easily saturated and after multiple doses the terminal half-life may increase from the normal 2-3 h to 10 h. A soluble salt, lysine acetylsalicylic acid, with similar pharmacological properties to aspirin, has been used by parenteral administration for postoperative pain. Aspirin in low doses (80-160 mg daily) is widely used in patients with cardiovascular disease to reduce the incidence of myocardial infarction and strokes. The prophylaxis against thromboembolic disease by low-dose aspirin is due to inhibition of COX-1-generated thromboxane A2 production. Because platelets do not form new enzymes, and COX-1 is irreversibly inhibited by aspirin, inhibition of platelet function lasts for the lifetime of a platelet (8-10 days). 

The basic items normally considered in a material balance of a cooling tower are the range, T, circulating water rate, C (in U.S. gpm), the ppm (parts per million of any soluble salts in C (Xc), and M, B, E and W. Xc is normally expressed in terms of equivalent chlorides,... 

---