Purity requirements

Starch for use in papermaking has to meet specific purity requirements in residual oil, protein, bran and ash content. Industrial starches have a protein content (N X 6.25), ranging from about 0.05% for potato starch to 0.3-0.6% for com starch, depending on separation efficiency during production. Excess protein content will induce foaming in dispersions of starch and affect the quality and strength of the coated surface. Starch for use in the paper industry should not contain more than 0.4% protein. Oxidized starches tend to have the lowest protein content. Residual oil will cause retrogradation due to complex formation with amylose. 

Starches often contain a substantial amount of salt (sodium chloride or sodium sulfate) as a residue from the modification process. Small quantities of salt in a hot starch dispersion will attack low-grade stainless steel and can cause severe corrosion of tanks, pipelines and coating application equipment. The salt content should not exceed 0.2% or 2000 ppm chloride. 

Purity criteria of colours for use in foodstuff are regulated in the EU under Directive 95/45/EC [4], According to the legislation food dyes that are placed on the market, must meet special purity requirements An example for C.I. Food Yellow 4 (tartrazine) is shown here  

Organic compounds other than coloring matter total not more than 0.5 % 

In the U SA the regulations for the purity of food dyes are specified in the Code of Federal Regulations 21CFR Part 74 and Part 82. The Food and Drug Administration (FDA) is responsible for the registration and certification of food dyes. 

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The value of many chemical products, from pesticides to pharmaceuticals to high performance polymers, is based on unique properties of a particular isomer from which the product is ultimately derived. Eor example, trisubstituted aromatics may have as many as 10 possible geometric isomers whose ratio ia the mixture is determined by equiHbrium. Often the purity requirement for the desired product iacludes an upper limit on the content of one or more of the other isomers. This separation problem is a compHcated one, but one ia which adsorptive separation processes offer the greatest chances for success. 

Purified terephthahc acid became commercially available from Amoco Chemical Co. in 1965, by which time a considerable polyester industry based on dimethyl terephthalate had already developed. The Amoco process involves purification of cmde terephthahc acid by a separate step to attain the high product purity required for polyester manufacture. The Amoco technology is the most-used worldwide, but other processes have been developed and are operating commercially. 

Equally strict purity requirements apply to dimethyl terephthalate, as shown in Table 21. Freezing point is a sensitive measure of purity and can be ... 

Sepa.ra.tion of Plutonium. The principal problem in the purification of metallic plutonium is the separation of a small amount of plutonium (ca 200—900 ppm) from large amounts of uranium, which contain intensely radioactive fission products. The plutonium yield or recovery must be high and the plutonium relatively pure with respect to fission products and light elements, such as lithium, beryUium, or boron. The purity required depends on the intended use for the plutonium. The high yield requirement is imposed by the price or value of the metal and by industrial health considerations, which require extremely low effluent concentrations. 

The choice of catalyst is based primarily on economic effects and product purity requirements. More recentiy, the handling of waste associated with the choice of catalyst has become an important factor in the economic evaluation. Catalysts that produce less waste and more easily handled waste by-products are strongly preferred by alkylphenol producers. Some commonly used catalysts are sulfuric acid, boron trifluoride, aluminum phenoxide, methanesulfonic acid, toluene—xylene sulfonic acid, cationic-exchange resin, acidic clays, and modified zeoHtes. 

Pressure Swing Adsorption. Carbon dioxide can be removed by pressure adsorption on molecular sieves. However, the molecular sieves are not selective to CO2, and the gases must be further processed to achieve the high purity required for "over the fence" use as in the urea process. Use of pressure swing adsorption for CO2 removal appears most appHcable to small, stand-alone plants (29). 

The high purity required of sificon and the small size of semiconductor devices place stringent limits on the chemicals used in processing sificon. By far the most important chemical is water which is used extensively to dilute etchants and clean wafers. Pure water (24) is required to have fewer than 0.025... 

Whereas Hquid separation method selection is clearly biased toward simple distillation, no such dominant method exists for gas separation. Several methods can often compete favorably. Moreover, the appropriateness of a given method depends to a large extent on specific process requirements, such as the quantity and extent of the desired separation. The situation contrasts markedly with Hquid mixtures in which the appHcabiHty of the predominant distiHation-based separation methods is relatively insensitive to scale or purity requirements. The lack of convenient problem representation techniques is another complication. Many of the gas—vapor separation methods ate kinetically controUed and do not lend themselves to graphical-phase equiHbrium representations. In addition, many of these methods require the use of some type of mass separation agent and performance varies widely depending on the particular MSA chosen. 

Feedwater The feedwater for a steam cycle must be purified. The degree of purity depends on the pressure of the boiler. Higher pressure boilers require higher feedwater purity. There is some trade-off between feedwater purity and boiler blowdown rate. However, increasing blowdown rate to compensate for lower feedwater purity is expensive, because blowdown water has been heated to the saturation temperature. Typical feedwater specifications for utihty boilers are given in Table 4. To some extent turbine steam purity requirements determine the feedwater purity requirements. The boiler-water siUca required to maintain adequate steam purity for higher pressure steam turbines is considerably less than the boiler could tolerate if deposition in the boiler were the only issue. 

Water Treatment. Water and steam chemistry must be rigorously controlled to prevent deposition of impurities and corrosion of the steam cycle. Deposition on boiler tubing walls reduces heat transfer and can lead to overheating, creep, and eventual failure. Additionally, corrosion can develop under the deposits and lead to failure. If steam is used for chemical processes or as a heat-transfer medium for food and pharmaceutical preparation there are limitations on the additives that may be used. Steam purity requirements set the allowable impurity concentrations for the rest of most cycles. Once contaminants enter the steam, there is no practical way to remove them. Thus all purification must be carried out in the boiler or preboiler part of the cycle. The principal exception is in the case of nuclear steam generators, which require very pure water. These tend to provide steam that is considerably lower in most impurities than the turbine requires. A variety of water treatments are summarized in Table 5. Although the subtieties of water treatment in steam systems are beyond the scope of this article, uses of various additives maybe summarized as follows ... 

Because no process has been developed for selectively removing impurities in vanadium and vanadium alloys in the metallic state, it is essential that all starting materials, in aggregate, be pure enough to meet final product purity requirements. In addition, the consoHdation method must be one that prevents contamination through reaction with air or with the mold or container material. 

High solids, water, high purity required... 

Indirect (French) Process. Ziac metal vapor for burning is produced ia several ways, one of which iavolves horizontal retorts. Siace all the vapor is burned ia a combustion chamber, the purity of the oxide depends on that of the ziac feed. Oxide of the highest purity requires special high grade ziac and less-pure products are made by blending ia Prime Western and even scrap ziac. 

Food and pharmaceutical grades of calcium carbonate are covered by the Food Chemicals Codex (7) and the United States Pharmacopeia (8) and subject to U.S. Food and Dmg Administration Good Manufacturing Practices (9). Both purity requirements and test methods are available (7,8). Calcium carbonate is listed in the U.S. Code of Federal Regulation as a food additive, and is authorized for use in both paper and plastic food contact appHcations. 

The number of columns ia a multicomponent train can be reduced from the N — 1 relationship if side-stream draw-offs are used for some of the component cuts. The feasibiUty of multicomponent separation by such draw-offs depends on side-stream purity requirements, feed compositions, and equihbrium relationships. In most cases, side-stream draw-off distillations are economically feasible only if component specifications for the side-stream are not tight. If a single component is to be recovered ia an essentially pure state from a mixture containing both lower and higher boiling components, a... 

The suitabiHty and economics of a distillation separation depend on such factors as favorable vapor—Hquid equiHbria, feed composition, number of components to be separated, product purity requirements, the absolute pressure of the distillation, heat sensitivity, corrosivity, and continuous vs batch requirements. Distillation is somewhat energy-inefficient because in the usual case heat added at the base of the column is largely rejected overhead to an ambient sink. However, the source of energy for distillations is often low pressure steam which characteristically is in long supply and thus relatively inexpensive. Also, schemes have been devised for lowering the energy requirements of distillation and are described in many pubHcations (87). 

Raw Material Purity Requirements. The oxygen process has four main raw materials ethylene, oxygen, organic chloride inhibitor, and cycle diluent. The purity requirements are estabHshed to protect the catalyst from damage due to poisons or thermal mnaway, and to prevent the accumulation of undesirable components in the recycle gas. The latter can lead to increased cycle purging, and consequently higher ethylene losses. 

The air process has similar purity requirements to the oxygen process. The ethane content of ethylene is no longer a concern, due to the high cycle purge flow rate. Air purification schemes have been used to remove potential catalyst poisons or other unwanted impurities ia the feed. 

Difficult Separations Some binary separations may pose special problems because of extreme purity requirements for one or both products or because of a relative volatihty close to 1. The y-x diagram... 

Membrane separation Medium to high purity Na, 95to 99.9% Small typical module produces 855 scfh at I75lb/in and 77°F Can use plant air as air source simple and safe to operate stable output maybe economical for low-capacity medium- to high-purity requirements excellent when some oxygen is required with the nitrogen temperature and pressure sensitive... 

Attempts by Kao and others to enhance transparency by chemically removing impurities from glass met with little success the level of purity required was indeed comparable with that needed in silicon for integrated circuits. In the event, the required purification was achieved in the same way in which semiconductor-grade silicon is now manufactured, by going through the gas phase (silicon tetrachloride), which can be separated from the halides of impurity species because of dilTerences in vapour pressures. This breakthrough was achieved by R.D. Maurer and his... 

In light ends fractionation it is usually just as important to remove light material from the heavier cut as it is to keep heavy material out of the lighter cut sidestreams are seldom withdrawn. The desired purity (expressed as per cent of impurity) of the overhead and bottoms is determined by product specifications or by the requirements of subsequent processing units. To meet these purity requirements, higher reflux ratios and greater numbers of plates between cuts are required than in crude distillation units. 

Steam stripping is not adequate for the bottoms purity required. More positive stripping is obtained by charging the tower bottom liquid to the reboiler. In a typical reboiler, 50% of the feed is vaporized and returned to the tower below the bottom plate. A fractionating tower equipped with a steam heated reboiler is shown... 

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