Weak Acid Processes

Stainless steels have excellent corrosion resistance to weak nitric acid and are the primary materials of construction for weak acid processes. Low carbon stainless steels are preferred because of their resistance to corrosion at weld points104. 

Catalyst baskets and gauze supports (where the temperature may reach 900°C) must be resistant to oxidation, nitriding and distortion from high temperatures. Typical materials of construction are high strength alloys made of iron-nickel-chromium, nickel-chromium and nickel-chromium-tungsten- 

High chromium (20% to 27%) stainless steel is used in the cooler condenser and the tail gas preheater. Compared to low carbon stainless steel, it has better corrosion resistance to nitric acid at elevated temperatures so the extra cost is justified by the longer life104. 

Zirconium is used in nitric acid service for cooler condensers, tail gas preheaters and reboilers. It rivals tantalum in its corrosion resistance to nitric acid at all concentrations up to the boiling point. Its resistance extends up to 230°C and 65 wt %. However it is susceptible to stress corrosion cracking, which can be prevented by avoiding high, sustained tensile stresses104. 

Duplex stainless steels (4% nickel, 23% chrome) may offer cost advantages in absorption columns. These materials provide the ductability of austenitic stainless and the stress-corrosion cracking resistance of ferritic stainless steel104. 

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Weak Acid. Stainless steels (SS) have exceUent corrosion resistance to weak nitric acid and are the primary materials of constmction for a weak acid process. Low carbon stainless steels are preferred because of their resistance to corrosion at weld points. However, higher grade materials of constmction are required for certain sections of the weak acid process. These are limited to high temperature areas around the gau2e (ca 900°G) and to places in which contact with hot Hquid nitric acid is likely to be experienced (the cooler condenser and tail gas preheater). 

Hydration of Ethylene Usings Dilute Acids. A review of the early work on the hydration of ethylene using dilute acids, the weak acid process, has been given (48). The reaction is favored by the use of low temperatures and high pressures. Temperatures in the range of 150—250°C are the most frequendy quoted (132—137), although temperatures as low as 80°C have been reported (138). 

Modern processes for strong acid are based on direct oxidation of ammonia with air or oxygen, with the first two steps being similar to the weak acid process. Various processes exist allowing co-production of weak and strong acid. 

Diisopropyl sulfate is an intermediate in the indirect hydration (strong- or weak-acid) process for the preparation of isopropanol from propylene. It has no other known industrial use. No data were available on levels of occupational exposure to diisopropyl sulfate (lARC, 1992). 

An increased incidence of cancer of the paranasal sinuses was observed in workers at factories where isopropanol was manufactured by the strong-acid process. The risk for laryngeal cancer may also have been elevated in these workers. It is unclear whether the cancer risk was due to the presence of diisopropyl sulfate, which is an intermediate in the process, to isopropyl oils, which are formed as by-products, or to other factors, such as sulfuric acid. Epidemiological data concerning the manufacture of isopropanol by the weak-acid process are insufficient for an evaluation of carcinogenicity (lARC, 1987). 

Isopropyl oils, formed during the manufacture of isopropanol by both the strong-acid and weak-acid processes, were tested inadequately in mice by inhalation, skin application and subcutaneous administration. Isopropyl oils formed during the strong-acid process were also tested inadequately in dogs by inhalation and instillation into the sinuses (lARC, 1977, 1987). 

Propanol is conunonly manufactured from propene. Strong and weak acid processes, used previously and involving potentially hazardous intermediates and by-products, have largely been replaced by the catalytic hydration of propene today. The catalytic reduction of acetone is an alternative process. 

Indirect catalytic hydration has been replaced by the direct hydration process in countries such as Japan, United States, and Western Europe. Indirect catalytic hydrogenation of acetone involves the feeding of 88-93% sulfuric acid and propene gas into a reactor to produce a mixture of isopropyl and di-isopropyl sulfates, which are hydrolyzed with water to 2-propanol. Principal by-products are di-isopropyl ether and isopropyl oils consisting mainly of polypropylenes of high relative molecular mass. It has gradually been replaced by the weak-acid process, in which propene gas is absorbed in, and reacted with, 60% sulfuric acid. The resulting sulfates are hydrolyzed in a single-step process. 2-Propanol is stripped and refined from the condensate, which also contains di-isopropyl ether, acetone, and polymer oils of low relative molecular mass. 

Examples of the lader include the adsorption or desorption of species participating in the reaction or the participation of chemical reactions before or after the electron transfer step itself One such process occurs in the evolution of hydrogen from a solution of a weak acid, HA in this case, the electron transfer from the electrode to die proton in solution must be preceded by the acid dissociation reaction taking place in solution. 

The process may now be continued. Methylarsonic acid, when reduced by sulphur dioxide in concentrated hydrochloric acid, gives dichloromethylarsine, CHjAsCl. If this arsine is added to aqueous sodium hydroxide, it is hydrolysed to the weakly acidic methylarsenous acid, CH3As(OH)j, which in the alkali... 

In addition to the Hquid—Hquid reaction processes, there are many cases in both analytical and industrial chemistry where the main objective of separation is achieved by extraction using a chemical extractant. The technique of dissociation extraction is very valuable for separating mixtures of weakly acidic or basic organic compounds such as 2,4-dichlorophenol [120-83-2] and 2,5-dichlorophenol [583-78-8] which are difficult to separate by... 

Wea.kA.cid Cation Exchangers. The syathesis of weak acid catioa exchangers is a one-step process when acryHc acid or methacrylic acid is copolymetized with DVB. If an acryHc ester is used as the monomer iastead of an acryHc acid, the ester groups must be hydrolyzed after polymerization usiag either an acid or base (NaOH) to give the carboxyHc acid functionaHty, or the sodium salt (4) of it. 

A problem common to produced water appHcations is the tendency for oil fouling of the resin. If weak acid or chelate resins are used, a two-step regeneration process is required which uses acid to remove calcium and magnesium from the resin, foUowed by a dilute NaOH solution to convert the resin to the sodium form. 

Process Licensors. Some of the well-known nitric acid technology licensors are fisted in Table 3. Espindesa, Grande Paroisse, Humphreys and Glasgow, Rhfyne Poulenc, Uhde, and Weatherly are all reported to be licensors of weak acid technology. Most weak acid plant licensors offer extended absorption for NO abatement. Espindesa, Rhfyne Poulenc, Weatherly, and Uhde are also reported (53,57) to offer selective catalytic reduction (SCR) technology. 

The bottoms from the stripper (40—60 wt % acid) are sent to an acid reconcentration unit for upgrading to the proper acid strength and recycling to the reactor. Because of the associated high energy requirements, reconcentration of the diluted sulfuric acid is a cosdy operation. However, a propylene gas stripping process, which utilizes only a small amount of added water for hydrolysis, has been described (63). In this modification, the equiUbrium quantity of isopropyl alcohol is stripped so that acid is recycled without reconcentration. Kquilibrium is attained rapidly at 50°C and isopropyl alcohol is removed from the hydrolysis mixture. Similarly, the weak sulfuric acid process minimizes the reconcentration of the acid and its associated corrosion and pollution problems. 

A Hquid-phase variation of the direct hydration was developed by Tokuyama Soda (78). The disadvantages of the gas-phase processes are largely avoided by employing a weakly acidic aqueous catalyst solution of a siHcotungstate (82). Preheated propylene, water, and recycled aqueous catalyst solution are pressurized and fed into a reaction chamber where they react in the Hquid state at 270°C and 20.3 MPa (200 atm) and form aqueous isopropyl alcohol. Propylene conversions of 60—70% per pass are obtained, and selectivity to isopropyl alcohol is 98—99 mol % of converted propylene. The catalyst is recycled and requites Htde replenishment compared to other processes. Corrosion and environmental problems are also minimized because the catalyst is a weak acid and because the system is completely closed. On account of the low gas recycle ratio, regular commercial propylene of 95% purity can be used as feedstock. 

Effect on Oxide—Water Interfaces. The adsorption (qv) of ions at clay mineral and rock surfaces is an important step in natural and industrial processes. SiUcates are adsorbed on oxides to a far greater extent than would be predicted from their concentrations (66). This adsorption maximum at a given pH value is independent of ionic strength, and maximum adsorption occurs at a pH value near the piC of orthosiUcate. The pH values of maximum adsorption of weak acid anions and the piC values of their conjugate acids are correlated. This indicates that the presence of both the acid and its conjugate base is required for adsorption. The adsorption of sihcate species is far greater at lower pH than simple acid—base equihbria would predict. 

The standard cation—anion process has been modified in many systems to reduce the use of cosdy regenerants and the production of waste. Modifications include the use of decarbonators, weak acid and weak base resins. Several different approaches to demineralization using these processes are shown in Figure 1. 

Fig. 1. Demineializei systems consist of various unit processes arranged to meet the system needs. I Lstrong acid cation exchanger I I Strong ha anion exchanger 0 Degasifier I Mixed bed I Weak acid cation exchanger 1 1 Weak base anion exchanger and I IConnterflow cation.

Most mordant dyes are monoazo stmctures. The most important feature of this class of dyes is excellent fastness to light and washing. Mordant dyes are available ia aU shades of the spectmm with the exceptioa of bright violets, blues, and greens. To be useful, the metal complexes must be stable, ie, must not demetallize when subjected to dyebath conditions and aU aftertreatment processes, especially repeated washings. Chromium forms stable chelate rings with mordant dyes which are not affected by treatment with either weak acid or alkaU (see Coordination compounds). 

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