Separation and purification of industrial

New applications of zeolite adsorption developed recently for separation and purification processes are reviewed. Major commercial processes are discussed in areas of hydrocarbon separation, drying gases and liquids, separation and purification of industrial streams, pollution control, and nonregenerative applications. Special emphasis is placed on important commercial processes and potentially important applications. Important properties of zeolite adsorbents for these applications are adsorption capacity and selectivity, adsorption and desorption rate, physical strength and attrition resistance, low catalytic activity, thermal-hydrothermal and chemical stabilityy and particle size and shape. Apparent bulk density is important because it is related to adsorptive capacity per unit volume and to the rate of adsorption-desorption. However, more important factors controlling the raJtes are crystal size and macropore size distribution. 

Major industrial adsorption processes using zeolite adsorbents may be classified as follows (I) hydrocarbon separation processes, (II) drying gases and liquids, (III) separation and purification of industrial streams, (IV) pollution control applications, and (V) nonregenerative applications. Some important commercial processes in each of these areas are discussed briefly. 

Large-scale separation and purification of industrial chemicals and biochemicals... 

The ability of porous solids to reversibly adsorb large volumes of vapor was recognized in the eighteenth century and early experiments were carried out by Scheele and Fontana but the practical application of this property to the large-scale separation and purification of industrial process streams is relatively recent. Perhaps the most familiar example of such a process is the use of an adsorbent column, packed with a suitable hydrophilic adsorbent, as a drier for the removal of traces of moisture from either gas or liquid streams. Similar processes are also in common use on a large scale for the removal of undesirable impurities such as HjS and mercaptans from natural gas and organic pollutants from water. Such processes are conveniently classified as purification processes since the components which are adsorbed are present only at low concentration, have little or no economic value, and are frequently not recovered. The economic benefit of the process is derived entirely from the increase in the purity and hence the value of the stream containing the major component. 

A.sahi Chemical EHD Processes. In the late 1960s, Asahi Chemical Industries in Japan developed an alternative electrolyte system for the electroreductive coupling of acrylonitrile. The catholyte in the Asahi divided cell process consisted of an emulsion of acrylonitrile and electrolysis products in a 10% aqueous solution of tetraethyl ammonium sulfate. The concentration of acrylonitrile in the aqueous phase for the original Monsanto process was 15—20 wt %, but the Asahi process uses only about 2 wt %. Asahi claims simpler separation and purification of the adiponitrile from the catholyte. A cation-exchange membrane is employed with dilute sulfuric acid in the anode compartment. The cathode is lead containing 6% antimony, and the anode is the same alloy but also contains 0.7% silver (45). The current efficiency is of 88—89%, with an adiponitrile selectivity of 91%. This process, started by Asahi in 1971, at Nobeoka City, Japan, is also operated by the RhcJ)ne Poulenc subsidiary, Rhodia, in Bra2il under Hcense from Asahi. 

The Parex, Toray Aromax and Axens Eluxyl processes are the three adsorptive liquid technologies for the separation and purification of p-xylene practiced on a large scale today. The MX Sorbex process is the only liquid adsorptive process for the separation and purification of m-xylene practiced on an industrial scale. We now consider a few other liquid adsorptive applications using Sorbex technology for aromatics separation that have commercial promise but have not found wide application. 

The potential differences, /, at different phase boundaries, as mentioned before, have been found to have many industrial applications. The application of electrophoresis to the separation and purification of proteins has also been discussed. Both electrophoresis and electroosmosis have attained a certain amount of industrial application. 

An equimolar mixture of two enantiomers is called a racemate. The separation of two enantiomers that constitute a racemate is called optical resolution or resolution. Their crystalline forms best characterize types of racemates. A racemic mixture is a crystal where two enantiomers are present in equal amounts. A conglomerate is a case where each enantiomer has its own crystalline form. Sometimes their crystals have so-called hemihedral faces, which differentiate left and right crystals. For over a hundred years, crystallization processes have been used for the separation and purification of isomers and optical resolution, both in the laboratory and on an industrial scale. 

Many industrial organic acids can be produced by fermentation, such as acetic, citric, and lactic acids. Succinic acid is a dicarboxylic acid of potential industrial interest as a platform chemical (1-3). Separation and purification of succinic acid by adsorption was tested to replace current precipitation methods and their associated waste disposal problems. Succinic acid is a valuable intermediate value chemical with a moderate market. For succinic acid to have an economic and energy impact, it will need to become a commodity chemical intermediate with a much lower price. This target price hasbeen estimated to be between 0.22 and 0.30 / lb ( 0.48- 0.66/kg) and is potentially achievable with advanced technology (1). At this price, succinic acid can be catalytically upgraded into other higher valued chemicals suchastetrahydrofuran, 1,4-butanediol, y-butyrolactone, 2-pyrrolidinone, and N-methylpyrrolidinone. 

In the food, medical, and pharmaceutical industries, amino acids, peptides, and proteins play very important roles. Their purities must be high because of using for human body. For separation and purification of amino acids, peptides, and proteins, adsorption and ion exchange processes are generally used in these industries. 

In China, zeolites A and X were first synthesized in 1959, followed by the industrial production of zeolite Y and mordenite. With the development of the zeolite industry, zeolites were applied in many fields as well in China. In the 1950s, zeolites were mainly used in drying, separation, and purification of gases. Since the 1960s, zeolites have been widely used as catalysts and catalyst supports in petroleum refining. At present, zeolites have become the most important adsorbents and catalysts in the petroleum industry. 

Because of the aforementioned properties, zeolite 5A after Ca2+ exchange has been largely used as an important selective adsorbent industrially to separate or to purify various liquids and gases. Owing to its fast adsorption of gases, zeolite 5A can be used not only for isothermal adsorption separation such as N2-H2 separation, N2-He separation, production of 02-rich air and N2-02 separation, and purification of H2, but also for temperature-variation separation. 

The separation and purification of fluid mixtures (gas or liquid) by adsorption is a major unit operation in the chemical, petrochemical, environmental, pharmaceutical, and electronic gas industries. A list of the key commercial applications of this technology is given in Table 1. The phenomenal growth in the development of this technology is demonstrated by Fig. 1, which shows a year-by-year tally of U.S. patents issued between 1980 and 2000 on five different topics of adsorption.f The total number of patents is overwhelming. 

Clarification of rough beer, vinegar and pasteurization of clarified beer by cross-flow ultrafiltration are also very common processes utilizing hollow fiber ultrafiltration. As seen in Table 1, an important number of membrane manufacturers specialize in medical and pharmaceutical applications. In pharmaceutical and biotechnology industries, hollow fiber membranes are used for the concentration, separation, and purification of physiological activators such as antibiotics, vaccines, enzymes, proteins and peptides, as well as blood purification (hemofiltration). As a physical barrier for bacteria and viruses, membranes are also a popular option for the production of purified water for hospitals and pharmacies. 

Separation and purification of gas mixtures by selective adsorption of one or more components of the mixture on a micro- and meso-porous adsorbent is a major unit of operation in the chemical, petrochemical, environmental, and pharmaceutical industries. The phenomenal growth in the development of this technology during the last 20 years is demonstrated by Fig. 22.1. It shows the number of the US Patents issued every year between 1980 and 2000 on gas separation by adsorption and adsorption for air pollution control. More than... 

In Chapter 4, we dealt with the thermodynamic, physical and chemical properties of pure liquids. However, in most instances solutions of liquids are used in chemistry and biology instead of pure liquids. In Chapter 5, we will examine the surfaces of mainly nonelectrolyte (ion-free) liquid solutions where a solid, liquid or gas solute is dissolved in a liquid solvent. A solution is a one-phase homogeneous mixture with more than one component. For a two-component solution, which is the subject of many practical applications, the major component of the solution is called the solvent and the dissolved minor component is called the solute. Liquid solutions are important in the chemical industry because every chemical reaction involves at least one reactant and one product, mostly forming a single phase, a solution. In addition, the understanding of liquid solutions is useful in separation and purification of substances. 

The establishment of a nuclear power industry based on fission reactors involves the production of a number of materials that have only recently acquired commercial importance, notably uranium, thorium, zirconium, and heavy water, and on the operation of a number of novel chemical engineering processes, inciuding isotope separation, separation of metals by solvent extraction, and the separation and purification of intensely radioactive materials on a large scale. This text is concerned primarily with methods for producing the special materials used in nuclear fission reactors and with processes for separating isotopes and reclaiming radioactive fuel discharged from nuclear reactors. 

One of the most common industrial methods for the production of sodium hydroxide depends on the electrolysis of brine in a diaphragm cell. The products of the electrolysis are chlorine, hydrogen, and cell liquor, which is a solution of sodium hydroxide and sodium chloride. A large fraction of the cost of commercial sodium hydroxide results from the concentration, separation, and purification of the alkali. The sodium hydroxide required in the sea water descaling process need... 

Physical equilibrium is also a very important aspect of thermodynamics which applies to the chemical-process industries. It is more often used, however, in those phases of a process which involve the separation and purification of the product once it is formed. The physical-equilibrium portion is applied almost entirely to the unit operations. It is used to calculate phase compositions, vapor-liquid equilibria, absorption, adsorption, extraction, and distillation and to estimate solubilities. This phase of chemical engineering is covered adequately in other texts. 

---