Fine chemicals, adsorptive separation

Preparative chromatographic processes are of increasing importance particularly in the production of fine chemicals. A mixture of compounds is introduced into the liquid mobile phase, and this then flows through a packed column containing the stationary solid phase. The contacting scheme is thus differential, but since the adsorption characteristics of the compounds in the mixture are similar, many equivalent theoretical stages are required for their separation. Chromatographic processes are mostly ran under transient conditions, such that... 

A good example of using adsorptive separation in a non-refining/petrochemical application is the separation of fructose from an aqueous solution of mixed sugars. This process allows the production of high concentration fructose which has a much higher sweetness to calorie ratio than simple glucose or sucrose. As in fine chemical and pharmaceutical applications we can often use adsorption when distillation is not possible or feasible or when the material is thermally sensitive. 

The second part of the book covers zeolite adsorptive separation, adsorption mechanisms, zeolite membranes and mixed matrix membranes in Chapters 5-11. Chapter 5 summarizes the literature and reports adsorptive separation work on specific separation applications organized around the types of molecular species being separated. A series of tables provide groupings for (i) aromatics and derivatives, (ii) non-aromatic hydrocarbons, (iii) carbohydrates and organic acids, (iv) fine chemical and pharmaceuticals, (v) trace impurities removed from bulk materials. Zeolite adsorptive separation mechanisms are theorized in Chapter 6. 

Among hybrid separations not involving membranes, adsorptive distillation (87) offers interesting advantages over conventional methods. In this technique a selective adsorbent is added to a distillation mixture. This increases separation ability and may present an attractive option in the separation of azeotropes or close-boiling components. Adsorptive distillation can be used, for instance, for the removal of trace impurities in the manufacturing of fine chemicals (it may allow for switching some fine chemical processes from batchwise to continuous operation). 

Native enzymes, which can spatially and chemically recognize substrate molecules, are powerful catalytic systems in many biochemical processes under mild reaction conditions. The preparation of artificial enzymatic catalysts with the capability of molecular recognition capability, by a molecular-imprinting method, which creates cavities with a similar shape and size to the template molecule in polymer matrices has been developed [1-14]. The technique has been mainly established in the field of analytical chemistry - molecular receptors [15-23], chromatographic separations [24-28], fine chemical sensing [29-33]. All of the methods rely on the selective adsorption of target molecules on imprinted adsorption sites. The number of papers reported per year on molecular imprinting is summarized in Fig. 22.1. 

Mordenite ( Nag(H20)24l [AlgSi-toCLr,]) is a very important catalysis, adsorption, and separation material, widely used in petroleum refining and the fine-chemicals industry. 

These studies, therefore, set the basis for a continuous system for extractive bioconversion of steroids. The interesting point that comes to notice is that the combination of two techniques such as aqueous two-phase separation, and adsorption, which are generally used individually for performing extractive bioconversions, could be advantageous for efficient product recovery in case of fine chemicals as well. 

Although size separation was pioneered by the use of Sephadex (Pharmacia Fine Chemicals) columns in aqueous systems (gel filtration), the development of high-performance SEC for water-soluble low-molecular-mass compounds (see also Chapter 7) has been disappointing. Most columns for aqueous use have been designed for separation of large biochemical molecules such as proteins and peptides. Complex interactions, such as adsorption, or ionic effects which lead to a non-SEC mechanism, may occur. The essential simplicity of SEC is therefore lost. 

These kinds of MMs are useful for MMRs and/or incorporation into microfluidic devices with thin-fllms. The potential applications of such zeolite MMs include separations in pharmaceutical and fine chemical synthesis, as well as in lab-on-chip devices, sensors, zeolite micro-FCs and adsorption screening tools (Pina et al, 2011). 

During the last decade, this technique has been used for the scale-up of purification steps based on adsorption chromatography. Among the possible applications are pharmaceuticals, fine chemicals, biochemical products, flavors and fragrances, and especially enantiomers, which are separated by means of suitable chiral stationary phases (see Table 2). 

Simulated Moving Beds (SMBs) are well established for the adsorption based separation of hydrocarbons as well as of fine chemicals, particularly enantiomers. This technology covers a broad range of production scales from the laboratory units, which use chromatographic columns with a 0.5 cm internal diameter, to the mnlti-ton prodnction nnits licensed by Novasep for chiral separations with colnmn diameters between 20 and 100 cm, to the largest SMB unit licensed recently in South Korea by the Institute Francaise du Petrol with a column diameter of 8 m for the prodnction of 700,000 tons per year of p-xylene. New applications are envisaged in the near fntnre, particnlarly in the emerging area of bio-separations, e.g. for the pnrification of enzymes, peptides, antibiotics and natural extracts. 

Comparing gas phase spectra of the free molecules with that of adsorbed ones, two observations are made loss of rotational fine structure, thus broadening of peaks, and a shift in energies on an IP scale. The extent of the energy shift reflects the state of the adsorbed molecule [101, 118]. However these shifts compared to the gas phase values cannot be directly related to chemical properties since they consist of mainly two parts that can be separated in contributions from physical adsorption, called relaxation shifts AErv and chemical adsorption AErond (Eq. 2.21) [97, 118]. 

Adsorption process has been widely used in many chemical and related industries, such as the separation of hydrocarbon mixtures, the desulfurization of natural gas, and the removal of trace impurities in fine chemical production. Most of the adsorption researches in the past are focused on the experimental measurement of the breakthrough curve for studying the dynamics. The conventional model used for the adsorption process is based on one-dimensional or two-dimensional dispersion, in which the adsorbate flow is either simplified or computed by using computational fluid dynamics (CFD), and the distribution of adsorbate concentration is obtained by adding dispersion term to the adsorption equation with unknown turbulent mass dififusivity D(. Nevertheless, the usual way to find the D, is either by employing empirical correlation obtained from inert tracer experiment or by guessing a Schmidt number applied to the whole process. As stated in Chap. 3, such empirical method is unreliable and lacking theoretical basis. 

Separation of a gas from the gas mixture is a key issue in the various industrial fields hydrogen recovery in petroleum refinery process, oxygen removal to prevent flame, air dehumidification to prevent moisture absorption, dehydration of fine chemical products, for example. Although there are various methods to perform gas separation [membrane separation, pressure swing adsorption (PSA) separation, cryogenic separation], the method using membranes attracts much attention. 

Chromatography is based upon the selective adsorption from solution on the active surface of certain finely divided solids. Closely related substances exhibit different powers of adsorption, so that separations, which are extremely difficult by ordinary chemical methods, may be effected by this means. When, for example, a solution of leaf pigments... 

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