Aromatics adsorptive separation

This chapter addresses the fundamentals of zeolite separation, starting with (i) impacts of adsorptive separation, a description of liquid phase adsorption, (ii) tools for adsorption development such as isotherms, pulse and breakthrough tests and (iii) requirements for appropriate zeolite characteristics in adsorption. Finally, speculative adsorption mechanisms are discussed. It is the author s intention that this chapter functions as a bridge to connect the readers to Chapters 7 and 8, Liquid Industrial Aromatics Adsorptive Separation and Liquid Industrial Non-Aromatics Adsorptive Separation, respectively. The industrial mode of operation, the UOP Sorbex technology, is described in Chapters 7 and 8. 

Since 1971 mainly adsorptive separation processes are used to obtain high purity -xylene (55,84—86). A typical commercial process for the separation of -xylene from other Cg aromatics produces about 99.8% purity -xylene at greater than 95% recovery. 

Aromatic and Nonaromatic Hydrocarbon Separation. Aromatics are partially removed from kerosines and jet fuels to improve smoke point and burning characteristics. This removal is commonly accompHshed by hydroprocessing, but can also be achieved by Hquid-Hquid extraction with solvents, such as furfural, or by adsorptive separation. Table 7 shows the results of a simulated moving-bed pilot-plant test using siHca gel adsorbent and feedstock components mainly in the C q—range. The extent of extraction does not vary gready for each of the various species of aromatics present. SiHca gel tends to extract all aromatics from nonaromatics (89). 

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. 

Poro-xylene is an industrially important petrochemical. It is the precursor chemical for polyester and polyethylene terephthalate. It usually is found in mixtures containing all three isomers of xylene (ortho-, meta-, para-) as well as ethylbenzene. The isomers are very difficult to separate from each other by conventional distillation because the boiling points are very close. Certain zeoHtes or mol sieves can be used to preferentially adsorb one isomer from a mixture. Suitable desorbents exist which have boiling points much higher or lower than the xylene and displace the adsorbed species. The boihng point difference then allows easy recovery of the xylene isomer from the desorbent by distillation. Because of the basic electronic structure of the benzene ring, adsorptive separations can be used to separate the isomers of famihes of substituted aromatics as weU as substituted naphthalenes. 

Kulprathipanja, S. (1995) Process for adsorptive separation of ethylbenzene from aromatic hydrocarbons. U.S. Patent 5,453,550. 

Zeolites have been used in the industrial adsorptive purification of aromahc petrochemicals since the early 1970s. The application of zeolites to aromatic adsorptive purification and extraction is a particularly suitable fit because of three major factors. The first is the inherent difficulty involved in separating certain aromatic components by distillation. Petrochemical production requires individual components be obtained in very high purity, often in excess of 99.5%. While distillation is the most popular method of separation in the petrochemical industry, it is not well suited for the final step of producing high purity single component streams from close boiling multi-component aromatics-rich mixtures. 

Neuzil, R. (1971) Aromatic hydrocarbon separation by adsorption. US Patent 3,558,730. 

This chapter reviews the adsorptive separations of various classes of non-aromatic hydrocarbons. It covers three different normal paraffin molecular weight separations from feedstocks that range from naphtha to kerosene, the separation of mono-methyl paraffins from kerosene and the separation of mono-olefins both from a mixed C4 stream and from a kerosene stream. In addition, we also review the separation of olefins from a C10-16 stream and review simple carbohydrate separations and various acid separations. 

As promised, this chapter outhnes numerous liquid-phase non-aromatic adsorption processes that enable one to economically separate a commercially desirable component from a mixture when the separation is impossible (given the closeness of their relative volatilities) by conventional means such as distillation. We review process that can separate a wide range of normal paraffins from a mixture of their corresponding feedstocks. In addition to this, we also review how to separate mono branched paraffins and olefins from similar feedstocks. Finally we review liquid-phase adsorption processes to isolate desired carbohydrates, fatty acids and citric acid from their feed source and for each separation we reveal insight on the corresponding operating conditions, process configuration and adsorbent necessary to achieve the separation. 

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. 

Chapter 7 gives a review of the technology and applications of zeolites in liquid adsorptive separation of petrochemical aromatic hydrocarbons. The application of zeolites to petrochemical aromatic production may be the area where zeolites have had their largest positive economic impact, accounting for the production of tens of millions of tonnes of high-value aromatic petrochemicals annually. The nonaromatic hydrocarbon liquid phase adsorption review in Chapter 8 contains both general process concepts as well as sufficient individual process details for one to understand both commercially practiced and academic non-aromatic separations. 

Development of processes of dehydrogenate -paraffin to -olefins and alkylate benzene with them (mid-1960s) Isomerization of Cg aromatics to /7-xylene (late 1950s) Adsorptive separation of /7-xylene in high yield and purity, making possible separation of other isomers by precise fractionation (early 1970s)... 

Embree, H. D., Chen, T. H., and Payne, G. F., Oxygenated aromatic compounds from renewable resources motivation, opportunities, and adsorptive separations. Chem Eng J 2001, 84 (2), 133-147. 

This case study involves the recovery of highly valued and high demand ethylbenzene (EB) and mixed-xylenes (comprising of p-xylene (PX), m-xylene (MX) and o-xylene (OX)) from a C8-aromatics mixture (C8A). As point out above, C8A is isomers mixture, so their separation (recovery) is not simple, that why there is only one commercial process of liquid-phase adsorptive separation available for EB recovery from C8A. [8] However, this process requires high investment cost and generates huge volume of waste adsorbent that may become an environmental problem. Therefore, another green process should be considered for the EB purification. The ratio of various properties of the key components (EB and PX) were tested to examine the possibly alternatives. The result showed, by vapor pressure ratio, the solvent-based extractive distillation can be employed for their purification. [7]... 

In a study of selective adsorption of sulfur compounds and aromatic compounds in a hexadecane on commercial zeolites, NaY, USy, HY, and 13X by adsorption at 55 °C and flow calorimetry techniques at 30 °C, Ng et al. found that a linear correlation between the heat of adsorption and the amount of S adsorbed for NaY.162 Competitive adsorption using a mixture of anthracene, DBT, and quinoline indicates that NaY selectively adsorbs quinoline, while anthracene and DBT have similar affinity to NaY, indicating that NaY is difficult to adsorptively separate sulfur compounds from aromatic hydrocarbons with the same number of the aromatic rings. 

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