Adsorption technology, commercial applications

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. 

Table 1. Key Commercial Applications of Gas Separation and Purification by Adsorption Technology...

Commercial as well as potential uses of inoiganic membranes multiply rapidly in recent years as a result of the continuous improvement and optimization of the manufacturing technologies and applications development for these membranes. Most of the industrially practiced or demonstrated applications fall in the domains of microfiltration or ultrafiltration. Microfiltration is applied mostly to cases where the liquid streams contain high levels of particulates while ultrafiltration usually does not involve particulates. While their principal separation mechanism is size exclusion, other secondary mechanisms reflecting the solution-membrane interactions such as adsorption are often operative. Still under extensive research and development is gas separation which will be treated in Chapter 7. 

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. 

Aluminas have been in use for many years as adsorbents. First introduced commercially in 1932 by Alcoa for water adsorption [1], activated aluminas have traditionally been known as desiccants for the chemical process industries. As early as 1901, references can be found for the use of synthetic aluminas in the chromatographic purification of biological compounds [2], In recent years aluminas have found widespread usage in applications as diverse as municipal wastes, polymers, and pharmaceuticals. Novel design of these materials is extending their separations capability into even more nontraditional areas of adsorption technology. 

At present adsorption technology is recognized to be the most common technology applied to reach ultra-low sulfur levels for fuel cells applications. Activated carbon is one of the most versatile adsorbents known with high removal efficiency, low costs reusability, and possible product recovery [16,140-155]. However, there are many other commercial adsorbents used for fuels desulfurization at ambient temperature and pressure, such as silica, alumina, zeolites and some metal oxides [156-166]. 

Although reactive adsorption technologies are well proven at lab scale, they still need to be applied commercially for continuous large-scale applications. It is essential to develop new materials for reactive adsorbents with improved chemical and mechanical properties such that they fulfill necessary requirement of matching of operating conditions for both reaction and adsorption. The reusability of an adsorbent for a longer duration with sustainable adsorption capacity is another crucial parameter, which needs to be improved. Also for the processes where more than one contaminant is present in the effluent stream, it is essential to develop multicomponent reactive adsorption systems however the interaction between adsorbates and adsorbent makes the selectivity in reactive adsorption process more complex. 

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. 

Ethylbenzene is a high volume petrochemical used as the feed stock for the production of styrene via dehydrogenation. Ethylbenzene is currently made by ethylene alkylation of benzene and can be purified to 99.9%. Ethylbenzene and styrene plants are usually built in a single location. There is very little merchant sale of ethylbenzene, and styrene production is about 30x10 t/year. For selective adsorption to be economically competitive on this scale, streams with sufficiently high concentration and volume of ethylbenzene would be required. Hence, although technology has been available for ethylbenzene extraction from mixed xylenes, potential commercial opportunities are limited to niche applications. 

Advanced affinity chromatography (AAC) media are used for the adsorption of metals from ground and waste waters. The AAC technology has been used in multiple applications and is commercially available from Affinity Water Technologies, formerly Ntec Solutions, Inc. 

The ultradeep desulfurization of the current commercial fuels has become a bottleneck in hydrogen production for fuel cell applications. It is urgent to develop a more efficient and environmentally friendly process and technology for the ultradeep desulfurization of the hydrocarbon fuels for fuel cell applications. Many approaches have been conducted in the improvement of the conventional HDS process or development of new alterable processes. These approaches include (a) catalytic HDS with improved and new catalysts, reactor, and/ or process (b) selectively adsorptive desulfurization and (c) ODS and others. 

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