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Adsorption and Membrane Processes

Zeolites are used to adsorb a wide range of solutes from vapour and gas streams. What is it about their structure that makes them suitable for selective adsorption? What isotherm(s) is/are used to model the adsorption capacity of zeolites?

Describe, with examples, three different industrial processes that successfully utilize zeolites. Include in your discussion for each application how the selected zeolite’s structure influences its performance and why zeolites are used in preference to other types of available adsorbents, in particular activated carbons.

Your report should be fully referenced within the text, including any pictures or diagrams copied and used. Include an Abstract at the beginning (no more than 8 lines). The Abstract should be followed by an Introduction, and then individual Sections (e.g., one on structure, one on isotherms, one each for the three industrial process etc.).

Finally, include a Conclusion section that provides an overall discussion of the key points arising from the entire report. The Conclusions should be no more than one page. Make sure you include at the end of the report a list of all the References used in the text (make sure that you also indicate throughout the report those sections of text and/or diagrams that are references, and from which reference(s) they came from).

On-line encyclopedias, such as Wikipedia, are not adequate as a reference - you must look at and quote the original source (e.g., journal article, book, technical report etc.).

Any blatant cutting and pasting of text will be penalized.

Please place a hard (paper) copy of your report in my mail slot in the Engineering School Office, on or before Wednesday 20th February 2013. Soft copies (e.g., email attachments) cannot be accepted.

No extensions to this deadline will be offered.

 

Solution 

Zeloite and Its Applications

Abstract

Studies on the conversion of low-cost materials into zeolitic materials have been promoted nowadays. The increasing requirement for a healthy environment, predominantly related to elevated quality of water required for drinking and elimination of pollutants from industrial, agricultural and municipal wastewater, are a main reason to investigate for latest materials. Almost all function of zeolites has been driven by environmental anxiety and zeolites play an important role in decreasing poisonous waste and consumption of energy. Zeolites have exceptional absorbent characteristics. Hence they are used in a diversity of applications with a worldwide market of quite a lot of million tonnes per year. Most important uses of zeolites are in petrochemical industry. It is also extensively used in ion-exchange for softening of water and to remove impurities from water. It is also used for separation and elimination of gases and solvents. Zeolites are also used in construction, agriculture, animal husbandry, medicine etc.

In this study a short synopsis of the industrial and environmental applications of zeolite covering the basics and latest developments is proposed.

Introduction

Zeolites are basically natural volcanic minerals with quite a few number of exceptional and important properties. When volcanic ash got deposited in ancient alkaline lakes it lead to the formation of Zeolites. The contact of the volcanic ash with the salts which are there in the lake water altered the ash into various zeolite materials.

In 1756, it was discovered by Axel Fredrick Cronstedt, the Swedish mineralogist, that stilbite which is natural mineral evidently lost water when heated. Thus he named the class of materials zeolites from the conventional Greek words which means 'boiling stones.' Zeolites were judged an ambiguous collection of minerals with distinctive characteristics and Cronstedt was considered principally for discovering the element nickel.

Zeolites have an atypical crystalline structure and they also have an exclusive capability to change ions. Structures of Zeolites have a considerable number of small channel as well as larger pores. The small channels are known microporosity because they have distinctive diameters of 0.5 to 0.7 nm which is only a little big than the diameter of a molecule of water. The larger pores are called mesoporosity. These channels have positive ions which can be replaced by other ions.

This changeover of ions allows zeolites to adsorb certain unsafe or unnecessary elements selectively from soil, water and air. Removal of calcium from hard water is a very classic example of this unique characteristic of zeolites. With the help of zeolites, calcium ions (Ca+2) can be removed and replaced with Sodium ions (Na+), which end in soft water. Zeolites also have powerful affinity for some damaging heavy metals like lead, chromium, nickel and zinc.

Suspended and colloidal particles can be trapped in the mesopores of zeolite. Dissolved organic molecules can also be adsorbed in these pores.

Industrial and environmental uses of zeolites are limitless because of its unique properties and characteristics. Zeolites can also be used in landfills and at industrialized sites, which can assist to avoid the discharge of a large amount of damaging or unnecessary elements into the environment.

Structure of Zeolite

According to Breck (1974), zeolites are crystalline, hydrated, micro porous aluminosilicates which are made from a noticeably expanding three dimensional arrangement of [SiO4]4- and [AlO4]5- tetrahedral which are associated to each other by the sharing of oxygen atoms (p. 4). Their composition can be taken as an inorganic polymer which is made from tetrahedral TO4 units, where T is Si4+ or Al3+ ion. Each O (Oxygen) atom is shared between two T atoms.

The organization of zeolite is on the basis of the crystallographic unit cell :  Mx/n      

[(AlO2)x(SiO2)y]· wH2O, where M is an alkali or alkaline earth cation, n is the valency of the cation, w is the number of molecules of water per unit cell, x and y are the total number of tetrahedron per unit cell, and the ratio y/x usually has values of 1 to 5, though for the silica zeolite, y/x can be raging from 10 to 100. (Bekkum, Flanigen, et.al., 1991)  

According to Lobo (2003), the O/T ratio in the structure of zeolite is always equal to 2 because it is derived from silicate type network structures. The O-T-O bond angle is close to the ideal tetrahedral bond angle of 109.5°. The T-O-T bond angle is much more flexible than the O-T-O bond angle and is usually around 140° to 165°.

According to Breck (1974), the principal structural component of zeolites is the TO4 tetrahedra and the secondary building units (SBUs) are the geometric arrangements of this principle tetrahedral arrangement. The secondary building units can be uncomplicated polyhedrals like cubes, hexagonal prisms, or cubo-octahedral (Bekkum, Flanigen, Jacobs & Jansen, 1991). The structures of zeolites can be produced by repeating SBUs. Zeolites can be categorized into eight groups. Fig. 1 and 2 shows the components and parts of the structure of zeolite.

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