Sorbent Regeneration Methods
In recent years, sorbent regeneration methods have become widely used in the process of transformer oil regeneration. A high cleaning and purification effect can now be achieved by skillful use of sorbents. The simplicity of the process can be an important advantage of the sorbent method of oil regeneration. Processing oil by means of sorbents allows a significant portion of particulate matter, acids and other impurities formed in the oil and insulation as the result of the aging process to be removed. Impurities are retained on the surface and internal pores of the sorbent materials.
Sorbent Regeneration Methods can vary and some include:
1. Percolation Method
2. Contact Method
3. Counter Method
1. Percolation recovery. Used/contaminated oil is filtered (percolated) through a layer of granulated sorbent material loaded in vertical cylindrical vessels.
2. Contact recovery. Used recoverable oil at 70-750S comes into contact (is mixed) with powdered sorbent materials for a specified time period. The oil is then released from the sorbent in the filter press. A small fraction of the sorbent and heat transfer provide a sufficiently high rate of mass absorption of contaminates and cleaned oil passes quickly from the filters. The major disadvantage of the contact cleaning method includes the need for disposal of large amounts of contaminated sorbent materials posing environmental hazards to underground water supplies. .
3. Counter recovery. In the third method – oil and sorbent move towards each other. The most promising method places sorbent in a moving bed where the process proceeds continuously without stopping for periodic replacement of the spent sorbent. This method however, involves the use of very complex and expensive equipment.
Types of Sorbents
Sorbents used in the regeneration of transformer oils primarily consist of macroporous absorbents. This type of sorbent materials have a highly developed surface (hundreds of square meters per 1g), wherein the active surface is not only outside, but also (primarily) on the inside formed by an enormous number of pores that permeate throughout the sorbent material. The nature and size of the surface pores of the sorbent are the major factor in determining the rate (efficiency) of absorption. The larger the surface pores, the higher the absorption rate. Another important factor is the quantity of absorbed molecules. Coarse and fine-porous sorbents (ceteris paribus) equally adsorb substances consisting of small size molecules substances with large molecules. As the size of the molecules increases, the sorbent efficiency will decrease.
Properties of synthetic and natural absorbents.
Synthetic absorbents. These include silica, alumina, silica-alumina catalysts, and others. Silica gel – a porous material with a large adsorption surface (400-500 m³ / d), which is a silica gel Si02, that is dried and calcined at 600°C (1112°F) and used in the regeneration of transformer oil and is a large macroporous silica.
Active alumina particles are of cylindrical shape with a diameter of 3-6 mm and a height of 10-25 mm. The total surface area of the active alumina reaches 370 m2 / g and a pore radius ranging from 25 to 55 A. The active alumina is a highly efficient absorbent with basic properties and consequently, has a high absorption capacity relative to the acidic products found in aging oils. Particularly, active alumina work well with respect to low molecular weight acids, causing acidic aqueous extract oil.
Natural adsorbent. The natural absorbents are materials that do not require further treatment and have significant absorption capacity with respect to the pairs, liquids or dissolved substances. These adsorbents and absorbents are widely distributed in nature. These include clays, bauxite and substances of organic origin (plant and animal: humus, lignin, lignite and coal, etc.).
The most important physical properties, that characterize the quality of the natural adsorbents include; bulk density, true and apparent density and porosity. Volumetric weight, i.e. mass per unit volume of the layer of powder or grains of the absorbent is usually not more than 1.5 g / cm3. And for the best natural absorbents, is decreased to 0.5 g / cm 3. Sorbent performance however, largely depends on the relative humidity of the sorbent material. It is important therefore, to monitor conditions of constant humidity. Typically for natural absorbents, the bulk density is determined in a dry condition. The true density of natural absorbents depends on the content of mineral and organic components and is usually between 1,600-2,700 g / cm 3.
Bauxite. Bauxite is a very practical absorbent used for regeneration of transformer and turbine oils. It is relatively inexpensive as natural bauxite because It contains a significant amount of alumina (70%). Bauxite is the raw material for the production of aluminum. Bauxite can be found in different colors – from white to dark red. During thermal activation of bauxite deposits at various temperatures between 500°-700°C (932°-1292°F), humidity drops to between 7-5% and it obtains good absorption properties. It is established that during the expansion of the temperature interval of activation, the absorption capacity of bauxite decreases. Grinding bauxite also contributes to reducing the activation temperature to approximately 100°-150°C (212°-302°F).
Bleaching Clay. Clays are the weathering products of igneous rocks (basalts, scoria and ash, and tuff) through decomposition under the action of water, carbon dioxide, and other factors. With long-term exposure to water the solute in igneous rocks, such as alkali, alkaline earth and ferrous iron are removed from rocks. As a result, intermediate materials, with more a friable and porous structure with visible absorption properties are formed. Due to incomplete weathering of igneous rocks, typical clays are formed (Bentonite, Fuller’s Earth, etc.). The final products of the process of weathering produces silicates with low alkali content (silikolit) or hydrates sesquioxides (bauxite, laterites and ferrolity). The absorption capacity of bleaching clay is related to its porous structure, high surface area, and the nature of the surface.
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