Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 240

Blending is a process which, in the current context, mixes the constituents of the fuel in such a manner as to ensure that there is a homogeneous mixture of all of the ingredients. The process can be carried out numerous times within a fuel manufacturing process when new excipients need to be added to the blend. Fuels as produced at the end of the process are actually blends of different streams since no one unit can, for example, produce specification-grade gasoline or diesel fuel. Thus, blending is the final operation in fuel production.

Blending consists of mixing the products in various proportions to meet specifications such as vapor pressure, specific gravity, sulfur content, viscosity, octane number, cetane index, initial boiling point, and pour point. Blending can be carried out in-line or in batch blending tanks. Air emissions from blending are fugitive volatile organic compounds (VOCs) from blending tanks, valves, pumps, and mixing operations. During the blending process, not only the physical and chemical properties of each blending component have to be considered in order to produce the specification-grade fuel and ensure that instability or incompatibility do not occur.

In fact, the major refinery products produced by the product blending process are gasoline, jet fuels, heating oils, and diesel fuels. The objective of product blending is to allocate the available blending components in such a way as to ensure all product demands and specifications are met at the least cost and to produce products which maximize overall profit. Gasoline blending is a refinery operation that blends different component streams into various grades of gasoline. Typical grades include 83 octane (blended later with an oxygenated fuel such as ethanol), regular 87 octane, and premium 92 octane. The Reid Vapor Pressure (RVP) is set depending on the average temperature of the location the gasoline will be used (cold temperatures require higher RVP than warmer climates). These two specifications are the most significant, and they are documented with each blend, to minimize the potential for octane giveaway.

Most refiners use computer-controlled in-line blending for blending gasoline and distillates. Inventories of blending stocks, together with cost and physical property data, are maintained in a database. Many of the properties of blend components are non-linear, such as octane number, so estimating final blend properties from the components can be quite complex. When a certain volume of a given quality product is specified, the computer uses linear programming models (LP’s) to optimize the blending operations to select the blending components to produce the required volume of the specified product.

See also: Blended Fuels.