boiling point of the gas liquids but below that of the oil. This process allows for the recovery of around 75% by volume of the butane derivatives, and 85 to 90% by volume of the pentane derivatives, and higher-boiling constituents from the gas stream.
The basic absorption process above can be modified to improve its effectiveness, or to target the extraction of specific gas stream liquids. In the refrigerated oil absorption method, where the lean oil is cooled through refrigeration, propane recovery can be upwards of 90% by volume, and approximately 40% by volume of the ethane can be extracted from the gas stream. Extraction of the other, higher-boiling liquids can be close to 100% by volume using this process.
The absorption method, on the other hand, uses an absorbing oil to separate the methane from the gas stream liquids. While the gas stream is passed through an absorption tower, the absorption oil (lean oil) soaks up a large amount of the gas liquids. The absorption oil (enriched oil), now containing gas liquids, exits the base of the tower after which the enriched oil is fed into distillers where the blend is heated to above the boiling point of the gas liquids, while the oil remains fluid. The absorption oil is recycled while the gas liquids are cooled and directed to a fractionator tower. Another absorption method that is often used is the refrigerated oil absorption method where the lean oil is chilled rather than heated, a feature that has the potential to enhance recovery rates.
More specifically, to allow the process to operate at low temperatures, the feed gas must be injected with ethylene glycol solution to avoid hydrate formation in the heat exchangers. The feedstock (the gas stream) is cooled by propane refrigeration and separated in a cold separator, typically at approximately -18°C (0°F). The separator liquid is sent to the ethane recovery unit (the deethanizer) while the separator vapor is routed to the absorber operating at 400 psig. Refrigerated lean oil is used to absorb the higher molecular weight hydrocarbons (the C3+ constituents) from the gas stream thereby producing a lean gas and a propane rich bottom which is sent to the deethanizer. The deethanizer operates at a lower pressure, typically at 200 psig, producing an ethane rich gas and a rich oil bottom containing the C3+ components.
The overhead stream from the deethanizer is compressed to the sales gas pipeline or used as fuel gas. The bottom product is further processed in a rich oil still which regenerates a lean oil to be recycled back to the absorber and an overhead distillate containing the C3+ constituents. The C3+ stream can be fractionated in a propane recovery unit (the depropanizer) which produces the propane and butane product. Because of the high-boiling material of the lean oil, a heater is used in the rich oil still. If necessary, the lean oil composition can be controlled using a lean oil still (not shown) to remove the heavy tails of the lean oil from the process.
See also: Absorption, Gas Cleaning, Gas Processing, Gas Treating.
Absorption Tower
An absorption tower (also known as a scrubbing tower) is a long vertical column that typically contains packed bed which is used for absorbing gases from gas streams. The gas is introduced at the bottom of the column and the absorbing liquid, often water, passes in at the top and falls down against the countercurrent of gas. In the tower, the higher-boiling (gasoline-range) hydrocarbon derivatives are partially absorbed by a liquid in the form of falling droplets.
The gas stream is introduced into the lower portion of the tower and flows upwards while making contact with a washing fluid. The tower is provided with one or several washing stages, each comprising one or several venturi tubes through which the gas is given a speed increase and the washing fluid is discharged through fluid nozzles in the gas flow path. Liquid distribution in the tower occurs in the gas flow in directions forming such angles to the vertical that the fluid flow will not fall backwards and it is preferential that the gas flow is not interrupted.
There are many devices that fall into the category of absorption tower – packed towers (packed columns) are most frequently used to remove contaminants from a gas stream. However, packed towers can also be used to remove volatile components from a liquid stream by contacting it with an inert gas (stripping). They are also used in distillation applications where the separation is particularly difficult due to close boiling components.
The packed tower consists of a vertical hollow column that is filled with small solid pieces (the packing) which occupy the space but leave voids between the pieces. Thus a liquid stream can be introduced into the top of the tower and flow down over the surface of the pieces, creating a large surface area for a given volume of liquid. The gas to be treated is introduced into the tower and flows through the voids where it comes into intimate contact with the surface of the liquid. The packings are divided into three principal types: (i) dumped packings, which is the loose packing method, (ii) stacked packings, which is the dense packing method, and (iii) structured packing, which is the ordered packing method.
The diameter of a packed absorption tower depends on the quantities of gas and liquid handled, their properties, and the ratio of one stream to the other. The height of the tower, and hence the total volume of packing, depends on the magnitude of the desired concentration changes and on the rate of mass transfer per unit of packed volume.
See also: Absorption Oil, Gas Cleaning, Gas Processing, Gas Treating.
Accuracy and Precision in Analysis
Analysis is the means by which any fuel (fossil or alternative, especially a biomass-derived) fuel is characterized and accuracy and precision are required otherwise the analytical data are suspect and cannot be used with any degree of certainly. This is also especially true of analytical data that is reused for commercial operations where the fuel may be sold on the basis of heat content.
Thus, the accuracy of a test is a measure of how close the test result will be to the true value of the property being measured. As such the accuracy can be expressed as the bias between the test result and the true value. However, the absolute accuracy can only be established if the true value is known. In the simplest sense, a convenient method to determine a relationship between two measured properties is to plot one against the other. Such an exercise will provide either a line fit of the points or a spread that may or may not be within the limits of experimental error. The data can then be used to determine the approximate accuracy of one or more points employed in the plot. For example, a point that lies outside the limits of experimental error (a flyer) will indicate an issue of accuracy with that test and the need for a repeat determination.
However, the graphical approach is not appropriate for finding the absolute accuracy between more than two properties. The well-established statistical technique of regression analysis is more pertinent to determining the accuracy of points derived from one property and any number of other properties. There are many instances in which relationships of this sort enable properties to be predicted from other measured properties with as good precision as they can be measured by a single test. It would be possible to examine in this way the relationships between all the specified properties of a product and to establish certain key properties from which the remainder could be predicted, but this would be a tedious task.
An alternative approach to that of picking out the essential tests in a specification using regression analysis is to take a look at the specification as a whole, and extract the essential features (termed principal components analysis).
Principal components analysis involves an examination of set of data as points in n-dimensional space (corresponding to a number (n) of original tests) and determines (first) the direction that accounts for the biggest variability in the data (first principal component). The process is repeated until n principal components are evaluated, but not all of these are of practical importance since some may be attributable purely to experimental error. The number of significant
