Hydrocarbon Compression

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Gas Processing Design

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The design of any of the distillation processes discussed requires choosing an operating pressure, bottoms temperature, reflux condenser temperature and number of trays. This is normally done using any one of several commercially available process simulation programs which can perform the iterative calculations.

Some typical parameters for design are shown in Table 9-4. The actual optimum to use for any given process will vary depending on actual feed properties, product specifications, etc.

 Gas Processing Design

 Gas Processing Design

In Table 9-4 the actual number of trays are included. This is because complete equilibrium between vapor and liquid is normally not reached on each tray. For calculation purposes the number of theoretical flashes may be quite a bit less than the number of trays. For smaller diameter towers packing is used instead of trays. Manufacturers supply data for their packing material which indicates the amount of feet of packing required to provide the same mass transfer as a standard bubble cap tray.

Some recent advances in structured packing are being used by some operators in larger diameter towers where they would have normally used trays. The structured packing is said to allow both smaller diameter and less height of tower.

Once the operating conditions are established for a tower, its diameter and height can be chosen using data available from tray and packing manufacturers. The details of tower diameter selection, tray spacing, and internal design are beyond the scope of this text.

Written by Jack

September 21st, 2009 at 2:03 pm

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Fractionation Gas Processing

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The bottoms liquid from any gas plant may be sold as a mixed product. This is common for small, isolated plants where there is insufficient local demand. The mixed product is transported by truck, rail, barge or pipeline to a central location for further processing. Often it is more economical to separate the liquid into its various components and sell it as ethane, propane, butane, and natural gasoline. The process of separating the liquids into these components is called fractionation.

Figure 9-4 shows a typical fractionation system for a refrigeration or lean oil plant. The liquid is cascaded through a series of distillation towers where successively heavier and heavier components (fractions) are separated as overhead gas. In this figure the liquid from the still of an absorption plant or the de-methanizer (or de-ethanizer) tower of an expansion or refrigeration plant is routed to a de-propanizer. If there is too high a fraction of butanes-plus in the propane, this can be reduced by adjusting the de-propanizer pressure upward or reflux condensing temperature downward. If the vapor pressure of the propane exceeds the required specification this means that the fraction of methane and ethane in the inlet stream is too high. This fraction can be adjusted downward by increasing the temperature or decreasing the operating pressure of the still or tower that feeds liquid to the de-propanizer.

 Fractionation Gas Processing

The de-butanizer works in a similar manner. The upstream tower (depropanizer) determines the maximum vapor pressure of the butane product. If the concentration of propane-minus is too large in the inlet stream, the vapor pressure of the butane overheads will be too high. Similarly, the concentration of pentanes-plus in the butane will depend upon the reflux condensing temperature and tower operating pressure. If the pentanes-plus exceed specifications, further reflux cooling or a higher operating pressure will be needed to condense pentanes-plus from the butane overheads.

The temperature at the base of the de-butanizer determines the vapor pressure of the gasoline product. If its vapor pressure is too high, the temperature must be increased or the tower pressure decreased to drive more butanes-minus out of the bottoms liquids.

If the feed to the fractionator contains recoverable ethane, such as is likely to be the case with a cryogenic plant, then a de-ethanizer tower would be installed upstream of the de-propanizer.

Written by Jack

September 21st, 2009 at 12:55 pm

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