Stabilizer cold-feed

Figure 6-5 shows a stabilizer with reflux. The well fluid is heated with the bottoms product and injected into the tower, below the top, where the temperature in the tower is equal to the temperature of the feed. This minimizes the amount of flashing. In the tower, the action is the same as in a cold-feed stabilizer or any other distillation tower. As the liquid falls... 

At the lop of the tower any intermediate components going out with the gas are condensed, separated, pumped back to the tower, and sprayed down on the top tray. This liquid is called reflux. and the two-phase separator that separates it from the gas is called a reflux tank" or reflux drum. The reflux performs the same function as the cold feed in a cold-feed stabilizer. Cold liquids strip out the intermediate components from the gas as the gas rises. 

A condensate stabilizer with reflux will recover more intermediate components from the gas than a cold-feed stabilizer. However, it requires more equipment to purchase, install, and operate. This additional cost must be justified by the net benefit of the incremental liquid recovery, less the cost of natural gas shrinkage and loss of heating value, over that obtained from a cold-feed stabilizer. 

It can be seen from the previous description that the design of both a cold-feed stabilizer and a stabilizer with reflux is a rather complex and involved procedure. Distillation computer simulations are available that can be used to optimize the design of any stabilizer if the properties of the feed stream and desired vapor pressure of the bottoms product are known. Cases should be run of both a cold-feed stabilizer and one with reflux before a selection is made. Because of the large number of calculations required, it is not advisable to use hand calculation techniques to design a distillation process. There is too much opportunity for computational eiToi. 

A gas-processing plant, as described in Chapter 9, is designed to recover ethane, propane, butane, and other natural gas liquids from the gas stream. A condensate stabilizer also recovers some portion of these liquids. The colder the temperature of the gas leaving the overhead condenser in a reflux stabilizer, or the colder the feed stream in a cold-feed stabilizer, and the higher the pressure in the tower, the greater the recovery of these components as liquids. Indeed, any stabilization process that leads to recovery of more molecules in the final liquid product is removing those molecules from the gas stream. In this sense, a stabilizer may be considered as a simple form of a gas-processing plant. 

It should be clear from the description of LTX units in Chapter 5 that the lower pressure separator in an LTX unit is a simple form of cold-feed condensate stabilizer. In the cold, upper portion of the separator some of the intermediate hydrocarbon components condense. In the hot, lower portion some of the lighter components flash. 

The ROD is similar to a cold feed stabilizing tower for the rich oil. Heat is added at the bottom to drive off almost all the methane (and most likely ethane) from the bottoms product by exchanging heat with the hot lean oil coming from the still. A reflux is provided by a small stream of cold lean oil injected at the top of the ROD. Gas off the tower overhead is used as plant fuel and/or is compressed. The amount of intermediate components flashed with this gas can be controlled by adjusting the cold loan oil retlux rate. 

The de-methanizer is analogous to a cold feed condensate stabilizer As the liquid falls and is heated, the methane is boiled off and the liquid becomes leaner and leaner in methane. Heat is added to the bottom of the tower using the hot discharge residue gas from the compressors to assure that the bottom liquids have an acceptable RVP or methane content. 

One way to utilize a stabilizer Is illustrated In Figure 5, which is simply the Figure 4 process with the liquids from K and 4 diverted to a stabilizer. The stabilizer could be either refluxed or cold-feed, as a further variation. This process reduces the recycle load significantly in the two lower compression stages, as compared to the previous processes. This process also provides an additional control for the crude oil vapor pressure which can be independently varied, since the fractionator split can be controlled and the fractionator bottom product is blended with the crude stream. It may be desirable to blend this stream into separator 1... 

When extruding CSM compounds a cold feed extruder is best, but a hot feed one will also work, but uniform warming of the compound is essential for dimensional stability. Higher die temperatures are needed for a smooth surface and lower feed zone temperatores provide better back pressure for air removal. Operating temperatures of the equipment for both cold and hot feed extruders are... 

A typical cold-feed stabilization system is shown in Figure 2.4. The crude is being produced to a high-pressure separator operating at 1000 psig (7000 kPa). In this separator, the majority of the very light components, such as methane, can be removed without significant loss of the heavy components, while the bulk produced water is also removed from the crude. The crude oil is then flashed to the stabilizer... 

FIGURE 2.4. Schematic of a typical cold-feed stabilization system. 

Since the reflux condenser provides the cooling for the stabilizer top, cold-feed is no longer required. As shown, the feed to the stabilizer is heated by exchanging heat with the stabilizer s bottom product. The feed is now introduced into the middle section of the stabilizer. 

The feed to the tower wiU normally enter the tower near the top of a cold-feed stabilizer and at or near the tray where the tower conditions and feed composition most nearly match the inlet feed conditions, in towers with reflux. The liquids in the tower fall down through the down-comer, across the tray, over the weir, and into the down-comer to the next tray. The temperature on each tray increases as the liquids drop from tray to tray. Hot gases come up the tower and buhhle through the liquid on the tray above, where some of the heavier ends in the gas are condensed and some of the Hghter ends in the liquid are vaporized. The gas gets leaner and leaner in heavy hydrocarbons as it goes up the tower the falling liquids become richer and richer in the heavier hydrocarbon components. 

For a cold-feed stabilizer, an inlet feed cooler may be required. Again, calculations are required to determine the design feed temperature and the heat duty of the exchanger. This exchanger is usually a shell-and-tube type with some type of refrigerant required to cool the feed sufficiently. 

The selection of equipment and the decision on a cold-feed versus a reflux stabilizer depend on several considerations. The availability of heat sources for the reboiler and streams for cooling the system influences the final decision. As usual, the economics of product recovery, capital investment, and operation costs will be major factors. 

For stabilizers with a reflux system, a feed heater may be required. If a feed heater is used, it is normally a shell-and-tube type that exchanges heat between the cold-feed and the hot bottom product, which is then cooled before going to storage. 

The need to obtain greater recoveries of the C9, C3, and C4S in natural gas has resulted in the expanded use of low-temperature processing of these streams. The majority of the natural gas processing at low temperatures to recover light hydrocarbons is now accomphshed using the turboexpander cycle. Feed gas is normally available from 1 to 10 MPa. The gas is first dehydrated to a dew point of 200 K and lower. After dehydration the feed is cooled with cold residue gas. Liquid produced at this point is separated before entering the expander and sent to the condensate stabilizer. The gas from the separator is... 

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