Condensate
Stabilization
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Condensate Stabilization
Contents: Introduction .......................................................................................................3 Multistage Separation........................................................................................4 Multiple Flashes at Constant Pressure and Increasing Temperature..................5 Cold Feed Distillation Tower............................................................................. Distillation Tower with !e"lu#..........................................................................$
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Introduction : The li&uids that are separated "rom the gas stream in the "irst separator ma' (e "lowed directl' to a tan) or ma' (e *sta(ili+ed* in some "ashion. These li&uids contain a large percentage o" methane and ethane, which will "lash to gas in the tan). This lowers the partial pressure o" all other components in the tan) and increases their tendenc' to "lash to -apors. The process o" increasing the amount o" intermediate C3 to C5/ and hea-' C 0 / components in the li&uid phase is called *sta(ili+ation.* In a gas "ield this process is called condensate sta(ili+ation and in an oil "ield it is called crude sta(ili+ation. In almost all cases the molecules ha-e a higher -alue as li&uid than as gas. Crude oil streams t'picall' contain a low percentage o" intermediate components. Thus, it is not normall' economicall' attracti-e to consider other alternati-es to multistage separation to sta(ili+e the crude. In addition, the re&uirement to treat the oil at high temperature is more important than sta(ili+ing the li&uid and ma' re&uire the "lashing o" (oth intermediate and hea-' components to the gas stream. as condensate, on the other hand, ma' contain a relati-el' high percentage o" intermediate components and can (e easil' separated "rom entrained water due to its lower -iscosit' and greater densit' di""erence with water. Thus, some sort o" condensate sta(ili+ation should (e considered "or each gas well production "acilit'. PARTIAL PRESSURES The "raction o" an' one component that "lashes to gas at an' stage in a process is a "unction o" the temperature, pressure, and composition o" the "luid at that stage. For a gi-en temperature this tendenc' to "lash can (e -isuali+ed (' the partial pressure o" the component in the gas phase that is in e&uili(rium with the li&uid. Partial pressure is de"ined as2
The partial pressure at a gi-en pressure and temperature is lower when there are more moles o" other components in the gas phase. The lower the partial pressure the greater the tendenc' o" the component to "lash to gas. Thus, the higher the "raction o" light components in the inlet "luid to an' separator, the lower the partial pressure o" intermediate components in the gas phase o" the separator, and the greater the num(er o" intermediate component molecules that "lash to gas.
MULTISTAGE SEPARATION 3
Figure 1 shows a multistage separation process. ' remo-ing molecules o" the light components in the "irst separator the' are not a-aila(le to "lash to gas "rom the li&uid in the second separator, and the partial pressure o" intermediate components in the second separator is higher than it would ha-e (een i" the "irst separator did not e#ist. The second separator ser-es the same "unction o" increasing the partial pressure o" the intermediate components in the third separator and so "orth. The simplest "orm o" condensate sta(ili+ation is to install a lowpressure separator downstream o" an initial highpressure separator. nless the gas well produces at low pressure less than 566 psi/ and the gas contains -er' little condensate less than 166 (pd/, the additional e#penditure "or this stage o" separation is almost alwa's economical when (alanced against increased li&uid production. I" -apor reco-er' "rom the tan) is re&uired (' en-ironmental regulations, the "lash separator will signi"icantl' reduce the horsepower re&uired. I" -apor reco-er' is not re&uired, the gas "rom the "lash separator ma' (e economicall' "easi(le to (e reco-ered and recompressed "or sales e-en i" it is not "easi(le to reco-er stoc) tan) -apors.
MULTIPLE FLASES AT CONSTANT PRESSURE 4
AN! INCREASING TEMPERATURE It is possi(le to sta(ili+e a li&uid at a constant pressure (' successi-el' "lashing it at increasing temperatures as shown in Figure %. 7t each successi-e stage the partial pressure o" the intermediate components is higher than it could ha-e (een at that temperature i" some o" the lighter components had not (een remo-ed (' the pre-ious stage. It would (e -er' costl' to arrange a process as shown in Figure %, and this is e-er done. Instead, the same e""ect is o(tained in a tall, -ertical pressure -essel with a cold temperature at the top and a hot temperature at the (ottom. This is called a *condensate sta(ili+er.* Figure 3 shows a condensate sta(ili+er s'stem. The well stream "lows to a high pressure, threephase separator. 8i&uids containing a high "raction o" light ends are cooled and enter the sta(ili+er tower at appro#imatel' %66 psi. In the tower the li&uid "alls downward in a process that results in man' "lashes at e-erincreasing temperatures. 7t the (ottom o" the tower, some o" the li&uids are c'cled to a re(oiler where the' recei-e heat to pro-ide the necessar' (ottoms temperature %669F to 4669F/. The re(oiler could (e either a direct"ired (ath, an indirect"ired (ath, or a heat medium e#changer.
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The li&uids lea-ing the (ottom o" the tower ha-e undergone a series o" stage "lashes at e-erincreasing temperatures, dri-ing o"" the light components, which e#it the top o" the tower. These li&uids must (e cooled to a su""icientl' low temperature to )eep -apors "rom "lashing to atmosphere in the storage tan). COL! FEE! !ISTILLATION TO"ER Figure 4 shows the cold "eed distillation tower o" Figure 3. The inlet stream enters the top o" the tower. It is heated (' the hot gases (u((ling up through it as it "alls "rom tra' to tra' through the downcomers. 7 "lash occurs on each tra' so that the li&uid is in neare&uili(rium with the gas a(o-e it at the tower pressure and the temperature o" that particular tra'. 7s the li&uid "alls, it (ecomes leaner and leaner in light ends, and richer and richer in hea-' ends. 7t the (ottom o" the tower some o" the li&uid is circulated through a re(oiler to add heat to the tower. 7s the gas goes up "rom tra' to tra', more and more o" the hea-' ends get stripped out o" the gas at each tra' and the gas (ecomes richer and richer in the light ends and leaner and leaner in the hea-' ends :ust the opposite o" the li&uid/. The gas e#its the top o" the tower, The lower the temperature o" the inlet li&uid, the lower the "raction o" intermediate components that "lashes to -apor on the top tra's and the greater the reco-er' o" these components in the li&uid (ottoms. ;owe-er, the colder the "eed, the more
heat is re&uired "rom the re(oiler to remo-e light components "rom the li&uid (ottoms. I" too man' light components remain in the li&uid, the -apor pressure limitations "or the li&uid ma' (e e#ceeded. 8ight components ma' also encourage "lashing o" intermediate components (' lowering their partial pressure/ in the storage tan).
Fi#ure $%&' Cold%(eed distillation to)er o( condensate stabilization s*ste+'
There is a (alance (etween the amount o" inlet cooling and the amount o" re(oiling re&uired. T'picall', the li&uid out the (ottom o" the tower must meet a speci"ied -apor pressure. The tower must (e designed to ma#imi+e the molecules o" intermediate components in the li&uid without e#ceeding the -apor pressure speci"ication. This is accomplished (' dri-ing the ma#imum num(er o" molecules o" methane and ethane out o" the li&uid and )eeping as much o" the hea-ier ends as possi(le "rom going out with the gas. !ISTILLATION TO"ER "IT REFLU, Figure 5 shows a sta(ili+er with re"lu#. The well "luid is heated with the (ottoms product and in:ected into the tower, (elow the top, where the temperature in the tower is e&ual to the temperature o" the "eed. This minimi+es the amount o" "lashing. In the tower, the action is the same as in a cold"eed sta(ili+er or an' $
other distillation tower. 7s the li&uid "alls through the tower, it goes "rom tra' to tra', and gets increasingl' richer in the hea-' components and increasingl' leaner in the light components.
The sta(ili+ed li&uid is cooled in the heat e#changer (' the "eed stream (e"ore "lowing to the stoc) tan). 7t the top o" the tower an' intermediate components going out with the gas are condensed, separated, pumped (ac) to the tower, and spra'ed down on the top tra'. This li&uid is called *re"lu#,* and the twophase separator that separates it "rom the gas is called a *re"lu# tan)* or *re"lu# dram,* The re"lu# per"orms the same "unction as the cold "eed in a cold"eed sta(ili+er. Cold li&uids strip out the intermediate components "rom the gas as the gas rises. The heat re&uired at the re(oiler depends upon the amount o" cooling done in the condenser. The colder the condenser, the purer the product and the larger the percentage o" the intermediate components that will (e reco-ered in the separator and )ept "rom going out with the gas. The hotter the (ottoms, the greater the percentage o" light components will (e (oiled out o" the (ottoms li&uid and the lower the -apor pressure o" the (ottoms li&uid, 7 condensate sta(ili+er with re"lu# will reco-er more intermediate components "rom the gas than a cold"eed sta(ili+er. ;owe-er, it re&uires more e&uipment to purchase, install, and operate. This additional cost must (e :usti"ied (' the net (ene"it o" the incremental li&uid reco-er'< less the cost o" natural gas shrin)age and loss o" heating -alue, o-er that o(tained "rom a cold"eed sta(ili+er.
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