vacuum distillation
the atmospheric bottom, also known as reduced oil, is sent to the vacuumunit where it is further separated into vacuum gas oil and vacuum residues.
vacuumdistillation improves the separation of gas oil distillates from the reduced oil attemperatures less than those at which thermal cracking would normally take place.
the basic idea on which vacuum distillation operates is that, at low pressure, theboiling points of any material are reduced, allowing various hydrocarbon componentsin the reduced crude oil to vaporize or boil at a lower temperature. vacuum distillation of the heavier product avoids thermal cracking and hence product loss and equipment fouling.
vacuum distillation can also be referred as "low temperature distillation".
in distilling the crude oil, it is important not to subject the crude oil to temperatures above 370 to 380 °c because the high molecular weight components in the crude oil will undergo thermal cracking and form petroleum coke at temperatures above that. formation of coke would result in plugging the tubes in the furnace that heats the feed stream to the crude oil distillation column. plugging would also occur in the piping from the furnace to the distillation column as well as in the column itself.
the constraint imposed by limiting the column inlet crude oil to a temperature of less than 370 to 380 °c yields a residual oil from the bottom of the atmospheric distillation column consisting entirely of hydrocarbons that boil above 370 to 380 °c.
to further distill the residual oil from the atmospheric distillation column, the distillation must be performed at absolute pressures as low as 10 to 40 mmhg (also referred to as torr) so as to limit the operating temperature to less than 370 to 380 °c.
the vacuum distillation column internals must provide:
1- good vapor–liquid contacting.
2- at the same time, maintaining a very low pressure increase from the top of the column top to the bottom.
therefore, the vacuum column uses distillation trays only where withdrawing products from the side of the column (referred to as side draws).
most of the column uses packing material for the vapor–liquid contacting because such packing has a lower pressure droping than distillation trays. this packing material can be either structured sheet metal or randomly dumped packing such as raschig rings.
the absolute pressure of 10 to 40 mmhg in the vacuum column is most often achieved by using multiple stages of steam jet ejectors.
vacuum distillation of theatmospheric residueyields additional and valuable distillates, which could otherwise be thermally destroyedif further distillation was attempted at atmospheric pressure and above. a typical vacuumdistillation unit is shown in figure below.
fig.a vacuum distillation unit.
hot rco either from the steam-heated storagetank or from the bottom of the atmospheric distillation column is pumped through aseries of preheaters (heat exchanger train) followed by heating in a pipe-still heater toraise the temperature to 360°c–370°c. this hot stream is then flashed in a multiplateddistillation column where vacuum (below atmospheric pressure) is maintained by steamejectors by medium pressure superheated steam as the motive fluid that entrains the tophydrocarbon vapours, which are condensed by water coolers.
usually, three numbersof ejectors are used the first stage sends the uncondensed vapour to the second stagefollowed by condensation. the uncondensed vapour then enters the third stage followedby condensation and the uncondensed vapour from the third stage is vented out through
a flare or stack.
a vacuum of 30–40 mm of mercury is maintained at the top of the columnand 100–120 mm at the bottom. condensates from these ejectors are collected ina drum, known as a hot well.
the oily layer is then sent to an oil–water separator vesselfrom which oil is drawn as vacuum gas oil (vgo), part of which is sent to the columnas the reflux and the rest to storage. condensates from the hot well and the separatordrum are drained to the sewer or water collection system. a few plates below the topplate of the column, an additional vgo is drawn and sent to the diesel/gas oil pool.
sois the next vacuum distillate, which is drawn a few plates below the gas oil draw plate.similarly, the other vacuum distillates drawn from the plates below are light oil (lo),intermediate oil (io), and heavy oil (ho) in this sequence.
lo is sent to the vis-breakingunit for the production of low viscous fuel oils, whereas so, io, and ho distillates arefurther stripped in a side stripper by steam to remove the lighter components to adjustthe flash points.
the bottom residue from the tower is called the short residue (sr),which is stripped by the bottom steam followed by cooling through a steam generatorand sent for storage in the de-asphalting unit.
a portion of the hot vacuum distillate isdrawn from the column and returned back after cooling to control the heat load of thecolumn. this stream is called the pump around. unlike circulating refluxes in the atmosphericcolumn, pump around is only one stream in the vacuum column.
to aid washingof the ho fraction, 4%–5% of the feed rco is overflashed. it is to be mentioned thatso, io, and ho are the lube oil base stocks (lobs), which are the main ingredients oflubricating oils in the market. valuable lube oil stock, known as bright stock, is obtainedfrom the sr by solvent deasphalting.
however, the quality of vacuum distillates variesfrom crude to crude and, as a result, many crudes may not yield lobs, but are convertedto fuel products.
it is commonly found that most of the middle east or gulf crudes aresuitable for the manufacture of lobs from vacuum distillates.
typical boiling ranges atatmospheric pressure and the draw temperature under vacuum are listed below.
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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