vacuum distillation
the atmospheric bottom, also known as reduced oil, is sent to the vacuum unit where it is further separated into vacuum gas oil and vacuum residues.
vacuum distillation improves the separation of gas oil distillates from the reduced oil at temperatures less than those at which thermal cracking would normally take place.
the basic idea on which vacuum distillation operates is that, at low pressure, the boiling points of any material are reduced, allowing various hydrocarbon components in 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 the atmospheric residue yields additional and valuable distillates, which could otherwise be thermally destroyed if further distillation was attempted at atmospheric pressure and above. a typical vacuum distillation unit is shown in figure below.
fig. a vacuum distillation unit.
hot rco either from the steam-heated storage tank or from the bottom of the atmospheric distillation column is pumped through a series of preheaters (heat exchanger train) followed by heating in a pipe-still heater to raise the temperature to 360°c–370°c. this hot stream is then flashed in a multiplated distillation column where vacuum (below atmospheric pressure) is maintained by steam ejectors by medium pressure superheated steam as the motive fluid that entrains the top hydrocarbon vapours, which are condensed by water coolers.
usually, three numbers of ejectors are used the first stage sends the uncondensed vapour to the second stage followed by condensation. the uncondensed vapour then enters the third stage followed by 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 column and 100–120 mm at the bottom. condensates from these ejectors are collected in a drum, known as a hot well.
the oily layer is then sent to an oil–water separator vessel from which oil is drawn as vacuum gas oil (vgo), part of which is sent to the column as the reflux and the rest to storage. condensates from the hot well and the separator drum are drained to the sewer or water collection system. a few plates below the top plate of the column, an additional vgo is drawn and sent to the diesel/gas oil pool.
so is 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-breaking unit for the production of low viscous fuel oils, whereas so, io, and ho distillates are further stripped in a side stripper by steam to remove the lighter components to adjust the 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 generator and sent for storage in the de-asphalting unit.
a portion of the hot vacuum distillate is drawn from the column and returned back after cooling to control the heat load of the column. this stream is called the pump around. unlike circulating refluxes in the atmospheric column, pump around is only one stream in the vacuum column.
to aid washing of the ho fraction, 4%–5% of the feed rco is over flashed. it is to be mentioned that so, io, and ho are the lube oil base stocks (lobs), which are the main ingredients of lubricating oils in the market. valuable lube oil stock, known as bright stock, is obtained from the sr by solvent deasphalting.
however, the quality of vacuum distillates varies from crude to crude and, as a result, many crudes may not yield lobs, but are converted to 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 at atmospheric 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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