Atmospheric Distillation -Part 2 #Download no.20 by James William
2.2 CRUDE DISTILLATION The purpose of crude oil distillation is primarily to split the crude into several disti distilla llate te fracti fractions ons of a certai certain n boilin boiling g range. range. Sharp Sharpnes nesss of fracti fractiona onatio tion n is of secondary importance. importance. The number of trays trays used for crude distillation is very small small compar compared ed to most most other other distil distillat lation ions. s. A crude crude distil distillat lation ion tower, tower, produ producin cing g 6 fractions has only 40 to 50 trays. rude can be separated into gasoline, naphtha, !erosene, diesel oil, gas oil, and other products, by distillation "igure "igure 4.#0 at atmospheric atmospheric pressure. pressure. $istillation is an operation in which vapors rising through fractionating dec!s in a tower are intimately contacted with li%uid descending across the dec!s so that higher boiling components are condensed, and concentrate at the bottom of the tower while the lighte lighterr ones ones are concent concentrat rated ed at the top or pass overhe overhead. ad. rude rude is genera generally lly pumped to the unit directly from a storage tan!, and it is important that charge tan!s be drained drained completely free free from water before charging to the unit. &f water is entrained in the charge, it will vapori'e in the e(changers and in the heater, and cause cause a high pressure pressure drop drop through that e%uipmen e%uipment. t. &f a slug of water should should be charged to the unit, the %uantity of steam generated by its vapori'ation is so much greater than the %uantity of vapor obtained from the same volume of oil, that the dec!s dec!s in the fractionat fractionating ing column could could be damaged. damaged. )ater e(pands e(pands in volume #600 times upon vapori'ation at #00* at atmospheric pressure.
2.2.1 Process Description (Figure 2.10 A. !e"t E#c$"nge &n order to reduce the cost of operating a crude unit as much heat as possible is recovered from the hot streams by heat e(changing them with the cold crude charge charge.. The number number of heat heat e(chan e(changer gerss within within the crude crude unit and cross cross heat heat e(change e(change with other other units will vary vary with unit design. design. A record should should be !ept of heat e(changer outlet temperatures so that fouling can be detected and possibly corrected before the capacity of the unit is affected.
%. Cru&e F'"s$ing $esa $esalt lted ed crud crude e is he heat at e( e(ch chan ange ged d agai agains nstt wh what at ever ever othe otherr he heat at sour source cess are are available to recover ma(imum heat before crude is charged to the heater, which ultimately supplies all the heat re%uired for operation of the crude unit. The heat input is controlled by having the heater transfer temperature reset flow of fuel to the burners. burners. The heater heater transfer transfer temperature temperature is merely a convenient convenient control, and the actual temperature, which has no great significance, will vary from +5* to as high as 4+0*, depending on the type of crude and the pressure at the bottom bottom of the fractionatin fractionating g tower. &t is noteworthy noteworthy that if the %uantity %uantity of
gasoline and !erosene in crude is reduced, the transfer temperature re%uired for the same operation will be increased, even through the -lift is less.
Figure 2.11 Cru&e "n& "cuu) Disti''"tion Unit
rude entering the flash 'one of the fractionating column flashes into the vapor which rises up the column and the li%uid residue which drops downwards. This flash is a very rough separation/ the vapors contain appreciable %uantities of heavy
ends, which must be reected downwards into reduced crude, while the li%uid contains lighter products, which must be stripped out.
C. Fr"ction"tion "lashed vapors rise up the fractionating column "igure .## counter 1 current to the internal reflu( flowing down the column. The lightest product, which is generally gasoline passes overhead and is condensed in the overhead receiver. 2Should the crude contain any non3condensable gas, it will leave the receiver as a gas, and can be recovered by other e%uipment, which should be operated to obtain the minimum flash 'one pressure. The temperature at the top of the fractionators is a good measure of the endpoint of the gasoline and this temperature is controlled by returning some of the condensed gasoline as reflu( to the top of the column. &ncreasing the reflu( rate lowers the top temperature and results in the net overhead product having a lower endpoint. The loss in net overhead product must be removed on the ne(t lower draw try. This will decrease the initial boiling point of material from this tray. &ncreasing the heater transfer temperature increases the heat input and demands more reflu( to maintain the same top temperature. (ternal reflu( which is returned to the top of the fractionators passes downwards against the rising vapors. ighter components of the reflu( are revapori'ed and return to the top of the column while the heavier components in the rising vapors are condensed and return down the column. )e have then an internal reflu( stream flowing from, the top of the fractionators all the way bac! to the flash 'one and becoming progressively heaver as it descends. The products heaver than the net overhead are obtained by withdrawing portions of the internal reflu( stream. The endpoint of a sidecut will depend on the %uantity withdrawn. &f the sidecut withdrawal rate is increased, the e(tra product is material which was formerly flowing down the fractionators as internal refle(. Since the internal reflu( below the draw off is reduced, heavier vapors can now rise to that point and result in a heavier product. hanging the drawoff rate is the manner in which sidecuts are !ept on end point specifications. Temperature of the drawoff dec!s is a fair indication of the endpoint of the product drawn at that point and an e(perienced operator may vary his drawoff rate to hold a constant dec! temperature and therefore a specification product. The degree of fractionation between cuts is generally udged by measuring the number of degrees centigrade between the 758 point of the lighter product and the 58 point of the heaving product. 2Some people use &9: and "9: but the &9: varies with stripping. The gap between gasoline and !erosene should be about 5* while between !erosene and light gas oil +* is normal. "ractionation can be improved by increasing the reflu( in the fractionators, which is done by raising the transfer temperature. There may be occasions when the internal reflu( necessary to achieve satisfactory fractionation between the heaver products is so great that if it was supplied from the top of the fractionators the upper dec!s would flood. An -&ntermediate irculating ;eflu( solves this problem. Some internal reflu( is withdrawn, pumped through a cooler, or e(changer, and returned colder a few dec!s higher in the column. This cold oil return condenses e(tra vapors to li%uid
and increases the internal reflu( below that point. &f we wish to improve fractionation between the light and heavy gas oil, we would increases the heater transfer temperature, which would cause the top reflu( to increases, then restore the top reflu( to its former rate by increasing the circulating reflu( rate. &t is to be noted that even through the heater transfer temperature is increased, the e(tra heat is recovered by e(changer with crude, and as a result the heater duty will only increases slightly. Sometimes fractionators will be -pulled dry. That is to say, the rate at which a product is being withdrawn is greater than the %uantity of internal reflu( in the fractionators. All the internal reflu( then flows to the stripper, the dec!s below the drawoff run dry, and therefore no fractionation ta!es place, while at the same time there is insufficient material to maintain the level in the stripper, and the product pump will tend to lose suction. &t is necessary then to either lower the product withdrawal rate or to increases the internal reflu( in the tower by raising the transfer temperature or by reducing the rate at which the ne(t lightest product is being withdrawn.
D. Pro&uct Stripping The flashed residue in the bottom of the fractionators and the sidecut products have been in contact with lighter boiling vapors. These vapors must be removed to meet flash point specifications and to drive the light ends into lighter and more valuable products. Steam, usually superheated steam, is used to strip these light ends.
T$e e**ect o* ste") Steam is fre%uently used in fractionating columns, strippers and sometimes in furnaces. &f the %uantity and temperature of the steam are !nown, its effect can be determined by calculating the partial pressure e(erted by the steam. This partial pressure is then subtracted from the total system pressure 2according to Dalton=s law and the calculations on the hydrocarbon e%uilibrium etc. are carried out at resulting lower pressure. &n other words steam has the same effect as lowering the pressure.
E#")p'e
et it be re%uired to calculate the top temperature of a fractionating column when the top product, of which the composition is given below, contains in addition 48 by wt of steam. Total pressure at the top of the column is 0 psia.
onse%uently the steam partial pressure amounts to #7.08 of 0 psia > +.?0 psia and the hydrocarbon partial pressure e%uals, therefore, 03+.?0 > #6.0 psia. The conventional dew point calculation as described before is now carried out at a pressure of #6.0 psia, disregarding the steam. )hen li%uid water is present on the top tray of the column, its vapour pressure must be subtracted from the total pressure. &n this case, however, the temperature must be !nown, and this has to be determined by trial and error.
E#")p'e 2 D"t" gi+en, Total pressure and temperature of a flashing system and the amount of hydrocarbon vapour to be flashed off per hour/ Re-uire&
Amount of steam needed for this operation.
9y trial and error the pressure at which the amount of vapour flashed off at the given temperature is found from the flash e%uation. "rom the relation@
The moles of steam can be found and conse%uently the re%uired weight of steam determined.
E. Des"'ting ost crude contain traces of salt which can decompose in the heater to from hydrochloric acid and cause corrosion of the fractionatorBs overhead e%uipment. &n order to remove the salt water is inected into the partially preheated crude and the stream is thoroughly mi(ed so that the water e(tracts practically all the salt from the oil. The mi(ture of oil and water is separated in a desalter, which is a large vessel in which may be accelerated by the addition of chemicals or by electrical devices. The salt laden water is automatically drained from the bottom of the desalter. &f the oil entering the desalter is not enough, it may be too viscous to permit proper mi(ing and complete separation of the water and the oil, and some of the water may be carried into the fractionators. &f, on the other hand, the oil is too hot, some vapori'ation may occur, and the resulting turbulence can result in improper separation of oil and water. The desalter temperature is therefore %uite critical, and normally a bypass is provided around at least one of the e(changers so that the temperature can be controlled. The optimum temperature depends upon the desalter pressure and the %uantity of light material in the crude, but is normally about #0* Cor3 #0* being lower for low pressure and light crudes. The average water inection rate is 58 of the charge. ;egular laboratory analyses will monitor the desalter performance, and the desalted crude should normally not contain more than one !ilogram of salt per, #,000 harrels of feed.
F. Pro&uct Dispos"' All products are cooled before being sent to storage. ight products should be below 60* to reduce vapor losses in storage, but heavier products need not be as cold. &f a product is being charged to another unit, there may be an advantage in sending it out hot. A product must never leave a unit at over #00* if there is any possibility of it entering a tan! with water bottoms. The hot oil could readily boil the water and blow the roof off.
2.2.2 Pro&uct Speci*ic"tions The composition of a distillation product is determined by performing laboratory tests on samples of that product. These test results are then compared with product specifications or standards that have been set for the product. &f the product is meeting specifications, column operations do not have to be adusted. 9ut, if the products are off3specification, a change in column operations must be made. Ene can see that the control of the tower is a rather complicated simultaneous solution of material and heat balances. At each draw we must draw the %uantity of material in the crude that boils within the specified boiling range. &f we draw too much, or too little, the product above or below will have to shift by that amount, thereby possibly putting it off specification. To stay on specifications the material balance must be maintained/ the %uantity of each product in the crude must be withdrawn at that particular draw tray. The second problem, the heat balance, must be solved so that the right product appears at the right tray with the proper degree of fractionation. The tower designers help with this problem by locating the draw trays according to the design crude and product slate. Dowever, cruder vary, product re%uirements vary, and the refinery must manipulate the heat and material balance to draw the right amount of product, with the proper distillation range or other product specifications. Specifications for typical products boiling ranges are shown in "igure .#. These products would come out of crude that has a crude assay T9: as shown in "igure .#+.
A. Initi"' %oi'ing Point (I%P The initial boiling point 2&9: of a petroleum product is that temperature at which the first drop of condensate is collected during a laboratory distillation test. &n a mi(ture of hydrocarbons, the first molecules to vapori'e are the light ones. So, the initial boiling3point test is used to chec! for light hydrocarbons that are present in a product. Suppose specifications on the bottom product call for an initial boiling point between #003##0F". ab tests show an &9: of 75F". Gou !now that light material boils at lower temperatures than heavy material. So, the bottom product in this e(ample contains material that is too light.
Figure 2.12 Accept"/'e Pro&ucts Cru&e Oi' AT. Toer
Figure 2.13 T4pic"' Cru&e Oi'
&n order to raise the &9: of a product, we must ma!e the product heavier. Ene way to raise the &9: of the bottom product is to strip some light components off with steam. Another way to raise the &9: is to increase the temperature of the feed or the reboiler temperature so more light components are vapori'ed.
%. En& %oi'ing Point (EP The end boiling point 2: of a petroleum product is the highest temperature reached during a laboratory distillation test, or the temperature at which the last
drop of li%uid vapori'es during the test. &n a mi(ture of hydrocarbons, the last molecules to vapori'e are the heavy ones. So, the end boiling point or end point test is used to chec! for heavy hydrocarbons that are present in a product. Specifications call for an overhead product with an : between #503#60F ". ab results indicate an : of #H0F ". Gou !now that heavy material boils at higher temperatures than light material. So, the top product does not meet specifications because it contains material that is too heavy. &n order to lower the : of a product, we must ma!e the product lighter. Ene way to reduce the : of the top product is to decrease the feed or reboiler temperature so that fewer heavy components vapori'e. Another way to bring : on3specification is to lower the top temperature by increasing the reflu( rate.
C. F'"s$ Point The temperature at which a petroleum product generates ignitable vapors is called the flash point. ight hydrocarbons tend to flash more easily than heavy hydrocarbons. A sample that contains traces of light hydrocarbons flashes at a lower temperature than a sample without these traces. A side draw product carries flash point specifications of #53#+0F ". The lab test shows a flash point of ##0F ". The sample contains material that is too light. )e can bring the product bac! to specification by decreasing the reflu( rate, or by using more stripping steam, or by increasing the reboiler temperature.
D. API 5r"+it4 Another specification for petroleum products is A:& gravity. A:& gravity is used to designate the IheavinessI or IlightnessI of products based on a scale in which #0F A:& gravity is the same weight as water. An oil that is e(actly the same weight as water would be measured at #0 F A:&. Jerosine is measured at about 4F A:&.
Suppose specifications call for a product with an A:& gravity of +03+5F. The product sample tests ?F. The product is too heavy.
E. Co'our :etroleum products are often color tested in the lab. ight hydrocarbons are light colored while heavy hydrocarbons are dar! in color. A light hydrocarbon product that is dar! colored probably contains too many heavy molecules. (cessive vapor rates can cause small drops of li%uid to become entrained in the vapor and be carried up the tower. ntrainment of heavy materials may contaminate the overhead product and ma!e it too dar! in color. Dydrocarbons will decompose and change color at very high temperatures. So, an off3color product may indicate that a tower is operating at too high a temperature.
.+ ;K$ $&ST&AT&EL E:;AT&EL "igure .#0 shows a diagram of a crude distillation column. 9efore the feed enters this column, it is heated by a series of heat e(changers and a furnace. Le(t, the feed is introduced into the column on the feed tray. ost of the lighter fractions immediately vapori'e, or flash and start rising up the tower. The heavier fractions remain in a li%uid state and wor! their way to the bottom of the column. Any light components that remain in the li%uid are removed with stripping steam. &n addition to overhead and bottom products, three other fractions are drawn from the side of the tower. ach of these fractions passes through a stripping column that uses steam to remove light components. Mapors from the stripping columns are reintroduced to the tower at a point above the draw3off tray.
2.3.1 Re*'u# R"te C$"nging Mapors at the top of the tower are cooled and condensed to li%uid. :art of this li%uid is returned to the tower as cooling reflu(. etBs consider the effect of ma!ing cut point changes in a crude unit by varying the reflu( rate. ;eflu( as a IcoolantI that removes heavy fractions by condensing them. Suppose the reflu( rate is increased from #,000 to #,00 barrels per hour, and the other tower operating conditions are held constant. This e(tra reflu( flowing down the tower causes the temperature on each tray to decrease. Some of the heavier hydrocarbons in the upward flowing vapors will now condense and fall bac! down the tower. The heaviest components are condensed out of the vapors on each tray in the column. As a result, the fraction formed on each tray will be lighter. The e(tra reflu( flowing down the tower reduces the temperature of the li%uid at the bottom of the column. )hen the bottom temperature decreases, the amount of light material vapori'ed out of the li%uid at the bottom of the tower is decreased. So the li%uid at the bottom of the column becomes lighter. Since the amount of product drawn to the stripper columns remains constant, increasing the reflu( rate causes more bottom product to be formed. 9ecause fewer vapors are now going
overhead, the amount of top product formed is decreased, or less. ighter overhead, bottom, and side draw products are produced by increasing the reflu( rate. &f we decrease the reflu( rate from #,000 barrels to ?00 barrels, the cut point changes are reversed. The temperature on each of the trays increases, and a higher tower temperature mean heavier products. So overhead, bottom, and side draw products become heavier. The amount of overhead product produced increases and the amount of bottom product formed decreases.
2.3.2 Fee& Te)per"ture C$"nging Low letBs consider how changing the temperature of the feed affects cut point changes in the crude unit. Suppose we raise the temperature of the feed and hold the reflu( rate and other tower variables constant. As the crude enters the column more of the feed is vapori'ed because of the higher temperature. Some of the heavy material that previously fell to the bottom of the tower is contained in these vapors. So the products formed above the feed tray become heavier. The increase in temperature causes the lightest materials in the li%uid at the bottom of the tower to boil out, so the bottom product becomes heavier. Since more vapor goes overhead when the temperature of the feed is increased, the amount of top product formed increases. 9ecause less li%uid falls to the bottom of the column when the feed temperature increases, the amount of bottom product formed decreases. So heavier overhead, bottom, and side draw products produced by increasing the feed temperature. &f we reduce the temperature of the feed, the cut point changes will again reverse. ess heavy material is vapori'ed when the crude enters the column, so the top and side draw products become lighter. The material that no longer vapori'es is actually lighter than the li%uid at the bottom of the column. )hen this material falls to the bottom of the column, the bottom product gets lighter. ighter overhead, bottom, and side draw products are produced by decreasing the temperature of the feed. )hen the feed temperature is reduced, the amount of top product produced decreases and the amount of bottom product formed increases.
2.3.3 Si&e Pro&uct (Dr" o** R"te C$"nging Another way to change the cut point in a crude column is to vary the amount of li%uid that is drawn to the stripper columns. Suppose we increase the !erosene draw by #00 barrels. )hen we open a stripper draw on the side of a crude unit less reflu( flows to the trays below the draw3off tray. ;educing the amount of reflu( going to the trays below the !erosene draw causes these trays to heat up. As the temperature on these trays increases more heavy material begins rising up the tower. 9ecause the temperature of the vapors rising to the !erosene draw is higher, the draw3off tray temperature will also be higher. Digh temperatures
produce heavy products, so increasing the !erosene draw ma!es this product heavier. The products formed below the !erosene draw also become heavier. Low suppose we want to ma!e the !erosene product heavier without changing the composition of the gas oil and reduced crude products. To do this we must increase the !erosene draw and at the same time not change the amount of reflu(, or li%uid on the gas oil tray and the trays below this point. )e can do this by decreasing the gas oil draw at the same time the !erosene draw is increased. )e draw less gas oil product to !eep the same amount of reflu( on the gas oil tray and the trays below this point. )ith the same amount of reflu( on these trays, the temperature profile in this part of the tower does not change and the gas oil and reduced crude products do not change composition. So to ma!e the !erosene product heavier without changing the composition of the gas oil and reduced crude products, the gas oil draw is decreased and the !erosene draw is increased. et=s reverse the situation and loo! at what happens when the !erosene draw is decreased. Low the amount of reflu( flowing to trays below the !erosene draw increases. An increase in reflu( causes more heavy components to condense out of the rising vapors because the temperature on the trays falls. ow temperatures produce light products, so reducing the !erosene draw results in a lighter !erosene product. The products formed below the !erosene draw also become lighter. Suppose we want a lighter !erosene product but do not want to change the composition of the gas oil and reduced crude products. Since there is more reflu( flowing down to the gas oil tray, we will have to increase the gas oil draw. &n this situation we are drawing more gas oil product to !eep the same amount of reflu( on the gas oil tray and the trays below this point. So to ma!e the !erosene product lighter without changing the composition of the gas oil and reduced crude products, the gas oil draw is increased and the !erosene draw is decreased. The composition of crude distillation products can be changed by changing, or varying the amount of li%uid that is drawn to a stripping column. Epening a stripper draw ma!es this product and products formed below this point heavier. losing a stripper draw ma!es this product and products below this tray lighter. To change the composition of a side draw product without affecting the composition of products below this point, you must adust two stripper draws. The first stripper draw adustment is made to change the composition of the product. The second adustment !eeps the composition of the other products from changing by maintaining the same amount of reflu(, or li%uid on these trays.
Read More:
Atmospheric Distillation -Part 1 Atmospheric DistillationPart 3 Atmospheric DistillationPart 4GLOSSARY Atmospheric Distillation- Part - Appen!i"
