Patent Application: US-22198587-A

Abstract:
the present invention is directed to an improvement in an alkylation process involving effluent refrigeration in which an enhanced boiling surface heat exchanger is utilized in the alkylation reaction zone so as to carry out the reaction at its optimum reaction temperature and at a positive compressor suction pressure .

Description:
referring to fig1 at 1 is a reactor shell equipped with an open - ended circulating tube 2 . in one end of the circulating tube is a propeller or pump impeller 3 and within the circulating tube is a chiller consisting of a tube bundle 4 having internal enhanced boiling surface provided with a distributing head 5 which encloses one end of the reactor . the impeller is mounted on a shaft 6 rotated through a reduction gear 7 by any suitable means such as an electric motor or steam turbine shown diagrammatically as 8 . circulation within the reactor is established by the impeller through the annular space between the shell and circulating tube 2 over the cooling tubes 4 and back to the impeller . olefinic hydrocarbons and isoparaffinic hydrocarbons , such as isobutane , are introduced to the system through lines 9 and 10 , respectively , being combined in feed line 11 prior to passage through heat exchanger 12 . recycled isobutane returned through line 13 is introduced into the feed in line 14 . fresh acid , such as sulfuric acid , is fed to the reactor through line 15 and recycle acid from settler 16 is returned through line 17 . the hydrocarbons supplied through lines 9 and 10 mixed with recycled isobutane added through line 13 are mixed in the reactor with the acid catalyst introduced through lines 15 , 17 and 18 . alkylation of the isoparaffinic hydrocarbons by the olefinic hydrocarbons takes place in the reactor 1 , while the mixture is being rapidly circulated and agitated by impeller 3 insuring a thorough and intimate mixture of the hydrocarbons with acid catalyst . the mixture of hydrocarbons and acid is discharged from the reactor through line 19 passing to the acid settler 16 where it is permitted to separate into a heavier acid phase and a hydrocarbon phase . the acid phase is recycled to the suction side of the pumping side of impeller 3 of the reactor through line 17 , while a portion of the acid separated in the settler may be discarded through the spent acid discharge line 20 to maintain a proper balance and proportioning of catalyst and reactants in the system . the hydrocarbon phase separated in the settler is discharged from the top of the settler through line 21 , and pressure upon these hydrocarbons is reduced by throttling at valve 22 , after which the liquid / vapor mixture is passed immediately through line 23 to the distributing head 5 of the reactor . the head 5 is divided by a partition 5a which causes the coolant to pass through the heat exchange elements or enhanced tube bundle 4 , then into the opposite side of the distributing head and out through line 24 . the temperature of the reaction will generally be less than 50 ° f ., and preferably in the range of from about 40 ° f . to 50 ° f ., and most preferably in the range of 40 ° to 45 ° f . upon passing valve 22 , pressure on the hydrocarbon phase of the effluent is reduced to the order of 0 psig to 10 psig , preferably 2 to 4 psig , causing a considerable portion of the lighter components of the effluent to vaporize and resulting in the cooling of the entire hydrocarbon effluent mixture . depending upon the pressure established within the tube bundle 4 of the reactor , the temperature of the hydrocarbon effluent phase will be reduced to be in the range of from about 15 ° to 25 ° f . by the reduction of pressure . this chilled effluent , which is a mixture of liquid and vapor , while passing through the enhanced chiller tubes 4 of the reactor absorbs the exothermic heat of alkylation reaction by indirect heat exchange resulting in vaporization of additional lighter components of the effluent . upon leaving the chiller tubes 4 of the reactor , the partially vaporized effluent passes from the opposite side of the circulating head through line 24 to suction trap 25 where the vapor and liquid portions of the effluent are separated . a liquid level control 26 manipulating valve 27 regulates the discharge of the liquid phase from the suction trap through line 28 . this liquid is returned by pump 29 through line 30 to heat exchanger 12 where it is is brought in heat exchange relation with the incoming feed stock . from the heat exchanger , the liquid passes through line 31 to the neutralization and fractionation steps diagrammatically shown as 32 . the vapors separated from the effluent in suction trap 25 pass out through line 33 to compressor 34 from which they are discharged through line 35 to condenser 36 where they are totally condensed . a portion of the condensate from condenser 36 is directed through lines 37 and 38 to isobutane flash drum 39 which is operated at the same pressure as suction trap 25 , both pressures being controlled by the suction pressure on compressor 34 which , in accordance with the present invention operates at a pressure which is greater than atmospheric and is equal to the pressure of the hydrocarbon phase after passing valve 22 . interposed in line 37 is a pressure reducing valve 40 which holds sufficient back pressure on the condenser 36 to make possible total condensation of the hydrocarbons . liquid hydrocarbons passing through valve 40 are thereby reduced in pressure causing partial vaporization and chilling of the hydrocarbons prior to their introduction into flash drum 39 . when propane is a component of any of the feed streams , a portion of the condensate withdrawn through line 37 is diverted through line 41 to the depropanizer of the fractionation section 32 . after depropanization , this stream is returned to the system through line 42 , pressure reducing valve 43 and lines 37 and 38 to the isobutane flash drum 39 . back pressure valve 43 in line 42 functions in the same manner as reducing valve 40 described above . the liquid hydrocarbons withdrawn from suction trap 25 and passed to fractionation are there separated into streams of propane , normal butane , light alkylate and alkylate bottoms . the product streams are normally removed from the system through lines 44 , 45 , 46 and 47 , respectively . the isobutane stream taken overhead from the deisobutanizer tower is recycled through line 48 , reduction valve 49 and line 38 to the isobutane flash drum from which it is directed to the reaction zone in reactor 1 . fresh isobutane feed to the system may also be brought in either through line 10 or through line 50 which connects through line 38 to the isobutane flash drum . all of the streams entering the isobutane drum 39 are subjected to reduced pressure established by the suction of the compressor and are thereby self - refrigerated . the vapors evolved in the isobutane flash drum by this self - refrigeration are passed through line 51 to the compressor , while the chilled liquid from the drum , principally isobutane , is directed through line 52 to pump 53 and then through lines 13 and 14 to the reactor . fig2 depicts a cross section of the enhanced chiller tubes in which outer surface 100 contacts the reaction mixture and inner surface 110 , containing the preferred enhancement of a porous layer , contacts the boiling fluid . the preferred enhanced tube for use in the alkylation contactor ranges in diameter from 0 . 75 to 1 . 25 inch , with 1 . 0 inch being most common . the tube wall thickness ranges in thickness from 0 . 08 to 0 . 15 inch , with 0 . 10 being preferred . although the tube material may be comprised of any thermally conductive material , ferrous or stainless alloy is commonly used , the preferred material being ordinary carbon steel . a comparison is made of two process conditions for the sulfuric acid alkylation reaction , one using contactor bundles with high flux tubing , and the other with conventional tubes . in this example , a typical 10 , 000 barrel per stream per day ( bpsd ) in a plant having four contactors operating in parallel with two settling tanks are employed . each settler is fed by two contactors . the total olefin and isobutane feed flow is 66 , 000 bpsd , while the refrigeration compressor ( approximately 6 , 000 hp ) has a suction pressure fixed at 17 psia , to avoid vacuum operation . the details of the comparison are shown in table i below . each of the four contactors has a volume of 13 , 000 gallons , and a chiller bundle with a heat transfer area of 8 , 500 ft 2 . the volume of each of the acid settler is 92 , 000 gallons . a high rate of internal circulation of the emulsion is maintained by impellers with a total power consumption of 1 , 200 hp . the total heat of reaction , including energy imparted to the fluid , is 41 million btu / hr . hydrocarbon from the settler is throttled and fed to the boiling side of the chiller . to remove the heat of reaction , the boiling flow , at 540 , 000 lb / hr , is approximately 50 % vaporized . the boiling stream containing about 18 % c 8 alkylate , enters at 25 ° f . and exits at 35 ° f . in order to maintain enough temperature difference to transfer the heat of reaction , consistent with an overall heat transfer coefficient of about 100 ( btu / hr )/( ft 2 ° f .) when utilizing high flux tubing , and 50 ( btu / hr )/( ft 2 ° f .) when using a bare tube bundle , the reactor temperature must be maintained at 43 ° f . and 55 ° f ., respectively . as seen in table i , the relative octane number ( ron ) increases about 0 . 7 points , due to the lower reactor temperature , for the case using the high flux tube bundle . in the examples , the reactor volume per bpsd of alkylate product is that typically used in commercial practice , namely , 4 to 4 . 5 gallons . case 3 in table i illustrates the effect of attempting to lower the reactor temperature by adding more contactors . since the overall coefficient for the high flux tube bundle is about two times higher than for the bare tubes , it is assumed that doubling the number of bare tube contactors would reduce the reactor temperature to 43 ° f ., the value achieved by the high flux tubes . in reality , several additional effects occur which tend to make the necessary area increase even greater than two fold . the first effect is that an additional 1200 hp mechanical energy enters the circulating fluid from the impellors and must be removed . this increases the heat load to about 44 million btu / hr . since the boiling flow is fixed , more must vaporize which increases the outlet temperature . the second effect is that the reactor space velocity decreases which tends to increase the conversion or yield of c 8 alkylate , and thus the boiling temperature of the mixture . a third effect is that the total boiling flow of 540 , 000 lb / hr is now distributed over eight or more bundles rather than four , which at least halves the tube side velocity and reduces the tube side heat transfer coefficient . the net result is that considerably more than double the number of reactors must be added which is not feasible in an existing plant . table i__________________________________________________________________________example of improved 10 , 000 bpsd alkylation process using high flux case 3 case 2 conv &# 39 ; t . tubes case 1 conventional reactor atitem high flux tubes temp - case 1 remarks case 3__________________________________________________________________________alkylate capacity bpsd 10 , 000 10 , 000 10 , 000 fixedtota1 feed flow bpsd 66 , 000 66 , 000 66 , 000 fixedno . chillers in parallel 4 4 8 + highvolume of each chiller 13 , 000 13 , 000 13 , 000 fixedgal . area ( heat transfer / 8 , 500 8 , 500 8 , 500 fixedchiller ft .. sup . 2no . acid settlers 2 2 4 doubledvolume of settler , gal . 92 , 000 92 , 000 92 , 000 fixedspace velocity , bpsd feed / 53 53 26 equals ( 42 gal / bbl ) barrels reactor volume ( 66 , 000 )/( 13 , 000 ) × ( no . chillers )% alkylate in boiling feed 18 18 20 %+ because space velocity decreasedtotal flow to boiling 540 , 000 540 , 000 540 , 000 fixedside , #/ hr . amount vaporized , 50 50 approx . 60 + increases becauseboiling side % heat load increasedtotal contactors heat 41 . 0 41 . 0 44 + duty increasedduty mmbtu because of increased agitator poweragitator power ( total ) hp 1 , 200 1 , 200 2 , 400 + increased because # of reactors increasedboiling inlet temp . ° f . 25 25 28 + increased because alkylate increasedboiling outlet temp . ° f . 35 35 40 + increased because alkylate and amount of vapor increasedcompressor suction psia 17 17 17 vacuum operation avoidedreactor temp . ° f . 43 55 43 not feasiblemean . sup . δ t ° f . 12 . 3 24 . 6 & lt ; 12 ° decreased because of alk . & amp ; % vapor increaseoverall coeff ., ( design ) 98 49 & lt ; 49 because of decreased boiling flow / chillerrelative octane # increase 0 . 72 0 0 . 72 only if reactor is at 43 ° f . reactor volume / bpsd used 5 . 2 5 . 2 10 . 4 + at least doubledreactor volume / bpsd 4 . 0 - 4 . 5 4 . 0 - 4 . 5trade practicereactor area / ratio 3 . 4 3 . 4 6 . 8 + ft .. sup . 2 / bpsdresidence time , sec . 1 , 630 1 , 630 3 , 260 + 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