US2834716A

US2834716A - Separation of hydrocarbons

Info

Publication number: US2834716A
Authority: US
Priority date: 1954-05-19
Filing date: 1955-03-29
Publication date: 1958-05-13
Anticipated expiration: 1975-05-13
Also published as: GB778699A; FR1104947A; NL97868C; BE537154A; GB778700A

Description

May 13, 1958 Filed March 29, 1955 A. CHAMPAGNAT ET'AL SEPARATION OF HYDROCARBONS 2 Sheets-Sheet 1 ORIFICE- Mlxae DEHYDQATOQ ompncs. 1 [4 DEcANTeR 504/ 2 56 DECANTE-R 59 (9 2 3 2. A m m m 75 ELIO ALFPD Gw PAaA/AT CHARLES 1 NET BY-W ZZ ms .21

14 TTOF/VEYS SEPARATION OF HYDROCARBONS Alfred Champagnat and Charles Vernet, Paris, France, assignors to The British Petroleum Company, Limited, London, England, a British joint-stock corporation Application March 29, 1955, Serial No. 497,642

Claims priority, application France May 19, 1954 17 Claims. (Cl. 196-17) This invention relates to a process for the separation of hydrocarbons. More particularly, the invention relates to a process for the extractive crystallization of mineral oil fractions, by means of urea.

It is well-known that urea forms crystalline solid adducts with straight chain and slightly branched chain hydrocarbons but does not form solid compounds with naphthenes, high branched compounds or aromatics. By the use of urea a method has been developed for refining petroleum fractions by resolution of the fractions into chemical types by forming the above solid compounds, removing the remaining liquid phase (known as the urea adduction raftinate), decomposing the solid compounds and recovering from the product the liberated hydrocarbons (known as the urea adduction extract).

It is also known that when mixtures of normal parafiins and normal olefins are subject to urea adduction that by reasons of the different reactivity of these types of compounds towards urea, a degree of separation may be ef fected, the parafims tending to pass into the extract and the olefins of similar boiling range tending to pass into the raifinate. Similarly a degree of separation may be effected between normal mono-olefins and normal diolefins, by urea adduction, since the former compounds tend to pass into the extract and the latter compounds tend to pass into the rafiinate.

Furthermore, it is known that the presence of methanol or other low molecular weight alcohols, in admixture with urea and fractions undergoing treatment, ac-

elerates the rate of formation of the solid derivatives.

it has also been stated in the prior art that mixtures of methanol and water may be employed as the activating agent.

According to some proposals the urea is employed in solution in a solvent or solvent mixture which, in general, is not miscible with hydrocarbons. Usually, operating in this manner, the solution of urea must be saturated at the temperature of the reaction with hydrocarbons and there must be a certain quantity of urea present in addition to that necessary for saturation (at the temperature of the reaction), this excess urea constituting the available urea for reaction with the normal and slightly branched chain paraflins to produce the solid adduct. Under these conditions, the reaction between the mixture of hydrocarbons of the petroleum fraction to be treated and the urea, partially in solution, gives rise to the formation of a dense slurry comprising an emulsion, generally of the oil in water type, and consisting of a continuous liquid-phase, comprising the saturated solution of urea, and a discontinuous liquid phase, formed of hydrocarbons that have not reacted with the urea. The emulsion carries in suspension two solid phases, that is, solid adduct crystals and surplus urea crystals which have not formed an adduct.

The adduct obtained usually takes the form of a mass of interlaced crystalline needles, of very large apparent volume. Thus the separation of rafiinate from the thick,

thixotropic complex mixture has been found very diflicult.

In certain processes filtration is used, which leaves on the filter a mass of adduct crystals and surplus urea crystals. However, this mass of crystals retains a large proportion of rafiinate, which has to be eliminated by washing with the aid of a solvent, if it is desired to obtain a good output and good selectivity in separation. These filtration and washing operations usually call for laborious and costly apparatusv Other processes have recourse to decantation for separating the rafiinate from the aforementioned adduct containing slurry. Nevertheless, this slurry is for the most part very stable and thus, although it is often possible to separate, by simple decantation, a major proportion of the raffinate, the remainder thereof is retained in the phase comprising urea solution (which contains also the surplus adduct and urea crystals) thereby adversely effecting the selectivity of the process.

Methods for assisting the separation of the raflinate from the slurry are described in the specifications filed in respect to British Patent Nos. 704,439 and 716,068. In British Patent 704,439 there is described the use of a ternary solvent which is non-miscible with the hydrocarbons, comprising in its preferred form water, methanol an ethylene glycol. In Belgian Patent No. 510,286, there is described the operation of a process using urea containing 0.2 to 5% (preferably 1%) by weight of biuret. The use of this ternary solvent and of urea containing biuret permits the formation of crystals of urea and complex of very small dimensions, so that the droplets of raffinate in suspension in the slurry join and are decanted more readily. Also according to British Patent No. 704,439, when using the ternary solvents described therein, the decantation of the rafiinate is completed by the addition to the slurry of a small quantity of aqueous methanol derived by distilling part of the urea solution.

It is an object of the present invention to provide an improved process for. the extractive crystallization of hydrocarbon mixtures by the use of urea solutions in which the raffinate is separated in convenient manner from the adduction product. It is a further object to provide a process for the separation of normal paraflins from mineral oil fractions. It is a further object to provide a new and useful apparatus for effecting phase separations.

As hereinbefore described, the complex slurry obtained takes the form of a thick, thixotropic mass, the thixotropy of the slurry being caused by the presence in great abundance of the adduct crystals and also by the presence of crystals of urea in excess of saturation at the operating temperature. These adduct crystals and urea crystals have a marked tendency to form, by becoming entangled, suiiiciently solid structures to offer opposition to the decantation of the droplets of unreacted hydrocarbons (that is, the rafiinate) present in the slurry. It has now been found that under certain conditions as hereinafter described, these tangled crystalline structures tend to disintegrate, thereby facilitating the separation of the liquid phases by decantation.

According to the present invention, there is provided a process for the extractive crystallization of a hydrocarbon mixture comprising hydrocarbons of different chemical type as hereinbefore described, which comprises treating said hydrocarbon mixture with a solution comprising urea whereby there is formed a complex slurry, said slurry consisting of an emulsion of a liquid phase comprising a urea adduction raflinate and a liquid phase comprising urea solution, said emulsion containing, in suspension, a solid urea adduct with or without solid urea, thereafter separating at least part of the urea adduction raffinate from the complex slurry and prior to and/or during said separation, subjecting said slurry to the action of mechanical vibrations whereby there is formed an upper liquid phase comprising rafiinate and whereby the solid materials pass into the lower liquid phase comprising urea solution, the separation of the upper phase being effected by decantation, and thereafter recovering the urea adduction extract from the remaining solid/liquid phase.

Preferably the process is applied to the treatment of hydrocarbon mixtures comprising normal parafiins in admixture with other hydrocarbons and wherein the normal paraffins are recovered in the extract. The process of the invention has been found to be particularly suitable for the treatment of mineral oil fractions, for example, petroleum distillation fractions for the recovery of normal parafiins.

In the decantation stage, wherein the rairinate is separated from the urea solution phase, it has been found that the conditions of vibration may be adjusted so as to separate rapidly almost the whole of the raflinate retained by the crystals. A

Preferably the mechanical vibrations imparted to the slurry have a frequency in the range 100 to 2000 per minute. Preferably the amplitude of said vibrations lie within the range 1 mm. to 5 cms. More particularly it is preferred that the frequency is of the order of 1000 per minute and the amplitude of the order of mm.

If desired, the decantation of the raffinate may be carried out in a tank equipped with movable bafiles having in association therewith means for imparting mechanical vibration to said baffles. Preferably, however, the decantation of the rafiinate is carried out in a tank supported in resilient manner, said tank having, in association therewith, means for imparting mechanical vibration thereto.

Suitably the tank is supported, at least in part by springs. Suitably the mechanical vibration is imparted by the action of an eccentrically loaded flywheel.

Preferably the extract hydrocarbons, after separation of the rafiinate, are recovered by heating the urea solution with the adduct held in suspension in order to destroy said adduct, thereby releasing the urea adducted hydrocarbons and regenerating urea. In this operation the droplets of raflinate, that had remained fixed to the crystals of the adduct, mix with the hydrocarbons obtained by the destruction of said adduct. In the treatment of mineral oil fractions, the extract, separated by decantation from the surface of the solution of regenerated urea, therefore contains some highly branchedchained parafiin hydrocarbons, naphthenes and/or aromatic hydrocarbons, derived from these droplets and constituting an impurity in respect of the normal hydrocarbons. When the urea solvent is suitably chosen, the extract hydrocarbons obtained are usually in the form of a mixture of about 70% *normal paraffins and 30% other hydrocarbons. This degree of purity, which is, in general, very much greater than that obtainable by the industrial separation processes at present in use in the pertoleum industry, makes possible, according to the invention, the provision of feedstocks for the manufacture of a large number of chemical derivatives.

According to a further feature of the present invention, a rafiinate fraction having a higher degree of freedom from normal paraffins is obtained. Thus according to this further feature, after separating, with the assistance of mechanical vibration, the major part of the railinate present in the complex slurry, the separated raffinate is also treated with a solution comprising urea whereby there is formed a second complex slurry of the type hereinbefore described, the process thereafter comprises separating the major part of the urea adduction raflinate from the second complex slurry and prior to and/or during said separation, subjecting said second slurry to the action of mechanical vibrations of the type hereinbefore described.

, According to a still further feature of the presentim.

vention normal paratfins of an improved degree of purity are obtained. Thus according to this further feature, after separating the major part of the urea adduction raiiinate with the assistance of mechanical vibrations, a quantity of a washing solvent (which is a solvent for the ratrinate) is added to the remaining complex slurry. Thereafter a solvent-containing slurry is formed of this mixture by any suitable method, such as mechanical stirring.

The stage thus reached is analogous to that which preceded separation of raiiinate from the complex slurry with the assistance of mechanical vibrations. Thus the solvent incorporated in the emulsion in the phase consisting of urea solution and adduct crystals is separable under the same conditions as was the rafimate.

Thus the solvent treated slurry containing, in suspension, the solid urea adduct in the presence or absence of solid urea, is treated for the separation of the major part of the washing solvent from the solvent containing slurry and, prior to and/or during said separation, the slurry is subjected to the action of mechanical vibrations of a frequency in the range -2000 per minute and of an amplitude of at least 1 mm. whereby the solid materials in suspension pass into the lower liquid phase, comprising urea solution, the separation of the upper phase from the remaining product being effected by decantation, and thereafter the urea adduction extract recovered from the remaining solid liquid phase.

In this manner it is possible to effect a washing treatment upon the crystals of the complex in suspension in the urea solution. This washing removes the major part of the microscopic droplets of the rafiinate which had not been separated by the first operation of decantation.

A washing solvent can be formed of any suitable mixture of hydrocarbons, but preferably a petroleum fraction is employed. However, as the washing operation is aimed at increasing the purity of the normal parafiins resulting from the destruction of the adduct, it is usually desirable to select a solvent having a distillation range that is distinct from that of the extract of normal parafiins to be obtained since, after washing the adduct by the petroleum fraction, and after separating the fraction from the remainder of the solvent slurry, microscopic droplets of this solvent fraction remains fixed to the crystals of the adduct.

During the thermal decomposition of the adduct in suspension in the urea solution, whereby the extract is freed from urea, the droplets of solvent pass into the extract phase and constitute an impurity in the normal paraflins. When the washing agent used is a petroleum fraction of different volatility, preferably a fraction that is more volatile than the fraction undergoing treatment, the final separation of the small amount of washing agent retained by the extract is easily achieved by distillation.

It has been stated that, in general, the process forming the subject of the invention yields, on an average, an extract consisting of a mixture of 70 partsby weight of normal parafiins and 30 parts by weight of raffinate hydrocarbons. By use of the washing stage hereinbefore described, an extract of even higher normal parafiins content is obtained.

The quantity of petroleum solvent required for the Washing stage is relatively small and in a petroleum refinery the used washing solvent, together with entrained impurities, may be incorporated in fuels or other commercial products manufactured in the refinery. Alternatively the impurities may be separated from this washing by solvent distillation.

There is thus achieved, according to the present invention, an extractive crystallization process which may be operated in continuous manner and which involves the following steps:

(1) Reacting the mixture of hydrocarbons with a urea solution.

(2) Subjectingthe product to mechanical vibration and decanting to remove the raflinate.

(3) Optionally, adding to the remaining phase a washing solvent subjecting the mixture to mechanical vibration' and decanting. to remove the bulk of the washing solvent.

(4) Recovering the extract hydrocarbons, preferably by heating the urea solution, containing adduct crystals in suspension, to regenerate the urea and release the extract hydrocarbons and separating the extract hydro carbon by simple decantation.

The operation of this process does not necessitate the use of expensive equipment, such as rotary filters, centrifuges and provides a means of obtaining, on the one hand, a rafiinate very largely free from normal paraffins and, on the other hand, an extract consisting predominately of normal paraflins.

Preferably the process according to the invention is applied to mixture of hydrocarbons, e. g. petroleum fractions boiling within the range 80-350 C.

Preferably there is employed as solvent for urea in the formation of urea solution, the solvents particularly described in British patent specification No. 704,439.

Preferably also the urea employed contains 0.2 to 5% .by weight of biuret, as described in Belgian patent specification No. 510,286.

The solvent employed for forming the urea solution preferably comprises water and/or methanol. A particularly suitable solvent comprises a major proportion of methanol, a minor proportion of water and a'minor proportion of ethylene glycol.

It has been found that best results are obtained by using a solvent consisting of a mixture of 75' parts by weight of 80% methanol and parts by weight of ethylene lycol. Preferably the urea dissolved in this solvent contains 1% by weight biuret.

it is preferred that the mixing of hydrocarbon feedstock and urea solution be carried out by pumping these materials in a closed circuit to which feedstock and urea solution are fed at different points on the circuit and from which the product is withdrawn at a further point on the circuit. Preferably also, the flow rate (by volume) of circulation of the reactants in the closed circuit substantially exceeds the total rate of feed of the feedstock and urea solution. Preferably the ratio of volume flow rate in the circuit to total volume feed rate of feedstock and urea solution lies in the range 2:1 to 20: Land usually about 10:1, flow rate in the circuit being measured at a point immediately prior to the take-off point of product. The closed circuit should include a heat exchanger for maintaining the desired reaction temperature. In general, this heat exchanger will function as a cooler.

If desired two or more closed circuits, of the type described above, may be employed in series for mixing the hydrocarbon feedstock and urea solution. desired, one or more stirred vessels may be inserted in the lines connecting closed circuits or leading from the final closed circuit.

Similarly when the process is operated with the inclusion of the step of washing the complex mixture remaining after removal of rarfinate, the mixing of solvent and urea solution may be carried out in one or more closed circuits with or Without the additional use of stirred vessels as described above.

According to one embodiment of the process of this invention, said process comprises fractionally distilling a crude petroleum or a petroleum distillate fraction in a fractionation column with removal of a side-stream consisting of a relatively lower boiling fraction and with separate removal of a relatively higher boiling fraction comprising hydrocarbons capable of separationv by urea adduction, feeding said higher boiling fraction to a urea adduction stage wherein said hydrocarbon mixture is treated with a solution comprising urea whereby there is formed a complex slurry, said slurry consisting of an emulsion of Also, if

a liquid phase comprising a urea adduction raifinate and a liquid phase comprising urea solution, saidemulsion containing, in suspension, a solid urea adduct with or without solid urea, thereafter separating at least part of the urea adduction rafiinate from the complex slurry by decantation, mixing the remaining complex slurry with a washing solvent comprising at least part of said lower boiling fraction, separating the major part of the washing solvent from the solvent containing slurry by decantation, and recovering the extract from the remaining liquid phase, the solvent decanted from the solvent containing slurry being fed back to the fractionating column at a level below that at which the said side-stream is withdrawn.

Slurry and/or of the solvent from the solvent containing slurry may be assisted by recourse to the method described in the specification of our copending British patent. The decantation of the rafiinate and/or of the solvent is effected by the application, to the mixture prior to or during separation, of mechanical vibrations as hereinbefore described.

The invention also comprises within its scope the provisionof an apparatus for the resolution, into separate liquid layers, of an emulsion containing a mass of entangled crystals in suspension,-said apparatus comprising. a vessel having means for the removal of an upper liquid layer, means for the removal of a lower liquid layer and means for imparting to said vessel mechanical vibrations having a frequency in the range to 2000 per minute.

Preferably the apparatus comprises a tank supported in resilient manner, and mechanically associated with an eccentrically loaded flywheel capable of imparting the said vibrations. Preferably said tank. is. equipped with vertical baffles to increase the overall route to be taken by the urea adduct crystalline mass in its passage through said tank. Preferably the tank has its inlet at one end and itsoutlet at the opposite end, said tank having a rectangular base to which is secured a plurality of parallel vertical bafiles fixed transversally to the overall direction of flow of the crystalline mass and, alternating therewith vertical bafiles attached to the side walls in such manner that the crystalline mass is constrained to pass alternately to and from the centre line of the tank to increase the overall route to be taken by the urea adduct crystalline mass in its passage through said tank.

The invention is illustrated but in no way limited with reference to the accompanying Figures 1-6 wherein:

Figure 1 is a flow sheet of an embodiment of the present invention.

Figure 2 is a plan view of a battle tank serving to separate the crystal slurry from the raflinate.

Figure 3 is a cross section across llilll' of Figure 2.

Figure 4 is an end view of the vat of Figures 2 and 3.

Figure 5 is a cross section of an orifice mixer as used in the apparatus described with reference to Figures 1 and 6.

Figure 6 is a ilow sheet of a second embodiment of the invention.

The circulation schematically represented in Figure 1 is particularly suitable for separating a petroleum fraction from the normal hydrocarbons and especially front the normal parafiins which it contains, with for example the production of a heavy kerosene to be used as fuel in engines working on the principle of jet propulsion. in that case the extractive crystallization by means of urea aims at sufliciently lowering the initial temperature of solidification of kerosene so as toprevent its congelation.

at high altitudes. it is mainly a matter of obtaining a rafiinate of sutficieutly low congelation temperature, and the by-product of the extractive crystallization in the form of the extract of normal aliphatic hydrocarbons need not be very pure. The invention is-described hereinafter with particular reference vto the treatment of a petroleum 7 fraction boiling in the kerosene range but is clearly in no way limited to the use of this feedstock.

The kerosene arrives through pipe line 1 and is passed by pump 2 through pipe line 3 into an apparatus 4 in which it is dehydrated. Apparatus 4 may be for example, a salt filter. The kerosene, deprived of its water, arrives through pipe line 5 in the first reaction circuit A. Circuit A comprises

pipe lines

6, 7, 8, 9, and 11; the pump 12 arranged between pipe lines 7 and 8 serving to circulate through the various pipe lines: an orifice mixer 13, shown in detail in Figure 5, arranged between pipe lines 8 and 9; lastly a cooler 14 arranged between

pipe lines

10 and 11. The circuit A is fed with kerosene by pipe line 5, as previously indicated, and with a urea solution by pipe line 15. The urea solution may be of the kind described in the above mentioned British patent Nos. 704,439 and 716,068.

The orifice mixer 13 consists of two flanged tubes 71 and 72. having bolted, therebetween, an annular plate 73 supporting a concentric tube 74 having a number of orifices as illustrated at 75.

The temperature of circuit A is maintained at about to 18 C. by means of cooler 14 when kerosene is subjected to the treatment. Kerosene and the urea solution arrive at separate points of the circuit A in order to avoid kerosene coming into contact with a fresh solution of urea and thereby preventing the sudden formation of large quantities of adduct crystals with the consequent danger of choking the pipe lines. For this purpose a pipe line 6 is provided between the points Where pipe lines 5 and 15 join the circuit A. Pump 12 disperses the urea solution and the kerosene in the large excess of material circulating in the closed circuit, thereby producing a thixotropic slurry; the intimate contact is achieved by orifice mixer 13.

The delivery of pump 12 is much greater than the sum of deliveries through pipe lines 5 and 15 feeding into circuit A so that the liquids introduced by pipe lines 5 and 15 meet in circuit A considerable excess of slurry containing the adduct already formed. In this manner, as previously indicated, the choking of pipe lines is avoided, and an intimate contact and very rapid reaction is achieved.

By pipe line 16 there is withdrawn from circuit A a quantity of slurry containing the adduct crystals which is equivalent to the sum of fresh kerosene fed throughpipe line 5 and of urea solution fed through pipe line 15.

It should be made clear that the slurry of the adduct consists of a liquid phase, namely the urea solution, in which the adduct crystals, possibly together with crystals of solid urea, are dispersed in great abundance. In the whole are suspended droplets of unreacted hydrocarbons (urea adduction raflinate). The heterogeneous system is very thick and thixotropic and the interlaced crystals otter mechanical resistance to the separation by gravity of the droplets of raflinate enclosed within them.

The slurry is passed through pipe line 16 to a vibrating decanter B which is shown in more detail in Figures 2, 3 and 4. The decanter B comprises a vibrating vat 17 in which are arranged continuous baffles 18 by which the slurry is forced to follow a winding path in the direction of the arrows 19 and by which the vibrations to which the vat is subjected are transmitted to said slurry.

As the slurry proceeds through the vat (from left to right in Figures 2 and 3) the rafiiuate 20 separates and rises to the surface and is withdrawn through pipe line 21. The lower phase 22, composed of a slurry of urea solution and suspended adduct crystals, passes into compartment 23 and thence to compartment 24 from which it is withdrawn through pipe line 25. The overflow 26 of compartment 23 to compartment 24 is regulated at a fixed height so that a constant level of the surface 27, separating the two phases, is maintained 'at the outlet.

The vibration may be elfected by typical devices, for example by means of a vibrating platform 28 on which the decanter vat 17 is firmly fixed. This platform 28 is agitated" by vertical springs 29 and horizontal springs 30 the tension of which may be regulated. The vibrations are produced by a flywheel 31 provided with an adjustable eccentric loading 32. and rotated by a motor 33 with a detachable coupling device 34. The flywheel 31 rotates at a speed which may be controlled by speed regulator 35; the axle 36 of flywheel 31 is supported by the two bearings 37 and transmits an alternating movement to the vibrating platform 28. The pipe connections between vibrating vat 17 and the fixed other parts are made by flexible tubings attached to the pipe lines serving for feeding and withdrawal.

It has been established that decantation by a vat such as represented in Figures 2-4 is very elfective. Thus, in general, a residence time of about 10 minutes, and often less, in the vibrating vat is suflicient for freeing the slurry of adduct crystals completely from the mechanically separable rafiinate enclosed therein. This separation is sufiicient if the process of extractive crystallization is not intended to produce an extract of normal paratfins of great purity. However, in spite of the most effective decantation by vibration, there always remain microscopic droplets of raffinate adhering to the adduct crystals in the solution of urea which cannot be separated by mechanical action. In the description, hereinafter, of the second embodiment of the invention, means for removing these droplets will be described.

In short, the vibrating decanter B separates theslurry arriving through pipe line 16 into a raflinate phase which flows through pipe line 21 into tank 38, and into a slurry of adduct crystals which leaves by channel 25.

The raflinate 39, intermediately stored in tank 38, still contains a small proportion of normal aliphatic hydrocarbons due to the fact that one reaction stage (circuit A) is insufiicient to completely eliminate the normal aliphatic hydrocarbons. The raflfinate 39 may be subjected to a second treatment with urea solution. To this end, a pump 40 draws it through pipe line 41 to pass it through pipe line 42 into a circuit C which is identical with circuit A and comprises

pipe lines

6', 7, 8', 9, 10', and 11, a pump 12, an orifice mixer 13, as shown in detail in Figure 5 and cooler 14.

The circuit C is operated in the same manner as circuit A; the hydrocarbons are fed through pipe line 42 and the urea solution by pipe line 43. The slurry of adduct crystals and rafiinate is delivered through pipe line 44. it is advantageous to maintain the reaction temperature in circuit C slightly lower than that prevailing in circuit A so as to allow those normal paraflins which do not easily combine with urea to form adducts.

The slurry leaving the second circuit C by pipe line 44 passes into a second vibrating decanter D similar to the vibrating decanter B shown in Figures 24. From the vibrating decanter D a rafiinate phase is withdrawn through pipe line 45. The slurry of adduct crystals is withdrawn through pipe line 46. The raflinate passes through pipe line 45 into a tank 47 from which it is withdrawn through pipe line 48. This ralfinate will usually be a kerosene of sutficiently low freezing point for use as fuel in aviation gas turbine engines.

The slurry of adduct crystals fed through pipe line 46, and that fed through pipe line 25, are blended in pipe line 49. Pump 50 draws the whole slurry through pipe line 49 and passes it through pipe line 51 to a steam heater 52 where the temperature of the adduct is raised to about 70 C. in order to ensure the decomposition of said adducts and to obtain, on the one hand, an extract phase consisting of straight-chain aliphatic hydrocarbons and, on the other hand, a regenerated solution of urea. The decomposed adducts are fed through pipe line 53 into decanter 54 in which the layers 55 and 56 are formed. The extract hydrocarbons 56 are withdrawn by pipe line 57 while the regenerated urea solution 55 flows out through pipe line 58 to bedrawn off by pump 59 .by which it is passed to pipe line 60 and thence to the two pipe-lines and 43, by which urea solution is fed to the circuits- A and C respectively. I

It may be noted that in the arrangement represented in Figure 1 the hydrocarbon fraction (kerosene) arriving through pipe line 1 is treated with urea solution in two stages (circuit A and circuit C). By feeding fresh urea solution into each circuit it is possible to separatefrom the feed stock the maximum amount of normal parafiins and in consequence to obtain kerosene of the lowest possible freezing point.

With the apparatus represented in Figure l the following results have been obtained:

By treating a heavy kerosene, boiling range 200-290 C., with a urea solution containing, by weight Percent Urea (with 1% of biuret) 42 Methanol 33 Ethylene glycol 12.5 Water 12.5

Feed Rafiinate Extract stock S. .G. at C 0.812 0.823 0.779 Freezing Point (I. P. 16/44) C -27 60 Yield "percent. 75

In Figure 6 is represented a second embodiment of the invention which is particularly intended to produce normal paraffins of very high purity. In this case the important consideration is not the separation from the feedstock of the maximum quantity of normal parafiins but rather to obtain normal paraffins of very high purity. To achieve this end a different arrangement of the reaction circuits and vibrating decanters, from that hitherto described, is desirable. It is, moreover, desirable to wash the adducts with a solvent which is more volatile than the treated feedstock. If a kerosene fraction is treated, the reaction and regeneration may be carried out at the temperatures described with reference to the plant illustrated in Figure 1.

Kerosene is fed through pipe line 101 by pump 102 and thence through pipe line 103 and a dehydration device 104 such as a salt filter. The dry kerosene is then introduced by pipe line 105 into a first reaction circuit A, similar to circuit A of Figure 1, comprising in addition to

pipes

106, 107, 108, 169, 110, and 111, a pump 112, an orifice mixer 113 (as illustrated in Figure 5) and a cooler 114. The urea solution is fed to the circuit A through pipe line 115 and thus enters circuit A at a separate point from the kerosene. The slurry formed in the first circuit A is passed through pipe line 116 into a second circuit C, identical with circuit A, containing

pipes

106', 107, 108, 109', 110, 111, pump 112, an orifice mixer 113 (as illustrated in Figure 5) and cooler 114. The purpose of this second reaction circuit C is to complete the reaction started in the first circuit A at a slightly lower temperature.

The procedure differs from that described with reference to Figure l in that the whole urea solution is passed into the first reaction circuit (circuit A) and that the raffinate and the slurry of complex crystals are not separated between the first circuit A and the second circuit C. v

The slurry formed by the reaction in circuit C is passed through pipe line 110 and stirred vessel 179 into the vibrating decanter B of the type previously described with reference to Figures 2-4. The rafiinate 120 decantedfrom the surface-overflows through pipe 1ine-121 into tank 138. The slurry of adduct crystals which-still contains a small quantity of raffinate is takenthrough pipe line and pump 161 which also receives through pipe line 162 a washing solvent which is more volatile than the treated kerosene. The mixture of the slurry'of adduct crystals and the washing solvent is driven through pipe line 163 in the orifice mixer 164 which is similar to the devices 13, '13, 1-13 and 113 used in the reaction circuits. This device 164 efiects an intimate contact between the solvent (arriving through pipe line 162) and the adduct arriving through pipe line 125. The slurry, which now contains the solvent, overflows through "pipe line 144 into the vibrating decanter D which is similar to decanter B.

At the upper part, the solvent charged with the ratfinate extracted from the slurry of adduct crystals flows through pipe line 145 into the tank 147 for spent solvent. The washed slurryleaves-device D through pipe line 146 and is taken by pump 150 to be passed through pipe line 151 to the steam heater 152 where itis heated to 70 C. and the adducts are decomposed into an extract phase (straight-chain aliphatic hydrocarbons) and a phase comprising the solution of urea. This mixture passes through pipe line 153 into decantation vat 154. The regenerated urea solution 155 is drawn by pump 159 through pipe line 158 and passedinto pipe line 115 mentionedabove which feeds the first reaction circuit A.

The extract phase 156 from tank 154 is passed through a pipe line 157 to an intermediate tank from where the extract containing a small quantity of solvent which had not been removed in the vibrating decanter D is passed through pipe line 166 and pump 167 to a steam heater 168 followed by a distillation column 169. The extract, deprived of solvent, leaves at the bottom of the column through pipe line 170. This extract is stored in a tank 171 from which it is taken by pipe 1ine 157.

Solvent vapours are withdrawn from the top of column 169 by pipe line 172 and condensed in condenser 173. The recovered solvent is passed by pipe line 174 to the storage tank 175. I

Likewise, the spent solvent from tank 147 containing ratfinate is drawn by pump 167 through pipe line 166 and passed for distillation into steam heater 168' followed by column 169. The recovered raftinate leaves at the bottom of the column through pipe line 176 to arrive in the rafiinate tank 138 from which the raftinate may be withdrawn through pipe line 148.

The'solvent vapors leave at the top of the column through pipe line 172 to be condensed in the condenser 173. The liquid solvent is passed through pipe line 174' to a storage tank'175. From tank 175 solvent is passed by pipe line 178 and returned to circulation by pipe line 162.

It will be apparent that the apparatus described by reference to Figures 1 and 6 each comprise the same number of reaction circuits (A and C for Figure 1, A and C for Figure 6) and the same number of vibrating decanters (B and D'inthe case of Figure 1, B and D in the case of Figure 6). The equipment for the regeneration of the urea solution is identical. It is thus possible to employ the same equipment with suitable connections to operate either of the embodiments illustrated in Figures 1 and 6.

'It is also clear that it is possible to add one or several vibrating decanters to produce either normal parafiins of increased purity or to obtain, at the same time, a raffinate completely deprived of normal parafiins.

In a petroleum refinery the redistillation of solvent may be carried out by passing the spent (raftinate containing) solvent coming from pipe line 166' to the principal column of the atmospheric distillation unit (replacing column 169) under the conditions set forth above. Fresh solvent is then withdrawn continuously from the 11 light distillate leaving the column to be passed into tank 175.

The invention is illustrated further but in no way limited with reference to the following examples. The frequency of vibrations imparted to vibrating tanks was 1000 per minute, the amplitude being 10 mm.

Example 1 The urea solution employed had the following composition, parts being by Weight: Urea, containing 1% of biuret 42%; methanol 33%; ethylene glycol 12.5%; water 12.5%.

With the apparatus illustrated in Figure 5 the following results were obtained in the treatment of a kerosene of The apparatus used was as described with reference to Figure 5 with the following modifications:

(a) Solvent Washing was carried out using two vibrating decanting vessels, solvent being passed back in countercurrent flow from the second vessel to the first vessel.

(b) Solvent recovery was elfected in a refinery atmospheric distillation unit.

The feedstock to the process was a gas oil derived from Middle East crude oil processed under the conditions and with the results shown in the following tables. Proportions are expressed as vol. percentages on feed. I Reaction stage:

Urea solution fed percent 240 Concentration of urea solution in grams/litre 405 Saturation temperature of the urea solution C 38 Final reaction temperature C 28 Reaction tim minutes 60 Temperature C 28 Decantation stage:

Time taken for decantation r minutes 12 Recovery of raflinate percent 85 Washing stage:

Washing solvent fed do 50 Decomposition stage:

Temperature of eifecting decomposition of the adduct C 70 Time taken for decomposition of the adduct minutes 30 Distillation stages:

Feed of fat solvent to the distillation for removal of extract percent 2.5 Feed of fat solvent to the atmospheric column for removal of raiiinate percent 52.3 Composition of fat solvent- Lean solvent do 47.5 Raifinate do.. 4.8 Yield of extract do 10 INSPECTION DATA F RECOVERED EXTRACT HYDRO- CARB NS Density at 20/4 C 0.770 Distillation A. S. T. M.:

Initial boiling point C 242 C 251 50% C 262 95% C 291 Final boiling point C 303 Refractive index at 20/13 C degrees 1.433 Viscosity at 20 C centistokes 3.8 Flashpoint (closed) C 93 12 Freezing point C +7 Aniline point C 92 Aromatic content percent by vol Less than 0.5 Total sulphur percent by weight 0.004 Number of carbon atoms/molecule 13-17 Degree of purity of normal paraflins percent vol 96 We claim:

1. A process for the extractive crystallization of a hydrocarbon mixture comprising hydrocarbons which form adducts with urea and hydrocarbons which do not form adducts with urea, which comprises treating said hydrocarbon mixture with a solution comprising urea dissolved in a solvent whereby there is formed a complex slurry, said slurry comprising a liquid phase comprising a urea adduction raifinate, a liquid phase comprising urea solution, and, in suspension, a solid urea adduct, thereafter separating at least part of the urea adduction raflinate from the complex slurry by decantation and within the period prior to and during said separation, subjecting said slurry to the action of mechanical vibrations of an amplitude in the range of 1 mm. to 5 cms. and a frequency in the range of 100-2000 per minute whereby there is formed an upper liquid phase comprising raifinate and whereby the solid materials pass into the lower liquid phase comprising urea solution, and after said separation recovering the urea adduction extract from the remaining mixture of solid and liquid phases.

2. A process as specified in claim 1 in which urea adduction extract is recovered from the remaining mixture of solid and liquid phases by heating said mixture whereby the urea adduct is decomposed, and thereafter separating a phase comprising urea adduction extract from a phase comprising urea solution.

3. A process as specified in claim 1 in which the slurry formed by treating the feedstock with a solution comprising urea dissolved in a solvent is separated by decantation while subjecting said slurry to mechanical vibrations of a frequency in the range about 1000 per minute of an amplitude of about 10 mm.

4. A process as specified in claim 1, for the recovery of a rafiinate of low normal paratlin content, which comprises further treating the upper phase, separated by decantation, with a solution comprising urea dissolved in a solvent whereby there is again formed a complex slurry, said slurry comprising a liquid phase comprising a urea adduction rafiinate, a liquid phase comprising urea solution, and, in suspension, a solid urea adduct, thereafter separating at least part of the urea adduction raiflnate from the complex slurry by decantation and, within the period prior to and during said separation, subjecting said slurry to the action of mechanical vibrations of an amplitude in the range of 1 mm. to 5 cms. and a frequency in the range of 1002000 per minute whereby there is formed an upper liquid phase comprising ralfinate and whereby the solid materials pass into the lower liquid phase comprising urea solution, and, after said separation, recovering urea adduction extract from the remaining mixture of solid and liquid phases.

5. A process as specified in claim 1 which comprises mixing the mixture of solid and liquid phases, which remains after decantation of the upper phase, with a washing solvent, separating by decantation the major part of the washing solvent from the solvent containing slurry so formed and prior to within the period prior to and during said separation, subjecting the slurry to the action of mechanical vibrations in the range 100 to 2000 per minute and of an amplitude in the range 1 mm. to 5 ems. whereby the solid materials in suspension pass into the lower liquid phase, comprising urea solution, and after said separation recovering the urea adduction extract from the remaining mixture of solid and liquid phases.

6. A process as specified in claim 5 in which the mixture of solid and liquid phases recovered after separation 13 of solvent is subjected to heat treatment, whereby the urea adduct is decomposed, thereafter separating a phase comprising urea adduction extract from a phase comprising urea solution.

7. A process as specified in claim in which the washing solvent is a petroleum distillate fraction.

8. A process as specified in claim 1 in which the hydrocarbon mixture undergoing treatment is a petroleum distillation fraction.

9. A process as specified in claim 8 in which the petroleum distillation fraction has an initial boiling point above 80 C. and a final boiling point below 350 C.

10. A process as specified in claim 1 in which the solution comprising urea dissolved in a solvent is formed by dissolving urea in a solvent comprising methanol.

11. A process as specified in claim 1 in which the solution comprising urea dissolved in a solvent is formed by dissolving urea in a solvent comprising a major proportion of methanol and minor proportions of water and ethylene glycol.

12. A process as specified in claim 1 in which the solution comprising urea dissolved in a solvent is saturated at the temperature of treatment with the hydrocarbon mixture.

13. A process as specified in claim 1 in which the urea solution comprises a small proportion of biuret.

14. A process as specified in claim 13 in which the amount of biuret employed constitutes 0.25.0% by weight of the urea.

15. A process as specified in claim 1 in which the hydrocarbon feedstock and solution comprising urea are mixed by pumping in a closed circuit, to which feedstock and the solution comprising urea dissolved in a solvent are continuously fed at different points on said circuit and from which product is continuously Withdrawn at a further different point.

16. A process as specified in claim 15 in which the volume flow rate of circulation of reactants in the closed circuit substantially exceeds the total rate of feed of the hydrocarbon feedstock and solution comprising urea dissolved in a solvent.

17. A process as specified in claim 15 in which the hydrocarbon feedstock and solution comprising urea dissolved in a solvent are mixed in a plurality of closed circuits connected in series.

References Cited in the file of this patent UNITED STATES PATENTS

Claims

1. A PROCESS FOR THE EXTRACTIVE CRYSTALLIZATION OF A HYDROCARBON MIXTURE COMPRISING HYDROCARBONS WHICH FORM ADDUCTS WITH UREA AND HYDROCARBONS WHICH DO NOT FORM ADDUCTS WITH UREA, WHICH COMPRISES TREATING SAID HYDROCARBON MIXTURE WITH A SOLUTION COMPRISING UREA DISSOLVED IN A SOLVENT WHEREBY THERE IS FORMED A COMPLEX SLURRY, SAID SLURRY COMPRISING A LIQUID PHASE COMPRISING A "UREA ADDUCTION RAFFINATE, A LIQUID PHASE COMPRISING UREA SOLUTION, AND, IN SUSPENSION, A SOLID UREA ADDUCT, THEREAFTER SEPARATING AT LEAST PART OF THE UREA ADDUCTION RAFFINATE FROM THE COMPLEX SLURRY BY DECANTTATION AND WITHIN THE PERIOD PRIOR TO AND DURING SAID SEPARATION, SUBJECTING SAID SLURRY TO THE ACTION OF MECHANICAL VIBRATIONS OF AN AMPLITUDE IN THE RANGE OF 1 MM. TO 5 CMS. AND A FREQUENCY IN THE RANGE OF 100-2000 PER MINUTE WHEREBY THERE IS FORMED IN UPPER LIQUID PHASE COMPRISING RAFFINATE AND WHEREBY THE SOLID MATERIALS PASS INTO THE LOWER LIQUID PHASE RECOVERING UREA SOLUTION, AND AFTER SAID SEPARATION RECOVERING THE UREA ADDUCTION EXTRACT FROM THE REMAINING MIXTURE OF SOLID AND LIQUID PHASE.