US2736685A - Process of petrolatum cracking in liquid and vapor phase - Google Patents

Description

United States Patent PROCESS OF PETROLATUM CRACKING 1N LIQUID AND VAPOR PHASE Howard L. Wilson, Raritan Township, Middlesex County, Fred W. Barres, Westfield, Brook I. Smith, Elizabeth, and Joseph F. Nelson, Westfield, N. 3., assignors to Esso Research and Engineering Company, a corporation of Delaware Application January 2, 1953, Serial No. 329,168

' 8 Claims. (Cl. 196-50) This invention relates to a method of cracking petrolatum or other wax-bearing petroleum fractions to form high yields of higher (Cs-C19) olefins that are principally straight chain or lightly branched alpha olefins most suitable for certain chemical reactions.

With the present process of thermally cracking petrolatum, a substantial increase in the selective formation of the higher (Cs-C19) alpha olefins can be obtained by the sequence of (l) adequate liquid phase cracking followed by (2') a shorter vapor phase cracking in suitably proportioned periods. This sequence is contrary to the teaching of prior art which claims that no substantial amount of liquid phase cracking should occur in converting paraffinic wax hydrocarbons or petrolatum to unsaturated hydrocarbon products.

According to the prior art, a wax-containing or petrolatum-containing oil feed should be vaporized as rapidly as possible to avoid any liquid phase thermal cracking and should be cracked in the vapor phasefor a more prolonged period above 2.5 seconds and mainly for a period of 3 to 6 seconds or longer. Also, in the prior art methods no effort was made to avoid the presence of parafims boiling below the boiling points of the C20 hydrocarbons (650 F.).

In the present process petrolatum essentially of low volatility, i. e., boiling essentially above 650 'F., is used as a feed for the thermal cracking. Such a feed should contain no more than about 30 wt. percent of hydrocarbons boiling below 950 P. if it is to be successively cracked first in the liquid phase then in the vapor phase, but with the'liquid phase cracking over an adjusted period wherein at least 12% and up to 20% of the total cracking takes place. This period of liquid phase cracking is preferably from two to three or more times the period of the vapor phase cracking. This sequence of'crackin'g can be accomplished in two series connected units or in a single unit with properly adjusted conditions. The liquid phase cracked products at 750 to 950 F. together with remaining uncracked products are advantageously atomized and quickly vaporized by admixed steam superheated to above 1000" F., then the subsequent vapor .phase cracking is carried out with the admixture of from 2 to 6 moles of steam per mole of hydrocarbon material subjected to the vapor phase cracking. Preferably the amount of steam should be adjusted so as to give hydrocarbon partial pressures at the coil outlet of 5 to '20 .p. s. 1. a.

A preferred method of carrying out the cracking, -as will be described with reference to the accompanying drawing, is accomplished in a single continuous unit. In the first portion of the tubular or coil reactor, the petrolatum feed is subjected to liquid phase cracking forabout 6 to 8 seconds, or for a period to give at least 12% of the total desired conversion, at temperatures below 1000 F., more particularly at 750 F. to 950 F. In passing the liquid phase cracked effluent to the second portion of the reactor, the hydrocarbons are abruptly "vaporized and their temperature is raised to permit vapor ICC phase cracking at temperatures in the range of 1000 to 1250 F. for a shorter period of only about 1 to 3 seconds.

Referring to the drawing, the total liquid petrolatum feed boiling above 650 F. to be subjected to cracking is introduced into the inlet of the cracking tube or coil 1 which is connected to vapor-cracking tube 2. The coil 1 may be located in a convection heating section A of furnace 3. The petrolatum feed in line 4 may have a preheat temperature of about 600 to 750 F. on being introduced into the inlet of coil 1 and put under a pressure of 10 to 30 p. s. i. g. In the time that the liquid petrolatum feed is forced from the inlet of coil 1 to the outlet thereof, the petrolatum is brought to a temperature in the range of 750 to 950 F. over a period of about 6 to 8 seconds to obtain from 12 to 20% of total desired conversion, then is admixed with steam from line 5 to be vaporized as it is admitted with the steam into the vapor phase cracking tube 2.

By supplying between 2 and 6 moles of steam per mole of petrolatum (average molecular weight 300-600) and having the steam superheated to a temperature in the range of about 1000 to 1200 F., rapid atomization and vaporization of the cracked liquid petrolatum at the inlet to cracking tube 2 is obtained.

The vapor phase cracking tube 2 is located in a radiant heating section B of the furnace 3. ln travelling from the inlet to outlet of tube 2, the vapor phase petrolatumsteam mixture is maintained at temperatures in the range of 1000 to 1250" F. for a period of less than 3 seconds to obtain the remaining desired conversion. The total conversion of the petrolatum entering the cracking coils is to amount to from 25 to 50 wt. percent thereof, preferably 30 to 45 wt. percent.

The cracked vapor effiuent leaves the cracking tube 2 by line 6 and may be promptly quenched by admixture of a fresh petrolatum feed from line 7. The cracked vapor stream is transferred then by line 8 into a lower part of a tar separator and primary fractionator 9. A portion of the tar may be pumped through line 10 into line 6 to act as a coke fiuxing agent. Residual tar 'is withdrawn by line 11 to storage. Steam may be admitted .into the bottom stripping section of separator 9 from line 1 2.

The upper part of separator 9 is provided with a suitable number of plates e. g. 7 to 15, for fractionation. Condensate which is collected on one of the lower plates .13 is withdrawn by "line 14 at a temperature of about 600 F. to the recycle feed accumulator 15 into which stripping stream 16 may be injected. In this manner the fresh petrolatum feed is freed of tars; and the fresh feed combined with recycle stock is given a suitable end point and boiling range for use as the cracking feed to coil 1 byway of line 17.

The overhead line 18 from the top of accumulator 15 returns vapors to the separator 9. The overhead vapors are withdrawn from fractionat'or 9 at a temperature of about 550 F. by line 19. This overhead vapor mixture includes C6 to C21 olefins to be recovered and some heavier hydrocarbons together with all light gaseous products. The vapor mixture is passed by line 19 through condenser '20 into a distillate accumulator 21. Uncondensed gases are Withdrawn overhead by line 22. Water is drained through line 23. A portion of the oil condensate is refluxed by line 24 to the top of fractionator 9. Another portion of the heavy condensate is withdrawn by line 25 to the fractionating column 26, which acts as the product splitter tower.

The lightest distillate of C4-C6 olefins is taken overhead from tower '26 by line 27. Intermediate distillates of a C7-C8 olefin fraction and a Cs-Ciz olefin fraction a'r'ewithdr'a'wn by lines 28 and 29.

The higher boiling C13+ olefin products are passed from the bottom of tower 26 by line 30 into the vacuum fractionator 31, in which a (ha-C19 olefin fraction is distilled and removed by line 32, to leave a Czo-C21 olefin bottoms, withdrawn by line 33. Each of these fractions may be cut as desired, for example, C13-C1 olefins may be taken overhead from tower 31.

Each of the distillate fractions collected has a special use as intermediates for making certain valuable chemicals, but in each instance, it is desirable to have the olefins as nearly straight chain as possible with mainly alpha (terminal) double bonds. This type of olefin, i. e., alpha straight chain type, is most useful as an alkylating agent for the synthesis of ketones, and as a feed stock in oxonation. It is also desirable to have these distillate products contain of the order of 90100% olefins so as not to require separation of other kinds of hydrocarbons, such as parafiins or cyclic hydrocarbons.

The thermal cracking principally in liquid phase followed by vapor phase cracking gives a product with substantially higher yields of C9 to C19 olefins which are close to 85% alpha olefins as compared to a cracking operation .in which no substantial amount of liquid phase cracking occurs. A comparison of typical products from solely vapor phase cracking and from conjoint liquid phase-vapor phase cracking of petrolatum are illustrated in the following Table I.

Still another advantage of the combined liquid and vapor phase cracking is shown by the lower yields of undesirable diolefins and aromatics. This is demonstrated by the data in Table I.

TABLE II Wt. Percent of Selected Diolefins and Aromatics on Total Cracked Products Example 2 Example 1 Liquid Phase Vapor Phase Followed by Cracking Vapor Phase Cracking Component:

Butadlene 2. 1 0. 9 C Diolefins 1. 1 0.5 Benzene 0. 3 0.1 Toluene 0. 8 0. 4

Advantages of the present invention will be understood from the comparison of the following two examples. In these experiments a continuously operating unit made up of commercial pipe and tubing with appropriate means of feed and temperature control were used. The experiments were conducted with a high boiling petrolatum containing 34.0 per cent of oil cracking tube feed. Under atmospheric pressure conditions this oil boils in the range of 827 to about 1100" F. with 5% high boiling residue.

Example 1 This run was made with a flow rate of 1.47 volumes of the liquid petrolatum per volume of reactor per hour. Steam was injected into the molten petrolatum to atomize the petrolatum and rapidly vaporize the petrolatum. The vaporized petrolatum-steam mixture was rapidly heated to a temperature above 950 P. so that the calculated residence time for the material in liquid phase was 0.408 second. In the vapor phase cracking tube the residence time was 4.22 seconds. The hydrocarbon partial pressure and temperature at the coil outlet were 8 p. s. i. g. and 1075 F. respectively. Thus the petrolatum was in the liquid phase for less than 9% of the total residence time. On account of the slower cracking rate in the liquid phase not more than about 1 to 2% of the total cracked products was caused by pyrolysis in the liquid phase.

Example 2 This run was conducted in the same equipment as used in Example 1. However, in this case the temperature profile during the cracking is considerably changed to permit a substantially longer liquid phase cracking than vapor phase cracking. In this experiment superheated steam at 1050 F. was injected into the efi1uent from liquid phase cracking section when the effluent reached a temperature of 925 F. Assuming a slight amount of vaporization of low boiling components in the petrolatum feed during the liquid phase cracking, prior to the steam injection, it was determined that the residence time of the petrolatum in the liquid phase while being heated through the temperatures of 750 to 925 F. amounted to 6.6 seconds, while the subsequent residence time in the vapor phase at above 925 F. was 2.27 seconds. The temperature and hydrocarbon partial pressure at the cracking coil outlet were 1100 F. and 6 p. s. i. a. respectively. Thus, in this experiment, the petrolatum was cracked in the liquid phase for about 75% of the total residence time and about 15% of the total cracking occurred in the liquid phase.

A comparison of products formed by the two different methods described in Examples 1 and 2 is given in the following table:

This comparison in Table 11 shows a substantial increase in selectivity toward the formation of higher olefins in the Ca-Crc range by having a substantial amount of liquid phase cracking prior to the vapor phase cracking. An analysis of the olefins also showed equivalent or better selectivity toward the formation of alpha olefins in the amounts of about -85% by using liquid phase cracking for a substantial period followed by vapor phase cracking for a shorter period.

An important feature of the conjoint liquid-vapor phase cracking is the preparation of the cracking feed. In order to maintain adequate liquid phase cracking, the feed to the liquid cracking coil has to be practically free of low boiling components which would readily vaporize at liquid phase cracking temperatures. Moreover, such low boiling components would contaminate the final olefin products in the C9-C19 range if they passed through uncracked.

At the same time it is preferred that the fresh petrolatum feeds used should boil above 650 F. and principally (95 vol. percent) in the range of 700 to 1100 F. (370 to 590 C.). These materials containing not more than 40% oil are composed largely of parafiins having 20 to 45 carbon atoms per mole.

It is advantageous to reduce the oil content of petrolatums down to below 20%, e. g. by removal of hydrocarbons with solvents, such as methyl ketone mixed with benzene or toluene and/or distillation. The oil portion increases the formation of branched olefins, dienes, and aromatics. For example, with 29.5% oil as compared to 5% oil in the petrolatum feed, the desired C12 alpha olefin product formation was nearly cut in half, and the diene formation was nearly doubled. Some of the oil components tend to form tars and coke.

A very practical step used for eliminating the oil components that form the tars is involved in the procedure of subjecting the fresh petrolatum feed to a fractional distillation in common with the cracked petrolatum product passed into the tar separator and primary fractionator 9.

In the simultaneous distillation of the petrolatum with cracked petrolatum in the presence of steam a suitable distillate cut having lower oil and sulfur content can be obtained. For example, a petrolatum feed containing 29.5% oil had a sulfur content of 0.14 wt. percent; and over vol. percent of this feed boiled above 1000 F. The petrolatum feed after deoiling to a substantially lower oil content had a sulfur content of 0.1 wt. percent; and less than 5 vol. percent of this feed boiled above 1100 F. This demonstrated that deoiling, which can be effected by undercutting, i. e. steam distillation to eliminate tars, is highly beneficial in reducing the heavy end oil content boiling above 1100 F.

Analyses showed that much of the sulfur content is concentrated in the oil portion and a substantial amount of sulfur is eliminated by the distillation. For example, in the oil obtained by deoiling, the sulfur content was 0.47 wt. percent compared to 0.1 wt. percent of sulfur in the deoiled petrolatum.

Suitable fresh feeds comprise motor oil petrolatums con taining high melting point semiand micro-crystalline waxes. They are obtained by solvent dewaxing lubricant base stocks of proper boiling ranges. Other inspections of the preferred fresh feeds are given below:

Gravity, API 30-40 Flash point, F 300-500 Viscosity, Saybolt 35-60 Universal 210 F.

It is desirable that the petrolatum feed stocks be low in sulfur content, preferably 0.1 wt. percent or lower. To insure this, the fresh feeds can be subjected to hydrodesulfurization by contact with suitable catalysts such as nickel-tungsten sulfide or cobalt molybdate type supported on alumina or activated alumina. The desulfurization is accomplished by passing the petrolatum over the catalyst at a space velocity of 0.3 to 3 volumes per volume of catalyst per hour under a hydrogen pressure of about 200 to 500 pounds per square inch gauge.

In any event, the pretreatment given to the fresh feed in the primary fractionator 9 and accumulator 15 beneficially removes oils and tars to a substantial extent and this aids in reducing the sulfur content.

The invention described is claimed as follows:

1. A process for cracking petrolatum for producing principally high yields of C9 to C19 alpha olefins, which comprises heating petrolatum distillate freed from tarry residue boiling above 650 F. in liquid phase at liquidphase cracking temperatures in the range of 750 to 950 F. for a period up to about 8 seconds to effect 12 to 20 wt. percent of total cracking while the petrolatum is maintained in liquid phase, dispersing the resulting liquidphase cracked petrolatum products into a stream of steam superheated to above 1000 F. at the inlet to a vaporphase cracking zone to vaporize the liquid phase cracked petrolatum products as they are admitted to said vapor phase cracking zone, then passing the steam containing said vaporized cracked products through said vapor phase cracking zone wherein said products are heated to a vapor phase cracking temperature in the range of 1000 to 1250 F. for a period to obtain the remainder of 25 to 45 wt. percent conversion of the petrolatum, then cooling the cracked products and fractionating from the cracked products C9 through C19 olefins.

2. A process according to claim 1, in which the liquid phase cracking period is at least twice the vapor phase cracking period.

3. A process according to claim 1 in which 2 to 6 moles of steam are admixed per mole of petrolatum feed with the liquid phase cracked petrolatum products.

4. A process for cracking petrolatum principally for producing high yields of C9 to C19 alpha olefins, which comprises distilling a petrolatum fraction from a fresh petrolatum feed boiling above 650 F. to obtain a distillate fraction thereof free of residual tars boiling principally in the range of 750 to 1100 F., heating the distillate fraction of the petrolatum free of residual tars under pressure at temperatures of 750 to 950 F. for a period up to about 8 seconds to effect substantial liquid phase cracking amounting to 12 to 20 wt. percent of the total cracking in an initial section of a cracking zone, dispersing the resulting liquid phase cracked petrolatum product into a stream of steam superheated to above 1000 F. at the inlet of a vapor phase cracking zone to form a mixed stream containing the liquid-phase cracked petrolatum product in vapor phase, passing said mixed stream through said vapor phase cracking zone in a lesser period at a temperature of 1000 to 1250 F. in said vapor phase cracking zone, then fractionating cracked product effiuent from the vapor phase cracking zone with admixed fresh petrolatum feed which is substantially free of hydrocarbons boiling below 650 F. to obtain a combined distillate free of tars, returning said combined distillate to said initial section of the cracking zone, and recovering C9 to C19 olefins fractionated from the cracked product substantially free of any admixed paraffins.

5. A process for conjoint liquid and vapor phase cracking of petrolatum to produce increased yields of C9 to C19 alpha olefins, which comprises preparing an initial fresh petrolatum feed having an initial boiling point above 650 F., stripping from said feed a distillate fraction of which vol. percent boils in the range of 750 to 1100 F. and which contains cracked petrolatum products higher boiling than 650 F. condensed during the stripping by the hot cracked products and steam, cracking said distillate fraction free of residual tars, in liquid phase at 750 F. to 950 F. for a period of about 6 to 8 seconds to effect 12 to 20 wt. percent of the total desired conversion of petrolatum to lower boiling alpha olefins, atomizing and vaporizing the liquid phase cracked product by admixing steam superheated to above 1000 F. at the inlet to a vapor phase cracking zone, then cracking the resulting liquid phase cracked product for a shorter period in vapor phase at 1000 to 1250 F. with the admixed steam to obtain a total conversion of 25 to 45 wt. percent of the petrolatum to lower boiling olefins, and subjecting the resulting vapor phase cracked products to fractionation with admixed portions of the fresh petrolatum feed.

6. The process of claim 5 in which the fresh petrolatum feed is prepared and distilled to give said distillate fraction a low sulfur content of below 0.1 Wt. percent.

7. The process of claim 5, in which distillate fraction of fresh petrolaturn and petrolatum cracked products is stripped free of hydrocarbons boiling below 650 F.

8. The process of claim 5, in which the liquid phase cracked product is subjected to vapor phase cracking under a lowered hydrocarbon partial pressure of 10 to 20 pounds per square inch absolute in the presence of the admixed steam at 1000 to 1250 F.

References Cited in the file of this patent UNITED STATES PATENTS Garner et al June 16, 1953 OTHER REFERENCES Scientific Principles of Petroleum Technology, Chapman & Hale, London, 1926, page 114.

Claims (1)

1. A PROCESS FOR CRACKING PERTOLATUM FOR PRODUCING PRINCIPALLY HIGH YIELDS OF C9 TO C19 ALPHA OLEFINS, WHICH COMPRISES HEATING PETROLATUM DISTILLATE FREED FROM TARRY RESIDUE BOILING ABOVE 650* F. IN LIQUID PHASE AT LIQUIDPHASE CRACKING TEMPERATURES IN THE RANGE OF 750* TO 950* F. FOR A PERIOD UP TO ABOUT 8 SECONDS TO EFFECT 12 TO 20 WT. PERCENT OF TOTAL CRACKING WHILE THE PETROLATUM IS MAINTAINED IN LIQUID PHASE, DISPERSING THE RESULTING LIQUIDPHASE CRACKED PETROLATUM PRODUCTS INTO A STREAM OF STEAM SUPERHEATED TO ABOVE 1000* F. AT THE INLET TO A VAPORPHASE CRACKING ZONE TO VAPORIZE THE LIQUID PHASE CRACKED PETROLATUM PRODUCTS AS THEY ARE ADMITTED TO SAID VAPOR PHASE CRACKING ZONE, THEN PASSING THE STEAM CONTAINING SAID VAPORIZED CRACKED PRODUCTS THROUGH SAID VAPOR PHASE CRACKING ZONE WHEREIN SAID PRODUCTS ARE HEATED TO A VAPOR PHASE CRACKING TEMPERATURE IN THE RANGE OF 1000* TO 1250* F. FOR A PERIOD TO OBTAIN THE REMAINDER OF 25 TO 45 WT. PERCENT CONVERSION OF THE PETROLATUM, THEN COOLING THE CRACKED PRODUCTS AND FRACTIONATING FROM THE CRACKED PRODUCTS C9 THROUGH C19 OLEFINS.

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