CA1109409A - Process for cracking hydrocarbons - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Hydrocarbons are cracked in the presence of a mist of molten material containing a basic compound of an alkali metal, a basic compound of an alkaline earth metal or a mixture thereof, the amount of the molten material being 0.01 - 1 part by weight per 1 part by weight of the hydrocarbons, then quenching the resulting cracked gas-containing mist to a temperature of not lower than the melting point of the molten material, and then separating the cracked gas from the molten material.

Description

This invention relates to a process for cracking hydro-carbons comprising quenching the gases produced by cracking heavy liquid hydrocarbons such as, for example~ crude oil~ heavy oil, bottom residue of vacuum distillation and the likeO
Various processes are known for cracking hydrocarbons such as from ethane up to asphalt at high temperatures for producing hydrogen, ethylene~ propylene~ butadiene> benzene, toluene, xylene and the like which are the so-called petrochemical raw materials.
In commercial processes for producing the petrochemical raw materials, there are often used light hydrocarbons such as natural gas, refinery off-gas, naphtha, and light oil produced by an atmospheric pressure distillation, but high-boiling oils such as crude oil, light oil from a reduced pressure distallation and the like are used only to a limited extent.
The main reason is that the heavier the raw material, the easier carbon and tar-like materials are formed as by-products upon cracking. The resulting by-products attach to and deposit in the cracking apparatus and in the quenching devices for the cracked gas and therefore a prolonged running becomes impossible~
A number of processes have been proposed for suppressing the formation of carbon, tar-like materials, and a number of pro-cesses have been proposed for preventing them from attaching to and depositing in the apparatus~ but these known processes have various drawbacks such as requiring large investment~ increased energy demand such as for steam and the like, and decreased energy recovery. Even if the above-mentioned drawbacks can be eliminated and prolonyed, continuous operation is possible, there would not be any economic advantage due to the use of high-boiling oilsO

-- 1 -- ~

Heretofore, processes have been known for cracking hydro-carbons in the presence of a molten salt or a molten metal. In the known processes, the molten salt is usually used as a heat transfer medium. ~ydrocarbons such as high-boiling oils are blown into a molten salt bath and receive a quantity of heat necessary for cracking from the molten salt at an elevated tem-perature. According to this type of cracking process, the amount of heat energy required for heating crude oil or a combination of crude oil and a diluting agent such as diluting steam up to a cracking temperature, together with the heat of vaporization of the crude oil and the decomposition reaction heat of the crude oil amounts to about 1000 Kcal. per KgO of crude oil.
The quantity of the molten salt required to circulate as a heat transfer medium amounts to some multiple of ten times the weight of crude oil~ according to calculation. Therefore~ the quantity of the molten salt retained in the apparatus is also large. From the view point of safety, it is not desirable to keep such a large quantity of a molten salt at high temperature in an oil cracking plant where a large amount of combustible and inflammable material is kept.
In addition~ circulating such a large quantity of molten salt renders design of and operation of the apparatus very difficult, and further the energy needed for the circulating amounts to a large quantity.
Since the above-mentioned disadvantages have not yet been eliminated, many attempts of utilizing commercially a molten salt for cracking a high-boiling oil have not been successful.
Among such attempted processes, there may be mentioned those disclosed in ~apanese Patent Publications Nos. 5656/1963, 29824/1964 (corresponding to U.S. Patent 3~192,018 in the name g S. Minami dated June 29,1965) and 3887/1965 (corresponding to U.S. Patent 3,210,268 in the name Henclal et al., dated October 5, 1965.) These processes still suffer from the above-mentioned dis-advantages from a commercial point of view. For example, Japanese Patent Publication No. 5656/1963 discloses alkali metal chloride as a molten salt which passes together with crude oil through a reactor at a high velocity. In this process a large quantity of the molten salt as a heat transfer medium has to be used so as to impart the required quantity of heat, and thereby the heat transfer medium cannot become a mist form, but become liquid drops of a fairly large drop size such as about 1000 microns in size, and these large size liquid drops are transferred.
Such process as above can prevent carbon from depositing on the reaction vessel wall, but the resulting carbon particles are of a large size and cannot be removed by water gas reaction under the cracking conditions. Therefore, the molten salt cannot be reused unless the carbon is removed from the molten salt by combustion of carbon or by other treatments.
Japanese Patent Publication Nos. 19244/1972 and 8711/1975 disclose processes for preventing carbon from depositing in a quenching device by employing molten metals or molten heavy metal salts. The former flows a molten metal in a manner of entrainment and thereby the particle size of the molten metal inevitably becomes large. Therefore, the process has drawbacks similar to those of Japanese Patent Publication No. 5656/1963. Worse still, the metals and their compounds are not subjected to water gas reaction so that a particular treatment is necessary for removing carbon.

According to the present invention, there is provided a hydrocarbon-cracking process comprising cracking the hydro-carbons in the presence of a mist of steam and molten material comprising a basic compound of an alkali metal, a basic compound of an alkaline earth metal, or a mixture thereof, the amount of the molten material being 0.01 - 1 part by weight per part by weight of the hydrocarbons and the average particle size of the molten material in the mist being not larger than 300 microns, quenching the resulting cracked gas-containing mist of the molten material to a temperature not lower than the melting point of the molten material, and separating the cracked gas from the molten material.
In the accompanying drawings, FIG. 1 and FIG. 2 show diagrammatically apparatus which may be used for carrying out the present invention.
A molten salt used in the present invention is selected from basic compounds of alkali metals, basic compounds of alkaline earth metals and mixtures thereof.
Representative basic compounds for molten salts are hydroxides of those metals such as sodium hydroxide, potassium hydroxide, lithium hydroxide, barium hydroxide and the like;
carbonates of those metals such as sodium carbonate, potassium carbonate, lithium carbonate and the like; double salts such as KNaCO3, KMgH(CO3)2 and the like; mixtures of those basic compounds such as a mixture of 50 molar % sodium hydroxide and 50 molar potassium hydroxide, a mixture of 50 molar % sodium carbonate and 50 molar % potassium carbonate, a mixture of 20 molar %
barium carbonate, 40 molar % calcium carbonate, and 40 molar %
lithium carbonate, and the like. It is preferable to use a mixture of different metal compounds so as to obtain a lower eutectic point. An example of such a mixture is an equimolar mixture of Li2CO3 (m.p. 618 C), Na2CO3 (m.p- 851 C) and K2CO3 (m.p. 891C) having..............................................

the eutectic point of 385C.
The mechanism by which deposition of carbon and tar-like materials in the cracking apparatus is prevented by using a mist of the molten salt is not clearly understood. Although it is not desired to limit the invention to any particular theory, it is believed that particles of the molten salt are so fine that the resulting carbon is very fine and porous and subject to the water gas reaction and furthermore the strong catalytic action for the water gas reaction facilitates the water gas reaction of carbon with diluting steam when the latter is introduced into the cracking apparatus and carbon is converted to a gas, and in addition, tar-like materials attaching to thewall of the apparatus are washed away by the molten salt mist which collides against the wall, and thereby formation blocks of deposited carbon is pre-vented.
The average particle size of the molten salt mist is usually not larger than 300 microns, preferably not larger than 100 microns. The molten salt mist may be produced by using a venturi nozzle as illustrated in FIG. 1. It is not easy to 20 measure average particle size of molten salt mist in a practical apparatus, but the average particle size can be calculated by the equation of Nukiyama and Tanasawa (cf. Perry's Chemical Engineer's Handbook, 4th Ed., 1963, Chapter 18, page 68). When there is used a nozzle whose shape is not suitable for applying directly the equation of Nukiyama and Tanasawa, a material easy for handling such as a water-air system is used and actual particle size of the water-air system is measured and thereby a correction co-efficient for the equation is determined, and then average particle size of an actual molten salt mist can be calculated.

Other methods for producing the mist may be to use a pressure nozzle such as a single hole nozzle, a collusion spray valve, a spiral spray valve, or the like, to use a rotating device such as a rotating disk, a rotating pan, a rotating spray or the like, to use a gas-atomizing spray such as an air or gas-atomizing nozzle or the like, or to use vibration.
Further, the mist may be obtained by dissolving or SU9-pending the salt in a crude high-boiling oil or water used as a diluting vapor and supplying the resulting solution or suspension to the cracking apparatus.
The structure or shape of the heat decomposition reactor is not critical as long as the molten salt mist can be uniformly dispersed in the reactor and can collide against the whole wall to renew the surface of the wall because one of the purposes of the present invention is to prevent carbon and tar-like materials formed upon cracking hydrocarbons from depositing in the apparatus.
Examples of reactors that can be used for carrying out the present invention are reactors usually used for cracking hydrocarbons such as tubular reaction furnaces having external heating, column reaction furnaces having internal heating, tubular reaction furnaces having internal heating and the like.
The structure or type of the quenching device that may be employed is not particularly limiting, and preferable types are multitubular heat exchangers and scrubber columns for circulating the molten salt.
The amount of the molten salt supplied as mist is 0.01 - 1 part by weight per part by weight of hydrocarbon. When the amount is more than the upper limit, there is no increased effect but the heat load for elevating the temperature of the molten salt becomes larger and in addition the above mentioned various dis-advantages appear, and further a larger amount of the diluting gas is necessary to make the molten salt a fine mist, resulting in a lowering of the efficiency of the reactor.
It is preferable to use steam so as to inhibit deposition of carbon. The preferable amount of steam is about 0.5 - 5 parts by weight per part by weight of hydrocarbon.
The molten salt is not essentially used as a heat transfer medium so that the quantity of heat that should be supplied is that required for the decomposition reaction. Pre-ferably, heat is supplied by introducing directly a superheated steam and/or a combustion gas into the reactor. Such heat medium may be used for forming a mist of the molten salt.
Superheated steam may be produced by blowing steam into a heat accumulating furnace, and high temperature combustion gas may be produced by burning, in a burner disposed before a venturi, a part of the crude oil or by-product gas or oil produced by decomposition of the crude oil. The resulting superheated steam or the high temperature combustion gas is introduced into a venturi. As the combustion gas, there may be used a partial combustion gas. In particular, where high-boiling oil by-products of high sulfur content are produced as the result of cracking and no appropriate use for them is found, it will be efficient to partially burn the high-boiling oil by-products for gasification and simultaneously utilize the heat energy of the partially burned gas for decomposition of crude oils. According to the above-mentioned procedures, less valuable high-boiling oils of high sulfur content as well as other petrochemical raw materials such as ethylene, propylene and the like can be converted to useful hydrogen and com~ustion gases.
The heat decomposition reaction is effected usuallyat about 600 - 900C for 0.001 - 1 sec., preferably about 0.01 -0.3 sec. The quenching temperature is usually 400 - 600C where a mixture of Li2CO3-Na2CO3-K2CO3 is used as a molten salt.
Now referring to FIG. 1, steam is introduced through a pipe 1 with a flow control by a control valve, heated by a heater

2 to form superheated steam and introduced into a venturi nozzle 7 as superheated steam.
The superheated steam becomes a high speed stream in venturi nozzle 7, and a molten salt is introduced into the high speed stream through a small hole 8 to become mist.
A crude oil passes through a pipe 3, is heated by a heater 4 and is introduced into a raw material feeding nozzle 9 arranged near the end of the venturi nozzle, and a diluting steam heated by a heater 6 is introduced into the nozzle 9 through a pipe 5, and the crude oil is atomized and fed to the end portion of the venturi nozzle. The crude oil is then mixed with the superheated steam containing a mist of the molten salt and intro-duced into a heat decomposition reactor 10 where the heat decom-position reaction proceeds rapidly. The state of dispersion of the mist in the reactor 10 can be observed through a sight glass 11. The cracked gas produced by a heat decomposition reactor 10 passes through a transfer line 12 and is introduced into a quenching pipe 13 to suppress side reactions of the cracked gas. The temper-ature in quenching pipe 13 is kept at a temperature not lower than the melting point of the molten salt so as to prevent solidification of the molten salt in the quenching pipe.
The cracked gas cooled by quenching pipe 13 is intro-~0 duced into a mist separator 14 having a cyclone structure and the molten salt is separated, stored in a reservoir 15 and thencirculated again to venturi nozzle 7 through a pipe 16 and the small hole 8.
The separated molten salt may be sent to the mist producing means by various methods such as utilizing a pressure difference, a pump, a steam lift or the like. The cracked gas separated from the molten salt at a mist separator 14 may be sent to the next step through a pipe 17 and separated and purified as, for example, is conducted in an ethylene producing apparatus.
The following examples are given for the purpose of illustration and not by way of limitation.

Arabian Light crude was cracked to produce high-boiling olefin. Referring to FIG. 1, steam at 3 Kg./cm was introduced at a rate of 8 Kg./hr. through a pipe 1 and converted to super-heated steam of 850C by a heater 2. The resulting superheated was introduced into a venturi tube 7 having a throat diameter of 9.5 mm. and the venturi tube was kept at 800C by an electric heater. The venturi throat portion was provided with a small lateral bore having a diameter of 2 mm. Through the small lateral bore was supplied a molten salt of an equimolar mixture of Li2 C03-K2C03-Na2C03 kept at 600C at a rate of 0.5 Kg./hr. to produce a mist of the molten salt by a high speed flow of 160 m./sec. at the throat portion.
Average size of the mist particle was 50 microns according to the equation of Nukiyama and Tanasawa. Arabian Light crude, introduced at a rate of 4.5 Kg./hr. through a pipe 3 and heated to 350C by a heater 4, and steam, introduced through a pipe 5 and heated to 500C by a heater 6, were mixed and then ejected into _ g the molten salt mist stream through a raw material feeding nozzle 9 of 2 mm. diameter.
A heat decomposition reactor 10 of 90 mm. diameter and 1 m, length was heated from outside hy an electric heater and the heating was controlled in such a manner that the temperature of the cracked gas at the exit of the heat decomposition reactor was 800C. The resulting cracked gas produced in the heat decom-position reactor 10 was introduced into a quenching heat exchanger 13 of 25 mm. inner diameter and 2~00 mm. length through a transfer 10 line of 25 mm. inner diameter and 600 mm. length, cooled in the quenching heat exchanger to result in an exit gas temperature of 500C, and subjected to separation of the molten salt mist in a mist separator 14 of a cyclone type. The resulting cracked gas was then cooled with circulating cracked oil and cooling water, measured by a flow meter and analyzed by chromatography.
The molten salt was stored in a reservoir 15 in an amount of 15 Kg. kept at 500C so as to prevent the solidification, , The results are as shown below.
(1) Crude oil Arabian Light crude Specific gravity 0.852 Sulfur contents 1.6% by weight Conradson carbon 3.1% by weight residue (measured according to ASTM D-189-52) (2) Operation conditions Crude oil feed 4.5 Kg~/hr.
Total steam feed 9.0 Kg./hr.
Amount of the recirculating 0.5 Kg./hr.
molten salt Cracking temperature 800C

r~

Pressure in the heat50 mmHg gauge decomposition reactor Residence time 0.5 sec.
Quenching temperature500C
Pressure 45 mmHg gauge Residence time 0.13 sec.
Yield and Composition of Cracked Gas % by weight H2 3.01 CH411,97 C2H4 23.81 C2H6 2.31

3 6 9.70 C H 0.0 c4 ~2.86 ~5 27.13 CO 1.32 C2 18.81 TOTAL 100.92 Oily matter 15.29 Under the above mentioned conditions the operation was continued for 624 hours. No problems such as rise of pressure occurred during the operation, and after the operation was stopped, the apparatus was dismantled and inspected, and no deposition of carbon and tar-like materials was found in the decomposition reactor, transfer line, quenching heat exchanger, or mist separator.
Carbon suspended in the molten salt was as fine as less than 1 micron in size and the concentration was 100 ppm.
COMPARATIVE EXAMPLE
The procedure of Example 1 was repeated except that the small lateral bore 8 was closed. After 4 hours, the pressure of the quenching heat exchanger increased markedly and after 5 hours, the operation became impossible. On inspecting the inside of the apparatus, it was found that fine carbon had deposited in the heat decomposition reactor and carbon of a dense structure which seemed to be the carbonized tar-like materials had deposited in the quenching heat exchanger. The total amount of deposited carbon amounted to 2.3% by weight based on the supplied Arabian Light crude.

Referring to FIG. 2, Arabian Light crude was cracked for producing low-boiling olefins by a high temperature medium.
To a combustion chamber 21 were introduced propane at a rate of 1 Kg./hr. through a pipe 18, oxygen at a rate of 3.64 Kg./hr. through a pipe 19, and steam at a rate of 8 Kg./hr. through a pipe 20, and there was produced a high temperature gas at 2000C.
This high temperature gas then passed through a venturi pipe 7 of 11.5 mm. in diameter, and a molten salt of an equimolar mixture of Li2CO3-K2CO3-Na2CO3 kept at 600C was introduced into the venturi pipe through a small lateral bore of 2 mm. in diameter at a rate of 0.5 Kg./hr. The molten salt thus introduced was formed into mist by the high speed stream of 340 m./sec. Then Arabian Light crude introduced through a pipe 22 at a rate of 4.5 Kg./hr. and heated to 350C by a heater and a steam introduced through a pipe 23 and heated to 500C by a heater were preliminarily mixed and then ejected into the molten salt mist through a raw material feeding nozzle 9 of 2 mm. in diameter.
A heat decomposition reactor lO of 30 mm. in diameter and 1 m. in length was externally heated by an electric heater and the temperature of the cracked gas at the exit of the heat decomposition reactor 10 was adjustec3 to 800C by controlling the heat loss.
The resulting cracked gas from the heat decomposition reactor 10 passed through a transfer line 12 of 30 mm. inner diameter and 600 mm. length kept at a wall temperature of 750 C.
The cracked gas was cooled in quenching heat exchanger 13 of 25 mm. inner diameter and 3m. length in such a way that the exit temperature was 500C. Then the cracked gas together with the molten salt mist entered a mist separator 14 of cyclone type and the molten salt mist was separated and collected in a reservoir 15. The cracked gas thus separated was led through a pipe 17.
The cracked gas was cooled with a circulating cracked oil and a cooling water, measured by a flow meter and analyzed by chroma-tography.
(1) Crude oil Arabian Light crude (2) Operation conditions Cracking temperature 800C

Pressure in the heat 80 mmHg gauge decomposition reactor Crude oil feed 4.5 Kg./hr.

Total steam feed 9.0 Kg./hr.

Amount of the recirculating 0.5 Kg./hr.
molten salt Residence time in the heat 0.1 sec.
decomposition reactor Temperature at the quenching 500C
heat exchanger Pressure 75 mmHg gauge Residence time at the 0.2 sec.
quenching heat exchanger v`~ ,.

(3) Yield and Com~osition of Cracked Gas % by weight 2 2.61 CH4 7.29 C2E~2 1.38 C2H4 19.27 C2H6 1.43 C3H6 10.46 3 8 0.00 C 5.59 24.74 CO 0.58 CO 13.70 TOTAL 87.05 Oily matter 24.55 Under the above mentioned conditions the operation was continued for 220 hours. No problems such as rise of pressure were encountered during the operation. After the operation was stopped, the apparatus was dismantled and inspected. No deposition of carbon and tar-like materials was found in the decomposition reactor, transfer line, quenching heat exchanger, or mist separator.
Carbon suspended in the molten salt was as fine as less than 1 micron in size and the concentration was 100 ppm.

Alternatively, when crude oil fed through a pipe 22 and steam fed through a pipe 23 were introduced into the venturi throat portion, a similar result was obtained.

Claims (3)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN
EXCLUSIVE PROPERTY OR PRIVILEGE IS CLAIMED
ARE DEFINED AS FOLLOWS:

1. A hydrocarbon-cracking process comprising cracking the hydrocarbons in the presence of a mist of steam and molten material comprising a basic compound of an alkali metal, a basic compound of an alkaline earth metal, or a mixture thereof, the amount of the molten material being 0.01 - 1 part by weight per part by weight of the hydrocarbons and the average particle size of the molten material in the mist being not larger than 300 microns, quenching the resulting cracked gas-containing mist of the molten material to a temperature not lower than the melting point of the molten material, and separating the cracked gas from the molten material.

2. A process according to Claim 1 in which the separated molten material is recirculated to the cracking reaction zone.

3. A process according to Claim 1, 2 or 3 in which the average particle size of the molten material in the mist is not larger than 100 microns.