US3546316A - Prevention of coke formation in steam cracking processes - Google Patents

Description

United States Patent O 3,546,316 PREVENTION OF COKE FORMATION IN STEAM CRACKING PROCESSES Ihor Koszman, Parsippany, N.J., assignor to Esso Research and Engineering Company, a corporation of Delaware No Drawing. Filed Feb. 9, 1968, Ser. No. 704,256 Int. Cl. Cg 1/16; C07c 3/08 U.S. Cl. 260-683 14 Claims ABSTRACT OF THE DISCLOSURE FIELD OF INVENTION This invention relates to an improvement in the thermal cracking of arsenic containing petroleum fractions. More particularly, this invention relates to an improved steam cracking process wherein coke formation in the cracking zones is greatly inhibited by the removal of substantially all of the arsenic from an arsenic containing petroleum a feed fraction prior to the cracking operation.

PRIOR ART Thermal cracking processes, conducted with or without the presence of steam, are well known to the art and are a major source of highly valuable unsaturated compounds, e.g., ethylene, butadiene, etc. Unfortunately, however, all of the petroleum feed fraction is not pyrolytically converted to desirable products. Thus, undesirable side reactions also occur, which give rise to acute difficulties and tend to substantially reduce the yield of desirable products from the cracking process. One of the most troublesome side reactions occurring in such processes is the formation of coke within the cracking zones, i.e., within the tubes in the cracking furnace, the reaction mixture being heated to cracking temperatures within these tubes. Generally, for example, when ethane is cracked, a portion thereof is converted to methylene radicals, i.e., CH radicals, and hydrogen. These radicals may either form ethylene, or by linking together, it is believed, to form polymer. The polymer then dehydrogenates under the conditions existing in the tubes and forms carbon deposits. Certain forms of this carbon, i.e., coke, are in an extremely hard particulate form. The coke adheres to the tube walls and tends to build up, thereby restricting the effective cross-sectional area of the tube and leading to large pressure drops. Additionally, because of the insulating properties of the coke, furnace temperatures must be increased in order to maintain desired reaction temperatures within the tubes. This reduces tube life substantially and increases the frequency of shutdown periods for replacing damaged tubes.

While coking is a normal feature of almost all steam cracking processes, it appears that the coking rate is substantially increased by the presence of certain metals, for example, arsenic. Thus, since arsenic is known to be present in various oil fractions (in several and changeable forms, e.g., arsine, arsenious oxide, etc., but hereinafter simply referred to as arsenic regardless of the form it takes), the coking rate, when such fractions are steam cracked, is substantially increased, a harder coke forms, and coke removal operations are, accordingly, more freice quent and more difficult. For example, onstream periods may generally range from about 1000-1400 hours but due to arsenic promoted coking, onstream periods have been reduced to less than about 500 hours, and as low as 350 hours. It has now been discovered, however, that coking can be substantially inhibited, and the product yield from the steam cracking of arsenic containing hydrocarbons can be greatly increased, i.e., by allowing greater run periods, by the substantial removal of arsenic from arsenic containing feeds.

SUMMARY OF THE INVENTION In accordance with this invention, therefore, an improved steam cracking process is provided wherein the formation of coke, in the steam cracking of an arsenic containing petroleum feed, is substantially reduced by treating the feed to reduce substantially the arsenic concentration therein. While not wishing to be bound by any particular theory, it is believed that arsenic, in whatever form it is present in the petroleum feed, tends to promote the formation of coke, leading to shorter run times and greater downtimes. Additionally, it has been found that normal decoking operations, e.g., air decoking, for tubes through which arsenic containing feeds have been cracked do not remove all of the arsenic containing coke. Consequently, since arsenic tends to concentrate in the coke, sufiicient arsenic remains to promote excessive coke formation in the subsequent cracking of feeds containing little or no arsenic. Therefore, the necessity for removing arsenic from the feed prior to steam cracking is clearly evidenced.

Arsenic removal can be effected in a variety of ways, well known to the art. Generally, any procedure adapted for the removal of arsenic from petroleum fractions will suffice to reduce the arsenic concentration of that fraction to levels insufficient to cause excessive coking when that fraction is thermally cracked. One skilled in the art will encounter little trouble in determining the optimum arsenic removal technique and optimum arsenic concentrations for the most efiicient results. Thus, once knowing that it is arsenic that causes excessive coking, the determination of the arsenic removal technique to reach satisfactory arsenic levels will be a relatively straightforward procedure. Preferably, however, the arsenic removal process is conducted in such a manner as to reduce the arsenic concentration of the steam cracking feed to less than about 50 p.p.b. (part per billion) by weight, more preferably, less than about 10 p.p.b., and still more preferably less than about 2 p.p.b.

Many arsenic removal techniques have been reported in the literature and a few will be discussed hereinbelow. For example, one method for removing arsenic from petroleum fractions involves the use of a basic compound, i.e., when dissolved in water the pH is above about 7, of alkaline or alkali earth metals, e.g., oxides, hydroxides, salts, such as the hydroxides of lithium, sodium, potassium rubidium, cesium, barium, calcium, strontium, etc., sodium salts such as acetate, phosphate, borate, carbonate, citrate, cyanide, chromate, formate, lactate, oxalate, perborate, tartrate, etc., and similar salts of the other alkaline and alkali earth metals. These compounds may be employed in aqueous solutions or solutions of polar solvents such as alcohols, ethers, ketones, etc., or used as a solid bed with no solvent, or used as a supported material on such suitable carrying materials as kieselguhr, alumina, silicates, magnesia, zirconia, titania, etc. The arsenic containing petroleum fraction, in the liquid or gaseous phase depending upon contact temperatures and feed to be treated, is contacted with the basic reagent at temperatures below about 500 F., preferably about -500 F. A fraction of reduced arsenic content is then recovered by normal procedures. More details on this method can be found in US. 2,779,715.

Another method for removing arsenic from petroleum fractions is reported in U.S. 2,781,297. This procedure involves contacting the arsenic containing petroleum fraction with a salt of copper and/ or metals lower than copper in the electromotive series, e.g., mercury, silver, palladium, platinum, gold, etc. Generally, the salts are those of acids, such as sulfates, chlorides, nitrates, fluorides, etc., but may also be organic acid salts such as acetates, propionates, butyrates, valerates, etc. The salt is preferably composited with a porous support, such as kieselguhr, silica gel, alumina, magnesia, silica, clays, and the like, and contact with the petroleum fraction may be had at any suitable temperature, e.g., but generally below about 500 F. The condition of the feed will again depend upon contact temperatures and the feed to be treated.

A third method for reducing the arsenic concentration of petroleum fractions involves the use of a nitrogen compound containing three attached groups and an unshared pair of electrons, e.g., aqueous ammonia, hydrazine, alkanol amines, aliphatic amines, alkylene polyamines, and the like, some typical examples of which are: ethyl amine, dimethyl formamide, formamide, ethanolamine, diethylene triamine, acetamide, tetraethylene penta-amine, pentaethylene hexa-amine, ethylene diamine, hexanol amine, etc. The nitrogen compounds may be used as such or in solutions of water, alcohols, ketones, etc. The petroleum fraction is contacted with the reagent at temperatures generally not above about 200 F. and an arsenic free petroleum fraction is recovered. In U.S. 2,867,577 further details on this process are reported.

Another method for removing arsenic from petroleum fractions involves the use of an acid impregnated support, the acid being present in amounts above about 20 wt. percent, preferably about 50-150 wt. percent, e.g., sulfuric acid impregnated silica gel as reported in US. 3,093,574. The petroleum fraction is contacted with the acid impregnated support at temperatures below about 500 F., but preferably below about 200 F and a petroleum fraction substantially reduced in arsenic content is then recovered. In addition to silica gel, other porous support materials may be acid impregnated, e.g., kieselguhr, clays, diatomaceous earths, etc., and similarly other acids may be employed, e.g., phosphoric acids, nitric, and the like, but preferably those acids that contain oxygen. (Various US. patents have been mentioned hereinabove as disclosing techniques for the removal of arsenic from petroleum fractions. The disclosure of these patents regarding treatment of the petroleum fraction for arsenic removal is hereby incorporated herein by reference.)

Still another, and preferred, method for removing arsenic from petroleum feeds involves the use of activated charcoal. The employment of activated charcoal or any other similar porous material, e.g., alumina, molecular sieves, silica-alumina, cobalt-silica-alumina, clays, e.g., atapulgite clays, diatomaceous earths, etc., is in a manner similar to that for employing silica gel, i.e., operating conditions are essentially the same. Additionally, the activated charcoal may be impregnated with acids such as sulfuric acid, or with metals such as copper, which tend to increase the arsenic removal process by allowing for a reaction, i.e., conversion to arsenates which are a readily removable form, in addition to the physical adsorption process already occurring.

The foregoing processes can be conducted with the petroleum feed fraction either in the liquid or vapor phase depending upon the state of the feed fraction at the particular temperature at. which the arsenic removal step is effected. Additionally, the feed may be processed one or more times through the arsenic removal medium in order to reach desired arsenic levels. The arsenic free petroleum fraction may be recovered by any well-known technique,

such as filtration, centrifugation, etc.

The foregoing arsenic removal processes are normally satisfactory for removing sufficient arsenic from the petro. leum feed to prevent excessive coking in the cracking operation. Thus, while the arsenic concentration in petroleum feed fractions generally ranges from about 200-800 p.p.b. and higher, the foregoing processes should be employled to reduce this concentration to previously specified eve s.

The cracking operation is well known and will be only briefly discussed here. See, for example, Chemical Week, Nov. 13, 1965, page 72 et seq. Generally, the petroleum feed fraction is admixed with steam, i.e., in amounts ranging from about 20 to mol percent steam, preferably about 20 to 60 mol percent, prior to entry into the steam cracking furnace. The furnace normally contains two sections, a convection section wherein the feed is vaporized, if not already in that form, and a radiant or cracking section, the feed being passed in admixture with steam in the absence of added catalysts through one or more tubes located within the furnace. The convection section is normally employed to increase heating efficiency and the petroleum-steam mixture is heated therein to temperatures of about 1000 to 1100 F How ever, these temperatures are below that at which the feed cracks since cracking is undesirable in the convection section. The heated feed then passes into the radiant section where the temperature is quickly raised to about 1200 to 1700 F, or higher as tube metal materials permit, and the feed is cracked. Residence times in the radiant section are carefully controlled to minimize coke formation, polymerization, and other undesirable reactions. Thus, residence times in the cracking section will range from 0.1 to 10 seconds, preferably about 0.1 to 1 second. Pressures within the tubes may range from about 0-50 p.s.i.g. but are not critical, and higher pressures, e.g., up to about 100 p.s.i.g., can be tolerated. Upon exiting the cracking section, the reaction products are immediately quenched to stop further reactions and/or minimize loss of primary conversion products.

The petroleum fractions which may be converted by this process may vary widely in boiling point range. Generally, however, the process is most applicable to hydrocarbon feeds consisting essentially of cyclic or acyclic saturated hydrocarbons. Thus, hydrocarbon feeds that may be utilized herein include such cyclic hydrocarbons as cyclopropane, cyclopentane, cyclooctane, and mixtures thereof. The acyclic hydrocarbon feeds include any alkane, namely, aliphatic hydrocarbons of the methane series or mixtures of alkanes with eycloalkanes. The preferred feeds for use are saturated hydrocarbons containing from 2 to 24 carbon atoms and, more preferably, alkanes containing from 2 to about 12 carbon atoms. Exemplary hydrocarbon feeds which can be used in the practice of this invention are butane, ethane, propane, isobutane, n-hexane, n-decane, n-dodecane, n-hexadecane, eiscosane, tricosane and light naphthas boiling, at standard pressures, within a range of from about to 430 F. In addition to the foregoing, gas oils having boiling points ranging generally from about 450 to about 800 F. and kerosenes having a boiling temperature ranging from about 430 to about 550 F. can also be utilized in the practice of this invention.

The determination of the cause of coke formation is often a difficult task but one method is to determine the concentration of various elements in the coke produced by a cracking operation. Since trace elements tend to concentrate in the coke, they may be more easily detected therein. Thus, it was found that the arsenic content of coke from a cracking operation where onstream periods had been reduced was about 20 ppm. to about 0.14% by weight and generally about 200-500 p.p.m., while the arsenic content of coke from a normally running process was only about 0-10 p.p.m., with values in the lower end of the range predominating.

In order to determine for certain that arsenic was indeed the cause of observed excessive coking rates several experimental tests were conducted. These tests were conducted in a 3 inch long by 1 inch diameter cracking tube of 310 stainless steel. A test feed of ethane and steam were separately preheated to about l000 F., mixed, and passed through the cracking tube which was maintained at temperatures of 1500l600 F. by electrical heating, thereby simulating a steam cracking furnace. The steam content of the mixture was 25 weight percent and flow rates were adjusted to achieve conversions matching commercial units, thereby simulating cracking chemistry.

In a series of tests, gram coke samples, in which sample B contained arsenic concentration levels in the range of 20500 p.p.m. and sample A contained arsenic concentration levels of an imperceptible amount, were placed in the cracking tube and a two hour ethane cracking test was conducted at constant sulfur levels of p.p.m. for all tests. Table I, below, shows the weight percent change of the coke samples after three such tests.

The results of Table I show that relatively high arsenic levels in the coke promote excessive coke formation, and the rate of coke formation of coke with high arsenic levels is at least 20 times that of coke with low arsenic levels.

iIIl another series of tests about 1 gram of arsenious acid was sprinkled inside a new cracking tube and a sulfur free ethane-steam mixture was cracked (this mixture, in the absence of arsenic, was found not to coke on fresh tube walls). After 2 hours 17.9 grams of coke had been formed. The tube was then air decoked (not all of the coke being removed to simulate commercial practices) and another 2 hour test conducted. 11.7 grams of coke formed in this second test. The test was repeated seven times, with air decokin-g after each test, before the tube reached a noncoking level. These tests demonstrate the great tendency of arsenic to adhere to tube walls and to continue to promote coking.

In another series of tests 500 ppb. arsine (AsH was added to the ethane test feed. The coking rate upon cracking the test feed was 23 :g./ 2 hrs., which is equivalent to a high coking rate in normal commercial steam crackers, e.g., when the feed contains excessive amounts of sulfur.

It follows from the foregoing test procedure that arsenic causes excessive coking and its removal is highly desirable. While ethane was used in these reported tests, since it is generally representative of the steam cracking feeds, similar results are obtained with other petroleum feeds.

What is claimed is:

1. In a process for thermally cracking an arsenic containing petroleum feed stock by passing the same in admixture with steam in the absence of added catalysts through one or more tubes of a cracking furnace, the improvement which comprises treating the arsenic containing feed stock so as to substantially reduce the arsenic content thereof thereby substantially reducing the formation of coke on the interior of said tube or tubes.

2. The process of claim 1 wherein the arsenic content of the feed is reduced after treating to less than about 50 p.p.b.

3. The process of claim 1 wherein the arsenic content of the feed is reduced after treating to less than about 10 p.p.b.

4. The process of claim 1 wherein the arsenic content of the feed is reduced after treating to less than about 2 p.p.b.

5. The process of claim 1 wherein arsenic removal is efiected by a process which comprises contacting the petroleum feed with a porous support under conditions effective for removing at least a substantial portion of the arsenic from the petroleum feed.

6. The process of claim 5 wherein the support is activated charcoal.

7. The process of claim 5 wherein the support is silica gel and is acidified with at least about 20 wt. percent of sulfuric acid.

8. The process of claim 5 wherein the support is kieselguhr and is acidified with at least about 20 wt. percent phosphoric acid.

9. The process of claim 1 wherein arsenic removal is effected by a process which comprises contacting the petroleum feed with an alkaline material selected from the group consisting of alkali metal and alkaline earth metal compounds, which when dissolved in water, have a pH above about 7.

10. The process of claim 1 wherein arsenic removal is elfected by a process which comprises contacting the petroleum fraction with nitrogen compound containing three attached groups and an unshared pair of electrons.

11. The process of claim 1 wherein arsenic removal is effected by a process which comprises contacting the petroleum fraction with a salt of a metal not higher than copper in the electromotive series of metals.

12. In a process for the production of olefinic products comprising passing an arsenic containing petroleum feed stock in admixture with 20 to mole percent steam in the absence of added catalysts through one or more tubes of a cracking furnace at a temperature between about 1200 and 1700 F., the improvement which comprises treating said arsenic containing petroleum feed stock prior to passage to said furnace to reduce the arsenic concentration thereof to less than about 50 parts per billion by weight thereby substantially reducing the formation of coke on the interior of said tube or tubes.

13. The process of claim 12 wherein said arsenic containing petroleum feed stock is selected from the group consisting of saturated hydrocarbons containing from 2 to 24 carbon atoms, light naphthas boiling within a range of from about 90 to 430 F., gas oil having a boiling point ranging from between about 450 to 800 F. and kerosene boiling between about 430 to about 550 F.

14. The process of claim 12 wherein said petroleum feed stock is ethane.

References Cited UNITED STATES PATENTS 2,203,470 6/1940 Pier et a1. 208 2,865,838 12/1958 Mills 208- 2,867,577 l/1959 Urban et al. 208--289 2,951,804 9/1960 Juliard 208-91 3,093,574 6/1963 Bertolacini et al. 208-91 DELBERT E. GANTZ, Primary Examiner C. E. SPRESSER, IR., Assistant Examiner US. Cl. X.R.