US2893941A - Removing and preventing coke formation in tubular heaters by use of potassium carbonate - Google Patents

Description

y 7, 1959 w. c. KOHFELDT ET AL 2,893,941

REMOVING AND PREVENTING COKE FORMATION IN TUBULAR HEATERS BY USE OF POTASSIUM CARBONATE Filed. Jan. 27, 1955 m zst 2 s 2:558

INVENTORS WALTER c KOHFELDT FREDERICK J. HEBERT sv yw ATTORNEY MGVING AND PREVENTING COKE FORMA- TION IN TUBULAR HEATERS BY USE OF P- TASSEUM CARBONATE Walter Clarence Kohfeldt and Frederick Joseph Hebert,

Baton Rouge, La., assignors to Esso Research and Engineering Company, a corporation of Delaware Application January 27, 1955, Serial No. 484,408

4 Claims. (Cl. 208-48) The present invention relates to a process for the cracking of hydrocarbons wherein the formation of coke during passage of the hydrocarbon material through the coils of a tubular heater or cracking coil is substantially avoided. The invention also relates to such a process wherein coke previously formed may be substantially removed during continuation of the substantially normal flow of hydrocarbon feed materials through a cracking coil system. More particularly, the invention relates to a process for cracking hydrocarbons in the presence of steam, and especially when accompanied by the injection of small amounts of potassium carbonate.

When hydrocarbon fluids are subjected to heating at elevated temperatures in the presence of steam to secure thermal cracking in a cracking coil furnace and related preheaters or heat exchangers, coke deposits tend to form on the inner walls of the tubular members forming the furnace coils to an extent which eventually requires that the apparatus be taken out of service until the coke deposits are removed mechanically, or otherwise. A variety of methods for removal of coke deposits of the nature contemplated have been suggested previously. In addi tion to mechanical systems wherein the tube ends are opened and the coke deposits removed by drilling or grinding, it has been proposed that the coke deposits be removed by soaking in the presence of boiling water followed by steaming and blowing with air while applying heat externally of the tubes. Chemical processes have also been suggested as where the coke deposit is first impregnated with sulfuric acid and thereafter subjected to the action of an alkali carbonate solution to generate carbon dioxide-gas within the interstices of the deposit, and substantially by expansion of the generated gas, causing spalling of the deposits adhering to the inner walls of the cracking coil tubes.

Each of the methods previously employed or suggested has-required that the normal function of the furnace and coil or tubes for the cracking of hydrocarbon materials be interrupted during the cleaning or coke removal operation. Such interruption of on-stream time of the cracking coil produces a. serious economic problem. In addition, the coke removal processes previously proposed have assumed the coke formation to be an inevitable result of the cracking process, and therefore each has been directed to removal of the coke after formation rather than through theprevention of formation. As aresult, the gradual decreased efiiciency of operation of a cracking coil as coke formation progresses has been an accepted condition of operation.

It is anobject of the present invention to provide for an improved process of thermal cracking in the presence of steam wherein the initial formation of coke deposits is essentially inhibited. It is a further object of the invention to provide an improved process wherein coke deposits, such as may have been formed in absence of the benefits of the present disclosures, may be substantially removed while continuing normal operation of a steam cracking procedure. The inventionand itsobjects 2,893,941 Patented July 7, 1959 2 may be more fully understood from the following description when read in conjunction with the accompanying drawing, wherein the flow path of a hydrocarbon feed material through an apparatus for thermal cracking of such material in the presence of steam is illustrated diagrammatically.

In the drawing, the numeral 1 designates a supply conduit leading from a source of a hydrocarbon material employed as a feed stock to a thermal cracking system, wherein cracking takes place in the presence of injected steam. The numeral 2 designates a preheater including a convection heating section 3 and a radiant heating section 4. Although not shown in detail in the drawing, each section may contain a series of banked tubular members arranged in groups. In the drawing, the convection and radiant tubes are designated respectively by the numerals 5 and 6. The supply conduit 1 is connected to one or more of the tubular members 5 at the inlet of the convection section 3. The convection tubes are in turn connected to the tubes 6 in the radiant section of the preheater 2, in a substantially conventional manner. Where two or more groups of convection tubes are employed, it is preferred that a similar number of groups of radiant tubes will be disposed in the radiant section of the preheater. As indicated by the arrows in the drawing, flow through the convection sections and radiant sections is in countercurrent relation to flame travel from burners 7, and to travel of the convection gases through the preheater. In the preheater as illustrated, burners are provided in the roof of the radiant section, flames and hot gases traveling downwardly through this section through and around the tubes and then passing upwardly through the convection section. The hydrocarbon material is substantially vaporized during passage through the tubes 5 and 6, the vapors being discharged from the uppermost tubes in the radiant section as by way of an outlet conduit 8, passed through a conduit, such as designated by the numeral 10, and thence into a separator drum 11.

Under certain circumstances, as Where the initial feed stock is a clean material completely vaporizable at the temperatures which may exist in the preheater furnace 2, the vapors discharge from the radiant section of the furnace 2 by way of the outlet conduit 8 and the conduit 10 may be passed directly into the convection section of the cracking furnace 16 by way of the bypass conduit 9 communicating between the conduit 16 and the conduit 13. Are shown, valves are provided in the conduits 10 and 9 and suitably located therein to permit either employment of the separatoror by-passing of the separator. In the separator drum 11, unvaporized and condensable portions of the feed material are withdrawn through a bottom conduit 12 while the uncondensed vapors are taken off overhead as by means of the conduit 13. In the steam cracking system contemplated by the present invention, steam is introduced into the separator drum in a lower portion thereof as by means of the steam feed conduit 14. Steam may also be introduced into the overhead conduit 13 as by means of the steam feed conduit 15.

The cracking operation is carried out with the feed material in a vapor phase in the coils or tubes of a conventional cracking coil furnace. In the drawing, the furnace is designated by the numeral 16 and includes a convection section 17 and a radiant section 18. The tubes or coils disposed in the convection section of the furnace are designated inthe drawing by the numeral 19, while the tubes or coils in the radiant section are designated by the numeral 20. In addition to the tubes 19 and-20, soaker tubes 21 may also be provided. When provided, these tubes are disposed at or just below the entrance to the convection section. Also as mentionedwithreference to the vaporizer coils or tubes 5 and 6, the respective furnace tubes 19 and 20 may be arranged as groups of tubes disposed in banks. When so disposed, the conduit 13 is connected by manifolding to the respective groups. Also as previously described with reference to the preheater or vaporizing structure, where the convection tubes are divided into two or more groups, the tubes 19 in the radiant section of the furnace will be divided into a corresponding number of groups, with each convection group of tubes connected to a corresponding group of tubes in the radiant section. Normally, the convection tubes 19 are arranged in the convection section 'of the cracking furnace so as to provide for flow therethrough which is countercurrent to the flow of combustion gases, while in the radiant section of the cracking furnace, provision may be made for flow through the radiant tubes in concurrent relation to flame travel from the heating burners provided, such as the burners 22 shown.

In the apparatus as shown in the drawing, the soaker tubes are connected to a transfer line 32, provision being made at'or adjacent to the connection to the transfer line 32 for the introduction of a comparatively cool liquid material for the purpose of quenching vapors issuing from the soaker tubes to a temperature below the cracking temperature attained in the furnace. Such a connection for the introduction of a quenching liquid is indicated by the numeral 33. The transfer line 32 is in turn connected to a fractionating column indicated in the drawing by the numeral 34 and preferably at an intermediate level therein. Product draw-off lines such as conduits 35, 36

and 37 shown, communicate with the fractionating tower the soaker tubes 21 may be omitted, in which case the outlets of the radiant tubes 20 may be connected directly to the conduit 32.

According to the present invention, in a conventional system such as illustrated in the drawing and described terials which have been found to reduce the formation of carbon or coke in the conduits and connections forming a flow path through the respective furnaces and opening into the fractionator for the vaporized feed material. These materials tending to reduce or remove coke formation may be introduced into the system at one or more points along the flow path. Such points of introduction are represented in the drawings by the conduit connections 41, 42 and 43. The conduit connection 41 provides above, means are provided for theintroduction of mafor introduction of the agent to reduce or remove coke along with the liquid feed entering'the preheater furnace 2 byway of the line 1. The connection 42 is made to the overhead conduit connection between the separator drum 11 and 'the convection section furnace tubes 19, while the third connection 43 is made to the inlet of the radiant tubes 20 in cracking furnace 16.' Each of the conduit connections 41, 42 and 43 is provided for communication with a supply conduit 44 including a means such as the indicated pump 45 for introducing the coke inhibition "conduit 44, alternately the coke inhibition agent may be introduced in conjunction with the introduction of steam through the conduit 15 by the conventional employment of a steam eductor 46 connected in the conduit 15 and to supply conduit 44 by line 47.

In a typical and convectional steam cracking operation, a cracking feed stock selected from among hydrocarbon materials such as heavy naphtha, kerosene, and gas oil would be fed into the preheater furnace 2 by way of the feed conduit 1 and thence into the separator 11 by way of convection and radiant heating coils 5 and 6, conduit connection 8 and conduit 10. Depending upon the nature of the feed stock, the material passing through the conduit 1 would be raised from a temperature at the inlet in the range of from F. to about 500 F., to a temperature at the outlet of the furnace 2 in the range of from about 600 F. to about 900 F. In the separator 11, those fractions not vaporized in the furnace 2 would be removed as bottoms through the line 12. Although not shown, this material might be recycled or removed from the system for special treatment. Steam added to the separator as by way of the connection 14 tends to increase the partial pressure of the feed material facilitating vaporization of the preheated material. The combined vapors from the separator drum 11 then pass by way of the conduit 13 into the convection section 17 of the cracking furnace 16 and through the tubes 19 therein. Conventionally, additional steam would be introduced into the vapor transfer line 13 intermediate the separator drum and the inlet of the convection tubes 19 in section 17.

In passing through the cracking coil furnace 16, the vapors from the separator drum 11 are subjected to elevated temperatures in the cracking range and according to theinitial feed stock in the range of from about 1050" F. to about 1800 F. The pressures employed in such a system may range from the lowest pressure feasible to maintain adequate flow through the system to substantially elevated pressures. In the tubes in the cracking coil furnace itself, the inlet pressures conventionally may be in the range of from the pressure substantially adequate to produce flow, to about 100 pounds per square inch gauge and even higher. Convenient and conven tional preferred inlet pressures and outlet temperatures in the cracking coil might be in the range of from about 50 pounds per square inch gauge to'about 100 pounds per square inch gauge and from about 1200 F. to about 1400 F. Passing from the cracking coil furnace as by way of the conduit 32 indicated in the drawing, the cracked vapors are introduced into a fractionator column such as is indicated by the numeral 34. In the fractionator, the vapors are fractionated and condensed in stages upwardly through the tower 34 separating them into their various fractions which are removed at intervals upwardly through the tower as by way of the product draw-off lines indicated by the numerals 35, 36 and37. The heavier most readily condensable portions of the connected vapor stream are removed as bottoms from the tower 34 by way of conduit 39, while the substantially uncondensed and uncondensable vapors or gases are removed overhead through the conduit 38. In order to avoid continuance of the cracking reaction during passage through the conduit 32, it is customary to quench the vapor stream therein by introducing a cooler liquid material as through the line 33. It is also conventional to supply this quenching liquid from a selected product stream or streams withdrawn from the fractionator. A

provision fora typical recycle quench is shown in the forming the cracking coil requiring the system to be shut down while the deposits-are removed-by-suchmethods as havebeen previously referred to. Valuable production time is thus lost. In addition, if the deposits form within-the tubes, the tubes tend to overheat, resulting in rapid deterioration of the metal and also in reduction of operating efiiciency. According to the method contemplated by the present invention, theoriginal formation of carbon may be reduced with a consequent extension of on-stream time for the system. Also, where the method according to the present invention has not been employed initially in the cracking operation, removal of tube deposits and coke may be accomplished effectively and some of the benefits of the invention thereby may be realized.

It has been found that by the injection of small amounts of potassium-carbonateatone or more points in the flow path for the feed materials or their vapors, initial formation of coke deposits may be inhibited or substantial re moval of previously formed deposits may be accomplished. As shown in the drawing, the potassium carbonate maybe introduced withthe feed stock as into the conduit 1 by means of conduit 41, into the vapor line 13 from the separator 11 asbymeans of the conduit 42 or at any other point in the flow path through the system such as is indicated by the connection 43 to the inlet of the coils 20 in the radiant section 18 of the cracking coil furnace 16. Any convenient system for introducing the inhibitor material may be employed in the apparatus as illustrated in the drawing. It is contemplated that the material will be introduced as a solution of potassium carbonate and preferably as an aqueous solution as by means of the pump 45 and the discharge conduit 44. Also represented in the drawing is a supplementary or alternative system for the introduction of a potassium carbonate solution. Such a system is represented as a steam eductor 46 in the steam conduit connection 15, whereby a potassium carbonate solution may be drawn from the line 44 as through the conduit connection 47. Preferably the solution is prepared and introduced into the system in such a fashion as to accomplish a concentration of potassium carbonate in the range of from about 20 to about 40 parts per million as based on the volume of the liquid feed stock initially fed into the system, although the proportion may be in the range of from about 5 to 100, or more, parts per million.

As a specific example of an operation according to the present invention, a cracking coil feed stock comprising gas oil was passed through the preheater furnace 2 'by way of the conduits 1, 5 and 6 at the rate of about 39,500 lbs. per hour, the temperature of the feed stock at the inlet to the convection coil section 5 was about 450 F., and was at a pressure of 150 pounds per square inch gauge. In the preheater furnace, the incoming feed stock was raised to a temperature of 800 F. at the outlet of the coils 6. After separation in the drum 11, the overhead vapors were passed by way of the line 13 to the inlet of convection tubes 19 in the cracking coil furnace 16. Steam was introduced into the system by way of the connections 14 and 15 to the extent of a total of about 9000 pounds per hour of which the major portion or 75-80% was introduced by way of the conduit 15. The temperature of the vapors entering the coils 19 was about 760 F. At the same time, an aqueous solution of potassium carbonate was introduced into the conduit 13 by way of the conduit connection 42. This solution consisted of potassium carbonate, in the proportion of 1% by weight in water and the solution was introduced into the conduit 13 at a rate of 15 gallons per hour providing a percentage introduction of 0.32 wt. percent of solution on the feed stock at the feed rate of 39,500 lbs. per hour. At this rate, the proportion of potassium carbonate injected to the feed stock was about 32 parts per million. In this example, the introduction of the aqueous solution of potassium carbonate was commenced after the system had been operated in a conventional manner with out the introduction of potassium carbonate, for a period of about seven months and just prior to what would have been the normal or conventional cut-off time for the operation. By inspection, the tubes in furnace 16 had indicated a number of hot spots, and several tubes had shown signs of extreme overheating. In addition, the pressure drop through the system from the inlet line 13 to the outlet of the conduit 32 had increased from a coil pressure drop of about 45 to 55 pounds per square inch gauge to a pressure drop of 66.7 pounds per square inch gauge, indicating that the coils were heavily coked.

Operation of the system with injection of the aqueous potassium carbonate solutionas described above was continued for a period of over fifteen days. During that time, the following table indicates the improved operation obtained during the treatment period, showing averaged data for two coil circuits.

TABLE I Potassium carbonate decoking test Pressures, p.s.i.g.

Day Time Coil Coil Inlet Outlet Delta P Pressure Pressure 1 8:00 am. (Just prior 83. 5 16.8 G6. 7

to start of test). 0 81 17. 4 63.6 79. 5 17.3 62. 2 78 18 60. 0 75 18 57.0 77. 5 19 58. 5 77.5 18. 5 59.0 75. 5 76. 5 77. 5 81 1 Before changing circuit flow to equalize radiant outlet temperature. 9 After changing circuit flow to equalize radiant outlet temperature.

In addition to the improved operation as shown by the foregoing table, visual inspection also indicated improved conditions in the cracking coil tubes. This was particularly noticeable in that the tubes in the radiant section 18 of the furnace 16 visibly darkened during the test indicating that coke deposits had been removed from the tubes. Also, when after the test had been completed, the tubes were opened for inspection, the tubes in the radiant section were found to be cleaner than normal with coke formation within the tubes ranging from no more than an extremely thin film at the inlet of the tubes to a deposit about A" in thickness at the outlet. This deposit compares with deposits found in normal and conventional operation wherein they would range up to a substantially completely blocked condition at the outlet. The reduction of coke took place also in the convection tubes 19 as well as in the soaker tubes 21. Consideralble amounts of loose coke in pieces ranging up to the circumference of the tubes and about 4" in length were also found at the outlet of the tubes in the radiant section as well as in the return bends and in the soaker tubes indicating that the action of the injected potassium carbonate solution had not only inhibited the formation of additional coke deposits, but had also acted to destroy deposits previously formed and to produce some spalling of the remaining deposits.

This example shows the particular utility of using substantial amounts of potassium carbonate, within the range of about 2040 ppm, to overcome severe coking. In this aspect of the invention, periodic injection of such quantities of potassium carbonate may be employed, for example, on a monthly or bimonthly basis. Alternatively, however, somewhat smaller amounts of potassium carbonate may be continuously injected. In this case amounts of about 5-10 p.p.m. may be employed so as to inhibit coke formation.

The mechanism by which the objects of this invention are achieved is not fully understood. It is probable that thepotassium carbonate is converted to the form of potassium oxide so that other potassium compounds capable of forming the oxide may be employed. More broadly, however, it is contemplated thatthe presence of potassium ions is the basis of the coke inhibition so that potassium compounds other than those specifically named may be employed. It is to be understood then that use of potassium carbonate constitutes the preferred mode of practicing this invention, without limiting it thereto.

What is claimed is:

1. In a process for treating liquid hydrocarbon materials which includes the steps of passing a stream of said materials through a confined flow path including a thermal cracking zone externally heated to produce thermal cracking temperatures in said stream of materials, thermally cracking said hydrocarbon materials in the presence of potassium carbonate and also in the presence of steam added to said stream during passage through said flow path, the step which comprises injecting said potassium carbonate into said stream and flow path at least at a point in said flow path upstream from said thermal cracking zone as an aqueous solution thereof and in a proportion of from about to about 100 parts of said potassium carbonate per million parts of said liquid hydrocarbon materials.

' 2;, A process-according ,to claim 1, wherein said potassium carbonategis present in the solution in the amount of-a'bout 1.0weightpercent: 5 j

3. A process according to claim 1, wherein said potassium carbonate in solution is injected in the amount of about 0.32weight percent of saidliquid'hydrocarbon materials passed into said confined flow path. 4. A process according to claim 1, wherein said potassium carbonate in solution is introduced into said liquid hydrocarbon materials in a proportion of between about 20 and about 40 parts of potassium carbonate per million parts of said hydrocarbon materials; 7

References Cited in the file of this patent UNITED STATES PATENTS 1,569,532" Berry Jan. 12, 1926 1,613,124 Owens Jan. 4, 1927 1,927,829 I-Iarnsberger et a1 'Sept. 26, 1933 2,015,420 Chave et al. Sept. 24, 1935 2,200,463 Alexander May 14, 1940 2,652,319 Sweetser et a1. Sept. 15, 1953 2,738,307 Beckberger Mar, 13, 1956

Claims (1)

1. IN A PROCESS FOR TREATING LIQUID HYDROCARBON MATERIALS WHICH INCLUDES THE STEPS OF PASSING A STREAM OF SAID MATERIALS THROUGH A CONFINED FLOW PATH INCLUDING A THERMAL CRACKING ZONE EXTERNALLY HEATED TO PRODUCE THERMAL CRACKING TEMPERATURES IN SAID STREAM OF MATERIALS, THERMALLY CRACKING SAID HYDROCARBON MATERIALS IN THE PRESENCE OF POTASSIUM CARBONATE AND ALSO IN THE PRESENCE OF STREAM ADDED TO SAID STREAM DURING PASSAGE THROUGH SAID FLOW PATH, THE STEP WHICH COMPRISES INJECTING SAID POTASSIUM CARBONATE INTO SAID STREAM FROM AND FLOW PATH AT LEAST AT A ING ZONE AS AN AQUOEUS SOLUTION THEREOF AND IN A PROPORING ZONE AS AN AQUEOUS SOLUTION THEREOF AND IN A PROPORTION OF FROM ABOUT 5 TO ABOUT 100 PARTS OF SAID POTASSIUM CARBONATE PER MILLION PARTS OF SAID LIQUID HYDROCARBON MATERIALS.

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