US3557241A - Decoking of onstream thermal cracking tubes with h20 and h2 - Google Patents

Abstract

THERMAL CRACKING OF HYDROCARBONS IN ADMIXTURE WITH STEAM IN TUBES ARRANGED IN A CRACKING FURNACE LEADS TO THE DEPOSITION OF COKE ON THE INTERIOR WALLS OF THE TUBES, WHICH COKE MUST BE PERIODICALLY REMOVED IN ORDER TO MAINTAIN CRACKING EFFECIENCY; THE COKE CAN BE REMOVED, WITHOUT SHUTTING DOWN THE FURNACE, BY CUTTING OUT THE FEED TO AT LEAST ONE TUBE AND PASSING THROUGH SUCH TUBE OR TUBES A DECOKING FEED OF STREAM AND/OR WATER AND HYDROGEN, WHILE MAINTAINING THE FURNACE ONSTREAM AND CONTINUING THE THERMAL CRACKING PROCESS IN TUBES THAT ARE NOT BEING DECOKED.

Description

2 sheets-sheet 1 J. A. KIVLEN ETAL Jan. 19, 1971 DECOKING oF oNsT'REAM THERMAL 'CRACKING TUBES WITH H20 AND H3 Filed oci. 1e, 1968 Jan. 19, 1971 .I, A. KIvLEN ETAI. 3,557,241

DECOKING OF ONSTREAM THERMAL CRACKING TUBES WITH H30 AND H2 Filed Oct. 16, 1968 2 Sheets-Sheet 2 FIGURE 2 DEcoKING RATE IN H2- H20 GAS MIxTuRES AT ATMOSPHERIC PRESSURE cc co/'SEQ H2O MOLE FRACTION j. K lnvenors BY Attorney United States Patent O 3,557,241 DECOKING F ONSTREAM THERMAL CRACKING TUBES WITH H AND H2 John A. Kivlen, Sparta, and Ihor Koszman, Parsippany,

NJ., assignors to Esso Research and Engineering Company, a corporation of Delaware Filed Oct. 16, 1968, Ser. No. 768,065 Int. Cl. C07c 3/00; C10g 9/12 U.S. Cl. 260-683 11 Claims ABSTRACT OF THE DISCLOSURE Thermal cracking of hydrocarbons in admixture with steam in tubes arranged in a cracking furnace leads to the deposition of coke on the interior walls of the tubes, which coke must be periodically removed in order to maintain cracking efciency; the coke can be removed, without shutting down the furnace, by cutting out the feed to at least one tube and passing through such tube or tubes a decoking feed of steam and/ or water and hydrogen, while maintaining the furnace onstream and continuing the thermal cracking process in tubes that are not being decoked.

FIELD OF THE INVENTION This invention relates to a process for decoking the tubes of a cracking furnace. More particularly, this invention relates to an improved process for decoking steam cracking tubes while maintaining the furnace onstream and continuing the cracking process. Still more particularly, this invention relates to an onstream decoking process whereby the hydrocarbon feed to at least one tube in the cracking furnace is cut out and a decoking feed comprising steam and/or water and hydrogen is passed through said tube or tubes and the coke is removed.

PRIOR ART The thermal cracking of petroleum feed streams, with or without the presence of steam, is well known to the art and is widely used as a source of valuable unsaturated compounds, e.g., ethylene, propylene, butadiene. Generally, when noncatalytic processes are conducted, it is desirable to employ steam as the principal diluent in order to control the reaction and reduce erosion and corrosion effects. While steam cracking has been technically and economically successful, several major drawbacks exist which militate against the development of the full potential of the steam cracking process. Essentially, these drawbacks center around the carbon forming tendency of the process gas, i.e., vaporized petroleum feed at reaction (cracking) temperatures.

Perhaps the most objectionable drawback relating to carbon formation is the deposition of coke on the interior of the tube walls through which the cracking mixture ows. Coke deposition is believed to be due to the formation of free radicals, e.g., when ethane is cracked methylene radicals can -be formed, which may then polymerize with other unsaturate components into long chain compounds and dehydrogenate to form coke on the tube walls. The coke tends to build up and, therefore, reduces the effective cross-sectional area of the tube necessitating higher pressures to maintain a constant throughput. More importantly, however, because coke is an excellent thermal insulator, its formation is accompanied by a sharp increase in furnace tube temperature, in order to maintain l 3,557,241 ce Patented Jan. 19, 1971 cracking efficiency, thereby resulting in a decrease in tube life and the limiting of the craking temperature that can be employed (which also limits conversions and yields). Eventually, the coke lbuildup is such that the furnace must be shut down for decoking with a consequent loss in production capacity.

The coking problem has been attacked by a process which basically involves the elimination of the hydrocarbon feed from at least one of the tubes and passing therethrough a decoking feed of steam and/or water while maintaining the remaining tubes onstream. Such a process is reported in U.S. 3,365,387 to Cahn et al., which is incorporated herein by reference. Now, however, by the process of this invention it is possible to increase substantially the rate of decoking using steam and/ or water and thereby return these tubes to onstream service more quickly.

SUMMARY OF THE INVENTION In acordance with this invention, therefore, an im,- proved process of the decoking of steam cracking tubes is provided whereby the hydrocarbon feed to at least one tube is cut out and a decoking feed comprising steam and/or water and hydrogen is passed through said tube or tubes while maintaining the remaining tubes onstream, i.e., in normal service, continuing to crack hydrocarbon feed in the remaining tubes, and maintaining the temperature level of the tube or tubes rbeing decoked essentially the same as the temperature level in the tubes remaining onstream. This invention has the advantage of lending a decoking capability to the steam cracking process which allows decoking of one or more tubes while maintaining furnace temperatures and continuing to make product. Additionally, this invention contemplates the decoking of only a single tube at a time in the furnace, thereby not substantially reducing the conversion capacity of the furnace as a whole, or the decoking of any number of tubes simultaneously or successively, or the decoking all of the tubes of the furnace simultaneously. Nevertheless, because the vapor load on downstream equipment increases with an increase in the number of tubes being simultaneously decoked by this method, it is preferred that only a minor portion of the tubes in a steam cracking furnace :be decoked by this method at any one time. After the decoking operation has been completed, the clean tubes are returned to normal service by reintroducing the hydrocarbon feed, adjusting the steam/water rate, and cutting out the hydrogen.

While not wishing to be bound by any particular theory, it is believed that the decoking reaction results from an interaction of steam and coke, according to Equation l:

i.e., the water gas reaction. However, at steam cracking temperatures this reaction is quite slow and would not !be expected to be economically feasible as a decoking method. However, it is believed that as the coke forms on the tube surfaces a diffusion process takes place between the coke and the metal tube. Thus, some coke goes into solid solution in the metal tube while trace amounts of metal, e.g., iron and nickel (the latter 1being present in substantial amounts in widely used stainless steel tubes), diffuse into the coke layer. These trace amounts of metals then tend to catalyze the water gas reaction allowing it to proceed at favorable rates at steam cracking temperatures. Additionally, it is believed that there is an induction period during which the water gas reaction does not proceed rapidly. This induction period is believed to be caused by the presence of sulfur (which is present in some amount in almost every steam cracking feed) which coats the trace elements thereby masking their catalytic effect. Only when the sulfur is removed do the trace metals become active catalysts. The process of this invention eliminates or substantially decreases this induction period due to the presence of hydrogen in the decoking feed, the hydrogen acting as a desulfurization agent:

Hg-l-S (in coke or adsorbed on Ni or Fe)- HZS-l-desulfurized cokel-l-sulfur free Ni or Fe (2) (Norm-ally, there will be no sulfur in the decoking feed and the reverse reaction of (2) will not occur.)

The quantities of steam and/or water that are used during the decoking process are predetermined to Imeet the following criteria.

(1) Sufficient steam and/or water are introduced to remove the heat normally going to the process gas, i.e., hydrocarbon and steam mixture, without exceeding the tube metal temperature allowances as determined by stress or oxidation limits for the tube material.

(2) The temperature of the decoking feed, i.e., steam/ water and hydrogen, entering the section of the furnace to be decoked i.e., cracking section, should be about 700 F. or higher. If water is employed, it must be vaporized and superheated while steam need only be superheated.

(3) The mass rate of decoking feed entering the furnace section to be decoked should preferably be greater than pounds per second per square foot of tube internal cross-sectional area when tube outlet pressure is in the order of -25 p.s.i.a. Higher mass rates at constant temperature reduce the time required for decoking. Higher operating pressures in the furnace tubes being decoked require higher mass rates of steam for the same decoking time. (It is noted, however, that in this invention it is preferable to conduct the decoking operation at pressures as low as possible, e.g., atmospheric, since lower pressures tend to increase the decoking rate.)

Operation in this manner insures that the temperature in the tubes being decoked is essentially the same as in the tubes remaining onstream, i.e., in normal service.

When decoking is completed, e.g., about 2 to 4 hours for a typical tube under optimum conditions, the supply of' steam and/or water and hydrogen can be cut off from the decoked tube and feed may be simultaneously reintroduced. The completion of the decoking operation can be monitored by any one of several methods, such as l) decrease in pressure drop across the section of the furnace being decoked, (2) decrease in tube metal temperature, or (3) rate of carbon monoxide formation (cf. Equation l, CO will increase during decoking but fall off sharply when little or no coke is left in the tube).

DRAWING DESCRIPTION This invention may be more fully understood from the following description when read in conjunction with accompanying FIG. l wherein the flow path of the reactants through an apparatus for thermal cracking of hydrocarbons is illustrated diagrammatically.

Referring to the drawing, the cracking furnace 10 comprises an upper, convection or preheat section 11 and a lower, cracking zone 12. Burners 13 are provided on the side walls and/or on the bottom of the furnace to supply heat. The number of burners provided is dependent upon the heat required and may vary considerably.

Although not shown in detail in the drawing, the furnace contains several conduits or passes in parallel. Each pass may contain a number of connected tubular members or tubes that provide a flow path through the convection section and into the cracking section. ln the drawing, one pass is shown, with the tubes in the convection section 11 designated by the numeral 15 and thecracking coils or tubes in the cracking zone 12 designated by the numeral 16. It is to be understood that the number of conduits or tubes in the furnace is a function of the size of the furnace and is dictated solely by design considerations.

Hydrocarbon feed stock is supplied to the steam cracker via supply conduit 20 and manifold or distributor conduit 21 to the several parallel cracking conduits or passes. A control valve 22 is provided on each conduit 23 connecting the feed distributor 21 to each of the cracking conduits or tubes. Steam,`or in the decoking operation, steam and/or water are supplied through inlet line 24and valve 25 to the conduit 23. (In some cases, steam and water are supplied through separate lines and not necessarily at the identical point in the convection section.) Hydrogen is mixed with the steam or water inlet from line 26 through valve 27.

The reaction products are discharged from the coils or tubes 16 of the cracking furnace via conduits 28 into conduit or header 29 from which they are discharged into conduit 30. In order to stop the cracking reaction promptly and thereby prevent or minimize side reactions, quenching agents such as higher boiling hydrocarbons and/or water are supplied through conduit 31 and control valve 32. The mixture of quenched reaction products and quenching agent is discharged via conduit 30 into fractionating tower 33. Aromatic tar product is withdrawn from the bottom of fractionating tower 33 through line 34 and product is taken overhead via line 35. Other intermediate boiling range fractions may be withdrawn as product or recycled to a higher plate in the fractionating tower as one or more reflux streams. The quench oil may be Withdrawn from the fractionating tower 33 through line 36 and passed through heat exchanger 37 where it is passed in indirect heat exchange relation to the hydrocarbon feed stock for preheat thereof or to water for steam formation while cooling the quench oil to a suitable temperature for discharge through line 31 and valve 32 into the reaction product stream in line 30 as described above.

The onstream decoking procedure requires the closing of one of the hydrocarbon feed valves 22 and the opening of the steam water valve 25. The amount of steam and water passed through the decoking conduit 24 is adjusted so that the steam temperature inside the pass is about 700 F. or higher at the point'of transition from convection'tubes 15 to cracking tubes 16. When sufficient time has elapsed to allow the coke to be removed from the inside of the tubes, valves 25 and 27 are closed and valv 22 is opened.

FIG. 2 depicts a graphical portrayal of the increase in decoking rate due to the presence of hydrogen in a steam/ water decoking feed. The figure shows carbon monoxide formation plotted against the mole fraction of water in a decoking feed of water and hydrogen as a function of temperature. As mentioned, decoking by steam/water involves the water gas reaction and, therefore, carbon monoxide make is directly related to decoking rate. Examination of FIG. 2 shows that an H2O mole fraction of zero, the rate of decoking is zero. However, for each temperature curve shown the rate of decoking steadily increases as the mole fraction of H2O increases until a certain point is reached, depending upon temperature, where the effect of hydrogen in the decoking feed begins to drop off and at an H2O mole fraction of above about 0.8 the effect of hydrogen is negligible and the decoking rate approximates that for water only. It is also shown that as temperature increases the optimum steam to hydrogen mole ratio increases. Thus, the steam to hydrogen mole ratio will vary with temperature. Nevertheless, it can be generally stated that steam to hydrogen mole` ratios of from 1/1 to 3/1 are preferred and more preferably 2/1 to 3/1.

These curves were developed by passing a steam-hydrogen decoking feed through a coked 310 stainless steel tube of 1" ID by 3 long. The tube wall temperature was maintained at 1600-2000 F. The H2O/H2 ratio was varied by mixing CP grades of oxygen and an excess of hydrogen outside the tube and combusting the mixture. By varying the rate of oxygen flow the H2O/H2 ratio could be varied instantaneously and, in turn, allowed the measurement of the decoking rate, i.e., by carbon monoxide make.

The steam cracking operation is vold and Well known (see, for example, Chemical Week, Nov. 13, 1965, p. 72 et seq), and will only be briefly described hereinbelow. Generally, the petroleum feed fraction is admixed with steam, i.e., in amounts ranging from about 20-95 mole percent steam, prior to entry into the steam cracking furnace which may be heated by any suitable means, e.g., gas firing, etc. The furnace itself normally contains two sections, a convection section wherein the feed is vaporized, if not already in that form and preheated, and a radiant or cracking section, the feed being passed in admixture with steam through one or more furnace tubes located within the furnace. The convection section is normally employed to increase heating eiciency and the petroleum-steam mixture is heated therein to intermediate temperatures, i.e., about 100G-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, i.e., the cracking zone, where the temperature of the reactants is quickly raised to about 1200-1700 F., preferably 1500-1700 F., or higher, as tube metal materials permit, and the feed is cracked. (Generally, raising the temperature of the reactants to the mentioned ranges requires heating the tubes to about 1400-2000" F., preferably 1600-2000 F. and higher as tube materials permit.) Residence times in the radiant section are carefully controlled to minimize polymerization and other undesirable reactions. Thus, residence times in the cracking zone will range from about 0.1-10 seconds, preferably 0.1-1 second. Pressures within the tubes may range from about -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 zone, the reaction products are immediately quenched to stop further reaction and/or minimize loss primary conversion products.

Now, since cracking occurs only in the radiant zone of the furnace, it is only this zone that requires decoking. And, since the flow of decoking feed through this Zone will be such as to maintain normal onstream temperatures, the tubes not being decoked can continue to crack feed with little or no disruption to the entire unit.

The petroleum fractions which may be converted by this process can vary widely and one skilled in the art will readily determine optimum conditions for different petroleum feeds. Generally, however, the process is most applicable to hydrocarbon feeds consisting essentially of cyclic or acyclic saturated hydrocarbons. Thus, hydrocarbons that may be utilized herein include such cyclic hydrocarbons as cyclopropane, cyclobutane, cyclopentane, cyclohexane, cyclooctane, cyclododecane, etc., and mixtures thereof. Acyclic hydrocarbon feeds include any alkane, namely, aliphatic hydrocarbons of the methane series or mixtures of alkanes with cycloalkanes. Preferred feeds are those saturated hydrocarbons containing from 2 to about 24 carbon atoms, more preferably alkanes containing 2 to about 12 carbon atoms, e.g., ethane, propane, butane, isobutane, hexanes, heptanes, etc., n-hexadecane, eicosane, and light naphthas boiling in the range of 90- 430" F., gas oils of 450-800 F., or higher boiling points, and kerosenes of 430-550" F. boiling points can also be effectively cracked in this process. Because coking can be readily controlled by practicing the invention described herein, higher temperatures in the cracking zone (radiant section) can be employed, the credit for these higher ternperatures being taken as increased yields or in the cracking of poor quality feed stocks, i.e., those that would normally give excessive coking.

What is claimed is:

1. In a process for thermally cracking hydrocarbon materials by passing the same in admixture with steam through a multiplicity of tubes arranged in a cracking furnace wherein the tubes are subjected to heat sufficient to raise the temperature of the reactant so that the reactants are cracked, and wherein coke deposits are formed on the interior Walls of said tubes, the improvement which comprises taking at least one of the tubes oifstream by cutting out the ow of hydrocarbon fed and passing a decoking feed containing hydrogen and a component selected from the group consisting of water, steam, and mixtures thereof, the mole ratio of said component to hydrogen being from about 1:1 to about 3:1, through the tubes in suicient amount to maintain the temperature of the decoking feed in the offstream tubes at essentially the same level as in the tubes remaining onstream, thereby effecting the removal of coke from the interior of the offstream tubes, and thereafter returning the offstream tubes to onstream operation.

2. The process of claim 1 wherein the hydrocarbon feed is cut out of all of the tubes and the tubes are taken otfstream for decoking and maintaining the temperature in the tubes at essentially the same level as that for onstream operation.

3. The process of claim 1 wherein the tubes are heated by radiant heat sufficient to raise the temperature of the reactants within the tubes to about 12001800 F.

4. The process of claim 1 wherein the hydrocarbon feed is cut out and the decoking feed is supplied to only one of the tubes at a time to remove coke therefrom without substantially reducing the conversion capacity of the cracking furnace as a whole.

5. In a process for thermally cracking hydrocarbon materials by passing the same in admixture with steam through a multiplicity of tubes arranged in a cracking furnace wherein the tubes are subjected to radiant heat sufficient to raise the temperature of the reactants within the tubes to about 12001800 F. and wherein coke deposits are formed on the interior walls of said tubes, the lmprovement which comprises taking at least one of the tubes offstream by cutting out the flow of hydrocarbon feed and passing a decoking feed containing hydrogen and a component selected from the group consisting of steam, water, and mixtures thereof, the mole ratio of said component to hydrogen being from about 2:1 to about 3:1, through the tubes in sufficient amount to maintain the temperature of the decoking feed in the oifstream tubes at essentially the same level as in the tubes remaining onstream, continuing the supply of decoking feed through the offstream tubes fol a period sufficient to effect removal of coke on the interior of the offstream tubes, and thereafter returning the offstream tubes to onstream operation.

6. The process of claim S wherein a minor portion of tubes are taken offstream at any one time for decoking.

7. The process of claim 5 wherein the reactants are heated to an intermediate temperature in a convection section of the cracking furnace and are cracked in a cracking section of the cracking furnace.

8. The process of claim 7 wherein the temperature of the decoking feed in the offstream tubes is at least about 700 F. as it passes from the portion of the tubes in convection section to the portion of the tubes in the cracking section.

9. The process of claim 8 wherein the mass rate of decoking feed entering the portion of the tube to be decoked is greater than 15 pounds per second per square foot of tube internal cross sectional area when the tube outlet pressure is of the order of 20a-25 p.s.i.a.

10. The process of claim 9 wherein only one of the tubes is taken offstream at a time in order to remove coke therefrom without substantially reducing the conversion capacity of the cracking furnace as a whole.

7 8 11. The process of claim 10l wherein the decoking feed OTHER REFERENCE 1S sulfur free' R f C t d Comprehensive Treatise on Inorganic Theroretical v e "ences e Chemisrry,Me110r,v01. 5, pp. 63 and 64, 1925.

UNITED STATES PATENTS 3,365,387 1/1968 Cahn et aL 208 48 5 DELBERT E. GANTZ, Primary Examiner 2,271,955 2/ 1942 Russell 208-48 J. M. NELSON, Assistant Examiner 2,289,351 7/1942 Dixon et al 208-48 2,547,221 4/1951 Layng 252-411 U.S. Cl. X.R.

3,156,734 1/1964 Happel 260-679 208-48

Images