IL26518A - Thermal cracking process with improved decoking - Google Patents

Description

PATENTS AND DESIGNS ORDINANCE SPECIFICATION 1AL CRACKLTRJ PROCESS WITH IMPROVED DECOKIKG miieo nptnn e* »eiii mx*e epip srpwa X ( we ) SSSO RESEARCH AITD MOIHBSRIFO COMPAHT, a Corporation organized and existing tinder the lavs of the State of Delaware, of Ilizabeth, ^ew Jersey, U.S.A. do hereby declare the nature of this invention and in what manner the same is to be performed, to be particularly described and ascertained in and by the following statement :- The present invention pertains to a method of decoking the tubes of a cracking furnace and particularly to the decoking of the coils or tubes of a steam cracking furnace. c, Various processes have been desribed in the prior art on high temperature thermal cracking or steam cracking of hydro-carbons including high boiling hydrocarbons such as residual oils and gas oils, and lower boiling hydrocarbons such as naphtha and hydrocarbon gases such as ethane, propane, etc., to produce olefins such as ethylene, propylene, diolefins such as butadi-ene, isoprene, etc., and aromatic hydrocarbons such as benzene, toluene, etc. In these processes the petroleum feed stock is vaporized, diluted with steam and cracked in a coil at a temper-ature of about 1200 to 1600°P. Residence times are relatively short, usually in the range of about 0.1 to 5 · 0 seconds where-upon the reaction products are immediately quenched to arrest further reactions and minimize loss of primary conversion prod-ucts.

Steam cracking of the hydrocarbon feed stock is effected by supplying the feed stock in vaporized or substantially vapor-ized form in admixture with substantial amounts of steam to suitable coils in a cracking furnace. It is conventional to pass the reaction mixture through a number of parallel coils or tubes which pass through a convection section of the cracking furnace wherein hot combustion gases raise the temperature of the reaction mixture. Each coil or tube then passes through the fired or radiant section of the cracking furnace wherein a multiplicity of burners supply the÷»heat necessary to bring the reactants to the desired reaction temperature and effect the desired reaction or conversion.

When hydrocarbon feed stocks are subjected to the members forming the cracking furnace coils. The problem of coke deposition on the inner surface of such cracking furnace coils has become a major concern in the steam cracking of hydro-carbons sine* such coke deposits not only interfere with heat flow through the tube walls into the stream of reactants but also with the flow of the reaction mixture due to reduction in the cross-sectional area of the tubes.

In the cracking section of a steam cracking furnace it is necessary to heat these tubes or coils to temperatures of the order of about l600 to 2000°F. in order to obtain reactant temperatures within the tubes of about 1200 to l600+°P. which are necessary to give yield patterns and conversion rates which are optimum for the production of the chemicals demanded by today's industry. The insulating effect of the coke deposits makes it necessary to operate the furnace and the tube metal at higher temperatures in order to obtain the desired cracking temperatures in the gas phase. Such higher temperatures cause more rapid deterioration of the heating coils or make necessary the use, if available, of more heat resistant and more expensive metals in such coils.

Aside from subjecting the feed stocks to the above-mentioned high temperatures, it is critically important to main-tain high throughput rates in order to minimize the time during which the hydrocarbons are subjected to these temperatures.

It is equally important in many cases to maintain relatively low pressures, i.e. pressures just high enough to insure a rap-id throughput rate, it being highly desirable to crack the hydrocarbon feed at a pressure approaching atmospheric. Accord-ingly, the pressure drop across the furnace, i.e. from the feed inlet to the product outlet, should be minimum. for the removal of such coke deposits have been proposed such as opening tube ends and drilling or grinding the coke deposits, treating the deposits with 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 gen-erate carbon dioxide gas within the interstices of the coke deposits and by expansion of the generated gas, causing spelling of the deposits adhering to the walls of the cracking coils or tubes. It has also been proposed to add materials such as po-tassium carbonate to the reaction mixture in order to reduce or remove coke deposits on thermal cracking coils.

Most of the methods previously employed or suggested have required that the normal function of the furnace and the coils or tubes for cracking of hydrocarbon materials be inter-rupted during the cleaning or coke removal operation. Such interruption of on-stream time of the cracking coil produces serious economic problems in view of the temperatures involved and the time required to take the unit of -stream, effect the necessary removal of the coke deposits, and again bring the unit on-stream. Normal decoking of the furnace tubes often requires a feed outage of one to three days or even longer.

In addition, the cycling of bringing furnaces on and off in-creases the wear of structural members, particularly tube sup-ports.

It is the object of this invention to provide an im-proved process for thermal cracking hydrocarbon feed stocks in the presence of steam.

It is a further object of this invention to provide removed without shutting down the cracking furnace and with only a minor reduction in production throughput of the system.

These and other objects will appear more clearly from the detailed specification and claims which follow.

It has now been found that coke deposits can be effectively removed from cracking furnace tubes by introducing steam and/or water at the inlet to one pass or tube of the cracking furnace while simultaneously reducing or eliminating normal hydrocarbon feed to that pass, the temperature within the off-stream tube being maintained at approximately the same level as within the parallel tubes on-stream. The remaining passes or tubes of the cracking furnace remain in normal service. It should be understood that it is within the scope of this invention to decoke more than one tube at a time, simultaneously or successively, so long as the total number of tubes out of service at any given time represents only a minor proportion of the total number of tubes in the cracking furnace. The quantities of steam and/or water are predetermined to meet the following criteria.

Sufficient steam and/or water are introduced to re-move the heat normally going to the process fluid without ex-ceeding the tube metal temperature allowances as determined by stress or oxidation limits for the tube material.

The temperature of the steam entering the section of the furnace to be decoked must be about 700°F. or higher. If water is introduced it must be vaporized and superheated to this temperature while steam must be merely superheated.

The mass rate of steam entering the section of the furnace to be decoked should preferably be greater than 15 pounds per second per square foot of tube internal cross -sec - 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.

After a sufficient period of time the supply of steam and/or water for decoking can be cut off from the decoked pass of the furnace and feed simultaneously reintroduced. With reasonably optimum steam mass rates and temperatures, a furnace pass may be decoked in 12 hours or less. After decoking of one pass of the furnace is complete, the same procedure may be used at any time deemed desirable to decoke additional passes, one at a time. As noted above, if so desired, two or more passes in a multipass furnace can be decoked in this man-ner simultaneously, but this does tend to detract from the major advantage of this scheme, namely, minimum upset to normal operation.

This invention may be more fully understood from the following description when read in conjunction with the accom-panying drawing wherein the flow path of the reactants through an apparatus for thermal cracking of hydrocarbon is illustrated diagrammatically .

Referring to the drawing, the cracking furnace 10 comprises an upper, convection or preheat section 11 and a low-er, 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 by the numeral 15 and the cracking coils or tubes in the cracki-ing zone 12 designated by the numeral 16. It is to be under-stood 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 con-duit 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 2 and 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.) The reaction products are discharged from the coils or tubes l6 of the cracking furnace via conduits 2β into con-duit or header 27 from which they are discharged into conduit 28. In order to stop the cracking reaction promptly and there-by prevent or minimize side reactions, quenching agents such as higher boiling hydrocarbons and/or water are supplied through conduit 29 and control valve 30. The mixture of quenched reac-tion products and quenching agent are discharged via conduit 28 into fractionating tower 31· Aromatic tar product is with-drawn from the bottom of fractionating tower 31 through line 32 and product is taken overhead via line 33· Other intermedi-ate boiling range fractions may be withdrawn as product or re-cycled to a higher plate in the fractionating tower as one or more reflux streams. The quench oil may be withdrawn from the fractionating tower 31 through line 3^ and passed through heat - water for steam formation while cooling the quench oil to a suitable temperature for discharge through line 9 and valve 30 into the reaction product stream in line 28 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 l6. When sufficient time has elapsed to allow the coke to be removed from the inside of the tubes, valve 25 is closed and valve 22 is opened. There are two indications that help show the progress of the coke removal: · Decrease in pressure drop.

· Decrease in tube metal temperature.

As a specific example of an operation according to the present invention, a steam cracking furnace containing ten passes or conduits was decoked by slowly backing out the hydro-carbon feed to one pass at a time while adding boiler feed water and adjusting normal steam flow. Three tests were carried out by varying the quantities of steam, water and time of de-coking, and a fourth test was made with constant water and steam rates in one. ass until a furnace shutdown some twenty hours later. On completion of the several tests, the furnace was shut down and the tubes were then removed from the furnace for examination. The following are summaries of the tests which were carried out and the condition of the tubes on ex-amination after the furnace was shut down.

In the following tables transition from convection tubes 15 to cracker tubes l6.

T.I. = Temperature indicator.

TEST NO. 1 Peed to Pass 8 backed out over a 2-hour period.

When steady conditions were reached, feed was returned to the pass.

C.O.T. Crossover Oil Plow Steam Flow Water Flow Coil Inlet Time °F. Temp . , °F . lb/hr. lb/hr . lb/hr . Pressure psia 1 .35 1405 1203 345Ο 995 0 50 40 1403 1203 1407 1203 50 1405 1175 975 650 56 55 1398 1115 975 650 I5.OO 1395 1062 975 970 05 1395 1027 2999 IO70 97O 6 Ψ0 10 1398 1035 2999 1096 970 61 15 1400 1045 l404 1043 1130 970 66 25 1405 1037 2547 II60 970 67 HELD DUE TO T.: I. PANEL FAILURE 16.20 1400 1043 1234 970 68 25 l400 1405 123 970 69 35 1405 40 1410 45 1407 I76O 970 82 50 1410 55 1410 2200 I7.OO 1405 I76O I29O 05 1405 1410 2480 I29O 95 15 1412 2200* 1385 218O 2754 100 25 13 23ΟΟ 320O 1385 710 1193 2300 2240 1405 675 741** 2240 40 1357 510 2240-4I60 45 1467 765 0 4l60-2240 50 55 1465 685 0 26ΟΟ 288Ο 94 18.00 STEADY CONDITIONS 18.10 PEED RETURNED TO PASS - END OF TEST Large areas of Pass 8 completely free of coke were found upon examination of the tube after the furnace shut down. Remaining areas had coke deposits of average thickness .128" (Micrometer Readings).

Pass No. 9 was used as a reference, i.e. was not decoked. Heavy coke deposit throughout its length having an average thickness .248" (Micrometer Readings) was found when examined after shutdown. Pass was typical of all passes not decoked.

* Manual feed control ** Feed out.

TEST NO. 2 Feed to Pass 7 backed out using a steam-water mixture over a 45-minute period. This condition was held for one hour and the feed again returned to the pass.

C.O.T. Crossover Oil Flow ί Steam Flow Water Flow Coil Inlet Time °F. Temp. , °F. lb/hr . lb/hr. lb/hr. Pressure psia 08.15 1400 1210 4o60 932 0 7 I398 1210 1287 1412 1207 2750 1287 95 1406 900 1405 880 40 1405 870 27ΟΟ* 2750 256Ο 100 45 1406 630 3490 50 1395 457 55 1385 620 09-00 I4l2 685 05 1430 720 500** 1428 695 0 2436 110 1440 718 0 1423 685 0 1405 655 0 1405 650 0 1405 655 0 3494 2499 112 4o HELD STEADY TO 10.05 45 THEN FEED IN Large areas of the tube of Pass No. 7 were completely free of coke. Remaining areas had coke deposits of average thickness .088" (Micrometer Readings).

* Feed on Manual.

** Feed out.

TEST NO. 3 Feed to Pass 6 was backed out using a steam/water mixture over a period of 15 minutes. After a further 30 minutes the steam flow was reduced for a 10-second period to induce ther- mal shock in the tube. This was repeated 5 minutes later.

Feed was again returned to the pass.

CO.T. Crossover Oil Flow Steam Flow Water Flow Coil Inlet Time °F. Temp. ,°F. lb/hr . lb/hr. lb/hr . Pressure psia 11.00 1420 1205 316O 945 0 52 ° v 05 1430 1090 2257 1290 1442 1057 *2057 2676 1290 1420 635 **1513 2430 256Ο l4l0 620 0 2440 1450 748 3300 112 1465 760 1463 745 40 1456 735 3300 45 721-605 ***(2900 50 505-660 (2100 55 1423 670-7 5 #*** 330Q 2630 12.00 1430 1120 05 1420 1215 Large tube areas of Pass No. 6 were found to be completely free of coke when examined after the furnace shutdown. Re- maining areas had coke deposits of average thickness .088" (Micrometer Readings).

* Manual feed.

** Feed out.

*** Steam rate reduction.

**** Feed in.

TEST NO. 4 Feed to Pass 5 was backed out very quickly using a steam/water mixture. This condition was maintained until the furnace shutdown some 20 hours later. Examination of the tube of Pass No. 5 found it to be completely free of all coke through- out its length.

These tests proved quite conclusively that: (a) Removal of coke was found to be related to the time length of decoking (Tests No, 1, 2 , 3, and 4). (b) Complete removal of coke was possible in a steam cracking furnace with one pass having a steam/water mixture only as the feed. There was no upset on the remain-ing nine parallel passes while one of the ten passes was being decoked.

It was further observed that there was no upset on any other equipment downstream of the cracking furnace during any of the above tests.

This invention is not to be limited by the illus-tration or examples since numerous variations are possible without departing from the scope of the appended claims.

Claims (6)

HAVING NOW particularly desorihed and ascertained the nature of our said invention and in what manner the earn© is ttf he performed, we declare that what we olaim is

1. A process for thermally cracking hydrocarbon materials by passing the same in admixture with steam through a multiplicity of tubes arranged in parallel in a cracking furnace wherein said tubes are subjected to radiant heat sufficient to raise the temperature of the reactants within the tubes to about 1200-l600°F. , which comprises taking a minor portion of said tubes offstream by cutting out the flow of hydrocarbon feed and passing steam and/or water to said tubes in sufficient amount to maintain the temperature within said tubes at essentially the same level as in the parallel tubes remaining onstream and to effect removal of coke accumulated on the interior of the offstream tubes, and thereafter returning said tubes to normal onstream operation* multiplicity

2. The process according to claim 1 wherein the/of is tubes -are heated to an intermediate temperature in a convection section by contact with hot combustion gases and then subjected to radiant heat sufficient to raise the temperature of the reactants within the tubes to about 1200 to l600+°F. and the temperature of steam in the offstream tubes is at least about 700°F. as the steam passes from the portion of said tubes in the convection section into the portion in the radiant heating section.

3. · The process as defined in claim 2 wherein the mass rate of steam entering the section 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 20-25 psia.

4. The process as defined in claims 1-3 wherein the feed is cut off and the steam and/or water is supplied to only one of said parallel tubes at a time in order to remove coke therefrom without substantially reducing the conversion capacity of the cracking furnace as a whole.

5. » The process as defined in claims 1-3 wherein the hydrocarbon feed is cut off and the steam and/or water is supplied to several of said parallel tubes in succession, the total number of said tubes which are offstream at any particular time representing only a minor portion of the total number of tubes in the cracking furnace.

6. The process for thermally cracking hydrocarbons substantially as described.