IL26384A - Method of operating regenerative furnaces and apparatus therefor - Google Patents

Description

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PATENTS AND DESIGNS ORDINANCE SPECIFICATION METHOD OF OPERATING REGENERATIVE FURNACES AND APPARATUS THEREFOR muy ipnm o^'-n^a D'jsna n ysn1? no»w We, UNION CARBIDE CORPORATION, a corporation organized under the laws of the State of New York, United States of America, of 270 Park Avenue, New York, New York, United States of America 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 :- This invention relates to a regenerative furnace and method of operating the same in the pyrolysis of hydrocarbons to produce acetylene and/or ethylene.

In the usual operation of a conventional regenera-tive furnace for the production of acetylene and/or ethylene, certain amounts of oil and heavy tars are produced in the cracked gas during the usual cracking cycle. This is particularly true when a liquid, such as naphtha or gas oil, is employed as the feed stock and cracked during the cracking cycle. Such liquids usually contain fractions that are aromatic or naphthenic and which do not crack into a gaseous product under the usual conditions maintained in such a regenerative furnace, and produce some tars and oils, having high dew points, which tend to condense and form a tarry deposit in the pas-sages of the outer ends of the furnace if their temperature falls below about 300°C. As a result, it is necessary to operate the cracking cycle so as to maintain the exit temperature of the cracked gas above such dew point temperature of about 300°C. and preferably under conditions such that the exit temperature of the cracked gas is 400°C. or higher to a-void a rapid build-up of tar in the outer ends of the passages of the furnace; The effect of tar deposition and build-up in the passages of the furnace is very detrimental in that it reduces the size of the passages, which increases the pressure dro therethrough, and if such tar build-up is not uniform in all of the passages, which normally it is not, it substantially changes the cross-sectional distribution of gas flow therethrough and adversely affects the cracking reaction which is dependent on a close control of the temperature and residence time of the gases in the passages. Variations in flow from passage to passage, due to an uneven tar build-up therein, causes over cracking in some passages and sometimes under-cracking in others, with an overall loss in cracking efficiency.

Although, as pointed out above, such tar deposits can be minimized or avoided by operating the cracking cycle so as to maintain the exit temperature of the cracked gas above 00°C, doing so results in a loss of sensible heat from the furnace and requires a corresponding increase in fuel consump-tion to maintain the desired temperatures in the reaction zone of the furnace, and is therefore undesirable. Also, it normally requires twelve to twenty-four hours of normal operation of such a regenerative furnace to bring the exit temperature of the cracked gas up to 400°C, and during this initial heat-up period the exit gas temperature normally will not be sufficiently high to avoid such tar deposit in the passages at the ends of the furnace during the cracking cycles, which is also undesirable.

In the usual operation of a conventional furnace, it has been the practice to periodically eliminate the carbon and tar deposit in the outer ends of the ceramic checkers of the furnace by heating the same to a temperature above the ignition point of the deposited material, in the presence of excess air, to ignite the deposit, which then burns out substan-tially completely to leave the passages through the ceramic checkers sufficiently clean for efficient operation of the furnace. This is done by continuing the heating cycle through the furnace in one direction for a sufficient length of time to carry the heat from the normally relatively narrow high temperature zone in the central portion of the furnace out- Ing cycle to similarly carry the heat from the central high temperature zone outwardly to the other end of the furnace. If the furnace is at operating temperature at the start of the burn-out cycle it is not necessary to add fuel as the heat stored in the ceramic in the center section of the furnace is sufficient and the air flow will carry the heat to the end of the furnace. However, if the furnace has previously been operating under temperature conditions normal for the production of acetylene and/or ethylene, it requires about 20 minutes for the heat to carry out to one end of the furnace to ignite the deposit at that end and about 10 minutes more to burn it out, and a similar 20 minute period of a reversed heating cycle to carry the heat put to the other end of the furnace and another 10 minutes to burn out the deposit at that end. Allowing a reasonable period to insure complete burn-out of each of the two deposits in a furnace, it requires about one hour for each furnace for such burn-out operation. Since such furnaces are usually operated in pairs, the foregoing means that it requires about two hours to so clean up an installation. During such time, the production of acetylene and/or ethylene must be discontinued, which disrupts plant operation and adds to production costs. It has been normal practice in a commercial plant to perform such burn-out operation on a routine basis for approximately once a week.

A primary object of this invention is to provide an improved regenerative furnace and method of operating the same to avoid or minimize the disadvantages referred to above which have been usual in the operation of conventional regenerative furnaces in the manufacture of acetylene and/or ethylefte.

Stated generally, this is accomplished by periodically introducing a free flame into each outer end of the furnace, prefer- ably at the beginning of a heating cycle, allowing it to continue to burn for a small fraction of the heating cycle and for a sufficient period of time to Ignite and burn-out the tar deposit in the checkers of the furnace, and then extinguishing the free flame and permitting the deposit to continue to burn for the remainder of such specific heating cycle or until the deposit is completely burned out, whichever is the first to occur. ' This will be explained in more detail hereinafter.

A further object of the invention is to provide such an improved furnace and method of operating the same in which such free flame is formed periodically and automatically in each outer end of the furnace, without interrupting the normal operation of the furnace.

Another object of the invention is to form such free flame by diverting at least a portion of the fuel gas, normally used in the heating cycle, alternatively to the outer ends of the furnace, where it is mixed with the air normally passed into the furnace during such heating cycle to form a flammable mixture, and igniting such mixture before it passes into the outer end of the checkers of the furnace.

Further objects of the invention are: to ignite such flammable mixture by means of an electric spark or arc j and to control such Ignition with an automatic timing device and in phase with the valve which admits fuel gas into each outer end of the furnace.

Still another object of the invention is to operate such a furnace so as to form such a tar deposit deliberately during each cracking cycle of the furnace and then to burn out such deposit on a subsequent heating cycle to utilize such burn-out as an additional source of heat during the heating c cle Other objects, features, and advantages will appear from the following specification and the drawing, which are for the purpose of illustration only, and in which: Figure 1, in diagrammatic form, partly in section, shows the furnace of our invention and the piping and some of the auxiliary apparatus thereof; Figures 2, 3> and 4 are diagrams illustrating certain of the steps in the practice of the method of the invention; and Figure 5 is an enlarged, fragmentary, sectional view showing an igniting device of the invention.

Referring to the drawing, Figure 1 shows a furnace 10, having a steel shell 11 provided with a heat insulating lining 12. Inside the lining 12 are three spaced regenerative masses of checkers 13 , 14, and 15, providing a combustion space 16 between the masses 14 and 15 and a combustion space 17 between the masses 13 and 14. The regenerative masses 13 and 15, for convenience of description, are hereinafter referred to as the left-hand ("LH") and righthand ("RH") end masses, respectively, and the regenerative mass 14 as the central mass. The RH mass 13 is provided with a plurality of passages 19 which extend longitudinally therethrough (shown in end-view in Figure 5) , and the masses 14 and 15 have similar passages therethrough. The right-hand and left-hand ends of t e furnace 10 are provided with plenum chambers 20 and 21 respectively.

A fuel supply pipe 23 supplies fuel gas through a main valve 24 to a manifold 25 provided with a pair of three-way valves 26 and 27. The valve 26 has connected thereto a gas supply pipe 28 which leads to a plurality of vertically spaced nozzles 29 which extend into the combustion space 17 to supply fuel gas thereto. Similarly, the valve 27 has connected thereto a gas supply pipe 30 which leads to a plurality of vertically spaced nozzles 31 which extend into the combustion space l6 to supply fuel gas thereto. Actually, the supply pipes 28 and 30 are each in the form of a yoke straddling the furnace and each is provided with a set of such nozzles on each side of the furnace 10, it being understood that the supply pipe 28 has a second set of nozzles similar to the nozzles 31, on the near side of the furnace, and the supply pipe 30 has a second set of nozzles similar to the nozzles 29 on the far side of the furnace. Although for convenience only three such nozzles have been shown in each bank thereof, it is to be understood that conventionally additional vertically spaced nozzles are usually provided in each bank thereof.

The three-way valve 26 also has connected thereto a secondary gas supply pipe 33 which leads to a manifold 34 communicating with the RH plenum chamber 20. Similarly, the three-way valve 27 has connected thereto a secondary gas supply pipe 35 which leads to a manifold 36 communicating with the LH plenum chamber 21.

Connected between the manifolds 3 and 36 are: an air supply pipe 37 communicating with a source of air (not shown), and having valves 38 and 39 therein; an exhaust pipe 4l leading to a stack 42 or other point of disposal, and having valves 3 and 44 therein; an off-gas pipe 45 adapted to convey cracked-gas from the furnace 10 to a suitable point of storage or use (not shown), and having valves 46 and 47 therein; and an in-gas pipe 48, having valves 49 and 50 therein, which is connected to a suitable mixer 51 to which is connected a diluent supply pipe 52 adapted to supply a diluent, such as steam, to the mixer and having a valve 53 therein, there also bein connected to the mixer a feed stock su l i e ^ adapted to supply a suitable hydrocarbon feed, such as, for example, propane, to the mixer, and having a valve 55 therein.

Also connected to the manifolds 3 and 36 are igniters 57 and 58, respectively, which are identical, the ig-nitor 57 being shown in detail in Figure 5. The igniter 57 includes a nipple 59 connected to the manifold 3 and into which is fitted an electric sparking device 60, supplied with a high potential electric current, such as 5000 to 10,000 volts, by an electric cable 6l. The sparking device 60 in-eludes a metal collar 62 threaded into the nipple.59 and onto which fits a tubular collar 63, the outer end of which is threaded to receive a coupling 64 in which is mounted an insulator 65 which houses the end of the cable 6l and the end of an ignition rod 66 electrically connected to the end of the cable and extending into the manifold 34. The collar 62 has a tubular extension 67 which surrounds the rod 66, the outer end being provided with perforations 68 to admit gas thereinto between the extension and the rod.

The three-way valves 26 and 27 are conventional solenoid-operated valves and are controlled by a conventional timer (not shown) . Each of such valves may be closed to shut off any flow of fuel gas therethrough, actuated to pass the full flow of fuel gas from the fuel supply pipe 23 to one of the combustion chambers l6 or 17* actuated to pass the full flow of fuel gas from the supply pipe 23 to one of the manifolds 34 or 36, or actuated to divide the flow of fuel gas between the plenum chamber 20 and the combustion chamber 17* or between the plenum chamber 21 and the combustion chamber l6.

In operation, the regenerative masses are first heated up to operating temperatures by a preheating step. By - tral mass 14 is heated to a temperature of about 2200°C. and the end masses 13 and 14 are heated to a temperature somewhat lower than 2200°P. adjacent to the combustion spaces 16 and 17 and to a much lower temperature, e.g., below 400°C. at their outer ends, with a fairly uniform temperature drop profile between the inner and outer end of each of the end masses.

Assuming that it is desired to start such normal operation of the furnace with a left-hand (LH) make step, the valving is adjusted to provide the connections illustrated in Figure 4, in which valves 46, 50, 5 / and 55 are opened and all other valves are closed. The hydrocarbon feed stock, such as propane, flows into the mixer 51 from the pipe 54, where it is mixed with a diluent, such as steam, from the pipe 52, to form the usual in-gas. The in-gas then flows through the pipe 48 and the manifold 36 into the left-hand (LH) plenum chamber 21 from which it flows sequentially through the LH mass 15, the combustion space 16, the central mass 14, the combustion space 17, and the RH mass 13, cracking taking place largely in the central mass 14, to form an off-gas containing the desired end product, e.g., acetylene and/or ethylene. The off-gas is quenched to a much lower temperature during its passage through the cooler RH end mass 13, and passes through the plenum chamber 20 into the manifold 34 and thence through the off-gas pipe 45 to storage or use. In a right-hand (RH) make step, some of such valving is reversed from that shown in Figure 4 to convey in-gas into the RH plenum chamber 20, through the furnace from right to left, out through the LH plenum chamber 21 , manifold 36, and off-gas pipe 48 to storage or use. Such make, or cracking, steps are generally conventional with a re-generative furnace of this general type. However, it is to be noted that this invention contemplates cooling the off-gas to an exit temperature below 400°C, which is normally below the dew point of tars in the of -gas, whereas in the conventional operation of such a furnace the exit temperature of the off-gas is maintained above the dew point of such tars to prevent their deposition in the furnace. In the preferred embodiment of this invention, such exit temperature of the off-gas is maintained below such dew point to deliberately foster the deposition of such tars in the furnace, which is an alternative objec mass will have been cooled considerably by the passage of relatively cool in-gas therethrough, the central mass 14 will have been cooled to about the minimum cracking temperature for the product desired, and the fiti mass t&l will have been heated fi/ ^ considerably a subsequent ous regenerative masses must be readjusted. Consequently, such a ¾H make step is followed by a LH heat step, as diagram-matically illustrated in Figure 3, and designated as "LH .

Heat-B" . With the time cycle usually used this temperature change in one step of the cycle is on the order of 100°F.

As illustrated in Figure 3, in a LH heat step valves 26, 38, and 44 are opened and all other valves are closed.

Air is conveyed through the air supply pipe 37 and the manifold 34 into the RH plenum chamber 20 and thence through the RH mass 13, cooling the same, and into the RH combustion chamber 17 where it mixes with fuel gas supplied thereto through the fuel supply pipe 28 and nozzles 29, to form a combustible mixture which automatically ignites and burns, the hot products of combustion passing therefrom through the central mass, from ri ht to left to heat it back u to crackin tem eratures and then through the LH combustion chamber 16, the LH regenerative mass 15, the LH plenum chamber 21, the manifold 36, and the exhaust pipe 41 to the stack 42 from which such products of combustion are discharged. Such a LH heat step is conventional. For a similar RH heat step, the valving is readjusted to pass air into the LH end of the furnace, through the LH mass 15 and into the LH combustion chamber 16 where it mixes with fuel gas introduced thereinto through the nozzles 31, ignites to form products of combustion which then pass left to right through the balance of the. furnace and the manifold 3 to the stack 42. to reestablish the heat balance fol- LH lowing a fc I make step.

The novel features of the method of our invention, relate to the conventional heat steps previously described. In a preferred embodiment of the invention, at the beginning of a LH heat step the valving is set as illustrated in F.igure 2, in which the valves 38 and 44 are opened and the three-way-valve 26 is adjusted to direct all fuel gas from the fuel supply line 23 through the supply pipe 33 to the manifold 34, so that such fuel gas mixes in the manifold with air therein, adjacent to the RH plenum chamber 20 to form a flammable pr combustible mixture. At the same time, electricity is momentarily supplied by the cable 6l to the ignition rod 66 and a spark or arc thereupon bridges between the rod and the tubular extension 67 (which is grounded through the manifold 34 and steel shell 11 of the furnace), to ignite the combustible mixture in the manifold. Such burning mixture passes through the plenum chamber 20 and into the passages 19 of the RH regenerative mass and therethrough. In its travel through such pas-sages the burning combustible mixture ignites the carbon and tars which have deposited therein during a preceding make step or steps and the same burns or is burnt off, leaving the walls of the passages substantially clean of such undesirable deposits. This is an important feature of the invention. need only be momentary, as the spark therefrom immediately ignites the combustible mixture, which continues to burn, and the electrical impulse is then discontinued by suitable switching (not shown) . A suitable timer (not shown) may be provided to synchronize automatically the opening of the valves 26, 38, and 44 with the supply of high voltage to igniter 57> and this is a further feature of the invention.

The step illustrated in Figure 2, which may be refer-red to as a burn-out step, is continued for only a brief period. Normally, a conventional heating step, as generally illus-trated in Figure 3 and described above, requires about one minute. We have found that by using such a separate burn-out step, as illustrated in Figure 2, for a period of 15 seconds or less, adequate burn-out is accomplished. At the conclusion of such burn-out step, the valving is reset as shown in Figure 3 and the regular heating step then continues until the end of its normal period of time. However, if such burn-out has not been completed during the separate burn-out step, the carbon and tar deposits in the passages 19 continue to burn during the regular heating step, since air continues to flow through such passages during the regular heating step, to eliminate such deposits, which is a further feature and advantage of our method. Such burn-out step is in fact a part of the main heating step, as the burning of the combustible mixture and the hot products of combustion therefrom transfers sensible heat to the walls of the passages 19 of the masses 13, 14, and carbon and tar deposits, as described above, furnishes additional heat, such carbon and tar deposits serving as fuel therefor, and this is a further feature and advantage of the invention in that it improves the overall heat efficiency of the operation.

As an alternative to the method described above, the burn-out step and heating step may be combined, without departing from the spirit of the invention. In such alternative practice, the valving is as illustrated in Figure 2, but the three-way valve 26 is adjusted to divide the supply of fuel gas between the pipes 28 and 33 , so that a portion of the fuel gas passes to the . plenum chamber 20, is there mixed with air and ignited as previously described to provide the burn-out feature, while at the same time the balance of the fuel gas supply is passed directly into the RH combustion space 17 through the pipe 28, where it mixes with the excess air from the plenum chamber 20 and the mixture ignites to provide the normal heating step previously described. After the brief period required for such burn-out, normally between and 1 seconds, the three-way valve 26 is readjusted to shut off the flow of fuel gas to the plenum chamber 20 through the supply pipe 33 and convey all of the fuel gas supply directly into the combustion space 17 through the pipe 28, after which the heat step continues normally as described above.

Although only a LH heat step following a LH/make A*4^ step, in which the ignited fuel gas and hot products of combustion pass from right to left in the furnace, has been de- LH AiJLA scribed, it will be understood that a ftjS/make step is followed by a similar, but reversed, RH heat step, in which the valving is reversed to put air In the left-hand end of the furnace, and fuel gas in the left-hand plenum chamber 21 for the burn-out of the left-hand end of the furnace and fuel in the LH combustion space 16 for the normal RH heat step.

In the usual operation of a regenerative furnace of this general type, it is conventional to use solenoid-type valves throughout the system and to control their operation automatically by standard electric timers to automatically provide the desired sequence of heat and make steps in both directions through the furnace, and we contemplate the use of such an automatic timing in connection with this invention. In addition, as indicated above, we also employ the same or other suitable timer equipment to control automatically the burn-out step described above, merely by controlling the sequence of operation of the three-way valves 26 and 27* and this is another object of the invention.

Although the burn-out step may be effected at the beginning of each heat step, and this is frequently desirable if the feed stock and operating conditions deposit any substantial carbon or tar during the normal make steps, if such deposit is light during any single make step, due to less stringent operating conditions, it is unnecessary to provide such burn-out step at the beginning of every heat step. Consequently, when conditions permit, our method is practiced by using the burn-out step at the beginning of a heat step, in each direction, only periodically during normal operation. We have found that under some favorable conditions of operation of the furnace, although the heat and make steps may be alternated with periods of one minute each, for example, the burnout step need only be effected every 10 or 15 minutes, or even less frequently, to maintain the furnace adequately clean of carbon and tar deposits. Consequently, we do not desire to be limited to usin the burn-out ste with ever heat ste but desire to include the use of such burn-out steps periodically at regular intervals during normal furnace operation, particularly with a timed sequence thereof correlated with the normal operation of the furnace, so that the entire operation is auto-matic An alternative embodiment of the invention is to add a differential pressure switch (not shown) which measures the pressure drop across the furnace from one plenum chamber to the other and which closes a control circuit any time this pressure differential becomes too great to bring the burn-out function into operation. This may either cut off as soon as the differential has been brought down or run for a predetermined period of time each time the differential reaches a point too high.

By the use of the method of this invention, the furnace can be kept adequately clean of carbon and tars at all times, with the result that the pressure drop through the passages 19 of the ceramic checkers remains substantially uniform and maintains the yield of cracked gas from the make steps at a maximum. This is done automatically and requires no manual control or attention during operation. The result is an improved efficiency of operation and also a reduction of nonproductive downtime and maintenance labor.

Also, although we have described a system in which the same fuel gas source is used to supply fuel for the burnout and normal heat steps, it will be understood that a separate fuel source may be employed to supply fuel to the plenum chambers 20 and 21 for the burn-out steps. In this case, the valving for the fuel supply for the normal heat steps and the valve for the fuel supply to the plenum chambers for the burnout steps are separate and independent, although timed to op- erate in the sequences described above. By this means, the air-fuel ratios of each may be set independently at optimum values, which is sometimes advantageous and another feature of the invention.

While we have shown and described a preferred embodiment of this invention, it will be apparent to those skilled in the art that its novel features may be applied in other equivalent forms, and we do not desire to be limited to such specific embodiment but desire to be afforded the full scope of the following claims.

Claims (17)

1. A method of operating a regenerative furnace having regenerative masses divided into a pre-heat section and a quenching section with a heat input section therebetween, said masses having a plurality of longitudinal passages therethrough and a plenum chamber of the outer end of each of the masses, which includes alternating heat and make steps through the furnace, the make step depositing undesirable foreign materials on the wall of the outer portions of the passages of the masses, including: a periodic bum-out of said deposits by each of adding fuel into the plenum chambers, igniting the mixture of fuel and air, and passing the resulting burning mixture into and through said longitudinal passages of the adjacent mass from the outer end thereof and towards the other mass to ignite and burn out said undesirable deposits in said adjacent mass.

2. A method according to claim 1, in which a burn-out step follows each make step in each direction.

3. A method according to claim 1, in which there is a burn-out step in each direction following a preselected number of make steps.

4. A method according to claim 1, in which the burn-out step is a defined part of the heat step.

5. A method according to claim 4, in which the burn-out is a part of a heat step after a preselected number of make steps.

6. A method according to claim 1, wherein the through one of the masses from the outer end thereof towards the other of said masses and into the space therebetween where it is mixed with a fuel gas to form a first combustible mixture which ignites and the products of combustion pass through the said other mass, and in which periodically and automatically, after a make step, mixing air and a fuel gas to form a second combustible mixture, igniting said second mixture and passing the same while ignited into the outer end of one of the masses and therethrough towards said space to burn said foreign materials from the passages of said one mass and add heat thereto, and repeating the same steps through the other of said masses in a reverse direction.

7. A method according to claim 6, in which at the beginning Of a heat step at least a portion of the fuel gas is diverted to an end of the urnace *and there mixed with air to form the second combustible mixture.

8. A method according to claim 7, in which, prior to the end of a heat step the diversion of such portion of fuel gas is discontinued and then continuing such heat step to its normal conclusion.

9. A method according to claim 6, in which at the beginning of a heat step all of said fuel gas is diverted to an end of the furnace and there mixed with air to form the second combustible mixture.

10. A method according to claim 1, wherein the air is added for a time beyond the time during which

11. A regenerative furnace having first and second end regenerative masses, such masses being spaced apart to provide a combustion space means between said masses, each of said masses .having a plurality of longitudinal passages therethrough, a plenum chamber at each end of the furnace, each communicating with an outer end of one of said end masses; means for conveying an in-gas to either of said plenum chambers; means for conveying an off-gas from either of said plenum chambers; means for conveying an exhaust gas from either of said plenum chambers; means for conveying air to either of said plenum chambers; means for conveying a fuel gas to said combustion space; means for conveying a fuel gas to either of said plenum chambers; and igniting means associated with each of said plenum chambers for creating a free flame therein.

12. A furnace according to claim 11, in which there are two end masses separated and a central mass between the two end masses to form a combustion space on each side of the central mass.

13. A furnace according to claim 11 or 12, further comprising a differential pressure switch operated by the pressure difference between plenum chambers to initiate a bum-out step when the differential pressure exceeds a predetermined value.

14. A furnace according to claim 13, in which the differential pressure switch initiates the burn-out

15. A furnace according to claim 11, in which there is provided timing means for automatically and periodically causing fuel gas and air to flow into each of said plenum chambers to form a combustible mixture and operating said igniting means to ignite the mixture.

16. A method of operating a regenerative furnace substantially as herein described with reference to the accompanying drawing #

17. A regenerative furnace constructed /&/rfif /af pj- MzA> kk® NdI4 t&tel substantially as herein described with reference to the embodiments illustrated in the accompany- i„g drawing. fl f^u k^ AGENTS FOR APPLICANTS