US2971752A - Method of heating and reforming hydrocarbons and other gases - Google Patents

Description

METHOD OF HEATING AND REFORMING HYDROCARBONS AND OTHER GASES Julius D. Madaras, Longview, Tex., assignor to Gas Incorporated, Detroit, Mich., a corporation of Michigan No Drawing. Filed Aug. 30, 1954, Ser. No. 453,164

9 Claims. (Cl. 263-52) This invention relates to a new method of heating gases in a contact type of heat exchanger such as a checker stove, pebble heater or other separately fired and cooled heat exchanger. This application is a continuation-inpart of my copending application Serial No. 125,932, filed November 7, 1949, now abandoned. By this new method heat transfer may be made more efiiciently from the heat storage material to the gases exposed to it. By my method more heat can be extracted within a given temperature range from a checker stove'than has been possible by using conventional methods of gas handling. Likewise more heat can be transferred at or near the maximum available temperature in the stove, thus producing a higher average temperature in an equal quantity of heated gas or a larger quantity of gas heated to an equal average temperature than can be produced by present methods.

This method, consequently, will provide a greater heat, chemical and mechanical efficiency in systems where it shall be used. In the thermal reformation of hydrocarbons, thermal cracking, use of. this method of. gas handling will not only give greater efficiency but will also provide a better quality of cracking measured by the conversion of methane and higher hydrocarbons to hydrogen and carbon or carbon gases at any given conditions of firing and preheat in the cracking stoves. It may be used on any gas that can otherwise be handled in the heaters by conventional methods.

In the particular case of a checker stove that is alternately heated by firing into it with gas and air to the desired temperature of preheat and then cooled by heat exchange with process gases of a different nature, the common practice is to fire the stove to a predetermined flue gas temperature with air and fuel leaving the heat storage material at a desired temperature, then isolate the firing gases from the stove and switch to the process gases, which may also be air but may also be any gas which is substantially non-reactive with the stove and its contents at the temperatures encountered. In this manner the heat storage material transfers heat to the process gas until the outlet temperature reaches a predetermined minimum value. Then the cycle is repeated. It is normal practice to have 'two or three such stoves in a bank to provide continuous production of hot process gas above a desired minimum temperature. The size of the stoves is determined by consideration of the process heat extraction rate, the firing heat input rate and the acceptable range of temperature swing between maximum allowable firing temperature and minimum process gas temperature. The process gas usually flows countercurrent to the direction of firing gases in such stoves, although by careful firing, variation of air fuel ratio, such stoves can be heated quite uniformly from one end. to the other. In such case the direction of gas flow is unimportant.

Process gas as it flows through the heat storage material in its principal stream, picks up heat from the States Patent surrounding material by heat transfer and radiation according to well known principles and thereby cools the hot material in its path and immediately nearby.. Simultaneously, as material in the major stream is cooled, heat is transferred from hotter material not in the major gas stream by secondary convection currents, radiation and conduction to the zone of the major stream or channel. This secondary transfer of heat is essentially inetficient and wasteful as it must include a decrease in temperature from that of the preheated checker material to that of the cooler material in the zone of major gas fiow.

It 1s characteristic of gas flow through any porous bed at any practical rate, whether it is a bed of lumps or a carefully placed checker stack that most of the gas will follow a particular course through the bed and much of the heat otherwise available at the maximum preheat bed material is not available directly at such temperature to the process gas being heated. Consequently the maximum temperature available at the flow pattern through the bed. These are necessarily costly and inefficient design limitations. With my method of handling gas they become unnecessary and full use of max1mum temperatures becomes available.

In the described case I take an individual conventional checker stove. However, the following described prine stove down to the prescribed outlet pressure required. The outlet valve is Icilosed and the cycle is repeated.

y pulsating the pressure of the gas as it flows throu h the stove the ma or flow of the gas is made to reach :11

In other words, a stove operated absorb available heat as they progress through the stove and the outlet temperature falls very rapidly as a result of extracting heat along the entire length of the stove from the start of the heat exchange cycle. By my mehod it is possible to extract more B.t.u. at a higher temperature from a given stove; hence with greater heat efficiency.

By pulsations of the gas pressure I shall describe the extreme situations. Under conventional stove operation there are no, or only slight, variations of the gas flow. There is a uniform pressure drop through the stove sufficient to move the required volume of gas through the stove. Under the extreme case of practice of my method the volume of gas admitted with each cycle of valve operation will be related to the supply pressure and the volume of void in the stove. The stove would be a cyclicly operated metering tank. This case provides maximum pulsation of the pressure as all gas is admitted to the highest pressure and exhausted to the lowest pressure. It also results in the minimum gas flow in any case. In between these extremes the full heat and temperature advantage of my method be enjoyed with a minor decrease in gas flow related to full use of the increased gas pressure described above and with a much larger gas flow measured in terms of the lower inlet pressure used in a conventional heater.

I have found that good results conclusive of the merits of my method may be obtained by mechanically operating the inlet valve as descriebd above on a time cycle and holding the outlet valve partially closed or the outlet otherwise restricted so that a substantially uniform minimum outlet pressure is achieved during each exhaust cycle as described above.

With the following conditions of pressure, 20 p.s.i.g. minimum on the outlet and 30 p.s.i.g. on the inlet supply, several times more steam could be heated to an outlet temperature of 2000 degrees minimum in an equal or shorter period of time in a refractory checker stove than could be heated while passing steam through at an equal rate. In both cases the stoves were preheated similarly and other conditions were similar.

The above described procedure is applicable to the thermal reforming of hydrocarbon gases to make hydrogen and carbon black or of hydrocarbon gases and steam or carbon dioxide to make hydrogen and carbon monoxide in refractory stove heat exchangers. Hydrocarbons heated to high temperatures above 2000 F. dissociate into hydrogen and carbon black or, in the presence of steam or carbon dioxide, dissociate and reform into hydrogen and carbon monoxide. handling gases has additional advantages beyond those of heat efficiency when applied to the thermal reforming of hydrocarbon such as natural gas. The reforming reactions are very endothermic and are rereversible. The equilibrium concentrations of the reformed elements of the gases increase with temperature. Therefore much greater percentage of conversion, as measured by the smaller percentage of hydrocarbons in the exhaust gases from the stove, is possible when using my method of gas handling as outlined above. The gases involved here are chemically reactive only with themselves and not with the refractory bed of the above.

It is clear through using the method of my invention why an improved quality of gas is produced. The reacting gases, for instance natural gas or natural gas and steam, are admitted through the hot refractory bed heated in excess of 2300 F. with the same pulsating pressure as-described above and by the same means. When properly operated, the reacting gases are heated to the highest available temperature in the checker bed in the shortest space and time by direct contact between the gases and the refractory. Very little of the available high temperature heat is wastefully transferred indirectly to the gas by secondary transmission as described above. These My method of gases must be reformed at the highest temperature available in order to 'get the best conversion of gas.

These gases taken directly from the reformer make excellent reducing gas for use directly without cooling in the gaseous reduction of iron and other oxide ores. In the case of cracking hydrocarbons to form hydrogen and carbon black, all or part of the carbon in the gas stream may be removed and recovered for other purposes as described in this invention.

My method can be applied to any situation where gaseous materials are reacted in a porous bed either for heating in the bed or for catalytic action of the bed.

An example of the application of repeatedly injecting gaseous materials into a porous bed of solids and exhausting the reacted gaseous product is that of cracking hydrocarbon gases. A properly insulated pressure vessel is filled with very high refractory bricks, lumps or plates in the manner of checker work, or forming straight fines or forming an irregularly piled up charge. The refractory may be, for example, alumina or silicon carbide bricks or plates of any desired shape which will stand very high temperatures without decomposing and will resist the chemical action of the gas.

The refractory charge is heated by firing gas into it, and after heating it to the proper temperature hydrocarbon gas is repeatedly injected into the retort and the gas product exhausted at a predetermined average flow rate. It is practical in this operation to restrict the exhaust flow of the gas without pulsation, and to pulsate only the inlet flow. This may be regulated at will however, and if desired the inlet flow may be made a steady stream while only the exhaust flow is pulsated. Either of these operations will cause the gas to pulsate throughout the refractory bed. After the refractory bed is cooled to a predetermined temperature the flow of hydrocarbon is stopped, and the refractory charge may be reheated and the cycle repeated. In this way channeling is eliminated and the time of contact between the gas and the refractory is regulated.

Some of the separated carbon black is deposited on the surface of the refractory and the rest is carried out by the cracked gas. A considerable amount of the deposited carbon black may be blown out of the charge by the following method. After exhausting the hydrocarbon, the exhaust valve may be opened wide and additional gas under pressure may be pulsated through the refractory bed to sweep or flush out the deposited carbon black.

The preheating of the refractory bed, the passing and pulsating of the hydrocarbon gas through it and the sweeping out of the carbon can all be mechanically controlled. By properly controlling the time of contact of the injected hydrocarbon gas with the refractory surfaces and controlling the time of exposure of the gas and carbon to the predetermined temperature in the refractory bed, length of remaining heated bed+gas velocity, different varying and desirable qualities of carbon black will be produced. In this way the quality of the carbon black will be controlled, as well as controlling the extent of cracking the gas.

It will also be practical with my method to collect the carbon black that passes out with the cracked gas as well as the carbon black that is swept out with the recirculated cracked gas. The carbon blacks obtained from these two operations will have different and useful characteristics. The carbon black swept out with the recirculated cracked gas will be more graphitic than the other due to long exposure to high temperature, and will be used for different purposes.

With my process it is also practical to pass the hydrocarbon gas first through a cooler refractory bed and partially crack it. The gas is then cooled and the car bon collected. The remaining partially cracked gas is then passed through a hotter refractory bed where the cracking is completed to any desired degree. The carbon is collected and the gas used for the above mentioned useful purposes.

My invention as described above may also be used for carburization of iron contained in sponge iron while the iron ore is being reduced. A desired and predetermined amount of carbon is left in the hot reducing gas after leaving the gas cracker or the catalyst bed. This gas, containing carbon black, is passed into the ore where the carbon black is partly absorbed by the hot reduced iron and partly deposited in the sponge iron bed.

Furthermore, this will make it possible to use the reducing gas at a higher temperature than otherwise practicable. The reaction of ironoxide with such hot reducing gas is exothermic and very eiiicient. The heat produced by the exothermic reaction may overheat the sponge iron so that the partially reduced iron oxide may become fused without being reduced. The carbon carried with the gas reacts with the iron oxide quite rapidly at about 1800 degrees Fahrenheit and absorbs the excess heat produced by the exothermic reaction of gaseous reduction, thereby keeping the temperature roughly around 1800 degrees Fahrenheit to 2000 degrees Fahrenheit. If there is a surplus amount of carbon in the reducing gas so that the carbonaceous reduction would excessively cool the sponge iron bed, the carbon will not react but the gaseous reduction will still go on, or excess carbon may be removed in advance. In this manner the temperature of the reducing iron oxide will be controlled. After the iron oxide is nearly or completely reduced, however, the excess sensible heat in the reducing gas having the carbon with it will heat the sponge iron and the carbon black will further carburize the iron. The free carbon deposited in the sponge iron will further carburize the iron, if so desired, when the sponge iron is being melted.

A modified and suitably designed gas cracker may also be filled with coke or other form of carbon lumps or blocks and used for cracking hydrocarbon gases. The carbon bed is first preheated by blowing air, oxygen or oxygen enriched air into it, in order to produce heat in the coke or carbon bed. Then hydrocarbon gas is passed through the hot carbon bed with a pulsating pressure as described above. The hot cracked reducing gas may then be passed through iron ore or other oxide and used for ore reduction. Also the gas of combustion from cracker preheat will contain reducing gases consisting mainly of carbon monoxide, and similarly can be used for ore reduction.

The filling of the carbon into the gas cracker and removal of the possible ash and other residue can be done in any suitable manner well known in the industry.

The method of pulsating the hydrocarbon gas through a refractory bed and carrying out a substantial amount of the separated carbon at very high temperature may be further utilized to carry out additional useful operations. For instance, as the cracked gas, with its carbon content is exhausted at an approximate temperature of 2500 degrees Fahrenheit, a predetermined amount of hot steam is passed into the stream of hot cracked gas and the mixture is passed through a catalyst bed of any proper composition.

The sensible heat of the excess gas or steam over and above any such amount which reacts in any given catalyst, will supply the extra heat required for the endothermic reaction between the carbon black and steam to form CO and H Also, if desired, CO or H or both may be formed in this way.

An example of calculating the heat and gas quantities involved is as follows: Reacting 1 lb. of carbon with 32 cu. ft. of H 0 requires the addition of 4760 B.t.u. per lb. of carbon. Let us assume that the hydrogen gas with its carbon content and steam are brought together at 2500 degrees Fahrenheit and passed through the catalyst where they react and are cooled to 1700 degrees Fahrenheit forming CO and H gas. In this case, the sensible heat in the 800 degrees Fahrenheit temperature drop of the carbon is 320 B.t.u. per lb. of carbon and 830 B.t.u. per 32 cu. ft. of H 0, making a total of 1150 B.t.u. usefully released for reaction. To complete the reaction an additional 3610 B.t.u. is needed. This amount of useful heat is supplied by 224 cu. ft. of hydrogen in cooling from 2500 degrees Fahrenheit to 1700 degrees Fahrenheit. This means that in order to react completely 1 lb. of carbon with steam in the catalyst bed, 224.cu. ft. of gas has to be present. During my operation of a plant embodying my invention, actually 2300 cu. ft. of gas was made by cracking 1000 cu. ft. of natural gas obtained from the East Texas Gas Field. Therefore, if 10.3 lbs. of carbon is carried out from cracking each 1000 cu. ft. of natural gas, this carbon can be reacted with steam to form CO and H and leave the total gas at 1700 degrees Fahrenheit which is quite an ideal temperature for gaseous reduction of iron oxide.

Actual operations have shown that this approximate amount of carbon in the cracked gas is easily obtainable. If more carbon is present it can pass into the iron ore Without plugging it or unduly cooling it by reacting with the iron oxide.

If the absorption of more carbon is necessary, the addition of surplus steam will form some CO gas and H which reactions will be endothermic to a lesser degree. Also oxygen or heated air may be introduced with the steam into the hot gas stream in any required and controlled amounts. The available oxygen burns with the carbon and supplies the heat balance required in addit.on to the useful sensible heat in the gases for reaction with the steam. This gas mixture then will be reformed to suflicient degree and will have suitable components to be used for gaseous ore reduction.

This gas after passing through the cataylst will be passed directly through the iron ore with the pulsating pressure as previously described to reduce the ore. In some particular cases, if so desired, this reducing gas may be passed through the iron ore without pulsation to accomplish a specific lesser degree of reduction.

Another application of my invention is the following: After the gas passes through the iron ore and reacts with it, it contains about 50% unreacted gas. The unreacted part of the gas is mostly H and a small amount of CO. This exhaust gas is then cooled and the H 0 condensed out of it and if so desired the CO scrubbed out by well known methods. The recovered gas will be mostly H and some CO. This gas is compressed and mixed with the natural gas stream. The mixture is again passed through the gas cracker where it is heated to the same temperature as the cracked natural gas, and leaves the gas cracker mixed with the cracked gas and carbon. In this manner, the hot exhaust gas leaving the cracker will contain only one-half the amount of carbon it does when no gas is recirculated or added to the natural gas.

The cracked gas may be added in any desired amount to the natural gas in order to serve a specific purpose, and may be passed into the gas cracker in one stream with the natural gas or in separate streams. The addition of this gas will enable the operator to exhaust the gas from the gas cracker at a lower temperature than indicated above and still obtain higher temperature gas leaving the second catalyst bed than indicated above, since there will be proportionately more hot cracked gas to supply the sensible heat for the reaction between the carbon and steam in the catalyst bed.

It is not necessary to scrub out the CO from the I recirculated gas in some instances for specific operations,

since the CO will crack with the carbon in the gas cracker into two volumes of CO which is a desirable reducing gas.

My invention for cracking hydrocarbon gases will also be practical and useful in cases where the resulting gas is used for other purposes than ore reduction after having been cooled and reheated, such as cases wherein the produced gas is a by-product and the carbon black is the desired main product. When the carbon black is to be saved, the hot exhaust gas from the gas cracker is cooled to a desired degree preferably by injecting water and steam into it, and the carbon black is separated from the gas stream either by a Cottrell precipitator or filtered out or gathered in any other practical manner.

It will be practical to partially burn hydrocarbon gas with oxygen at a controlled temperature in order to pro duce controlled quality carbon black, soft lamp black, similar to channel black and others. In the flow the burned gas and carbon will be cooled by the unburned natural gas. The carbon black will be filtered out from the stream and the rest of the uncracked gas will be further cracked. This can be done in several controlled stages injecting a controlled amount of oxygen or air after filtering out the carbon black at each stage. The final reforming of the gas may then be done as described above by reacting the hydrocarbon gas with the CO produced by the injection of and burning with oxygen.

When it is not desired to save the carbon black, a surplus amount of hot steam may be injected into the hot exhaust cracked gas which then will form mostly CO instead of CO with the carbon. This reaction requires a lesser amount of heat addition than is necessary to form CO, and can be carried out at a lower temperature. The practical approximate amount of heat required to react carbon with steam to form CO and H requires the addition of 1600 B.t.u. per lb. of carbon. This absorption of carbon or completion of cracking of the partially cracked hydrocarbon gases, may bealso be done in two stages. l-lot steam is first injected into the hot cracked or partially cracked gas stream as it leaves the gas cracker and before it goes into the catalyst bed. After the gases leave the catalyst bed more steam at a lower temperature is injected to cool the gases which are then passed through another suitable catalyst bed to complete the reaction to CO Hot reduced sponge iron may be carburized and cooled in a similar manner. Considering a mass of hot reduced sponge iron in a vessel as a porous bed or a checker work as described above, cold hydrocarbon gases or liquids can be injected into the charge and the prodnets of reaction exhausted through the charge. Heat is absorbed from the hot iron in two ways; first, by the sensible heat transfer to the injected and product gases from the hot sponge iron, which, if freshly reduced, is at about 1400" to 180Q F. and second, by heat absorption by the gases in reforming from hydrocarbons to hydrogen and carbon insofar as reforming occurs. 1ncan= descent sponge iron is an excellent catalyst surface for thermal cracking, and while the temperature in a charge of sponge iron remains sufficient, a vessel charged with this material functions in the same manner as the reformers described above for a single cycle of cooling.

Sponge iron, while it is porous to gas fiow, is quite effective in filtering carbon black formed in the above described cracking procedure. This carbon acts as a strong carburizer of the hot iron and depending on the final temperature of the reduction of the iron, I have deposited carbon in excess of 5% in the sponge mass of which about one half was reacted with the iron to form iron carbide; the remainder was carbon black deposit in the sponge. The amount of iron carbide depends a great deal upon temperatures of iron and gas, methane content, amount of gas circulated and other factors.

In a production operation where the final sponge iron reduction temperature is established and fixed the amount of carbon deposited may be very closely controlled by selection and control of the uncracked hydrocarbons in the inlet gases. Sponge iron may be cooled by sensible heat transfer alone by circulating cold reducing gas in the described manner, or the circulated gas may contain any percentage of uncracked hydrocarbons up to By controlling this balance between cracked and uncracked reducing gas the amount of carbon deposited can be closely controlled, and depending on the finishing temperature of the hot sponge iron, the amount of combined carbon can be controlled.

This cooling can be done by passing the gases described through the hot sponge iron without the pulsating pressure I have described above, but in an industrial operation pulsation will be substantially more effective, the cooling will be more uniform and faster than without pulsation.

At 400 F. or less, sponge iron with its deposited and combined carbon is well protected for handling in the air or under other oxidizing conditions. At higher temperatures a heavy coating of carbon deposited in the above described manner will permit substantial handling of sponge iron without observable losses through reoxidation.

I claim:

1. Method of heating gases which comprises preheating a checker heater to a relatively high temperature, introducing a gas into said preheated checker heater at higher pressure, and increasing the pressure within the checker heater, then exhausting the gaseous products to lower pressure, repeating the cycle of introducing the gas, increasing the pressure and exhausting the gas until the temperature of the checker heater is substantially reduced, then again preheating the checker heater to the said high temperature, and continuing with the cycle.

2. In a method for operating a checker heater, the steps which comprise passing a gas at higher pressure through a preheated checker heater, pulsating the gas as it passes through said checker heater, and exhausting the gaseous products at a lower pressure.

3. Method according to claim 2 wherein the gas comprises a hydrocarbon gas.

4. Method according to claim 2 wherein the gas comprises a mixture of hydrocarbon gas and oxidized gas.

5. Method according to claim 2 wherein the gas passed into the checker heater comprises a mixture of hydrocarbon gas and partially oxidized reducing gas.

6. In a method for operating a checker heater, the steps which comprise passing a gas at higher pressure through a preheated checker heater, pulsating the gas as it passes through said checker heater, and exhausting the gaseous products at a lower pressure, said exhaust pressure being not substantially lower than atmospheric pressure.

7. Method according to claim 6 wherein the gas comprises a hydrocarbon gas.

8. Method according to claim 6 wherein the gas passed into the checker heater comprises a mixture of hydrocarbon gas and partially oxidized reducing gas.

9. Method according to claim 6 wherein the gas passed into the checker heater comprises a hydrocarbon gas, hydrogen, carbon monoxide, and carbon dioxide, thereby reforming said gas mixture to obtain from said hydrocarbon gas and said carbon dioxide additional amounts of hydrogen and carbon monoxide.

References Cited in the file of this patent UNITED STATES PATENTS 417,273 Parkinson Dec. 17, 1889 874,265 Volney Dec. 17, 1907 1,319,589 Jones Oct. 21, 1919 2,166,207 Clark July 18, 1939 FOREIGN PATENTS 482,831 Great Britain Apr. 5, 1938

Claims (1)

1. METHOD OF HEATING GASES WHICH COMPRISES PREHEATING A CHECKER HEATER TO A RELATIVELY HIGH TEMPERATURE, INTRODUCING A GAS INTO SAID PREHEATED CHECKER HEATER AT HIGHER PRESSURE, AND INCREASING THE PRESSURE WITHIN THE CHECKER HEATER, THEN EXHAUSTING THE GASEOUS PRODUCTS TO LOWER PRESSURE, REPEATING THE CYCLE OF INTRODUCING THE GAS, INCREASING THE PRESSURE AND EXHAUSTING THE GAS UNTIL THE TEMPERATURE OF THE CHECKER HEATER IS SUBSTANTIALLY REDUCED, THEN AGAIN PREHEATING THE CHECKER HEATER TO THE SAID HIGH TEMPERATURE, AND CONTINUING WITH THE CYCLE.