US2174196A - Process for the manufacture of ethylene - Google Patents

Description

D. A. ROGERS 2,174,196

Sept. 26, 1939.

FROCESS FOR THE MANUFACTURE OF ETHYLENE x Filed Dec. 6, 1937 2 Sheets-Sheet 1 f ff F06/ wm'zer /5 U/m Zd Wagens BY l ATTORN Y Sept. 26, 1939. D. A. ROGERS 2,174,196

PROCESS FOR THE MANUFACTURE OF ETHYLENE Filed Dec. 6, 1937 2 Sheets-Sheet 2 Fl/e/ 651525 0/7 ATTORNEY Patented Sept. 26, 1939 4UNITED STATES PATENT OFFICE Donald Atwater Rogers, Petersburg, Va., assignor to The Solvay Process Company, New York, N. Y., a corporation of New York Application December 6, 1937, Serial No. 118,320

3 Claims.

This invention relates to the manufacture of ethylene. It is particularly directed to the operation of regenerative cracking units in the vaporous or gaseous phase thermal cracking of hydrocarbons ofhigher molecular Weight than ethylene.

The manufacture of ethylene by thermally cracking hydrocarbons such as ethane, propane, butane, and higher hydrocarbons or mixtures thereof such as gas oil, kerosene, crude petroleum, or topped petroleum requires for efficient operation the maintenance of relatively high temperatures in the cracking zone. Temperatures in the range 780 to 1050o C. have been found to be satisfactory and temperatures around 800 to 950 C. are preferable. Attempts to attain such cracking temperatures by heating the hydrocarbons or mixtures containing them through heat transfer Walls of steel or other metals results in rapid destruction of the apparatus with consequent heavy replacement cost; iron surfaces furthermore have a deleterious effect upon the desired reaction. As a consequence the use of metal surfaces is not desirable. Refractory materials on the other hand do not possess the combination of tensile strength and heat conductivity essential for indirect heat transfer operations on an economical basis.

The use of regenerative types of equipment possesses the advantage that highly refractory materials may be employed and a relatively long equipment life may be obtained.

The use of previously proposed regenerative cracking processes, which involve, in alternate periods, heating refractory material to cracking temperature with fresh hot combustion gases and introducing hydrocarbon into the heated material for cracking, is open to the objection that an efficient utilization of the heat supplied is not obtained and consequently fuel costs are high.

The present invention has for an object the improvement in regenerative cracking of hydrocarbons at the high temperatures employed in the manufacture of i ethylene whereby an efficient utilization of heat is made possible and operating costs may be substantially reduced.

In accordance with the present invention the air required in the heating or blast period of a regenerative cracking process is preheated by contact with refractory material which has been employed for cooling the products of a preceding cracking period. Thus the process of my inven tion involves a cycle having a blast period, in which refractory material of considerable heat capacity is cooled by ingoing air, and an inde- (Cl. 26o-683) pendent cracking period, in which the cracked products are cooled by contact with this material. The heat of the hot products of cracking is transferred indirectly to hydrocarbon to be cracked in a succeeding stage so that the sensible heat of the high temperature products of cracking is conserved and utilized for further cracking.

It is important that the gaseous product of cracking contain as high a concentration of ethylene as possible in order to avoid handling excessive quantities of gases and to obtain the highest absorption efficiency when the ethylene is extracted from the mixture by absorption media such as sulfuric acid. Accordingly the product of cracking should contain a minimum of combustion products. To accomplish this result the cracking step is an independent step, apart from the heating or combustion steps and the cracked product is collected and recovered separately from combustion products. Complete freedom 0 from combustion products may be obtained by displacing such products from the system with steam before each cracking period and avoiding the introduction of substantial amounts of air during the cracking period. ,25

In its preferred aspects the invention involves a cracking of hydrocarbon mixed with steam. An amount of steam from 1 to 3 times the weightof the total hydrocarbon present preferably is employed. The steam serves the purposes of 30 maintaining refractory surfaces in a clean condition and of reducing the tendency of ethylene formed in the cracking step to polymerze or form benzene.

The invention is applicable to cracking processes in which the hydrocarbon to be cracked is heated to cracking temperature by direct c0'n tact with hot refractory and to processes in which a part or all of the heat required for cracking is supplied to the hydrocarbon from such refractory through the interposition of steam as a heat transfer medium.

Although a very considerable proportion of the heat of cracked products may be conserved by passing them through a bed of refractory material and later passing air for use in the blast period in the same direction through the refrac tory bed, it is preferred to employ a flow of air opposite in direction to the flow of hydrocarbon products through this refractory bed in order that a maximum efficiency of heat recovery may be attained.

It is preferred also to pass the hot combustion gases through a second zone of refractory material in the blast period to cool the gases to below 400 C., and to pass initially relatively cool (below 300 C.) steam or mixture of steam and hydrocarbon to be cracked through this refractory zone in a direction countercurrent to the blast gases. In this manner the blast gases serve not only to heat the cracking zone but to preheat the steam and in some cases the hydrocarbon to be cracked.

It has been found that by operating a regenerativev cracking proces for the manufacture of ethylene in accordance with the invention. cracked products may be cooled from a cracking temperature between 780 and 1050 C. to a temperature between 150 and 400 C.

The following description of the preferred embodiments Illustrated in the accompanying drawings will further explain the nature of the invention.

In the drawings Fig. 1 designates one combination of refractory cracking and heat exchange units which may be employed for carrying out my invention;

Figs. l-a and l-b are ow diagrams for a representative two-period cycle for operating the apparatus of Fig. l, showing the blast period and the cracking period, respectively;

Fig. 2 illustrates a second combination of regenerative heat exchange cracking units especially adapted for use of steam to supply the heat required for cracking;

Figs. 2-a and 2-b are flow diagrams for a representative two-period cycle in the apparatus of Fig. 2;

Fig. 3 shows a third combination of apparatus embodying the invention, arranged for operating with a four-period cycle of a partially reversible type, employing separate chambers for preheating steam and hydrocarbon; and

Figs. 3 4, 3-b, 3 6, and 3-d are ow diagrams for a representative four-period cycle in the apparatus of Fig. 3.

It should be appreciated that the drawings are in diagrammatic form and that for simplicity conventional features of construction known to the art of cracking hydrocarbons have been omitted. However, the relative dimensions of the packed units approximate those which may be used advantageously on a commercial scale. It will be understood, of course, that these dimensions are largely a matter of choice in that the packed chambers may be made longer and narrower to increase the influence of counter-current operation or may be made broader and shorter to reduce pressure drop through the units. The units shown are proportioned for operating with broken refractory packing without creating an excessive pressure drop through the units. If other types of refractory are employed the preferred proportions may be different. Thus checkerwork which has a relatively large amount of free space may render the use of taller chambers desirable.

With special reference to Fig. 1 of the drawings, the numeral I designates a refractory cracking unit provided with a packed section 2 comprising brick crushed to between 1 and 2 inch mesh. Since in normal operation the upper portion of the packing is maintained at relatively high temperatures compared with the lower portion, more than one type of packing may be employed. 'Ihus in the lower half a fire-clay brick capable of withstanding moderately high temperatures and selected especially to withstand the pressure of the mass of material upon it may be used. while in the upper half a crushed material especially selected to withstand the high temperatures in this zone, for instance claybonded silicon carbide, may be employed.

Unit I has inlet 3 for steam, inlet 4 for oil, and outlet 5 for blast gas.

Steam inlet l has valve 6 for controlling tlm'. introduction of steam into the unit. Hydrocarbon inlet I preferably is provided with atomizer I and oil inlet pipe l controlled by valve 9, so that oil may be introduced into the cracking unit in the form of either vapor or a mixture of vapor and atomized liquid, as will be hereinafter explained. Outlet conduit 5 to the stack may have a valve I0. While in cracking unit construction it is common to employ a stack valve at a point remote from the cracking unit in order not to injure the valve structure by the high temperature gases passing therethrough. this is not essential in the present structure since the gases leaving the cracking unit'are at a relatively low temperature.

A conduit II unites cracking unit I to a second refractory unit I2 of the same diameter but provided with a relatively short section of packing I3. The packing may be of the type described in connection with unit I and may be entirely of firebrick or may have an upper layer of more resistant refractory material. Preferably the lower portion consists of somewhat larger fragments than the balance of the section. This unit has a fuel inlet Il provided with a valve I5 above the packing in the unit. Any suitable distributing nozzle I5a may be provided. Below the packed section there is a second conduit l5 to which are connected air inlet I I having valve I8 and steam inlet I 8 having valve 20. Conduit I5 leads to a product recovery system, which may be of any suitable type such as that shown and described in the application of Frank A. Porter and John Moyle Duncan, Serial No. 153,496, entitled Process for the manufacture of ethylene, filed July 14, 1937. Flow through conduit I6 may be controlled by a valve ZI.

The apparatus illustrated in Fig. 1 may be cmployed for the manufacture of ethylene from gas `oil in the following manner.

The blast period:

The flow during the blast period of the cycle is shown in Fig. l-a. Temperatures indicated are representative of those occurring in a normal blast period, i. e., one that follows a cracking period as distinguished from an initial heating.

With valves I 0, I5 and I8 open and the rest of the valves closed, air at ordinary atmospheric temperature, for example around C., is admitted through conduit I6 to unit i2 and passes up through the refractory I3 therein. In the combustion space above refractory I3, the air is mixed with fuel introduced at inlet I4, for instance tar obtained from the cracking operation, if desired after removal of more valuable constituents. The fuel may be ignited by any suitable ignition device (not shown), for instance a small mass of refractory shielded from the flow of cracking mixture so that it retains its high temperature from one blast period to the next. The ratio of air to fuel may be adjusted to give a combustion gas temperature of about 1050 C The hot combustion gases produced by the combustion of fuel in the combustion space of unit I2 pass through conduit II into cracking unit I, down through the refractory in this unit, and out through blast gas outlet 5.

Introduction of air and fuel is continued until the refractory in the dome of unit I attains a temperature of about 1000 C. The valves I5 and I8 are then closed.. At this point in the initial warming up refractory at the top of section I3 will have attained a temperature around 300 C. or higher because of the combustion taking place thereabove; otherwise the refractory of this section will be relatively cool. Refractory 2 of unit I will have been heated by the hot combustion gases to a temperature around 1000D C. at the top and around 300" C. at the bottom.

The combustion gas purge:

After the blast period has been completed, valve 20 may be opened for a short period to admit steam and expel, via the stack, combustion products and air from units I and I2. This purge fills the refractory units with steam and places them in condition to begin a cracking period.

The cracking period:

Valves 20 and I0 are closed and valves 6, 3 and 2l are opened. Gas-oil, preheated to a temperature of about 200 C. and sufficient pressure to maintain it in liquid phase, enters at oil inletv 8 and passes into atomizer 1 where it is partially vaporized by sudden reduction of pressure, and the unvaporized portion is suspended in the resultant vapor. The mixture of hydrocarbon liquid and vapor passes through inlet 4 into the mixing and vaporizing space between the upper and lower packing of section 2. Steam at a temperature of about 105 C. passes through inlet 3, is heated by the lower packing in section 2 and mixes with the oil to complete the vaporization thereof. The ratio of steam to oil may be suitably controlled at about 1.7 to 1, byweight. The average temperature at the point Where steam mixes with oil should be sufficient so that liquid oil will not accumulate from one cracking period to another, preferably not at all. The steam, containing oil vapor, continues up through refractory material in section 2 into zones of successively higher temperature. The hydrocarbon vapor is first superheated and then, as it attains a temperature around 780 C., is cracked to form ethylene. The cracking continues as the mixture passes into the top of unit I and through conduit' II into unit I2.

The iiow is preferably regulated to give a cracking time between-about 0.006 and 0.3 second depending upon the cracking temperature. Thus in the neighborhood of 1050 C. the shorter periods within the range should be used and at 780 C., the longer. For an 850 C. cracking temperature a time of about 0.05 second may be employed to advantage; either`the time or temperature can be varied considerably-from these values without greatly affecting the ethylene yield. By operating in this manner a gas containing from 25 to mol per cent of ethylene may be produced.

In unit I2 the mixture passes down through packed section I3, which, as above noted, is of relatively low temperature, and is cooled to a temperature in the neighborhood of 200 C. The cooled products pass out through outlet conduit I6 to suitable product recovery apparatus for condensation of condensable products after which ethylene may be passed to suitable storage.

As the cracking period continues, refractory section 2 is gradually cooled and the heat imparted to the,mixture of steam and oil vapor diminishes. As a consequence the maximum temperature attained by the mixture may gradually fall. Thus, at the commencement ofy the period the mixture may be heated to `a temperature of 950 C. and after some time the maximum temperature may have fallen to about 850 C. On the other hand, the temperature of the product leaving at outlet I6 may rise to the neighborhood of 300 C. during a corresponding period. A fall in the maximum temperature attained by the cracking mixture may be compensated to some extent by reducing the rate of flow.

The product purge:

When the maximum temperature of the mixture at a reasonable rate of flow has dropped to 850 C., valve 9 may be closed. Valves 6 and 2| may be left open for a short purge to eliminate hydrocarbons from units I and I2.

A normal cycle blast period:

Valves 6 and 2| are closed and valves I0, I5, and I8 opened to begin a second blast period.

Since refractory material in section I3 has abstraeted heat from the cracked products, the temperature of this material will now be considerably higher than at the commencement of the cracking period and ingoing air may be heated from atmospheric temperature to around '150 C. in this refractory. The high temperature of this air permits a corresponding reduction in the amount of fuel required in each blast period. Furthermorethe temperature of packed section 2 may be only about 150 C. at the bottom so that the combustion gases may be cooled to around 200 C. before leaving unit I. As the blast period continues, the temperature of refractory I3 falls and that of 2 rises until at the end of the blast period the air may be preheated to around 360 C. and the blast gases cooled to about 300 C.

1f the cracked products are chilled too far in packed section I3 tarry products will deposit; hence it is desirable to avoid too effective cooling in the lower part of this section. By the provision of relatively large refractory fragments exposing a relatively small surface area, in this portion of the packed section, the heat removal may be controlled. vThus air entering at atmospheric temperature may cool the surface of the refractory material but the interior will remain hot. If a thermal soaking period is provided this heat will pass to the surface. The purge period and valve operation will provide some soaking time and the purge steam will also supply some heat to the refractory surfaces. Further time may be provided by allowing the apparatus to remain idle. Such a procedure tends too to eliminate localized extremes of temperature.

Alternatively the lower part of this refractory may be kept warm by preheating the entering air,

for instance by heat exchange with blast gas, or

by withdrawing air from a hotter zone of the packing andrnixing it with the entering air in sufficient duantities to maintain the desired temperature.

The refractory temperature should not be substantially below the dew point of tarry or oily constitutents. Small quantities of condensate appearing early in the cracking period may be evaporated later in the period but even such condensation is undesirable since it provides a wet surface to which solid particles can adhere and these solid particles remain as firmly attached deposits when the liquid portions are evaporated. In initially starting operations it may be desirable to conduct a few cycles using steam along in the "cracking periods, in order to warm-up the refractory in the cooling unit.

The proportion of a complete cycle required for the blast period, the cracking period, and the purges will vary considerably depending upon the design of the apparatus, the relative mass flow rates of blast air and process steam, the type of fuel employed, and the type of initial material employed. Other operating factors, such as the temperature of the steam and oil used, may also affect these values. For illustrative purposes, however, the cracking period may be considered to consume from 14% to 30% of the cycle and the blast from 71% to 55%, the remaining 15% of the cycle being used up in purging the system and manipulating the valves.

The apparatus of Fig. 2 is substantially the same as that of Fig. 1 except that the oil inlet is located at the top of the first unit. Since a space for oil vaporization within section 2a is not necessary, this section need not be interrupted. To this extent the construction of the apparatus is simplified.

The various elements of the apparatus of Fig. 2 have been designated by the same numerals as corresponding elements in Fig. 1 with the letter "a" added.

'Ihe blast period:

Air and a fuel, such as tar, may be introduced into unit 12a via inlets I1a and Ila, respectively,

and the mixture ignited to produce a hot combustion gas mixture at a temperature around 2000 C. This hot mixture passes through conduit Ila to unit la, down through heat storage section 2a, and out through 5a to the stack. The flow is illustrated in Fig. 2-a. This period is continued until the refractory at the top of section 2a has attained a temperature of about 1950 C. The blast gases may be cooled to a temperature around 200 C. by the refractory in section 2a at the beginning of the blast period; as this refractory becomes heated, the exit gas temperature rises and may reach about 300 C. at the end of the blast period.

The combustion gas purge:

When the blast is completed, steam is introduced at 19a to purge the system of combustion gas and air. This steam passes up through unit Iza, down through unit la, and out at 5a. to the stack. In this manner the system is prepared for the rst cracking period.

'I'he cracking period:

Oil at an initial temperature around 200 C. and sufficient pressure to maintain it in liquid phase, is introduced at 8a and steam is introduced at 3a in a ratio of steam to oil of about 1.7 to 1 by weight. The steam passes up through the hot refractory in section 2a and is heated to about l900 C. The oil, partly in the form of vapor and partly in the form of a suspension, mixes with the steam and is thereby heated to cracking temperature. The rates of oil and steam flow are preferably controlled to give a temperature of about 825 C. for the mixture. This mixture passes through conduit Ha to unit 12a and down through section I3a therein. Since in the blast period section I3a was contacted with cold entering air, this packing is relatively cold and cools the cracked products to a temperature in the neighborhood of 200 C. The minimum refractory temperature may be controlled as discussed in connection with the first embodiment described. Cracking may be conducted until the maximum temperature of refractory in section 2a has fallen from about 1950 C. to about 1850 C. and the maximum steam temperature has dropped to around 1800 C. As the cracking period continues, refractory material in section 13a becomes heated and by the end of the cracking period the products leaving at ISa may be around 300 C.

The cracking temperature may be maintained substantially constant throughout the cracking period by varying the flow of oil only about 6% from the average rate, thus it may be 6% faster at the beginning of the cracking period and 6% slower at the end. When a greater variation is necessary to maintain a constant cracking temperature, it is advantageous to terminate the cracking period. As in the case of the apparatus of Fig. 1, however, the process may be operated with variation of temperature rather than variation of ow of materials through the cracking zone.

When the cracking period is terminated, the flow of steam may be continued for a brief period to purge the system of hydrocarbons.

In the above operations the cycle may be timed so as to consume about 27% of operating time in the cracking period and about 58% in the blast period; the other 15% being used for purges and valve operation. Of course this last percentage is kept as small as permissible for the particular mechanism employed.

In Fig. 3 a three-unit system is shown. The central unit is subjected to blasting and only this unit is employed for heating the steam prior to cracking. The other two units are employed alternately for preheating the blast air, cooling cracked products, and superheating ingoing hydrocarbon vapors. Thus the heat given up to refractory by the products of cracking is returned partly as preheat of blast air and partly as superheat of ingoing hydrocarbon.

Specifically the apparatus comprises three regenerative units, 5|, 52, and 53, connected by conduits 54 and 55 and provided with heat storage sections 56, 51, and 58 containing refractory packing. Units 5I and 53 are provided with fuel inlets 59 and 60, oil inlets 6| and 62, air inlets 63 and 64, outlets 65 and 66 for the cracked products, and outlets 61 and 68 to the stack. Unit 52 is provided at its base with a steam inlet SS and an outlet to the stack.

The operation of this apparatus in a representative cycle is shown in Figs. 3-a, 3-b, 3 0, and 3-d.

The first blast period (Fig. 3-a) Air and fuel are introduced at 63 and 59, respectively, into unit 5|. -Combustion gases at about 1620 C. pass through conduit 54 into unit 52, down through section 51 where they give up most of their contained heat to the refractory packing of which this section consists and out through 10 to the stack at about 200 C. at the beginning and about 300l C. at the end of the period. By this operation the packed section 51 is heated at its top to a temperature around l570 and at its bottom to a temperature around 250 C. If the blast period is prolonged, the depth of the zone at approximately maximum temperature is increased. The most satisfactory depth will depend upon the amount of oil to be cracked in the succeeding cracking period, and this in turn will vary depending upon the operating characteristics of the particular apparatus involved. Packed section 56 serves no function in the initial heating up step but in subsequent blast periods will serve to preheat the air to from 625 C. at the beginning of the blast period to 525 C. at the end. In the initial heating up, section 58 may be blasted to heat it to about 800 C. at the top.

The combustion gas purge:

After the blast period steam is introduced at 69 and passed through packed sections 51 and 56 to expel combustion products. Since the amount of such products is exceedingly small compared with the amount of cracked products produced, the purged gases may be sent to product recovery. It is preferred, however, to send such products through outlet 61 to the stack. During the purge steam also may be passed through packed section 58 in unit 53 and through outlet 68 to the stack to insure against the presence of combustion gases in unit 53. y

The first cracking period (Fig. 3-b) After the purge, oil preheated to 200 C. under pressure and partially vaporized as in the embodiments of Figs. 1 and 2, is introduced at 62 and steam is introduced at 69. The vaporization of the oil is completed and the vapors are superheated to a temperature of about 725 C. in packed section 58. Steam introduced at 69 is heated in section 51 to about 1520" C. The preheated oil vapor mixes with the preheated steam in the vapor space above packed section 51 in unit 52 and the mixture at a temperature around 825o C. passes into unit 5I, down through this unit where it is cooled by contact with refractory 56 to a temperature around 200 C. and thence by Way of outlet 65 to the product recovery system. This period may be continued until the ingoing steam is heated to about 1420 C. and the ingoing oil is heated to only about 625 C. By this time the refractory section 56 will have been heated to about '775 C. at the top and about 250 C. at the bottom and the products leaving the system will be around 300 C.

Purge of products:

The ilow of steam may be continued after the l cracking period is ended to expel cracked products from units 52 and 5|. At the same time by opening outlet 66 a portion of the steam may be caused to flow through unit 53 to expel oil vapors from this unit. The steam flow is then stopped.

The second blast period (Fig. 3c) and purge:

When the system has been purged of hydrocarbon` vapors, air and fuel may be introduced at 64 and 60 respectively and combustion products may be withdrawn at 10. Section 51 is thus again heated to about 1570 C. at the top by 'the combustion gases. When this temperature has been attained, the blast is discontinued.

Packed section 58, which has been cooled to about 675 C. in the previous cracking period by contact with ingoing oil, is further cooled by the air for combustion. The combustion air at the same time is heated to a temperature of about 625 C. at the commencement of the blast period. These temperatures may fall gradually about C. during the blast period so that at the end the air is preheated to about 525 C. The system is now purged by passing steam in at 69, up through section 51, down through sections 56 and 58, and out to the stack.

The second cracking period (Fig. 3-d) and purge:

When the purge is completed the flows of oil and steam are started through inlets 6I and 69, respectively. .A The oil is heated in section 56 to a temperature of about 725 C. and the steam is heated in section 51 to a temperature around 1520 C. The steam and oil vapors are mixed in the vapor space above packed section 51 and pass thence into unit 53 at a temperature around 825 C. The mixture then passes down through section 58 where it is cooled by contact with the refractory material to about 200 C. before being sent to the product recovery system. As the cracking period continues the maximum temperaturesof the steam and superhea-ted oil vapor may fall about 100 C. each. The cracking period is then terminated. After the cracking period the system may be purged by means of steam as previously described.

The above described arrangement of regenerative units may be operated on a schedule involving around 32% cracking period and 53% blast period; the remaining 15% being proportioned among the purges and valve operations.

The cracking temperature may be maintained substantially constant through the cracking period by varying the flow of oil only about 14% from the average rate, thus it may be 14% faster at the beginning of the cracking period and 14% slower at the end.

It is possible to revise the operation of the arrangement of Fig. 3 so that the periods follow in the order 3-a, 3-d, 3-c, 3-b. Such a regimen has the disadvantage that it reduces the heat efficiency slightly but has the advantage that it provides a thermal soak for the cooling unit after the refractory in this unit has been cooled by blast air and before it is used to cool reaction products. Hence, aside from provision of relatively large refractory fragments in the lower parts of the sections 56 arid 58, no further provision is required to prevent tar deposition as a result of overcooling.

I make no claim to the feature of superheating the hydrocarbon vapor to be cracked, by contact with refractory material which has been employed for cooling the products of the preceding cracking period as described herein, since this is the subject matter of application Serial No. 178,303 of John Moyle Duncan, entitled Process for-the manufacture of ethylene, filed' on the same date as this application.

I claim:

1. In the manufacture of ethylene from hydrocarbons of higherv molecular weight by a cyclic regenerative process of thermal cracking in the 'presence of steam wherein heat of hot combustion gases is supplied to a cracking zone in one period of a cycle and steam and hydrocarbon to be cracked are supplied to said zone in .another period of the cycle and cracked therein to form a reaction mixture at a temperature between 780 and 1050 C., the improvement which comprises passing air into contact with hot refractory `A to heat said air and cool said refractory, burning a fuel with the heated air to form hot combustion gases, passing the hot combustion gases into contact with refractory B to heat said refractory to at least the cracking temperature of the hydrocarbon to be cracked and cool said gases, and removing the cooled combustion gases apart from products of cracking, in the vone period, and in the other period passing steam and hydrocarbon into contact with refractory B to heat the steam and hydrocarbon to the cracking temperature of the hydrocarbon to be cracked, passing the resultant mixture into contact with refractory A to transfer the heatv absorbed from refractory B to refractory A and thereby cool the products of cracking, and recovering the cooled products apart from combustion gases.

2. In the manufacture of ethylene from hydrocarbons of higher molecular weight by a cyclic regenerative process of thermal cracking in the presence of steam wherein heat of hot combustion gases is supplied to a cracking zone in one period of a cycle and steam and hydrocarbon to be cracked are supplied to said zone in another period of the cycle and crackedA therein to form a reaction mixture at a. temperature between 780 and 1050 C., the improvement which comprises passing air into contact with hot refractory A to heat said air and cool said refractory. burning a fuel with the heated air to form hot combustion gases, passing the hot combustion gases into contact with refractory B to heat said refractory to at least the cracking temperature of the hydrocarbon to be cracked, and then into contact with additional refractory to cool the gases at least to 400 C., and removing the cooled combustion gases apart from products of cracking, in the one period, and in the other period passing steam at a temperature below 300 C. first into contact with said additional refractory to superheat the steam and then, in admixture with hydrocarbon, into contact with refractory B to heat the steam and hydrocarbon to the cracking temperature of the hydrocarbon to be cracked, passing the resultant mixture into contact with refractory A to transfer the heat absorbed from refractory B to refractory A and thereby cool the products of cracking, and recovering the cooled products apart from combustion gases.

3. In the manufacture of ethylene from hydrocarbons of higher molecular weight by a cyclic regenerativeprocess of thermal cracking in the presence of steam wherein heat of hot combustion gases is supplied to a cracking zone in one period of a cycle and steam and hydrocarbon to be cracked in a weight ratio between 1:1 and 3:1

are supplied to said zone in another period of the cycle and cracked therein to form a reaction mixture at a temperature between 780 and 1050 C., the improvement which comprises passing air at about atmospheric temperature into contact with hot refractory A to heat said air and cool said refractory, burning a fuel with the heated air to form hot combustion gases, passing the hot combustion gases into contact with refractory B to heat said refractory to at least the cracking temperature of the hydrocarbon to be cracked, and then into contact with additional refractory to cool the gases at least to 400 C., and removing the cooled combustion gases apart from products of cracking, in the one period, and in the other period passing steam at a temperature below 300 C. rst into contact with said additional refractory to superheat the steam, the passage of said steam through said additional refractory being in a direction countercurrent to the direction of flow of the aforesaid combustion gases, and then, in admixture with hydrocarbon, into contact with refractory B to heat the steam and hydrocarbon to the cracking temperature of the hydrocarbon to be cracked, passing the resultant mixture into contact with refractory A countercurrent to said air to transfer the heat absorbed from refractory B to refractory A and thereby cool the products of cracking at least to 400 C., and recovering the cooled products apart from combustion gases. 3o

DONALD ATWATER ROGERS.

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