US2121733A - Purifying gases and apparatus therefor - Google Patents

Description

June 21, 193,8. F. G. COTTRELL PURIFYING GASES AND APPARATUS THEREFOR 2 Sheets-Sheet l Filed Aug. 14, 1935 `une 21, 1938'.

F. G. COTTRELL 2,121,733

PURIFYING GASES AND APPARATUS THEREFOR Filed Aug. 14, 1955 2 sheets-sheet 2 Patented June 21, 1938 PURIFYING GASES AND APPARATUS THEREFOB Frederick G. comen, washington, D. o., assigner to Research Corporation, New York, N. Y., a corporation of New York Application August 14; 1935, Serial No.'36,241

8 Claims.

This invention relates to a process of, and to apparatus for, thermally altering a gas.

In essence, the process ofthe present invention comprises initially heating a portion of a body of gas-traversable heat-absorbing material to an elevated temperature T2 at least equal to the thermal alteration temperature of the gas to be treated, whereby to create within the body of heat-absorbing material a thermal alteration zone between less highly heated zones, thereafter forcing the gas, at a materially lower entrant temperature T1, through the body in one direction, whereby the gas is heated substantially to T2 in passage through said thermal alteration zone and thereafter is cooled substantially below thermal alteration temperature, preferably` substantially to T1, by contact with a cooler portion of said body, the thermal alteration zone thereby being moved through said body in the direction of sage of said gas, reversing the direction of gas flow before thethermal alteration zone has been substantially moved out of said body, whereby the thermal alteration zone is .moved through said alternate movements of the gas whereby the thermal alteration zone is reciprocated within said body. Where the thermal alteration process is not productive of enough heat to offset losses of heat, from the body, by conduction and radiation, and it is desirable or necessary to maintainL the thermal alteration zone at or above a predetermined temperature, auxiliary heat may be introduced within the body, either continuously or intermittently.

Preferably, the size ofthe individual solids constituting the body of heat-absorbing material is so chosen with respect to the length of the path of gas flow therethrough and with respect to the area of section of the mass normal to the path of gas ow that the gas in its traverse experiences a relatively small drop in pressure but a close approach to thermodynamic reversibility of heat exchange with the solids. The identity of the 45 heat-absorbing material selected for use as the aforesaid body depends in part upon the temperature of the thermal alteration zone to be maintained therein and in part upon the gas or gases to be treated thermally therein. I It must be a. solid which does not materially fuse or deform at the desired thermal alteration temperature and which Ais chemically suitable. When the latter is -relatively low, almost any form of crushed rock, gravel, sand or metallurgical slag may be used; lidwhen higher maximum temperatures areto be gas fiow at a rate slower than the rate of pas,

body in the reverse direction, and repeating the4 maintained, it is suitable to employ broken or crushed bricks made of fire-clay or of silica. In some special cases particles of the common refractory oxides (CaO, MgO,'Al2O3, FezOa, S102, TiOa, ZrOz, CrnOa, etc.) singly or in combination are found to be of value. These substances are herein referred to generically as heat-resisting solids. Specifically included within the meaning of this term are, of course, any of the metals which are suitable or adapted to such use. \More over, the invention is not, in general, restricted to beds which are homogeneous with respect to particle size or composition. Upon occasion there may be used super-imposed layers in which individual particles differ from layer to layer in size, in shape and in chemical composition. Moreover, the solid particles or objects constituting the body of heat-absorbing material may, where found desirable, be formed of or contain materials capable of catalytically accelerating the desired alteration; the whole of the bed may consist of such catalytic material, or the latter may be intermingled throughout the body with other, non-catalytic, heat-absorbing material, or portions only (for example, the more nearly -central portions in which the zone of maximum heat more particularly resides) may be composed of, or contain, such catalytic heat-absorbing solid material. In the sense here contemplated, the so-called particle of catalytic material may either consist throughout of the catalytic substance or it may include a chemically inert core of heat-absorbing material coated or covered with, or carrying, the catalytic substance.

The thermal alteration contemplated in accordance with the present invention may be the thermal decomposition or dissociation of a constituent of the gas being treated, 6r a molecular re-arrangement; it may and frequently does include high-temperature oxidation, althoughl the process is by no means confined to a treatment wherein oxidation takes place. When high-temperature oxidation is contemplated and the gas being treated does not contain suflicient oxygen therefore, air or other oxygen-supplying gas is to be supplied, as by admixing the same with the gas being treated, in an amount to supply the needed oxygen, prior to introduction of the gas into the thermal alteration chamber, or by introducing tli'e oxygen-supplying gas directly into said chamber for ultimate admixture with the gas being treated. l

The process of the present invention nds utility in any one of a number of industrial operations. For instance, it is adapted to be used in the destruction of noxious foul and odorous constituents of gases such, for example, as frequently are discharged from furnaces, crematories, incinerators, driers, carbonizers and burners now used or proposed for the disposal of various agricultural, chemical, municipal, domestic and industrial waste products. It is to be understood, however; that the technical fields in which logically an application of the process lies are not conflnedto the treatment of the gases cited above, nor is the object of the invention restricted to the destenching of gases before discharge into the air. To illustrate an application of the invention to a somewhat different purpose, there may be cited a gas purification treatment of manufactured gas for municipal use in any of the usual retorts,

coke ovens or gas-making plants. In many plants manufacturing carburetted water-gas and oilgas, the primary product frequently contains certain substances which tend to form gums and resins which collect as objectionable deposits in pipe lines and gas meters in the distribution system. These organic gumand resin-forming constituents of the gas may frequently be thermally decomposed by subjecting the gas to relatively low temperatures for rather short durations of time.

This reference to the purification of manufactured city-gas containing gum-forming constituents is given to illustrate other types of applica.- tion of the method and apparatus than the destenching purpose discussed above. This example should suiiice to typify a large variety of technical operations which logically are grouped into a class which, from the standpoint of this invention, is termed gas to be treated, foul gas to be purified, or gas to be subjected to thermal treatment. 'I'he process consists simply in providing technical means by which a selected gas may be subjected to a thermal treatment which may readily be called recessive or retrocedent;

which term may be specifically defined as subjecting a gas to a predetermined temperature rise followed by a substantially equivalent re-cooling whereby the gas is restored after treatment to substantially its initial temperature.

The invention will, hereafter, be more specifically described with reference to the application of the process to the destruction of noxious constituents of gases discharged from driers operating upon waste organic materials.

The apparatus according to the present invention consists, in essence, in an elongated, gastight, heat-insulated vessel divided into a plurality of communicating chambers, for example, into two chambers with a communicating passage between them, each chamber containing a mass of heat-absorbing solids (preferably, promiscuously deposited relatively small heat-absorbing solids) bedded therein and having a free space immediately adjacent a surface at either extremity of the mass, means intermediate the extremities of the vessel for'introducing heat into a zone thereof, means associated with free spaces at the remote extremities of the respective masses for the positive admission and discharge of a gas therethrough, means associated with said admission and discharge means for forcing a gas into and through the heat-exchange mass of one chamber, through the adjacent free space and into and through the heat-exchange mass of the adjacent chamber, and means for reversing the direction of ow of the gas. Preferably, the aforesaid heatintroducing means comprises a burner, located in a free space (which may be a passage between adjacent chambers of the vessel), provided with means for the supply of combustible fuel, e. g. a fuel gas, or a fuel oil, or powdered fuel, and of a combustion-supporting gas, e. g., air. However, other forms of heating, such, for instance, as electric heating, are within the scope of this invention. The selection of a fuel type for the establishment of the plug of heat in the apparatus after an interruption in operation and as a source of auxiliary heat during operation Ais generally determined by local conditions. Gas, natural or manufactured, is always a convenient fuel; in many places its cost operates against its use. Fuel oil is cheap in many places and the apparatus required for use in the present invention can usually be adapted to the use of oil both for heating before operation and for auxiliary heat during operation. In localities where gas and oil are both expensive, a solid fuel in powdered form is indicated and can be used. Powdered fuel of course contains solid particles of ash. This adds to the lamount of non-combustible suspended matter in the gas to be treated. Where the gas to be treated contains a considerable amount of non-combustible suspended matter, increased operating difiiculties due to a small amount of ash from powdered fuel are not significant. In every actual case, choice of the source of heat such as gaseous, liquid or solid fuel or electrical energy will be governed largely by consideration of price and availability. In the present specific illustration it is assumed that fuel oil is used for the initial establishment of the plug of heat; and that gas is used for auxiliary heat during operation.

The invention will now be described in greater particularity, and with reference to the accompanying drawings, in which:

Fig. 1 represents, partly in section and partly diagrammatically, a complete apparatus in accordance with the present invention, and

Figs. 2 and 3 represent, partly in section and partly diagrammatically, modified forms of the apparatus shown in Fig. 1.

In Fig. 1 there is shown a chamber I provided with a thermally insulating material 2 which chamber serves as a container for two beds 3 and 4 of heat-absorbing solid particles. 'I'hese particles may be ordinary pebbles or any heat-resisting material, crushed and screened to a suitable f size. 'I'he upper bed 3 is supported on a suitable set of grids or grate bars 5 and the lower bed 4 upon the grids 6. When the grids and beds are disposed within the chamber I, as shown, three open gas spaces are formed, 1, 8 and 9. `Covered hand-holes I 0 and II are provided for the removal-of the heat-absorbing solids comprising beds 3 and 4, and other covered hand-holes I2 and I3 are provided to permit the renewal of these beds. A gas conduit I4 is, provided in communication with space 'I and a similar conduit I5 connects with space 9. Flow of gas through I4 and I5 is controlled by valves I6, I1, I8 and I9, so arranged that I4 and I5 each may be brought into communication with a chimney 20 and with a foul gas feed line 2| through which the contaminated gases to be treated are delivered to the apparatus. An opening 22 is provided, communicating with space 8, connecting with a fuel line 23 controlled by fuel valve 24. A draft line 25 is also provided, to mix air with the gaseous liquid or powdered fuel traversing 23. Flow of draft air regulated by valve 21 is set in motion by the low pressure fan or draft blower 28.

Draft line 25 also connects, when valve 28 is open, with foul gas line 2|.

It frequently is convenient to insert the fan or blower30 into foul gas line 2l to provide sufllcient pressure to force the foul gases through the apparatus. Also, it is, upon occasion, desirable to introduce fan or blower 3I in the exhaust line 20, which, operating slightly below atmospheric pressure, forces the gases to be treated through the apparatus. Whenever the foul gas received from the process generating it is not delivered under suilcient pressure, either fan 30, fan 3 I, or both, may be employed to maintain foul gas in motion through the apparatus. The choice of location for this blower is guided by the temperature of the gas as received and by its chemical composition with particular reference to its corrosive effect upon the blower mechanism.

Further, there is provided a purge pipe 82 connecting with space 8, 32 being controlled by a purge valve 33 which when open connects 32 with exhaust pipe 43. Purge pipe 32 communicates also through economizer valve 34, by way of conduit 35 with economizer chamber 36, a shell having an insulating lining 31 and filled with a bed of heat-absorbing solids 38 supported on a set of grids or grate bars 39 located above an open space 40. In communication with open space 40 there is provided an economizer exhaust 4I controlled by the economizer exhaust valve 42 which connects with economizer exhaust or chimney pipe 43. Open space 40 also connects with conduit 46 controlled by valve 45 and permits draft blower 28 to force air into the bottom of economizer 36. Draft preheat line 48, thermally insulated and controlled by water-cooled valve 41, connects draft line 25 with economizer 36.

The principles underlying the present invention can be more clearly explained by illustrating the manner in which the destenching operation preferably is carried out with the apparatus showninFig-1, Let it be assumed that fish-scrap is being dried in any of the usual commercial driers used for this purpose, and that the gases discharged from this drier are delivered to the destenching apparatus at the inlet to blower 30 initially at 200 say, and sensibly at atmospheric pressure'. Destencher I first is preheated in arather definite manner. All valves are closed except 24, 21 and 33 which are open. Blower 28 is set in motion. Fuel, e. g., manufactured gas, from the fuel supply line passes open valve 24, mixes with induced draft from fan 28 and is burned in the open space 8, the products of combustion discharging to waste through the open purge valve 33. With control of the flow through valves 21 and 24 the walls' of the chamber contiguous with open space 8 and the solid particles in the uppermost layers of bed 4 are rapidly heated. Grate bars 5 and the lower layers of particles in bed 3 are also heated. When the materials surrounding open space 8 have been heated to a chosen destenching temperature, say 1500 valves I6 and I1 are opened and valve 33 is closed. The products of combustion from open space 8 now discharge along two paths, a fraction passing upwardly through bed 3, conduit I4, open valve I6 to the chimney 20; the remainder discharging downwardly through bed 4, discharge pipe I5, open valve I1 and to chimney 20.

When these products of combustion, fully heated, startinstantly to ow through the beds of pebbles initially sensibly isothermal, there is set up in these beds a temperature distribution having a unique form or shape which conveniently may be called a thermal wave". Where the solid particles comprising the beds 3 and 4 consist of a mass of small pebbles-a preferable material in this relation-this thermal wave is relatively short measured along a line of gas iiow and presents a distinctly steep wave-front,

i. e., the temperature distribution is marked byk high temperature gradients.

Operation of burner 22 is continued, and hot gas is forced through beds 3 and 4 until two zones, one in the lower portion of bed 3, the other in the upper portion of bed 4, each a foot or so thick, have been heated up to the destenching temperature, 1500 F. Thereupon fuel valve 24 and air valve 21 are closed, the operation of blower 28 is stopped, valve I6 is closed, valve I8 is opened and blower 30 is put 'in operation. The foul gases from the sh-scrap drier, subjected to pressure by passage through fan 30, enter the destencher by way of pipe 2 I, open valve I8, conduit I4 and into the open space`1. Gas entering here is forced to iiow downwardly through beds 3 and 4 in succession, discharging through the open space 9 below the grate 6 and to exhaust through the conduit I5 and open valve I1 and to the chimney 20. The foul gas in its passage downwardly through beds 3 and 4 encounters heated pebbles and is heated to 1500 F. The gas flows downwardly through the bed following in its path the mutually connected interstices between the individual particles composing the bed, and during its passage heat is interchanged between the gas and the heated solid particles or pebbles. When the individual particles are small and the interstitial gas passages between them are therefore small, the transfer of heat from solid to gas takes place with a close approach to thermodynamic reversibility. In fact, the technical success and the eiectiveness of the process depend in large measure upon the close approach to reversibility which obtains `in the interchange of heat between the gas and a heat-absorbing mass of the type employed.

The two mechanically separated beds 3 and 4 act thermally as a single continuous mass of heat-absorbing particles since little or no heat transfer occurs in open space 8. The temperature distribution established in the initial heating exhibits a central zone or section of highly heated pebbles a foot or so thick in vertical extent, bounded both above and below by .two thermal wave fronts reversed and oriented back to back. This central highly heated section or zone, together with its bounding Wave fronts, conveniently may be termed a plug of heat or a thermal piston. As the gas continues to traverse the pebble bed, the plug of heat moves downwardly with relatively little change in its shape or in its size. The upper portion of bed 3 is cooled down to the temperature of the incoming foul gas, viz., 200 F. The heat initially stored in the two beds is translated or carried downwardly into bed 4. During this down-flow of gas through the pebble beds, the temperature of the gas discharging through exhaust line I5 is essentially at the temperature of the bed prior to the initial heating, and remains so, until the plug of heat in its downward motion approaches the bottom of bed 4. When this api :cach has become close, as indicated by an incipient rise in temperature of the effluent gases, valves I6 and I9 are opened and valves I1 and I8 are closed. After this valve operation, the foul gas enters by way of open valve I9 and follows |5 into the open space 9 below bed 4. The gas then passes upwardly, traversing beds 4 and 3 in order and discharges through I4 by way of open valve I6 and into the chimney exhaust 20. Exhaust fan 3| may be absent.

During the up-fiow of gas through thc apparatus, the plug of heat moves upwardly and during this up-iiow the sensible heat contained largely in bed 4 at the time of valve reversal is re-translated from bed 4 upwardly and into bed 3, the thermal phenomena taking place being essentially the reverse'of those observed during the course of the down-flow, with the difference now that the gases discharged through conduit |4 exhibit a temperature at exit close to 200 F. in place of the temperature of the bed prior to the initial heating. When the plug of heat approaches the upper surface of bed 3, as again indicated by an incipient rise in exhaust 'temperature, the down-flow is restored, valves |1 and I8 being opened and I6 and I9 being closed, the destenching process being continued by continuing an alternating succession of up-ows and down-flows, described, by suitable manipulation ofthe valves IB, |1, I8 and I9.

During the operation of the process, heat is lost from the beds 3 and 4 by conduction through the walls and the maximum temperature in the plug of heat tends gradually to diminish. This diiculty can be avoided by maintaining in open space 8 the combustion of a small amount of gas from the gas supply line through the opened and adjusted fuel valve 24. When the foul gas to be puried is merely contaminated air, fuel gas introduced through valve 24 burns satisfactorily in open space 8 and admission of draft air through draft line 25 is unnecessary.

In the specific apparatus embodiment herein illustrated there has been shown an apparatus embodying two beds of refractory heat-exchange bodies. It is to be appreciated, however, that one, two or more than two beds can be employed if desired, it being a feature of the invention that the gas is contacted with heat-exchange bodies maintained at a plurality of different temperatures in a plurality of zones thereof, one zone being maintained at or above the thermal alteration temperature of the gas, or constituent of the gas, to be thermally altered.

It is to be understood that the apparatus shown in the drawings is described solely to illustrate a single method of carrying out the concept of the present invention. In the design and construction of apparatus suited to a realization of the destenching process it frequently, upon occasion, may be desirable to employ apparatus of other specific construction. In a number of the applications of the invention to denite industrial processes, it is convenient to construct the necessary apparatus after a fashion illustrated by Fig. 2. In Fig. 2 are shown two similar gas-tight chambers and |02, each lprovided with a refractory lining |03 and |04 and partly filled with a mass of heat-absorbing particles, frequently pebbles, |05 and |06. The lower parts of chambers |0| and |02 may be so formed as to provide conical bottoms of a shape to expose upwardly directed annular free-surfaces of pebbles |05 and |06. These annular free-surfaces have ample cross-sectional area so that gas may enter and leave the beds with little consequent pressure-drop. The free-surfaces |01 and |08 cornmunieate immediately with the annular open gas spaces |09 and ||0 respectively. The two open spaces and ||2, respectively above the beds |05 and |06, are connected by an insulating cross-over conduit |I3. The annular open gas space |09 adjacent the free-surface |01 at the lower part of bed |05 is in communication with a conduit ||4 controlled by a valve ||5 connecting with inlet pipe ||1. Inlet pipe |I1 communicates also under control of valve I8 by way of conduit I9 with the annular open space ||0 adjacent the free-surface |08, at the lower part of bed |06. A chimney or exhaust flue |20 connects through valve 2| with conduit ||4 and through valve |22 with conduit 9. Chamber |0| is provided with a purge outlet |28 controlled by purge valve |29 open to atmosphere and connecting open space with atmosphere when |28 is open. A similar purge pipe |30 controlled by a purge valve 3| connects open space ||2 with atmosphere when 3| is open. A burner 22 (similar to the burner in Fig. 1) is provided opening into cross-over conduit ||3. This burner connects with a fuel supply line regulated by fuel valve 24. The burner may also be fed with a supply of induced draft regulated by a draft valve 21 and flowing in through draft line 25 from a draft blower (such as 28 in Fig. 1) not shown in Fig. 2.

The carrying out of the process of the present invention in apparatus such as that suggested in Fig. 2 essentially calls for the same operations as those described above with respect to Fig. 1: a portion of one bed of heat-exchange materialis initially heated to a temperature at or above the thermal alteration temperature of the gas, or gas constituent to be thermallyaltered, and then the alternate movements of gas, with consequent movements of the thermal alteration zone, are effected. Auxiliary heat, when the same is necessary or desirable, is provided at any suitable point within the vessel constituting the two chambers, but preferably in an already heated zone therein which is between less highly heated zones.

It is in all cases desirable, and in many cases necessary, to employ for the heat-absorbing mass particles relatively small in size. The smaller the linear dimensions of the individual particles composing the heat-absorbing mass, the more restricted and the sharper the wave-front, the greater the space gradients of temperature in the wave, the higher the thermodynamic efficiency of heat interchange, and the nearer the approach to thermodynamic reversibility exhibited by the heat-absorbing bed in operation. 'I'here is encountered, of course, an apparent difficulty in attempting to employ such a highly efficient heat-absorbing assemblage of small particles, arising from the fact that the small interstitial spaces presented to gas flow tend to increase the back pressure of a given bed. When the particles used are quite small, such as small gravel, crushed fire brick or sand, this pressure drop is very great. It has been found, however, that the total or over-all resistance experienced by the flow of a given amount of gas through any such bed may be made to be as small as may be desired by choosing a suitable shape, size and form for the heat-absorbing bed. That is to say, if the cross-sectional area of` the bed normal to the direction of flow of the gas therethrough is made large and at the same time the length of path of the gas flow through it be made short enough, the resistance to flow in any practical case can be held below any desired limit. As a matter of fact, it has been found that the operative e'ectlveness of the various types oi apparatus used for carrying out the present invention is controlled in large measure by the close approach to thermodynamic reversibility obtaining in heat transfer with beds of high thermal reactivity, which means broad and short beds made of small individual particles, through which the gas can ow without experiencing an objectionably large pressure drop.

In operating the process with apparatus such as shown in Fig. l or in Fig. 2. the plug of heat as it is translated up and down the duplex beds 3--4 or III-I I2 gradually enlarges itself, occupying an increasingly greater portion of the beds. Continued operation as described leads ultimately to the state of affairs where the plug of heat has encroached on the whole of both beds and one no longer is able to continue the regular destenching operation except by discharging exhaust gas at an elevated temperature and with a consequent loss of heat from the apparatus and with a resultant loss of thermal efficiency.

When this state has been reached, when using apparatus shown in Fig. l, both valves I6 and I1 are closed and valve 34 is opened. The entering foul gas, under pressure from the blower 30, thereby is caused to split its stream, the two resultant fractions entering open valves I3 and I9 at the same time, and a flow of foul gas is maintained in conduits I4 and I5 simultaneously, thereby causing gas entering from I4 to flow downwardly through bed 3 and gas from conduit I5 to flow upwardly through bed 4, the two fractions of the split stream rejoining each other in the open space 8 and exhausting by way or open valve 34 and conduit 35 into the economizer 36. This purged gas flows downwardly through bed 33 and discharges through exhaust conduit 4I, open chimney valve 42 and conduit 43. As the foul gas to be treated ows in split stream downwardly through 3 and upwardly throughl, the enlarged plug of heat is, as it were, compressed and concentrated towards its own center and a large extent of the upper portions of bed 3 and of the lower portions of bed 4 is recooled and brought back to an isothermal condition (temperature 200 F. in this particular case). During this process the heated gas leaving chamber i may pass through economizer 36 and its sensible heat be vdeposited or stored in the heat-absorbing bed 38. The initial foul gas of course during this step is fully destenched lby passage through the three beds 3, 4, and 33.

After compressing in this fashion an enlarged plug of heat back to a small and workable size, regular operation is re-initiated as described above. It has been found that the maximum temperature in the center of the plug of heat is decreased in this plug-compressing or purging process; consequently, it is necessary in such case to increase the flow of fuel gas through valve 24 during the immediately following passage of foul gas through the beds 3 and 4 in direct or reverse ilow, in order to restore the desired destenching temperature in the central open space (III, H2 and H3). In restoring destenching temperature after a purging step, economizer 33 permits, with an improvement in fuel economy, the use of at least a portion of the withdrawn heat. To accomplish this, valve 45 is opened, valve 21 is closed, air is forced from blower 23 through 43 into open space 40 in econornizer 36; this air is caused to traverse bed 33, being heated thereby, valve 34 is closed, valve 41 is opened and this preheated air is discharged through conduit 4l into burner 22. In this manner a considerable fraction of the heat removed during the preceding purging step is restored to the system.

In the immediately preceding paragraph there has been described a procedure for the consolidation oi' the plug of heat involving withdrawal of a portion of the latter from about the middle thereof. It is to be understood that, as an alternative procedure, the same net result can be effected by removing degraded heat at either end of the so-styled plug of heat, one end, e. g., adjacent space 6, being trimmed through valved purge pipe 32' while the plug" largely occupies one extreme portion of the system and the other end, e. g., adjacent space 1, being trimmed at a later stage through valved purge pipe 32"' while the plug largely occupies the other extreme portion of the system.

In the immediately preceding paragraph there has been described a procedure for the consolidation of the "plug of heat involving withdrawal of a portion of the latter vfrom about the middle thereof. It is to be understood that, as an alternative procedure, the same net result can be effected by removing degraded heat at either end of the so-styled plug of he'at, one end, e. g., adjacent space 3, being trimmed through valved purge pipe 32' while the plug largely occupies one extreme portion of the system and the other end, e. g., adjacent space 1, being trimmed at a later stage through valved purge pipe 32" while the "plug largely occupies the other extreme portion of the system.

It is to be understood that wherever in this specification and in the appended claims reference is made to open spaces as bounding and as adjoining an assembled mass of particles, or a bed, the term is to be understood as used to mean functionally open with respect to gas permeability, and any space lled with a mass of particles individually large compared with the parimmediately adjacent outer extremities of the heat-exchange material, which are lled with such relatively large-sized heat-resisting objects. Thus, if, for example, open spaces 8 and 9 are illled with pieces of crushed rock averaging three inches in diameter, while beds 3 and 4 are themselves formed with one-inch pebbles, the specific resistivity presented to gas flow by the interstices between the three-inch particles is only one-ninth the specic resistivity of beds 3 and 4. In this case, spaces 8 and 9, although fllled with large'lumps, are regarded as being open spaces insofar as their response to the flow of gas is concerned. When the discrepancy in size is relatively great, intermediately sized objects may be bedded between the two widely diierently sized bodies.

Such layers of large dimensioned solid particles, once heated, act as a reservoir for heat and because of their relatively large size react thermally much more slowly to impressed changes in temperature. They thereby tend to minimize accidental irregularities and random iluctuations in operation. Of course the refractory walls of the open spaces serve this purpose to an extent. The addition of layers of larger particles superimposed on main beds increases this thermally stabilizing eiect normally exhibited by the walls CII of the chamber and acts to increase the consequent thermal buffering action.

In attempting to purify or treatgases carrying certain specific contaminants, a measurable, if usually brief, time is required for the destruction of these contaminants, the time rate of destruction being in general a function of temperature, gas composition, pressure, time of contact and the possible chemical reactivity of the particles contacted. In such cases an added layer of superimposed lumps is of distinct technical value. The normal purpose of the plug of heatfwhich is employed in the destenching process is triple: (1) The steeply ascending wave-front serves to preheat the initially foul gas in passage to the destenching temperature; (2) the middle highly heated portion or central maximally heated zone serves to hold the gas at destenching temperature until thermal decomposition or oxidation of impurities has been effected; and (3) the steeply descending wave-front at the far boundary of the plug of heat serves to re-cool the gas to a low temperature before its discharge from the apparatus to economize heat. In the case now considered, when an extension of time is required for the destruction of a given contaminant, there is formed and maintained in the beds a plug of heat having a somewhat broad central section. If the beds serving as heat-interchanging material are composed exclusively of small particles, operation with a centrally enlarged plug of heat is accompanied by an increased and often unpermissible drop in gas pressure as the foul gas traverses the apparatus. It has been found, however, that by the use of the described layers of large-size solid lumm superposed upon beds |05 and |06 (or filling the open space 8 in the case of apparatus as shown in Fig. 1) and by controlling the several heating steps involved, the process can be operated successfully while maintaining and moving one of the wave fronts back and forth in and without its leaving chamber and while also maintaining the opposing wave front continually resident within chamber |02. If the duration of the direct flow and of the reverse flow of gas through the apparatus is properly controlled, these two wave fronts can be moved back and forth, each reciprocated within its appropriate bed; all the while maintaining the temperature of the large solid lumps and of the gas throughout the two layers superimposed on beds and |06 uniformly at destenching temperature. Due to the large size of these lumps little drop in pressure is experienced by the gas in its passage through them. Thus a plug of heat having an unusually wide maximally heated central zone interposed between two sharp and steep wave fronts can be maintained and yet an increased loss of pressure, which would naturallyk result from such operation in a bed of homogeneously sized small particles, can be avoided.

The successful operation of the process is dependent in great measure upon the realization of a relatively high thermal efficiency of the heat-exchange with the type of heat-absorbing mass of solid particles employed. 'I'he desired efficiency can be attained as a direct result of the small individual size of these heat-absorbing particles. Furthermore, in order to prevent an inadmissibly large drop in pressure occurring when the gas is forced through beds of such small particles, it is usually necessary to construct these beds of restricted vertical height, and they must be enclosed within containers or chambers of relatively large diameter and broad cross-sectional area. Practical considerations further require that the solid particles forming the bed be so arranged that they may be periodically renewed. The rapid temperature changes occurring in the beds may, in the case of some materials, cause spalling and breakage oi.' particles. Many of the gases whose purification is contemplated entrain an amount of suspended matter of dust or of fume frequently to cause gas stoppage in portions of the bed, channeling of the gas and at times a complete illing of the interstices with entrapped dust and dirt. Asa practical limitation, it has been found necessary in some cases to orient the beds in space to present lines of gas flow directed vertically, and to provide the beds y with terminal free-surfaces of large area for the entrance and elilux of the gas into and from the beds immediately contacting open gas spaces. It is recognized of course that these features of a heat-absorbing bed of solid particles have been fully described in U. S. Patent No. 1,940,371 issued December 19, 1933, and those features are not herein claimed, per se, as a constituent part of this invention.

It is to be noted, relative to the apparatus shown in Fig. 2, that the chambers |03 and |04 may be constructed without the conical or hopper bottoms specifically illustrated in that ilgure of the drawings, and that entrant and exit gases may be admitted and discharged otherwise than as specically illustrated. Thus, the walls |0| and |02 of said chambers may be supported upon the ground or a floor, or upon suitable supports thereon, the pebble beds |05 and |06 being supported above the ground or floor by the interposition therebetween of suitable masses of relatively large-sized objects with means for admitting gas and for discharging gas freely to and from said beds |05 and |06 through such masses of large-sized objects. In a specific installation this. has been accomplished by .extending the walls |0| and |02 to the ground, filling the bottoms of the chambers, except for spaces described immediately below, with large-sized rocks, and supporting the pebble beds |05 and |06 immediately upon these rocks. Adjacent the bottoms of the walls and within the same, annular open spaces, equivalent broadly to annular spaces |09 and |0 of Fig. 2, were created by the use of suitable foraminous retaining material, and gas inlet and gas discharge conduits were led into these annular open spaces.

Fig. 3 illustrates a simpler form of apparatus adapted for the carrying out of the process of the present invention. According to that embodiment, a chamber 20| is provided with a thermally insulating lining 202 and encloses a heatabsorbing bed of solids 203 which may be supported upon grate bars 204 or may be supported upon a mass of relatively large-sized heat-resisting objects as described above in relation to Fig. 2. The chamber 20| may be provided with covered hand-holes 205 through which the solids may be introduced to form the bed 203 and with a covered hand-hole 206 through which the solid particles may be withdrawn for renewal. The gas to be purified enters through conduit 201 under pressure from blower 208 and is forced through inlet-pipes 209, 2|0, 2|| under control of inlet valves 2|2, 2| 3. Treated gas discharges through the water-cooled exhaust valves 2H, 2|5 and chimney 2 6. Fuel is introduced through the fuel line 2|8 under control of the fuel valve 2|9 through the spray or mixer 2|?. A second auX- iliary fuel line 223 is inserted into conduit 209v between the blower 208 and the inlet valves 2I2 and 213. This line is connected with a supply of gas and is controlled by valve 224. A valve 220 is inserted in the foul-gas inlet 201 and a breather-pipe 22| connects with 201 between valve 220 and blower 208. Admission of air through breather line 22| is controlled by breather valve When the upper layers of the bed and the inner walls of the open space above 203 have been heated to a suitable thermal decomposition temperature, as described above, valve 2H is closed and 2|5 opened. Hot products of combustion are forced downwardly through bed 203,. The upper layers of bed 203 are thus heated and a plug of heat is established at the upper part of bed 203. Valves 2I9 and 222 are then closed and the gas to be treated is admitted at ordinary temperature to the apparatus by opening valve 220. 'I'he cold gas enters the upper open space, cooling it, and passes through 203, transporting the plug of heat downwardly through the bed and cooling its upper layers to substantially inlet temperature.

When the plug of heat approaches grate bars 204, valves 2l5 and 2I2 are closed and valves 2i3 and 2 I4 opened. The gas now enters 203 through the bars 204, flowing upwardly through it, re'- transporting the plug of heat upwardly through the bed.

It is to be observed that in the embodiments illustrated in Figs. 1, 2 and 3 of the attached drawings a feature of the invention is the provision of the heat-exchange filling in masses each of which is relatively broad, measured on a surface normal to the direction of gas flow there-l through, thereby providing a relatively short path of gas flow through the mass (e. g., of Fig. 3) or masses (e. g., of Figs. 1 and 2).

Sufficient structural and operative details have been given to permit the reader in any practical case to design apparatus, to operate it and to carry out the invention here described.

I claim:

1. Process for thermal alteration of a gas, which comprises initially heating a portion of a body of gas-traversable heat-absorbing material to an elevated temperature T2 at least equal to the thermal alteration temperature of the gas to be treated, whereby to create within said body a thermal alteration zone contiguous with another zone having a temperature materially below said thermal alteration temrature, forcing a stream of gas, at an entrant temperature T1 which is materially lower than said thermal alteration temperature, through the body in one direction, whereby the gas is heated substantially to T2 in passage through said thercal alteration zone and thereafter is cooled substantially below thermal alteration temperature by contact with a cooler portion of said body, the thermal alteration zone thereby being moved through said body in the direction of gas flow at a rate slower than the rate of passage of said gas, reversing the direction of gas now before the thermal alteration zone has been substantially moved out of said body, whereby the thermal alteration zone is moved through said body in the reverse direction, and repeating the alternate movements of the gas, whereby the thermal alteration zone is reciprocated within said body.

2. Process as defined in claim 1, in which the size of the individual solids constituting the heatabsorbing body is so regulated with respect to 4. Process for thermal alteration of a gas,

which comprises initially heating a portion of a body of gas-traversable heat-absorbing material to an elevated temperature T2 at least equal to the thermal alteration temperature of the gas to be treated, whereby to create within said body a. thermal alteration zone contiguous with another zone having a temperature kmaterially below said thermal alteration temperature, forcing a stream of gas, at an entrant temperature Ti which is materially lower than said thermal alteration temperature, through the body in one direction, whereby the gas is heated substantially to T2 in passage through said thermal alteration zone and thereafter is cooled substantially to below thermal alteration temperature by contact with a cooler portion of said body, the thermal alteration zone thereby being moved through said body in the direction of gas ow at a rate slower than the rate of passage of said gas, reversing the direction of gas ilow before the thermal alteration zone has beenv substantially moved out of said body, whereby the thermal alteration zone is moved through said body in the reverse direction, repeating the alternate movements of the gas, whereby the'thermal alteration zone is reciprocated within said body, and maintaining the thermal alteration zone at T2 by addition of heat thereto.

5. Process of removing a contaminant from a fouled gas, which comprises initially heating a portion of a. body of gas-traversable heat-absorbing material to an elevated temperature at least equal to the thermal decomposition temperature of the contaminant to be removed from the gas, whereby to create within said body a thermal decomposition zone bounded by zones having temperatures materially below said thermal decomposition temperature, forcing a stream of the gas, at an entrant temperature which is materially below said thermal decomposition temperature, through the body in one direction, whereby the gas and contaminant are heated to the thermal decomposition temperature of said contaminant in passage through said thermal decomposition zone and thereafter the gas is cooled substantially to entrant temperature by passage through a cooler portion of said body, the thermal decomposition zone vthereby being moved through said body in the direction of .gas flow at a rate slower than the rate of passage of said gas, reversing the direction of gas ow before the thermal decomposition zone has been substan-l tially moved out of said body, whereby the thermal decomposition zone is moved through said body in the reverse direction and repeating the alternate movements of the gas, whereby the thermal decomposition zone is reciprocated Within said body.

6. Process of removing a contaminant from a fouled gas, which comprises initially heating a portion of a body of gas-traversable heat-absorblng material to an elevated temperature at least equal to the thermal decomposition temperature of the contaminant to be removed from the gas, whereby to create within said body a thermal decomposition zone contiguous with another zone having a temperature materially below said thermal decomposition temperature, forcing a stream of the gas, at an entrant temperature which is materially lower than said thermal decomposition temperature, through the body in one direction, whereby the gas and contaminant are heated to the thermal decomposition temperature of said contaminant in passage zone is reciprocated Within said body, and maintaining the temperature of the thermal decomposition zone by addition of heat thereto.

7. Apparatus for the thermal treatment of a gas, which comprises two gas-tight heat-insulated stoves each defining a chamber containing a mass of relatively small heat-exchange solids freely bedded therein anu having entrance and exit free spaces immediately adjacent free surfaces at either extremity of the mass; a heatinsulated conduit communicating between one free space of one stove and one free space of the other stove and constituting with the two free spaces an open intermediate zone; a heating means in association with said open intermediate zone; a valved purging conduit communicating with said open intermediate zone; means associated with each of the other free spaces for the positive admission and discharge of a gas therethrough, means associated with said admission and discharge means for forcing a gas into and through the heat-exchange mass of one stove,

through the open intermediate zone and into and 20 through the heat-exchange mass of the other stove; and means for reversing the direction of ow of the gas.

8. Apparatus as defined in claim '7, characterized in that each heat-exchange bed is relatively broad, measured in relation to the normal path of gas ow therethrough. v

FREDERICK G. CO'ITREIL.

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