US1709764A - Flue-gas-recirculating system for cracking-still operations - Google Patents

Description

April 16, 1929. H.- A. ATWATER ,L709764 FLUE GAS RECIRCULATING SYSTEM FOR CRACKING STILL OPERATIONS Filed May 9, 1927 2 sheets-sheer l Ill INVENTOR 'ATTORNEY Apnl 16, 1929. H. A. ATWATER FLUE GAS RECIRCULATING SYSTEM FOR CRACKING STILL OPERATIONSl Filed May 9, 1927` l ZvSheets-Sheet 2 VQN IFI.

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Fatented Apr. i6, i929.

HARRY A. ATWATER, OF KANSAS CITY, MISSOURI, ASSIGNOR TO COMBUSTION EQUIP- MENT COMPANY, 0F KANSAS CITY, MISSOURI, A PARTNERSHIP COMPOSED 0F A. W. MACKIE, R. V. DOWNS, ANDHARRY I` A. ATwATER.

FLUE-GAS-RECIRCULATING SYSTEM FOR CRACKING-STILLOPERATIONS.

Application filed May 9, 1927. Serial No. 189,959.

The present invention pertains to oil cracking operations, and particularly to that phase of such operations which relates to the maintenance of the proper heat conditions at the heating surfaces of the cracking units. l

Accordingly the principal object of the invention is to provide a more efficient as wlell as more economical system, in which a more uniform temperature and practical control of the same are rendered possible by a recii-, culation of a portion of the flue gases in a p` .culiar and effective manner through the con bustion chamber of the furnace.

It is furthermore an object lof the invent tion to devise not only a method but an inrproved apparatus of such a character as to improve generally the cracking still opera-y:` tion for .which the system is especiallyI adapted.

lVith these general objects in view, the invention will now be described by reference to the accompanying drawings illustrating one form of furnace and still construction adapted for the embodiment of my improved system, after which those features and combinations deemed to be novel will be particularly set forth and claimed.`

In the drawings: i

Figure l is a vertical sectional elevation illustrating a burner and cracking still construction embodyingr the system herein described; and

Figures 2 and 3 are horizontal and vertical sectional views; representing sections taken on`the lines lI-II and III-HI respectively of Figure l.

In the drawings is illustrated one commercial form of cracking still unit comprising a tube bank 4 mounted between drum tube headers 5l and spanning the upper portionof the furnace combustion chamber A. the drum lube headers being in turn connected to a horizontal upper drum 6, as clearly shown in Figure 1. This form of cracking unit'as well as the type of furnace construction shown are merely forv illustrative purposes, as it is to be understood that the invention is equally well adapted for other forms and arrangements of cracking stills and other types of furnaces, either with or withoutv the dutch oven forming an extension 7 from the lower portion of the furnace combustion `or tire chamber A.

heit, which is vin contrast to ordinary boiler operation Where the temperature in the combustion chamber frequent-ly reaches three thousand degrees Fahrenheit, the only limits under such conditions being the temperatures to which the refractory linings may be subjected Without excessive maintenance expense, and the attempt is also made to reduce as far as possible the temperature of the flue gases as they leave the biler heating surface in order to keep down the loss of heat up the stack.

Non7 since the temperature limit for the cracking operation must not ,exceed `fifteen to sixteen hundred degrees Fahrenheit, which is much lower than that produced by the efficient combustion of most fuels, it has always been necessary to reduce materially the temperature of the combustion gases, usually by admitting excess air into the combustion chamber at such a rate as will produce .a

volume suflicient for this cooling function. At the outlet from the combustion chamber l into the fines a restriction as to temperature also exists since this must not fall below eight hundred t0 eleven hundred degrees Fahrenheit if an eicient cracking tempera-` terchange of heat by radiation, and to accomplish this it becomes necessarylto make the heat transfer principally by convection. Various methods and complex structures have been devised for this purpose. some even providing a separate room or chamber for the stillumit, but allsuch expedients are open to the same general economical objection as rcgards too great a loss of heat to the stack on account of the great volumes of gas leaving y the cracking unit at comparatively high temperatures. y A It is apparent that these temperature reduction requirements which` have heretofore been met by providing excess air couldbe met just as satisfactorily by substituting any other suitable gas inasmuch as the oxygen for supporting the fuel combustion is -not derived from this excess air supply. Hence a cooling diluent composed for the most part of nitrogen and carbon dioxide may be employed just as eiiiciently for the same purpose, while air is admitted in volumesxs'uiieient merely for furnishing the oxygen required forburning the fuel.

This is the principle followed in the present invention, as practiced by the apparatus illustrated in the drawings where the iiue gases, after leaving the space occupied by the tube bank 4, arepdischarged through suitable exit openings 8 into the iue passage or conduit 9, as many of which are provided as may be necessary, for conducting the gases to the stack 10. In the present instance, however, a fraction of these gases is diverted for recirculating purposes. into one or more branch conduits 12 by the action of a suction fan 14 in communication withsaid branch conduits, and leading ofi' from the latter are separate branch passages 15 and 16 communieating with gas intake compartments 17 and 18 at the top and bottom, respectively, of the oven extension 7, as clearly shown in F igure 1. By proper control, by means of dampers 19- in the passages 9 and 12 and similar dampers 19l and 19 in the passages 15 and 16, respectively, fthe correct and desired amount of iiue gases can be reeirculated'and the balance of the waste gases rejected and! delivered to the stack 10. A marked saving of fuel is effected by this method of recirculation, for it is readily seen that the diluting gas is required to be heated up in the combustion chamber only from a temperature ranging from eight hundred to eleven hundred degrees, in contrast to the increasein temper-` ature required to bev imparted to air entering at atmospheric temperature.

The mere discharge of the recirculated fiue gas into the combustion chamber, without other regulations, will not accomplish the most efficient results, as it is necessary to effect a thorough mixing and practica ly uniform distribution of the gases in the combustion chamber in order to secure as even and uniform temperatures as possible. Since the temperaturel of the gases resulting from the fuel combustion greatly exceeds that of the recirculated fine gases, andthere is a correspending difference in density and hence a tendency to travel in separate strata instead of mixing or diffusing, the problem of thoroughly mixing the two and to do this without the upper one being in' the form of arches 23 and the lower one taking the form of a` shelf 24, preferably projectingsomewhat further into the combustion chamber A than the arches 23. All the air required for combustion is admitted into the burner chamber 22. By this means theflue gases are discharged into the'combustion lchamber from the compartments 17 and 18,in relatively thin sheets or streams'between the fuel burning zone and two other sources of heat, viz, the refractory linings of the top and bottom walls of the combustion chamber. Inthis connection it will be recognized that the absorption of heat by these refractory linings is mainlyby radiation from the combustion zone,v and such radiation takes place without any appreciable heating eiiect upon the gases, while very little loss of heat occurs `byconduction through` the walls. The fioor and arches of the combustion chamber are also maintained at a high temperature by the absorption of heat from the burning fuel, and hence it isk apparent that the construction ofthese several chambers or compartments is such as to quickly and effectively heat not only the combustion gases travelling through the burner compartments but also the two'thin streams of recirculated Hue gases by the principle of convection as they flow past and in contact with these hot wall surfaces; and by the time the combustion gases have travelledthrough and out of the burner chamber orcompartlment their temperature has reached such a point that any heat loss to the recirculated flue gases will be insulicient to interfere with 'complete combustion. These two comparatively thin streams or sheets of recirculated gases also permit more intimate contact between the recirculated gas and the combustion gas, which results both in a more complete intermixing of the gases and also in a more rapid rise in the temperature of the ue gas, so that the ultimate result is a homogeneous mixture of the gases at a fairly uni-y form temperature ready for passage into the zone occupied by the heating surfaces of the cracking still units.

In practice, I prefer under some conditions to employ a checker wall structure 26 at the discharge points between the compartments 17 and 18 and the combustion chamber A. As illustrated in Figure 3,'the openings 27 in these walls are so constructed and arranged as to interpose a'slight resistance to the movement of the gases and thus maintain a substantial volume of the gases entirely across the face of the wall; in addition, the arrangement of the sizes of the openings 27 is adapted to distribute the gases practically evenly to the spaces in front ofthe walls corresponding tothe several burners.; However, the provision of any wall structure of this nature may not always be essential, and when. used its specific construction will simply be that best jects of ,the invention. --By ythe described method or process ofrecirculation', not only is a great economyeifected in fuel consumption, but' numerous other advantages are secured, among which is the greatly increased capac1ty obtained for the cracking units, due to the rapid increase in temperature as the Hue gases are recirculated through the combustion chamber between the hot combustion gases and the highly heated refractory surfaces, resultling in a rapid exchange of-heat.4 This leads also to a more thoroughl intermingl-ing ofthe gases, and consequently a practically h omogeneous mixture at a nearly uniform temperature. Since theinitial temperatures of the gases are not sofar apart as in former practices, this serves 'to reduce and practicallyeliminate the chilling effect 0n the combustion of the fuel, more perfect combustion results, and .excessive and destructive temperature changes in the refractory surfacesV are prevented; as a consequence therev is not such a wide variation in the resultant temperatures; leading to hot or'cold spots at the still f heating surface.- Allthis has the final result of not only increasing the output or capacity of the still units,.but` t e (tuality of the product is improved dueto the etterl Control of the operation and greater uniformity of temperature of the gases entering into the still heating action.n y

It is equally apparent thatthe construction or arrangement o .the system can be altered in any desired` or required way to meet varying conditions of operations, and is in no sense to be limited to the details herein illustrated, and accordingly I desire to reserve the right to make all such changes or modifications which may fairly fall within the scope of the appended claims. v

I What I claim is:

-1. Flue gas recirculati-ng apparatus for cracking stills comprising, in combination with the `furnace combustion chamber havmg an initial fuel combustion zone and a second heat exchange zone for the still heating sur- A faces, means for conducting Vand returning a fraction of the Wasteflue l gases from saidl second zone to said combustion chamber, including separate discharge ducts communicating with said combustion chamber at opposite sides of said initial combustion zone, and partition structures on opposite sides4 of above and below said combustion zone in position to shield said initial combustion zone from the chilling effect of said flue gases on iirst entering said chamber. 2. Flue gas recirculatingapparatus for cracking stills comprising, in combination with the furnace combustion chamber having a heat exchange zone for the stillheatingfsurfaces andan initial fuel combustion zone and also flue gasreceiving compartments on'opposite sides of above and below said combustion zone, and a conduit for conducting a' fraction of the exhaust gases from said heat exchange zone and terminating in ducts communicating with each of said flue gas receiving compartments. 1

3. Fluo'. gas recirculating apparatus' for crackingy stills comprising, in combination with the furnace combustionchamber having an initial fuel combustion zone and a second heat exchange zone for the still heating suresl faces, partition structures on opposite side-s of vsaid combustionlzone and\forming gas receiving compartments above an'y below said combustion zone, means for conducting a po'rtion of the waste flue gases fromsaid 4second zone to said gas receiving compartments, and bale structures of said flue gases on their llowout of said compartments. f

In witness whereof I hereunto ailix my signature. Y

HARRY A. ATWATER.

`for modifying the movementl

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