US2981531A - Particle size control - Google Patents

Description

April 25, 1961 R. PYZEL 2,981,531

PARTICLE SIZE CONTROL Filed May 20, 1958 BLOWER INDIRECT HEAT EXCHANGER ROBERT PYZEL INVENTOR.

PARTICLE SIZE CONTROL Robert Pyzel, 85 East End Ave., New York, N .Y.

Filed May 20, 1958, Ser. No. 736,640 '18 Claims. (Cl. 263-53) The present invention relates to the control of particle size distribution in fluidized beds, and particularly in such beds where chemcal reaction is taking place between the fluidized particles thereof, as, for example, in the production of hydraulic cement.

In processes employing fluidized beds of solid material and the reaction of solid feed materials into finished product, the accretion of newly-formed product upon the surface of previously present partcles will causea gradual growth in the size of the particles in the bed. An example of a process of this type is found in my prior Patent No. 2,776,132, which issued January 1, 1957.

Continued growth of the particles composing the fluidized bed by accretion causes a progressive shifting in the particle size distribution of the bed to a coarser condition. If such growth continues unchecked, the resultant increase in size of the particles makes them too heavy, in relation to their surface area exposed to the fluidizing gas stream, to be fluidized by practical amounts or velocities of the fluidizing gas.

Generally successful controls for this growth of the particles may be employed, such as, the regulated introduction of relativelyfine-sized product particles or other nonreactive material to the bed, such as disclosed in my aforesaid Patent No. 2,776,132. However, such controls have not been found entirely satisfactory in themselves for all circumstances.

It is generally to be expected that if new material ac cretes at an equal rate on the surface of two substantially round particles of different diameters, theparticle having the lesser diameter will exhib;t a 'higher percentage of increase in diameter for a given interval, although the quantitative additions to their respective diameters may be the same. Therefore, in .abed exhibiting the substantially homogeneous nature found in fluidized beds, and receiving new materialin a manner permitting an equal rate of accretion, it is generally expected that theprogression of particle size growth .will .be calculable to some reliable extent.

However, in some instances, the growth ofthe various .sizes of particles isnot even in that theirrespective rates of growth cannot be predicted according to their relative initial diameters. This is particularly true in processes wherein the amount of heat required to cause the accretion of new material is relatively high per unit of reacting mass. In these cases of uneven growth, the rate of growth of the larger particles exceedsthe expected rate, and the rate of growth of the finer particles falls below the expected rate. As a result, the particle size distributlonof the bed is unfavorably'aifected in that the largest particles grow larger -at an excessive rate com- .pared'to the average rate of growth of all the particle sizesintherbed, while the smallest particles grow at a rate 'lessithan .the average, with the result that most of the mass ofthe bed becomesconcentrated in'the largest particles whichhave grownat an excessive rate and in the .smallest ,particles which ,have not ,grown fast enough,

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2,981,531 Patented Apr, 25, 1961 the form of intermediate particle sizes. Control of the fluidization of a bed with such an unbalanced particle size distribution may become difi'icult or even impossible.

While the reason for this occurrence is not fully understood, it appears that this alteration of the particle size distribution due to uneven growth is a function of the heat content of the material in its respective particle sizes. At any given temperature, a large particle of material, having a higher ratio of mass-to-surface area, will exhibit a higher ratio of heat conteLt-to-surface area than will a smaller particle at the same temperature, as a result of the smaller particles lower'ratio of mass-to-surface area.

Therefore, since a small particle offers a larger proportion of surface in relation to its heat content, it contains less heat with which it can support the reaction of product-forming reactants on its surface. Consequently, a small particle may become overloaded with productforming materal, and cannot then support the reaction by delivering sufficient heat to the reactants. In this case, a longer time is required for the small particles and the reactants thereon to acquire heat from the principal reaction-heat source, or from the remainder of the bed, to complete the reaction of the product-forming materials.

in contrast, the larger part.cles are less readily overloaded, and can accommodate more reacting material on their surface in a given length of time than can the finer particles. Consequently, the larger particles continue their growth at a rapid rate by accretion of new product material while the finer particles with the partially-reacted materal thereon are retarded in growth by the time required to receive heat for completing-the reaction. Although the reaction delay associated with the finer particles may be almost immeasurable, the cumulative effect of a succession of such delays causes the alteration in the rate of growth and in the particle size d.'stributio n.

In general, the preferred form of the present inven tion, as illustrated in conjunction with the production of hydraulic cement, comprises establishing a fluidized bed of fin shed cement product particles and maintaining the bed at cement-forming reaction temperatures by combustion of fuel within the bedor by other suitable means. Cement-forming raw materials are fed to the fluidized bed to react therein and to accrete on the surface of the product particles, and finished product so formed is discharged from the bed.

A portion of the heat suppl.ed for maintaining cementforming reaction temperatures in the bed is supplied ;to the unreacted cement-forming materials prior to their introduction into the bed. The cement-forming rawvmaterialsare heated to a temperature at which they require relatvely little further heat to complete their conversion into cement product. Therefore, accretion on the surfaces of the smaller product particles takes place at rates more nearly equal to the rate of accretion on the surfaces of the larger product particles.

A better understanding of the invention may be derived from the following drawing and description in which the figure is a partly schematic view of the fluidized reactor for producing hydraulic cement.

As shown in the figure, a vessel 1 having a gasdistributing grid 2 in the lower region thereof isprovided with an upper gas outlet 3 and a lower gas inlet 4. The gas inlet 4 communicates, by means of a conduit 5, with a combustion chamber 6 having a burner 7 and an air intake 8 therein. i

Air intake 8 receives preheated air througha conduit 9, having a valved branch 10 extending to the burner 7. Conduit 9 extends to the air intake 8 from an indirect heat exchanger ,11 which receives hot gases from the gas outlet 3 by way of :aconduit 12. Atmosphericair is forced. through: the exchanger- .11 and the remainder-of the system by a suitable blower 13. The discharged product gases leave the exchanger 11 through an outlet 14 without direct contact with the incoming atmospheric air to preclude contamination of the incoming air or dilution of its oxygen content.

,In the vessel 1, a burner 15 extends into the region above and adjacent the grid 2, and receives a fuel, such as oil, gas or pulverized coal from a fuel supply conduit 16. Fuel supply conduit 16 communicates by way of a valve 17 with a fuel source 18, which also supplies the burner 7 by means of a valve 19 and a second fuel supply conduit 20. Alternatively, other suitable means for the principal heating of the bed may be employed.

Adjacent the combustion chamber 6, the conduit 5 re ceives cement-forming raw materials from a hopper 21 through a feeder 22 and a conduit 23 for entrainment and heating in the combustion gases and preheated air which passes through the conduit 5 from the combustion chamber to the vessel 1.

Vessel 1 is provided with a seed product and recycle product inlet 24 in the region above and adjacent the grid 2 for the introduction of an initial bed of product particles or seed, and the subsequent introduction of regulated quantities and sizes of finer product particles. The introduction of fine product particles during operation of the process serves as a primary control over the particle size distribution, and therefore the fluidizing characteristics of the bed.

Vessel 1 is provided with a product discharge which is shown here as a simple overflow discharge 25 but which may take any suitable form. The overflow discharge 25 establishes the over-all depth of the bed of .fluidized material as well as providing means for removing finished product from the bed.

The finished product particles discharged from the fluidized bed may be passed to a suitable separator where the coarser particles are separated from the finer particles and discharged as the final product of the process,

while the finer particles are returned to the vessel 1 .through the product inlet 24 to maintain the proper ratio of seed particles to raw cement-forming material in the fluidized bed and also to assist in maintaining the proper size distribution of the particles comprising the bed. If

desired, some of the separated coarser particles may be ground to the desired size and returned to the fluidized bed through the product inlet 24 along with the fine particles from which the coarser particles were separated.

them, or at least any susbtantial amount of them, out of the vessel through the gas outlet 3.

In operation, the blower 13 is first started and delivers air through the heat exchanger 11, conduit 9, air

intake 8, combustion chamber 6, conduit 5, inlet 4, and

through grid 2 into the vessel 1. From the vessel 1, the

air passes through gas outlet 3, conduit 12 and heat exchanger 11 to the atmosphere, or to a dust collecting system, if desired.

After the flowing stream of air has been established, as above described, seed particles of finished cement product are introduced into the vessel through the product inlet 24 and are immediately fluidized by the flowing stream of air. The introduction of the cement product particles are continued until the fluidized mass of such cement product particles reach the approximate level of the overflow 25. During the filling of the vessel with seed particles, or upon completion of the filling thereof,

the burner 7 is ignited and, receiving appropriate quantities of combustion air and fuel through the valved conduit 10 and through conduit 20, respectively, begins the heating of the bed. When the filling of the bed is completed, and the temperature thereof has been raised sufficiently high by heat from the burner 7 to cause ignition of the fuel used (which may be about 900 F. for oil), the burner 15 is started. When the desired reaction temperature is reached in the fluidized bed, the burner 7 is cut back, or cut off entirely, and the feeder 22 is operated to feed unreacted raw cement-forming material into the stream of air passing through the conduit 5.

The air introduced into the combustion chamber 6 through intake 3 is preheated in the heat exchanger 11 by the hot gases escaping from the gas outlet 3 of vessel 1 by the described circuit. The raw cement-forming material is entrained by the gases flowing throughthe conduit 5 and carried through the grid 2 into the fluidized bed, where it undergoes the cement-forming reaction and accretes on the surface of the seed particles, while product particles rising through the fluidized bed to the upper surface are discharged from the vessel through overflow 25.

When the proper rate of feed or raw material through the pipe 24 has been established for normal operation of the process, the burner 7 is turned up, or again ignited as the case may be. The raw material in transit in the conduit 5 is then preheated both by the heated air from the heat exchanger 11, and by the direct effect of the flame of burner 7 and its combustion products. The intensity of the preheating of the raw cement-forming material is then increased and the rate of heating of the burner 15, or its equivalent, is decreased. The desired ratio of heating of the fluidized bed by the burners 15 and 7 is obtained by regulation of the valves 17 and 19. These valves will be so set that sufiicient fuel is supplied to the burner 7 that the unreacted raw cement-forming material introduced into the conduit 5 will be preheated, by the time it reaches the grid 2, to a temperature at least equal to the calcining point of the calcinable materials thereof, but not preheated to that temperature at which substantial calcination of the calcium carbonate or sulfate content thereof will occur. The balance of the heat necessary to maintain the fluidized bed at the cement-forming reaction temperature will be supplied by bur er 15.

The raw cement-forming material will be ground to the industry standard, namely, through a 200 standard Tyler mesh screen. The ratio of the mass of material in the fluidized bed to the mass rate of feed of the raw cement-forming material per hour will be of the order of 10 to 1, that is, if 3000 pounds of fluidized particles consisting predominantly of finished hydraulic cement are maintained in the bed, raw cement-forming material will be fed to the bed at the-rate of 300 pounds per hour.

The size distribution of the particles introduced into the vessel 1 to establish the initial fluidized bed, as percent total weight, and On the basis of Tyler standard mesh screen may be as follows:

Percent 6 mesh 6 6+ 8 mesh 19 8+l0 mesh 43 l0+l4 mesh 24 -,14+20 mesh 8 between -1 O and +14 mesh while the other half may have a size of between l4 and +20 mesh. If at any time during the operation the particle size of the material of the bed becomes too large, appropriate adjustments will be made in the size of the recycled finished product particles to re-establish the desired size distribution in the fluidized bed.

The term calcining pointfis used herein to define the temperature at which the calcination of the calcium carbonate content of the raw material will take place, but only to the extent that the free calcium oxide in the cement-forming materials, when charged into the fluidized bed through grid 2, will not exceed one percent, and the term substantial calcination is used to define that degree of calcination at which the free calcium oxide in the materials entering the bed through grid 2 exceeds about six percent. Thus, it is my intention to preheat the raw cement-forming materials before they are passed through the grid 2 into vessel-1 forreaction to a temperature which will cause them to undergo a limited degree of calcination, intermediate these conditions, resulting in a free calcium oxide content of at least one percent, but not exceeding about six percent.

With the hydraulic cement raw materials in contemporary use, and at substantially standard conditions of CO vapor pressures, substantial calcination rates generally are encountered above 1650 F., material temperature. Therefore, the temperature of the preheated materials preferably is held below 1700, but above 1350 F.

If the raw cement-forming material is heated to a temperature at which substantial calcination is obtained prior to its introduction into the fluidized bed, the velocity of the reaction of the material into cement product is measurably reduced, unexpectedly, in comparison to the reaction velocity attained when substantially uncalcined materials are charged into the bed.

To the best of my knowledge, at present, this phenomenon appears to be related to the timing of the formation of the calcium oxide. When the calcination step occurs in the presence of the silicates and other constituents at cement-forming reaction-temperatures, the resultant calcium oxide is presented to the other reactants in its nascent state. Apparently the calium oxide exists momentarily as a highly reactive radical, rather than as a molecule, much as in the familiar cases of the nascent forms of hydrogen and oxygen. Presumably under such conditions, the greater reaction velocity is due to free energies present during the transient state of the atoms at the instant of molecular rearrangement.

Consequently, to obtain maximum benefits, the raw cement-forming material should be preheated to a temperature at least equal to about the calcination point of the calcinable material but for a length of time insufficient to cause substantial calcination of such material and then immediately, and while at such tem erature and before substantial calcination of the calcinable material has taken place, introduced into the fluidized bed. If the pre heating is permitted to continue at such a temperature or above for a length of time such that there is a substantial calcination of the calcinable material, the efficiency of the over-all process will be impaired, and wi l more than offset any advantages in the operation of the fluidized reactor which might be expected as a result of substantial precalcining of the-raw materials.

The unreacted raw material introduced into the fluidized bed at a high temperaturewhich is, however, below its substantial calcining temperature, will immediately be brought into contact with the hot seed particles of various size in the fluidized bed, and, by such contact, and by the heating effect of the hot gases in the bed, have its temperature raised to the cement-forming reaction temperature, when it will react almost instantaneously to form cement, which will accrete onto the surfaces of the seed particles of various size of the finished cement in the bed. Under such conditions, the rate of accretion of layers of new product material on the surfaces of the seed parti-- cles will be more nearly equal for all particle sizes in the fluidized bed, and the growth of the particles will exhibit the desirable inverse relationship to their diameters referred to above. With this balance established, the particle size distribution of the fluidized bed is then responsive to direct and positive control by the introduction of only fine product particles as nuclei in the fluidized bed. Thus, a predetermined particle size distribution in the bed for the most effective operation of the process may be more easily maintained, without constant adjustment of the size of product particles introduced during operation, and with a minimum introduction rate of such fine product particles.

The heating of the raw cement-forming material to the calcining point, but not to that temperature where substantial calcination takes place, also facilitates the formation of new product particles by those cementforming material feed mixes which otherwise experience difiiculty in completing their reaction to cement unless they are associated with the larger of the seed product particles. Since these materials, when so preheated, contain a maximum of energy as heat, but less than the amount necessary to cause any substantial precalcination, and thereby retain the advantage in reaction velocity apparently due to the development of the nascent state of calcium oxide at the cement-forming temperatures,

when processed in accordance to the present invention,

the resultant reactivity created in the reacting materials permits them to form a certain amount of independent product particles without reliance on the seed particles previouslypresent in the bed. 'The production of these new seed product particles Within the fluidized bed, where it occurs in measurable amounts, correspondingly reduces the requirements for the introduction or recycling of the finer fractions of discharged product, with a resulting improvement in the over-all economy of the process.

The initial establishment of a fluidized bed of product particles has been described in conjunction with the production of hydraulic cement. However, it i to be understood that operation is contemplated using any operable bed of particles which are of a composition compatible with the product to be formed. This might be a mixture of product and raw material particles, or a bed of raw material alone, where aggregation or clinkering is not a problem, particularly when the invention is applied to processing products other than hydraulic cement. Although direct combustion is disclosed for the further heating of the raw materials, just before entry into the bed, to a temperature above that attained in the conventional heat exchanger 11, it is to be understood that other means may be used for this purpose.

Various changes may be made in the details of the present invention Without sacrificing the advantages or departing from the scope thereof.

I claim: l. A method for producing hydraulic cement in a fluidlzed system comprising initially establishing and thereafter maintaining a fluidized bed predominantly of cement product particles, maintaining the fluidized bed at Cement-forming reaction temperatures, preheating cementformmg materials including calcinable material to a temperature at least equal to about the calcination point of brought into heat-exchange relation with at leasta portion of the gases forming said gaseous stream.

3. Amethod of producing cement in accordance with the method of claim 1 in which the fluidized bed is heated to cement-forming reaction temperature partly by internal combustion of fuel therein and partly by the introduction of heated gases into it.

4. A method of producing cement in accordance with the method of claim 3 in which the heated gases are formed'at least in part of products of combustion.

5. A method of producing cement in accordance with the method of claim 3 in which the heated gases include a mixture of air and hot products of combustion.

6. ,A method for producing hydraulic cement in accordance with the method of claim 1 in which the cement-forming materials are preheated by suspension in hot gaseous products of combustion.

7. A method of producing cement in accordance with the method of claim 6 in which hot gases are discharged from above the fluidized bed, a stream of air is passed in heat-exchange relation to said discharged hot gases and is heated thereby, and at least a portion of said heated air is used for combustion of fuel to form said hot gaseous products of combustion.

8. A method of producing cement in accordance with the method of claim 7 in which another portion of said heated air is admixed with the hot gaseous products of combustion and the cement-forming materials are suspended in the resulting mixture and the suspension thereby formed introduced into the fluidized bed.

9. A method of producing cement in accordance with the method of claim 8 in which fuel is introduced into said fluidized bed and the mixture of said other portion of air and hot gaseous products of combustion contain sufficient air for combustion of the fuel introduced into the fluidized bed.

10. A method for controlling the particle size distribution in a fluidized bed wherein newly-formed product accretes on the surface of product particles which comprises initially establishing and thereafter maintaining a fluidized bed of particles of a composition compatible with the composition of the ultimate product, maintaining said bed at product-forming reaction temperatures, preheating product-forming material to at least the minimum temperature required to initiate an intermediate reaction for a length of time insufficient to cause substantial intermediate reaction to occur, introducing the preheated material while at such temperature and before substantial intermediate reaction has taken place into the fluidized bed for completion of the intermediate and product-forming reactions therein and adherence of the product formed thereby upon the previously present particles of the bed, introducing relatively fine-sized particles of a composition compatible with the composition of the ultimate product into the fluidized bed for adherence thereto of newly-formed product thereon, and withdrawing a portion of the fluidized bed as the final product.

11. A method for controlling the particle size distribution in a fluidized bed where cement-forming materials are reacting to form cement and the newly-formed cement accretes onto the surfaces of cement product particles, which comprises initially establishing and thereafter maintaining a fluidized bed predominantly of particles of finished cement of the same chemical composition as the cement to be produced, maintaining said bed at the cement-forming reaction temperature, preheating cement-forming materials containing calcinable material to a temperature at least equal to about the calcination point of the calcinable material for a length of time ir sufficient to cause substantial calcination of such material, introducing thepreheated material while at such temperature into the fluidized bed before substantial calcination of the calcinable material has taken place for completion of the calcination of the calcinable material thereof and subsequent reaction to cement and adherence of the newly-formed cement to the cement product particles of the bed, introducing relatively fine-sized particles of finished cement particles into the fluidized bed, and withdrawing a portion of the fluidized bed as the final product. 7

12. A method of producing cement in accordance with the method of claim 11 in which the cement-forming materials are charged into the fluidized bed while suspended in a gaseous stream, hot gases are discharged from above the fluidized bed, and such hot discharged gases are brought into heat-exchange relation with at least a portion of the gases forming said gaseous stream.

13. A method of producing cement in accordance with the method of claim 11 in which the fluidized bed is heated to cement-forming reaction temperature partly by internal combustion of fuel therein and partly by the introduction of heated gases into it.

14. A method of producing cement in accordance with the method of claim 13 in which the heated gases include a mixture of air and hot products of combustion.

15. A method for producing hydraulic cement in accordance with the method of claim 11 in which the cementforming materials are preheated by suspension in hot gaseous products of combustion.

16. A method of producing cement in accordance with the method of claim 15 in which hot gases are discharged from above the fluidized bed, a stream of air is passed in heat-exchange relation to said discharged hot gases and is heated thereby, and at least a portion of said heated air is used for combustion of fuel to form said hot gaseous products of combustion.

17. A method of producing cement in accordance with the method of claim 16 in which another portion of said heated 'air is admixed with the hot gaseous products of combustion and the cement-forming materials are suspended in the resulting mixture and the suspension thereby formed introduced into the fluidized bed. v 18. A method of producing cement in accordance with the method of claim 17 in which fuel is introduced into said fluidized bed and the mixture of said other portion of air and hot gaseous products of combustion contain sufficient air for combustion of the fuel introduced into the References Cited in the file of this patent UNITED STATES PATENTS 1,961,311 Uhle et a1. June 5, 1934 2,352,738 Ruthrufi July 4, 1944 2,409,707 Roetheli Oct. 22, 1946 2,469,989 Pyzel May 10, 1949

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