US2281811A - Drying process - Google Patents

Description

May 5, 1942.

DRYING PROCESS Filed June 12, 1940 W. B. TOWN ER INVENTOR ATTO R N EY Patented May 5, 1942 William B. Towner, Westtown,

General Chemical Company,

Pa., as'slgnor to New York, N. Y.,

a corporation of New York Application June 12, 1940, Serial no. 340,066

6 Claims. This invention relates to methods for drying liquors to form substantially dry solid products recoverable from such liquors by heating of the same to eliminate vaporizable constituents. For illustrative purposes, principles of the invention of disodium phosphate liquor to form substantially anhydrous disodium phosphate. 7

Anhydrous disodium phosphate is usually made by concentrating and dehydrating, in some way may be discussed in connection with the drying or other, a disodium phosphate liquor containing disodium phosphate in solution and in suspension. In modern practice, drum drier's are used economically to eliminate water from any kinds of slurries and liquors. One conventional type of drier comprises a rotatable horizontally mounted metallic drum equipped with suitable steam pipe and valve connections to provide for internal steam heating. Adjacent the lowermost periphery of the drum is a trough placed lengthwise of the drum and into which is run whatever liquor is to be dried. Relation of drum and trough are such that on drum rotation a portion of the drum surface becomes immersed in the liquor in the trough, and on emergence from the bath certain quantities of liquor adhere to the outer drum surface. The apparatus includes a longitudinal stripping knife arranged to contact a portionof the drying surface at a point 'just'prior to resubmergence of such portion in the liquor bath. In this was, dried material is shaved off the drier by action of the knife.

It has not been possible to secure consistent commercially satisfactory results by drying runof-plant disodium phosphate liquors by customary operation of drum driers. In the case of many liquors, dried product adheres tenaciously to the metal surface of the drum making necessary the application of excessive pressure to the knife in order to scrape off dried product. Even heavy knife pressure is ineffective, and in many situations substantial quantities of material still adhere to the drum after passing the knife, indications being that some portions of the disodium phosphate become in a sense semi-fused to the metallic drum surface. Heavy knife pressure requires use of a large amount of power to turn the drier, and also results in rapid wearing away of the knife edge with necessity for frequentshut-downs for resharpening, realignment,

or replacement. The disadvantages give rise to high power costs and low production per unit drier. Further, the substantially anhydrous product obtained is in an unsatisfactory powderlike form.

dry and vapor pressure at thedrying surface I have found that difficulties such as those enumerated, arising in the drying of run-of-plant liquors, may be overcome by forming on the heating surface of the drying equipment what, for descriptive purposes, may be considered as a double layer of the material to be dried.. More particularly, I havediscovered that the soughtfor objects of the invention may be attained by forming on a heated dryingsurface an initial relatively thin layer of the liquor to be dried, heating the layer to convert the same to an at least partially dried crust, forming on such crust a second relatively thin layer of such liquor, and then continuing heating of the composite material on the said drying surface to form substantially dry solid material. I find that, as a result of practice of the procedure herein described, there is formed on the drying surface a relatively thick crust which, I believe from observations, resists tendency to rupture justsumciently so that when the material becomes substantially becomes suflicient, the crust cracks, rises, and breaks away cleanly from the drying surface. This action takes place to such an extent that the stripping knife acts more or less as a guide to direct the dried trough or bin.

As applied to the drying of disodium phosphate liquors, the invention may be understood from the following description taken in connection with the accompanying drawing, in which Fig. 1 shows diagrammatically the apparatus used;

Fig. 2 is a plan view of a supplemental feed trough;

Fig. 3 is an elevation of the trough taken on the line 3-3 of Fig. 2;

Fig. 4 is a vertical transverse section taken on the line 4-4 of Fig. 2; and

Fig. 5 is a fragmental horizontal section taken on line [-5 of. Fig. 3-.

In Fig. 1, II designates a transverse vertical section of, a horizontally disposed metallic drum rotatable about axis II and equipped with suitable steam pipe valve connections, not shown, to provide for internal steam heating. Beneath the drum is a primary, longitudinally disposed feed trough l2 into which is run; thru avalve-controlled pipe I, the liquor to be dried from a suitable supply reservoir l6 equipped with adequate heating coils to keep the liquor to be dried at the desired working temperature. An agitatorll may be usedto prevent undue settling in trough l2. Arrangement of trough l2 and drum material into the collecting 'A movable rubber gasket 51, shown in III is such that on rotation of the drum, a portion of the drum surface becomes immersed in the liquor in the trough, and on emergence from the bath certain quantities of liquor adhere to the outer drum surface in the form of a relatively thin layer. A longitudinal stripping knife is positioned to contact the drying surface at a point just ahead of resubmergence of the drum surface in the liquor bath in trough I2, drum rotation being in the direction of arrow 2 l. Dried material is directed by the flat surface of the knife into an elongated collecting bin 2 Liquor overflow'from'trough I2 runs thru pi e into a sump 26, communicating by pipe 21 with the inlet side of pump which returns from sump 26 thru pipe to the reservoir is. The apparatus as so far described may be considered conventional.

Practice of the present invention involves use of a secondary or supplemental feed trough 33 of length corresponding with the length of the drum drying surface. This trough is mounted, by suitable supports not shown, in a position immediately adjacent the drying surface and preferably slightly above the axis of rotation of the drum, all as indicated in Fig. 1; The supplemental trough is shown in detail in Figs. 2-5. Trough 33 comprises channel iron 35 to the opposite ends of which are attached rods 31 atiording means thru which the trough, by supporting framework not shown, may -be held in position against the drum surface. Aswill appear from Fig. 4, the flat surface as oi" channel bar 35 forms the back side of the trough channel. Attached to the channel barat the lower horizontal edge of flat face as is a longitudinal triangular bar 52, to the sloping face of which is bolted a spring steel plate M extending the length of the trough. Resting on the horizontal face of bar 82 and also on the upper portion of plate is a sheet of relatively heavy canvas pump 30 and the indicated relation of supply tank it, both troughsand the associated pipe connections, it is possible to conveniently keep up a rapid circulation of a relatively large volume of liquor, regardless of rate of liquor drying,

vthus preventing possible clogging of the conduits and so-called "freezingf of the liquor.

When applying the principles of the invention to the drying of disodium phosphate, the liquors used may be produced in any suitable way. For example, the disodium phosphate liquor to be dehydrated maybe made by melting the dodecahydrate in its own water of crystallization, and then concentrating the melt by heating. In

large-scale operation, a suitable liquor to be dehydrated may also be the disodium phosphate liquor resulting from filtration, and subsequent concentration if necessary, of the liquor formed or other suitable flexible material securely held in place by a longitudinal metallic plate at and associated bolts 'which pass thru the canvas and are threaded into bar 52. The width of the canvas is such as to afford a lip flexibly con tacting with any'material on the immediately adjacent face of drum l0, and the length of the canvas strip corresponds with the length of the trough. r

- As shown in Figs. 2 and 3, L-shaped end plate 54 is boltedrigidly to channel face 30 as at 55.

Figs. 2 and 5 and partly in elevation in Fig. 3,

is sandwiched between the inner face of fixed end plate 5d and a movable supporting plate 50 on which are fixed studs 6i projecting thru, the

rubber gasket and into longitudinalslots 52 of end plate 50. The turned-in edge as of the rubber gasket is attached as at 08 in liquid-tight relation to the adjacent vertical edge of canvas strip 55. As shown in Fig. 3, the turned-in end of the gasket and the corresponding edge of inner supporting plate 59 are arcuateconiorming with the curvature of the drum. By loosening and tightening wing-nuts E0 on studs at, gasket at and inner supporting plate 59 are movable as a unit toward or away from the drying drum.

' Feed of liquor to trough 33 1S effected by means of pump 30, pipe .36, and pipe l5 containing a control valve l0. Constant working level in the trough is maintained by overflow pipe l8 emptying into sump 26; Valve-controlled pipe 00 provides a by-pass around trough 33. Bymeans oi plan in' in the usual process for making hydrated disodium phosphate.

Especially when dehydrating disodium phosphate liquor by means of drying equipment such as a drum drier 'or similar apparatus where the drying surface passes thru and picks up an adhering layer of the liquorto be dried, it is advantageous to take into consideration the matter of concentration of the liquor used. It is preferred to carry out make-up of the disodium phosphate prior to drying so that the liquor fed into the drum drier supply troughs has a concentration of not less than 49 B. at temperatures in the range of 200 to 220 F. To facilitate formation, on the drum or other drying surface, of liquor layers of the more satisfactory physical characteristics, it is preferred to operate so that the concentration of the liquor in the feed troughs is not in excess of 51 Be. at temperature of 200-220 F. Formation of such liquors may be efiected by suitable control of the previously mentioned concentrating operation. Temperature of the liquor in the feed troughs and as actually fed onto the drying-surface may vary considerably and is for the most part a matter of operating convenience, i. e. such temperature should be high enough to maintain the mass in the system in readily flowable condition.

Furthermore, when using the invention for advantageous results, i. e. high production per unit drier, it is preferred to feed to the drying surface a disodium phosphate liquor containing,

by weight of the liquor, not more than 0.02% monosodium phosphate and. not more than 0.05% trisodium phosphate, calculated on a 50 B. at 212 F. basis.

In instances where it is desirable to .treat or adjust the liquor to be dehydrated to form a mass of the composition indicated, such treatment may be effected at any time during makeup. However, the more practicable procedure is to first bring the liquor, ordinarily by boiling weak liquor, to a concentration of about 50 Be. at 212 F., determine the monoscdium or trisodium phosphate content by known tests, and then calculating the amount of treating material needed-to bring the liquor within the indi cated monosodium-trisodium phosphate range. Adjustment may be accomplished by adding to the liquor sodium hydroxide or sodium carbonate on the one hand, or by addition of phosphoric acid on the other. Where the monosodium phosphate content exceeds 0.02% by weight on a 50'B. at 212? F. basis, requisite quantitent exceeds 0.05% on a 50 connecting circulating in the liquor. Feed to first formed on the drum a layer 2,281,811 ties of sodium hydroxide or sodium carbonate may be added to reduce the monosodium phosphate content to not same basis.- In this situation it is preferred to employ sodium carbonate since even though an excess of sodium carbonate were used, practically no trisodium phosphate would be formed, and hence there is no danger of forming a liquor containing more than 0.05% trisodium phosphate. -In cases where the composition of the liquor is such that the trisodium phosphate con- B. at 212 F. basis, the liquor is treated with a sufficient amount of preferably relatively pure phosphoric acid to reduce the trisodium phosphate content to not more than 0.05% on the same basis. Should the liquor to be dried have concentration of say 49' Be. at 212 F. (instead of 50 B. at 212 F. as above indicated), determination of monosodium or trisodium phosphate content may be made as before. Conversion of such analysis to 50 B. basis may be had by multiplying such analysis by 50/49, and according to the value thus obtained the amount of treating material to be used may be calculated.

In practice of the invention, reservoir I is filled with liquor to be dehydrated and by suitable control of heating coils in the reservoir the liquor is kept at whatever temperature is desirable to facilitate flow of liquor thru the system. In drying disodium phosphate, it will be understood the liquor introduced into tank I6 is, in accordance with the best embodiments of the invention, preferably of a concentration and composition previously noted. Pump 30 is put in operation and circulation of the liquor is started thru troughs l2 and 33 and the associated pipe connections indicated in Fig. 1. It will beunderstood that in normal operation both troughs are kept fllled to overflowing and there is a continual movement of liquor thru the troughs and pipes regardless of the rate of drying on the drum. Overflow of trough I2 is fixed so that the lower portion of the dry. ing drum is immersed and overflow from trough 33 are adjusted so that there is a continual flow of liquor over lip II of trough 33 to such an extent that the material already on the drum surface, as the latter passes lip ll, becomes as thoroughly wetted as feasible with liquor from trough 33 without permitting any material runto an appreciable extent more than 0.02% on the off of liquor down the side of the drum. In this way it will be understood that by reason of the relationship of the drum and trough l2, there is of liquor to be dried. During movement of a portion of the drum surface from trough l2 up to lip 5| the material on such portion becomes at least partially and in most instances'quite thoroughly dried. Such given portion of the drum passes under lip 5| and on account of the previouslyementioned liquor flow over lip 5|, there is formed on the partially dried material already on the drum a second layer of the liquor to be dried. On further rotation of the drum heating of material on the drying surface continues, steam supply to the interior of the drum being such that all of the material on the drum surface has dried to the desired degree by the time a given portion of the drum reaches stripping knife 20.

Prior to development of the invention it was found that when using a run-of-plant disodium phosphate liquor containing for example 0.07% monosodium phosphate, from commercial viewpoint the results obtained with respect to power consumption, dried product removal, physical form of the product, and knife readjustments and replacements were unsatisfactory and use of a drum drier equipped with conventional single trough feed, e. g. trough l2, was entirely impracticable. Further, experience shows dried material removal from the drying surface became dif ficult and unsatisfactory where he trisodium phosphate content exceeded the indicated 0.05% by weight. It was then found that by proceeding as already indicated by forming on the heated drying surface a layer of liquor, heating the layer to convert the same to an at least partially dried crust, then forming on the crust a layer of the liquor-and continuing heating the material on the drying surface to substantially dry the same, remarkably satisfactory results might be obtained even though, in the case of disodium phos-- phate liquors, the monosodium or disodium phosphate content might be outside the above indicated preferred operating range. In one operation employing the double" feed of the invention, and using the same drum drier, the same steam pressure, and the same run-oi- ,plant liquor containing 0.07% monosodium phosphate, production or 1320 pounds per hour of disodium phosphate having about 0.5% moisture ccntent was obtained. In this procedure the liquor in troughs I 2 and 33 was at concentration of about 495 B., temperature of the liquor was about 211 F., average horse-power needed was 9, and R. P. M. of the drum was 2.6. In another similar run, using double feed, the same drum drier, the same steam pressure, but a run-ofplant liquor which has been treated with sodium carbonate to form a In this case average R. P. M. of 'th drum was 2.6, average horsepower needed was slightly less than 8, concentraticn of the liquor was about 49.7 B., and temperature averaged 210 F. In both of these operations, using the run-of-plant liquor and the sodium carbonate treated liquor, most of the composite crust formed on the drum cracked and chipped off of its ing the stripping knife, and only very moderate Using the same liquor and the same drum equipped for the double layer feeding of the invention production was about 18 tons per 24 hours and drum speed was only 2.5 R. P. M.

No satisfactory explanation for the improved results aflorded by the invention has been developed. It has been observed, however, that when operating the drum with a single layer coating, 'the thickness of. product particles is only 0.003-0.007 of an inch, whereas thickness of flaked product of the double coating procedure of the intention is generally 0.018-0.23 of an inch. It is thought that, since the initial coating is usually almost dry by the time the second layer is applied, the first coating provides a rough surface which holds much more solution than isretain by the first application, and the result is formation of a composite crust from three to' six times as thick as is obtained by single layer application. that the substantial increase in thickness of coat ing secured when double feeding .is used affords a deposit whichresists rupture tendency sufiiciently so that 'whe n' the material. is substantially dry the vapor pressure on the surface of the drum and beneath becomes sufficient tocra'cl-zand raise, the crust awayfrom the dryingsurface in relatively large flakes or sheets.

The double layer'feed method of the invention may be used to dry other liquors such as trisodiumphosphate, sodiumsulfate, calcium acetate, calcium chloride, magnesium chloride, and sodium benzene sulfonate. Further, the'invention may be practiced in connection with use of equipment other than. a drum drier. For

example, a heated metal belt drier, or-a rotating disk drier, or anyother type of drier ordinarily including a knife for removal of dried material from the heating surface may be employed instead of a drum'drier.

I claim:

1. In the process for drying disodium phosphate liquor by procedure involving use of a drum drier, the steps comprising forming on the drying surface of the drum a layer of disodium phosphate liquor, heating said layer to convert the same to an at least partially dried. crust, forming on such crust a layer of disodiumv phosphate liquor, and continuing heatingfiofzthe disodium phosphate'material on the drurliidrw ing surface to thereby form substantially-dry material readily removablefromaid surface;

2. In the processlfor drying disodium pho Hence, it is possible,

phosphate liquor" containing, by vveight of said liquor, monosodlum. phosphate in amount not I more than 0.02% and trisodium phosphate in at temperature of 200-220,F. and containing, by weight ofsaid liquor, monosodium phosphate amount not more than 0.05%, on a 50 B. at 212 F. basis, heating said layer to convert the same. to an at least partially dried crust, forming on such crust a layer of said disodium phosphate liquor, and continuinaheating of the di sodium phosphate material on the drum drying surface to thereby form substantially dry mate-= rial readily removable from said surface.

a. In the process for'drying disodium. phosphate liquor by procedure involving use of a drum drier, the steps comprising forming on the drying surface of the drum 2. layer of disodium phosphate liquor of concentration of 49-51" Be.

in amount not more than 0.02% and trisodium phosphate in amount not more than 0.05%, on a. 50 as. at 212 F. basis, heating said layer to convert the same to an at least partially dried crust, forming on such crust a layer of said disodium phosphate liquor, and continuing heat= ing of the disodium phosphate material on the drum drying surface to thereby form substan= drum drier, the steps comprising rmin'gaon-the' drying surface of the drum-a la ofidisodium phosphate liquor of concentra'ti of not i-less than 49 B. at temperature of 200-'-220 j.F.-, heating s'aid layer to convert the same to an -a't least partially dried crust, forming-on such crust a layer of said disodium phosphate liquor, and

material on the drum drying surface to thereby form substantially dry material readily removable from said surface.

3. In the process for drying disodium 'phosphate liquor'by procedure involving use of a' drum drier, the steps comprising forming on the drying surface of the drum a layer of disodium tially dry material readily removable from said surface.

5. In the process for drying disodium phos= phate liquor on a heated drying surface, thesteps comprising forming on said surface a relatively thin layer of disodium phosphate liquor,

heating said layer to convert the same to an at least partially dried crust, forming on such crust a relatively thin layer of disodium phosphate liquor, and continuing heating of the disodium phosphate material on said drying surface to 2 thereby form substantially .dry material readily removable therefrom.

'6. In the process for drying disodium phosphate liquor on a heated drying surface, the steps comprising forming on said surface a relatively thin layer of disodium phosphate liquor continuing heating of the disodium phosphate of concentration of-not less than 49 B. at temperature of 200-220" F., heating said layer to convert the same to an at least partially "dried crust, forming on such crust a relatively thin layer of said disodium phosphate liquor, and continuing-heating of the disodium phosphate material on said drying surface to thereby form substantially dry material readily removable therefrom.

WILLIAM B. TOSIVNER.

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