US2192831A - Drying of peat and the like - Google Patents

Description

March 5, 1940. 1'. GRAM Er Al.

DRYING oF FEAT AND THE LIKE March 5, 1940. T. GRAM Er AL DRYING 0F PEAT AND THE LIKE Filed Aug. 18, 1937 2 Sheets-Sheet 2 J? 'Re lll/lill rllllll/lll/ Fyfw.

Patented Mar.. 5, 1940 UNITED STATES DRYING 0F PEAT AND THE LIKE Thomas Gram and Nils Testrup, London, England, assignors to Techno-Chemical Laboratories Limited, London, England Application August 18, 1937, Serial No. 159,793 In Great Britain August 26, 1936 3 Claims.

This invention relates to the drying of peat and other fibrous materials. for instance grass, in subdivided condition While suspended in a gaseous medium such as air and passed therewith through heated tubes.

The invention constitutes an improvement upon a process for drying peat based upon the inventions described in United States patent specilications Nos. 1,766,843, 1,755,373 and 1,840,857.

When carrying into effect the invention covered by Patent No. 1,766,843, it was found that in order to dry peat from a water content of 50-60% to 10% a relatively long drying time was required, necessitating the material travelling through several banks of tubes of considerable length in series in order to effect the necessary drying. This requirement was further accentuated by the introduction of the invention covered by Patent No. 1,755,373, when smaller temperature differences were imposed by the use of doubleor multiple-eect heating. A total or developed length, e. g. 20,000 to 30,000 times the internal diameter of the plain tubes employed was needed. In order to obtain the highest evaporation it was necessary to work with a very high ratio, namely, 3:1, between the weight of the material carried and the weight of the gaseous carrying medium. It was found that the material travelled mainly in the centre of the tubes where the distance from the tube wall was the greatest: as a result, the diameter of the tubes had an influence upon the drying time and upon the ratio between the weight of material carried in suspension and the weight of the gaseous carrying medium. Investigation in these cases showed that in making up a given heating surface, upon which, with the temperature differences employed, the heat transmission largely depends, the adoption of tubes of greater length tends somewhat to increase the heat transmission, whilst an increase in diameter partly counteracts this owing to the concomitant increase in the distance of the heated walls from the centres of the tubes. The predominating eiiect of increasing the diameter` of the tubes, however, having regard to the fact that a certain minimum velocity of the carrying medium has been found necessary for moving the material in suspension init, is to require a greater quantity of carrier medium per unit of time for each unit oi heat transferred in that time to the material to be dried: thus, the thermal ballast edect of the carrier medium is increased and so also is the heat required for drying.

It was therefore desirable to adopt the smallest practicable tube diameter and 1%" was adopted. However, the employment of tubes of small diameter for the drying of peat containing fibres entails the risk of choking. It was found that plain tubes 'of 1%" diameter did not choke provided suiciently intimatesubdivision 5 of the peat was effected before feed.

The introduction of spiralised tubes led to great improvements in the drying of peat: it was anticipated that this, by giving a better disposition of the various sizes of particles, would increase 10 the heat transfer from the tubes to the material, and that it would make it possible to reduce the ratio between the weight of carried material and that of the carrying medium. Practical experience, over some years, of units having spiral tubes 15 of the helically distorted form of Figurefi of Patent No. 1,840,857, of 11/2" diameter arid 35 feet long confirmed this. It was found that the said ratio couldv be reduced to about 1/sth of its former value, i. e. to one-halfzone, and it was 20 also found (ignoring practical obstacles afterwards dealt with herein) that the drying time could be reduced considerably, 'leading to great simplification and cheapening of the apparatus by shortening the length of tube to a mere 20th, 25 e. g. in one specic case, to only 1400 times the internal diameter.

However, it was found that the long bres in the material caused choking of the tubes, and in order to overcome this didlculty the necessity of 80 removing the said bres from the peat by screening prior to the drying was conrmed. For an appreciation of the diiliculty it is to be borne in mind that whilst the physical character and composition of peat vary greatly according to the 80 age and the district of the deposit from which it originates, it may be stated, in all cases when such peat is delivered to drying plants of the kind with which this invention i's concerned, it is usually a moist or wet conglomeration of (a) 40 particles of amorphous nature and particles of more or less cellulosic nature, i. e. showing short bred structures up to 2 length and most of them much under that length, and (b) long iibred structures, hereinafter called long fibres over 2" long, and frequently reaching one foot length or more, dispersed in the substrate formed by the amorphous and short-fibre mixture composition as in (a) above. The percentage of the long iibres in the peat as a whole, as explained, commonly uctuates from one sample to another between as high as 30% or more to as low as 1%. The risk of choking when using the peat unscreened, although it is a variable risk, has remained ever present and real.

-The screening operation for the removal of the long libres as above defined, involves greatly increased cost as the screening of wet or semi-wet peat involves quite elaborate plant comprising screens and conveyors which are subject to heavy maintenance costs. Furthermore, the fibres or screen-reject material could not be dried with double-effect, and even when using screening, occasional choking occurred requiring plant stoppages and the use of clearing devices.

The present invention consists oroadly in drying peat in a tubular drying plant or structure in such a way as to avoid choking of the drying elements while avoiding necessity for libre screening and to enable if desired multiple-effect drying to be elfected on the whole of the material.

The invention also consists in a method or means for drying the materials referred to while carried in suspension by a gaseous medium through heated tubes of such a shape that the material is subjected to rotational action and caused to travel adjacent to the tube walls, in which the tubes are adapted to avoid choking thereof.

The invention also consists in the drying of peat in disintegrated condition while carried by a gaseous medium through heated conduits according to which the peat is disintegrated and while still containing long fibres occurring as a natural'characteristic of raw peat is directly lntroduced into the suspension medium for the drying.

The invention also consists in apparatus f or the drying of peat, consisting of one or more tubular driers substantially of the kind set forth in Patents Nos. 1,766,843, 1,755,373 and 1,840,857, comprising spiral tubes, in combination with disintegrator means coupled directly to an initial drying stage.

The invention also consists in drying apparatus of the kind referred to comprising a plurality of series-connected stages embodying helically distorted or internally spiralised heated tubes the total developed length thereof through the several stages bearing to the internal diameter of said tubes the approximate ratio of 1400:1, and the internal tube diameter being at least 2" nominal and not substantially in excess of 41/2".

The invention also consists in a method of drying peat substantially as hereinafter indicated and/or set forth in the claims hereof.

The invention also consists in apparatus for the drying of peat substantially as hereinafter described and/or claimed.

Referring to the accompanying drawings:

`Figure 1 is an elevation showing the general layout of one construction of drying plant according to the present invention;

Figure 1a is a fragmentary section;

Figure 2 being a plan of the same construction.

The invention, as indicated above, is based upon experimental observations having their foundation in our experience with plain-tube drying structures indicating that if the internal diameter of a spiral tube of the form of Figure 4, (Patent No. 1,840,857) referred to above, were to be increased to at least 2.1/2" internal diameter (nominal) with a twin spiral of about 24" lead and l2" spiral pitch so as to avoid deposit of material, loss of efficiency would result. Such would be the case if enlargement of tube diameter and consequently cross section were, as seemed certain, to necessitate reduction of the number of tubes to maintain a given heating surface and reversion toward the former high material-carrier gas ratio from the much lower demonstrated optimal ratio, In the end, however, it was established that thermal eiliciency was preserved if merely the length of the tubes was extended to, e. g. 58 feet i. e. in proportion to the diameter increase. When the velocity of gaseous ilow through the tubes of this arrangement is at least 30 feet per second, it is found that removal of the peat fibre by screening is unnecessary and that continuous Working can be maintained with high eillciency and without choking; while the whole of the peat can be treated by doubleor multiple-eifect drying-a result hitherto unattainable.

The amount of carrying medium per unit of heat transferred could be kept at a suitable iigure and high efficiencies of drying, which have characterised the type-of plain tube apparatus the subject of our earlier patents, are at once preserved and rendered available for the drying of unscreened peat and like fibrous materials in a practically more economical Way than hitherto.

In the practical application of the foregoing considerations according to one form, a drying apparatus is constructed, as to its main essentials, substantially in accordance with the foregoing patents, that is to say, it embodies features affording economical operation with low temperature differences, the use of a plurality of drying effects, and includes in the driers tubes constructed so as to maintain the peat particles throughout their entire passage through them in close relationship. with their heated walls.

The layout of apparatus shown in Figures 2, 3 and 4 of the drawings of Patent No. 1,755,373, may thus be generally followed with the important modification which consists in eliminating the conveyor N1, the mixer N2, and preferably also the conveyor O2, bringing the macerator or disintegrator into close proximity with the iirst or low temperature drier and connecting the two so that the disintegrated peat is delivered directly, preferably by gravity, into the upwardly impelled gaseous medium near the base of said drier. Conduits such as O3 and O4 including conveyors for obtaining recirculation of a portion of the dried peat through the same drier are also eliminated.

Dealing thus with peat which is of a relatively high water content and only roughly disintegrated, the driers and tubes thereof are required to cope' with more intractable material than hitherto attempted.

Usually the peat apart from its fibrous content should be suiliciently iinely divided to pass through a sieve of about 1/4 mesh; such a condition ensuring proper suspension in the gaseous medium at speeds of the order mentioned. The tubes of the driers are made of the spiral form already mentioned and of a bore not less than 2", and the length of the tubes is increased beyond the 35 feet usually employed hitherto. As a1- ready indicated, the risk of choking is reduced if the Velocity is sufficiently high; and tube lengths up to 100 feet or more may be employed; such also enables say three shells to replace five with shorter tubes in a drying stage. By proportionate increase of tube lengths and decrease of the number of shells, one may also cheapen very substantially the construction of peat drying apparatus for any given duty by reducing the cost of construction of the shells and the number of devices required between shells operating in series to constitute a drying stage.

By way of further example a double-effect drying apparatus may comprise two sets of tubes 100 feet long constituting a first or steam heated' effect and one set of the same length constituting the second or lower temperature effect, all the tubes being 2;/2 to 3 inch diameter. Alternatively, a total of five sets of tubes traversed in series by the material may be retained, using tubes of 100 feet long and l11/4 inch bore.

It has been established that not only is there a lower limit below which the tube diameters are insuihcient to enable unscreened material to be handled as above, but also there is an upper limit beyond which increase in tube diameter adversely affects the efilciency as expressed by the heat consumption for unit weight of water evaporated. These limits are related to tube lengths in practically operable apparatus by the rule that the total or developed length of heat transmitting tube through which the material is carried in the process of drying, i. e. the sum. of the lengths of the tubes in the several passages or stages, bears to the internal diameter of the tubes a more or less definite ratio whilst for the reasons already stated the nominal internal tube diameter is not less than 2" or substantially in excess of l1/2".

The optimal ratio applicable to the drying of peat with double-effect and representing the optimum for eflicient drying of that material in that way from a moisture content of about down to 10% is l400:1. However, when operating temperature diierences and tube diameters are settled drying is dependent upon the time during which the material is in heat exchange relationship with the active surface of the driers, i. e., upon the total time during which the material is passing through heated tubes. The rate of movement is itself determined as above by considerations of obtaining adequate suspension, and is for peat of the order of 30' per second when the material is free from lumps or particles larger than about 1/1" mesh; so that for a less extensive drying than about 50% to 10% (or for drying through higher ranges in which evaporation occurs more rapidly) shorter tubes may be employed, i. e., a ratio of less than 1400:1 can be adopted whilst affording an adequate drying time.

Again, for any given reduction of moisture content, ii double-eiect drying be not adopted the average temperature differences change: their increase (as in single-effect drying using only live steam heated tubes) enables proportionately shorter tubes to be used with equal eiilciency. Tripleor multiple-effect drying (again assuming a fixed reduction of moisture content) utilising lower temperature differences calls on the other hand for a longer drying time if equal eiliciency is required, and as the speed of movement of the material cannot practically be reduced below about the figure stated, namely, 30' per second, proportionately longer tubes are required. The ratio of tube length to diameter thus, whilst determinative of the efliciency of drying in any given tube or drier (singledoubleor multipleeffect) is variable generally over a certain range on either side of an optimum value, below which range the plant shows a deiinite thermal inemciency, and above which the plant, e. g. by multiplication of parts, tends to become unwieldy and more costly than is justified by such thermal eiliciency improvement as may be secured. This range, in the case of double-enect drying of the kind described above and applied -to peat for reduction .of the water content from about 50% to 10%, is from about 500:1 to 2000zl.

In the illustrated example oi' an apparatus ac- Y cording to the present invention and intended for shells A1, B1 and C1 containing tubes A2, Bz and C2 respectively of the spiralised form already mentioned and as shown in the sectioned areas of Figure 1a, being approximately 21A" in maximum internal diameter and 88 feet long, so that their total or developed length is approximately 264 feet and the preferred ratio, diameter to length, as above is secured.

To deal with the gradually increasing volumes due to evaporation the shells A1, B1. C1, contain successively an increasing number of tubes.

The peat to be dried after disintegration in the appliances of any suitable character indicated generally at N is without further treatment fed down a chute O into a chamber O1 in the lower part of the drier C. From this point it is carried upwardly in suspension-- through the heated tubes C: at a velocity of about 40 feet per secondby a current oi' air or other gaseous medium impelled by a fan K1 to a cyclone separator J1 from which the separated partly dried material is discharged by a conduit L1, the -air or other carrier mediumbeing discharged by a vent pipe M to atmosphere. The tubes Cs are externally heated by a liquid which is circulated through the shell C1 and through heat exchanger means co-operating with a scrubbing device, collectively indicated by references E and E1, to impart to said liquid, via another liquid, heat recovered by the condensation of vapours evolved in the drying of material in the other tubes A2, Ba in a manner already known per se.

By way of the conduit L1 the partly dried material is admitted to the lower ends of the tubes Bz in the shell B1 and after passing upwardly through these tubes and being further dried it is separated by a second cyclone J2 and passed by the conduit La to the tubes Az of the third shell A1 after traversing which it is finally separated by the cyclone Ja and discharged at La'in the dried condition for disposal in any manner.

Speed of the material in the tubes of the driers B and A is about 33 ft./sec. and the total time of drying in the tubes of the three stages is about 7.5 seconds.

For the driers A and B which are heated by live steam passed in parallel through the shells A1, B1 around the outsides of the tubes therein, the gaseous suspension medium, for example an air and vapour mixture, follows a closed cycle. Carrying with it the material admitted through La it is impelled by the fan Ka upwardly through the drum A, through the cyclone J3 (Where the material is discharged) from there via the conduit P1 to the drier B (with the material admitted through L1) the cyclone J2 (where the material is separated) and conduit P2 to the lower part of the scrubber E, from the upper part of which the cooled medium is drawn back through the conduit Ps to the suction of the fan K2.

In the above apparatus the material is subjected only to disintegration to a desired iineness and suitable mixing to obtain homogeneity, and before admission into the lower temperature or rst effect drier C is not subjected to any other process than is essential to its being fed with suiiicient uniformity into the rising current of gaseous medium at the base of the shell thereof.

The whole of the material so introduced is subiected to double-eiiect drying with its attendant high eillciency. y

Inasmuch as the ratio discussed above is variable with considerations of required efflciency, moisture content conditions and temperature differences, when dealing with peat as the material to be dried, the range may be further varied in the operation upon other fibrous materials susceptible to drying whilst carried in suspension through heated tubes. So also, adherence to or departure from the optimum ratio, ascertainable by test, becomes chieiiy a matter dependent upon other e. g., economic considerations, already discussed. f

We claim:

1. In drying moist disintegrated peat having long bres and being free from lumps or particles larger than about 1A in. mesh, introducing said material after disintegration and with its longbre content directly into a current of gaseous fluid, passing it thereby in suspension therein upwardly in streamsat a speed of not less than about feet per second with continuous rotational movement through externally heated tubular conduits having an internal diameter of no less than 2 and no more than 41/2", the length of said conduits being at least 1400 times as great as the internal diameter thereof.

2. In drying moist disintegrated peat having long fibres and being free from lumps or particles larger than about A in. mesh, introducing said material after disintegration and with its long nbre content directly into Aa current of gaseous uid, passing it thereby in suspensiontherein upwardly in streams at a speed of not less than about 30 feet per second with continuous rotating movement through externally heated tubular conduits having an internal diameter of no less than 2" and no more than 41/2, the total combined length of said conduits being at least 1000 times as great as the internal diameter thereof for the drying of peat from a water content of 55% to a water content of 10% while employing an average temperature difference oi' 30 C. between the heating medium' and the gaseous carrying medium.

3. In drying moist disintegrated peat having long ilbres and being free from lumps or particles larger than about 1A in. mesh, introducing said material after disintegration and with its longflbre content directly into a current of gaseous fluid. passing it thereby in suspension therein upwardly in streams at a speed of not less than about 30 feet per second with continuous rota.- tional movement through externally heated tubular conduits having an internal diameter of no less than 2" and no more than 41/2", the length of said conduits being between about 500 and 2000 times as great as the internal diameter thereof for the reduction of water content from about to 10%.

THOMAS GRAM. NILS TESTRUP.

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