Process Aspects of Regeneration

7/27/2019 Process Aspects of Regeneration

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PROCESS ASPECTS OFREGENERATION IN A

MULTIPLE HEARTH FURNACE

Hankin Environmental Systems Inc. isthe exclusive licensee for Nichols Technology

Hankin Environmental Systems Inc.One Harvard Way, Suite 6 PO Box 9597 Hillsborough, NJ 08844 USA

Phone: (908) 722-9595 E-mail: [email protected] Fax: (908) 722-9514Website: www.hamkinenv.com


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PROCESS ASPECTS OF REGENERATION IN A MULTIPLE HEARTH FURNACE

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TABLE OF CONTENTS

Section Description Page

I Introduction ...................................................................................................... 3

II Multiple Hearth Furnace Description................................................................ 4

III Reactivation Process - Zones and Outline of Sizing Factors........................... 7

IV Determining the Basic Factors Required forRational Sizing Calculations .......................................................................... 10

A. Drying Zone.............................................................................................. 10B. Pyrolysis Zone.......................................................................................... 11C. Reaction Zone .......................................................................................... 12

V Calculation of Hearth Area and Furnace........................................................ 19

A. Reaction Zone Alternatives ...................................................................... 19B. Pyrolysis Zone.......................................................................................... 25C. Drying Zone.............................................................................................. 27

VI Some Brief Experiments Regarding Reaction Rates..................................... 29

VII Conclusions.................................................................................................... 31

References..................................................................................................... 36

Table of Appended Figures............................................................................ 33

Appended Figures.......................................................................................... 34


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SECTION IINTRODUCTION

Discussion of the methods of calculating the size multiple hearth furnace required for a given carbonactivation requirement by usual chemical engineering process calculations will serve as a usefulframework within which to discuss variations in what diverse adsorption processes require of thereactivator, methods of determining the work to be done in reactivation, and some possible directionsfor exploration aimed at reduction of carbon losses and/or improvement of quality of reactivatedcarbon.

The effort to show a rational method of size calculation may be useful in itself for those who are

required to do such sizing.

All figures are appended at the back of the paper.


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SECTION IIMULTIPLE HEARTH FURNACE DESCRIPTION

Figure 1 shows a cross section of multiple hearth furnace. Since the equipment is well known only afew brief points about mechanical and process aspects peculiar to this type of apparatus will be made.

Briefly, the multiple hearth furnace is a device to accomplish heat and mass transfer, between gasesand solids, passing the gases and solids counter currently through a series of compartments, orstages, in each of which the gas travels in mixed laminar flow over solids, spread in thin furrowedlayers, that are periodically raked, to both mix the solids and advance them through the compartment.

Heat transfer is both by direct convection and radiation from the gas to the thin wide spread area of

solids, and by indirect transfer to the brick walls and parallel brick compartment roof followed byre-radiation to the solids, plus minor amounts of conduction through the hearth floor from the gases ofthe compartment next below.

Mass transfer is solely by convection as the gases pass over the solids in laminar flow.

Burners, supplying oxidizing or reducing products of combustion of fuel burned, nozzles injectingsteam, or air injection nozzles apart from fuel burners can be placed at any hearth.

Thus temperature and atmosphere can be changed quite sharply from one hearth compartment to thenext.

If the carbon reaction zone covers two or more hearths, early reaction while the carbon is heavily ladenwith residue from adsorption may be carried out at one temperature and atmosphere, and final reactionas the carbon nears the regenerated state may be carried out under a different atmosphere ortemperature.

As to solids flow, it is apparent that the carbon is spread relatively thinly over a large area and stirredperiodically. For a given furnace, and at given volumetric feed rate, the faster the center shaft isturned, the shorter the retention time. The conveyance of material through the furnace is by positivedisplacement. All particles have equal retention time except for some slippage and/or some short

circuiting giving about a + 5% range to particle retention time under conditions of normal bed depth.The faster the center shaft is turned the more frequent the stirring and the shorter the interval in whichparticles lying on top of the bed remain there. Up to a three minute interval this time between stirringshas not been grossly or obviously significant in reactivation results. Neither has the effect of intervalsbetween stirrings been really fully investigated.


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There is, however, a danger in turning the shaft too fast for a given feed rate. The average bed depthbecomes too thin to completely cover the distance between one rabble tooth and another.

The effect over rapid stirring excessively reducing bed depth is shown in Figures 2 and 3. This can bea source of excessive carbon loss.

It should be noted that the rabbled bed of carbon is left in furrows, or corrugations.

The interface between carbon and gases, which is the determinant of capacity, is actually 1.2 to 1.4times the nominal -- or plan view hearth area, as the angle of repose of the carbon goes from 30o to45o.

It is noteworthy that the rabbling action itself is extremely gentle.

In recent test runs reactivating powdered carbon, some light agglomerates were fed, along with themain carbon stream. After passing six hearths they were visible still as agglomerates in the productcollection drum, but so friable that they could not be picked out between two fingers but had to bespooned out of the main mass for examination.

Similarly some years ago a client took the trouble to pass animal bone char through a cold 8-hearth14'-3" O. D. multiple hearth at several tons/hour rate, and then into a bucket elevator and back throughthe furnace for 30 cycles. Bone char shows more or less the same results on standard NBS abrasion

testing as does some common commercial active carbons.

As a result of this test the client reported that furnace attrition loss (disappearance plus creation of 60mesh particles) was 0.3% per cycle.

This makes one wonder how the commonly reported 5% losses in carbon regeneration are created.

The amount of carbon in the act of falling from hearth to hearth is at all times constant and, in granularcarbon applications, of negligible area for heat and mass transfer. A furnace of 175 sq ft/hearthnominal area, 227 sq ft of actual furrowed surface with 4 ft hearth spacing, would have in suspensionbetween each hearth a lb of material at all times. This would have a total particle surface area for 30

mesh average size material of about 12.5 sq ft, but it falls in a relatively tight stream, not totallydispersed in the gas and probably represents at most 1 or 2 sq ft of actual surface exposure.


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This is considered fortunate as individual particles showered through active gas would receive heatthrough all sides of the particle. Also they would contact gas of the concentration of reactant's extent

that is normal to the main body gases above furnace hearths, which is very strongly reactive. Surfacegasification and not depth of pore penetration would be expected.

In the hearth bed, even the surface particles receive heat from one side only, half their surface area,and conduct much of that away through the downward facing side.

The gas composition around the particles in the bed of the reaction zone are believed to be quitedifferent from that of the main stream of gases over the bed due to the laminar gas flow and stagnantfilm at the bed surface.


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SECTION IIIREACTIVATION PROCESS - ZONE AND OUTLINE OF SIZING FACTORS

It is usually considered that thermal regeneration of carbon occurs at 1500oF to 1850oF by reaction ofwater vapor and/or carbon dioxide with whatever is left of the adsorbate after the adsorbate, along withthe original carbon, has been heated to these temperatures.

Clearly reaction cannot begin until the material has been heated to reaction temperature, and heatingcannot begin until the carbon has been dried.

Thus, there are three steps which may, with best design and operation, occur in three separate furnacezones rather than overlap. They are a reaction zone, a heating zone and a drying zone, considering

them from the bottom of the furnace upward, following the path of the gas flow, and also naming themin their probable order of importance rather than their order of occurrence.

The reactions expected are:

1. C + H2O CO + H2 (endothermic 4800* BTU/lb carbon)K equilibrium 1700oF = 47.8

2. C + CO2 2 CO (endothermic 5950* BTU/lb carbon)K equilibrium 1700oF = 52.5

3. CO2 + H2 CO + H2O (endothermic 37.3 BTU/cu ft any constituent)K equilibrium 1700oF = 0.73

*Assumes carbon is graphite, whereas most of the reactions in regeneration are with a modified formof amorphous carbon.

Even at 1500oF equilibrium constants for the first two reactions are high enough (about 10) to expectreaction to go essentially to completion except for kinetic rate limitations.

The reaction zone might be expected to be sized by volume of rabbled carbon bed considering that thecarbon gasification reactions that occur in it are governed by kinetics and reaction rate limited.

Actually it is sized by hearth area. The area exposed to the gases controls mass transfer of reactantsfrom the gas phase to the carbon, and heat transfer to support the endothermic reactions.


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Reactions occur both on and below the surface of the rabble bed, but at diminished rate with eachsuccessive layer of depth due to a combination of shielding from heat transfer and shielding from

reactants, by layers of carbon nearer the surface.

Within the normal range of bed depths previously discussed, no variation of carbon gasification rate isfound with varying frequency of stirring, varying retention time or depending on bed depth beyond theminimum required for full hearth area coverage.

If one wishes, one could consider the reaction zone to be determined by a volume of carbon bed, butone would have to assume a certain limited bed depth as being effective - probably about 2" and thusvolume would be reduced in effect to area times a constant.

The hearth area for the reaction zone can be established by determining an amount of carbon to begasified per hour and dividing by a rate of gasification expressed in pound fixed carbon gasified perhour per square foot.

The question of sizing the reaction zone then becomes a matter of deciding how much carbon is to begasified per pound of product and at what rate: subjects to be discussed in some detail further on inthe paper.

Once the conditions in the reactivating zone are established, the temperature, volume, mass andcomposition of the gases entering the pyrolysis zone from the activation zone are fixed.

Conditions in the pyrolysis zone, if it does not overlap with the reaction zone, can be altered from thosethat would result from simply flowing reaction zone gases across the pyrolysis zone hearths.Temperature can be lowered by adding steam or raised by adding air (to react with CO and H2 in thegases from the reaction zone) or by firing fuel.

Often, depending upon design and operation and adsorbate character, the pyrolysis zone shades intoor overlaps with the reaction zone, the transition being gradual as thermal decomposition of theadsorbate subsides and water vapor plus carbon dioxide reactions increase in rate.

The sizing of the pyrolysis zone and also of the drying zone above it, is simply a matter of heatexchange area required to transfer into the rabbled bed the heat required for heating carbon,adsorbate and water: evaporating water; heating carbon and adsorbate; plus any heat of reaction forpyrolysis of the adsorbate.


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There are all kinds of possibilities for minimizing the area required by raising gas temperatures andthus raising the temperature difference creating heat transfer, or minimizing fuel by burning either CO

and H2 from the reaction zone, or organic vapors given off during charring of the adsorbate.

The exact method of calculation will be discussed later with an example.

At present let it be said that the preparation step of drying the carbon is not at all a factor in carbonlosses and quality within the usually applied range of conditions.

Before detailed sizing calculations data must be obtained and decisions made on various points thatmay effect quality, losses and furnace size.


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SECTION IVDETERMINING THE BASIC FACTORS REQUIRED

OR RATIONAL SIZING CALCULATIONS

A. DRYING ZONEAll that is really needed for calculating the size of the drying zone is to know the pounds per hourof water that will enter the furnace with the spent carbon and must therefore be evaporated.

Usually the furnace is preceded by a slurry dewatering device that is in common use by othersand one accepts the reported experiences of others who use the planned device, as to thepercent moisture to be expected in the feed.

It was once common to wash the carbon "in situ" in fixed bed adsorption columns, using hotwater for the wash, and then allow the carbon to either drain by gravity or be more rapidly purgedof water by blowing a small volume of compressed air through the column. For this condition, weexpected 37% to 45% moisture.

Lately, it is more common to deliver a slurry of carbon to an upwardly inclined screw conveyor,allowing to water the drain to the lower end of the screw conveyor, counter to the flow of carbon.For this condition we have come to expect 45% to 55% moisture in the dewatered carbon.

A fairly wide range of expected moisture content is given for both devices because the moisturecontent will be affected by two or three factors beside the dewatering device used. When carbonis dewatered in a stationary bin, the temperature of the final wash water, the use or non use ofcompressed air to break the film of water between particles, and the time allowed for dewatering,all affect the result. With the dewatering screw, the primary variable is the particle size of thecarbon and whether or not the space between the screw and the shell of the screw conveyoris periodically cleaned of fine particles and possible mold growth, to allow free drainage of water.

Also when the adsorbate loading is relatively high, more of the pore volume in the carbon is filledwith adsorbate and there is less pore volume available for moisture. Hence moisture contents inthe lower end of the ranges stipulated above, can be expected. 0.3 to 0.5 pounds adsorbate perpound of original carbon would qualify as being a relatively high adsorbate load.

In the case of either dewatering device it must be remembered that moisture content is usuallystated on an "as received" basis, so it is the weight of water divided by the weight of water plusweight of original carbon plus weight of adsorbate. Therefore, one needs to know the weight ofadsorbate per weight of original carbon in order to calculate the pounds of water to beevaporated per hour in producing a given rate of regenerated carbon product.


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Perhaps less attention is being paid to the dewatering of carbon than might be justified at presentfuel prices, because the "as received" basis of stating moisture content is somewhat deceptive.

It seems surprising to note that 40% moisture is b pound of water per pound of dry solids and50% moisture is one pound of water per pound of dry solids; not 10% more moisture nor 25%more moisture but 50% more water to evaporate.

B. PYROLYSIS ZONEFor proper calculation of the heat transfer required during pyrolysis, the weight of adsorbate perunit weight of original carbon should be known and the heat of reaction for thermaldecomposition of the adsorbate should also be known.

Generally an exact knowledge of these two facts is not critical in the sizing of the pyrolysis zonesince the assumption of a half pound of adsorbate per pound of original carbon (the maximumwe have ever experienced) and of 250 BTU/lb of adsorbate endothermic heat of pyrolysis wouldnot significantly affect the total furnace size. It is difficult to envision a situation where the heat ofpyrolysis of the adsorbate or the amount of adsorbate being different from the above assumptionwould cause any problem.

Knowing the weight of adsorbate per unit weight of original carbon has greater importance forestimation of the drying zone, and for consideration of the heating value of organic vapors whichcan be used in the drying zone as a cost-free source of heat; than this has for sizing the pyrolysiszone.

Maximum removal by pyrolysis is desired because the more adsorbate removed by pyrolysis, theless char will be left to be removed by reaction with H2O and CO2.

Removal of char by reaction has generally been incomplete in respect to pores of about 30Angstrom unit diameter and smaller. Removal of char by reaction is generally accompanied byloss from the outside of the particle and enlargement of macro pores.(1)(2) Macro poreenlargement is not only a weight loss but weakens the particle, inviting additional handling loss insubsequent cycles.

While maximum removal by pyrolysis is desirable, such removal should probably not bemaximized by maximizing char temperature during pyrolysis.Graphitization begins at about 1600oF and it would seem desirable to keep the fresh char aschemically reactive as possible, to encourage gasification reactions to react with such charpreferentially to reaction with original active carbon structure. The original carbon structure haspresumably been made less chemically reactive by stripping reactive portions and exposure to


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high temperature during the manufacturer's activation.

All this leads to a desire for a procedure to maximizing organic removal by pyrolysis.

Goos(3) working in wood charcoal has suggested that the most rapid rate of temperature riseresults in the lowest char yield. He reports the ability to reduce charcoal yield from wood from35% to 14% by reducing the time of pyrolysis from days to seconds. If that applies, the multiplehearth is a naturally excellent tool, as once the spent carbon is dry, each piece as it comes to thesurface will rise in seconds to within roughly 100oF of the gas temperature. Usually gases overthe hearth where pyrolysis is occurring are at 1500oF to 1800oF.

If, as will be shown later, design and operating conditions are carefully established for the

purpose, the solids as a whole, will heat from 200o

F to 1500o

F in a small area, which dependingon shaft speed, may represent 5 to 20 minutes retention time. A fluid bed would be better yet butwould involve other problems especially if pyrolysis were not done as a separate step.

All that can be recommended at present is to avoid overheating during pyrolysis and avoid slowbaking.

To help avoid overheating during pyrolysis, the H2O content of the gases is kept high and COand H2 minimized, endothermic reaction will begin at 1500 and increase in rate with increasingtemperature and prevent the carbon from recycling full gas temperature. The gas temperatureshould be limited to 1700oF or thereabouts.

To avoid slow baking during periods of less than design throughput, the effective furnace areacan be shortened by either moving the reaction zone up and letting the last hearths at thefurnace bottom be simply steam purged, or by reducing temperatures drastically over the dryingzone to leave the carbon wet until just before reaching the reaction zone.

In design calculations, it seems desirable to seek means for minimizing the pyrolysis zone areaand thus the time the material will take to pyrolyze, by maximizing heat transfer rate in this zone.

C. REACTION ZONEThe object is clearly to remove all the char left by pyrolysis of the adsorbate without any reactionwith the original carbon structure.


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1. Determination of Char formed by Adsorbate Pyrolysis: The first requirement for designdata is to know how much char will be left by pyrolysis.

The adsorbate weight can be determined by knowing the particle density, as determined bymercury displacement, for the carbon in its virgin state and again after it has been spent.The ratio of spent particle density to virgin particle density minus one, equals the poundsadsorbate per pound original carbon.

Spent carbons should also be devolatilized by heating in covered crucibles to 1740oF for72 minutes.

Again the ratio of spent and pyrolyzed particle density to virgin carbon density equalspounds char formed per pound original carbon.

The indicated char formation must of course, be corrected for ash gain or loss from thevirgin state to the pyrolyzed state.

It is of some academic interest to compare the two ratios for an indication of char yield interms of pounds char per pound adsorbate, as gathering experience in this regard can helpthe designer to guess what to expect of different classes of compounds and might help instudying various pyrolysis conditions.

If mercury density apparatus is not readily available, careful ABD (Average Bulk Density)determinations have proven to give closely similar results.

One advantage of using average bulk density instead of particle density is that exactly thesame sample can be used for virgin density determination, spent and dried, used for spentdensity determination and pyrolyzed in one or more crucibles and again used for pyrolyzeddensity determination.

With mercury particle densities the material is made useless for adsorption or pyrolysis sothe problem of getting three identical portions of carbon sample arises.

The carbon sample for the above purposes must be carefully prepared by properly riffling

or quartering a small sample of carbon. Commercial carbon particle densities and averagebulk densities vary too much from time to time, lot to lot and top of a bag to bottom of a bagto run the spent and pyrolyzed densities on random samples and compare with publishedvirgin densities or densities from another random portion of the same bag of carbon.


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Often the problem arises of determining expected char gasification requirements when theplant producing the stream to be treated by carbon is not yet built. The above procedure

can easily be used on pilot size samples or even with bench work, producing simulatedwaste streams. With particle density determination by mercury displacement, only about 20grams of carbon need be spent in adsorption service.

If the adsorption system is not yet designed, and it is uncertain whether the carbon will bespent to equilibrium with the stream entering the adsorption process, it is of course safestto do furnace design work from a sample spent to exhaustion. This sample may havereleased lighter components of a mixed adsorbate and replaced them with heaviercomponents, but the heavier components usually show the highest char yield and will thusresult in safe furnace sizing, on both counts - maximum loading and maximum char yield

from adsorbate.

The above procedure of determining adsorbate load and char to be expected frompyrolysis is much preferred to calculation of adsorbate load from column tests byintegration of treated volume and reduction of stream components. The latter method toooften overlooks stream components adsorbed but unmonitored because they are notintentionally removed as an object of treatment, or includes material removed from thestream but biologically oxidized on the carbon and so no longer present at reactiva-tion. Also measurements of stream components often given results in terms BOD, COD,TOC or color which cannot be accurately translated into pounds adsorbate per poundoriginal carbon.

Most important, stream change measurements leave the furnace designer to guess, fromexperience, how much of the adsorbate will be easily removed by simple or destructivedistillation, and how much will remain as hard to remove residual char from pyrolysis. Thisnot only affects furnace sizing but also anticipated losses.

2. Extra Carbon to be Gasified from Original Structure, Reaction Condit ions andReaction Rate: Having determined the amount of char deposited by adsorbate pyrolysis,two related questions remain before rational calculation of furnace size can be undertaken:What will the carbon reaction rate in lbs/hr/sq ft of hearth area be, and how much carbon

is to be gasified in this way?a. Gasification of Overall Carbon Structure

It must be expected that the original carbon structure will compete with the char leftfrom the adsorbate for the privilege of reacting with the available H2O and CO2molecules. Even if all of the original carbon's structure was uniformly covered byfresh char from the adsorbate (a condition which probably never exists, deviation


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from it depending upon the distribution of various adsorbate species in the solutionbeing purified) one would still expect the outside of the structure and the lining of the

very large diffusion pores to be cleaned of fresh char before the adsorption macropores and adsorption micro pores were cleared of fresh char. Such pores are theroute by which H2O and CO2 will travel to reach the adsorption pores. Juhola(2)indicates a much faster reaction rate for a carbon of coarse pore structure than forone of the fine pore structure which seems reason-able support for this contention.

Since the reaction we desire is not with fixed carbon but with a soft, freshly madechar from pyrolysis, which may still contain some hydrogen, nitrogen, sulfur or otheratoms, there might be hope of a reaction condition that would be highly selective,giving a faster reaction rate with the new soft char than with the original active carbon

structure. A very brief preliminary investigation of this possibility will be reported atthe end of the paper, though it is quite inadequate to serve as more than a hint of thedesirability of further work in this direction.

It is this author's suggestion, based on general experience, that until betterinformation is available the expectation of gasifying 0.25 pound of original carbonstructure per pound of char formed by pyrolysis in a covered crucible at 1740 oFwould be a reasonable basis for design.

A sugar refinery that uses granular active carbon to polish away the last traces ofcolor after the heaviest load of color has been removed on bone char, reports (4) lessthan 1% carbon makeup required per cycle. It can be anticipated from the conditionsof service that the amount of adsorbate per pound of carbon would be extremelysmall. It is well known that cane sugar adsorbate gives an extremely low yield offixed carbon from pyrolysis. It was practice in this industry to adsorb such color oncharred animal bones and pyrolyze in closed retorts at approximately 900oF. Unlessthe adsorbate loading was extremely heavy, no great problem of build-up of fixedcarbon was observed though the adsorbent was used through 200 cycles ofregeneration.

This plant report can be taken as an example of how little loss may be attributable tomechanical handling and other mechanical losses in a well designed and well runplant.

It has been the author's general experience that the greater the adsorbate loadingand the greater the fixed carbon deposited from such adsorbate, the higher the


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carbon make-up requirement can be expected to be.(4)

With due allowance for plants which regenerate intermittently, regenerate at smallcarbon through-put rates and where design or operation is known to be less thanideal, this suggestion of 0.25 pound original carbon loss per pound of fixed carbonformed by pyrolysis, would in all instances given a calculated carbon make-uprequirement from non-selective gasification that would not be in obvious conflict withthe observed results.

The work of Juhola(2) indicates an average carbon volume decrease during thereaction step of about 1.8%. The total weight loss in baking and reactivation tooriginal average bulk density was about 20% and when baking was done at 1700oF

with nitrogen only about 40% of the adsorbate remained as char. This would indicatethat in relation to the 80% of the sample that was original carbon weight, the charformed by pyrolysis was about 10%. The overall 1.8% loss figure then indicates theloss of 0.18 pounds original carbon per pound char formed by pyrolysis. This is asgood agreement between laboratory experiments and the author's rough appraisal ofcommercial practice which is aimed at producing a safe allowance for maximumlosses, as might reasonably be expected.

b. Selecting Reaction Zone ConditionsIt has often been the practice to select reaction zone conditions to maximize thegasification rate so as to use the smallest possible size piece of equipment for agiven reactivation capacity. It has proven safe to use a figure of 0.6 pounds fixedcarbon gasified per hour per square foot when considering carbons of fine porestructure. There is less experience with carbons of coarse pore structure but thefigure of 1.2 pounds fixed carbon gasified per hour per square foot has been verifiedin laboratory and commercial practice as being safely achievable for such coarsepore structured carbons.

The higher rates were achieved with one commercial carbon, and over three cycles

of use the rated seemed to be diminishing. It may be a mistake to attribute the highgasification rate to pore structure. Catalytic mineral content and other causes mayexist.

These figures of 0.6 or 1.2 lbs/hr are both to be used only if one believes the fixedcarbon gasification rate should be maximize in order to minimize equipment size. It


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seems reasonable, however, that pressing the reaction rate to a maximum wouldminimize the effect of any difference in chemical reactivity between the freshly

formed char from pyrolysis and the original carbon structure.

Gasification rates are increased by increasing temperature and increasing H2O andCO2 concentration at the reacting surface of the carbon. Mizushima (5) indicates thatgraphitization of carbon begins at temperatures as low as 1650oF and takes someeffect in as little as three minutes. He prepared his starting samples, whichpresumably had no graphitization, by baking at 1300oF.

Limitation of reaction zone gas phase conditions to approximately 1700oF (perhaps1600oF material temperature) might be desirable both to preserve a difference in

chemical reactivity between freshly charred adsorbate and original carbon structure,and in case through lack of control, some charred adsorbate is left to be removed ona subsequent reactivation cycle.

It has generally been observed that high gasification rates due to high reactant gasconcentrations and high temperature tend to result in product reactivated carbon thathas an increased ratio of molasses number to iodine number. Seemingly the coarsepores are cleaned out or enlarged but the fine pores are not.

The preliminary experimental work reported at the end of this paper indicates that thelower the temperature of reaction, the greater the reaction rate of fresh char frompyrolysis in ratio to the reaction rate with original carbon structure, at least underconditions of 40% water vapor by volume.

In the report of West(1) of the University of Colorado on activation of Wyoming coal itis seen that when activating with only CO2 and nitrogen, gasification occurs in themacro pore structure (pores larger than 28 Angstrom units diameter) and not in themacro pore structure.

It is interesting to note, however, that in his comparison with the results of others,reaction in the macro pore structure seems to be reported by all those who use H2O

and nitrogen on various carbons and not by those who use CO2 without H2O.

In the work of Juhola(2) reviewing the several reactivation runs made before the useof HCl leeching, it can be observed that in very few of these runs is there anyincrease in the iodine number beyond the iodine number achieved during the bakingportion of the run. In some cases, the iodine number actually decreases during the


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activating step. The common denominator of those runs without HCl leeching that doproduce an increase in the iodine number during the reaction, seems to be a ratio of

steam to CO2 exceeding 3 to 1. Even after HCl leeching a high proportion of steamto CO2 seems to give a greater increase in iodine number between the bakedcondition and the activated condition.

All of the above is less than complete and conclusive survey of conditions but isenough to point towards two conclusions.

(1) If one feels that the adsorption process for which the reactivated carbon isintended requires the maximization of iodine number, high steam concentrationin relation to CO2 is probably desirable. If only molasses number is of interest

there may be no reason for stress on this point.

(2) Lower temperatures than the 1800oF gas temperature, which gives maximumgasification rate, may well be desirable to maximize the selectivity of thegasification reactions toward the newly formed char from pyrolysis and awayfrom the original carbon structure. The small experimental work to be reportedat the end of the paper will, however, indicate a rapid decrease in reaction ratewith decreasing temperatures and therefore the need for a compromise inorder not to require very excessive hearth area in the reaction zone.

Tentatively gasification rates in the range of two-thirds of those suggestedabove, specifically 0.4 pounds fixed carbon gasified per hour per square footfor small pore structure carbons, may be a reasonable suggestion to minimizecarbon make-up requirements due to non-selective burning.

If a 1% reduction in carbon losses could be achieved in this manner, adding50% to the hearth area, in the reaction zone only, would generally show a veryattractive financial return.


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SECTION VCALCULATION OF HEARTH AREA AND FURNACE SIZE

After review of the above factors, appropriate experimental work and decisions as to whether to stressminimum equipment size or stress flexibility of reaction conditions and minimization of loss, one isready to calculate required hearth area for each of the zones and sum them up to reach total heartharea.

Furnace calculations should be made from the bottom upward following gas flow. Where heat transferfrom the gases to the rabbled bed is to be calculated, the following similar equation is used:

q = UA (LMDT)

Where: q = heat to enter the bed - BTU/hrU = an empirical heat transfer coefficient BTU/hr - sq - oFA = nominal hearth area

(LMDT) = log mean temperature difference between gas and solids on a single hearth

The gas temperature is considered constant on a given hearth, equal to a thermo-couple reading, andequal to the temperature at which the gas will leave the hearth after giving up the heat required fortransfer to the bed and for superheating vapors and for equipment heat losses.

U has been found to be about 3 at low temperature such as 200oF - 500oF, and about 20 at 1700oF to1800oF and to vary more or less linearly with temperatures between these ranges.

As an example, an arbitrary instance has been chosen, starting with the following assumptions:

Original Carbon (dosage to adsorption) 2000 lbs/hrAdsorbate Loading 0.3 lbs/lb original carbon 600 lbs/hrChar from Adsorbate - 40% of adsorbate 240 lbs/hrGasification Conditions: Fine pore retention considered moderately important 1700oFH2O equals 3 times CO2H2O = CO2 to equal or exceed 40% by volume

Reaction rate in gas phase for above conditions (see experimental data to follow) - 0.4 lbs/hr - sq ft.

Fuel - natural gas, considered as CH4 - 1000 gross BTU/cu ftAdsorbate Heating Value - 10,000 BTU/lbFeed Moisture - 40% as received

The calculations will be shown only far enough to illustrate principals and choices, and the final results


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shown in Figure 4 with gas values and temperatures approximated. (H2O + C and CO2 reaction heataveraged with weight toward C +H2O reaction - to simplify illustrative calculations.)

A. REACTION ZONE1. Hearth Area Calculation: Hearth area required for reaction is calculated as 240 lbs/hr x

1.25 - 0.4 lbs/hr sq ft = 750 sq ft. Adsorbate char was multiplied by 1.25 factor to includeexcess carbon gasified. The expected loss of original carbon is indicated as 25% of 240lbs/hr char from adsorbate or 60 lbs/hr or 3% of the original carbon.

The calculated hearth area requirement indicates a choice of 2 hearths 232 ft insidediameter, 3 hearths 20 ft inside diameter, or 4 hearths 162 ft inside diameter. Final choiceshould be made later when the size of other zones is known. In this case 4 162 ft inside

diameter hearths were chosen.

2. Selection of Fuel, Air and Steam FlowsIt is apparent that steam and air, and probably fuel, will have to be added to each andevery hearth of the four, to allow control of the atmosphere and temperature as gasificationproceeds.

The bottom hearth of the 162 ft diameter furnace will have 200 sq ft, a larger area thanothers above it because there is only one drophole plus the area of the center shaft tosubtract from the gross area of a 162 ft circle.By gasifying 0.4 lbs/hr sq ft, we can expect 80 lbs of carbon or 6b moles of carbon to reacton this bottom hearth.

Case A: Air Plus Steam - No Fuel - Maximum Reaction Rate at 1700oF. If only air andsteam were to be added to this hearth, 80 lbs of carbon burned will release 1,126,000BTU/hr, 300,000 BTU/hr* are needed for heat losses and 394,000 BTU/hr needed for thesensible heat in the gases produced from admission of the required 12,000 SCFH of air.This leaves 432,000 BTU available to heat 12, 375 SCFH of steam, or 589 lbs/hr of 7 lbssteam/lb carbon gasified. The gases leaving the hearth would then consist of:

12,376 SCFH H2O

2,520 SCFH CO29,480 SCFH N2

24,381 SCFH Total

The water vapor is almost 6 times the CO2 content and approximately 50% of the total gasvolume.


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The actual reaction course would be for H2O and CO2 to react with the carbon, producing

H2 and CO in the gas phase. Even where gases with surplus O2 are passed over fixedcarbon beds in multiple hearth furnaces, one sees a bed blanketed by a blue CO burningflame. There is a stagnant film over the carbon bed and no O2 will be able to penetrate theCO and H2 coming from the bed to actually reach the carbon, unless air jets are focuseddirectly on the carbon.

The author has actually operated multiple hearths under the conditions described abovewith no terrible effect upon the product or the losses.

It is a matter of judgment, however, to say that at 1700oF the maximized reaction rates with

60% H2O and no CO or H2 would probably be undesirable.

Case B: Reduced Air Plus Steam - No Fuel.An alternative is to add less air and less steam, but still no fuel. At 400 lbs/hr steam, theendothermic reaction of steam and carbon will require, for 80 lbs carbon gasified per hour,384,000 BTU/hr; and 300,000 BTU/hr must be allowed for furnace heat losses. Therefore684,000 BTU/hr available at 1700oF will be required. To produce this heat 3,144 cu ft/hr ofCO and H2 must be burned and so 7,486 cu ft /hr of air must be injected to cause suchburning. Heat to raise the air to 1700oF was included in figuring 215 net BTU/cu ft CO andH2 burned.

The exhaust will consist of:

6,824 SCFH H2O1,376 SCFH H21,994 SCFH CO2

532 SCFH CO5,914 SCFH N2

16,640 SCFH Total

_______________

*This figure is quite variable depending on furnace construction and especially number of rabble arms and whether rabble arms areinsulated. The figure used throughout these simplified example calculations is for reasonably typical construction.


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Note that: CO2 x H2= 0.755

H2O CO

So K equilibrium at 1700oF (K = 0.73) has been satisfied by application of the reaction CO2+ H2 H2O + CO to any arbitrary initial assumption of how much of the air admitted reactedwith CO and how much with H2. Now the H2O is 40.4% of the gas volume and 3.4 timesthe CO2 content. Total CO2 and H2O is 53% of the gas volume which is still somewhathigh for our desired reaction rate, but still less air and steam would result in less than 3parts H2O per part CO2.

Still other choices are open. It could be decided to burn natural gas at stoichiometric fuel

air ratio.

Case C: 1320 SCFH Methane Burned with Stoichiometric Air - No Steam.Without any steam, the fuel firing rate would be set by heat balance. Needing 300,000BTU/hr for equipment heat losses, and 416,000 BTU/hr for carbon endothermic reactions,we must burn 1320 cu ft/hr of methane.

The gas entering and leaving the bottom hearth would then be:

Entering LeavingH2O 2,640 SCFH 1,000 SCFH

H2 1,640 SCFHCO2 1,320 SCFH , 510 SCFHCO , 810 SCFHN2 9,910 SCFH 9,910 SCFH

13,870 SCFH TOTAL

With the calculated result about 10% total H2O and CO2 and CO and CO2 ratio exceeding0.5 - the desired reaction rate of 0.4 lbs/hr sq ft will be certain not to occur. An estimate of0.1 to 0.2 lbs carbon gasified/hr/sq ft would be the more likely outcome.

One should remember that these calculated compositions are gas phase compositions.Some differential of concentration must exist to achieve transfer of H2O and CO2 into thebed and transfer of CO and H2 out of the bed. The H2O and CO2 concentration at thesurface of the pores of the carbon would under this circumstance be very small so thereaction rate would slow to leave more oxidant gases and produce less CO and H2.


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Case D- Fuel as Case C Plus 480 lbs/hr Steam and Excess Air.The option then remains of adding steam while still firing stoichio-metrically as in Case C or

of adding air and steam.

Adding air alone to the above would increase CO2 and H2O, reduce CO and H2 but raisethe temperature. To maintain the desired 1700oF, steam would have to be added with theair.

Adding 480 lbs/hr of steam (6 times fixed carbon gasified - a rule of thumb foundconvenient) would require that an extra 370,000 BTU/hr be developed.

Adding only air would require burning 1720 cu ft of H2 and CO. Gas composition leaving

the hearth would then be, after adjustment to make:

[CO2] [H2][CO] [H2O] = 0.73 = K equilibrium at 1700oF

H2O 12,180 SCFHH2 ,640 SCFHCO2 1,230 SCFHCO 90 SCFHN2 13,190 SCFH

27,330 SCFH Total

Now the H2O + CO2 = 50% of the total gas volume and H2O equals almost ten times CO2which comes near enough to satisfying our criteria.

Case E: 2000 SCFH Methane Burned with Stoichiometric Air Plus 480 lbs/hr Steam. Ifinstead, we added the same 480 lbs/hr of steam and burned fuel stoichiometrically toprovide the required extra 370,000 BTU/hr, the gas composition on the bottom hearthswould after burning 680 additional cu ft of gas additional to Case C become:

H2O 11,990 SCFH

H2 2,090 SCFHCO2 1,640 SCFHCO 360 SCFHN2 15,045 SCFH

31,125 SCFH TotalThis choice, burning fuel instead of just adding air has increased the total gas volume,


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lowered the concentration of oxidizing gases from 50% to 44% and lowered the H2O toCO2 ratio to 8:1 instead of 10:1.

There is no information available to guide a clean choice between the last two conditions.The cost of fuel burned instead of air may not be an important consideration since if thefuel is not required here, it may be required* in the drying zone to offset the smaller hotgas volume resulting when air only is added. Some would suggest the higher H2O toCO2 ratio and the higher oxidant concentration of example D to be preferable. Some wouldfeel that difference to be slight and prefer not to lower the CO and H2 concentrations so farbecause of the risk of adding too much air or having local areas where CO and H2 mightnot be available to react with air as it is admitted.

Lacking a good reason to choose, the designer would plan for burning of fuel and let theoperator experiment with increased air to fuel ratio at the burners. We will carry on ourexample based on Case E.

The above examples are intended to clarify the following points.

1. Steam is obviously necessary. Fuel alone fired stoichiometrically will not give theconcentrations of CO2 and H2O required for a reasonable reaction rate. Air alone willproduce a reactant gas that is all CO2 and no H2O which the literature indicates wouldproduce only external attack and macro pore structure attack but not micro pore

reactions.

2. The steam requirement is best related to the gas composition desired. That is percenttotal oxidants and H2O to CO2 ratio are good criteria for calculating steam requirement.This in turn is related to the fixed carbon being gasified. No inherent connection withlbs/hr of carbon passing through the reaction zone or pounds product/hr, is seen.

In the example to be followed just a bit further, Case E, the steam usage after another550 sq ft of hearth area on 3 more hearths, would be 1800 lbs/hr of 0.90 lbs steam/lboriginal carbon, But one can readily see that if the fixed carbon that were produced bypyrolysis were half or double the amount shown in the example, so also would the totalsteam usage recommended be halved or doubled.


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3. Burners and steam admission should be available on each hearth of the reaction zoneto provide operational flexibility and the choice of maintaining the specified temperature

and composition of each reaction hearth. If the gas composition shown above asleaving the lower reaction hearth simply passed over the next hearth above, practicallyno reaction could occur as the reactions are endothermic and the gas was alreadycooled nearly to reaction temperature. Cooling the gases 300oF would not supplyenough heat for equipment heat loses, and that would leave the gases too cold forreaction.

In the example to be carried forward, a good case could be made for redistributing thesteam and fuel and air so that the gasification rates, or even temperature would bedifferent between the last hearth of the four reaction zone hearths and the first hearth.

One might wish to slow the reaction of the last hearth, where the carbon is nearly freeof char from pyrolysis and increase the reaction rate on the first of the reaction zonehearths, expecting the original carbon structure to be shielded by char. A contrarycase for maximizing steam and temperature on the last hearth and allowing a lowersteam to CO2 ratio on the first of the reaction hearths can also be made, if only as asteam and fuel saving device. The one approach we would oppose is higher gastemperatures on the upper reaction hearth or accumulation of CO and H2 which wouldslow the reaction rate and bring the carbon closer to gas temperature.

Experimentation in this area of distribution of gas composition and temperature wouldbe useful. Unfortunately, most experimental reactions are made with constanttemperature and composition, perhaps overlooking the fact that the nature of thereaction carbon is changing as the reactivation progresses.

B. PYROLYSIS ZONECarrying forward Case E above, the gases leaving the reaction zone and entering the fifth hearthfrom the bottom are 1700oF and total 130,000 SCFH. The next hearth will have 175 sq ft ofnominal area. Since the reaction rate on the hearth below was requiring 0.4 lbs/hr - sq ft x 5200BTU/lb or 2080 BTU/sq ft and U was 20, the carbon must be heated to 100oF below gastemperature or in other words to 1600oF.

The temperature of material entering the last pyrolysis hearth will be assumed and then checkedby trial and error calculation.


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Assume 2000 lbs/hr original carbon plus 600 lbs adsorbate enter this hearth at 200oF, dry, andthat the adsorbate decomposes at an average temperature of 1000oF, requiring 250 BTU/lbs

endothermic heat of reaction, and having 0.33 specific heat, which is the same as the carbon.

The heat required to be transferred into the carbon is:

600 lbs adsorbate x 250 BTU/lb = 250,000 BTU/hr600 lbs adsorbate x 1/3 x [1400oF - 200] = 280,000 BTU/hr2,000 lbs carbon x 1/3 x [1600oF - 200] = 935,000 BTU/hr

Total 1,465,000 BTU/hr

If the hearth is fired with fuel and air to maintain 1700oF, the gas temperature will be 1700oF all

across the hearth and (LMDT) will be 445o

F. A heat transfer coefficient of 18 can be expected.From the previously given q = UA (LMDT), we can calculate a heat transfer of 1,4000,000BTU/hr which quite nicely checks the assumption of 200oF temperature of carbon entering thehearth.

The fuel fired in this case will be about 2,700 CFH of natural gas at 1000 gross BTU/cu ft.

However, if we assume no firing on this hearth, the heat supply is the entering gas volume of130,000 SCFH which gives up only 286,000 BTU/100oF reduction in temperature.

If we assume a temperature of 1450oF for the material entering, the heat required to heat this to1600oF is 2240 lbs char plus carbon 1/3 x 150 = 112,000 BTU/hr.

The gas will supply this heat plus an estimated 150,000 BTU/hr equipment loss by cooling 100oF.The gas temperature across the hearth will be 1600oF and (LMDT) as the carbon approaches

1600oF and will be 0. Actually, the carbon just won't quite heat to 1600oF - about 1575oF mightbe a reasonable guess. Instead of the above, assume the carbon is heated from 1400oF to1600oF on the first furrow or two of the reaction zone. Then we need only heat the carbon to1400oF on the last hearth of the pyrolyzed zone. Now an assumed temperature for carbon andadsorbate entering the last pyrolysis hearth of about 900oF and no other temperature assumption

will satisfy the heat transfer equation or heat balance equations.

The point here is that the pyrolysis zone can be spread over two or three hearths or all done onone hearth depending very critically on whether the last hearth of the pyrolysis zone is fired ornot.


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Assuming the last hearth of the pyrolysis zone is fired as in the above instance to maintain1700oF, by adding air to react with the products of pyrolysis, then this hearth alone is the entire

pyrolysis zone and only drying remains.

The air will have plenty of fuel from the pyrolysis to react with. The adsorbate was stated to be600 lbs/hr at 10,000 BTU/lb of 6,000,000 BTU/hr. 240 lbs of char was formed at about 14,000BTU/lb or 3,300,000 BTU. So the organic vapors released must have had a heating value of2,700,000 BTU or just enough to serve in place of the natural gas mentioned above.

C. DRYING ZONEThe gas entering the drying zone now consists of 157,000 SCFH of gas with a mean specificheat of 0.023 BTU/st cu ft and actually at 1700oF. Cooling these to 850oF, exhaust gas

temperature will release 3,000,000 BTU/hr.

The feed entered as 2600 lbs carbon and adsorbate with 40% moisture or 2/3 lbs water/lb dryfeed or 1740 lbs water.

The heat for evaporation, superheating of water vapor to 850oF, heating feed solids from 60oF to200oF plus equipment heat loss will also be about 3,000,000 BTU/hr.

No use has been made of the 2,200,000 BTU/hr from residual CO and H2 from the reaction zone.

This can be used as fuel value in any required afterburner, or a higher exhaust gas temperaturecan be allowed to shorten the drying zone.

The hearth area for drying can be calculated as previously suggested and is found to be about300 sq ft assuming 850oF exhaust. Two hearths of 175 sq ft each will provide some safety factorin the drying zone.

If the furnace in practice behaved exactly according to theory, the extra drying zone area wouldresult in the material being heated to 300oF to 500oF before entering the pyrolysis hearth, whichwould be no great change.

The points all of these calculations are intended to bring out are:

1. If the adsorbate will give no organic vapors at all, not even by steam distillation, until thecarbon is bone dry, no afterburner should be needed. Such adsorbates are seldomencountered.

2. Adding air to the top hearth of this furnace to consume residual CO and H2 could have


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raised the gas temperature to about 1300oF to 1400oF. Pyrolysis would be partly onHearth #2 and partly on Hearth #3 and reaction would begin on Hearth #3. The reaction

rate wanted would then be slightly reduced from 0.4 lbs carbon gasified/hr/sq ft. This mightavoid a separate afterburner.

3. If the feed moisture content had been 40%, 2600 lbs/hr water instead of 1740 lbs/hr, therewould have been an increased requirement for heat transfer of 963,000 BTU/hr, and onemore hearth would be needed. Virtually all of the CO and H2 left from the reaction zonewould have to be burned to satisfy heat requirements. Beyond 50% H2O in the feed, forthis example, additional fuel would be required.

Had Case A, B, or D for the reaction zone been followed, much less fuel would be used in

the reaction zone, no surplus fuel would be available at 40% moisture feed (in fact somefuel saved in the reaction zone in some cases would be needed for the drying zone) andthen any additional moisture over 40% moisture adds to the fuel bill.

If the adsorbate load were less than shown in this example, its heating value less, orunavailable, then increasing or decreasing moisture content of the feed would surelytranslate directly into increasing or decreasing fuel cost. The same is true in cases whereall the exhaust gas including moisture content must be reheated in an afterburner.

In short, whether or not decreasing feed moisture content can save fuel depends on anumber of other circumstances than the actual level of feed moisture content.

4. The relative hearth areas for drying pyrolysis and reaction change drastically withadsorbate loading and also with burner locations, steam and air entries. One can easilyenvision that if the example chosen called for 20% adsorbate loading and 20% of that tobecome fixed carbon, there would be one reaction hearth, one pyrolysis hearth and still achoice of two or three drying hearths at this furnace diameter.

In fact, a smaller furnace diameter with half as much area per hearth would have beenchosen to give two reaction hearths, still one pyrolysis hearth and perhaps five dryinghearths.


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SECTION VISOME BRIEF EXPERIMENTS REGARDING REACTION RATES

Commercial active carbon made from bituminous coal and of fine pore size, screened to 12 x 16 meshsize was pyrolyzed in covered crucibles at 1740oF for ten minutes, split and a portion spent in aconcentrated solution of Di-Nitro Toluene. The spent portion was pyrolyzed at 1200oF in a coveredcrucible and found to have gained weight from the virgin to the pyrolyzed condition so that 14% of theweight of a the spent pyrolyzed sample was char and 86% original carbon.

The samples, virgin and char laden, were exposed to H2O, N2 gas mixtures in the apparatus shown inFigure 5 at various temperatures. Sometimes the reaction boat was entirely filled with carbon, butgenerally it was filled with sand as shown and covered with a carbon layer averaging one particle deep.

Figure 6 shows the reaction rate of virgin carbon of fine pore structure made from bituminous coal,when exposed at 1600oF, with 20% H2O in the gas, using a single layer of carbon, as compared to a3/8" deep layer with the boat containing all carbon and no sand. Note that the reaction rate more orless doubles when the full bed depth is used.

Figure 7 shows the same comparison with 40% H2O and 1800oF, with carbon that was first devolatizedat 1740oF.

Unfortunately no deeper boats were available in time for experimental proof that still deeper layerswould not further increase reaction rate, but other work did indicate very little reaction in the bottomlayer, at this depth.

Figure 8 shows the results of treating the same carbon with gases containing 40% moisture at varioustemperatures, and also the gasification rates for the same carbon after being laden with char fromDi-Nitro Toluene as described above.

The units for the gasification rate scale have been multiplied by two as stated in the label for theordinate. The plot is exactly correct as shown, but this method of stating gasification rate units makesthe actual numbers on the ordinate correspond to what we would extrapolate for full depth of carbonfrom the previous Figures 6 and 7 and correspond at least reasonably with what we observe in

commercial multiple hearth furnaces.

In Figure 9 the ratio for the two gasification rate curves of Figure 8 has been plotted again temperature.

There is considerable indication that at least for Di-Nitro Toluene as an adsorbate and with only H2O


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present as a reactant, lower temperatures favor reaction with the char in preference to the originalactive carbon structure. In furnace design, however, this must be balanced against the desire to keep

the required hearth area of the reaction zone from becoming excessively large and costly.

The unfortunate part of this work is that there is no way of determining how much of the highgasification rate shown for char-laden active carbon is reaction with the char itself and how much isreaction with the original carbon structure. This lack of knowledge must be corrected in futureexperiments.

At the present stage Figures 6 through 9 are shown only as preliminary information that may usefullyindicate general trends. It would particularly be wrong to depend heavily upon the exact scale values.


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SECTION VIICONCLUSIONS

There is no simple answer to reactivation of various carbons with various adsorbates, especially sincedifferent adsorption processes are differently affected by a change in the carbon structure andadsorption properties.

Complete and conclusive laboratory studies of reactivation are generally impractical because severalcycles of carbon expenditure and regeneration would be required, probably considering several gradesor brands of carbon in parallel.

The multiple hearth furnace has considerable flexibility in operation, but to make full use of this thefurnace, as initially constructed, must be of more than the minimum size and must be equipped withburners and steam injection connections at more than the minimum number of locations.

Reactivation furnace sizing and process results to be expected during reactivation, are a function of theadsorbate, the char left by pyrolysis of the adsorbate, initial or added mineral constituents which maycatalyze or suppress reactions, the active carbon used, and the feed moisture content. Additionally,plant circumstances such as need or lack of need for retention of iodine number, scale of operation,cost of losses, and value of capital, should be taken into account during design.

Small scale reactivators can be designed from precedence without serious hazard of inability toreactivate carbon at all, especially if the carbon and the adsorption job are similar to previous wellknown cases. It is unlikely that such precedence sizing and precedence operation will give optimumresults in relation to carbon losses or quality.

Large scale installations deserve some laboratory investigation of the regeneration effects of theparticular carbon and particular adsorbate involved, and custom designed to suit the user's need forcarbon quality, his economic situation, and to allow flexibility for optimal development during multiplecycles of in-plant carbon reactivation.

There are several avenues of approach to more economical and/or more effective reactivation in

multiple hearth furnaces, particularly control of pyrolysis and maximized adsorbate removal in that stepwhile avoiding graphitization, investigation of the possible importance of high steam to CO2 ratios,investigation of reaction rate in relation to losses and per structure effects, and continued investigationof variations in the conditions under which fixed carbon gasification occurs while the carbon changesfrom "heavily char laden" to "nearly regenerated."


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REFERENCES

1. West, Ronald E., "Effect of Porous Structure on Carbon Activation," USEPA, Water PollutionControl Research Series, Project No. 17020 DDC, June, 1971.

2. Juhola, A. J., "Optimization of the Regeneration Procedure for Granular Activated Carbon,"USEPA, Water Pollution Control Research Series, Project No. 17020 DAO, July, 1970.

3. Goos, A. W. et al, "Some Experiments in Sawdust Carbonization," Forest Products ResearchSociety, March, 1948.

5. Mitzushima, Sanchi, "Rate of Graphitization of Carbon," Proc. 5th Carbon Conference, Volume 3,

Page 439.


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PROCESS ASPECTS OF REGENERATION IN A MULTIPLE HEARTH FURNACE_____________

Page 33

TABLE OF APPENDED FIGURES

Figure Page

1 - Furnace 34

2 - Bed Too Thin 35

3 - Rabbling with Proper Bed Depth 36

4 - Results of Example Process Calculation 37

5 - Apparatus (4) Tube Furnace Reaction Studies 38

6 - Gasification Rate of Carbon (Single Layer and Multi-Layer) 39

vs Time 1600oF - 20% H2O40

7 - Gasification Rate of Carbon (Single Layer and Multi-Layer)vs Time 1800oF - 40% H2O

8 - Gasification Rate of Char Laden and Virgin Devolatilized Carbon vs Temperature 41

9 - Ratio of Gasification Rate, Char Laden to Virgin Carbon vs Temperature 42

(From here tl the end, nine 9 figures must be done on AutoCAD and inserted)


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Figure 1Furnace


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Figure 2Bed Too Thin


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Figure 3Rabbling with Proper Bed Depth


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Figure 4Results of Example Process Calculation


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Figure 5Apparatus (4) Tube Furnace Reaction Studies


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Figure 6Gasifi cation Rate of Carbon (Single Layer and Multiple Layer)

Vs. Time 1600oF 20% H2O


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Figure 7Gasification Rate of Carbon (Single Layer and Multi-Layer)

Vs. Time 1800oF 40% H2O


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Figure 8Gasifi cation Rate of Char Laden and Virgin Devolatized

Carbon Vs. Temperature


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Figure 9Ratio of Gasification Rate, Char Laden to

Virgin Carbon Vs. Temperature

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Process Aspects of Regeneration

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