"July, 1987.""Portions of this work were used by KAC as partial fulfillment of the requirements for the Master of Science degree at The Institute of Paper Chemistry.""This paper will be presented at the 1987 AIChE Meeting in Minneapolis, MN in August, 1987.

Clay, David Telford

Crane, Katherine A.

Scholarly Materials And Research @ Georgia Tech

THE INSTITUTE OF PAPER CHEMISTRY, APPLETON, WISCONSINIPC TECHNICAL PAPER SERIESNUMBER 245AN EMPIRICAL EQUATION OF THE VOLATILIZATION OF KRAFTBLACK LIQUOR DROPLETS DURING BURNINGKATHERINE A. CRANE AND DAVID T. CLAYJULY, 1987An Empirical Equation of the Volatilization of KraftBlack Liquor Droplets During BurningKatherine A. Crane and David T. ClayABSTRACTKraft black liquor burning phenomena were studied in aconvective single particle reactor. The droplets were suspendedfrom a microbalance in a downward flowing hot gas stream. Thelinear portion of the mass loss history during the burn mainlycorresponded to the volatiles burning stage. An empirical rateequation was determined statistically from the normalized slope ofthis linear region. Included in the equation were initial dropletsize and gas phase oxygen concentration. The mass loss rate wasclosely proportional to the initial droplet surface area. Gastemperature was not found to be statistically significant.INTRODUCTIONChemical recovery is an integral part of the kraft pulp-ing process because it regenerates the cooking chemicals. Pulpingproduces both pulp and a weak black liquor containing organics andinorganics. The pulp continues on to the papermaking process.The weak black liquor is concentrated into strong black liquorwhich is then combusted in the recovery boiler. The organic-1-fraction is oxidized for steam generation which is used millwide.The inorganic fraction is reduced to recover Na2S, a pulpingchemical, and Na2CO3, in the form of smelt. The smelt dissolvesin an aqueous stream to form green liquor which is then causti-cized; the conversion of Na2CO 3 into NaOH. The regeneratedcooking chemicals, Na2S and NaOH, then return to the digester formore pulping.Very little quantitative information is known concerningthe fate of black liquor droplets in the recovery boiler. Thestages that the droplet experiences were identified by Hupa (1985)and Grace (1985). The three main burning stages are drying, vola-tiles burning, and char burning.The drying stage is evaporation of water in the blackliquor as a result of heat transferred to the droplet. The vola-tiles burning stage consists of three major processes: volatilesevolution, volatiles combustion, and the formation of char in aninorganic matrix. A great deal of swelling and bursting occursduring the drying stage, with sustained swelling only during thevolatiles burning stage. The char is then consumed during thechar burning stage. When the carbon content is sufficiently lowthe char structure collapses, and the inorganics, in the form ofmolten smelt, coalesce. The stages overlap slightly, especiallyin the larger droplets. For this work, visual events were used to-2-define the beginning and the end of each stage. These techniqueswill be discussed later.Single droplet studies done with black liquor at TheInstitute of Paper Chemistry (IPC) (Miller 1986, Moreland 1985,and Robinson 1986) suggested several variables important to blackliquor combustion. These were droplet temperature, dropletinitial size, gas temperature, and gas composition. Research doneon single droplets (Hupa 1985) combusted each droplet in aradiantly heated stagnant air environment and measured the timesfor each stage. Hupa's times will be compared with those fromthis investigation.The present investigation had two goals. The first wasto determine an empirical equation for the linear region of thedroplet mass loss history and the second was to define whatoccurred in this region.EXPERIMENTALThe apparatus used for this work can be seen in Figure 1.It is a convective downflow single particle reactor. The gas isheated in an electrical furnace, travels through an insulatedpipe, passes through a flow straightener, and enters the singleparticle reaction area. The flow rate of the gas is adjustable.-3-There is a damper to divert the gas flow while the sample isplaced in the reactor. The sample is suspended on a wire con-nected to the electronic microbalance. The microbalance and thegas analyzers are connected to an Apple II+ microcomputer withan analog/digital interface. Heat transfer to the sample isprimarily through convection, with some radiative heat transmittedfrom the surrounding walls. A 16 mm motion picture is taken ofeach burn through the optical trench. The times for each stage ofthe burn are obtained from the movies.(Figure 1 here)At the start of each burn the kraft black liquor isweighed and wrapped around the wire. The black liquor, fromThilmany Pulp and Paper Co., Kaukauna, WI, had been concentratedto 71.8% solids under vacuum. The black liquor is shaped into anapproximate sphere. A wire coil and screen, the basket, surroundsthe sample to provide constant drag throughout the burning, as thedrop does not swell uniformly. Once the drop has been formed, thereactor is opened by lowering the lower section, the damper isclosed to divert the hot gas, the sample in the basket is hookedonto the microbalance wire, the reactor is closed, the 16 mmmotion picture camera is started, and the computer is started.The computer causes the damper to open, allowing the hot gas toreach the sample at a defined time zero. The camera runs for up-4-to 16 seconds, while the data acquisition program runs for 20seconds.The experimental design was based upon the central-composite-rotatable three-variable statistical design. Thisdesign examined the effects of reactor temperature, T, (660 to860°C), initial mass of the droplet, Mi, (4 to 41 mg), and oxygencontent in the gas stream, 02, (0 to 21%), on the rate of vola-tiles evolution. Replications were done at the center point(10.5% 02, 763°C, 22 mg) to determine the lack of fit and thereproducibility of the data. The design assumes that the lack offit is constant for all conditions.RESULTSDroplet Mass LossThe plot of the mass of the droplet over time for a typi-cal burn can be seen in Figure 2. The initial rise in the curveis due to the microbalance adjustment to the drag force on thebasket and wire assembly. Mass loss data does not reflect theburning process for approximately the first second. The slope ofthe linear region is the rate of mass loss during the volatilesburning stage, the primary rate measurement used in this work.(Figure 2 here)-5-For the purpose of data consistency the following stagetransition points were defined. Drying started the moment thedroplet was exposed to the hot gases, and ended at the firstvisual evidence of ignition. The droplet was assumed to be 100%solids at ignition, although residual moisture contents of 5 to10% are possible. Volatiles burning started at ignition and endedat the point of maximum swollen volume. Maximum volume usuallyoccurred near the end of the linear mass loss. Char burningstarted at maximum volume and ended at the final collapse of thechar structure into a smelt bead.The points of ignition, maximum volume, and smeltcoalescence are noted on the plot. The droplet ignited initiallynear the wire under all the tests except those at 0% 02. At 0% 02the particle was under pyrolysis conditions and did not ignite orcombust. After the volatiles burning stage, mass is continuouslylost as carbon is consumed. The weight regain noted prior to thesmelt bead formation is due to reoxidation of Na2S formed duringthe burning processes (Grace, 1985).Linear Mass Loss RateThe slope of the linear region SL, during the volatilesburning stage, was found by linear regression through the includedpoints. It was normalized with respect to initial liquor solids.-6-NSL = SL / (SOL * Mi) (1)where SOL is the initial solids fraction andMi is the initial droplet mass in mg.This was done so that particles of different size could bedirectly compared. The diameter, Di, was calculated from thedensity of black liquor and the mass of the droplet. The dropletwas assumed to be a perfect sphere. When data are plotted for asingle oxygen content, as in Figure 3 at 10.5% 02, the plot islinear with minimal scatter. The equation for the line isNSL = 1.04 - 0.242 Di (2)with an r2 of 0.80. A plot of SL vs. diameter was much more scat-tered. Figure 4 shows the plot of NSL against Di for all thedata. Each of the five gas compositions tested is plotted separa-tely. The scatter is partially due to the variation of tempera-ture in the gas stream, T, in which the burns were done. When allthe burns were combined the statistical equation wasNSL 1634 + 0.034 - 0.0054 02 - 0.316 (3)P1 Diwith an r2 of 0.85. 02 is the percent oxygen in the bulk gasstream. This equation was determined using stepwise linear-7-regression from a statistical software package for IBM or com-patibles. Each variable was entered in the equation depending onits F-test value. New F-test values were calculated after eachstep. After all the variables with F-test values high enough toenter the equation were entered, the program stepped backward,removing all the variables with F-test values below the F-toremove value. Only one variable was entered or removed from theequation per step. The interaction terms 02 /Di, Di*02, Di*T, and02*T, were included to test the variable interactions. Di*02,,Di*T, 02*T, and T were not statistically significant.(Figure 3 and 4 here)NSL was obtained according to equation (1) by dividingthe rate of mass loss by the initial mass, Mi. This is equivalentto dividing by the cube of the initial diameter, Di, and a con-stant. This means that in equation (3), the rate of mass loss isproportional to the square of Di. In other words, the rate ofmass loss during volatiles burning is dependent on the initialsurface area of the droplet.Figure 5 shows the comparision of model to data. Theother two levels of oxygen, 4.3% and 16.7%, behaved in a similarmanner. They were not included on the plot to improve clarity.Figure 6 shows the agreement between the statistical equation andthe data.-8-(Figure 5 and 6 here)It was not too surprising that the reactor temperaturewas not a significant variable. Hupa (1985) had found only aslight influence of reactor temperature on the volatiles burningtime. In both this work and that of Hupa the temperature of theparticle surface, the reaction zone, was not measured. Its rela-tionship to the volatiles burning rate may be a more significanttemperature to measure.TimesThe times for drying, volatiles burning, and char burningare listed in Table 1. Hupa's (1985) times are compared to theones from this work for a 23 mg droplet, 3.08 mm in diameter, com-busted in air. Hupa presented his findings in plots with thelargest initial droplet 2.0 mm in diameter. His times weregraphically extrapolated and should only be considered as an esti-mate. The difference in droplet sizes is due to the necessity ofcomparing burns under similar combustion conditions.Even though Hupa's times can only be taken as an esti-mate, they are significantly longer than the present data. Onereason for the difference in the drying times is that Hupa hadmuch more water to dry. Hupa's liquor was 60% solids and this-9-work was at 71.8% solids. Another reason for the difference isthat Hupa had a lower heat flux in the furnace than the presentwork, 76 kW/m2 compared to 87 kW/m2 based on rough calculationsand experimental verification for IPC's flux (Miller 1986).Table 1. Comparision of Movie to Hupa Times for the Combustion ofa 3.08 mm Diameter DropletTime (sec)Movie HupaDrying 1.7 3.8Volatiles Burning 2.8 4.2Char Burning 3.5 5.0Total 8.0 13.0The volatiles burning time is 50% longer. The above fac-tors are expected to influence the drying region but do notexplain the differences in the volatiles burning region. Externaldiffusion of oxygen through the bulk gas or differences betweenthe liquors may have contributed to the different times. If dif-fusion was important, then changing the velocity of the gas streamfrom stagnant (Hupa) to flowing (present) would reduce the timesfor this stage by having higher net 02 content at the particlesurface. It is not possible at this stage to comment on differ-ences between the two liquors.-10-DISCUSSIONThe present results provide the first data on mass lossrates during the volatiles burning stage for a black liquordroplet. They are, however, insufficient to support a quantitativeexplanation of the controlling phenomena. The data suggest thatthe initial droplet surface area is a key factor in the volatilesburning stage. Since the actual surface area is continuallychanging, the data do not differentiate between external or inter-nal controlling processes. Externally controlled kinetics (i.e.,through the gas temperature) had no significant effect. Exter-nally controlled mass transfer (i.e., the 02 content), or possiblythe 02 concentration effect on the reaction kinetics, did slightlyinfluence the mass loss rate during volatiles burning. Futuremeasurements of dynamic surface area and temperature should helpclarify relative rates of external and internal processes.CONCLUSIONSSeveral conclusions can be drawn from this preliminarywork. The first is that the linear region in the droplet's massloss history mainly corresponds to the volatiles burning stage.The second is that the rate of mass loss in the linear region isclosely proportional to the surface area of the initial droplet.Quantification of particular rate limiting steps for the volatiles-11-burning region will require future work. Finally, for closelysimilar reactor conditions, Hupa's volatile burning times (1985)were longer than those from this work. Differences in air flowrate past the particles appear to be an important difference be-tween these works.Portions of this work were used by (KAC) as partialfulfillment of the requirements for the Master of Science degreeat The Institute of Paper Chemistry.LITERATURE CITEDHupa, M; Solin, P. Combustion Behavior of Black Liquor Droplets.TAPPI International Chemical Recovery Conference, Book3:335-344 (1985).Grace, T.M.; Cameron, J.H.; Clay, D.T. Role of the Sulfate/SulfideCycle in Char Burning - Experimental Results and Implications.TAPPI International Chemical Recovery Conference, Book 3:371-379(1985).Miller, P.T.; Clay, D.T. Swelling of Kraft Black Liquor DuringPyrolysis. AIChE Forest Products Division, 1:152-159 (1986).Moreland, B.A.; Clay, D.T. The Influence of Water on Black LiquorCombustion. Preprints of TAPPI Pulping Conf., Book 2:389(Nov. 1985).Robinson, M. Characterization of Black Liquor Droplet Drying inAir, Steam, and Humid Air. Doctorate Thesis in Progress.Appleton, WI, The Institute of Paper Chemistry, 1986.-12-SINGLE PARTICLEREACTORGas BypassGas Out toAnalyzersFromHeater-lower unit(moveable)Note: All gas passages are 50mm x 50mmSingle Particle Reactor.Figure 1.r-.qC)CO0C)M0cfi- .-40oa*33)60o0.-'40,uCat W0I II IIV1 I.t-r-r~T-Z~Nr J i-r-r-r ' t, t & a * 5v sft ig % In^(sW) asotiNC,W-/ 4::.mL m 0r.' .. InC) cE0/ -4 ':+ / N A-+ B,I -I ~ .. . I -. I , -~ 0)'= m0..4CU/UCD^ a CD v? - _o to Of o so oN/ CDIl v u -o/* o(t-aas) adols paZ.L 2WJON(0 0o1. cCDN-+X-X <1x000 *4X4* *Uii <1.+ -@00U++0- 0ui~n a~)0,.i0, -(r) a. Om. ._ f o_ Wa~ a, -oU ccu1-0.4*r-<O 0I I I I I I IC; &b 0 tu ( C Cu CU0 0 0 0 0 oo(;-3as) adOti pazTIeWJONC0).0no0)j a + 0.n.cu0. CI mir (a twn cn cv u -o o o o 0 0 o.cA-. ,10300coo aJw4-·,- IB- So o0.-E uE T,EboUt(aI1.40(;-:ase adds paZTZewIWoNC(00\ (0*\1S =,~~* \-( ~~u N\) * '\e ',mU 0 » *-o* -\ - .0 a)*\'°1% Co P1 CU '4 00 0 0 0 0 0 0~~~~~(;-a3s) adoIs pazIUQWJON paBTpaJd.

An empirical equation of the volatilization of kraft black liquor droplets during burning

https://smartech.gatech.edu/bitstream/1853/2403/1/tps-245.pdf

An empirical equation of the volatilization of kraft black liquor droplets during burning

Abstract

Similar works

Full text

Available Versions

Scholarly Materials And Research @ Georgia Tech