THE VAPOR DRYING PROCESS. Robert D. Graham Wood Technologist Oregon Forest Products Laboratory Corvallis, Oregon

Transcription

1 THE VAPOR DRYING PROCESS by Robert D. Graham Wood Technologst Oregon Forest Products Laboratory Corvalls Oregon Presented at the Annual Meetng of the Western Dry Kln Clubs Corvalls Oregon May

2 THE VAP0. DRYING PIZOCESS INTRODUCTION Vapor dryng s essentally the same as kln dryng except that organc vapors are employed n the place of steam. At present no attempt s made to control the equlbrum mosture content whch s very nearly zero throughout the process The basc dryng problems stll preval although they may be altered to some extent when hgh temperatures (over F.) are used; the effect of hgh temperatures on wood has not been fully nvestgated. DEVSLOFMENT OF THE P DCESS Attempts were made to use organc vapors for dryng wood as early as 870 but they faled because of mperfect technque. Smlar attempts were made n 90 usng low bolng pont dryng agents such as ether and carbon dsulphde; apparently no commercal applcaton resulted. In 937 a vapor process was employed for the removal of ptch. The ntal work on the present process was begun by Mr. Mone Hudson n the late 930's and resulted n the ssuance of a number of patents. The frst commercal nstallaton was completed n 945 at the plant of the Taylor-Colqutt Company Spartanburg S. C. At present there are four commercal plants n operaton and another s under constructon. Two plants one n Australa and the other n New Zealand are used exclusvely for the dryng of lumber. The remanng two operatng plants whch are n the Unted States are used for dryng wood pror to preservatve treatment.

3 2 DE7)CRIPTION OF PROCESS AND EQUIPMENT The equpment and the general flow dagram for the process are shown n Fgure. The organc lqud s boled n the evaporator and the vapors pass nto the cylnders where a porton of the vapors condense on and n the wood. As the organc lqud contnues to condense the temperature of the wood s rased above the bolng pont of the water whch vaporzes and passes out of the dryn- chamber wth the surplus of organc vapors. The mxture of water and organc vapor s condensed and the lquds pass to the separator. The mmscble and lower specfc gravty organc lqud floats on the water and drans contnuously to the condensate return tank from whch t s pumped to the evaporator for re-use. The amount of water removed s measured whch wth the volume of the charge and ts ntal mosture content permts the operator to determne when the materal has reached the desred average mosture content. At the completon of the dryng perod a vac= s used to recover the organc lqud and then the materal may be steamed n an attempt to releve stresses. The temperature employed n the process can be vared ether by selectng an organc lqud havng the desred bolng pont or by the use of a vacuum whch reduces the bolng pont of both the organc lqud and water. The temperature of vapors leavng the dryng chambers (effluent vapor temperature) controls the steam supply to the evaporator. APPLICATION OF THE PROCESS The vapor dryng process vas developed orgnally for condtonng (partally dryng) materal pror to treatment wth a wood preservatve and t s stll used largely for ths purpose. Consderable success has

4 Temperature control bulb WATER out n -----r-rwfing CHAMBER AND CHARGE organc- vapor Fgure. Flow Dagram of Vapor Dryng Process

5 Ce C.) CD cf)! o Fnal Modture Content n Fer Cent C72.3 o r 220 o o o 270 F :. S : ; r r 6 r \ \ \ \ # \ F...- I!! \ I N I I "."'.' 2 Y5( f 3. s s. S N! t. - q2nh a.4_ I j t I.. Cntlng Range degrees F r O 230 =-5n c..0 0 I I s 3 t J t...m.l... \ \..\IC"... 3 \ 3' ' ---- \. '... a --- f a \ t ese rchlore...z.-...:-.--y-ina...q h.lla a_... Ia I I ' r \ I \ ' I ' Dryng Tme n Hours 250 Fgure 2 Relaton-shp between temperature end tme requred to vapor dry 3 x C nch southern pne roof plank from an ntal mosture content of 00 per cent wth varous dryng agents. (Adapted from Dryng Lmber by the V:;spor Process by Yone Se Hudson 950 ProcUnzs of "2) tmcrca. d-preservers3 Assocaton)

6 3 been acheved n dryng crosstes partcularly oak and gum whch present dffcult ar seasonng problems and t appears that thecheckng pattern attaned durng dryng wll prevent the development of large checks and splts after the tes are placed n servce. The process can be used for dryng treatng and redryng lumber that s treated wth water-borne wood preservatves. Also a solvent recovery process has been developed n whch the materal s dred wth organc vapros treated wth a soluton of a preservatve n ths same organc lqud and the organc lqud s recovered leavng the preservatve n wood. Ths development should prove partcularly useful n the preservatve treatment of wood pror to lamnaton where a dry nonbleedng surface s essental to a good glue board. The dryng of lumber to a specfed mosture content s a feld n whch ths process may prove commercally feasble. VAPOR DRYING LUA3 Organc lquds havng bolng ponts n excess of 300 F. have been used successfully for dryng 3 x 8 nch southern yellow pne lumber. The tme-temperature relatonshp for dryng ths materal based on three organc lquds s shown n Fgure 2. Unpublshed data developed through cooperaton of the Taylor- Colqutt Company and the Oregon Forest Products Laboratory usng expermental equpment (dryng cylnder of 2 nches T.D. by 0 feet) ndcate that () -nch ponderosa pne lumber can be dred from an average ntal mosture content of 50 per cent to 0 per cent n 8 hours when an alter. nate heatng (260 F.) and vacuum cycle s used; (2) -nch Douglas- r

7 lumber can be dred from an average ntal mosture content of I.7 per cent to 0 per cent n 6 hours usng a contnuous cycle at 260 F.; (3) 2-nch Douglas-fr lumber can be dred from an average ntal mosture content of 47 per cent to 2 per cent n 2 hours wth a contnuous cycle usng a vacuum of 7 nches of mercury. Although no dffculty was encountered n dryng -nch Douglasfr at 260 F. consderable honeycomb developed when 2-nch materal was dred at ths same temperature. The use of a vacuum of 7 nches of mercury whch reduced the temperature to 235 F $ apparently elmnated ths nternal checkng. The shrnkage of the materal durng dryng has been normal whch n vew of the hgh temperatures used and an equlbrum mosture content of vrtually zero would appear to favor the development of seasonng defects. Snce such defects do not develop when the proper dryng cycle s employed the theory has been advanced by Mr. H dson that "the duraton of stress appled to ;rven lamnae s so short because of rapd dryng that the strength-tme lmt of resstance of the wood to falure s not exceeded." The expanson of the wood at these temperatures also may have a sgnfcant effect on the ntensty of the stresses developed. The present practce s to steam the charge after dryng and recovery of the organc lqud n an attempt to releve stresses. In some nstances steamng has been successful but more expermental work must be done wth Douglas-fr to develop a satsfactory steamng cycle A more successful approach to ths problem of relevng stresses appears to be through control of the water vapor - organc vapor mxture n the dryng chamber. Ths mght be accomplshed by recrculatng the vapors wthn the chamber at a controlled temperature and pressure and f neces-

8 sary by addng water to the system n order to establsh equlbrum mosture content condtons whch appear to be theoretcally possble at these hgher temperatures. A study now s beng made to determne what equlbrum mosture content condtons f any do exst. RDG: fh 5-8-5