Drying Curves

 

 Adamson University College Of Engineering Chemical Engineering Department

Unit Operations Lab 2

Experiment No. 2 DRYING CURVES Submitted by:

Group  !riones" Neil #yan $. %aguia" Ni&&a C. #odrigue'" (ohn %aul )agleo" Gelo *angalang" (oey )odina" )a. !etina $.

Submitted to:

Engr. #u+i #udgi

(anuary ,-" 2,/

 

Experiment No. 2 DRYING CURVES AS!RAC!

0hee o+ 0h o+1e 1ect ctiv ivee of this this or& or& is to an analy aly'e 'e the the possi possi+il +ilit ity y of apply applyin ing g th thee dryin drying g cu curv rves es generali'ation methodology to the conductive3convective hot plate drying of cellulose. 0he experiments ere carried out at different heated plate temperatures and air velocities over the surface of the samples. 0his &ind of approach is very interesting +ecause it permits comparison of the results of different experiments +y reducing them to only one set" hich can +e divided into to groups4 the generali'ed drying curves and the generali'ed drying rate curves.

 

!RANS"I!!AL LE!!ER 

Octo+er /" 2,5

Engr. Al+ert Evangelista Chemical Engineering Department Adamson University Ermita" )anila

Engr. Evangelista4

6n compliance ith the fulfillment of the re7uirements on the su+1ect 8Unit Operations 9a+ 2:" the group ould li&e to present this experiment report entitled 8 DRYING CURVES: in accor acc orda danc ncee i ith th yo your ur inst instru ruct ctio ions ns.. 0he 0he main main purpos purposee of th this is exper experim imen entt re repo port rt is to determ det ermine ine heat heat flo flo rate rate through through the +are +are and lagged lagged pipes" pipes" to determ determine ine the thermal thermal conductivity of lagging material +y assuming the heat input to +e that heat flo rate through lagged lag ged pipe pipe and to determ determine ine the effici efficiency ency of insulat insulating ing materi materials als.. ;e hope that thi thiss experiment report ill meet you approval.

#espectfully 5 and @5MC (aafar and )ichalos&i ,--B. 6n most cases" experimental data could +e fitted to ithin JM up to a relative humidity of .@" and in some instances over the hole humidity range. 0o cope ith the hygroscopic +ehavior at high relati relative ve humidi humiditie tiess ith ith colloid colloidal al materi material" al" hich hich sells sells ith ith increa increasin sing g moistu moisture re content" *chuchmann et al. ,--B have recommended that Pln , P IB +e chosen as the dependent varia+le rather than y itself in the correlation. 6t is unise to extrapolate sorption correlations +eyond their tested range of relative humidities" oing to changes in hygroscopic +ehavior at extremes in this range compared ith that at intermediate values. 0he manner in hich a material dries out depends not only on its structure +ut also on its physical form. 0he drying of small ood chips is controlled essentially +y moisture? vapor transport through the +oundary layer veneers and thin slats of the same ood +y the dry fraction of the exposed surface hile the drying of +oard tim+er" +y the internal moisture?transport mechanisms ithin the tim+er itself. Early experiments on drying materials in sample trays in an air stream have noted that initially" the drying rate as almost the same as that of a free li7uid surface under the same conditions and remained relatively constant as the material dried out Qeey ,-@2B. 0his period of drying is folloed +y one in hich the drying rates fell off sharply as moisture content as reduced to the e7uili+rium value even though the drying conditions remain unchanged. 0his mar&ed difference in +ehavior has led to the division of drying into the constantratee period  rat period  and falling-rate  and falling-rate period " re resp spect ectiv ively ely.. 0he 0he R&nee R&neeRR in th thee dryin drying g cu curv rvee  +eteen these to periods is &non as the critical point . *ometimes" these periods are referred to as unhindered drying and and hindered drying " respectively" to indicate hether  the material itself plays a controlling role in restricting moisture loss. Appearance of the initial period can +e mas&ed +y the induction effects at the start of drying as a moist solid arms up or cools to a dynamic e7uili+rium temperature" hich is the 1etbu'b !emperature"  if the surface is only heated convectively. 0his surface temperature is

mainta mai ntaine ined d as long long as the surfac surfacee is suffi sufficie cientl ntly y et to ef effec fectiv tively ely satura saturate te it. An example of a drying curve is shon in -i,u -i ,ure re 3.

 

-i,ure 3# An e4amp'e o/ a dryin, %ur&e#

0he reasons for the appearance of a drying period of constant" or near?constant" drying rates are complex" particularly as it is unli&ely that there exists a film of li7uid moisture at the surface except in rare circumstances van !ra&el ,-B. 6ndeed" a constant?rate  period can +e o+served if the dimensions of the et and dry patches p atches on the surface are sufficiently small compared ith the thic&ness of o f the +oundary layer. 0he re7uirement is that moisture?vapor pressure at the surface maintain the saturation value at the mean surface temperature" and thus the rate of moisture loss over unit exposed surface 

B

is4

here pG is the partial pressure of the moisture vapor in the +ul& of the gas and p * is the value of the gas ad1acent to the moist surface. 6n drying calculations" it is more useful to use humidities ratios of the mass of moisture vapor to that of dry gasB" and the a+ove expression transforms to

here Sy is a mass mass transf transfer er coeff coeffici icient ent +ased +ased on the humidi humidity ty differ difference ence T is the humidity potential coefficient hich effectively RcorrectsR for the introduction of a linear humidity driving force Qeey ,-@B and < * and