tray dryer experiment

ABSTRACT

I n th is exper iment , we use t ray d ryers to d ry the wet sample .

We choose the tea herbs as our sample because we be l ieve tha t tea

herbs requ i res shor t d ry ing t ime, and the mo is tu re con ten t w i l l

dec rease fas te r w i th t ime. The measurement o f the mo is tu re con ten t

i s based on the we igh t o f the sample wh ich inverse ly p ropor t iona l

w i th the tempera tu re & t ime. Thus , we can de te rmine the d ry ing ra te

o f the sample . Based on the da ta f rom the exper iment , the mass and

mo is tu re con ten t i s decreas ing w i th t ime, whereas the d ry ing ra te i s

va ry ing rap id ly w i th t ime.

INTRODUCTION

Dry ing i s the p rocess o f the rma l l y remov ing vo la t i l e subs tances

(e .g . , mo is tu re ) to y ie ld a so l id p roduc t . Mechan ica l methods fo r

separa t ing a l i qu id f rom a so l id a re no t cons idered in therma l d ry ing .

When a wet so l id i s sub jec ted to therma l d ry ing , two p rocesses occur

s imu l taneous ly ; f i r s t i s the t rans fe r o f energy , mos t l y as hea t f rom

the sur round ing env i ronment to evapora te the sur face mo is tu re , and

second i s the t rans fe r o f in te rna l mo is tu re to the sur face o f the so l id

and i t s subsequent evapora t ion due to the f i r s t p rocess .

Severa l t ypes o f d ryers a re used in d i f fe ren t f i e lds and

spec ia l t i es . The common type o f d ryer in many indus t r ia l

app l i ca t ions tha t invo lve d ry ing opera t ion i s t ray d ryers (o r cab ine t

dryers ) . The d ryers a re made o f t rays he ld in a cab ine t , wh ich i s

connec ted to a source o f hea ted a i r by gas , d iese l o r b iomass . The

food , wh ich i s to be d r ied , i s sp read ou t , genera l l y qu i te th in ly , on

t rays wh ich the d ry ing takes p lace . Hea t ing may be by an a i r cu r ren t

sweep ing ac ross the t rays , by conduc t ion f rom hea ted t rays o r

hea ted she lves on wh ich the t rays l i e , o r by rad ia t ion f rom hea ted

sur faces . Mos t t ray d ryers a re hea ted by a i r , wh ich a lso removes the

mo is t vapours . The tempera tu re o f amb ien t a i r i s con t ro l led by

thermos ta t wh ich i s usua l l y se t in the range o f 50 to 700 °C. The a i r

en te rs the bo t tom o f the chamber be low the t rays and then r i ses ,

th rough the t rays o f food be ing d r ied , and ex i t s f rom an open ing in

the top o f the chamber . F resh a i r i s a l lowed to en te r the cab ine t and

mo is t a i r i s exhaus ted by us ing a dehumid is ta t to con t ro l an exhaus t

fan and a i r i n take shu t te rs . The d ryer t rays a re t igh t - f i t t i ng in the

cab ine t to p reven t a i r f rom bypass ing the mate r ia l to be d r ied .

Depend ing upon the food and the des i red f ina l mo is tu re , d ry ing t ime

may be o f the o rder o f 10 o r even 20 hours . T ray d ryers requ i re low

cap i ta l and ma in tenance cos ts bu t have re la t i ve ly poor con t ro l and

p roduces more var iab le p roduc t qua l i t y .

However , i n rec i rcu la t ion des igns , the mo is tu re laden a i r , a f te r

evapora t ing wate r f rom food , wou ld have to be d r ied be fo re be ing

undergo rec i rcu la t ion , o r e lse i t wou ld soon become sa tu ra ted and

fu r ther d ry ing o f the food wou ld s top . In such a case pass ing i t

th rough a des iccan t such as a bed o f s i l i ca ge l , o r condens ing

mo is tu re ou t by pass ing the mo is t a i r over co ld p la tes o r co i l s cou ld

d ry the a i r . No te a lso tha t i f we a re no t go ing to d ry the exhaus t a i r

fo r rec i rcu la t ion , then the exhaus t ven t shou ld no t be c lose to the

f resh a i r i n take a rea , o therw ise the mo is t exhaus t a i r w i l l be d rawn

back th rough the d r ie r and d ry ing e f f i c iency w i l l be los t .

Thermometers a re ins ta l led w i th the sens i t i ve e lements d i rec t l y in

t he ma in a i r cu r ren t approach ing the d ry ing t rays , and o f ten a lso in

the a i r cu r ren t leav ing t rays .

One o f the mos t impor tan t p rob lems encounte red i s no t to supp ly the

same dry ing ra te a t the a l l pos i t i on w i th in the t ray d ryers . The o ther

i s the fas t d ry ing o f food in the pos i t ion where the a i r f i r s t en te rs to

the sys tem and the s low d ry ing o f food in the o ther pos i t ion .

There fo re , add i t iona l hea te rs and fans may be p laced above o r

a longs ide the t rays to inc rease the ra te o f d ry ing . T ray d ryers a re

used fo r sma l l - sca le p roduc t ion o r fo r p i lo t -sca le work .

OBJECTIVES

The purpose o f th i s exper iment i s to de te rmine the d ry ing ra te

o f the sample by d ry ing method us ing t ray d ryer bes ides lea rn to use

and opera te the t ray d ryer .

THEORY

Dry ing occurs w i th the he lp o f evapora t ion by supp ly ing hea t to

the wet feeds tock . There a re two op t ions fo r med ium o f hea t t rans fe r

tha t i s th rough convec t ion by d i rec t d r ie rs o r conduc t ion by con tac t

o r ind i rec t d r ie rs . The remova l o f wa te r f rom dry ing i s to inh ib i t the

g rowth o f m ic roorgan isms and there fo re the food i s we l l p reserved

bes ides reduc ing the we igh t and bu lk o f food fo r cheaper t ranspor t

and s to rage . When car r ied ou t co r rec t l y , any a l te ra t ion o f nu t r i t i ona l

qua l i t y , co lour , f l avour and tex tu re o f rehydra ted foods w i l l no t be

a f fec ted too much .

Dry ing methods and p rocesses can be c lass i f i ed as ba tch ,

where the mate r ia l i s inser ted in to the d ry ing equ ipment and d ry ing

p roceeds fo r a g iven per iod o f t ime, o r as con t inuous , where the

mate r ia l i s con t inuous ly added to the d ryer and d r ied mate r ia l

con t inuous ly removed. Dry ing p rocesses can a lso be ca tegor ized

accord ing to the phys ica l cond i t ions used to add hea t and remove

wate r vapour . In the f i r s t ca tegory , hea t i s added by d i rec t con tac t

w i th hea ted a i r a t a tmospher ic p ressure , and the wate r vapour

fo rmed i s removed by the a i r . In vacuum dry ing , the evapora t ion o f

wa te r p roceeds more rap id ly a t low p ressures , and the hea t i s added

ind i rec t l y by con tac t w i th a meta l wa l l o r by rad ia t ion ( low

tempera tu res can a lso be used under vacuum fo r ce r ta in mate r ia ls

tha t may deco lour i ze o r decompose a t h igher tempera tu res ) . In

f reeze d ry ing , wa te r i s sub l imed f rom the f rozen mater ia l .

The f i r s t cons idera t ion in se lec t ing a d ryer i s i t s operab i l i t y ;

above a l l e l se , the equ ipment mus t p roduce the des i red p roduc t in

the fo rm a t the des i red ra te . The qua l i t y requ i red in a f in i shed

p roduc t , and i t s necessary phys ica l charac te r i s t i cs , a re de te rmined

by i t s end use . The f ina l cho ice i s then made on the bas is o f cap i ta l

and opera t ing cos ts . A t ten t ion mus t be pa id , however , to the cos ts o f

the en t i re i so la t ion sys tem, no t jus t the d ry ing un i t a lone .

Therma l l y sens i t i ve mate r ia ls mus t be d r ied a t low tempera tu re

under vacuum, w i th a low- tempera tu re hea t ing med ium, o r ve ry

rap id ly . The d ryer mus t a lso opera te re l iab ly , sa fe ly , and

economica l l y . Opera t ion and ma in tenance cos ts mus t be excess ive ;

po l lu t ion mus t be con t ro l led ; energy consumpt ion mus t be min im ised .

As fa r as the d ry ing opera t ion i t se l f i s concerned , ad iaba t i c d ryers

are genera l l y less expens ive than non-ad iaba t i c d ryers , in sp i te o f

the lower therma l e f f i c iency o f ad iaba t i c un i t s .

Un fo r tuna te ly there i s usua l l y a lo t o f dus t ca r ry over f rom ad iaba t i c

d ryers , and these en t ra ined par t i c les mus t be removed a lmos t

quan t i ta t i ve ly f rom the d ry ing gas .

In herbs and med ic ina l i ndus t ry , mos t med ic ina l and cu l ina ry

herbs a re so ld d r ied . When d ry ing la rge quan t i t i es o f herbs fo r

commerc ia l sa le , g rowers shou ld use a fo rced-a i r d ryer to p reserve

the i r co lo r , f l avour , o i l con ten t , and med ic ina l p roper t ies . To ach ieve

th is , good a i r c i r cu la t ion w i th in the d ryer i s impor tan t , as i t reduces

d ry ing t ime and a l lows the use o f lower tempera tu res , bo th o f wh ich

can p reven t the degrada t ion o f chemica l cons t i tuen ts dur ing the

d ry ing p rocess . There- fo re , dependab le tempera tu re con t ro l and the

ab i l i t y to p rov ide h igh a i r f l ow th roughout the p lan t mate r ia l a re

impor tan t . Dry ing tempera tu res and t imes d i f fe r by p lan ts , and p lan t

par ts , thus mos t herbs shou ld be d r ied a t low tempera tu res , idea l l y

a round 90-100°F . When ou ts ide cond i t ions a re ho t and humid ,

however , i t may be necessary to ra ise tempera tu res to as h igh as

130°F . The s ize o f the d ryer shou ld be based on the herb p roduc t ion

a rea and how much w i l l be harves ted a t one t ime.

The d ryer shou ld have we l l - spaced racks to ensure tha t a l l

s ides o f the p lan t rece ive su f f i c ien t a i r f l ow and the p lan t mate r ia l

d r ies even ly . The she lves shou ld be cons t ruc ted o f food g rade

sc reens o r covered w i th an accep tab le food g rade mater ia l . The

d ryer shou ld be f ree o f dus t , d i r t , i nsec ts , and roden ts . Ev idence o f

any o f these in the f ina l p roduc t can lower i t s va lue o r render i t

unmarke tab le .

The d ryer shou ld be ab le to ho ld an en t i re harves t and d ry i t

fas t enough tha t the c rop does no t spo i l . E f f i c iency and economic

concerns mus t be taken in to cons idera t ion to ensure the cos t -

e f fec t i veness o f i t s opera t ion . L ikew ise , a d ryer can be cons t ruc ted

f rom a new or ex is t ing shed ou t f i t ted w i th a hea te r , fan , and

dehumid i f ie r . Add i t iona l l y , a g rower may choose to cons t ruc t one

f rom sc ra tch .

In th is exper iment , tea leaves a re chosen as herbs tha t undergo

d ry ing p rocess by opera t ing t ray d ryer . The bes t method to d ry herb

w i l l depend la rge ly on the phys ica l charac te r i s t i cs o f the mate r ia l ,

the vo lume o f herb in the d ryer , and the re la t i ve humid i t y o f the

ou ts ide a i r .

D i f fe ren t herbs w i l l requ i re d i f fe ren t tempera tu res . Typ ica l l y ,

the h igher the amb ien t humid i t y , the h igher the tempera tu re tha t i s

requ i red to d ry the mate r ia l tho rough ly . In t roduc ing d r ie r a i r w i l l

reduce the amount o f t ime the mate r ia l i s in the d ryer , thus reduc ing

the cos t o f d ry ing the mate r ia l .

The mo is tu re con ten t o f d r ied p roduc ts i s ve ry impor tan t , and i f

i t i s too h igh , mou lds and yeas ts tend to g row. The mo is tu re con ten t

may be checked us ing sca les and an oven . The fo l low ing i s the

express ion tha t we may use in o rder to ca lcu la te the mo is tu re

con ten t o f our d ry ing tea leaves ;

Other impor tan t parameter fo r d ry ing p rocess i s the d ry ing ra tes , v

(m in - 1 ) ;

v = X t 1 – X t 2

T 2 – T 1

EXPERIMENTAL PROCEDURE

1. A l l o f the equ ipment wh ich used in th is exper iment were

sw i tched on .

2 . R inse the empty t ray us ing wate r be fo re s ta r t

3 . The mass o f the empty t ray were measured and ta red . The da ta

was recorded .

4 . The d ry tea herbs were spr ink led w i th enough o f wa te r to make i t

we t .

5 . The tea herbs were p laced on to the empty t ray and pu t in to the

t ray d ryers to be p repared to be d r ied .

6 . The in i t i a l mass o f the wet tea herbs were measured and

recorded be fo re be ing d r ied .

7 . The in i t i a l tempera tu re and humid i t y wh ich shown on the t ray

d ryers taken and recorded . The in i t i a l a i r f l ow ins ide the t ray

d ryers were measured us ing the anemometer and the read ings

was recorded .

8 . The d ry ing p rocess i s s ta r ted by p ress ing the ‘HEAT’ bu t ton on

the t ray d ryers .

9 . A l l o f the da ta readed and recorded every 3 m inu tes in one hour

t ime o f the d ry ing p rocess o f the tea herbs .

10 . The mo is tu re con ten t and the d ry ing ra te were ca lcu la ted .

11 . The g raph o f d ry ing ra te aga ins t mo is tu re con ten t was p lo t ted .

APPARATUS

Tray dryers

Analytical balance

Water

Anemometer

Stopwatch

Dry tea herbs

RESULT

Mass o f t ray = 1391 .3 g

Mass o f d ry tea leaves = 99 .4 g

T i m e

( m i n )

H u m i d i t y

( % r F )

T e m p e r a t u r e s

( ° C )

M a s s o f

s a m p l e ( g )

A n e n o -

m e t e r

( m / s )

M o i s t u r e

C o n t e n t

D r y i n g

r a t e

( m i n - 1 )I I I I I I

0 10 .6 0 .5 41 .9 39 .5 109 .4 1 .47 0 .1198 0 .0084

5 8 .4 0 .5 45 .3 43 .0 105 .3 1 .21 0 .0778 0 .01004

10 8 .0 0 .5 45 .8 44 .3 100 .4 1 .51 0 .0276 0 .00326

15 8 .0 0 .5 45 .7 44 .6 98 .8 1 .49 0 .0113 0 .00104

20 8 .0 0 .5 45 .6 44 .7 98 .3 1 .49 0 .0061 0 .001

23 8 .0 0 .5 45 .6 44 .8 98 .0 1 .52 0 .0031 0 .00037

26 8 .0 0 .5 45 .7 44 .7 97 .9 1 .52 0 .002 0 .00067

29 8 .0 0 .5 45 .6 44 .7 97 .7 1 .51 0 0

0 5 10 15 20 25 30 35

90

95

100

105

110

115Mass versus Time

0 5 10 15 20 25 30 350

0.020.040.060.080.1

0.120.14

Moisture Content versus Time

0 5 10 15 20 25 30 350

0.0020.0040.0060.0080.01

0.012Drying Rate (1/min) versus Time (min)

SAMPLE CALCULATIONS

1. Mo is tu re con ten t ,x ca lcu la ted us ing the fo l low ing fo rmu la :

Mo is tu re con ten t ,x = mass1−massfinal

mass final

Example :

Mo is tu re Conten t 1 , x 1 = 109.4−97.7

97.7

= 0.11975

Mo is tu re Conten t 2 , x 2 = 105.3−97.7

97.7

= 0.07779

2. A f te r ca lcu la ted the mo is tu re con ten t , the d ry ing ra te ca lcu la ted

us ing the fo l low ing fo rmu la :

Dry ing Rate , v = Δ xΔ t

Example : For tea herbs sample :

Dry ing Rate , v = x1− x2

∆ t

= 0.11975−0.07779

5

= 0 .008392

DISCUSSION

Based on g raph mass versus t ime , i t shows tha t the mass o f the

tea herbs a re decreas ing w i th t ime. Th is i s the same w i th g raph o f

mois tu re con ten t ve rsus t ime , where the mo is tu re con ten t i s

decreas ing w i th t ime.

Dry ing occurs in th ree d i f fe ren t per iods , o r phases , wh ich can

be c lear l y de f ined . The f i r s t phase , o r in i t i a l pe r iod , i s where

sens ib le hea t i s t rans fe r red to the p roduc t and the con ta ined

mo is tu re . F rom bo th g raph , the f i r s t phase can be seen c lear l y a t

t ime be tween the f i r s t m inu te un t i l m inu te 9 . The second phase , o r

cons tan t ra te per iod , i s when the f ree mo is tu re pers is ts on the

sur faces and the ra te o f evapora t ion a l te rs ve ry l i t t l e as the mo is tu re

con ten t reduces . Th is i s seen f rom the g raph a t m inu te 9 un t i l m inu te

12 . The th i rd phase , o r fa l l i ng ra te per iod , i s the phase dur ing wh ich

mig ra t ion o f mo is tu re f rom the inner in te rs t i ces o f each par t i c le to

the ou te r su r face becomes the l im i t ing fac to r tha t reduces the d ry ing

ra te .

P r inc ip les o f t ray d ryer s ta tes tha t dur ing the ear l y s tages o f

d ry ing , the cond i t ions in the d ryer , wh ich i s a t h igh humid i t y and

modera te tempera tu re , a re idea l fo r the g rowth o f m ic roorgan isms.

The qu icker the d ry ing t ime the be t te r the f ina l m ic rob ia l qua l i t y o f

the p roduc t . F rom the g raph o f dry ing ra te versus t ime , we can see

tha t the chang ing pa t te rn i s no t cons is ten t o r in mannered pa t te rn .

Hence , i t i s we l l sa id tha t the d ry ing ra tes i s no t depend ing much

upon t ime.

CONCLUSION

Based on the da ta p lo t ted on each g raph , we can conc lude tha t

the mo is tu re con ten t and mass o f the tea herbs a re bo th decreas ing

w i th t ime. Never the less , the d ry ing ra tes o f the herbs a re vary ing

w i th t ime.

RECOMMENDATION

I n o rder to ob ta in be t te r resu l t s , the re a re a few methods o r

recommendat ions tha t may be cons idered . F i rs t l y , the d ry ing ra tes

can be inc reased w i th two d i f fe ren t op t ions ; by inc reas ing the a i r

f l ow o r inc reas ing the a i r tempera tu re . However , the tempera tu re

mus t no t be se t to too h igh fo r i t may cause damage o r change to the

p roduc ts . Th is i s t rue fo r herbs and sp ices , as h igh tempera tu re w i l l

have the r i sk o f los ing de l i ca te f lavours o r co lours .

As a mat te r o f fac t , fan d r i ven d ryers may suck in f i ne dus t

par t i c les in dus ty a reas , caus ing con tamina t ion o f the p roduc t . In

ve ry dus ty a reas , powered d ryers may need a mus l in f i l t e r over the

a i r i n le t . Herbs and sp ices mus t be d r ied to a mo is tu re con ten t tha t

i s low enough to p reven t the g rowth o f m ic ro -o rgan isms such as

mou lds and bac te r ia .

Mos t commerc ia l d ryers a re insu la ted to reduce hea t losses ,

and they rec i rcu la te ho t a i r to save energy . Many des igns have

energy-sav ing dev ices , wh ich recover hea t f rom the exhaus t a i r o r

au tomat ica l l y con t ro l the a i r humid i t y . Computer con t ro l o f d ryers i s

inc reas ing ly soph is t i ca ted and a lso resu l t s in impor tan t sav ings in

energy .

I f the exhaus t a i r i s no t d r ied fo r rec i rcu la t ion , then the exhaus t

ven t shou ld no t be c lose to the f resh a i r i n take a rea , o therw ise the

mois t exhaus t a i r w i l l be d rawn back th rough the d r ie r and d ry ing

e f f i c iency w i l l be los t .

Good a i r c i r cu la t ion w i th in the d ryer i s impor tan t , as i t reduces

d ry ing t ime and a l lows the use o f lower tempera tu res , bo th o f wh ich

can p reven t the degrada t ion o f chemica l cons t i tuen ts dur ing the

d ry ing p rocess . The d ryer shou ld have we l l - spaced racks to ensure

tha t a l l s ides o f the p lan t rece ive su f f i c ien t a i r f l ow and the p lan t

mate r ia l d r ies even ly . The d ryer shou ld be f ree o f dus t , d i r t , i nsec ts ,

and roden ts . Ev idence o f any o f these in the f ina l p roduc t can lower

i t s va lue o r render i t unmarke tab le .

REFERENCES

1 . ht tp : / /www. fao .o rg /docrep /V5380E/V5380E09.h tm

2 . ht tp : / /www.eno tes .com/how-produc ts -encyc loped ia /g reen- tea

3 . B i rd ,R .B ,S teward .WE,and L igh t foo t ,EN, T ranspor t Phenomena,

John Wi ley and Sons , Inc ,1960

APPENDICES