Falling film evaporator

VACUUM Classified A b s t r a c t s

III - -

Vacuum

Processing

Techniques

~

IIl

Contd

Falling Film Evaporator Un,ted States A multi-stage e v a p o r a t o r is described which has been specially designed for operation at low pressures and t e m p e r a t u r e s I t is d d f e r e n t from the so-called multiple effect e v a p o r a t o r where the h o t v a p o u r s of one stage are used to h e a t t h e processed m a t e r m l in t h e n e x t I n t h e p r e s e n t design all stages h a v e a c o m m o n h e a t s u p p l y and v a p o u r e x h a u s t and u m t s of identical construction are used for each stage E a c h unit consists of a vertical t u b e chest, t h e t u b e s c o m m u m c a t m g between an u p p e r and a lower c h a m b e r The h q m d to be processed is fed rote the u p p e r c h a m b e r and is directed from there t h r o u g h s h o r t removable t u b e s to the inner walls of each e v a p o r a t o r t u b e in order to f o r m a failing film At the b o t t o m end of the t u b e chest t h e unv a p o n s e d liquor falls into a suinp, whence it is p u m p e d t h r o u g h a riser pipe to the top c h a m b e r for re-circulation t h r o u g h the e v a p o r a t o r The v a p o u r s are w i t h d r a w n t h r o u g h a d r u m fitted to t h e b o t t o m chamber, which i m p a r t s a centrifugal Inotlon to the v a p o u r s and causes entrained droplets to sweep along t h e walls of the drum, until t h e y fall into a s u m p in the d r u m The concentrate collecting in this s u m p is p u m p e d into the top c h a m b e r of the second stage unit The rate of evaporation in the first stage is higher t h a n in all s u b s e q u e n t stages T h u s t h e second unit deals w i t h a hquld of considerably higher density t h a n t h a t of the fresh liquid introduced into t h e first unit Variable speed p u m p s , valves or other devices m a y be employed for Inalntasinng u n i f o r m or providing variable operating conditions T w o e m b o d i m e n t s of the invention are shown Sornmazre. Ddtalls d ' u n d v a p o r a t e u r ~ dtages multiples de construction dlffdrente de l ' d v a p o r a t e u r coinmundm e n t connu p o u r la c o n c e n t r a t m n de hqinde a u x t e m p 6 r a t u r e s c o m p a r a t I v e m e n t basses sous vide A Laboratory-Scale Continuous V a c u u m Flash Evaporator United States T h e e v a p o r a t o r described here was specially developed for the reinoval of volatile tlavour c o n s t i t u e n t s from fruit p u t es and juices and consists essentially of a Inetering p u m p , a s t e a m jacketed stainless steel tube, ¼ inch wide and 9 It long, serving as a preheater, a s t e a m heated evaporating tube, a cyclone v a p o u r - h q i n d separator, a spiral t u b e surface condenser, an after-cooler and t w o receptacles, one for the residue and one for the distillate D r y Ice and liquid air t r a p s were fitted to the v a c u u m line After p u m p i n g d o w n t h e metering p u m p was set for the desired processing rate (up to 1 hire per nunute), s t e a m w a s Introduced Into t h e jacket of the p r e h e a t e r (1 atmosphere) and Into t h e jacket of the e v a p o r a t o r t u b e and t h e t e m p e r a t u r e of t h e v a p o u r emerging from t h e e v a p o r a t o r t u b e was checked on a t h e r m o m e t e r At a t e m p e r a t u r e of 35-40°C corresponding to 42-55 rain H g w a t e r containing t h e volatile flavours was r e m o v e d in a single-pass operation at a rate of 5 to 500/0 (based on weight of feed material) in the case of thick puree and 5 to 8 0 % in t h e case of juice A table, reproduced below, gives the essential performance d a t a of the a p p a r a t u s

Abstract No.

and References

200/Ill

J A. Cro~s & ~[alonmer Bro~ Co Inc U S Pat 2,570,211

2Ol/m

Feed Rate and .Percent Water Evaporated by Cont**zuous V a c u u m Flash Evaporator Materml -Processed

Feed Rate ml /m~n

Water Evaporated %*

Water Water Water Water Tomato Juice Tomato Juice Strawberry Puree

100 200 400 1,000 160 415 125

75 37 5 21 0 150 55 0 25 1 38 0

* Based on Weight Of Feed Material. The a m o u n t of volatile organic I n a t e n a l which could be recovered b y flash vaporlsation was assessed expertm e n t a l l y b y distflhng a solution containing 0 005% ethanol and a n o t h e r containing 0 005% phenol. These c o m p o u n d s were chosen because t h e y differ greatly from w a t e r in their b e h a v l o u r during distillation Tins concentration was chosen as it corresponds a p p r o x i m a t e l y to t h a t of t h e volatile orgamc flavours in fruits I n t h e case of t h e ethanol solution it was found t h a t an evaporation of 10% of t h e feed r e m o v e d approxamately 55o/o of t h e alcohol I n the case of the phenol solution 10% of phenol was recovered w h e n 10%o of t h e feed was evaporated I n a n o t h e r series of e x p e r i m e n t s pur~e samples obtained from several varieties of strawberries were stripped b y e v a p o r a t i n g a p p r o x i m a t e l y 4 0 % of w a t e r (based on t h e original weight of the puree) The residue was diluted w i t h distilled w a t e r to t h e original volume and stripped again E a c h distillate was t h e n concentrated m a Stedinan fractlonating column until a final distillate was obtained equal to 1/30 of t h e weight of the orlganal purde These concentrates were t h e n analysed for c a r b o n representing the essence After analysing t h e distillate from a third strapping it became clear t h a t nearly aU volatile organic substances winch could be recovered from this material had been obtained in the first t w o s t n p p m g s and t h a t over 8 0 % of the total w a s recovered in the first stripping alone Somma~re U n d v a p o r a t e u r ~ ' flash ' continu, de tallle de laboratolre est d d c n t et sa performance determlnde concernant l'dhrmnation de coinposds aromataques etc

October, 1952

Vacuum Vol. I I No. 4

AItmle b) K P Dlmiek & B Makowel Food Teehnol 5, 1951, 469 -517

431