1 6 8 J O U R N A L OF T H E A. S. S. B. T.
Personnel
O n c e the system was settled i n t o its simple r o u t i n e , it was felt that if a full time o p e r a t o r were needed, a second class station classification would be p r o p e r . However, only a few m i n u t e s of direct labor were r e q u i r e d per shift, thus, the beet e n d foreman or the e x t r a m a n p e r f o r m e d the necessary details.
W i t h the installation of the " A u t o - J e t " filters, interest in the new e q u i p m e n t r a n high. As a result, there was an almost u n n o t i c e d refresher course in filtration a m o n g the p l a n t p e r s o n n e l . T h e s e two situations created a very favorable p e r s o n n e l c o n d i t i o n . T h e new system was immediately a n d closely i n t e g r a t e d with the rest of the factory processes.
T h i s u n e x p e c t e d use of t o p caliber p e r s o n n e l was an i n t a n g i b l e , but nevertheless, positive benefit w i t h respect to station p e r f o r m a n c e . As the foremen o p e r a t e d the station i n c i d e n t a l to their o t h e r duties, there was no dirct l a b o r charge to the filters. A very tangible economic benefit is obvious.
Precoating
H y d r a u l i c a p p l i c a t i o n of a reliable filter m e d i a of asbestos a n d filter aid is p r e d i c a t e d u p o n filling the filter shell with a filtered syrup before circulating the precoat slurry. In this way, the precoat m a t e r i a l enters the filter a n d immediately deposits evenly u p o n the leaves.
For the first thick juice filters, 714 p o u n d s of grade 50 asbestos was a d d e d to a b o u t 250 gallons of previously filtered juice. T h e asbestos was most evenly dispersed by a 3 m i n u t e circulated flow t h r o u g h t h e precoat p u m p a n d directly back to the precoat tank. Simultaneously, 50 pounds of filter aid ( J M Celite 512 or Dicalite Special Speed Flow) was slurried in a b o u t 125 gallons of juice in a n o t h e r m i x i n g t a n k .
T h e asbestos slurry was t h e n circulated t h r o u g h the filter at a high flow rate for 3 m i n u t e s . T h e filter aid slurry was t h e n a d d e d to the pre coat tank a n d circulated for a n o t h e r 8 to 10 m i n u t e s , a l t h o u g h excellent clarity was o b t a i n e d in 5 to 8 m i n u t e s . T h e flow r a t e was t h e n reduced to n o r m a l , a n d filtration of melter syrups was b e g u n immediately.
For refilters, the p r o c e d u r e was identical except t h a t 10 p o u n d s of asbestos fiber were used.
Filtering
Permeability of t h e filter cake was m a i n t a i n e d by a v o i d i n g abuse of the cake. Abuse was m i n i m i z e d by c o n s i d e r a t i o n of solids conditioning, o p t i m u m flow rates, even density feed syrups, g r a d u a l pressure changes, e l i m i n a t i o n of air, a n d a c o n t i n u o u s a n d even flow.
To r e d u c e the d i s t u r b i n g influence of air passing t h r o u g h the cake, the filter shells were continuously v e n t e d . T h e melters, p u m p s , and re- circulation systems were modified, w h e r e possible, to r e d u c e entrapment of air in the filter feed slurries.
R e c i r c u l a t i o n of the filtered juice (1, 2) p r o v i d e d an even, continuous flow, despite irregularities in materials e n t e r i n g t h e h i g h r a w melter. Sudden pressure changes were thus avoided.
D u r i n g t h e first few weeks, a flow recorder controller aided in evah»a' tion of the filters a n d confirmation of desirable or o p t i m u m flow *ateS
F i l t e r i n g times were a b o u t 24 h o u r s for the first filters a n d 80 to 140 hours
VOL. IX. No. 2, J U L Y 1956 169 for refilters. Flow rates of 7 to 8 gph sq. ft. were about optimum. Flow rates as high as 12.5 are possible, but cycle times are seriously shortened and filtrate quality is less assured.
With lespect to avoiding excessive flow rates, maintenance of full filter- ing area was a temporary problem. T h e longer cycle times for rcfilters permitted time for some organic type scale formation. 1 his stale is common to all t>pes of filters operating on high density melter syrups. For example.
this is indicated on plate and frame filters by the loss of softness and pliabilitv of the textile media and by stale accumulations on the plate fates a n d in the drainage channels.
With the "Auto-Jet" filters, a slow but progressive blinding (due to scaling) was noted on the refilter screens. Fortunately, however, a 1-hour hot caustit recirculation through the filter removed the scale and restored the lull filtering area of the screens. Present experience indicates that a caustic leaning at about 1-week intervals would assure a high level of filter efficient y.
For evaporation economy reasons, high density syrups are desirable.
Poiseuille's basic liberation equation warns us, however, of the adverse effects of viscositv upon flow rates. As should be expected, high densities present problems with any filter.
Although a filtering temperature of 98° C. aided in depressing viscosity.
there was difficulty in filtering syrups with a lirix much above 65° (hot) . At the relatively high-flow rates used (7 8 g p h / sq. ft.) the lirix of the syrups averaged 71.5 (corrected) for the two campaigns.
II much further improvement in syrup density is to be made, it must come primarily from measurement and control of viscosity. Some test work at Alvarado has indicated promise in this respect.
Solids conditioning of the syrups was, of course, continuously reviewed.
Filter aid addition to the inciters was readjusted, as necessary to maintain the cake porosity, d e p e n d i n g u p o n the beets and the carbon addition.
Facilities were provided to add the carbon and filter aid to the high raw melter in a regulated and continuous manner. Filter aid to the refilter supply was also regulated.
Sluicing
A thorough cleaning of the leaves was assured with 3 minutes of sluic- ing. Only 1,000 to 1,100 gallons of thin juice were used as the sluice fluid.
T h e sluiced cake was drained to the carbonation Oliver filter mud tank.
-_ This system caused no dilution to the beet end and no appreciable disturb ance to the Oliver filters.
Following the campaign, the leaves were all examined. T h e first filter . screens were discolored but not blinded. T h e refilters had a small per- centage of scale blinded area. A caustic treatment, a five-minute, dilute*
. muriatic acid rinse, and a final soda neutralizing rinse removed all organic:
and carbonate scale. T h e screens were then as bright and clean as when initially installed.
References
(1) C o s s a i r t , G. W. 1955. Self sluicing unit points way to pushbutton filtra- tion. Food Eng. 27:59-61.
(2)COSSAIRT, G. W. 1953. Observations on leaf type filters, Calif. Field Crops T e c h . Conf., Opr. Dept. Session. 69-118.