Refini Ref ining ng an and d Degu mm ing Systems for Edib Edible le Fa ts an
Oils
RO Y A. CAR R, Hunt-Wesson Foods, Inc., Inc., 1645 West Valenc Valencia ia Drive, Drive, Fullerton, Ca lifornia 92634
Composition
ABSTRACT
Crude edible oils consist primarily of triglycerides, or triacyl glycerols, which are esters resulting from the union of o ne unit of glycerine glycerine with with three uni ts of fatty acids. acids. This is the por tion of the crude oil oil,, approximate ly 95%, 95%, that we wish wis h to recove recoverr and use as neutra l oil in the manu facture of finished products. products. The re maining nontriglyceride portion contains variable amounts of impurities, such as free fatty acids (FFA), nonfatty materials generally classified as "gums," phospholipids (phosphatides), tocopherols, color pigments, sterols, meal, oxidized materials, waxes, moisture and dirt dirt.. Mos Mostt of these impuriti es are detrimental to finished product color, flavor, foaming and smoking stability; hence, must be separated from the neutral oil by a purificati on step. step. Of these impurities, our primary concern is with the adequate removal of the FF A and phosphatides. phosphatides. As the name suggests, FFA is the amount of fatty acid, occurring naturally or produced during storage or process-
Crude edible fats and oil oilss contai n variable variable amount of nonglyce ride impuriti es, such as free fatty acids, non-fatty materials generally classified as "gums," and color pigments. Most of these impurities are detrimental to end product fres fresh h and aged aged quality char acterist acte ristics, ics, hence must be elimi nated by a purifica tion processs bef ore the finished fats and oils are suitable proces for huma n consump tion. The obje ct of this proce process ss is to remove these objectionable impurities with the least possible loss of neutral oil and tocopherals. Key theoretical and practica practicall factors for degumming and refining crude edible oils are discussed with parti cular reference to processes, flow charts, control systems and analytical testing requirements. In addition to typical large volume oils, such as soya and cotton, techniques are also reviewed for smaller volume oils, including palm, laurie and corn.
an uncombined state. The quantity of FFA present is a good measure of the quality of the crude oil, as well as the purified oil. Some crude oils, such as palm oil, may contain as much as 5% FFA by the time they are readied for processin ces sing. g. FFA cont ent can be lowered by alkali alkali treat ment or by other methods to be described later. Phosphatides consist of polyhydric alcohols combined with fat ty aci acids, ds, phosphoric acid, and a nitrogen-c ontainin compound. Their primary classifications are phosphoglycerides, phosphoinositides, and phytosphinogosines. Lecithin and cephalin are common phosphatides found in edible oils. In lecithin, the nitrogen base is choline, while hydroxyethylamine is the nitrogen base for cephalin. Soybean, corn, cottonseed and rapeseed are the major oils which con tain significant quantities of phosphatides. phosphatides. Alkali trea tmen t used for FFA r eduction is als also o capable capable of removremoving most of the phosphatides from these crude oils On the other hand, tocopherols are important minor constituents of vegetable oils. They perform the important function of serving as natural antioxidant protectors of oil keeping quality by retarding the development of rancidity. One of the tocop herols is referred to as Vit amin E. Proper processing processi ng allows allows most of the tocopherols to remain in the finished oils. Animal fats, however, lack natural tocopherols and require the addition of antioxidant agents. Thus, the object of the pur ification step step is to remove the objectionalbe impurities, with the least possible damage to the neutral oil and tocopherols, and min imu m los losss of oil during processing.
INTRODUCTION The primary system used to purify crude edible source oils is the co nven tio nal caustic soda refin ing process, in which a solution of sodium hydroxide is continuously mixed with crude oil and separated as soapstock from the neutral oil by centrifugal action. As an option, crude soybean oil may be degummed before refining by a water treatment, followed by centrifugation to remove the hydrated gums. If oils are are no t adequately refined, subsequent operations such as bleaching, hydrogenation, winterization and deodorization will be troublesome, and finished products will fail to meet quali ty objectives. Ineff icie nt refining will also reduce the yield of neutral oil and adversely affect manufacturing profits. Hence, the refining process has the greatest impact on the quality and economic performance of toda y's edible oils manufactu ring facilit facilities. ies. Factors impo rtan t to an efficient refining operation include the source oils supply, co mposi tion, receiving and storage techniques; type of crude oil purification system selected; processing conditions and equipment used in the selected system; and control methods used for yield and qualit y perfor mance during processing~ processing~ Each of these these factors will be reviewed for their effect on refinery performance.
SOURCE OILS Supply
Receiving
Of all edible fats and oils available to the world, and more specifically the United States, soybean oil is by far the most important factor in the supply of source oils to
Domestic crude oils are generally purchased and graded for quality according to indust ry-accepted rules, rules, such as the National Soybean Processors Association Trading Rules Book, or the Trading Rules by the National Cottonseed Products Associati Association. on. Prior to the unloadin g of crude oils oils from tank cars, trucks or barges, a representative sample is taken for comparison with the official loading sample of the l ot submitt ed by the mill. Soybean oil, as one example, example, is usually evaluated evaluated b y the American Oil Chemists' Chemists' Societ (AOCS) methods for percent FFA, neutral oil loss, bleached color, odor and green color. Hard fats receipts, such as tallow and palm oil, can be damaged by extended heating periods in colder climates, prior to unloading. An unloadi ng shed shed with heated agitator agitatorss can reduce the heating/ unlo adi ng period from two days to ca. five hours.
refiners.. At the present time, consum ptio n of soybean oil refiners oil in North America is higher than all other sources of fats and oils combined. Cottonseed, laurie (coconut and palm kernel), corn, palm, peanut and safflower oils are also used by U.S. refiners. On a worldwide basis, soybean oil contributes approximately 31 31% % of the total pro ducti on of vegeta vegetable ble oils. By 1985, projec tions of USDA data indica te that soyoils. bean oil's share may increase to over 34% of the total. Sunflower and rapeseed oils, combined with soybean oil, willl probably supply more t han half of the world's producwil tion of edible oils. oils. Other oils of inter nation al importanc are palm, laurie, cottonseed, peanut and olive oils. 76
76 6
JOURNAL OF THE AMERICAN OIL CHEMISTS' SOCIETY
F o l l o w i n g t h e s e s a m p l i n g a n d a n a l y t i c a l s t ep s , t h e weighed recepts should be segregated by oil type, color, F F A o r r e f i n i n g lo s s , a n d , a s n e c e ss a r y , b y t h e p r e v i o u s h i s t o r y o f t h e s u p p l i e r . S o m e c o m p a n i e s a s s ig n t h e i r q u a l i t y c o n t r o l ( Q . C. ) d e p a r t m e n t t h e r e s p o n s i b l i t y o f a u t h o r i z i n g the unloa ding of all receipts. If poo r qualit y receipts are significant, a supplier /quality mon itorin g system will highlight the suppliers causing the problem. An ongoing segregat i o n a n d b l e n d i n g p r o g r a m w i ll m i n i m i z e t h e c o m p l i c a t i o n s o f t h e p u r i f i c a t i o n s t e p s d u e t o v a r i a b i l i t y i n th e r a w m a terials.
Storage Storage of crude oils is necessary to build up sufficient i n v e n t o r i e s t o m i n i m i z e o i l l o s se s a s s o c i a t e d w i t h f r e q u e n t start ups and shutdo wns during short refining runs. The storage objective is to ev entually deliver oil to the refining o p e r a t i o n a s c lo s e a s p o s s i b l e t o t h e q u a n t i t y a n d q u a l i t y o f the c rude oils received. P r o b l e m s c a n s t a r t f r o m t h e m o m e n t c r u d e o i ls a r e u n l o a d e d . P h y s i c a l l o s s o f o i l ca n b e c a u s e d b y f a u l t y p i p i n g d e s i g n , l e a k i n g v a l ve s a n d i n a d e q u a t e v o l u m e t r i c o r w e i g h i n g e q u i p m e n t . C r o s s - c o n t a m i n a t i o n o f o i ls c an o c c u r f r o m pumping errors due to op erator mistakes, poor equipment, or faulty system design. Improper storage conditions will r e s u l t i n s i g n i f i ca n t q u a l i t y d e g r a d a t i o n f r o m F F A r i s e , colo r reversion o r fixation, and damage to the oils' natural k e e p i n g q u a l i t y o r s h e l f li f e . Q u a l i t y d e g r e d a t i o n d u r i n g s t o ra g e is g e n e r a l ly c a us e d b y m o i s t u r e , t e m p e r t u r e , m e t a l s o a ps , o x y g e n a n d e q u i p m e n t c o n d i t i o n o r d e s i gn . M o i s t u r e i s t o b e a v o i d e d f o r s e v er a l r e a s o n s , t h e m o s t i m p o r t a n t b e i n g t h a t i t ca u s e s f a t h y d r o l y s i s w i t h a r e s u l t ing rise in F FA . This deter iorat ion will prod uce higher ref i n i n g l os s e s, h e n c e h i g h e r p r o c e s s i n g c o s t s f o r c r u d e o i l s C o n t a m i n a t i o n b y m o i s t u r e c a n a l so d e g u m s o y b e a n o il i n the storage tanks. In orde r to keep moistu re content low, the m oisture of the crude oil should be carefully checked when received. Any crude with a high moisture (e.g., above 0 . 1 % ) s h o u l d b e r e j e c t e d o r u s e d as s o o n a s p o s s i b l e a f t e r r e c e i p t . H e a t i n g c o i l s m u s t b e k e p t i n g o o d r e p a i r t o p re vent moistu re leakage into storage tanks, and the storage tanks sh ould be rubbed ou t regularly to prevent a buildup of moisture and sludge. Oils should be stored at as low a temperature as possible. When sto red in prop er tanks which will not be used for s o m e t i m e , t h e y s h o u l d b e a ll o w e d t o c o o l d o w n t o a m b i ent temperature . Those oils in tanks from w hich pumpings are being mad e regularly should be k ept liquid at a temperature just high enough to prevent graining. This is usually on the order o f 5 C above the com ple te melting p o i n t o f t h e s t o c k . T h e r a t e o f o x i d a t i o n o f a n o il i n c re a s e rapidly as temperature increases. A general rule is that for e a c h 1 0 C i n c r e a s e in s t o r a g e t e m p e r a t u r e , t h e o x i d a t i o n r a t e t r i p le s . A d d i t i o n a l l y , w i t h e l e v a t e d t e m p e r a t u r e s , w a t e r solubility increases in the oil, and the rate of hydrolysis i n c r e a s es , c a u s i n g a n i n c r e a s e i n t h e r a t e o f F F A r i se a n d metal soap formation. D u r i n g p r o c e s s i n g , o i l s c o n t a c t m e t a l s u r f a c e s o f st o r a g e and handling equipment in the presence of FFA and moisture. All metals, with the exception of aluminum and n i c k e l, e x e r t v a r y i n g d e g r ee s o f c a t a l y t i c p r o o x i d a n t e f f e ct . T h i s d e t r i m e n t a l e f f e c t i s a c c e n t u a t e d b y in c r e a s e s i n temperature and the formatio n of metal soaps by the intera c t i o n o f m e ta l s w i t h F F A Storage or hand ling activities which increase the chances of oxyg en coming into intimate contact with the oil should be el im in at ed . A tzP.j.9_~ oil g,xn--a-bsorb.~2~._.l~y~volume of o x y g e n w h e n s t o r e d u n d e r ~ r . I f a n y si g n i f ic a n t p o r t i o n o f this abffbrbe-d- ai r reacts w it h the oil, flavo r deter ior atio w i l l r e su l t . W h e n s t o r i n g o i l u n d e r a i r , t a n k s s h o u l d b e f i l l e d com plet ely so that the oil surface-to-volu me ratio is mini-
VOL. 55
mized and the loss in quality is reduced. Oil storage under n i t r o g e n i s d e s ir a b l e b e c a u s e o f t h e s t a b i l i t y p r o t e c t i o n . I t i g o o d p r a c t i c e t o p e r i o d i c a ll y m o n i t o r o i ls in e x t e n d e d s t o r a g e f o r F F A , b l e a c h e d c o l o r , p e r o x i d e v al u e a n d m o i s t ure content. Lots showing deterioration trends should be refined promptly. Storage tanks should be emp ty, free of foreign matter w i t h c o i l s t e s t e d f o r l e a k s , b e f o r e f i l li n g w i t h n e w o i l . T a n k design should include side-entering agitators, a cleanout manho le at grou nd level, bottom -filling capability, adequate inventory and temperature measuring equipment. S~aao p r e s s u r e t o t h e r h e a t i ng c o i l s s ho ul d_ . h e _ 0 1 ~ s i g m a ~ u m a n d t h e f l o w controiied by temperature indicator controller (TIC). W here possible, provide separate piping and tank systems for expensive oils with critical physical prope r t i e s. P r o v i d e a d e q u a t e i n s u l a t i o n a n d h e a t i n g t o p i p i n g a n d t a n k s u s e d t o h a n d l e o i l s i n c o l d e r cl i m a t e s . T a n k f a r m d e si g n s h o u l d p r o v i d e t h e c a p a b i l i t y o f b l e n d i n g d e g u m m e d a n d n o n d e g u m m e d o il s p r i o r t o r e f i ni n g .
CRUDE OIL PURIFICATION C r u d e o i l s w h i c h h a v e b e e n c a r e f u l l y g r a d e d , s e g r e g a te d and stored are now ready for the first processing steps towards their conversion to finished products. The objective o f this step is to refine or cleanse the crude oils of undesirable impurities, with minimum loss of the neutral oils and natural to cophero ls. We use the term "ref ining" for any purification treatment designed to remove free fatty acids, phospha tides, gums or oth er gross impurities in the oil. The prim ary crude oil processing system used in the U.S. to achieve this objective is a com binat ion of degumming a n d c a u s t i c s o d a c e n t r i f u g a l r e f in i n g . C r u d e o i l m a y b e d e gummed before refining by a water treatment followed by centrifugation to remove the phosphatides as hydrated gums. Crude or degumm ed oil is treated with caustic soda to saponify impurities, which are subsequently removed as s o a p s t o c k b y a p r i m a r y c e n t r i f u g e . T h e r e f i n e d oi l i s w a s h e d t o r e m o v e t h e l a s t tr a c e s o f s o a p i n a s e c o n d a r y centrifuge~ Refined, water-washed oil is finally dried under v a c u u m a n d i s r e a d y f o r a d d i t i o n a l p r o c es s i n g , u s u a ll y bleaching. Pro cess ors h a v e t h e o p t i o n o f a p p r o ac h i n g t hi s purificatio n step in two ways. Soyb ean oil phosph atides can either be recovered as valuable by-products through degumming, o r considered as both ersom e imp urities which m u s t b e r e m o v e d f r o m t h e o i l t o p r o d u c e f i n is h e d p r o d u c t s with satisfactory quality attributes.
DEGUMMING D e g u m m i n g i s t h e t r e a t m e n t o f c r u d e o i ls w i t h w a t e r , salt solutions or dilute acids such as phosphoric to remove phosp hatides, waxes and othe r impurities. Some crude oils such as soybea n, contain significant quantities o f phosp h a t i d e s. D e g u m m i n g e x p l o it s t h e a f f i n i t y o f p h o s p h a t i d e s f o r w a t e r b y c o n v e r ti n g t h e m t o h y d r a t e d g u m s , w h i c h a r e insoluble in oil and readily separated by centrifugal action, Because of a strong demand (ca. 1 00,000 tons per year) for the lecithin em ulsifying agent derived from the hydrat ed g u m s , c r u d e s o y b e a n o il is f r e q u e n t l y d e g u m m e d p r i o r t o r e f i n i n g. T h i s s o y b e a n o i l b y - p r o d u c t is t h e o n l y c u r r e n t commercial source of lecithin used as eumulsifiers in margarine, salad dressings, shortening, peanut butter and many other products. The principa l phosphatides, lecithin and cep halin, are triglyceride com pou nds with one fatty acid radical replaced by phosphoric acid. The position of this phosphoric acid r a d i c a l i s i m p o r t a n t . W h e n i t is a t t a c h e d t o a n o u t e r c a r b o n link with the glycerol molecule, it is termed an Alpha-
NOVEMBER, 1978
CARR: REFINING AND DEGUMM1NGSYSTEMS FOR EDIBLE FATS AND OILS
Lipoid. If found in the center position, the compound is called a Beta-Lipoid. Alpha-Lipoids are hydrateable and may be removed by water degu mming prior to caustic refining. The Beta-Lipoids are no t hydrateabl e and can not be removed during degumming. Degumming by batch water treatment followed by centri fugati on is com mon ly practiced in the U.S. Water, at ca. 2% of the oil volume, is contacted with crude soyb ean' 0il by mechanical agitation in a mix tank. Agitation is continue d for ca. 30 _~ nu te s at 60-70 C to complete gum hydration, being careful to prevefit air entra inmen t during mixing. In a continuou s hydrat ion system, the preheated oil is treated with water and mixed in a hold tank for ca. 15 min. The hydrated gums are a complex mixt ure consisting mainly of phosphatides, neutral oil and water, while the degummed oils are unaltered chemically from the neutral oil in the source crude oils. It is important to add only the amount of water necessary to precipitate the gums, as any water excess causes unnecessary oil losses through hydrolysis. Phosphatide removal efficiency can range from 80 to 95% of the crude oil content. Following hydratio n, the content s of the mix tank ar pum ped to degum ming centrifuge s, such as the De Laval SRPX-317. The mixture of oil and gums continuously separates into sludge and degummed oil phases. Initially, the centrifuge oil discharge pressure control valve is adjusted to ob tain a 40% to 50% moisture conten t in the sludge phase for satisfactory separation. Back pressure is then fine-tuned to achieve the desired characteristics in the sludge and degummed oil phases. Hermetic centrifuges can deliver 65% to 80% Aceto ne Insolu ble (A.I.) gums on a dry basis wit hout sacrificing quali ty in the de gummed oil Res idu al unh ydr ata ble p h o s ph o l i p i ds r e ma i n in g i n degummed oil must be subsequently removed by an Alkali refining process. The sludge phase from the centrifuges is vacuum dried at ca. 90 C for 1-2 rain. by semibatch and continuous drying film evaporators to a 0.5% moisture level, then cooled to 50 C prior to pumping to the lecithin work tank. Residence time and temperature control are critical to the production of light colored products. Approximately 7% soybean oil and 3% vegetable fatty acids are blended with the lecithin in the work tank to obtain fludity at room temperature. As an option, this final blend can be bleached with peroxides prior to filling for sale as lecithin. For commercial food-grade lecithin , the cru de oil should be filtered prior to the d egumming operation. Degummed oil from the centrifuge is passed continuously through a vacuum dryer and on to degummed oil storage. It may then be sold as degummed crude oil or transferred to the refining process. Removal of gums prior to alkali refining often improves the overall yield, because phosphatides act as emulsifiers in a caustic sotuti on and increase the quan tit y of neutral oil entrained in the soapstock.
REFINING SYSTEMS Of all the unit operations to which vegetable oils are subjected during conversion to finished products, the refining process has the most impa ct on quality and ec onomic performance. If oils are not a dequately refined, subsequent operations such as bleaching, hydroge nation, winterizing, deodorization, etc. will be trouble some, and finished products will fail quality standards for fresh and aged performance. Inefficient refining will also reduce the yield of finished products and adversely affect manufacturing profits. Commercial worldwide purificati on systems include physical refining, miscetla refining, the Zenith process and alkali refining.
76
Physical Refining When dealing with high FFA and low phospholipid conten t c rude oils, physical refi ning, or steam refi ning, can provide an econo mic advanta ge over caustic refining~ In Europe, physical refining of high-FFA oils has been utilized for many years to deacidify source oils down to lower FF levels of 0.2-0.5%, prior to the tra diti onal caustic refining process. Domestic interest in this system increased coincidental to the rise in palm oil's share of the world edible oils market. A fullscale plant was constructed in 1973 at Portland, Oregon, to steam refine crude palm oil The process, similar to deodorization, is a steam distillatio n of water-de gummed and blea ched oil. Source oil is pumped through a deaerator to a deodorizer-deacidifier with a dwell time of cao One hour at 260C and 1 mm mercury vacuum. Inj ection of steam volatilizes most of the free fatty acids, which are condensed and collected in a tank separate from the neutral oil. When used with high-acidity crude oils, physical refining can reduce the loss of neutral oil in by-products, reduce the num ber of unit operations in the purification process and eliminate the acidulatio n problems associated with the soapstock by- product prod uced by alkali refining. On the othe r han d, this process is not as successful as alkali refining in readily achieving low FFA conten ts in the refined oil and satisfactory oil colors during the subse quent bleaching step. Thorough removal of prooxidants and phosphatides is crucial to preparing quality oils by steam refining techni-
Miscella Refining Facilities with an existing oilseed solvent extraction system may find miscella refinin g to be an advanta ge by using one solvent recovery unit for both purposes. It ca n be performed by either a continuous or a batch process for most U.S.-utilized fats and oils such as soybean, cottonseed, palm, safflower, sunflower, tallow and coconut~ The process usually starts with the blending of one to two parts of an inert solvent with one part of source oil for refining in a mi x tank~ Two solvents are presentl y being the De Laval, Ranchers and De Smet systems and acetone in the Vaccarino Process. The blend may then be treated with chemicals to cond ition gums and pigments prior to the addition of caustic soda. Properly conditioned miscella is homogenized, heated to 65 cooled to 45 C and then refined miscella and soapstock. T he former, or light phase, is filtered and either stripped of hexane to produce the refined oil or passed on to the ne xt miscella-based process such as winterization. Miscella refining, when compared to conventional methods, can result in lower refining losses, lighter colored oil and elimination of the need to water wash and vacuum dry refined oil. Today, more than a dozen plants are miscella refining vegetable oils arou nd the world.
Zen ith Proces Anot her refining method, used by a number of refineries in Europe, is the Zen ith Process. After a pretre atmen t with concentrated phosphoric acid, followed by sludge removal if necessary, the oil is neutra lized as droplets, rising by gravity through a lye column. The alkali lye is weak and keeps the formed soap in solution. This virtually eliminates the format ion of emulsions and any saponification o neut ral oil. It: is claimed that wat er washing of the n eutralized oil is not necessary, that the yield is very much improved and that the quality of the refined oil is excellent~
Alkali Refining Despite the develop ment of other technique s such as the
768
J O U R N A L O F T H E A M E R I CA N O I L C H E M I ST S ' S O C IE T Y
" s o d a a s h , " " m o d i f i e d s o d a a s h , " " c a u s t i c s o d a -s o d a a s h , " " a m m o n i a , " " m i s c e l l a ," a n d " s t e a m r e f in i n g " s y st e m s , th e v a s t m a j o r i t y o f r e f i n e rs i n t h e U . S . a r e c u r r e n t l y u s i ng t h e conve ntional caustic soda refining me thod . The present s y s t e m i s t h e r e s u l t o f a g r a d u a l a p p l i c a t i o n o f s c i e n ce t o t h e b a s i c a r t o f b a t c h r e f i n i n g in o p e n k e t t l e s a s p r a c t i c e d d u r i n g t h e f i rs t t h i r d o f t h i s c e n t u r y . T h e k e y t e c h n o l o g ic a l b r e a k t h r o u g h w a s th e d e v e lo p m e n t o f a " c o n v e n t i o n a l c a u s t i c s o d a r e f in i n g " p r o c e ss i n which a solution of sodium hy droxide was mixed with crude oil and separated by centrifugal action in a continuous process. This new c ontinuou s refining system bec a m e a c o m m e r c i a l r e a l i t y i n 1 93 2o T h e p r i m a r y a d v a n t a g e of the con tinuou s process over the batch process is m a r k e d l y r e d u c e d c o n t a c t t i m e b e t w e e n o i l a n d c a u st ic . This, in turn, reduces the sapon ification of neutral oil and r e s u l ts i n a h i g h e r y i e l d . P l a n ts w i t h d a i l y c a p a c i t i e s o f 2 0 tons or mo re w ill generally use the continu ous system in p r e f e r e n c e t o t h e b a t c h r e fi n in g m e t h o d . T h r o u g h t h e y e a r s , r e f i n in g e f f i c i e n c y o f th i s b a s i c s y s t e m h a s b e e n i m p r o v e d b y ( a ) u p g r a d i n g t h e a ve r a g q u a l i t y o f c r u d e o i ls b y s e l e ct i v e b r e e d i n g a n d m i l li n g a d vances; (b) evaluating th e effects of variables in each refining step (c) providing improv ed equ ipm ent for flow control, pump ing, heat exchange, mixing, separation and i n s t r u m e n t a t i o n . A s a re s u l t , t o d a y ' s c o m m e r c i a l c a u s ti c soda refining system is a relatively simple opera tion with t h e n e c e s s a r y f l e x i b i l i t y t o e f f i c i e n t l y r e f in e a l l t h e c r u d e o i ls p r e s e n t l y u t i l i z e d i n t h e U . S .
C O N V E N T I O N A L C A U S T IC SO D A R E F IN I N G S Y S T E M Basic Process The following is a sum mar y of the system. Crud e oils are received in tan k cars, tank trucks, barges, or from the d e g u m m i n g o p e r a t i o n . R e c e i p t s a r e s a m p l e d , g r ad e d a n d t h e n t r a n s f e r r e d t o a p p r o p r i a t e s t o r a g e ta n k s . A s n e e d e d , they are pum ped by oil type to a day tank in preparation f o r r e f in i n g . C r u d e o i l f ro m t h e d a y t a n k i s c o n t i n u o u s l y m i x e d w i t h a p r o p o r t i o n e d s t r e a m o f d i l u t e c au s t i c s o d a s o l u t i o n a n d h e a t e d t o o b t a i n a b r e a k i n th e e m u l s i o n . Soa pstoc k is continu ally separated from the neutral oil by centrifugal action. T he resultant refined oil is mixed with h o t , s o f t w a t e r a n d a g a i n c e n t r if u g a l l y s e p a r a t e d t o r e m o v e s m a l l a m o u n t s o f r e s i d u a l s o a p . T h is w a t e r - w a s h e d r e f i n e d o i l, c o n t a i n i n g t r a c e s o f m o i s t u r e , i s t h e n p a s s e d t h r o u g h a continu ous vacuum -drying stage and on to the refined oil storage tank. F i v e k e y f a c t o r s d e t e r m i n e t h e s u c c e ss o f a n y e d i b l e f a t s a n d o i ls r e f i n in g o p e r a t i o n s ( 1 ) u n i f o r m f e e d s t o c k ; ( 2 ) p r o p e r q u a n t i t y o f r e fi n in g r e a g e n t ; ( 3 ) p r o p e r m i x i n g o f reagent an d oil; (4) pro per residual conta ct time and temperature contro l; and (5) efficient centrifugation. Each of the unit operations within the caustic soda process will now b e r e v i e w e d f o r f e a t u r e s i m p o r t a n t t o t h e o v e r a l l r e f in i n g system.
Crude Oil Preparation C r u d e o i ls a r e p u m p e d f r o m s e g r e g a t e d s to r a g e t o tem por ary holding tanks (day tanks) by oil type, quality and q uan tity, as necessary for pro duc tion planning requireen ts. B e c a u se f e e d s t o c k u n i f o r m i t y i s e s se n t ia l t o steady-state refining operation, these day tanks should be equip ped with slow speed, side-entering agitators. This i p a r t i c u l a r l y im p o r t a n t f o r p h o s p h a t i d e -c o n t a i n i n g so u r c e o i l s , such a s n o n d e g u m m e d s o y b e a n o i l . T a n k s s h o u l d b e s i z e d t o p r o v i d e h o m o g e n e o u s b a t c h e s o f c r u d e o i l, s u f fi cient for 24 hr min imu m c o n t i n u o u s r e f i n in g r u n s . S m a l l e r b a t c h e s i n c r e a s e r e f in i n g lo s s e s f r o m f r e q u e n t s t a r t u p s a n d s h u t d o w n s . S e v er a l d a y t an k s m a y b e n e c e ss a r y t o
VOL. 55
provide sufficient time to prepa re the crude oil batch, test t h e o i l a n d s e l e c t t h e a p p r o p r i a t e r e f in i n g c o n d i t i o n s . P r i o r to the refining start up, the crude oil in the day tan k should be evaluated for FF , Neutral O il or Cup Loss, and Bleach Test on cup refined oil. C r u d e o i ls w i t h s i g n i fi c a n t le v e ls o f p h o s p h a t i d e s , s u c h a s s o y b e a n o i l , ar e u s u a l l y t r e a t e d w i t h 3 0 0 t o I 0 0 0 p p m o f f o o d - g r a d e , 7 5% p h o s p h o r i c a c i d , a t le a s t f o u r h r ' p r i o r t o t h e r e f in i n g s t e p t o i n c r e a s e t h e e f f i c i e n c y o f p h o s p h a t i d e r e m o v a l d u r i n g c a u s t ic r e f i ni n g ~ T h e P h o s p h o r i c a c i d a l s o a c ts a s a se q u e s t e r i n g a g e n t b y c o m b i n i n g w i t h m e t a l i o n s i n t h e o i l, r e t a r d i n g t h e i r c a t a l y t i c p o t e n t i a l f o r o x i d i z i n g t h e n e u t r a l o i l a n d f a c i l it a t i n g t h e i r r e m o v a l d u r i n g s u b s e q u e n t processing. F o r o p t i m u m r e f in i n g p e r f o r m a n c e , t h e a c e t o n e in s o l u ble or phosphatide con tent of degumm ed soybean oi s h o u l d b e b e l o w 9 . 3 % , . W h e n t h i s l ev e l is e x c e e d e d , d e gumm ed oil should be blended with nondegum med soybean o i l to 1 .0 % m i n i m u m a c e t o n e i n s o l u b l e le v e l. C r u d e o i l s u c h a s p a l m , p a l m k e r n e l , c o c o n u t a n d c o r n sh o u l d b e r a p i d l y h e a t e d b y a n i n - li n e h e a t e r t o 8 2 - 8 8 C i m m e d i a t e l y b e f o r e t h e y a r e p u m p e d t h r o u g h a C u n o t y p e c ru d e o i filter to the c austic-oil mix ing step.
Caustic Storage and Preparation R e f i n e r s u s u a l l y r ec e i ve c o n c e n t r a t e d c a u s t i c a t 5 0 ~ in t a n k c a rs o r t a n k w a g o n s a n d u n l o a d t h e c o n t e n t s t o t h e c a u s t ic s t o r a g e t a n k . P r i o r t o t h e r e f i n in g r u n , c o n c e n t r a t e c a u s t i c i s t r a n s f e rr e d t o t h e c a u s t i c d i l u t i o n t a n k a n d blended w ith water to obtain the desired concentration. After tho roug h mixing, the strength is checked with a hyd rom eter b y the operator. When on target, a sample is s u b m i t t e d t o t h e l a b o r a t o r y fo r p e rc e n t N a O H a n d d e gr e e B ~ , p r i o r t o s t a r t u p . S o m e i n s t a l l a t i o n s u t i l i z e a c a u s ti c water propo rtiona ting system, such as the Bran & Lubbe metering un it, to replace the caustic dilution tank. Diluted caustic of the desired concentration is pum ped throug h a s t r a i n e r, h e a t e x c h a n g e r a s n e c e s s a r y , an d f l o w i n d i c a t o r / c o n t r o l l o r , t o t h e c a u s t ic - o i l m i x i n g " T . " K e y c o n t r o l p o i n t s f o r c a u s ti c p r e p a r a t i o n a r e c o n c e n tration, flow rate or "treat," and temp erature. Selection of c o n t r o l l e v e ls i s d e t e r m i n e d b y t h e t y p e o f c r u d e o i l t o b e r e f i n e d , l a b o r a t o r y t e s t s , p a s t r e f in i n g e x p e r i e n c e w i t h similar oils and refining equip ent available. In general, the m i n i m u m a m o u n t o f t h e w e a k e s t s t r e n g t h c a u s ti c n e c e s s a r t o a c h i ev e t h e d e s i r e d e n d p o i n t c o n t r o l is u s e d t o m i n i m i z e saponification of neutral oil and prevent "three-phasing " or emulsions during separation. Caustic stren gths of 17 to 1 ~ Bd are usually prescribed f o r o i ls o t h e r t h a n p a l m , p a l m . k ~ r n e l a n d . c o c o n u t . T h e l a t t e r r e q u i r e w e a k e r c a u s t ic o f c a . 1 2 ~ B d _ t o o p t i m i z e c e n t r if u g a l s e p a r a t i o n , r e d u c e s a p o n i f i c a t i o n o f n e u t r a l o i and minimize emulsions. The treat selected for the crude oil to be refined wil vary with the F FA conten t o f the oil and the level of c a u s t i c " e x c e s s " o v e r " t h e o r e t i c a l , " d e t e r m i n e d f o r e a ch o i type from previous experience. The theore tical quan tity of c a u s t ic i s b a s e d o n t h e r a t i o o f m o l e c u l a r w e i g h t s o f s o d i u m h y d r o x i d e t o o l e i c a c i d . M o s t o i l s a r e r e f in e d w i t h 0 . 1 0% t o 0 . 1 3% e x c e s s , b u t t h e r e a r e i m p o r t a n t e x c e p t i o n s . L a u r i e and palm oils requ ire a minim al excess of ca. 0 02%, because t h e y a re r e f i n ed f o r F F A r e d u c t i o n p u r p o s e s o n l y . C o t t o n s e e d o i l is p r i m a r i l y r e f in e d f o r c o l o r r e d u c t i o n a n d r e q u i r e s a la r g e r e x c e s s , c a . 0 .1 6 % . D i l u t e d c a u s t i c , f o r u s e w i t h l a u r i c a n d p a l m o i ls , i s u s u a l l y p r e h e a t e d t o 6 5 C t o m i n i m i z e emulsion formation in the separators. A s m o o t h , r e p r o d u c i b l e f l o w o f c a u s ti c t o t h e m i x i n g i s o f p r i m e i m p o r t a n c e t o e f f i c i e n t r ef i n i n g . P u l s a ti n g d e l i v e r y w i l l c a r r y t h r o u g h t h e m i x e r s a n d p r o d u c e v a ry i n g m i x t u r e d e n s i t i t e s i n t h e s e p a r a to r s . R e f i n e r s n o w u t i l iz e
NOVEMBER, 1978
C A R R : R E F I N I N G A N D D E G U M M I N G S Y S T EM S F O R E D I B L E F A T S A N D O I L S
ratio units w ith pro portio nal band, reset, and rate action to achieve a nonpu lsating reagent delivery to the crude oil. T h i s r a t i o c o n t r o l l e r p e r m i t s o n l y t h e s p e c i fi e d q u a n t i t y o f c a u s t i c t o e n t e r t h e c r u d e o i l s t r e a m , e v e n i f t h e r e a r e fl u c t u a t i o n s i n t h e c r u d e o i l f l o w . In a d d i t i o n , r o t o m e t e r s a r e i n s t a l l e d in t h e r e a g e n t l i n e f o r v i s u a l o b s e r v a t i o n a n d e m e r g e n c y m a n u a l c o n t r o l . P r o m p t , a c c u r a t e r e a d in g s o f o i l a n d c a u s t i c f l o w r a t e s a r e e s s e n ti a l f o r t h e r e f i n i n g o p e r a t o r t o c o n t r o l h i s r e fi n i n g e f f i c ie n c y .
Caustic-Oil Mixing A f t e r t h e c a u st ic r e a g e n t h as b e e n p r o p o r t i o n e d i n t o t h e crude oil at the mixing T, it must b e sufficiently blended to insure adequate contac t with the free fatty acids, phosph atides and color pigments. Caustic reacts as follows with free f a t t y a c i d s t o f o r m s o a p s t o c k , w h i le h y d r o l y z i n g p h o s p h a t i d e s a n d r e m o v i n g u n sa p o n i f ia b l e m a t t e r f r o m t h e crude oil: R C O O H + N 2O H = R C O O N A + H 2 0 . A f t e r m i x i n g , t h e s o a p - o i l b l e n d i s h e a t e d a s n e c e s s a r y to a t e m p e r a t u r e s e l e c t e d f o r o p t i m u m s e p a r a t i o n i n t h e r e fi n i n g centrifuges. T h e u s u a l m i x i n g s y s t e m p r o v i d e s a h i g h s p e e d , i n li n e m i x e r f o r a l l o il s , p l u s a n o p t i o n a l b a n k o f d w e l l m i x e r s f o r s o f t o i ls , s u ch a s s o y b e a n . S u i t a b l e p i p i n g c o n n e c t i o n s a r e supplied to allow the inclusion to zero to four dwell mixers i n t h e m i x i n g s y s t e m . L a u r i c a n d p a l m o i l s a r e re f i n e d b y t h e " ~ o r ~ - ~M ' i x " ... pr .oc .. ..es . which requires low speed , sho rt ~ ha e m i x i n g w i t h o n l y t h e t n l i n e m i x e r . T h i s m i x t u r e s h o u l d b e i m m e d i a t e l y d e l i v e re d t o t h e s e p a r a t o r , t o m i n i m i z e e m u l s i f i c a t i o n a n d s a p o n i f i c a t i o n o f n e u t r a l o i l. S o f t o i ls r e c e iv e h i g h s p e e d m i x i n g b y t h e i n l i n e m i x e r t o o b t a i n i n t i m a t e c o n t a c t b e t w e e n t h e c a u s t i c a n d o il . T h e g u m s a r h y d r o l y z e d b y w a t e r in t h e c a u s ti c s o l u t i o n a n d b e c o m e insoluble in the oil. Inline mixing is followed by a delay p e r i o d i n a n u m b e r o f d w d l m i x e r s, v ar i ed t o a c h ie v e th e refined oil end poin t co ntrol. So ybea n oil usually requires three dwell mixers for adequate phosph atide removal One addition al m ixer is generally used for cotton seed oil to maximize color removal O n c e t h e m i x i n g o p e r a t i o n h a s b e e n c o m p l e t e d , th e soap-oil mixture sh ould be delivered to the separa tors at a t e m p e r a t u r e s u i t a b le f o r o p t i m u m s e p a r a ti o n . M o s t s o f t o i m i x t u r e s a r e h e a t e d t o 7 4 C to p r o v i d e t h e t h e r m a l s h o c k n e c e s s a r y t o " b r e a k " t h e e m u l s i o n o f t h e o i l - c a u s ti c - s o a p mass. Too high an oil tem peratu re during the additio n of caustic can increase the saponification rate of neutral oil a n d r e d u c e t h e y i e l d o f r e f i n e d o i l . T h e d e g r e e o f r e f in i n g i c o n t r o l l e d b y t h e c a u s t i c t r e a t, p l u s t h e c a u s t i c - o i l m i x i n g t e m p e r a t u r e a n d t i m e . H a r d o i l s , s u c h a s p a l m a n d la u r i c o i l s, s h o u l d b y p a s s t h e c a u s t i c- o i l h e a t e r . Som e refiners use pH to ensure that the reaction m ixture contains the prop er caustic treat before it is sent to the refining centrifuge, pH targets will range from 9.8 for lau rc o i ls t o 1 0 . 8 f o r c o t t o n s e e d o i l . P e r i o d i c m o n i t o r i n g , e i t h e r manually by the operator or by a continuous pH unit, minimizes un der refining and excess losses from n eutral oil emulsification, sapon ification and entrainm ent.
Soap-Oil Separation All of the previous o perations are in prepa ration for the p r i m a r y s e p a r a t i o n s t e p , t h e k e y t o r e f i n e r y y i e l d ef ficiency. From the caustic-oil mixer, the resultant soapin-oil suspension is fed to high speed centrifuges for separation into light and he avy density phases. These separators a r e d e s i g n e d t o d i v i d e s u s p e n s i o n s o f i n s o l u b l e l i q u id s a n d s o l i d s in s u s p e n s i o n w i t h d i f f e r e n t s p e c i f i c g r a v i ti e s . T h e l i g h t p h a s e d i s c h a r g e is c o m p r i s e d o f r e f i n e d o i l c o n t a i n i n g traces of m oisture and soap, while the heavy phase is pri-
769
a r l y i n s o l u b l e s o ap , m e a l , f r e e c a u st ic , p h o s p h a t i d e s a n d small quantities of neutral oil. Centrifugal separa tion is several thousa nds times faster than gravity separation used in batch refining, and the se pa rat ion e f f ic i e n c y i s c o n t r o l l e d p ri m a r i l y b y t h e tem peratu re of the oil-caustic feed. The basic principle is tha t an obje ct in flight will travel in a straight line unless p r e v e n t e d b y a n o u t s i d e f o r c e . F o r e x a m p l e , a w e i g h t tw i r l ing on a string will exert a force awa y from the center of r o t a t i o n . T h e s t r in g p u l l s b a c k o n t h e w e i g h t , c a u s i n g it t o t a k e a c i r c u l a r p a t h . T h e f a s t e r t h e s p i n , t h e g r e a te r t h e force generated, until the force beco mes great enough to break the string. A centrifuge contains a bow l or hollow cylinder turning on its axis. As the flow of material to be treated by cent r i f u g a l fo r c e e n t e r s t h i s r o t a t i n g b o w l , i t i s fo r c e d o u t w a r d s to the disc stack. The flow then sep arates and the soap, h a v i n g a h e a v i e r s p e c i f ic g r a v i t y , i s t h r o w n o u t t o t h e b o w l p e r i p h e r y . T h e l i g h t e r s p e c i f ic g r a v i t y p h a s e , w h i c h i s t h e desired neutral oil, is disPlaced to the center of the bowl and eventually discharges from the neck of the top disc. In t h e m e a n t i m e , t h e s o a p p h a s e f l ow s o v e r t h e t o p d i s c a n d out the soaps tock discharge port. In either a top feed or bo tto m feed centrifuge, the composition of the phases can be adjusted by changing the p o s i t i o n o f t h e n e u t r a l z o n e , o r i n t e r fa c e , i n t h e c e n t r if u g e . In the old atmo spheric centriguges, zone changes were obt a i n e d b y m o d i f y i n g t h e d i a m e t e r o f th e h e a v y p h a s e d is c h a r g e p o r t a n d u s i n g v a r i o u s ri n g d a m s i ze s . M o s t c e n t r i f u g e s i n t h e U . S . a r e n o w o f t h e p r e ss u r e o r h e r m e t i c type, in which zone changes can be readily achieved by adjusting the back pressure a pplied to the light phase disc h a r g e. R e g a r d l e s s o f th e s y s t e m e m p l o y e d , c o m p l e t e s e p a ration of the two phases can never be achieved. F o r g o o d o p e r a t i o n , t h e i n t e rf a c e s h o u l d b e n e a r t h e m i d p o i n t b e t w e e n t h e c e n t e r a nd p e r i p h e r y o f t h e b o w l . It s position depen ds upo n the refined oil discharge back press u r e . I n c r e a s i n g t h i s b a c k p r e s s u r e r e d u c e s th e s o a p c o n t e n t i n t h e o i l p h a s e , b u t i n c r e a s e s t h e n e u t r a l o i l l o s t in t h e soapstock. Conversely, reducing the back pressure decreases the n eutral oil loss in the soap ph ase, bu t increases soap in r e f i n e d o i l to a l e v e l w h i c h i s b e y o n d t h e c a p a c i t y o f th e subsequ ent water-washing step. Key factors improv ing the com pleteness of separatio n are: (a) greater difference in s p e c i f i c g r a v i ty o f t h e p h a s e s ; ( b ) l o w e r v i s c o si t ie s , ( c ) higher temperature; (d) shorter travel distance for the heavy p a r t i c l e s ; (e ) i n c r e a s e d c e n t r i f u g a l f o r c e , a n d ( f ) l o n g e r centrifuge dwell time. For cen trifuge start up cond itions, mos t refiners use re fined o il back pressure gu idelines selected from previous r e f i n i n g e x p e r i e n c e . O n c e t h e p r o c e s s s t a r t s t o l in e o u t , r e f i n e d o i l b a c k p r e s s u r e is c a r e fu l l y a d j u s t e d u n t i l t h e r e f i n e d o i l , a s v i e w e d t h r o u g h a l ig h t e d s i g h t g l a ss , b e c o m e s s l i g h t l y t u r b i d f r o m i n c l u d e d s o a p p a r t i c l e s . A h ig h s p e e d t e s t t u b e c e n t r i f u g e m a y t h e n b e u s e d t o d e te r m i n e t h e q u a n t i t y o f h e a v y p h a s e l e f t i n t h e r e f i n e d o i l. T h e s p i n t e s t provides a rapid gu ide to separa tion efficiency and back p r e s s u r e a d j u s t m e n t . S p i n t e s t e v a l u a t i o n s a n d b a c k p re s s u re a d j u s t m e n t s a r e c o n t i n u e d u n t i l t h e s g a 9 c o n t e n t i n the refined oil is com patib le with water-washing capability, u s u al ly 3 00 p p m m a x i m u m L o w l o s s o il s , s u c h a s d e g u m m e d s o y b e a n o i l , s h o u l d b e s e p a r a t e d b y c e n t r i f u g e s e q u i p p e d w i t h b o w l f l u sh u n i t s t o d i l u t e t h e v i s c o u s so a p p h a s e . T h i c k , s t i c k y s o a p s t o c k f r o m the refining centrifuge can reduce separa tion efficiency by i n c r e a s i n g th e s o a p p h a s e b a c k p r e s s u r e o r b l o c k i n g t h e s o a p s t o c k d i s ch a r g e li n e . H o t w a t e r m a y a l so b e a d d e d t o the soap discharge port to low er soapsto ck viscosity to the d e s i r e d l ev e l f o r s a t i s f a c t o r y c e n t r i f u g e o p e r a t i o n . Separa tion efficiency for palm and pa lm kernel oils is u s u a l ly i m p r o v e d b y m e t e r i n g w a t e r 0 r b r i n e i ~ 0 t h e s o a p -
77 0
JOURNAL OF THE AMERICAN OIL CHEMISTS' SOCIETY
in-oil suspension as the mixture enters the refining centrifuge. Water is gene rally used for cru de oils contai ning les than 6% FFA. For higher FFA content crude oils, a 10% sodium chloride brine solution may be used at flow rates not exceeding 5% of the crude oil through put. The "UltraShort-Mix" method can be used for high acidity palm and laurie oils to shorten contact time further by introducing caustic directly into the hollow spindle of the centrifuge. Once steady state is achieved, key control features such as flow rates, temperatures and pressures should be frequently monitored and adjusted as necessary. Permanent log sheets for processing data and equipment maintenance are very important for long term satisfactory performance. The refined oil phase, containi ng minu te quantities of soap is pumped conti nuousl y from this primary refining step to the se condar y refining stage, namely, water washing. Soapstock or "foots" formed during the alkali neutralization of FFA and removed during this primary centrifugation can be sold to soapmakers for further processing, or can be split by sulfuric acid to form a mixture of crude FFA, phospholipids, proteins and other impurities. This mixture can be sold to animal feed manufactures as "acidulated soapstock" or used as a raw material for the produ ction of purified products, such as fatty acids.
Water-Washing, Vacuum Drying an d Storage Refined oil from the primary centrifuge is reheated as necessary to 88 C. Hot softened water, or recovered steam condensate, is proportion ed i nto the refined oil at a rate of 10 to 20% by weight of the oil flow. This water-oil combinati on passes through a high speed inline mixer to obtain inti mate contact for maximum soap transfer from the oil to the water phase. The soapy water-oil mixture continues through secondary separators, such as a De Laval B-214-C centrifuge. Similar to the action of the refining centrifuge, water-washed oil is discharged as the light phase and the soapy water solution is the heavy phase. The waterwashing operation removes ca. 90% of the soap content in the refined oil and a single washing pass is usually sufficient. Wash water temperature is important to efficient contro ls soap removal an d oil losses in the wash water. The control of the water-washing separator is easier than the refining cent rifuge and trou ble is seldom serious. Breakover will occur onl y because of carelessness, such as the use of an oversize discharge ring, poor refining condi tions , low oil or water temperatures, emulsion in refined oil or an excessively dirty bowl. The only critical factors are the selection of the correct discharge ring and the relationship of the tempe rature s of the oil and the water. The wash water must be as hot as, or preferably 5 t0.8 C hotter than, the oil to preve nt emulsi ons, Problems, such as emulsi on or very high soap content in the washed oil, are typically caused by impr oper separation at the refining centrifuge. Washed oil at ca . 82 C is passed through nozzles into the evacuated sectiori of a continuou s vacuum dryer, which controls the moisture content of the washed oil below 0.1%. A typical dryer operates at 70 cm Hg and is equipped with high level alarm and automatic shutdown capability. Before entering the refined oil storage tank, the dried oil is con tin uou sly cooled to ca. 49 C. If extended storage is necessary, a nitrogen blanke=t ma3?be applied to the surface of the oil to minimize oxidation.
EQUIPMENT The overall refining equi pment package should be designed and selected for its capability of producing a high yield of q uality produ ct, with operating flexibility to handle all types of edible fats and oils. For most refining installations, hermetic centrifuges are the cornerstone of
VOL. 55
the equipment presently utilized to achieve these requirements. I n a ddition to their efficiency and flexibility, hermetic centrifuges provide a closed, air free system which elimin ates the risk of oxidi zing the oil du ring refining. For example, peroxide values of all-hermetic refined oils are ca. 0.2 m.e. vs. 2 to 4 m.e. values for b atc h refined oils
Centrifuges Typical hermetic centrifuges are the De Laval VO, SRG and PX series. VO hermetic machines, such as the VO 194, are widely used by vegetable oil processors for lower volume applications. The interface between the light and heavy phases is controlled by varying the back pressure of the light phase discharge. Capacity of the VO 194 is ca. 5000 Ibs/hr. SRG-214 he rmeti c machines, with ca. four times the capacity of the VO 194, can be used in vegetable oil refining wherever the VO 194 is used, except for miscella refining. The B-214 is used for waterwashing in larger refineries, due to its large capacity of ca. 40,000 lbs/hr. It uses the same principle of balanced columns as the SRG-214, but since it is not hermetic, it differs in the met hod of controlling the interface location. To vary the interface position in the bowl, the diameter of the discharge ring must be changed. The ring traps the heavier layer of water in the bowl, thereb y confining the lighter layer of oil under the top disc for discharge from the center of the bowl. PX hermetic machines are used to separate liquids with high solids content and are designed for self cleaning operation. When solids build up in the bowl, a bottom sliding section is hydraulically opened to discharge the solids from the bowl. Within a few seconds, the bowl b otto m section returns to its original position an normal operat ion is resumed. Throughput capacity, depending upon the type of feed product and centrifuge selected, varies from the range of 25,000 to 50,000 lbs/hr. Refining centrifuges are often equipped with breakover warning systems or automatic sealing devices to minimize high loss of oil in the soap phase during periodic disr uptions in the neutral zone balance. A breakover occurs when the interface suddenly moves toward the bowl periphery and no longer contains the column of oil under the top disc. Large amounts of oil are discharged through the soap outlet in a partial breakover. When a total breakover occurs, the large oil flow quickly forces all the soap from the bowl and all the oil follows through the soap outlet port. Breakovers can happen when the centrifuge bowl is clogged, the refined oil outlet back pressure is too high, or the refined oil tempera ture or caustic concent ration drops considerably. Breakovers can usually be corrected by quickly opening the back pressure valve and readjusting until the soapstock and refined oil are discharging properly. The bowl may have to be reprimed or back flushed to correct large breakovers.
PER FORMA NCE Refinery performance control systems should be designed to monitor the refining yield efficiency over the system, evaluate the quality level of the refined oil and initiate pr ompt adjust ment of processing conditions for out of control incidents. Yield
Refining efficiency is generally considered to be the yield of dry neutral refined oil as a percentage of the available neutral triglyceride content of the crude oil. The former is either measured volumetrically and adjusted a necessary for temperature by specific gravity tables, or weighed by scale tanks. Crude oil qua nti ty is determi ned in a similar manner and then adjusted to available neutral triglyceride by a laborat ory loss evaluation~ The labor atory loss metho d, and hence the efficiency termi nology , varies
NOVEMBER, 1978
CARR: REFINING AND DEGUMMINGSYSTEMS FOR EDIBLE FATS AND OILS
with the oil type. Gum- con tai nin g crude oils, such as soybean, are usually evaluated by the chromatographic method for lab loss, and the refining efficiency is expressed as the ratio of neutral oil produced over the calculated neutral oil in the crude oil. The lab loss basis for cottonseed and corn oils is the AOCS cup loss method, and refining efficiency i expressed as "savings over cup." Palm and laurie oils refining efficiency is controll ed by the refining factor, a ratio of plant loss to FFA cont ent of the crude oil. Key features requiring close supervisory control are properly calibrated tanks, accurate daily inventories of stocks in all refinery vessels, stock transfer control systems to eliminat e overfilling tanks or mixin g stocks, and a responsive maintenan ce program to prevent stock leaks from valves and piping connections. Impro per refining conditions can also lose neutral oil through excess saponification, emulsions and process upsets. During refining, the dilu ted caustic supply and the caustic-oil feed to the primary centrifuge should be checked for sodium content at least once per shift. Soapstock should be monitored for neutral oil content at least every four hours. Wash water from the secondary centrifuge should be composited over a shift and analysed for neutral oil content.
Quality Refined oil quality standards are established for each oil type, compatible with the finished product quality objectives of the individual company. Final refined oil control samples are generally taken downstream from the vacuum dryer. At this point in the process, moisture content of the oil should not exceed 0.1%. In most casesr0.05%FFA and 50 ppm soap co ntent maxi mums are the primary endpoi nt lim its for all refined oils. Phosphat ide con tain ing crude oils, such as soybean, are also controlled by the residual gum level. These gums precipitate when the refined oil sample is treated with acetone. The quantity of precipitate can then be measured by visual comparison with a standard tube or by nephelometric equipment, to determine whether or not the oil is fully refined. In the base of cottonseed oil, completeness of refining is also controlled by comparing labo rator y bleach test colors of plan t refined oil vs. similar test on the laboratory refined crude oil samples.
Feedback In order for refinery personnel to adequately control yield and quality performance, process inst rumen tati on and analytical infor matio n should be prompt ly returned to their
771
control station. Any response delay will increase the amou nt of time that the process operates out of control. Some refiners reduce this feedba ck lag by utilizing the Sodium Balance Method for estimating loss at the primary centrifuges. Metering equ ipme nt, such as the Elliott or Sullivan system, can be used to Compare flowrates of crude oil input with refined oil outpu t for instantaneo us estimates of refin ing loss performance at any time thr oughou t the run. Despite all the improvements in equipment and instrumentation, the prime control factor is the attentiveness of the refinery personnel. Various mot ivation techniques have been tried, but the best approach is to provide management feedback for operating personnel to increase job satisfaction and knowledge. Yield and quality control can be improved by i nvolving operators in refini ng efficien cy goal setting, providing trend charts for tracking performance and clearly defining corrective actions for out of control in cidents. The same approach can be utilized to upgrade the responsiveness of the plant mainte nance program to plant equipment problems.
Troublesh oo ti n Periodic process control problems will arise, and operating personnel should be trained to quickly identify causes of these problems from the symp toms observed. Consistently poor yields can be caused by frequent start ups and shut downs from short refining runs, discontinuous refining due t o plugging filters or centrifuge bowls exceeding the centrifuges design capacity and improper processing conditions. Excess neutral oil loss to soapstock may be due to high centrifuge back pressure, improper feed temperatures or caustic flow and plugging of centrifug e bowls. High neutral oil loss in the wash water can be caused by the use of an improper ring dam or low water-oil feed temperature. High refined oil FFA or bleach color indicates underrefining, which can be due to low caustic strength or flow, insufficient caustic-oil mixing, or a rise in crude oil FFA or temperature. Conversely, a refined oil bleach color significantly lower than the crude oil refined and bleach color indicates unnecessary overrefining. If high soap content is observed in the vacuum dried oil, check primary centrifuge refined oil solids by a spin test and adjust ba ck pressure a necessary. Also investigate for low wash water rate and the oil-water mixing temperatu re. High moisture cont ent in the dried oil is usually due to low vacuum in the vacuum drier, poor separat ion at the water wash centrifuge or imprope oil-water mixing temperat ure.
