DRY FRAC
DRY FRAC
TIONAT
TIONAT
ION
ION
1. Historical perspective
1. Historical perspective
In edible oil processing, aIn edible oil processing, a fractionation process consistfractionation process consists of a controlled cooling of s of a controlled cooling of the oil, thereby inducing athe oil, thereby inducing a
partial, or ‘fractional’, crystallization. The remaining liquid (olein) is then separated from the solid fraction
partial, or ‘fractional’, crystallization. The remaining liquid (olein) is then separated from the solid fraction
(stearin) by means of a
(stearin) by means of a filtration or centrifugation.filtration or centrifugation.
This kind of
This kind of fractfractionationation process has been applied for ion process has been applied for almosalmost !" t !" yearyears. In s. In most litemost literaturerature,, HippolyteHippolyte
Mège-Mouriès
Mège-Mourièsis credited #ith the in$ention of %a is credited #ith the in$ention of %a patented method to produce certain fats of patented method to produce certain fats of animal origin&.animal origin&.
In fact, he concocted the production of a sort of margarine fat, by separating a liquid fraction from ordinary
In fact, he concocted the production of a sort of margarine fat, by separating a liquid fraction from ordinary
tal
tallo# lo# aftafter er gengentle tle coocoolinling. g. 'ut 'ut #it#ith h only tempeonly temperatrature ure difdifferferencence e as as the the dridri$in$ing g forforce, ce, a a frafractictionaonall
crystallizat
crystallization of a fat ion of a fat is a perfectly natural, spontaneous phenomenon. o it #as also obser$ed that in palmis a perfectly natural, spontaneous phenomenon. o it #as also obser$ed that in palm
(kernel) oil har$ested in tropical regions, small crystals #ould appear upon cooling and form a crystal
(kernel) oil har$ested in tropical regions, small crystals #ould appear upon cooling and form a crystal
suspension in the #ooden barrels during shipping to chillier estern *urope. These slightly denser solids
suspension in the #ooden barrels during shipping to chillier estern *urope. These slightly denser solids
e$entually settled, and such fractions could effecti$ely replace hardened fats in margarines +. In a more
e$entually settled, and such fractions could effecti$ely replace hardened fats in margarines +. In a more
e$ocati$e t#ist, #e could therefore consider these #ooden shipping drums the $ery first oil
e$ocati$e t#ist, #e could therefore consider these #ooden shipping drums the $ery first oil crystallizerscrystallizers, #ith, #ith
-ust the
-ust the peaceful ocean #a$es peaceful ocean #a$es pro$iding the pro$iding the necessary agitation to necessary agitation to keep the keep the mi in mi in suspension. /oreo$er,suspension. /oreo$er,
the natural fractional crystallization of fats #hen mildly cooled is echoed in the term ‘#interization’, referring
the natural fractional crystallization of fats #hen mildly cooled is echoed in the term ‘#interization’, referring
to the habit of lea$ing large oil tanks quiescent in #intertime to induce some mild crystallization and obtain a
to the habit of lea$ing large oil tanks quiescent in #intertime to induce some mild crystallization and obtain a
liquid fraction #ith impro$ed cold stability, in an economic fashion +0.
liquid fraction #ith impro$ed cold stability, in an economic fashion +0.
1espite the apparent spontaneity of the process itself, it took until the years 23" for the fractionation
1espite the apparent spontaneity of the process itself, it took until the years 23" for the fractionation
industry (and technology) to boom, #hen
industry (and technology) to boom, #hen the production of palm oil the production of palm oil in outh4*ast 5sia hea$ily increased andin outh4*ast 5sia hea$ily increased and
eport taes on processed palm oil #ere reduced. 5t that time ho#e$er, the boundaries of the technology
eport taes on processed palm oil #ere reduced. 5t that time ho#e$er, the boundaries of the technology
#ere mainly determined by the phase separation. In the early stages of fractionation technology, the olein
#ere mainly determined by the phase separation. In the early stages of fractionation technology, the olein
and stearin fractions of oils and fats had to be separated by settling, using only the force of gra$ity to bring
and stearin fractions of oils and fats had to be separated by settling, using only the force of gra$ity to bring
about a separation bet#een the hea$ier solid phase and the lighter liquid phase, #hich left the settled solid
about a separation bet#een the hea$ier solid phase and the lighter liquid phase, #hich left the settled solid
phase containing large quantities of entrained (trapped) liquid oil, almost certainly more than 6!7 +8. In the
phase containing large quantities of entrained (trapped) liquid oil, almost certainly more than 6!7 +8. In the
las
last t decdecadeades, s, the the concontintinuouuous s de$de$eloelopmepment nt of of sepseparaaratiotion n tectechnihniqueques, s, frofrom m $ac$acuum uum belbelt t filfiltratratiotion n toto
centrifuges and membrane press filters, has put fractionation on the map as a $ersatile and economic
centrifuges and membrane press filters, has put fractionation on the map as a $ersatile and economic
modification technique. 5lthough some specific techniques relying on use of detergents are still applied for
modification technique. 5lthough some specific techniques relying on use of detergents are still applied for
$ery particular production, actually only t#o main fractionation technologies are used in the 0st century’s
$ery particular production, actually only t#o main fractionation technologies are used in the 0st century’s
edible oil industry9
o Dry ractio!atio!, also kno#n as crystallization from the melt, is fractional crystallization in its most
simple form, and the economy of the technology allo#s it to be used for production of commodity fats. 1ry fractionation has long been regarded as an unpredictable, tedious and labor4intensi$e process. :o#e$er, the relati$ely cheap dry fractionation technique has e$ol$ed to the modification technology of the 0st century +;, as #ithout additi$es, polluting effluents or post4refining in$ol$ed, the sustainability and safety of the process is second to none.
o "olve!t ractio!atio!, already patented in the 2!"’s, in$ol$es the use of heane or acetone to let
the high4melting components crystallize in a $ery lo#4$iscous organic sol$ent. This can be helpful #ith respect to the selecti$ity of the reaction, but mainly offers ad$antages in the field of phase separation9 much purer solid fractions can be obtained, e$en #ith a $acuum filtration. 'eing a more epensi$e process, it is less common than dry fractionation and only comes into the picture #hen a $ery high added $alue of (at least one of) the resulting fractions makes up for the high cost.
In this contribution, the emphasis is on dry fractionation technology.
#. T$e Crystalli%atio! "tage
&ri!cipal co!cepts5 controlled crystallization of the melt is the backbone of any dry fractionation process, and the success of this process relies principally on the phase beha$ior of the constituent triglyceridess. Therefore it is probably useful to dedicate some #ords to that part of the physical chemistry of fats and oils that lies at the basis of the fractionation technology.
If anything is clear from bro#sing through the Lipid Library , it is the fact that a natural oil is a $ery comple miture of different triglycerides (not to mention diglycerides, free fatty acids, phospholipids, sterols, and uncountable other minor constituents). The mi of triglycerides does ha$e a repercussion on the melting beha$ior of a fat9 it does not ehibit a sharp melting point, but often displays a steady softening (or increasing liquid content) #ith increasing temperature, until it is completely liquid. The position and #idth of this melting range on a temperature scale is determined by the type of constituting triglycerides and compositional heterogeneity of the oil9
o The type of the triglycerides9 in a $ery simple approach, the longer the fatty acid moieties, the larger
the total molecule and consequently, the more energy (i.e. higher temperature) #ill be required to con$ert such triglycerides from a solid to a liquid ‘state’. 5 double bond in the carbon chain decreases the melting point dramatically, ho#e$er, and this is #hy oils containing a high proportion of unsaturated fatty acids are generally liquid.
o The broader the spectrum of triglycerides present in the oil, the broader the melting range. ome of
the triglycerides #ill only solidify (or melt) at !<=, #hereas others #ill still be hard as candle #a at room temperature.
5nother matter to take into consideration is the respecti$e concentration of each of these triglycerides. This makes t#o $ariables to consider if a certain triglyceride #ill remain in the melt or crystallize9 the temperature and its concentration (in fact, -ust the same principle applies for sugar in coffee). o, ideal solubility calculations based on the melting temperature and enthalpy of the pure solute, as #ell as the absolute temperature as the principal $ariables, can ser$e as first approach the solubility of a triglyceride in a sol$ent. 'ut this premise of ideality is the real issue in fractional crystallization of oil9 these are not regular solutions, the triglycerides are not dissol$ed in an ‘inert’ sol$ent> they are dissol$ed in a melt, i.e. other triglycerides. Thus, the fact that the sol$ent and solute ha$e quite some structural similarity leads to considerable de$iations from the ideal solubility. The most rele$ant is the occurrence of %intersolubility& in a solid state9 the property to form a solid ‘solution’ in #hich the constituting triglycerides cannot be separately determined, nor di$ided9 it beha$es as one phase. =onsequently, such intersolubility of the triglycerides often presents the largest fundamental problem in se$eral fractionation processes, as the actual goal of fractionation is to separate different triglycerides selecti$ely.
It is fair to state that other typical fat crystallization phenomena such as polymorphism are of secondary importance compared to intersolubility> typical fractionation conditions are generally sufficiently restricted in time and temperature range to only allo# one type of molecular arrangement to form. ?or palm oil, this is typically in a @’4form from start to finish.
Intersolubility is also increased at higher degrees of supercooling, so if the fractional crystallization is to occur selecti$ely, the challenge for the process engineer is to steer clear from such conditions and keep the melt -ust deep enough in metastable conditions to create a dri$ing force for crystallization of the triglycerides of interest, but not too deep as to pre$ent formation of solid solutions or uncontrolled crystal gro#th. If the crystal gro#th is #ell controlled, the crystal aggregates result in sharply discrete and dense spherulitic structures, sometimes measuring up to se$eral millimeters in diameter, #hich are fairly uniform in size and shape (Figure 1).
Figure 1. Aolarized light microscopic picture of typical spherulitic crystals de$eloping in palm oil fractions.
To be complete, the influence of minor components is not one to be captured in one phrase, but o$erall ‘impurities’ such as diglycerides ha$e a negati$e effect on crystal gro#th and filterability of the formed solids. 5lso product4#ise, it should be kept in mind that dr y fractionation is a ‘rubbish in, rubbish out’ process, so a
feedstock #ith a lot of impurities #ill ne$er result in t#o clean fractions. Co!'ucti!g ractio!al crystalli%atio!
The basic principles sketched in the former paragraphs help to eplain the key aspects of a crystallizer, the technological heart of the fractionation installation. It should be able to gently cool do#n a mass of oil (up to "" tonBbatch) and keep the resulting crystal suspension as homogeneous as possible. Cote that such gentle cooling means in fact imposing $ery lo# supercooling conditions, and it #ill result in a formation of fe#er and larger crystals, because the said conditions simply rule out the eistence of a mass of tiny crystals. ?at crystallization is a fairly eothermic reaction (up to D" kE can be released for e$ery kg of crystals formed), so the efficiency #ith #hich this energy can be r emo$ed is an important design feature. ?or most industrial crystallizers, this ranges bet#een 0" and 0"" Bm0
.F.
The problem #ith occurrence of ecessi$e intersolubility at higher degrees of supercooling eplains the $ery need for a (in terms of temperature) homogenizing crystallizer. Too steep temperature gradients #ithin the crystallizer #ill lead to formation of $iscous solid solutions near the cooling #all (the coldest spot), #hile oil entrapped in dead zones or #ith insufficient heat echange #ith a cooling #all could encounter too high temperatures to crystallize at an acceptable rate. Cote that t#o etreme temperatures #ill not result in an a$erage degree of crystallization, but in a $iscous slurry of badly filterable solid solution4crystals in a matri
of unstable, hazy liquid. 5t these unfortunate occasions, it is comforting to remember that a dry fractionation process is ""7 re$ersible, and that upon heating, this mess #ill re$ert to the melt again, ready for another attempt...
In order to preser$e the selecti$ity and to a$oid the temperature gradients #ithin the melt, the cooling rate under crystallization conditions is quite slo# (".048<=Bh), depending on the sensiti$ity of the reaction and the performance of the crystallizer. ensiti$ity of the reaction might be a little of a sub-ecti$e term, but it can be quite elegantly illustrated by Figure #. This sho#s a 1ifferential canning =alorimetry profile of palm oil (full line abo$e) and palm olein (dotted line belo#).
Figure #. 1= cooling profile (4!<=Bmin) of palm oil and its deri$ed palm olein fraction. *othermal peaks are sho#n up#ards.
The up#ard peaks in the profile sho# at #hich temperatures and ho# much heat of crystallization #ill be released upon cooling (here at !<=BminG). ?or palm oil, the sharp peak on the right is created by the fast solidification of predominantly trisaturated triglycerides. It is largely (and not solely, as some intersolubility #ill ine$itably occur) this fraction that is crystallized in the first step or ‘cut’ in multistage palm oil fractionation. 5lthough the absolute temperatures in this diagram are not really rele$ant for fractionation
because of the high cooling rate, the diagram suggests clearly ho# simple fractional crystallization can be9 you put the temperature in bet#een the t#o peaks and #ait for the solidification to take place. hen this first trisaturated fraction is remo$ed, ho#e$er, the remaining palm olein sho#s one large peak, and it is instantly clear that a slo# fractional crystallization of a distinct fraction is a lot harder to accomplish in this matri9 there is a lot more material to crystallize in a narro# temperature region, and intersolubility #ill ha$e its say.
The cooling medium remo$ing this heat of crystallization from crystallizers is typically clean cooling to#er #ater, sometimes mied #ith some propylene glycol to be able to #ork at subzero conditions (as in fish oil fractionation). =ooling by ammonia e$aporation can also be considered, but $ery often turns out to be too epensi$e for a classic installation. The cooling #all itself can be double4-acket, stainless steel cooling fins (plates) or pipes. Cormally, a cooling surface of at least ;m0 per m8 oil is epected to assure proper heat
transfer for bulk edible oil fractionation.
In programming the cooling regime, generally three ‘temperature’ parameters can be kept in check9 the cooling medium temperature, the oil temperature or sometimes also HT, the temperature difference bet#een oil and cooling medium. The subtlety of the process then eists in kno#ing #hich cooling rate and cooling modus to apply at #hich eact stage in the process, for there is no art in crash4cooling the lot. This is #here the process technologist can make a true impact on the outcome of the process, as a difference in cooling regime could lead to a different process cycle time, filterability of the slurry and o$erall economy of the process.
?or a gi$en cooling rate and heat echange surface, the appropriate remo$al of released crystallization heat to the cooling medium is also function of the agitation and the $iscosity of the bulk. The first being an eternally imposed process feature, the type and intensity of agitation is indeed a typical ‘design’ matter and linked #ith the concept of the crystallizer. It should be sufficient to enhance heat transfer and fa$or crystal initiation and secondary crystallization, but on the other hand ecessi$e agitation could damage the structures of the crystals being formed, #hich might present problems in the subsequent filtration stage.
iscosity on the other hand, is an intrinsic state of the oil (though to be fully correct, an edible oil crystal suspension does ehibit some shear4thinning beha$ior). *cessi$e $iscosities #ill reduce mass and heat transfer bet#een solid and liquid, hence decreasing the crystal gro#th rate, also by limiting molecular diffusion. The flo# properties of the oil are not only a function of solids concentration present in the liquid, but are also greatly influenced by the interactions bet#een the different crystal entities such as net#ork formation andBor ‘gelly layers’ +!. In normal fractionation conditions, the $iscosity of crystal suspensions -ust prior to filtration ranges from 8"" to 0""" mAa.s depending on the application, though in palm kernel fatty acid fractionation, $iscosities o$er !",""" mAa.s are not eceptional.
Figure (. 5 de$eloping oilBcrystal suspension. The darker #akes indicate ‘spontaneous separation’ of the liquid from the bulk upon stirring.
(. T$e "eparatio! "tage
5lthough the triglyceride separation theoretically is already established during crystallization, it is clear that the separation stage itself effecti$ely determines the product yields as #ell as the stearin quality. 5s more residual olein can be epelled from the solids cake, the final stearin #ill be more concentrated in crystals and #ill turn out ‘purer’ and #ill display higher and steeper melting. The olein quality is determined entirely by the amount and selecti$ity of crystallization in the preceding stage. In some applications, the formed crystals are often not sufficiently stress4resistant and get squeezed through the filter medium. Jb$iously, such contamination of crystals in the olein phase affects the efficiency of the fractionation process negati$ely and results in a liquid phase #ith inferior cold stable properties. J$erall, the ‘permitted’ degree of olein dilution in the stearin cake determines the choice for the applied separation technology, eemplified in Ta)le 1.
Ta)le 1. Diere!t separatio! syste*s or pal* oil ractio!atio! +,.
acuu* iltratio! Ce!triugal !o%%les Me*)ra!e press /1, )arg0I &al* Oil !0 !0 !0
I &al* Olei! !34!6 !34!6 !34!6
I &al* "teari! ;"4;0 83 8"480
"oli's i! cae /20 ;3 4 3!
Olei! Yiel' /20 60 63 D0
/embrane press filtration, as also used in for eample sludge de#atering systems, is by far the most used separation technology in dry fractionation no#adays. uch filters consist of a large steel frames that can easily hold up to !" filter plates together, each plate counting for up to 6m 0 of filtration surface and o$er
""K filter chamber $olume (Figure 3).
Figure 3. 5 membrane press filter used in dry fractionation
Lsually, the filter chambers are first filled #ith the crystal suspension, and doing so a large portion of the liquid olein is already passing through the filter cloths. Then #atertight membranes (one membrane per
chamber) attached to the internals of the plates are gradually inflated (#ith #ater, liquid oil or air) like internal balloons to the desired pressures, reducing the chamber $olume and pushing out residual liquid, #hich is immediately e$acuated $ia internal channels in the plate to#ards collecting tanks. The $olume reduction of the chamber thus literally compacts and dries the cake. Typical final squeeze pressures are 3 or !barg, 8" and e$en !" barg membrane press filters are a$ailable on the market to attain higher purity in speciality fat cakes. It is also good to note that the mass fraction of solids in the filter cake decays eponentially as a function of the distance to the filter cloth, and consequently thinner filter chambers and longer squeezing times can be helpful ( yet costly) means to reduce the entrainment significantly.
The #hole filtration plus squeezing operation can $ary from 8" to 2" minutes. 5fter this, the filter plate package is opened so the solid cakes can -ust drop by gra$ity into a stearin tank underneath the filter to melt.
Figure 4. :opper and stearin tank, mounted underneath the membrane press filter.
3. T$e Fractio!atio! &la!t Asse*)ly
Figure , presents a general lay4out of a present4day dry fractionation process. Jften multiple crystallizers are used in (o$erlapping) series. This is not only a matter of capacity, it is also in order to maimize the use
of the filter> by a good planning of the crystallization times of filtration, the epensi$e (batch) filter should be in constant operation.
?igure 3. Kay4out of a typical dry fractionation process.
The reduction of dead time of a filter can also be established by means of a crystallized offer buffer tank> each crystallizer can be quickly drained and made ready to recei$e the net batch of oil, #hile the cooled buffer tank #ill send set $olumes of crystal slurry to the filter, #hene$er it is ready. =ontinuous filtration systems ha$e been a $ery elegant strategy in dry fractionation as #ell, although currently, the demand for purer solid fractions as obtained by filter chamber compaction has pushed continuous belt filters some#hat out of the dry fractionation market.
It should be kept in mind that fractional crystallization of a triglyceride oil is a relati$ely slo# process and is therefore the time determining stage> some simple fractionations can be established in about !hr crystallizer residence time, #hereas more comple oils can require up to 8 days of cooling and crystal maturation before being sent to the filter.
In essence, the goal of fractionation is to create the biggest possible difference bet#een t#o fractions. Aalm oil is by far the most fractionated oil in the #orld. Mi$en the broad spectrum of triglycerides and also its naturally high amount of palmitic acid that gi$es a fat ‘body’ at room temperature, the separation of palm oil into sharply defined fractions usually happens in a multi4stage process (Figure 5).
Figure 5. /ultistage fractionation of palm oil #ith possible food applications for the $arious the fractions
The first step of dry fractionation of palm oil yields olein fractions #ith a cloud point belo# "<=. The olein fractions are used as a substitute for soft oils in frying, cooking and salad oils or are being further fractionated. Together #ith a further de$elopment of single4stage palm oil fractionation by technological impro$ements, there is an increased tendency to eecute a double or triple fractionation of palm oil in order to produce fractions #ith specific characteristics such as high I superoleins (IN3!) and hard palm4mid4 fractions (hard A/?) (IO83).
The latter fraction can ser$e as a feedstock for the production of typical cocoa butter equi$alents (='*), #hich are non4lauric fats similar in their physical and chemical properties to cocoa butter. They are often prepared by sol$ent fractionation +6, though the more contemporary de$elopments #ithin dry fractionation (better suited crystallizers, impro$ed separation technologies) are closing the gap bet#een the quality of sol$ent4 and dry4fractionated hard A/?.
5nother technological field of interest is the use of plug flo# reactors that allo# the fractional crystallization in a continuous fashion, offering considerable reduction in operation costs (such as steam usage and cooling po#er). Indeed, -ust as in any other edible oil processing technology, there is a continuous quest for economization and process optimization. :eat reco$ery systems, crystal seeding installations, optimized miing procedures and elegant plant lay4outs can all contribute to maimize capacity and minimize costs for a dry fractionation plants.
Jne final comment is that the process technologist should al#ays remember that a fractionation process yields t#o products, and thus that the sum of the $alue of the t#o fractions should al#ays eceed the processing cost and feedstock cost. This is #hy the feasibility of multistage fractionation is not only a matter of technological kno#4ho#, but also a matter of ha$ing markets for all ‘by4products’ generated along the #ay.
