Despite the apparent spontaneity of the process itself, it took until the 1960s for the fractionation industry (and technology) to boom, when the production of palm oil in Southeast Asia heavily increased and export taxes on processed palm oil were reduced. At that time, however, the boundaries of the technology were 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 gravity to bring about a separation between the heavier solid phase and the lighter liquid phase, which left the settled solid phase containing large quantities of entrained (trapped) liquid oil, almost certainly more than 75%. In the last decades, the continuous development of separation techniques, from vacuum belt filtration to centrifuges and membrane press filters, has put fractionation on the map as a versatile and economic modification technique. Although some specific techniques relying on use of detergents are still applied for very particular production, actually only two main fractionation technologies are used in the 21st century’s edible oil industry:
1. Dry Fractionation, also known as crystallization from the melt, is fractional crystallization in its most simple form, and the economy of the technology allows it to be used for production of commodity fats. Dry fractionation has long been regarded as an unpredictable, tedious and labor-intensive process. However, the relatively cheap dry fractionation technique has evolved to the modification technology of the 21st century, as without additives, polluting effluents or post-refining involved, the sustainability and safety of the process aresecond to none.
2. Solvent Fractionation, already patented in the 1950s, involves the use of hexane or acetone to let the high-melting components crystallize in a very low-viscosity organic solvent. This can be helpful with respect to the selectivity of the reaction, but mainly offers advantages in the field of phase separation: Much purer solid fractions can be obtained, even with a vacuum filtration. Being a more expensive process, it is less common than dry fractionation and only comes into the picture when a very high added value of (at least one of) the resulting fractions makes up for the high cost. In this contribution, the emphasis is on dry fractionation technology.
The Crystallization Stage
Principal Concepts
A controlled crystallization of the melt is the backbone of any dry fractionation process, and the success of this process relies principally on the phase behavior of the constituent triglyceridess. Therefore, it is probably useful to dedicate some words to that part of the physical chemistry of fats and oils that lies at the basis of the fractionation technology. The fact that a natural oil is a very complex mixture of different triglycerides (not to mention diglycerides, free fatty acids, phospholipids, sterols, and uncountable other minor constituents). The mix of triglycerides does have a repercussion on the melting behavior of a fat: It does not exhibit a sharp melting point, but often displays a steady softening (or increasing liquid content) with increasing temperature, until it is completely liquid. The position and width of this melting range on a temperature scale is determined by the type of constituting triglycerides and compositional heterogeneity of the oil:
1. The type of the triglycerides in a very simple approach, the longer the fatty acid moieties, the larger the total molecule and consequently, the more energy (i.e. higher temperature) will be required to convert such triglycerides from a solid to a liquid ‘state’. A double bond in the carbon chain decreases the melting point dramatically, however, and this is why oils containing a high proportion of unsaturated fatty acids are generally liquid.
2. The broader the spectrum of triglycerides present in the oil, the broader the melting range. Some of the triglycerides will only solidify (or melt) at 5°C, whereas others will still be hard as candle wax at room temperature.
Another matter to take into consideration is the respective concentration of each of these triglycerides. This makes two variables to consider if a certain triglyceride will remain in the melt or crystallize: the temperature and its concentration (in fact, just the same principle applies for sugar in coffee). So, ideal solubility calculations based on the melting temperature and enthalpy of the pure solute, as well as the absolute temperature as the principal variables, can serve as a first approach to the solubility of a triglyceride in a solvent. But this premise of ideality is the real issue in fractional crystallization of oil: These are not regular solutions, the triglycerides are not dissolved in an ‘inert’ solvent; they are dissolved in a melt, i.e. other triglycerides. Thus, the fact that the solvent and solute have quite some structural similarity leads to considerable deviations from the ideal solubility. The most relevant is the occurrence of “intersolubility” in a solid state: the property to form a solid ‘solution’ in which the constituting triglycerides cannot be separately determined, nor divided: It behaves as one phase. Consequently, such intersolubility of the triglycerides often presents the largest fundamental problem in several fractionation processes, as the actual goal of fractionation is to separate different triglycerides selectively.
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 allow one type of molecular arrangement to form. For palm oil, this is typically in a β’-form from start to finish. Intersolubility is also increased at higher degrees of supercooling, so if the fractional crystallization is to occur selectively, the challenge for the process engineer is to steer clear from such conditions and keep the melt just deep enough in metastable conditions to create a driving force for crystallization of the triglycerides of interest, but not too deep as to prevent formation of solid solutions or uncontrolled crystal growth. If the crystal growth is well controlled, the crystal aggregates result in sharply discrete and dense spherulitic structures, sometimes measuring up to several millimeters in diameter, which are fairly uniform in size and shape. To be complete, the influence of minor components is not one to be captured in one phrase, but overall impurities such as diglycerides have a negative effect on crystal growth and filterability of the formed solids. Also product-wise, it should be kept in mind that dry fractionation is a ‘rubbish in, rubbish out’ process, so a feedstock with a lot of impurities will never result in two clean fractions.
Conducting Fractional Crystallization
The basic principles sketched in the former paragraphs help to explain the key aspects of a crystallizer, the technological heart of the fractionation installation. It should be able to gently cool down a mass of oil (up to 100 ton/batch) and keep the resulting crystal suspension as homogeneous as possible. Note that such gentle cooling means in fact imposing very low supercooling conditions, and it will result in a formation of fewer and larger crystals, because the said conditions simply rule out the existence of a mass of tiny crystals. Fat crystallization is a fairly exothermic reaction (up to 180 kJ can be released for every kg of crystals formed), so the efficiency with which this energy can be removed is an important design feature. For most industrial crystallizers, this ranges between 120 and 200 W/m2·K.
The problem with occurrence of excessive intersolubility at higher degrees of supercooling explains the very need for a (in terms of temperature) homogenizing crystallizer. Too steep temperature gradients within the crystallizer will lead to formation of viscous solid solutions near the cooling wall (the coldest spot), while oil entrapped in dead zones or with insufficient heat exchange with a cooling wall could encounter too high temperatures to crystallize at an acceptable rate. Note that two extreme temperatures will not result in an average degree of crystallization, but in a viscous slurry of badly filterable solid solution-crystals in a matrix of unstable, hazy liquid. At these unfortunate occasions, it is comforting to remember that a dry fractionation process is 100% reversible, and that upon heating, this mess will revert to the melt again, ready for another attempt.
In order to preserve the selectivity and to avoid the temperature gradients within the melt, the cooling rate under crystallization conditions is quite slow (0.2-3°C/h), depending on the sensitivity of the reaction and the performance of the crystallizer. Sensitivity of the reaction might be a little of a subjective term, but it can be quite elegantly. This shows a differential scanning calorimetry profile of palm oil (full line above) and palm olein (dotted line below).
DSC cooling profile (-5°C/min) of palm oil and its derived palm olein fraction. Exothermal peaks are shown upwards. The upward peaks in the profile show at which temperatures and how much heat of crystallization will be released upon cooling (here at 5°C/min). For 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 will inevitably occur this fraction that is crystallized in the first step or ‘cut’ in multistage palm oil fractionation. Although the absolute temperatures in this diagram are not really relevant for fractionation because of the high cooling rate, the diagram suggests clearly how simple fractional crystallization can be: You put the temperature in between the two peaks and wait for the solidification to take place. When this first trisaturated fraction is removed, however, the remaining palm olein shows one large peak, and it is instantly clear that a slow fractional crystallization of a distinct fraction is a lot harder to accomplish in this matrix: There is a lot more material to crystallize in a narrow temperature region, and intersolubility will have its say.
The cooling medium removing this heat of crystallization from crystallizers is typically clean cooling tower water, sometimes mixed with some propylene glycol to be able to work at subzero conditions (as in fish oil fractionation). Cooling by ammonia evaporation can also be considered, but very often turns out to be too expensive for a classic installation. The cooling wall itself can be double-jacket, stainless steel cooling fins (plates) or pipes. Normally, a cooling surface of at least 4 m2 per m3 oil is expected to ensure proper heat transfer for bulk edible oil fractionation.
In programming the cooling regime, generally three temperature parameters can be kept in check, the cooling medium temperature, the oil temperature or sometimes also ΔT, the temperature difference between oil and cooling medium. The subtlety of the process then exists in knowing which cooling rate and cooling modus to apply at which exact stage in the process, for there is no art in crash-cooling the lot. This is where 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 overall economy of the process.
For a given cooling rate and heat exchange surface, the appropriate removal of released crystallization heat to the cooling medium is also function of the agitation and the viscosity of the bulk. The first being an externally imposed process feature, the type and intensity of agitation is indeed a typical design matter and linked with the concept of the crystallizer. It should be sufficient to enhance heat transfer and favor crystal initiation and secondary crystallization, but on the other hand excessive agitation could damage the structures of the crystals being formed, which might present problems in the subsequent filtration stage.
Viscosity, on the other hand, is an intrinsic state of the oil (though to be fully correct, an edible oil crystal suspension does exhibit some shear-thinning behavior). Excessive viscosities will reduce mass and heat transfer between solid and liquid, hence decreasing the crystal growth rate, also by limiting molecular diffusion. The flow properties of the oil not only are a function of solids concentration present in the liquid, but are also greatly influenced by the interactions between the different crystal entities such as network formation and gelly layers. In normal fractionation conditions, the viscosity of crystal suspensions just prior to filtration ranges from 300 to 2000 mPa·s depending on the application, though in palm kernel fatty acid fractionation, viscosities over 50,000 mPa·s are not exceptional.
If you want to know the literature of dry fractionation, please click the link below: