Crystallography Theory: Importance of process on proper "Winterization"

I was taught to winterize my crude by dissolving it in ethanol at a 10:1 ration of solvent to crude at 80C. Next I place the mixture in a "So-Lo" freezer overnight and filter out solids in a funnel the next day. Repeat. Repeat. Lots of work, especially if you are extracting with CO2 and have crude with high content of solids. Surely there must be a better way.

My first contribution to the industry was the Sambo filter trolley, which given its simplicity and extensive filter area proved successful for small and medium refiners. For large refiners we recommended filter presses for which we developed an effective SOP.

The major drawback today is the initial need for 3 or more winterization steps. It is not enough to filter the solution 3 times with the three standard micron ranges (-45/+20, -20/+10, -10/+1.5). You must chill your solution each time before you filter again. What is creating this phenomenon where solids do not drop out of suspension all at once? Lets begin:

"Winterization" is a misnomer in the industry. What we are doing is called "Fractionation," so please call it as such.

Fractionation exploits the difference in solubility instead of melting point as in winterization. Crude is dissolved in a solvent such as Methanol, Isopropanol, Acetone, Hexane, or Ethanol in solvent to crude ratios of 3:1, 4:1, and 10:1. To a limit, the more dissolved a mixture, the better fractions separate from one another. If we use too much solvent, particles may remain dissolved at any temperature. We have always advocated the need for testing solvent ratios, though I now know other factors are at play.

To illustrate the solubility factor, consider the typical CO2 fractionation SOP. We warm crude, mixed with a solvent and allowed to cool to room temperature. Solids soluble in warm solvent become insoluble at room temperature and drop out of suspension. You can now filter out solids. No refrigeration required.

A secondary factor in the process is the solvent used. Ethanol is used arguably because it is edible and performs the process well. A laboratory with proper equipment and procedures should not endanger the public in the first place by contaminating their products with residual extraction chemicals. Soybeans, for example, are extracted using Hexane and produce and oil free of solvent safe to consume. If Hexane (or any other solvent) works better at fractionating, we should consider it. Fortunately ethanol works well, but Hexane and Acetone are important alternatives to consider. Solvents used in primary extraction and fractionation play unique roles so should be viewed differently.

Phosphatides, phospholipids, gums, all the same animal, play an important role in fractionation. These gums are contaminants and impede the growth of proper characteristics of fat and wax crystals during fractionation. Also, gums interfere with the separation of solids from the oil, creating additional losses when waxes are filtered out. Gums also slow down the filtration rate. Thus the first step post extraction is the complete removal of gums.

 Now that we have a clean oil low in gums, it helps to understand how crystals in fractionation behave. Fat and wax crystals are polymorphic, belonging to three categories: alpha, beta prime, and beta.

Alpha crystals are the first to form and are more prevalent when crude is chilled rapidly. These crystals are randomly oriented and have an approximate diameter of 5 microns. These crystals have the lowest melting point and are highly unstable.

Beta prime crystals are needle-shaped and cross-linked to form three-dimensional structures. These are typically 1 micron long with diameters from 3.8 to 4.15 microns. Sizeable amounts of oil can be trapped in the interstitial spaces between these crystals. Beta crystals do not form three-dimensional structures like beta prime, so they are the optimum crystal to grow to avoid losses.

Beta crystals are 25-50 microns long and have a diameter of 4.2 micron. They have the greatest stability of all three crystals and highest melting point. This is due in part to the crystals unidirectional formation.

One lesson learned here is the importance of growing mainly beta crystals. When we are forced to filter down to 1 micron particles, we either did not fully fraction out all waxes or we did not give crystals the proper conditions to transition from alpha to beta.

The formation of alpha nuclei depends on rate of cooling; a rapid chilling produces nuclei in large numbers. Many nuclei create many crystals. The goal here is to create many enormous crystals, not an enormous quantity of small crystals. Also, it is possible to seed nuclei from another batch of crystals to jump-start the process.

So how does all this work in practice?

There are no practical examples that I am aware of, but theory suggests:

1. Completely melting waxes allows crystal shape to reset and destroy all nuclei. This is already standard practice. Now you know why.
2. We must place crude in a chiller while it is 15-20 degrees above its melting temperature. This promotes rapid creation of nuclei. Whether this is possible to do so on a solvent/crude mixture depends on melting point of waxes and boiling point of solvent. What is the melting point of your waxes?
3. Once oil is dissolved in solvent and chilled if it will form alpha crystals.
4. Under suitable conditions, beta prime crystals convert to beta. This depends on rate of cooling, cooling time, storage temperature, agitation, solvent ratio, and saturation of solids.

The process of slowly cooling the oil allows fractions to form beta crystals that are large, easy to filter, and reduce losses.

The crystal formation process would explain why we need to filter our crude and winterize several times. Normal SOP's I've seen is to chill your mix to -86C as quickly as possible. My belief is that the rate of cooling is the fundamental problem causing this issue, followed by not enough cooling time and also the presence of gums and other contaminants that hinders crystal nuclei formation.