The Final Treatment

Washing or polishing followed by distillation offers the purest biodiesel around
By Raj Mosali | May 01, 2012

Biodiesel is traditionally produced via esterification of fatty acids and transesterification of triglycerides with an alcohol such as methanol and sodium methylate. The transesterification of triglycerides is comprised of three sequential, reversible reactions wherein triglycerides react to form diglycerides, monoglycerides and glycerol.

All biodiesel processes, whether traditional or nontraditional, involve a settling process as the penultimate step. During the settling process, the biodiesel and glycerin mixture is settled using a decanter or a centrifuge to separate the biodiesel from the glycerin. The separated biodiesel is then taken through the final steps, which, in some cases, involve two stages: washing/polishing and distillation. Washing/polishing is mandatory and biodiesel distillation is optional. This article discusses both the washing/polishing and distillation stages.

Washing and polishing mean the same thing. The term washing is used if water is used to do the washing step, and the term polishing/filtration is used if powder, ion exchange resins or some other media is used instead. This step is necessary to wash or polish off the excess glycerin or soap in the freshly separated biodiesel to meet the ASTM specification.

Biodiesel distillation is an optional step. As the name indicates, the distillation process distills the fuel to a colorless methyl ester. Both of these steps are individually addressed below in detail.

What is Washing or Polishing?

A fatty acid in the oil and base (catalyst) reacts to form a new compound, called soap, and water. Compounds such as soap, in which the hydrogen (proton) of an acid has been replaced with a metal ion are often called salts. The reason that such compounds exist is that materials such as sodium hydroxide (NaOH) or potassium hydroxide (KOH) can split apart, or dissociate, in a fashion that gives Na+ and OH- or K+ and OH-, in which the protons and electrons are not evenly distributed, leading to charged particles. Thus, having the same charge, Na+ or K+ can replace H+ here.

And during the settling process, stray amounts of glycerin end up in the final product. Both soap and glycerin have to be taken out for biodiesel to meet the ASTM biodiesel specification, D6751.
You can take the soap/glycerin out by washing it with water or mixing the biodiesel with magnesium silicate and using specialty soap-absorbing chemicals.

Whether one is washing or polishing, the step is extremely critical. This step should be flexible enough to be able to address excess soap or glycerin in the process. If there is a glitch in the production or the settling process, a properly planned and designed washing/polishing process could act as a catchall process to fix the issues.

Comparing operational costs of both water wash and dry polishing (using magnesium silicate, clay or ion exchange resin), the water wash is cheaper. Currently, the per-gallon cost of water washing in a 10 MMgy plant ranges between 2 and 4 cents. This includes the cost of water, disposal or treatment, energy and labor. Polishing, however, costs between 6 and 9 cents a gallon. This includes the cost of filtrate, disposal/landfill costs, and labor.

Washing with water is preferred where water is either available in abundance or there is a means to dispose of the wash water, such as through a nearby ethanol plant or wastewater treatment plant, or another location where the disposing laws are not as stringent as some other locations.

With the added upfront costs of building a water wash system with some water treatment capabilities in place, and considering the amount of filtrate to be disposed, a water wash system makes sense in a typical biodiesel plant that is larger than 10 MMgy. Plants with production capacities of less than 10 MMgy are better off most times with a polishing system.

A fairly recent addition to the ASTM 6751 specification, voted in a few years ago, is cold soak filtration. Cold soak filtration is when biodiesel is chilled to a point and reheated back to room temperature. The chilling and reheating process will result in the formation of mushy crystal-like material called sterols. This material will not blend with diesel fuel and will clog the fuel filters on vehicles. To address this and other related issues, ASTM issued the cold soak filtration guidelines.

With a water wash process, cold soak filtration should still be addressed outside of the wash step. In the case of polishing, cold soak filtration could be addressed at the same time the polishing step is performed. Jatrodiesel does this today at number of its plants.

Biodiesel Distillation

We believe biodiesel distillation is the future of biodiesel. Various updates currently proposed in ASTM requirements related to unreacted components, and most of the other requirements that might pop up in the future related to metals, salts and so forth, can all be addressed today with biodiesel distillation.

Distillation is chosen for various reasons, from a desire to change corn oil biodiesel’s red color to the need to remove high sulfur and metal content from yellow grease or some animal fats, for example. Biodiesel resulting from distillation is the purest form of the fuel.

Through distillation, unreacted oil components—mono-, di- and triglycerides, metals, catalyst, salts and pigmentation—are left over as the column bottoms and need to be drained off or sent out along with crude glycerin.

Biodiesel from various feedstock distills at various temperatures. The range is between 210 degrees Centigrade to 250 degrees C. As you can see, the biodiesel distillation process operates at a very high temperature compared to other parts of the production process.

The typical and cost-effective provider of heat to this process is a thermal fluid (or thermal oil) heater. A thermal oil heater has the capability to operate at a very high temperature, if designed ahead of time to handle high temperatures. The thermal heating fluid is the heat-transferring liquid instead of, for instance, steam in a steam boiler or water in a water heater. The thermal fluid that is used as the heat transfer liquid in the heater should be capable of handling very high temperatures; if not, the high temperatures will disintegrate the thermal fluid, requiring expensive heating fluid replacement.

Due to the high-temperature operations, heat transfer equipment such as heat exchangers and condensers should meet the American Society of Mechanical Engineers specifications or be built to ASME specifications. Also, due to the high temperature of biodiesel during the process, a potential hazard for flashing exists. At very high temperatures, and with enough vapor present, any liquid can flash with sudden exposure to air. Care should be taken during the design process to address these issues for proper and safe operation.

Another consideration often overlooked in the biodiesel distillation process is structural related. The reactors or columns involved should be able to withstand constant expansion and contraction. They must also have enough thickness and support to handle high heat. Insulation is also extremely important. Other parts of the biodiesel production process could survive with little or no insulation, but in the case of distillation, insulation is a critical component and it is well worth the investment to improve the efficiency and reduce operation costs.

A couple of glaring disadvantages with biodiesel distillation are loss of yield due to distillation, which could range anywhere from 1 to 5 percent by weight, depending on the feedstock and the biodiesel conversion process; and the high temperatures inherent with distillation result in the failure of oxidative stability, so an additive needs to be added for the distilled biodiesel to meet the ASTM specification.

When a typical customer uses clear biodiesel resulting from a distillation process, they rarely would like to buy anything else. It is not just the perception, but a clear biodiesel blends better than normal biodiesel even though both clear and normal biodiesel meets the ASTM specification. Biodiesel distillation does not do much to the cloud point of the biodiesel, but by restricting mono- and diglycerides to next to nothing, the blending and related filter plug issues are considerably reduced. Some of the larger oil companies that are buying wet gallons restrict mono- and diglyceride values, and metals. The specification is easily achievable with biodiesel distillation instead of a hit-and-miss approach by trying to fix the existing biodiesel process on the frontend.

To summarize, water is scarce and disposal of wastewater is increasingly regulated. The regulations and related paperwork might be onerous for a water-wash-based system compared to a polishing system’s once-a-year chemical profile one needs to prepare for the hauling company. Eventually, the costs to treat or regulate water could be such that polishing will be the only way to go.

Biodiesel distillation produces the purest form of biodiesel. If you are in the planning stages to build a plant, including the biodiesel distillation process would greatly reduce the headaches resulting from changes to the ASTM specification and any blending-related issues experienced by the customers.

Author: Raj Mosali
President, Jatrodiesel Inc.
(937) 847-8050 ext. 201
[email protected]

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