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Report Catalogue Data

  Report Class   General Public Report
  Analysis Type   Situation Analysis
  Issue Category   Technology Analysis
  Publish Date   07_21_2008
  Last Update   03_20_2009
  Reference Code   GPR-SA.TA.BT-20080721-AWW

Biodiesel Technologies
Analyzing Water-Free Washing Bio-diesel

By Opubo G Benebo
More Update Post: 07_22_2008; 07_24_2008; 07_25_2008

Water-free washing biodiesel, the process of removing transesterification reaction residues in biodiesel, is an alternative approach to the method for removal of the residues with the use of water that led to the description of the process as "washing biodiesel". Washing biodiesel as a process came about as a method developed to remove the reactants-residues in biodiesel produced by the transesterification reaction of vegetable oils containing some Free Fatty acids, FFA. More specifically, the "washing biodiesel" processes involved the spraying of water on biodiesel liquid, resulting in the passing of water droplets through the biodiesel accompanied with the extraction of the reaction-residues from the biodiesel. The washing is usually accomplished by one of two methods: The mist washing method and The bubble washing method; though the recognition of both method as not being most efficient resulted in the development of fuel-bubbling washing biodiesel for both batch and continuous washing.

in any event, the methods of spraying water on biodiesel, as explained, resulted in the generic use of the phrase "washing biodiesel", by which the term "washing biodiesel" has now come to signify any process of removing residues in biodiesel by any means that accomplishes the task. Consequentially, the process resulted in the further qualification of the washing techniques with "water"  as in "water washing" and "water-free (also waterless and Dry) washing".  The latter method is currently supported by the use of adsorbents such as magnesium silicate, zeolite, resins, and others. Choosing any one of these adsorbents for the dry-washing of biodiesel therefore requires conversance with the science basics of these adsorbents physical and chemical interactions with the transesterification reaction-residues, to the extent impacted also by the properties of these residues.

Residue Substances Properties
The transesterification reaction is such that the concentrations of the reactants can be controlled to limit the concentrations of the reaction residues and therefore the scope of methods employed for the washing. Depending on the method of production of the biodiesel, and in particular the adopted method of quenching the reaction, the reaction-residues to be washed out may include all or some of the following: Soap when the vegetable oil has some free fatty acids, FFA; lye - unreacted Sodium (or Potassium) hydroxide; Alcohol - either methanol or ethanol; and Glycerin traces still floating within the biodiesel after the initial draining off of the glycerin. The method for washing the residue substances therefore derives from the properties of these reaction-residues irrespective of the chosen method of purification.

The salient characteristic of the soap molecules is that the molecule is an amphiphile. The removal of the soap from the bio-diesel therefore is possible with one of two methods: The use of a substance

that is of opposite polarity as the hydrophilic group of the amphiphiles, and The use of a substance that adsorbs the hydrophobic group of the molecule.

 The Sodium (or Potassium) Hydroxide or lye is naturally ionic and therefore has the preference of forming ionic bond with substances that have opposite ionic charges in polarity. The molecules are also of very defined geometry of easily calculable circumscribing circle that defines the radius of an enclosing sphere; and therefore the minimum size tunnel through which it can pass. However, equally important of note is that these may also be removed by the method chosen for quenching the reaction, and so does not present a situation of absolute need for washing.

Residue methanol or ethanol is of linear structure of well-defines length and effective diameter, and can therefore be represented with a cylindrical shape. Of course, this class of residue could also be separated from the biodiesel by distillation, and so does not present a situation of absolute need for washing.

Glycerin or glycerol molecules generally are linear backbone substances with branches of OH extending from the backbone. Effectively therefore, these molecules also have fixed calculable effective diameter for a  psuedo-cylinder descriptive of the size of the molecule.

Dry Washers Physical and Chemical Properties
The effectiveness of a dry washing adsorbent derives from the physical and electrical properties of the substances, which are factors in the considerations of their uses.

Magnesium Silicate is the most often used of these dry washer adsorbents.  Magnesium Silicate is a mineral crystal, common called Talc and is the main compound in Talcum Powder. The adsorbents is of monoclinic crystal structure and is hydrous and fibrous. The characteristic of monoclinic crystals is that one of the surfaces is slightly obtuse oriented therefore allows for access into the crystal interior. Further, these crystals have the tendency to remain polarized after been subjected to intense electric fields - essentially providing a means of designing polarized products out of them.

Zeolite, the common name of aluminosilicate, is often of well-define structures, that are seemingly spherical. These adsorbents have internal cavities and opening into them, and sometimes through them. These open cavities occludes - that is accommodate and holds within - a variety of cations including such as Na+, K+, Ca2+, Mg2+. These cavities and the tunnels or "opening" to them are of well-defined shapes and sizes as well and so are very selective in the types and sizes of ions that can be occluded; and this selectivity is the salient property of these adsorbents in their uses as molecular sieves and makes them suitable for washing bio-diesel production impurities. Further

aluminum silicate often are mixed with the talc or magnesium silicate, with the latter taking on the shape of the mineral zeolites.

Resins, the last of the most frequently used adsorbents for the purification of bio-diesel, are synthesized polymer products. Most resins are simply ion-exchange resins that are polymer beads. These have been synthesized such that there are porous openings all over the beads of very specific sizes to ensure selectivity of chemicals, and have also been made to exchange trapped ions of a specific chemical property and in exchange give up a different chemical ion. 

Selecting a Dry Wash
In view of the properties of the adsorbents suitable for separation of the reaction-residues from the biodiesel, water-free washing biodiesel can be carefully planned on the basis of the type of free fatty acids in the vegetable oil by which is determined the carbon chain length for choosing an adsorbent that can completely occlude the residue-molecule; the concentration of the ion-exchange spots for adsorption of the lye molecules; and the degree of cross-link branches that should trap the glycerol molecules as they flow past the substances.

An equally significant factor in the selection of an adsorbents is the strength of the polarity of the ionic groups of the adsorbent: The amount of electrostatic force exerted on the ionic groups of the amphiphiles in the reactor-effluent biodiesel stream is directly proportional to the electro-ionic strength of the ionic group because the electrostatic force of attraction between the ionic group of the amphiphiles and of the adsorbents are directly proportional to the product of the two electrostatic forces. A factor related to the strength of the polarity of the adsorbents is  the ions-group count per adsorbent. Clearly the efficiency of the adsorbents in removing amphiphiles increase with ionic-group increases in surface density, because the more ionic groups there are, the greater the number of amphiphiles will be removed. Moreover, the net electrostatic force of attraction prior to the ionic-bonding will be higher as a result of the net force exacted on an amphiphile by a single particle of the adsorbent.

Washing Pellets
The use of these adsorbents has often entailed the mixing of the biodiesel and the Magnesiun Silicate or Zeolite powders or ion-exchange resin. The mixing of the biodiesel and the powder or ion-exchange resin however subsequently required filtration, though the former was more intense filtration than the latter. However, a much better approach is to use Washing [Biodiesel] Pellets which are pellets formed of binders and the adsorbents, that enable readily accomplishing water-free washing biodiesel purification.

Ultimately with the knowledge of all these properties, design of the separator can be more precisely undertaken.

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