Why Washing is Required?: A by-product of BioDiesel reaction is the formation of glycerol or glycerin and soap, formed as the free fatty acids (FFA) react with the catalyst. Any water contained in the oil, catalyst or alcohol will cause an increase in the free fatty acid level by hydrolysis of the oil and therefore more soap will form. While it is critical to eliminate the presence of water in the trans-esterification step, there are a number of options available to the biodiesel producer to purify the raw biodiesel but in simple terms, either a “Water-wash” or “Waterless wash” method may be employed.
Water Wash (WW) Process: Previously, soap was removed from biodiesel using a water wash (WW) process. It is not recommended any more. This requires the raw biodiesel to be intimately mixed with water and gently agitated. The mixture is then allowed to settle so the water phase separates from the methyl ester or biodiesel phase. In this method, soap is extracted into the water phase as are most of the metals and glycerin. However, the reactions are not complete and there are many downside issues producers need to be aware of should this method be used.
Waterless or Dry Wash Method: Given the excessive volume of water required for the WW method, producers have turned to alternative Dry Wash Methods such as using ion exchange resins, cellulosics and inorganic compounds (e.g. magnesium silicate). When resin is used, a combination of adsorption and ion exchange mechanisms are utilized. Both ion exchange and adsorption take place in the resin bed. Trace metals and polar contaminants like glycerin and monoglycerides are removed via ion exchange where a hydrogen ion replaces the contaminant, soap and glycerin are removed via adsorption.
Dry Washing is done by resins which are user friendly and easy to handle while charging, during process operations, regeneration and unloading after exhaustion. These resins are thermally stable up to 120^oC (250^oF) and can be regenerated with suitable quantity of MeOH. It is important to note that the Purification of methyl ester / B100 process stream with resin is a polishing step. It is not a substitute or alternate for bulk phase separation of the methyl ester and glycerin. These biodiesel purification resins are the highly productive, most cost-effective polishers with proven track records across globe.
Resin Used for Dry Wash: It is a Macroporous Strong Acid cation exchange resin with sulphonic acid functional group in hydrogen or sodium ionic form. It is a Gel type Strong Acid cation exchange resin with sulphonic acid functional group in either the sodium or hydrogen form. Ion Exchange resin technology plays a crucial role in the purifying of raw biodiesel fue. These are offered in dry form. These resins achieves highest quality biodiesel as per the norms of EN or ASTM. (European and American norms).
These are engineered to handle desired pressure drops and flow kinetics. These are specifically designed pore size for effective processing of long chain hydrocarbons (for Macroporous type). These are highly compatible with variation in the process feed, temperature and pH. These treat diverse bio-diesel qualities originating from varied sources of triglycerides. These can regenerated spent media thereby achieving lower process and disposal cost. It is a simple and continuous process compared to messy water separation or other competing processes.
These have good life, which depends on Ester feed quality, Process design, Levels of soap and catalyst in resin feed. Higher levels of soap and catalyst in feed will exhaust the resin capacity and hamper its absorption ability for water and methanol. Effectiveness of the bulk separation process of methyl ester and glycerin. Quality and maintenance of process equipment.
Methanol Regeneration Procedure: These remove glycerol / glycerin from raw biodiesel fuel through the combined mechanisms of adsorption and ion exchange. The very polar glycerin molecule prefers the polar environment rather than the biodiesel phase that consists of fatty acid methyl esters which are nonpolar. The adsorption capacity for glycerol depends heavily of the methanol content of the raw biodiesel. While a higher methanol level will result in lower operating capacity to adsorb glycerol, the adsorptive capacity of the resins is dependent on a low percentage of methanol in the raw biodiesel fuel to be purified. Methanol content needs to be greater than 1%, 2-4% has worked well in many installations.
These two mechanisms (1% and 2-4%), ion exchange and adsorption, are independent of each other as are their respective break-through points. Which breakthrough occurs first depends on the level of the impurities in the raw biodiesel. The glycerol breakthrough point can be measured by regularly determining the glycerol content in the treated biodiesel using either the ASTM D-6584 or the European EN14105 test methods. In addition to the glycerol breakthrough there will be an ionic breakthrough point where resin will be removing Na and K metal ions from the alkali catalyst used in the trans-esterification reaction, plus other metal ions such as S, P, Fe and Mg commonly found in the raw oils used as the starting feedstock.
There are two methods for applying methanol for regeneration of the bed:
Method 1: Draining the biodiesel from the column. Introduce methanol from the bottom such that all the resin is covered. Re-circulate the methanol downflow for 4-6 hours. Drain the methanol and send back to the methanol recovery unit to be used in subsequent trans-esterification reactions. Introduce biodiesel from the bottom and allow 1-hour conditioning before restarting the service cycle.
Method 2: Alternatively pass methanol, down-flow through the column at one-half the service flow rate. This methanol can be captured once it exits the vessel and returned to the methanol distillation tower to be used in subsequent trans-esterification reactions. Begin downflow introduction of raw biodiesel and begin next service cycle.
The advantage of these procedures is that there will be virtually no liquid waste since the methanol can be re-cycled and re-used.