Glycerin Distillation. The Glycerin Distillation unit is designed to steam fractionate the raw or crude glycerin into a major Distillate I fraction, a lesser Distillate II fraction for recycle or separate disposal and other residual matter. The Unit 16 operates continuously, 24 hours per day, except when the heavier residues are periodically distilled off to remove them from the system. Evaporation is carried out at low pressures around 13 mbar and the presence of steam in the tower promotes glycerin flow and internal column recirculation. The major fraction of the distilled glycerin is continuously bleached with activated carbon to obtain pharmaceutical product – Distillate I. Besides the bleached main distillate, a small amount, about 3 %, of Distillate II is produced which contains water and low boiling impurities.
Process Description. The raw glycerin is preheated by exchange with hot glycerin product and then flashed at a pressure of around 100 mbar to degas and evaporate some of the moisture present in the raw glycerin. The resultant more concentrated glycerin is then fed to specially designed 4 brag steam injectors at the base of the glycerin column. Steam at 16 barg is used to reboil the column. Typically the column operates as low as 13 mbar pressure with the live steam promoting the fractionation process.
The vapours flowing from the top of the distillation column to the overhead condenser still contain the remaining glycerin (~3%) and virtually all the low boiling impurities, as well as much of the water vapour.
Any remaining vapours and the non-condensable gases which enter with the crude glycerin are drawn off by the steam ejectors, compressed to reduce the negative pressure to an intermediate stage and condensed in the first condenser together with the driving steam. A water ring seal pump will compress the non-condensable gases to atmospheric pressure. A metering pump serves to add NaOH (Sodium Hydroxide, ie, Caustic Soda) to the raw glycerin in order to adjust the pH of the feed if necessary.
The majority of the glycerin is collected as Glycerin I from the center section of the column. It is then bleached to remove the final colorants by passing it through a bed of activated carbon, followed by a polishing filter to catch any entrained activated carbon. After passing through the bleachers, the bleached glycerin is freed from any existent activated-carbon fines in a polishing filter, being cooled and pumped provided by Customer.
After condensing, this stream is discharged as Distillate II with a glycerin concentration of around 90%. It is of inferior quality and thus used for industrial purposes.
Over time, non-distilling components (salts, soaps etc.) increase in the base of the still (ie, higher concentrate).
This residue is periodically bled off to a secondary still where any incumbent glycerin is recovered and the residue is dispensed from each batch into drums. The recovered glycerin is returned to the main still.
The distilling process consists of a continuous 50-60 MT/day glycerin refining unit. The design of the plant encompasses crude distillation and glycerin bleaching, producing USP pharmaceutical grade glycerin. The plant consists of a pre-dryer, a structured packing distillation column, and a continuous bleaching unit. The process also has two post-addition stills for residue recovery.
The process was designed for either soap lye or splitters crude as per British specification BS2621 for soap lye crude and British specification BS2622 for the splitters crude. Crude will be supplied to the refining unit as individual streams or a blend of the two based on raw material availability. The process was modified recently for all “vegetable” based biodiesel crude. The maximum allowable methanol content from the biodiesel crude is 0.5 % max.
Equipment required for the glycerine refining process:
- USP and Crude Glycerin Storage
- Steam Pre Dryer
- Distillation Column & Condensing Units
- Post Distillation Residue Recovery
- Bleaching Columns ( 3 )
- Steam Vacuum Jet System (Grahm)
- WonderWare & Allen Bradley PLC Controls
