How Are PTFE Heaters Used in Heating Bio-Diesel Reactors for Transesterification?

Biodiesel is generated by reacting vegetable oils or animal fats with methanol in the presence of a strong alkaline catalyst such as potassium hydroxide. The reaction mixture is heated to speed up the process. The heater has to endure the corrosive mix of hot alcohol and caustic.

Transesterification Reactor Design
Biodiesel is commonly prepared via transesterification in a hot tank reactor with stirring. The reaction mixture (triglycerides, methanol and a catalyst such as KOH or NaOH) is kept at 60–70°C to obtain practical reaction rates. Heating can be provided either by an immersion heater directly into the reaction mixture or by an external heat exchanger with a recycle loop. The chemical environment is very aggressive for most metals: heated methanol can leach alloying elements, and the strong alkali damages aluminium, zinc and even stainless steel in the long run.

Immersion heating is widely used for small to medium size biodiesel plants, due to its simplicity and cheaper capital cost. The heater that is selected should be resistant to corrosion by methanolic KOH or NaOH, should not catalyse any undesired side reactions and should be operable in the presence of the high viscosity of the oil-feedstock mixture during the initial phases of heating.

Why PTFE Heaters Can Withstand the Harsh Transesterification Environment
PTFE immersion heaters are particularly useful for heating biodiesel reactors as PTFE is resistant to strong alkalis (KOH and NaOH) and alcohols (methanol) up to 110°C. The material does not corrode, pit or scale even after prolonged exposure to the alkaline methanol solution. PTFE, unlike metallic heaters, e.g. , carbon steel or copper, will not release metal ions that catalyse saponification or create soaps that limit biodiesel yield.

In a PTFE heater biodiesel reactor transesterification application, the non-reactive surface also prevents the catalyst from accumulating or degrading on the heater. This assures that the reaction will occur without any local hot patches or unwanted catalysis. Corrosion proof heater allows continuous production and no downtime for cleaning or replacement of failing metal heating elements.

Processing of viscous feedstock and prevention of saponification
The mixture of oils or fats with methanol can be quite viscous before the reaction reaches working temperature especially if utilising waste cooking oil or animal fats with a high level of saturation. The physical design of PTFE heaters is designed to endure the mechanical load of stirring in viscous fluid, such as heavy-duty sheaths and reinforced mounting flanges. The smooth low-friction PTFE surface also minimises the adherence of sticky residues from the feedstock.

Process Tip: Sufficient agitation must be maintained during heating to avoid localised saponification on the heater surface. Poor mixing can create significant local concentrations of caustic catalyst around the heater sheath that may react with free fatty acids to form soap deposits. Although PTFE itself does not catalyse saponification, soap layers can nonetheless form as a purely chemical reaction result. The risk can be avoided by using the correct type of agitator (e.g. pitched-blade turbine or anchor impeller) and placing the heater outside stagnant zones. If soap build-up occurs, the non-stick qualities of PTFE make it easier to clean than on metal surfaces.

Technical Performance and Operating Limits 
PTFE has high resistance against potassium hydroxide and sodium hydroxide solution at temperatures up to 110°C and methanol at boiling point (65°C). Since the transesterification of biodiesel is normally performed at 60–70°C, the PTFE heaters are well within their safe temperature range. For procedures that employ ethanol rather than methanol (boiling point 78°C) or those that require higher temperatures (e.g. 80–90°C for some feedstock pre-treatment steps), PTFE remains entirely appropriate, as long as temperatures do not exceed 110°C.

Another advantage in biodiesel production is that PTFE heaters are immune to galvanic corrosion (corrosion which can occur when different metals are present in the reactor, for example, a stainless steel vessel with a copper heating coil). PTFE is an electrical insulator, hence no galvanic pair is generated and that failure mode is completely removed.

Operating Condition PTFE Compatibility
1% w/w KOH in methanol (65oC)Excellent Excellent Sodium hydroxide in methanol (1% w/w, 70°C)
Vegetable oil/methanol combination, 60°C Excellent (no swell)
Localised caustic + free fatty acidsGood PTFE is resistant, however saponification can occur in bulk liquid
Summary
PTFE heaters offer a lasting, low maintenance heating solution for the harsh chemical environment of biodiesel transesterification. These heaters can withstand hot methanolic KOH and NaOH solutions, do not leach metal ions and do not cause deterioration of the catalyst, therefore maintaining consistent reaction conditions and great product quality. Renewable fuel production requires durable, corrosion-resistant equipment and PTFE immersion heaters are up to the task – even in small, medium and pilot scale biodiesel operations.