ROTOVAP BASICS

What is a Rotovap?

Rotary Evaporators are essential tools in a processing facility. The primary use for a rotary evaporator is the recovery of ethanol after a winterization or extraction process. It can be used to recover many other solvents as well as separate certain compounds, but for the simplicity of this guide, we will just be discussing ethanol recovery.

 

Understanding the process:

Before we discuss setup and operation, it is important to understand the chemical process that is happening inside of the rotovap. The following example illustrates the process: A mixture containing extract and ethanol must be separated before further distillation can occur. To separate the mixture, heat is applied while simultaneously pulling a vacuum. This causes the boiling point of the ethanol to drop significantly. As the evaporation flask turns, the mixture creates a thin film which is spread across the interior of the flask. This speeds up evaporation and forces the ethanol to turn into a vapor. That vapor is pulled into the path of the condenser by the vacuum pump. As the ethanol vapor enters the condensation path, it re-condenses back into a liquid and drops into the collection flask. The original extract is left in the evaporation flask while the ethanol is left in the collection flask. The ethanol can then be re-used for winterization or extraction.

 

Rotovap Components:

To successfully perform evaporation you will need the following components:

-Rotovap: The size you need will depend on how many liters you need to recover although the most common sizes are 5L, 10L, and 20L.

-Chiller: Chillers for rotovaps are specific in that they have a much a higher cooling capacity compared to refrigerated circulators. But, they usually do not have as large of a temperature range. For evaporation, you will not need extremely cold temperatures but you will need high cooling capacity. The solvent vapor will be extremely warm when it hits the condenser. To combat this, the increased cooling capacity will keep your chiller at a stable temperature.

-Vacuum Pump: For best results, a chemical resistant oil-less diaphragm pump should be used with a rotovap. These pumps do not have as much depth as rotary vane (oil) pumps but, they are much more resistant to contamination. You will quickly destroy an oil pump if you use it with a rotovap.

-Cold Trap: The cold trap is placed between your vacuum pump and rotovap to protect the pump from further contamination. Cold traps are essential for rotovaps because they catch any vapors that pass through the condenser. The cold trap will be the coldest point in the system which means any vapors that get pulled into it will recondense inside the trap leaving your vacuum pump dry and contaminant free.

 

Basic Operating Procedure:

This operating procedure is meant to be a basic starting point for anyone new to rotary evaporation. Once mastered, you can adjust the parameters to suit your specific needs.

“Continuous Feed” Operating Procedure

-Apply light grease to all joints and inspect components to make sure everything is in working order

-Connect chiller to condenser and set to 0C (32F). You should always fill a rotovap from the top down (chiller output to top of condenser, chiller input to bottom of condenser).

-Connect vacuum pump to cold trap and connect cold trap to vacuum port on rotovap.

-Turn on heat bath and set to 40C (113F)

-Once heat bath and coil have reached desired temperature, turn on rotovap motor and set speed to approximately 100 RPM (does not have to be precise)

-Start vacuum pump and allow it to pull down for a few minutes before beginning injection

-Once vacuum has been pulled to a sufficient level (in the red on the gauge, does not have to be precise), using the injection valve, inject 200-500 ml of liquid in the rotovap, and then close the valve. Allow this small amount to begin evaporating. You will see the chiller begin to rise in temperature 1-3 degrees. This is called “priming the rotovap”

-Once the temperature stops rising (I usually wait until it starts to drop back down again), open the injection valve again very slowly so that a small amount of liquid begins to enter the rotary flask. At this point you must try to match the input speed to the output speed. What this means is that you should be injecting the same amount of liquid into the rotovap, as the amount of liquid that is dripping off the coil into the receiving flask. You will know that the input speed and output speed are matched because the chiller will stay at a stable temperature. If the chiller temperature begins to rise, then you are injecting too much liquid. Dial back the valve until the chiller stabilizes.

-Once everything is stable, you can walk away from the rotovap and allow the evaporation to continue without supervision till complete

 

Questions and Answers

Q: What is a J-KEM DVR-200 used for?

A: The J-KEM DVR-200 is a digital vacuum regulator that maintains the specified vacuum level throughout the evaporation process. This will allow you to pinpoint specific solvent boiling points and will also speed up the process, and prolong the lifespan of the vacuum pump. It also allows you to data log the vacuum level over time.

Q: What kind of tubing do I need?

A: Gum rubber is what we recommend for all vacuum tubing. Silicone or PVC tubing is what we recommend for chiller tubing. We also recommend using tubing insulation to provide the most stable temperatures possible.

Q: Can I use an oil vacuum pump with a rotovap?

A: You can but you really shouldn’t. You can use an oil vacuum pump but it will quickly become ruined. Diaphragm vacuum pumps are lined with PTFE on the inside making them highly chemically resistant. If you use an oil vacuum pump with a rotovap, you will immediately notice that the pump oil will turn black and start to smell like ethanol. This is because the vapors are getting trapped in the oil case and contaminating the inside of the pump. This will dramatically decrease the lifespan of the pump and will force you to change oil very frequently.

Q: Do I need a cold trap and how does it work?

A: Yes, a cold trap is always recommended when using a rotovap. A cold trap is used to trap any vapors that might be might be pulled into the vacuum pump during an evaporation. Ethanol (Alcohol) is very difficult to re-condense and some ethanol vapor will always pass through the condenser during evaporation. The cold trap is used to trap this vapor before it reaches the vacuum system. This prolongs the lifespan of the vacuum pump considerably. Cold traps also trap extremely volatile compounds like Terpenes. For this reason, the CT80 is always recommended but the CT40 also works well. A 1.5 gallon cold trap will work also but must be fed dry ice to keep the trap cold.

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