What Comprises a Complete Rotary Evaporator System?

18 April 2018 at 16:34

If you don’t have a lot of experience using a rotary evaporator, you may not be familiar with the overall setup and the role each part plays in the application of the apparatus. A rotary evaporator is a rather complex piece of equipment itself, plus additional items are required to ensure it functions as it should.

The main purpose of a rotary evaporator is to increase the rate of evaporation of a solvent. It does this via three methods:

  1. The vacuum decreases the pressure in the system, lowering the boiling temperature of the solvent.
  2. The rotation increases the surface area of the solvent.
  3. The water bath heats the solvent.

In order to its job efficiently, a rotary evaporator system typically comprises the following main components:

  • Rotary evaporator and glassware
  • Chiller or other source of coolant
  • Vacuum pump
  • Vacuum controller or regulator
  • Tubing to connect the parts

In this post, we’ll break down this complex apparatus into its separate components and explain the role each plays in the overall setup.

Rotary Evaporator and Glassware

First, let’s take a look at the main piece of equipment, the rotary evaporator itself. We’ll also talk about the glassware in this section as this is typically included when you purchase a rotary evaporator.

The LabTech EV311 Rotary Evaporator.

The LabTech EV311 Rotary Evaporator.

The rotary evaporator in the image above shows what you can expect from a basic model. Included in this one is the machine itself with a digital display and control for rotation speed. You also have a water bath with its own digital display and control for temperature. Note that different models will vary in terms of displays and controls, including where they are located. Some may have additional items or features included, such as integrated vacuum control (more on that below).

When it comes to the water bath, you may have a choice between uncoated stainless steel or PTFE coated stainless steel. The latter is designed to improve durability and reduce corrosion.

As mentioned, glassware is also typically included. You usually get an evaporating flask, a condenser, and a collection flask. When you purchase a system, among your options might be the type of glassware you want bundled with your rotovap.

Chiller or Other Source of Coolant

The condenser part of the rotary evaporator is used for cooling the evaporated solvent and transferring it to the collection flask.

Some applications, such as when using low boiling point solvents, require the use of a dry ice condenser. Some people just use dry ice condensers because it's more convenient -- if they have an accessible source of dry ice -- and negates the need for a chiller. However, if you aren’t using a dry ice condenser, you’ll need to supply the condenser with a coolant. Typically a coolant is supplied by a chiller, some examples of which are shown below.

Examples of chillers.

A variety of chillers (from left to right): an H Series Recirculating Water Chiller, a 6000 Series Portable Chiller, and an RC Compact Recirculating Cooler.

This will cool a liquid (coolant), for example water, and provide a recirculating supply of this liquid to the condenser. This means that the condenser always has chilled liquid running through ready to condense the evaporated solvent.

When choosing a chiller, the most important factor to consider is its cooling capacity. If this is lower than the capacity of your rotovap's heating bath at the desired coolant temperature, it probably won't be able to keep up.

Another important thing to take into account is the pressure that will be produced when the coolant is pumped through the condenser. Glassware can break under too high a pressure, so you may want to go with a chiller whose pump has a maximum pressure output below the pressure rating of the glass.

Vacuum Pump

A vacuum pump is an important part of the rotary evaporator setup because it reduces the pressure within the system. This lowers the boiling temperature of the solvent enabling faster evaporation. One of the implications of this is you can work with a lower temperature bath than you would otherwise. This may mean you can skip out on having switch to oil and use water instead. It also means you reduce the risk of temperature-sensitive compounds in your mixture reacting. Finally, working at lower temperatures is beneficial from a safety standpoint.

The pump is typically purchased separately, although some models include one.

An RV 10 Control Auto Rotary Evaporator.

This RV 10 Control Auto Rotary Evaporator comes with a pump.

An ideal pump for a rotovap system should be chemical resistant and oilless to ensure as long a life as possible. Diaphragm (or membrane displacement) pumps are most commonly used.

Pump examples.

A variety of pumps (from left to right): an ME1 Diaphragm Pump, a VP Series Vacuum Pump, and a MVP 10 basic Vacuum pump.

The pump you choose for your application will also depend on the pump rate required. Factors affecting this include the solvent being evaporated, the temperature of the waterbath, and the volume of the flask being used.

It’s possible to connect more than one rotovap system to a single pump using a switchbox. A switchbox can also double as a vacuum controller (more on that below), enabling you to control the pressure in up to three systems individually.

Some pumps combine a chiller system so you only need one piece of equipment for both functions.

Vacuum Controller or Regulator

A vacuum controller or regulator is needed to help ensure you have the right level of pressure within your system. If it’s too high, the vacuum pump won’t be doing its job of speeding up evaporation. It the pressure is too low, you could see bumping or foaming.

What’s more, having precise control over the pressure of your system means you can find the optimal pressure for the desired evaporation rate and more easily create a reproducible process. Plus, it may mean you don’t have to make other adjustments, such as altering the temperature of your water bath in order to reach the evaporation rate you need.

A vacuum controller is built into some rotovap models, although this feature often bumps up the price by a significant amount.

If your rotovap doesn’t include a controller, you can use a digital vacuum controller, a manual vacuum regulator, or even a vacuum valve with a Woulff bottle:

Vacuum controller examples.

Left to right: A digital LabTech VC1000 Vacuum Controller, a Heidolph Manual Vacuum Controller, and a Heidolph Vacuum Valve.

Alternatively, if you simply don’t have the budget for a vacuum controller or regulator, or need a quick fix, it is possible to juryrig one using a T-fitting, a shutoff valve, and a pressure gauge.

There are a couple of cases in which a vacuum controller is not required. For example, if the vacuum pump's minimum pressure hits the sweet spot for the solvent then you can get by without one. Alternatively, if you have an RPM-regulated pump, then the pump will stop once the desired vacuum level is reached. It then runs intermittently at a low speed to keep the vacuum constant. This negates the need for a controller.

Tubing to Connect the Parts

To forge connections between certain parts of the rotary evaporator system, you’ll need tubing. Specifically, you need it to run from the vacuum to the condenser and from the chiller to the condenser and back.

Tube set for Hei-VAP Industrial models.

Tube set for Hei-VAP Industrial models.

Some tubing is sold specifically for use in a rotary evaporator system, but you could save some money and simply buy some at a hardware store. The vacuum tubing just needs to be structurally sound enough to not collapse in on itself under vacuum. Ideally, the tubing you use for the coolant should be insulated (although the products sold specifically for this purpose often aren’t).