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Optimizing Büchi® Rotary Evaporator Applications

Introduction

Selecting the correct operating parameters for Büchi Rotary Evaporators is an important part of your application. Many variables must be considered, i.e. choosing the appropriate bath temperature, vacuum pressure, rate of evaporation, and accessory chiller—so it's no surprise to find more vapors collecting in the lab environment than in the receiving flask!

The following discussion offers a complete guide for optimizing your Büchi Rotary Evaporator application.

Choosing the bath and condenser temperature (20-40-60 Rule) The first topic of discussion is the 20-40-60 Rule. The recommended water bath temperature is 60°C. This minimizes evaporation from the bath, reduces the risk of burns, and minimizes the loss of heat energy into the lab environment. The bath temperature should be 20°C higher than the vapor pressure temperature of the solvent. In this example, a vacuum pressure is chosen to produce a vapor pressure temperature of 40°C.

(Calculating vacuum pressure will be discussed in the next section.) The condenser temperature should be 20°C lower than the vapor pressure temperature of the solvent. In this example, the vapor pressure temperature is 40°C so the condenser temperature should be 20°C.

To conserve water and maintain lower condenser temperatures, Lauda® Chillers may be used as an alternative cooling method over standard tap water methods. (Choosing an appropriate Chiller will be discussed in later sections.)

The above example illustrates the 20-40-60 Rule for efficient use of rotary evaporators under ideal conditions. There are instances, however, when temperature-sensitive samples or high-/low-boiling solvents prohibit operation under these parameters. A more generalized rule is the 20°C ? T Rule, which states that the condenser should be 20°C lower than the vapor pressure temperature and the bath temperature should be 20°C higher than the vapor pressure temperature.

By following these simple guidelines, costs can be reduced and results can be optimized.

Choosing Vacuum Pressure

Vacuum Controllers play a critical role in regulating evaporation experiments: without one, experiments can only run at maximum vacuum pressure. Vacuum Controllers reduce the risks of sample loss and lab contamination, and they may be programmed to reduce bumping and foaming and perform automatic distillations.

Once a Vacuum Controller has become part of your lab set-up, how is the appropriate vacuum for the solvent calculated?

The vapor pressure temperature under vacuum of most common solvents can be calculated by using the following formula introduced by Reckhard1 in 1958. Refer to Table A for a listing of physical constants for common solvents.

Log P = 3.006 [Ts Tp]
_______

[b Tp]

P = vacuum pressure
Ts = boiling point (°K) at a pressure of 1013 mbar (atmospheric pressure)
Tp = vapor pressure temperature (°K) at pressure p (mbar)
b = constant b

Example:

What vacuum pressure should be set for the evaporation of ethanol at a vapor pressure temperature of 40°C?

Bp = 78°C
Constant b = 0.159

Log P = 3.006 [Ts Tp]
_______

[b Tp]

Log P = 3.006 [351 313]

[0.159 313]

= 2.24

p = 174.7 mmbar

The nomogram (Table B) can also be used to extrapolate the appropriate vacuum pressure for an application.

Calculating the rate of evaporation

The rate of evaporation is dependent on the difference between the vapor pressure temperature (boiling point under vacuum) and the bath temperature as well as the rotational speed of the evaporation flask. The rate of evaporation of any solvent can be calculated by using the following formula:

Evaporation rate of solvent = Evaporation rate of water (g/hr.) 2,260

Heat of vaporization of solvent (j/g)
The evaporation of water can be extrapolated from the graphs listed below.

*The evaporation rate of water was determined experimentally. Example:

When using a 20-liter Industrial Rotary Evaporator, what is the best expected evaporation rate for Hexane if the vapor pressure temperature is 40°C and the bath temperature is set to 60°C?

Evaporation rate of solvent = Evaporation rate of water (g/hr.) 2,260

Heat of vaporization of solvent (j/g)
Evaporation rate of water with (? t = 20°C, rpm = 225) = 2.0 L/hr. or 2,000 g/hr.
Heat of vaporization of Hexane = 371 j/g
Density Hexane = .659 g/cm3
Evaporation rate of solvent = 2,000 g/hr. 2,260

371 j/g
= 12,183 g/hr.
V = 12,183 g/hr.

.659 g/cm3
= 18,487 cm3/h
Choosing a Recirculating Chiller

Brinkmann™ Instruments offers a complete line of Lauda Recirculating Chillers to provide the necessary cooling capacity for any application. Recirculating Chillers offer lower cooling temperatures and higher heat removal capacity than tap water cooling.

What criteria should be considered when selecting a Lauda Recirculating Chiller?

The first consideration should be the outflow pressure. Büchi condensers are rated to withstand up to 1 Bar of positive pressure, therefore it is important that the Recirculating Chiller have a pressure reducer to limit the outflow pressure of the pump. Most Lauda WK Series Chillers have a built-in pressure reducer; however, an optional pressure reducer is available for Lauda Chillers that do not include this feature.

The second consideration, and perhaps the most important, is the heat removal capacity required by the system. Once the rate of evaporation is known, it is possible to calculate the necessary cooling capacity.

Heat Removal Capacity (BTU/hr.) = Rate of Evaporation (kg/hr.) x HvapSolvent(cal/g) x 3.97 BTU/hr.

Heat Removal Capacity (watts) = Heat Removal Capacity (BTU/hr.) x 0.293 (watts/BTU)

Heat Removal Capacity (watts) =

Rate of Evaporation (g/hr.) x HvapSolvent (joules/mol)

Molecular weight (g/mol) x 3,600 sec./hr.

Example:

From the previous example, we determined the rate of evaporation for Hexane to be 12,183 g/hr. The vapor pressure temperature in this example was assumed to be 40°C, therefore the condenser temperature should be 20°C.

What heat removal capacity is required for Hexane to be evaporated under these conditions?

Heat Removal Capacity (watts) =

Rate of Evaporation (g/hr.) x HvapSolvent (joules/mol)

Molecular weight (g/mol) x 3,600 sec./hr.

=

12,183 g/hr. x 31,912 j/mol

86.2 (g/mol) x 3,600 sec/hr.

=

1,253 watts

Based on the following graph, the Lauda Recirculating Chiller Model WK-1200 would provide sufficient cooling capacity for this application. As a general guideline, the following table can be referenced for selecting an appropriate Lauda Chiller.
Laboratory Rotary Evaporators Industrial Rotary Evaporators
R-220 R-187
Maximum Positive Pressure (bar): 1 1 1
Recommended flow rate (L/hour): 30 to 40 80 to 100 150 to 200
Cooling Capacity at 12C (W): 500 2,500 3,500
References
  1. Reckhard, H., "Eine Methode zur Berechnung der Siedpunkte bei Vakeemdestillationen (A method for calculating boiling points in distillation under vacuum)." Erdol und Kohle 4, 234-241 (1958).


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