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Putting the Rule of 20 Into Practice

By Aimee O'Driscoll, 08 May 2020

In a previous post, we introduced the “rule of 20,” also known as the “20/40/60 rule” or “Delta 20 rule.” Here we explain the rule in greater detail and discuss how to put it into practice.

What Is the Rule of 20?

The rule of 20 provides a general guideline for the temperatures at which to set your bath and coolant. In general, your bath temperature should be 20°C higher than the boiling point of the substance you want to evaporate. And your coolant should be 20°C or more lower than the vapor temperature.

For example, if the boiling point of your substance is 40°C, a bath temperature of 60°C and coolant temperature of 20°C should provide an optimal setup.

So why are these temperatures advised? The main reason for having these guidelines is to help you strike a balance between process efficiency and energy usage. Distillation efficiency increases when you have a higher bath temperature and lower coolant temperature. However, the higher your bath temperature and lower your coolant temperature, the more energy you use.

At some point, the additional energy input won’t have much effect on efficiency and as such, is not worth expending. The rule of 20 generally allows for an efficient distillation system without wasting energy. 

Note that there are other considerations when determining temperatures that may override the rule of 20. For example, you may be dealing with heat-sensitive compounds, limiting how high you can set your bath temperature.

Putting the Rule of 20 Into Practice

In some cases, the 20/40/60 version of the rule is taken literally. In that case, you set your bath temperature to 60°C, set your coolant to 20°C, and then tweak your vapor temperature to 40°C using your vacuum controller. 

However, this is not necessary nor practical in many situations. For one, you may not have that level of vacuum control to work with, or you might be relying on facility water for your coolant. Using the rule of 20 on an adjustable temperature scale works too. For example, if your vapor temperature is 30°C, you can set your bath 20°C higher at 50°C and your coolant 20°C lower at 10°C.

If you are using a vacuum controller to tweak your vapor temperature, you’ll need to know what pressure to use. Here are the required vacuum pressures for some common solvents:

 

Solvent

Vacuum [mbar] for bp of 30°C

Vacuum [mbar] for bp of 40°C

Vacuum [mbar] for bp of 50°C

Acetic acid

26

44

72

Acetone

370

556

815

Acetonitrile

153

226

315

n-Amylalcohol, n-pentanol

6

11

20

n-Butanol

14

25

44

tert-Butanol

78

130

231

Chlorobenzene

22

36

56

Chloroform

332

474

665

Cyclohexane

154

235

347

Dichloromethane

699

Atm. press.

Atm. press.

Diethyl Ether

838

Atm. press.

Atm. press.

Diisopropyl Ether

251

375

545

Dioxane

68

107

165

Dimethylformamide (DMF)

6

11

17

Ethanol

97

175

276

Ethyl Acetate

153

240

366

Heptane

77

120

183

Hexane

264

335

525

Isopropyl alcohol

78

137

231

Isoamyl alcohol

9

14

29

Methanol

218

337

607

Pentane

834

Atm. press.

Atm. press.

n-Propanol

37

67

115

Pentachloroethane

8

13

21

Tetrachloromethane

179

271

398

Tetrahydrofuran (THF)

234

357

539

Toluene

48

77

118

Trichloroethylene

119

183

275

Water

42

72

120

Xylene

15

25

40


References:

Buchi: https://static1.buchi.com/sites/default/files/downloads/Multivapor_Solvent_List_0.pdf?9902cc3c546bcb4a09999d96e2b4a2d66f0b16d5

University of Wollongong: https://documents.uow.edu.au/content/groups/public/@web/@sci/@chem/documents/doc/uow093125.pdf