Five cases where the bigger the condenser, the better

The distillation rate is one of the most important indicators of how good your distillation is. So, I’ve dedicated this post to the relationship between distillation rate and condenser surface area. I show you evidence of how a larger condenser increases the distillation rate and provides additional benefits to your work. I also discuss the limitations of larger condensers. And I list several instances where a bigger condenser could really be a beneficial companion for your rotary evaporator.

You know, I’ve got to tell you, I’ve always really hated vans. I found them so ugly and thought they waste so much gas, I swore to myself to never get one when I was still a young man. Well, fast-forward into my forties and I was driving around in my van like no tomorrow. How could I have ever fitted kids, suitcases, sports equipment, dog cages in a regular car? I had to cave in that size does matter and the extra gas money sure beats making one trip several times.

I mean really, the main goal of any father is to get their kids from A to B as quickly and as comfortably as possible. Well, similarly, the main goal of anyone performing distillations is to get them done as quickly and efficiently as possible. To achieve this, the distillation rate is one factor that you should watch very closely.

Distillation refers to a combined process of evaporation through heating and condensation through cooling. The distillation rate measures the volume of solvent processed through evaporation and then condensation in a certain period of time. The distillation rate is expressed in L of solvent/h.

The efficiency of your distillation or rotary evaporation depends on parameters such as rotation speed, evaporation flask size and cooling capacity. In this blog post, I’d like to focus on the condenser surface area.

What does condenser surface area have to do with distillation efficiency?

Increased cooling surface means greater heat transfer from vapor into the cooling medium. In other words, the greater the surface area, the more vapor condenses on the coils. According to Fourier’s equation:

heat flow, surface area, fourier equation, rotary evaporation, condensor, distillation

𝑄 = Heat flow [W]
U = Thermal conductivity [W/m2K]
A = Surface area [m2]
ΔT = Temperature gradient between input and output [K]

As we’ve already seen, evaporation and condensation contribute to the overall distillation rate and affect performance. To keep the system in equilibrium, you need both sufficient condenser loading and a higher evaporation rate.

Five cases that benefit from larger condensers

Certainly, a single young man does not need a van. But this type of car is probably a good solution for a large family. Similarly, a large condenser is not fitting for any distillation. But for some cases, it can bring huge benefits.

Greater surface area enables concentration of solvents which would otherwise be discharged into the atmosphere or collected in secondary receiving flask. It would be beneficial to use a high-performance condenser when:

  • condenser loading is above the standard maximal condensation level
  • the cooling water inlet temperature is inadequate in removing sufficient energy from the vapor (the cooling water is too warm – ΔT < 20 °C)
  • distilling low-boiling solvents
  • you require shorter operating times
  • operating at higher temperature differences (ΔT > 20 °C)

Now how can you go about achieving higher distillation rates with your large, high-performance condenser?

Well, distillation rate can be improved by increasing the temperature difference between heating and vapor temperature, as between vapor and cooling temperature.

For example, you can increase the heating bath temperature and the temperature while the cooling temperature stays constant. Or you could increase the heating bath temperature while decreasing the cooling temperature and keeping the pressure constant. Or you could decrease the pressure and the cooling temperature while keeping the heating bath constant.

Importantly, the above alterations can only be performed with the high-performance condenser, as there would be an overloading of a standard condenser.

But you know me, I got to see it to believe it. So I set out to test how condenser surface area really affects distillation efficiency with an experiment. There I compared a standard condenser with a surface area of 1500 cm2 and a high performance (HP) version with 3000 cm2. For all experiments, I evaporated 1200 mL of acetone in a 2L flask at a speed of 280 rpm on a Rotavapor® R-300 .

I set up my parameters as follows:

heating bath, pressure, chiller, condenser, distillation, rotary evaporation, lab evaporation, laboratory evaporation

And obtained (unsurprisingly), the following results:

distillation rate, condenser surface area, distillation, rotary evaporation, lab evaporation

The results clearly indicated that greater condenser surface area substantially increased the distillation rate of the HP condenser in comparison to the standard condenser. In fact, I managed to distill as much as 8.2 L/h of acetone with the HP condenser.

With increased condenser loading, I also managed to decrease the time needed for a distillation by 20%!

Some more good news: only a negligible amount of solvent entered the vacuum pump , which helps to reduce the wear of the vacuum pump parts and protects the environment. And if you really value your material, take a look at the graph below:

condenser, distillation rate, rotary evaporation, laboratory evaporation

Again, as expected the condensation rate of the HP condenser is sufficient to condense all vapor generated by the increased evaporation rate, decreasing product loss.

So there you have it. Experimental proof that the bigger the condenser, the better for your distillation. Do keep in mind that there are limitations before making the investment. For example, to prevent re-boiling in the receiving flask, the pressure setting needs to be adjusted accordingly. This means the solvent boiling point needs to be above ambient temperature. To ensure you achieve equilibrium between the cooling and heating at a higher temperature difference, you also need a stronger chiller. Heating power can also be a limiting factor if the difference between the heating bath and vapor temperature would exceed the heating power at 100% bath operation.

It does seem I prefer bigger in my older posts. I’ve written about bigger TVs and their connection to resolution in chromatography ; bigger refrigerators and their relation to condenser overloading in freeze-drying. But boy, I’ve also talked about how big hockey players don’t always win championships and how small particle sizes are the ones that improve your column efficiency in chromatography.

And since the kids are out of the house, my wife and I have downsized our car. So I am well-balanced after all!

Till next time,

The Signature of Bart Denoulet at Bart's Blog