Degassing Deep Dive, Part 5

Degassing Deep Dive - Post 5

Claim: I don't think <1% of solvent will affect my distillation.

Many times, a client won't want to spend a pass on a trace amount of lights. However, it is not possible to skip the degas pass. So, the question arises: how significant is the effect of <1% solvent in a wiped film distillation?

In my Wiped Film Distillation Master Class, I teach a section on degassing. In that section I include this proof (abbreviated here):

Consider a feed that is 0.4 wt% Ethanol, 29.6 wt% C18 monoglyceride, 40 wt% C18 diglyceride and 30 wt% triglyceride. The goal is to distill the monoglyceride away from the other two. In order to perform the distillation at low temperature, it is important to apply 0.02 Torr.

However, because of the ethanol, the pressure in the still will be much higher than that.

According to the ideal gas law, the pressure inside of an enclosed vessel is equal to moles of gas times the constant, R, times the temperature and divided by the volume of the vessel. It is useful to use the ideal gas law to approximate what is actually occurring inside the still.

The moles of distilled monoglyceride can be ignored, because they condense on the internal condenser as soon as they vaporize. They don't have opportunity to fill the container.

However, the ethanol will not condense, because at 1 Torr the boiling temperature of ethanol is -31°C. Therefore, the ethanol will fill the entire space of the still body, until it reaches the cold trap.

So, how much does 0.4wt% ethanol affect the vacuum of the still body?

To come up with an answer, I used the following reasonable assumptions. The still used is the Pope 2" still (see picture). The volume is 3 L (if you have smaller flasks on the still). The feed rate is 500 g/hr. It takes 10 seconds for the ethanol to vaporize and clear the still. This tells us how much ethanol is in the still at any given time. The temperature of the head space is 25°C.

Based on those numbers, there is 1.21 * 10^-4 moles of ethanol in the still. That translates to a pressure of 0.74 Torr.

The desired pressure in the still is 0.02 Torr, but the actual pressure is very much higher because of the 0.4wt% ethanol.

This result tracks with what is observed on the pressure gauge of the still when materials with trace lights are run.

What many operators do to combat this problem is slow down the feed. The less feed entering the still per minute, the less moles of ethanol are present. But the tradeoff is loss productivity. If the example above were decreased to 100 g/hr, the distillation would probably work ok. But it would take 10 hours to run 1 kg of feed material.

Remember from Post 3 that rotovaps and reactors provide feed materials that contain 0.2-5% super volatile components remaining. This shows us that almost every feed we run should have a degas pass completed prior to running.

Picture courtesy of Pope Scientific Inc.