Many small distillation systems (usually blown glass type short path, wiped film systems) have very small vacuum pumps backing a 50mm inlet diffusion pump. Diffusion pumps are meant to pump at high vacuum — that is at pressures less than 1mtorr (1micron or 1 x 10-3 torr). In fact, when pressures at a diffusion pump inlet are higher than 50mtorr, the diffusion pump will not operate effectively. At these pressures, the pump will tend to both backstream oil into the inlet piping due to high turbulence and fore-stream the diffusion pump oil into the backing pump. This results in an expensive loss of oil.
Furthermore, at pressures above 10mtorr, the diffusion pump is really providing no additional pumping speed. In these cases, a turbo pump would be a much better choice.
To make matters worse, the backing pump—which is connected to the diffusion pump exhaust—is often the only roughing pump in the system. This means that all of the gases pumped by this pump must pass through the small and complicated geometry inside the diffusion pump, which is a huge restriction in the pumping speed. This causes a radical restriction in how much outgassing from the terpenes can be pumped through the roughing pump, thereby severely restricting the feed rate of bulk oil through the system during the first pass.
Be aware that the terpenes are condensable and are thus best pumped by condensing them. However, achieving the condensing conditions that raise throughput of the system is often either overlooked or difficult to achieve or maintain. A very good condenser with excellent cooling capacity and some means to slow or remove fouling of the condensing surfaces could have a profound effect on the efficiency of the system during the terpene cut.
To optimize the operation of your system, it is advisable to consult with a Leybold engineer. They can help you improve the efficiency of your current equipment without needing to buy all new equipment. Find out more about the tailor-made vacuum solutions our engineers can advise on, on our Vacuum Systems product page.