Vacuum system issues 1

[MatthewL]

All,

Looking for some insight into a problem at the plant I work at. We use a three ejector + vacuum pump on our reactor during polyesterification of block polyether/polyamide. Typical vacuum is around 1.5 to 3 mmHG absolute at the vessel. Recently (in the last month or so), the vacuum is no longer steady, instead it shows a saw-tooth pattern, with a slow rise in pressure followed by a rapid drop. The peak to trough change is about 0.5 mmHG, and the cycles are between 2 and 6 minutes. I ran a vacuum check today on the empty vessel with the same results. During the vacuum check I closed every valve feeding the vessel except the manual charge valve from the upstream vessel (it has an automatic valve and there was a batch in the upstream vessel) and the vacuum line. The vacuum system has a precondenser, then two steam ejectors, then a barometric condenser, then another steam ejector, then the final barometric condenser, and finally a liquid ring vacuum pump. Steam pressure to the ejectors is constant and has not changed (verified with both local pressure gauge and PT), the water temp to the barometric condensers is constant, and the liquid ring vacuum pump is on a continuous flush (2.5 GPM) and the ring fluid is cooled through a small HX using chilled (55 F) water. Typical ring fluid temp is around 75 F. The pump itself is not running hot after 2 hours of run time. Has anyone else seen a similar issue? If so what was the cause? If not, any ideas on what is causing it? Thanks for any help!

Regards,

Matt

Quality, quantity, cost. Pick two.

 

[Compositepro]

This could be a temperature control issue where a cooling valve open periodically, for example, and it causes a perturbation in the pressure. This should not be too hard to find. A less obvious cause would be refluxing, where liquid condenses in the piping, and when enough collects, it runs back down into the reactor causing a pressure increase.

 

[MatthewL]

Compositepro,

Thanks, your first idea was right on. An operator mentioned the same thing, and sure enough, the precondenser (shell and tube style) tube temperature change correlated exactly when overlaid with the vacuum reading. Looks like I am at or near a critical point for vapor pressure on the precondenser. I don't think it is reflux, since the precondenser has a large (100 gallon) drain leg that holds the condensate, and if we did get reflux back into the vessel, I would expect to see batch quality suffer, which I haven't seen yet.

Regards,

Matt

Quality, quantity, cost. Pick two.