Processes for preparing acetic anhydride

In one embodiment, the invention is to a process for producing acetic anhydride. The process includes at least a step of carbonylating methanol and/or a methanol derivative with carbon monoxide to form acetic acid, and contacting ketene with the acetic acid to form a crude acetic anhydride product stream and a vent stream having unreacted ketene. The process further comprises a step of directing at least a portion of the vent stream to a liquid ring vacuum compressor having a ring seal fluid comprising acetic acid The unreacted ketene in the portion of the vent stream contacts the ring seal fluid to form supplemental acetic anhydride.