Vacuum pump operation in a maleic anhydride recovery system

#This invention concerns an improved method for sealing liquid ring-type vacuum pumps used to maintain reduced pressure during distillation of crude maleic anhydride.

#Maleic anhydride is conventionally produced by the vapor phase oxidation of hydrocarbon feedstocks. Crude anhydride can be recovered from the oxidation product by absorption into an organic solvent followed by reduced pressure distillation of the anhydride-rich absorbent. The distilled crude maleic anhydride is taken overhead and the anhydride-lean absorbent bottoms are recycled to the absorber. A portion of the crude maleic anhydride is recycled to the distillation vessel and the balance is charged to a lights stripper, also operated at reduced pressure. In the lights stripper low boiling contaminants are distilled overhead. Some maleic anhydride is carried over with the low boiling impurities. Accordingly, as used herein, the term "anhydride distillation system" includes the distillation of anhydride-rich absorbent to obtain a crude anhydride product and the distillation of the crude product to strip low-boiling contaminants.

#In conventional process designs, the anhydride distillation system is maintained at reduced-pressure by a vacuum system which includes a vacuum pump or pumps connected to the distillation system through condensers. Vacuum pumps are preferred because when steam jets are used, sufficient water travels back up the feed line to hydrolyze carried over maleic anhydride to solid, insoluble acid, which then can cause severe plugging problems. In addition, any product passing to the jets is lost to further product recovery. Thus, vacuum pumps connected through condensers are preferred over steam jets.

#The following detailed description of the invention refers to the accompanying drawings in which:

#FIG. 1 illustrates a liquid ring-seal vacuum pump, and,

#FIG. 2 schematically illustrates a maleic anhydride plant design employing a liquid ring-seal vacuum pump sealed in accordance with the invention.

#This invention concerns an improved vacuum system for reduced pressure distillation of a maleic anhydride-rich organic absorbent and the crude anhydride product. In particular, it has been found that vacuum pump fouling can be eliminated by using a liquid ring-seal type vacuum pump and a liquid maleic anhydride absorbent sealant, which after use, is recirculated to an absorbent regeneration zone for further product recovery.

#U.S. Pat. No. 3,818,680 thoroughly describes the recovery of maleic anhydride from a gaseous mixture by contacting the mixture with an organic solvent to absorb the anhydride, and then distilling the anhydride-rich absorbent under reduced pressure. U.S. Pat. Nos. 2,574,644; 3,040,059; 2,893,924 and 3,891,680 describe similar processes using various other organic absorbents. In each of these disclosed processes, the anhydride is separated from the absorbent in a regenerating zone by distillation, usually at reduced pressure maintained by a vacuum system using a vacuum pump or pumps.

#Among other factors, this invention is based upon the discovery that vacuum pump fouling in the anhydride distillation system can be minimized and product recovery can be increased by feeding absorbent into the pump chamber of a liquid ring-seal-type vacuum pump and withdrawing a small stream of anhydride-rich absorbent from the separator of the vacuum system for removal of absorbed product in an absorbent regenerator. Sealing the vacuum pump in this manner prevents organic material from fouling the pump chamber, and increases product recovery.