Feed pipe backmixing

The feed should be introduced into zones of the highest turbulence, i.e. near the tip of the stirrer at the plane of the agitator or slightly above this plane the velocity of injected reactants should prevent backmixing of the reaction mixture into the feed pipe if the addition time will be too long, consider a multi-injection system with nozzles located at different angular positions. 

The feed pipe must now be designed. The impeller tip speed is 30ft/sec thus, the feedpipe velocity must be at least 30% of the impeller tip speed to avoid feedpipe backmixing. Thus, the feedpipe velocity must be 0.3 x 30 = 9ft/sec = 2.74m/s. The volume of semi-batch feed is 2150/80 = 27 L. This volume is fed over 15 minutes thus, the feed rate of the semi-batch feed is ... 

Once the reaction is established, three key factors must be addressed for successful development and scale-up of homogenous reactions 1) the effect of concentration on yield for competitive-consecutive reactions 2) the effect of feed rate or addition time on yield and 3) feed pipe backmixing. 

Jo, M.C., W.R. Penney, and J.B. Fasano (1994). Backmixing into reactor feed pipes caused by turbulence in an agitated vessel. AIChE Symp. Ser. 299, 90,41-49. 

Feed pipe backmixing and reaction possible cause. 

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