Market forecasts for bioproducts are upbeat. One even predicts that industrial biotechnology will be a $160 billion global business by 2010. Industrial biotechnology first took off in the early 1980s; what gives today's activity a buzz of excitement is the repeat of something else from that era: high energy prices.
Install a flow meter in the recycle, cascade the flow set point with a pressure control loop. With proper tuning, the valve characteristics are lost in the flow control loop, which should be tuned fast with PI.