A system is being developed to remove urea from the blood of patients with renal failure. A prototype fixed bed reactor is set up using urease immobilised in 2-mm-diameter gel beads. Buffered urea solution is recycled rapidly through the bed so that the system is well mixed and external mass transfer effects are negligible. The urease reaction is: (NH2)2CO + 3H2O 2NH + HCO3 + OH - . Km for the immobilised urease is 0.54 g 1-1. The volume of beads in the reactor is 250 cm3, the total amount of urease is 10-4 g, and the turnover number is 11,000 g NH4+ (g enzyme)-'s-1. The effective diffusivity of urea in the gel is 7 x 10-6 cms-1. The reactor is operated continuously with a liquid volume of 1 litre. The feed stream contains 0.42 g1- of urea and the desired urea concentration after enzyme treatment is 0.02 g1-!. Ignoring enzyme deactivation, what volume of urea solution can be treated in 30 min? First-order reaction: ra = k1CA 1 Sphere Mii 36736, coth 36, 1) () = First-order reaction: ra= k1CA 01 belos ki Sx V DA R ki Sphere 01 = 3 V D A