8 5 P P 7 44' + = instantaneous overall effectiveness factor given by = dimensionless time = ( k d -t ) = dimensionless radial position = r / R = density of catalyst particle, g/cc = space time, (s)(g cat.)/(cc) = dimensionless group = (3h) = dimensionless group = (3hA) = (4.A) = dimensionless concentration = (CJC,) Selected state and/or output variables can be made undisturbable, i.e., invariant, to arbitrary, unmeasured changes in specific input variables by properly designed feedback and feedforward controllers. Simulation and experimental applications to a computer-controlled, pilot plant evaporator gave results superior to conventional Controllers.Necessary and sufficient conditions for undisturbability are expressed in terms of the structure of the coefficient matrices of the state space model and equivalently of the corresponding eigenvector matrix. The design procedure normally includes arbitrary specification of all closed-loop eigenvalues and up to r elements of each eigenvector, where r is the number of control (manipulated) variables.