This paper analyses physical limits of multistage production or consumption of mechanical energy (work) in sequential heat-mechanical operations characterized by finite rates. The benchmark system, where these limits are evaluated, is a cascade of imperfect stages through which a resource fluid flows with a finite rate. Each stage consists of a fluid at flow, an imperfect work generator or consumer and the environment. The problem investigated is that of limiting yield or consumption of work by the fluid that interacts sequentially with the environment in a finite time. A discrete, finite-rate model subsumes irreducible losses of work potential caused by thermal resistances. Dynamic limits on work are obtained which bound one-stage or multistage energy convertors with production or consumption of power. These limits are expressed in terms of classical exergy and a residual minimum of entropy generation. A discrete generalization of classical exergy is found for systems with finite number of imperfect stages and finite holdup times. For this generalized exergy a hysteretic property is valid, meaning a difference between the maximum work delivered from engine mode and the minimum work added to the corresponding heatpump mode of the system.

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A commonly used method to dry fine solid particles is drying in a fluidized bed. This paper presents the optimization problem of fluidized drying of fine solids. A drying process proceeding in a three-stage cascade of fluidized cross-current dryers was considered. Solid flows from stage to stage, and fresh gas is introduced to each stage of the cascade. The hydrodynamics of bubble fluidized bed and kinetics of heat and mass transfer are taken into account. The bed hydrodynamics is described by a two-phase model. The drying process considered proceeds in the second period of drying. To optimize this problem a generalized version of a discrete algorithm with constant Hamiltonian was used. The optimization procedure is presented in the paper. In optimization calculations, gas parameters (temperature, humidity and flow rate) minimizing total process cost are sought. The results of calculation are presented as graphs. The results obtained and the conclusions drawn are discussed.

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