Assessment of overall heat transfer coefficient models to predict the performance of laboratory-scale jacketed batch reactors

Heat transfer models for agitated, jacketed, laboratory-scale batch reactors are required to predict process temperature profiles with great accuracy for tasks associated with chemical process development such as batch crystallization and chemical reaction kinetics modeling. The standard approach uses a reduced model which assumes the system can be adequately represented by a single overall heat transfer coefficient which is independent of time; however, the performance of reduced models for predicting the evolution of process temperature is rarely discussed. Laboratory scale (0.5 and 5 L) experiments were conducted using a Huber thermoregulator to deliver a thermal fluid at constant flow to a heat transfer jacket. It is demonstrated that the relative specific heat contribution of the reactor and inserts represent an increasing obstacle for these transient models with decreasing scale. However, a series of experiments implied that thermal losses were the limiting factor in the performance of a single coefficient reduced model at laboratory-scale. A diabatic model is presented which accounts for both thermal losses and the thermal inertia of the reactor vessel and inserts by incorporating a second coefficient and a modified heat capacity term. The mean absolute error in predicted process temperature was thereby reduced by a factor of 8, from 2.4 to 0.3 K, over a 150 min experiment

Predictions of Heat Transfer and Flow Circulations in Differentially Heated Liquid Columns With Applications to Low-Pressure Evaporators

Numerical computations are presented for the temperature and velocity distributions of two differentially heated liquid columns with liquor depths of 0.1 m and 2.215 m, respectively. The temperatures in the liquid columns vary considerably with respect to position for pure conduction, free convection, and nucleate boiling cases using one-dimensional (1D) thermal resistance networks. In the thermal resistance networks the solutions are not sensitive to the type of condensing and boiling heat transfer coefficients used. However, these networks are limited and give no indication of velocity distributions occurring within the liquor. To alleviate this issue, two-dimensional (2D) axisymmetric and three-dimensional (3D) computational fluid dynamics (CFD) simulations of the test rigs have been performed. The axisymmetric conditions of the 2D simulations produce unphysical solutions; however, the full 3D simulations do not exhibit these behaviors. There is reasonable agreement for the predicted temperatures, heat fluxes, and heat transfer coefficients when comparing the boiling case of the 1D thermal resistance networks and the CFD simulations