The existence of metastable phases of ice I and liquid in the domain of ice III permits the use of optimized paths for pressure shift freezing (PSF) and pressure induced thawing (PIT) processes, crossing the ice III domain, without actual crystallization of this ice polymorph. The use of higher pressures and lower temperatures before pressure release, in the case of PSF, and the use of higher pressures for PIT, increases the real temperature gradients of both processes, therefore reducing the total processing time. In this paper, a computational model has been developed to simulate heat transfer phenomena during the precooling (in PSF) and the heating (in PIT) steps of these optimized processes. The high-pressure-low-temperature (HPLT) processes are assumed to be conducted in a high-pressure tubular reactor designed to work in a semi-continuous mode on an industrial scale. Results showed that important vessel lengths (40-370 m) are needed when the sample velocity ranges from 1 to 6 m/min. These vessel lengths are feasible thanks to the modular structure of the suggested high-pressure system. (C) 2007 Elsevier Ltd. All rights reserved.