Wiped film evaporators: Segmental assessment of wetting behavior and heat transfer performance

摘要

An analysis of different operating points during the evaporation of a subcooled feed in a wiped film evaporator (WFE) allows a conceptual segmental assessment, distinction and quantification of various wetting behaviors and heat transfer situations during the liquid film flow. In detail, single-phase heating, evaporation from a fully covered surface and evaporation from a partly covered surface may be distinguished. For each section the wetting situation is extracted. This knowledge is essential for operating wiped film evaporators within a suitable and appropriate range to prevent e.g. product damage. The experiments were conducted at a stainless steel and steam-heated wiped film evaporator with a spring-loaded, inclined comb wiper system and monoethylene glycole as evaporation fluid. Process pressure, wiper type and rotational velocity were kept constant. Variations of feed rate and driving temperature difference mark out the characteristic operating points. In case of sufficient wetting of the evaporation surface, overall heat transfer coefficients of 1800 W/(m(2).K) are reached. By decreasing feed rate and assessing the achieved distillate rate minimum sump loads can be derived for a given driving temperature difference. This quantifies the position of film break-up leading to a partly covered heat transfer surface. Active heat transfer areas are extracted from the experimental data and quantified to get a more precise view on suitable process conditions of WFE and its principal operation. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.