The role of vapor formation in high-intensity drying - Description and comparison of two models

High-intensity paper drying processes involve the complex phenomena of multiphase transport in porous media. The role of simultaneous gas-liquid flows in the fibrous web during high-intensity drying is of special interest. In discussing high-intensity drying, we will specifically consider impulse drying, an emerging technology in which gas-liquid flows may be especially important. Numerical models have been developed to predict the transient heat transfer, vapor pressure development, and vapor-liquid flow in high-intensity processes. Here, we compare results from independent modeling efforts at two institutions, including a highly idealized moving boundary model for impulse drying (IPST) and an extensive generalized model for paper drying (SUNY-Syracuse) applicable to conventional and high-intensity conditions. Though neither model can fully capture the complex details of impulse drying, the results give some insight into possible heat transfer and flow mechanisms in impulse drying and related high-intensity paper drying processes. Both efforts point to the importance of vapor formation. Pressurized vapor can displace liquid water out of the web. Sheet permeability is an important factor controlling the development of internal vapor pressure. Numerical results also show that the physics of vapor formation and liquid flow are similar to that of a heat pipe. As drying progresses, the location of the heat pipe advances through the web. Continued vaporization and condensation in the web are major heat transfer mechanisms.