How Bubbly Mixtures Foam and Foam Control Using a Fluidized Bed

In hydrocracking and other foaming reactors, the foam rises to the top because it has a higher gas fraction than the bubbly mixture from which it comes. The high gas hold-up in foams is undesirable in chemical reactors because it strongly decreases the liquid residence time and in hydrocracking reactors also promotes the formation of coke. To study foams we built a cold slit bubble reactor which when used with aqueous anionic surfactants gives rise to foam. This reactor reproduces the foaming processes which are characteristic of the commercial system CANMET from Petrocanada. We discovered a critical condition for foaming; when the gas velocity exceeds a critical value which depends on the liquid velocity, a foam interface appears at the top of the reactor, with foam above and bubbly mixture below. The interface is very sharp and it moves down the reactor as the gas velocity is increased at a constant liquid velocity. This is the way reactors foam, with the bubbly mixture being consumed by foam. The foam may be destroyed by increasing the liquid velocity backing up against the foaming threshold. The reactor partitions into two phase, two phase ow with bubbly mixture below and foam above. The bubbly mixture is dispersed gas in water plus surfactant; the phase above is a foam through which large gas bubbles rise. Constant state theories for the bubbly mixture, the foam and the position of the foam interface are derived and semiempirical correlations are presented. Foaming may be strongly suppressed by uidizing hydrophilic particles in the bubbly mixture below the foam. The suppression is achieved by increasing the liquid holdup by bed expansion; by increasing the wetted area of solid surface (walls and particles) and by decreasing the gas hold-up by increasing the e ective density of the liquid-solid mixture. Industrial foaming reactors Applications of foams and foaming are found in many industries like the otation of minerals, enhanced oil recovery drilling in oil reservoirs, insulation, construction and re ning processes such as Vacuum distillation and Delay-Coker reactors. However, foaming and defoaming are not yet well understood. The modern theory of foams could be said to start with [1973] study of Bikerman. A consolidation of the advances in the study of foams and foaming is in the collection edited by Prud'homme and Khan [1995]. Foams trap gas and are not wanted in many applications; Garrett [1993] has edited a collection of papers on defoaming. Our application area is to unwanted foaming in foaming bubble reactors and the practical application of our study is the suppression of foaming. To achieve this goal we propose fundamental studies of recent observations which we have made in foam suppression studies in the slit bubble reactor described in section two below. Pruden [1993] has reported 80v% gas hold-up in the reactor of the commercial demonstration plant used in the CANMET process for a wide range of gas velocities and reactor temperatures. The overall gas hold-up values which were observed are consistent with a severe foaming condition in the reactor using a typical commercial gas velocity. In the CANMET process, foam formation was detected in the commercial unit only because of the higher gas velocities employed. Under typical hydrocracking conditions using catalytic slurry reactors of the bubble column type and catalysts, foam may be present in the reaction zone. Foaming can occur at temperatures higher than 400 C which are typical in industrial reactors.