Foam control utilizing hydraulic forces of an accelerated liquid surface and characterization of durable foams by application of an external force
The dissertation presents two distinct but related studies of foam. The initial investigation involves the evaluation of a method of controlling foaming in aerated fermenters; the second involves the evaluation of a method of characterization of durable foams.
Since foaming has frequently been a problem of considerable magnitude in aerated waste treatment and various other aerated cultures, a method was conceived which involved the disruption of foam at the liquid-foam interface during carriage as a result of an accelerated converging flow through a down flow draft tube. The principles of the method were based on the creation of differential shearing forces at the foam-liquid interface to cause disruption of foam at the interface and subsequent re-entrainment during the convergence of flow through the down flow draft tube.
The geometry of the inlet to the draft tube, recirculation rate, and hydraulic head over the inlet were principal variables affecting the degree of foam control. Foam control performance increased with increased diameter of the inlets as a result of the reduced tendencies of foam to span or 2 bridge inlets of larger size. Increased recirculation rate and low hydraulic head over the inlet offered the greatest potential relative to foam re-entrainment; however, the conditions of increased recirculation rate and low hydraulic head resulted in air being drawn into the draft tube in addition to the foam-liquid mixture. The foam subsequently re-appearlng at the liquid surface possessed a dense, fine hubble structure which was resistant to control.
The method proved successful on a short-term basis in as much as operational times were extended to about six times those required to generate comparable volumes of foam while employing aeration only. For extended operational periods, the method resulted in resistant foams which defied control by the process.
During the course of the foam control investigation a method was sought which could practically be employed to evaluate the durability properties of foams for routine characterization measurements.The natural collapse periods for foams resulting from the employed alkyl benzene sulfonate (ABS) solutions were of such duration to be impractical for repetitive measurements. A method was conceived whereby the destruction of teams could be accelerated by application of an external force. The method involved application of a weighted disc assembly to a definite volume of foam confined in a 3.75-inch inside diameter foam generation column. The principle of the method was based on the hypothesis that a measure of the durability properties of teams could he obtained relative to the rate of collapse under applied stress,
The method was evaluated on foams generated from solutions containing ABS concentrations ranging from 20 to 120 ppm employing aeration rates of 0.75, 1.5, and 2.45 l/min, Measurements consisted of recording the level of disc descent at one-minute intervals until the disc reached the liquid surface, The results were presented as disc descent rates or slopes of the disc descent curves for each measurement.
Experiments employing static decay methods were performed to observe the behavior of foams subjected to destruction due to natural causes, Measurements consisted of observing the remaining foam level at one-half hour intervals for a total period of eight hours. The results were presented graphically as foam collapse vs time curves and quantitatively in terms of the average lifetime of the teams.
The results of the experiments employing the weighted disc and the experiments employing static decay methods agreed insofar as the durability properties of foam were enhanced by increased ABS concentration and low aeration rate. The results of both methods indicated that, on a comparative basis of the data and by a statistical analysis of the data, aeration rate had a more pronounced effect on foam character than ABS concentration for the ranges of the variables employed in the investigation.
The periods required for completion of the measurements of the weighted disc experiment: ranged from about 10 to 25 minutes which represent a considerable saving in time in comparison to the length of experiments employing static decay procedures. The rapidity and simplicity of the method employing the weighted disc appear to make this method applicable for routine characterization measurements of durable foams.