Regularities in pressure filtration of fine and colloidal suspensions

A number of interesting and potentially useful regularities have been observed in high pressure batch filtration of fine and colloidal suspensions carried out to equilibrium under a wide variety of physical and chemical process conditions. Two such regularities are described here. The first regular behavior, demonstrated by a large number of colloidal suspensions, can be represented by the Pareto profile, which relates filtration rate with solid content of filter cake at equilibrium. The profile is found to be a strong function of material fineness but is seemingly independent of physical and chemical process conditions. Since it can be treated as a constrained performance benchmark for the filtration of a suspension, it is germane for evaluating the filtration process in terms of two of its more important process measures, namely, kinetics and maximum extent of dewatering that is achievable. The Pareto regularity reflects the fact that in general it is not feasible to improve both measures simultaneously in a batch filtration which is driven to equilibrium, and any process modification may improve one measure but invariably at the expense of the other. The second regular behavior, namely, self-similarity in pressure filtration is demonstrated for filtration of wide variety of varying materials and process conditions. Simple transformation and scaling of slurry filtration data with critical solid volume fraction and critical time at the transition from cake formation stage to cake consolidation stage translate the filtration curves into a form which are remarkably self-similar. This self-preserving behavior is demonstrated for a wide variety of experimental data under varying physical and chemical process conditions for different colloidal systems. Some implications of the regularities are discussed

The existence of a critical salt concentration for particle release

A critical salt concentration (CSC) was found to exist in the water sensitivity of Berea sandstone. If the salinity of the permeating fluid falls below the CSC, the sandstone permeability is significantly reduced as a result of clay particles being released from the pore walls and blocking the pore throats. However, when changes in salinity occur above this threshold value, clay particles are not released from the pore walls in the sandstone and therefore no reduction in core permeability occurs. The CSC was determined by core flood experiments in conjunction with particle analysis of core effluent samples. The CSC exists only in the case of monovalent cations and is virtually nonexistent for cations having a valence greater than one. Even among monovalent cations the CSC varies considerably and it decreases with increasing ion exchange affinity of the clay for the counterion. For polyvalent counterions, the release of clay particles is effectively prevented due to the strong ion-exchange affinity of clay for the counterion. The critical salt concentration was also found to depend on the temperature, but not on the flowrate, of the electrolyte solution. The temperature dependence has been explained by using the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory of stability of colloidal dispersions.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/24708/1/0000129.pd

Sandstone water sensitivity: Existence of a critical rate of salinity decrease for particle capture

A critical rate of salinity decrease (CRSD) was found to exist in the water sensitivity of Berea sandstone. The CRSD was determined by conducting core flood experiments in which the salinity was decreased exponentially by placing mixers of various volumes in the inlet stream. Above the CRSD, drastic reductions in core permeability occur due to a log jam, or bridging effect, at the pore constriction. Below the CRSD, the released clay particles pass through the constriction without being captured and therefore very little reduction in core permeability occurs. A critical particle concentration was also identified above which bridging occurs. Based on the experimental observations, a mathematical model was developed to delineate the regions of high and low permeability reductions. The model predictions are in good agreement with experimental measurements.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/25441/1/0000891.pd

Colloidally-induced fines release in porous media

A critical value of the total ionic strength (CTIS) has been found to exist for mixed salt solutions flowing in porous media. If the ionic strength drops below this value, significant amounts of fines are released in-situ due to colloidal forces, causing drastic formation damage. For a NaCl/CaCl2 system, the CTIS is strongly dependent on the relative amount of CaCl2 present in the solution. The concept of a critical salt concentration (CSC) and the analysis based on DLVO theory has been extended to mixed salt systems to estimate the CTIS. The difference between critical flocculation concentration (CFC), and the present definition of CTIS has been pointed out. Predictions of this analysis are consistent with experimental observations.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/28497/1/0000294.pd

A mean Phi model for pressure filtration of fine and colloidal suspensions

A working model for engineering analysis of pressure filtration is presented. Based on the filtration characteristics of fine and colloidal suspensions, the process was divided into two stages. A time-invariant spatially uniform volume fraction of solids approximation is invoked in the growing filter cake stage (stage 1). A time-dependent spatially uniform volume fraction of solids assumption is made in the cake consolidation stage (stage 2). The two models, named collectively as Mean Phi (M-P) model, have a common physical basis, seamless continuity between the stages and internal consistency. The M-P model has only three parameters: terminal or equilibrium volume fraction of solids in the filter cake that is related to its compressive yield stress, critical volume fraction of solids, which joins stage 1 and stage 2, and a permeability factor, which is common to stages 1 and 2. The model is validated with a large number of colloidal suspensions filtered under highly diverse physical-chemical process conditions. A Pareto profile is identified that relates the timescale of filtration and the extent of dewatering achieved, the two most important performance indices of the process

Reduction in gravity-induced collision frequencies of particles dispersed in a viscoplastic fluid

It is shown that the gravity-induced collision frequency of non-Brownian particles dispersed in a quiescent viscoplastic liquid is strongly dependent on the yield stress. The reduction in collision frequency with the increase in yield stress is due to the decrease in particle sedimentation velocity as well as the immobilization of smaller particles