Filtration dynamics

Includes cyclonic, dynamic, filtration, inertial impaction (wetted targets, packed towers, turbulent targets), spray chambers, and venturi. 

Dynamic filtration modules present a relative movement between the membrane and the module, or between the membrane and a rotor. Thus, it is possible to adjust the shear stress independently of the feed flow rate and of the transmembrane pressure drop. 

Dynamic filtration modules are basically of two types rotating disc filter (RDF) and vortex flow filter (VFF). In the latter, the filtration module has a cylindrical shape and has a rotating concentric cylindrical mesh in its interior. The rotational movement of the internal cylinder generates a Taylor-Couette flow in the annular gap (Roth et al., 1997), creating Taylor vortices that minimize concentration polarization and mesh fouling. Continuous perfusion processes based on this type of filter and operating continuously for up to 100 days have been reported (Mercille et al., 1994). 

Kroner KH and Nissinen V, Dynamic filtration of microbial suspensions using an axially rotating filter, J. Membr. Sci. 1988 36 85-100. 

Jaffrin MY, Ding LH, Akoum O, and Brou A, A hydrodynamic comparison between rotating disk and vibratory dynamic filtration systems, J. Membr. Sci. 2004 242 155-167. 

Lee S, Burt A, Russoti G, and Buckland B, Microfiltration of recombinant yeast cells using a rotating disk dynamic filtration system, Biotechnol. Bioeng. 1995 48 386. 

The most common methods of fouling prevention are wastewater pretreatment, the control of hydrodynamic conditions in the apparatus (low flux, high feed flowrates, turbulence promoting, dynamic filtration), or special construction of the module (cross-flow filtration). 

Dynamic Filtration. Dynamic (or cross-flow) filtration is a considerably more complex separation process than static filtration as the rate of filtration is a strong function of the flow of the drilling fluid. A number of studies (see reference 128 and the references cited therein) have demonstrated the various factors that control dynamic filtration rates. 

The elaborate nature of the equipment needed to study realistic dynamic filtration on representative rock samples has largely precluded routine measurements of the dynamic fluid loss of drilling fluid samples at the rig site during drilling operations. Figure 36, for example, shows the equipment used by Fordham and co-workers (129, 135) to measure dynamic filtration rates of water-based drilling fluids. Similarly large and complex equipment has been used recently by Jiao and Sharma... 

Figure 37 shows a schematic of the dynamic filtration of a drilling fluid. The drilling fluid is filtered across the filter medium at the pressure difference APfd while being subjected to flow (assumed laminar in the following discussion), which gives rise to a shear stress rc at the cake-fluid interface where the shear rate is yc. 

At longer times, the cumulative filtrate volume Vflinear dependence on time, indicating that the filter cake has reached a constant thickness. The limiting (constant) dynamic filtration rate is a marked function of 7C and rc. The limiting dynamic filtration rate Qm (= dVfa/ Adt as t - oo) has been scaled by 7C and rc by (135)... 

Figure 37. Schematic of dynamic filtration of drilling fluid against a filter medium.

Figure 40. Scaling of limiting rate of dynamic filtration with (A) shear stress tc and (B) shear rate yc. (Reproduced with permission from reference 135. Copyright 1989 Advance Publications Ltd.)...

Figure 42. Dependence of (a) limiting dynamic filtration rate (Q/d) and (b) inverse of filter cake thickness (l/hc) on shear rate of drilling fluid. (Muds 2 and 3 from reference 139 with Fordham and Ladva s data from reference 135.) Lines show least squares fit to data from muds 2 and 3.

Figure 43. Time dependence of cumulative filtrate volume from dynamic filtration into media of high and low permeability. Filtration into Ohio sandstone at various APfd (MPa) 1. 0.14 2. 0.55 3. 0.60 4. 0.97 5. 1.45 6. 1.93 7. 1.93. Filtration into Portland limestone at 8. 1.93 MPa. (Reproduced with permission from reference 140. Copyright 1992 Academic.)...

The dynamic filtration theory of Outmans (127) requires experimental terms such as particle-particle stresses, particle friction factors, and thickness of a shear zone within the filter cake that would be difficult to determine. However, the qualitative picture of dynamic filtration presented by Outmans, namely, irreversible adhesion of solid particles up to a certain thickness that is determined by the shear stress (or shear rate) at the surface of the cake, accords with the experiments of Fordham and co-workers (129,135). Once a filter cake has formed under dynamic conditions, it is difficult to remove it by subsequent changes in yc or vm. Figure 44 shows the effect of changes in the flow rate on cumulative filtrate volume. The limiting filtration rate obtained when the initial flow rate of the drilling fluid was 1.8 m3/h remained unaltered when the flow rate of the drilling fluid was increased to 7.0 m3/h in a step-... 

Effect of Composition on Fluid Loss. There have been many studies and tests of the effects of various additives (usually polymeric) on the static fluid loss of drilling fluids. There have been many fewer studies on the effect of these additives on dynamic filtration rates and very few that compare their effect on both dynamic and static rates. 

A comparison of the effect of well-known fluid loss additives on the rates of static and dynamic filtration was made by Kreuger some 30... 

Invasion and Drilling Related Formation Damage. The influx of particulates and filtrate into the near wellbore region of permeable formations during static or dynamic filtration has a number of consequences, two of the most important being displacement of wellbore... 

Figure 53. Time dependence of normalized permeability of water-saturated sandstone samples during dynamic filtration of various water-based drilling fluids and injection of filtrate. (A) 20 gjL bentonite suspension (B) 40 gjL bentonite suspension (C) injection of fresh-water filtrate (D) 40 g/L bentonite in 0.51 M NaCl. (Reproduced with permission from reference 150. Copyright 1992 Society of Petroleum Engineers.)...

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