OTHER TYPES OF SEPARATORS

Coalescers As noted earlier, adding coalescing internals to a decanter can improve decanter performance by promoting growth of small drops. The same concept can be applied in a separate coalescer vessel to treat the stream feeding the decanter. Systems of type III or type IV (Table 15-24) in particular may benefit, i.e., applications involving a need to break a secondary dispersion. Coalescers typically are packed with a granular material, a mesh made of metal wire or polymer filaments (or both), or fine fibers in woven or nonwoven com- posite sheets. The typical flow configuration is upflow if the light phase is dispersed and downflow if the heavy phase is dispersed. Coalescers containing fairly large media such as beds of granules or wire mesh may be able to tolerate a feed containing some fine solids. Coalescers containing fine granules or fine fibers require that the feed be free of solids to avoid plugging, so prefiltration may be necessary. For more detailed information, see Li and Gu, Sep. and Purif. Tech., 42, pp. 1–13 (2005); Shin and Chase, AIChE J., 50(2), pp. 343–350 (2004); Wines and Brown, Chem. Eng. Magazine, 104(12), pp. 104–109 (1997); Hennessey et al., Hydrocarbon Proc., 74, pp. 107–124 (1995); Madia et al., Env. Sci. Technol., 10(10), pp. 1044–1046 (1976); Davies, Jeffreys, and Azfal, Brit. Chem. Eng. Proc. Tech., 17(9), pp. 709–712 (1972); and Hazlett, Ind. Eng. Chem. Fund., 8(4), pp. 625–632 (1969). In most applications, the packing material should be wetted by the dispersed phase to some degree for best performance; however, this will depend on the size of dispersed droplets. For very fine droplets on the order of 10 µm or smaller, surface wetting is not the primary coalescence mechanism [Davies and Jeffreys, Filtration and Separa- tion, pp. 349–354 (July/August 1969)]. In these cases, the packing pro- motes coalescence by providing a tortuous path that holds dispersed drops in close contact, facilitating drop-drop collisions. In other cases involving larger drops, a drop interception and wettability mechanism becomes important; i.e., the media provide a target for drop–solid sur- face collisions, and the surface becomes wetted with drops that merge together and leave the media as larger drops. In this case, an interme- diate (optimum) wettability may be needed to most effectively pro- mote the growth and dislodging of drops from the media [Shin and Chase, AIChE J., 50(2), pp. 343–350 (2004)]. In general, the degree to which flow path/collision mechanisms and/or surface wettability are important for good performance depends on the drop size distribu- tion and dispersed-phase holdup in the feed, as well as system physi- cal properties and whether surfactants or fine particulates are present. (See “Stability of Liquid-Liquid Dispersions” under “Liquid-Liquid Dispersion Fundamentals.”) All this affects the choice of media, media size and porosity, and coalescer dimensions as a function of throughput. For a given application, some experimental work gener- ally will be needed to sort this out and identify an effective and reli- able design.

In cases where wettability is important, various types of sand, zeo- lites, glass fibers, and other inorganic materials may be used to facili- tate coalescence of aqueous drops dispersed in organic feeds. Carbon granules, polymer beads, or polymer fibers may be useful in coalescing organic drops dispersed in water. The packing material should resist disarming by impurities, meaning that impurities should not become adsorbed and degrade the surface wettability characteristics over time. This can happen with charged or surfactantlike impurities; Paria and Yuet [Ind. Eng. Chem. Res., 45(2), pp. 712–718 (2006)] describe the adsorption of cationic surfactants at sand-water interfaces, a phenome- non that can alter surface wettability. In a few cases, the packing needs to age in service to develop its most effective surface properties.

Madia et al. [Env. Sci. Technol., 10(10), pp. 1044–1046 (1976)] describe a chromatography method for screening potential media with regard to surface wettability. The method involves measuring the retention times of water and heptane (or other components of interest) by using columns filled with the packing materials of interest (reduced in size if needed); the longer the relative retention time, the greater is the wettability of the packing for that component. The authors used gas chromatography of water and heptane to characterize coalescence for an oil-in-water dispersion; but it should be possible to characterize other systems by using this approach, and liquid chromatography methods might be used for components with low volatility.

For granular bed coalescers, typical granule sizes include 12  16 Tyler screen mesh (between 1.4 and 1 mm) and 24  48 Tyler mesh (0.7 to 0.3 mm). Smaller sizes sometimes are used as well. Typical bed heights range from 8 in to 4 ft (0.2 to 1.2 m), with the taller beds used with the larger granules. Layered beds may be used. For example, the front of the coalescer may contain a thin layer of fine media with low porosity and high tortuosity characteristics to facilitate drop-drop colli- sions of very small droplets, followed by a layer of coarser media having the wetting characteristics needed to further grow and shed larger drops.

For fine-fiber coalescers, the coalescing media normally are arranged in the form of a filter cartridge. Wines and Brown [Chem. Eng. Magazine, 104(12), pp. 104–109 (1997)] describe a coalescing mechanism in which a drop (on the order of 0.2 to 50 µm) becomes adsorbed onto a fiber and then moves along the fiber with the bulk liq- uid flow until colliding with another adsorbed drop at the intersection where two fibers cross. Fiber diameter and wettability are important properties as they affect porosity (tortuous path) and wettable surface area. Like a packed-bed coalescer, a filter-type coalescer may be con- structed in layers: an initial prefilter zone to remove particulates and minimize fouling, a primary coalescence zone where small droplets grow to larger ones, and a secondary coalescence zone with greater porosity and having surface-wetting characteristics optimized to grow the larger drops.

Pressure drop, an important consideration in the design of any coa- lescer, depends upon media size and shape, bed height or filter thick- ness, and throughput. Methods for calculating pressure drop through packed beds and porous media are described in Sec. 6. For approxi- mately spherical media, the pressure drop due to frictional losses, assuming incompressible media, may be estimated from

= + Reparticle= ≤ 10

(15-191)

where L is the length of the packed section, V is the superficial veloc- ity of the total liquid flow, dmis an equivalent spherical diameter of the

media particles (given by 6 times the mean ratio of particle volume to particle surface area), and ϕ is the volume fraction of voids (flow chan- nels) within the bed [Ergun, Chem. Eng. Prog., 48(2), pp. 89–94 (1952)]. Also see Leva, Chem. Eng. Magazine, 56(5), pp. 115–117 (1949), or Leva, Fluidization (McGraw-Hill, 1959). The minimum value of ϕ for a tightly ordered bed of uniform spherical particles is 0.26, but of course for real media this will vary depending upon the particle size distribution and particle shape. The second term in Eq. (15-191) often is neglected at Reparticle≤ 1. For fiber media, dmcan be

thought of as a characteristic fiber dimension. For discussion of pres- sure drop through fiber beds, see Shin and Chase, AIChE J., 50(2), pp. 343–350 (2004); and Li and Gu, Sep. and Purif. Tech., 42, pp. 1–13 (2005). In practice, pressure drop data may be correlated by using an equation of the same form as Eq. (15-191), ∆PL = aV + bV2_, where a and b are empirically determined constants. Media and equipment suppliers generally will have some experimental data showing∆PL versus flow rate.

Centrifuges A stacked-disk centrifuge or other type of cen- trifuge may be a cost-effective option for liquid-liquid phase separa- tion whenever use of a gravity decanter/coalescer proves to be impractical because rates of drop settling or coalescence are too low. This may be the case for type III and type IV systems (Table 15-24) in particular. Factors involved in specifying a centrifuge are discussed in “Centrifugal Extractors” under “Liquid-Liquid Extraction Equip- ment.”

Hydrocyclones Liquid-liquid hydrocyclones, like centrifuges, utilize centrifugal force to facilitate the separation of two liquid phases [Hydrocyclones: Analysis and Applications, Svarovsky and Thew, eds. (Kluwer, 1992); and Bradley, The Hydrocyclone (Pergamon, 1965)]. Instead of using rotating internals, as in a centrifuge, a hydrocyclone

Vρcdm  µ 1.75ρcV2  dmϕ3 150(1− ϕ)2_µV  d2 mϕ3 ∆P  L

generates centrifugal force through fluid pressure to create rotational fluid motion (Fig. 15-68). Feed enters the hydrocyclone through a tangential-entry nozzle. A primary vortex rich in the heavy phase forms along the inner wall, and a secondary vortex rich in the light phase forms near the centerline. The underflow stream (heavy phase) exits the cyclone through the apex of the cone (underflow nozzle). The overflow stream (light phase) exits through the vortex finder, a tube extending from the cylinder roof into the interior. The feed split can be adjusted by changing the relative diameters of the vortex finder and underfow nozzle. A hydrocyclone is not completely filled with liq- uid; an air core exists at the centerline. A commercial-scale hydrocy- clone multiplies the force of gravity by a factor of 100 to 1000 or so, depending on the diameter and operating pressure. Hydrocyclones traditionally have been used for liquid-solid separations, but by adjust- ing their design (cone angle and length, vortex finder length, and so on) they can be applied to liquid-liquid separations [Mozley, Filtration and Sep., pp. 474–477 (Nov./Dec. 1983)].

Since the fluid flow is turbulent at the top of the unit and the rota- tion of liquid within the device produces a high shear field, mixtures with low interfacial tension tend to emulsify or create foam within a hydrocyclone. However, hydrocyclones may be well suited for type I or possibly type II mixtures containing some solids, especially if only a rough cut is needed. The flow pattern established within a hydrocy- clone normally requires that a considerable part of the feed leave in the overflow outlet. For this reason, hydrocyclones are generally more efficient for feeds containing only a small fraction of heavy phase, although some authors indicate they can be effective for feeds with a small fraction of light phase through careful specification of hydrocy- clone geometry.

The main operating variables for a hydrocyclone are the feed pres- sure, the feed flow rate, and the split ratio, i.e., the relative amounts of fluid exiting top and bottom. The split ratio may be adjusted by speci- fying the size of the underflow and overflow nozzles. Choosing a material of construction wetted by the heavy phase for the cone may improve the effectiveness of the device. Experimental work is needed to determine the efficiency of the separation as a function of the split ratio for a series of flow rates and hydrocyclone geometries [Sheng, Sep. and Purif. Methods, 6(1), pp. 89–127 (1977); and Colman and Thew, Chem. Eng. Res. Des., 61(7), pp. 233–240 (1983)]. If testing indicates satisfactory performance, hydrocyclones can be relatively inexpensive and simple-to-operate units (no moving parts). Because sufficient centrifugal force cannot be generated in large-diameter units, scale-up consists of connecting multiple small units in parallel.

Units are sometimes placed in series to provide multiple stages of sep- aration. Hydrocyclones are used on ships and drilling platforms for removing oil from water [Bednarski and Listewnik, Filtration and Sep., pp. 92–97 (March/April 1988)]. Numerical simulations of hydro- cyclone performance and flow profiles are described by Bai and Wang [Chem. Eng. Technol., 29(10), pp. 1161–1166 (2006)] and by Murphy et al. [Chem. Eng. Sci., 62, pp. 1619–1635 (2007)].

Ultrafiltration Membranes These are microporous mem- branes with pore sizes in the range of 0.1 and 0.001 µm [Porter, “Ultrafiltration,” in Handbook of Industrial Membrane Technology (Noyes, 1990)]. In this size range, the pores may be used to “filter out” and concentrate micelles from a liquid feed without disrupting (breaking) the micellar structure. Such a membrane may also be used to remove micrometer size droplets from a dilute dispersion. How- ever, if the dispersed-phase content is too high, the membrane may become fouled owing to deposition of a coalesced layer that obstructs the pores. This can be a particular problem when removing oil droplets for an oil-in-water dispersion using a polymeric membrane.

The feed solution is fed to the membrane module under pressure (normally less than 6 bar). The majority of the continuous phase flows through the pores of the membranes by pressure difference and collects on the permeate side as a clarified solution. The micelles or micro- droplets are rejected and flow with the remaining continuous phase, tan- gentially along the membrane surface, to the retentate outlet of the membrane module [Voges, Wu, and Dalan, Chem. Processing, pp. 40–43 (April 2001)]. The shear at the surface of the membrane should be high enough to stop the micelles from aggregating on the polymeric surface of the membrane, but low enough to avoid breaking the colloidal particles. Ultrafiltration membranes can be very efficient at removing col- loidal particles of an emulsion but normally will not stop dissolved oil from permeating. Since most membranes are polymeric, they are more stable in the presence of water, so they are best suited for aque- ous systems. Since they produce only one well-clarified phase (the per- meate), they should be applied to processes with stable micelles where clear continuous phase is required and where losses of continuous phase with the micellar phase can be tolerated. The use of ultrafiltra- tion membranes in an extractive ultrafiltration process for recovery of carboxylic acids is discussed by Rodríguez et al. [J. Membrane Sci., 274(1–2), pp. 209–218 (2006)].

Selecting the membrane best suited for a given application is best accomplished experimentally. The membrane material must be com- patible with the feed, and the module should exhibit high permeation flow while maintaining good micelle rejection. The pore size and the molecular weight cutoff reported by the manufacturer are good indi- cations of membrane performance; but since other factors such as membrane/solute interaction and fouling impact the separation, this information is only a starting point. Key operating parameters include temperature, feed flow rate, and permeate-to-feed ratio. Scale-up consists of adding membrane modules to handle the required production rate [Eykamp and Steen, Chap. 18 in Handbook of Separation Process Technology, Rousseau, ed. (Wiley, 1987)].

Electrotreaters In an electrostatic coalescer, an electric field is applied to a dispersion to induce dipoles or net charges on the sus- pended drops. The drops are then attracted to one another, facilitat- ing their coalescence [Waterman, Chem. Eng. Prog., 61(10), pp. 51–57 (1965); and Yamaguchi, Chap. 16 in Liquid-Liquid Extraction Equipment, Godfrey and Slater, eds. (Wiley, 1994)]. This technology is applicable only to a nonconductive continuous phase and an aqueous dispersed phase. Once the water drops are sufficiently large, they set- tle to the bottom of the vessel while the clarified oil phase migrates to the top. The top and bottom zones are kept quiet and out of the elec- tric field. In cases where inlet salt content is high, a multistage, coun- tercurrent desalting system can be used. Units with ac or dc voltage are available.

Electrostatic separators are high-voltage electrostatic devices that can arc under certain conditions. For this reason, a careful review of safety considerations is needed, especially for applications involving flammable liquids. Evaluating feasibility and generating design data normally involve close consultation with the equipment vendors. This technology is applied on a very large scale in the petroleum industry for crude oil desalting.

Tangential Feed

Overflow

Underflow Air Core