02 BOPs / Woods D.R 2008 rules-of-thumb-in-Engineering-practice (epdf.tips)

4.20 Membranes: Reverse Osmosis, RO 131

4.20

Membranes: Reverse Osmosis, RO

x Area of Application

a = 6 to 25; feed concentration 0.05 to 20 % w/w; with suggested economic feed concentration I 0.5 %; 99 % purity possible. Diameter of the target species: 0.2 to 0.8 nm. Must overcome a difference in osmotic pressure. The osmotic pressure coefficient in mass ratio units for different solutes = 20–80 MPa kg/kg at 25 hC. The higher the valence, the better the rejection.

x Guidelines

Driving force for the rate of separation: hydrostatic pressure.

Membrane: Asymmetric: homogeneous or microporous; active dense 20 to 50 mm layer of cellulose acetate with total thickness 100 mm; composite of a homogeneous polymer film on microporous substructure of polyamide or polysulfone, or asymmetric skin. Usually homogeneous.

Pressure: 1.4–10 MPa (1.4–4.2 for brackish water; 5.6–10 for seawater). Inlet pressure i twice the inlet osmotic pressure.

Temperature I 45 hC.

Capacity/unit: I 7 L/s.

For cellulose acetate membranes: 1/U+ = A p/B r = 1–500 with usual value 300, [dimensionless].

For aromatic polyamide membranes: 1/U+ = 0.7–20. A = permeate hydraulic permeability, g/s m2 MPa. p = total operating pressure, MPa (1.4–10 MPa).

B = target solute transport coefficient, mm/s (10–6 –10 mm/s). r = mass density of the feed stream.

Hydraulic permeability, A: 0.0005–8 g/s m2 MPa (0.1–10 for cellulosic). Permeate flux 0.001–0.1 L/s m2 ; for cellulose acetate: 0.006–0.0075 L/s m2 ; for

hollow fiber: 0.001–0.002 L/s m2 ; for thin film composite: 0.007–0.009 L/s m2 . Permeate flux increases about 3 % for every 1 hC increase. Permeate flux decreases by 10 to 50 % depending on the concentration polarization. Permeate flux is reduced because of particulates and bacterial adhesion so that flux for tubular I spiral wound I hollow fiber.

Configuration: see Section 4.15.

Usually spiral wound but some use hollow fiber and tubular (used for low volume, high value commodities), pleated sheet, tubular monolithic elements, or plate and frame.

Use cross flow batch with 100 % recycle; continuous with recycle ratio 15–30/1 or multistage (often three stages). Criterion to backwash and clean: operate until a given concentration or volume reduction is reached in the retentate or a given purity or volume is achieved in the permeate.

Membrane life, 2–4 years.

132 4 Homogeneous Separation

Cycle time: pretreat to prevent scaling or buildup or operate a short cycle, 2–12 h, cleaning with dilute nonionic detergent. Degree of pretreatment: hollow fiber i spiral i tubular.

Reverse osmosis: 0.0029 L/s m2 of membrane area; for waste water with concentration up to 10 000 mg/L.

x Good Practice

Consider pretreating hydrophobic membranes for aqueous use.

x Trouble Shooting

“Permeate flow I design”: physical fouling (incorrect/incomplete pretreatment/ scaling/biofouling)/chemical fouling: (pH shift/incorrect anti-scalant dosage). “Permeate quality degradation”: failure of mechanical seal/chemical attack of membrane by pH, chlorine or biodegradation/concentration polarization/post-con- tamination.

4.21

Membranes: Nanofiltration

x Area of Application

Molar mass cut off = 200 = 0.2 kDa. The usual range is 0.01–1 kDa. Can handle fluid with significant osmotic pressure, sugars, dissociated acids and divalent salts although the latter two are better handled by RO.

x Guidelines

Driving force for the rate of separation: pressure.

Membrane: asymmetric thin film, aromatic polyamide, cellulose membrane; membrane usually negatively charged to reject anions.

Pressure: (between UF and RO) = 0.3–1.4 MPa. Configuration, see Section 4.15. Usually spiral wound.

x Trouble Shooting

See Section 4.20.

4.22

Membranes: Ultrafiltration, UF

Related topic filters, Sections 5.13 and 5.14.

x Area of Application

a = k2 D2/k1 D1 = 6 to 60; liquid feed concentration 0.04–20 % w/w; 99.9 % purity possible. k = partition coefficient; D = diffusivity. Diameter of target species 0.8– 200 nm and usually 1–10 nm; removes soluble macromolecules, colloids, salts

4.22 Membranes: Ultrafiltration, UF 133

and sugars but cannot separate dissolved salts, species with molar mass I 1000 or species exhibiting a significant osmotic pressure. Feed concentration I 20 % dissolved organics. MMCO = 0.3–500 kDa.

x Guidelines

Driving force for the rate of separation: hydrostatic pressure.

Membrane: Most UF membranes are polysulfone: asymmetric microporous with thin skin 0.1 to 1 mm supported on a porous layer 50 to 250 mm. Pore size 0.001–0.2 mm. This is too porous for RO. Pore size prevents concentration polarization (limiting RO) but performance is limited by gel polarization with xgel 0.2–0.4. xgel = 0.25–0.35 for macromolecules; xgel = 0.75 for colloids. Need to have membrane life i 1 year.

Pressure: 0.1 to 0.7 MPa. Hydraulic permeability, A: 0.8–800 g/s m2 MPa. Feed concentration: 0.05–15 % w/w.

Temperature I 90 hC and pH 0.5–13 for polysulfone. Capacity/unit: 0.1–25 L/s. Select diameter or channel spacing so that diameter of the target species is 0.1 of the diameter or channel spacing; except for spiral wound where, for 0.75 mm spacing the particles must be I 5–25 mm; or 0.006–0.034 diameter of spacing; for 1 mm spacing particles should be I 25–50 mm or 0.025–0.05 of the diameter or

channel spacing.

Permeate flux : depends on the membrane and configuration: hollow fibers/ polysulfone: 0.005–0.016 L/s m2; spiral wound/polysulfone: 0.08–0.14 L/s m2; tubes/polysulfone: 0.06–0.2 L/s m2.

Liquid permeability increases 25 % for every 10 hC increase in temperature. Power depends on target species and configuration: water treatment 1.8 kJ/L

permeate; food application: 32 kJ/L permeate; electropaint: 60 kJ/L permeate. Configuration: hollow fiber 6 kJ/L; plate and frame 9 kJ/L; spiral 3–6 kJ/L; tube 15 kJ/L; or hollow fibers: 100–280 W/m2; plate and frame: 180–280 W/m2; spiral wound 25–120 W/m2.

Configuration: see Section 4.15.

Usually hollow fibers provided there are no particles. Other options include spiral wound, plate and frame, and tubular (use for small flow, high value and severe fouling applications).

For laminar flow operation of hollow fiber, plate and frame and spiral wound, keep the operating pressure 0.1–0.2 MPa; for turbulent flow operation of plate and frame, spiral wound and tubes, operate at 0.5–0.7 MPa.

Use deadend for low concentrations of particles i 0.1 mm.

Use cross flow batch with 100 % retentate recycle, continuous bleed with recycle ratio 15–30/1 and multistage: when a concentrated retentate is desired or when particle diameter I 0.1 mm.

Criterion to backwash/or clean: when 90 % reduction in retentate volume is achieved; given quality or volume of permeate; the permeate flux I 15 % of initial flux; or when the viscosity of the retentate is 100–300 mPa s. This corresponds to concentrations for pigments of 30–70 %; for microorganisms of 1–10 %.

Membrane life, 2–4 years.

134 4 Homogeneous Separation

Cycle time: clean by backwashing with permeate every 6, 24, 170, 360, 1000 h (depending on the criteria, the membrane, operating conditions and on the amount of pretreatment), Example 8 h on and 1–2 h off to clean; or short pulses every 150–300 s so that steady state flux operation is never achieved.

Clean by steam, detergents, solvents, acids or bases. Another option to lengthen cycle time is to add solids to the feed to mechanically wear away the gel layer. The choice affects membrane life. Steam cleaning gives 50–150 cycles before membrane replacement; non-steam cleaning gives 200–500 cycles.

x Good Practice

For membranes that are not hydrophobic; check the isoelectric or zero point of charge point of the species in solution compared with the charge on the membrane and consider changing the pH of operation so that the surface charges are the same.

For hydrophobic membranes treating aqueous feeds, consider pretreating the membrane to make the membrane surfaces hydrophilic.

x Trouble Shooting

“Permeate flux I design”: physical clogging ( inadequate pre-screening/backwash problems/aeration/recirculation/increase in influent solids loading); chemical fouling (change in water quality/inadequate cleaning).

“Permeate quality I design”: failure in mechanical seal, breakage of the membrane or hollow fibres/post contamination via regrowth/degradation of membrane by pH or chlorine.

4.23

Membranes: Microfiltration

Related topic filters, Section 5.14.

x Area of Application

Particulate diameter 0.05 to 800 mm and usually 0.1–10 mm; feed solids concentration I 75 %w/w; I 50 % v/v. Remove solid or gelatinous particulates by pore size in the membrane. Pore size: 0.2–1 mm with the membrane cut-off sizes in the range 0.05–10 mm.

x Guidelines

Driving force for the rate of separation: pressure.

Membrane: symmetric or asymmetric microporous. ceramic, sintered metals or polymers with pores 0.2–1 mm. Symmetric polymers have a porosity of 60 to 85 %; asymmetric ceramic membranes, porosity 30 to 40 %, are used for high pressure and higher temperature I 200 hC. Pressure 0.03–0.35 MPa. Pressure: 0.3–0.5 MPa for ceramic. Hydraulic permeability, A: 70 to 10 000 g/s m2 MPa, capacity/unit: 0.001–1 L/s. Liquid permeate flux: 0.001–0.2 L/s m2 with the perme-

4.24 Chromatography 135

ate flux through ceramic membranes 2–3 times higher than through symmetric polymeric or sintered metal membranes and 5–10 times higher than through asymmetric polymeric membranes because ceramic operates at higher pressure. Configuration: see Section 4.15.

Use tubular for feed concentrations of 10–80 % w/w; spiral wound or thin channels for low concentrations with particulates I 100 mm. For more see UF, Section 4.22.

x Good Practice

See UF, Section 4.22.

x Trouble Shooting

See Section 4.22.

4.24 Chromatography

The chromatographic separation approach is often used to describe “a method of running” an IX column or an adsorption column. These are not considered here; see Sections 4.13 and 4.11 and 4.12 respectively. Two uses of the chromatographic approach are considered here: affinity or immunosorbent (that is similar to adsorption/IX but is usually applied to bioseparations) and size exclusion or gel chromatography, SEC.

In affinity chromatography the feed is always liquid, the feed is a stream containing the target species, the target species complexes with ligands that are immobilized on the packing and so is “attached” to the column packing and is removed from the column by flowing an appropriate eluant through the column, in this way the operation is cyclical with bed loading, washing, eluate flow, wash and then the cycle repeats. Example, target species is an enzyme then the complexing agent might be an inhibitor; for an antibody as a target species an antigen might be the complexing agent.

In SEC separation, the feed may be gas or liquid, the feed is a carrier fluid to which pulses of feed containing the target species are injected, the smaller species in the injected feed usually diffuses into the pore spaces in the packing such that the exit from the bed of the low molar mass species is delayed. The larger species with the larger molar mass do not diffuse into the pores and so are carried out of the bed first by the carrier fluid. By astutely selecting the packing and the carrier a separation is obtained. If the molar mass of the target species differs from the other species in solution by a factor of i 1000, then the separation is relatively easy and is referred to as bulk SEC.

x Area of Application

The target species can be complexed with immobilized ligands then use affinity chromatography.

136 4 Homogeneous Separation

Separate species in the liquid phase whose molar masses differ by factor of 1000 via bulk CS (example, desalt a solution of proteins. we can separate species in the liquid phase whose molar masses differ by at least 20 % via SEC. Use SEC for gas phase separation if the relative volatility (based on vapor pressure of key components) avp is I 1.1. See Distillation Section 4.2.

Usually chromatography is used as the last separation because of the large volume of inert carrier fluid needed and the large size column.

x Guidelines

For Bulk CS, flowrate 500 mL/s m2. The high molar mass target species is diluted by the carrier fluid by a factor of 1.5 to 10. Select the inert stationary solid particle packing, such as a crosslinked dextran gel, crosslinked polyacrylamide gel, aga- rose-based gels. The carrier fluid is usually a dilute buffer (0.02 M) to eliminate any potential ion exchange effects that might occur between the target species and the inert packing.

For SEC, the space velocity is 0.1 to 0.2 BV/h; flowrate 2–50 mL/s m2 or slightly turbulent flow to prevent excessive zone spreading. The injection pulse may be 10–30 s. Feed volume = 1 to 2 % of total column volume.

Feed concentration I 8 to 10 %. Multistaging is easy with up to 500 theoretical stages. Scale up based on total liquid flowrate. HETS is a function of the packing diameter, and the local fluid maldistribution. HETS = 0.1 - 0.3 cm. Residence times in a column are in the order of 3 min.

x Good Practice

Ensure the packing is uniform. Include a tapered porous frit at the inlet to give uniform feed flow.

x Trouble Shooting

“Poor separation”: [poor resolution]*/maldistribution of packing.

[Poor resolution]*: carrier concentration incorrect/wrong carrier/temperature too high/flow rate too fast.

5

Heterogeneous Separations

In heterogeneous phase separation we start with at least two phases. The chapter starts with general guidelines, Section 5.0, Sections 5.1 to 5.4 address the separation of gas from liquid, gas from solid, liquid from liquid and gas liquid liquid. Section 5.5 gives an overview of options to separate liquids from solids and the details are given in Sections 5.6 to 5.17. Section 5.18 gives an overview of options to separate solids from solids and the details are given in Sections 5.19 to 5.30.

General Guidelines

Five general guidelines are listed below and Table 5.1 gives an overall guide to specific options, classified by the type of phase to be separated.

1.Consider shifting from heterogeneous phase separation to homogeneous phase separation.

2.If possible, separate the gas phase first, then the liquid, and then solid–solid ($).

For solid-solid separation ( Section 5.18):

3.Consider using dense media separation to preconcentrate before grinding to final liberation size ($).

4.Use feed assay and liberation size as criteria to guide selection of options ($).

5.Try froth flotation as a first option. Suggested heuristics are given by Woods (1994) p 5–71.

5.1

Gas–Liquid

x Area of Application

Liquid dispersions: rain i 100 mm; fog is between 1–100 mm.

Knockout pot: drop diameter i 100 mm; feed concentration i 1 % liquid v/v. Zig-zag baffled chamber: i 100 mm; i 0.01 % v/v but keep superficial gas velocity I 1 m/s to prevent re-entrainment.

Rules of Thumb in Engineering Practice. Donald R. Woods

Copyright c 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

ISBN: 978-3-527-31220-7

1385 Heterogeneous Separations

Table 5.1 Selection guide (section number in parentheses).

Minor feed	Major feed component
component
	Solid	Immiscible liquid	Gas/vapor
solid	general (5.18)	settler (5.8)	general (5.2)
	zone refine (4.5)	thickener (5.10)
	screen (5.30)	screen (5.7)
	classifier (5.22–5.25)	hydrocyclone (5.9)
	separator (5.26–5.29)	DAF (5.16)
	flotation (5.19)	filter (5.14)
	electrostatic (5.20)	centrifuges (5.12, 5.13)
	magnetic (5.21)	ultrafiltration (4.22)
		microfiltration (4.23)
liquid	general (5.5)	general options (5.3)	knockout pots
	dryer (5.6)		zig-zag baffled chambers;
	screen (5.7)		wet cyclones; spray
	leach (5.15)		chambers, venturis, cross
	expeller (5.17)		flow; wetted packing (5.1)
gas/		stripping (4.9)	homogeneous:
vapor			membranes (4.15)

Wet cyclone: 10 to 400 mm; 4 to 60 % liquid v/v.

Spray chamber: 10 to 100 mm; 0.1 to 8 % v/v; with collection efficiency decreasing from 90 % to 50 % as mist diameter decreases.

Venturi: I 100 mm; I 0.1 % liquid v/v; with collection efficiency decreasing from 95 % as mist diameter decreases.

Mesh demister: 10 mm; 0.001 to 0.01 % liquid v/v; see size enlargement, Section 9.1. Cross flow packed column: I 10 mm; 0.001 to 0.1 % liquid v/v;

Afterburner: I 0.8 mm; I 0.1 % liquid v/v; Steam traps: separate condensate from steam.

Float: for continuous flow required, limited to low pressures.

Float and thermostatic: to eliminate large volumes of air, low pressures and some air elimination.

Open bucket: for pulsing or widely varying pressures; low pressures and some air elimination. Rarely chosen.

Inverted bucket: for dirty lines or dirty steam, eliminate large volumes of air, lowest initial cost/kg flowrate condensate; caution about frost, if i 2 hC subcooling occurs, then there is poorer upstream heat transfer.

Balanced pressure or thermostatic: very large capacity at relatively low cost, not for superheat that could damage bellows, provides good upstream heat transfer.

Thermal expansion: when subcooling is required.

Thermodynamic/kinetic energy/impulse: OK for freezing conditions; low cost, poor air handling, keep initial steam pressure I 1.2 MPa-g.

5.1 Gas–Liquid 139

x Guidelines

If only a cost estimate is needed, then start with a cost correlation based on gas flowrate. Do not spend additional time sizing.

Knockout pot (drums and accumulators for high ratios of liquid/gas, as in distillation column overheads) use horizontal cylinder: size vapor space to provide the residence time for drops to settle out. Vapor volume between 20 to 50 % with a minimum of 0.3 m. Design vapor phase cross-sectional area to allow drops to settle in assigned length of the drum. Assume drops 0.1 to 200 mm. The maximum superficial density-weighted gas velocity, for vertical vessels is vo max = k((rL – rG )/rG )0.5 where “k” is usually 0.13. khoriz. = 1.25 kvertical.

Use superficial design value of 0.5 to 0.85 % of vmax. (This separation superficial gas velocity is used to design/size many types of equipment: distillation columns, demisters. Table 4.1 compares values for “k” for different applications.)

Design liquid volume usually 80 % with sufficient volume for 300 s residence time to satisfy process control requirements. Typical length to diameter ratio of 3:1 to 5:1. Liquid vortex breaker.

Knockout pot (for low ratios of liquid/gas, as in demisters) use vertical cylinder. Size the same as horizontal with height of the vapor space 1.5 q diameter with 15 cm minimum above the top of the inlet nozzle and use lower value for k. For the liquid phase: maximum liquid level at least 18 cm below bottom of inlet nozzle; liquid residence time about 300 to 600 s.

The following units are sized using approaches given in Section 5.2 for gas– solid separations: wet cyclone, spray chamber, venturi, cross flow packed column. Steam traps: ball float, open bucket, inverted bucket, liquid expansion and thermodynamic. Size on condensate flowrate. The cooler the condensate, the larger the flowrate.

Float: continuous discharge, operating principle of buoyancy, OK for low loads but not high pressure. Not for water hammer. Range 0.06–5 kg/s condensate. Usually size based on 2 q usual flowrate; if handling air or wide variation in flowrates, size on 8 q usual flowrate.

Inverted bucket: intermittent discharge, operating principle = weight of the bucket, robust, OK for high pressure and corrosive condensate, use check valve before trap. Can handle some water hammer. Insulate for winter use. Range 0.05–2.3 kg/s condensate. Usually size based on 2 q usual flowrate; if handling air or wide variation in flowrates, size on 8 q usual flowrate.

Balanced pressure, thermostatic: operating principle = vapor pressure of fluid inside bellows. Not for superheated steam, corrosive condensate or waterhammer. No adjustment needed for fluctuating steam pressure. Range 0.03–0.1 kg/s condensate. Size on 2 q usual flowrate of condensate.

Thermodynamic/kinetic energy: intermittent, operating principle is Bernoulli’s principle/impulse, poor air handling, larger sizes more susceptible to back pressure. Usually for steam pressure I 1.2 MPa but i 0.06 MPa. Affected by ambient temperature. Discharge pressure I 0.5 steam pressure. Range 0.06–0.3 kg/s. Size on 3–4 q usual flowrate of condensate.

140 5 Heterogeneous Separations

Select inverted bucket traps based on condensate flowrate, pressure differential and “safety factor allowance” for variations from “usual” condensate flowrate.

xGood Practice

Install a demister.

xTrouble Shooting

Knock out pots: “Poor separation”: [ foaming]*/insufficient residence time/feed and exit nozzles at wrong location/faulty design.

[Foaming]*: liquid downflow velocity through the foam is too low, Turner (1999). A general set of causes is given in Section 1.12.

Steam traps: install trap below condensate exit (or with a water seal if the trap is elevated), use a strainer before all traps, use a check valve for bucket traps. Slant pipes to the trap. Use a downstream check valve for each trap discharging to a common header. Pipe diameter j trap inlet pipe diameter. Prefer to install auxiliary trap in parallel instead of a bypass. Do not group thermodynamic traps because of their sensitivity to downstream conditions.

Float and thermostatic: usually discharges continuously, low pitched bubbling noise. High pitch noise suggests live steam is blowing.

Balanced thermostatic: leave about 0.6 m of uninsulated pipe upstream of trap. Diagnostics: when bellows placed in boiling water the expansion should be 3 mm.

Inverted bucket: use initial prime to prevent steam blowing. Prime a bucket trap when first put on-line. Diagnostics sounds: when it is functioning well: loud initially, then lower pitch bubbling and then silence. Discontinuous discharge. When steam is blowing through the trap, the sound is a steady bubbling if primed with a light load or constant rattling; or continuous high pitched whistling. Diagnostic for loss of prime: close outlet valve for several minutes, then open valve slowly and operation should return to normal. If this fails then check seat and valve.

Thermodynamic: about 6 cycles/minute.

x Trouble Shooting

The major faults are wrong trap, dirt, steam locking in the trap, group trapping, air binding and water hammer. Too large a trap gives sluggish response and wastes steam. Too small a trap gives poor drainage, backup of condensate. There is a DT across all traps. “No condensate discharge”: strainer or line plugged/steam off/valves plugged/no water or steam to the trap/trap clogged/ wrong trap selected/worn orifice/steam pressure too high (inverted bucket)/orifice enlarged by erosion (bucket trap)/incorrect Dp across the orifice (inverted bucket)/air vent clogged (inverted bucket or thermostatic air vent on float trap)/ valve seat choked (inverted bucket)/flabby or elongated bellows (thermostatic)/ superheated steam caused burst joints or scale (thermostatic). “Cold trap + no condensate discharge”: strainer or line plugged/steam off/valves plugged/no water or steam to the trap/trap clogged. “Hot trap + no condensate discharge”: bypass open or leaking/trap installed at high elevation/broken syphon/vacuum in heater