Chapter 4 Membrane Equipment

Transcription

1 Chapter 4 Membrane Equipment

2 The Components of Membrane Technology MEMBRANES MODULES Membrane Housing SYSTEMS Modules, Pumps, Piping, Tanks, Controls, Monitoring, Pretreatment t t Facilities, Cleaning Facilities DESIGN & OPERATION Series/Parallel Tapered Cascade Continuous/Batch Crossflow/Dead End

3 4-1. Introduction - Module : the smallest unit into which the membrane area is packed. A number of modules are arranged together as a system. (single pass, recirculation, cascade system, etc.) - Module configuration flat plate-and-frame module Spiral-wound module tubular tubular capillary hollow fiber - The choice of module configuration as well as the module arrangements is based on : i) Type of separation problem ii) Ease of cleaning, maintenance, operation iii) Compactness of the system iv) Possibility of membrane replacement

4 Membrane Module Type I (a) Hollow fiber membrane (b) Flat sheet membrane (c) Tubular membrane (d) Spiral wound membrane

5 Membrane Module Type II Tubular type Plate & Frame type Spiral Wound type Hollow fiber type

6 4-2 Plate-and-frame module

7 4-2 Plate-and-frame module

8 4-2 Plate-and-frame module - Packing density ( membrane surface area per module volume ) : about m 2 /m 3 - Stop disc is introduced to reduce channeling and to establish an uniform flow distribution ( Fig. VII-3 ). - Channel height : mm ( DDS module ) - The permeate from each pair of membranes can be visually observed in the plastic tubing coming from each support plate. Easy detection of leaks in a damaged membrane pair Sample analysis Flux measurement as a function of stack length - A gasket is used to transform the flat plate into a long tortuous narrow channel ( Fig. VIII-4 ). -Replacement of membranes on site is relatively easy.

9 4-2 Plate-and-frame module

10 4-2 Plate-and-frame module - The energy consumption is between tubular and spiral-wound module.

11 5-3 Spiral-wound module - It is in fact a plate-and-frame system wrapped around a central collection pipe, pp in a similar fashion to a sandwich roll.

12 4-3 Spiral-wound module

13 4-3 Spiral-wound module

14 4-3 Spiral-wound module - Packing density : 300-1,000 m 2 /m 3 > plate-and-frame module P ti l h i th h t k lti i l i bl - Particles hang-up in the mesh network, resulting in cleaning problem. This makes it difficult to process feeds containing suspended particles. Spiral-wound modules work best on relatively clean feed streams with a minimum of suspended matter.

15 4-3 Spiral-wound module - one-tenth rule must be modified for the spiral-wound module due to the prefthe spacer which hreduces the free volume in the channel sence of One-tenth rule: the largest particles that can be processed in a membrane module should be less than one-tenth th the channel height ht, taking into account the flocculation. i) 30 mil (=760 μm) ) spacer prefiltration ti down to 5-25 μm ii) 45 mil (=1,140 μm) spacer prefiltration down to μm -Larger channel height : Decrease packing density Minimize pressure drops Reduce feed channel plugging

16 4-3 Spiral-wound module - A number of spiral-wound modules are assembled in one pressure vessel and are connected in series via the central permeate tube.

17 - The relatively high pressure drop at high flow rate can give rise to a tele-scoping effect the spiral pushes itself out in the direction of flow. This can damage the membrane Anti-telescoping device

18 4-3 Spiral-wound module The state of turbulence in the spiral wound module is not too clear. The superficial velocity (v) volumetric flow rate = cross sec tional area ( a b ) a : width of the flat sheet-glued portion b : channel height v = cm/sec Re = 100-1,300 (Laminar Region) But additional turbulence contributed by the spacers, which can be substantial, should also be taken into account.

19 4-3 Spiral-wound module - The presence of a feed spacer has a large influence on the mass transfer and the pressure drop.

20 4-3 Spiral-wound module - The state of turbulence : ΔP = f(q) n n Q : flow rate (L/min) ΔP : pressure drop (kpa) If n = 1, laminar flow If n = , turbulent flow In Fig , pressure drop at v = 25 cm/sec (12 L/min) is psig - E Q = ΔP xq The combination of low flow rates, high pressure drops, and relatively high turbulence makes the spiral module one of the more economical modules in terms of power consumption

21 4-4 Tubular module 4. Tubular Module The difference between tubular, capillary and hollow fiber module arises mainly from the dimensions of the tubes employed.

22 4-4 Tubular module In contrast to capillary and hollow fiber, tubular membranes are not self-supporting. supporting. The membranes are placed inside a ceramic, a porous stainless steel and plastic tube with the diameter of the tube being more than 10mm.

23 4-4 Tubular module - The feed solution always flows through the center of the tubes while the permeate flows through the porous supporting tube into the module housing. - Large channel diameter handling fairly large particles. Rule of thumb ; the largest particles that can be processed in a membrane module should be less than one-tenth the channel height,taking into account the flocculation. - Re > 10,000 operate under turbulent conditions Recommended Velocities for UF : 2-6 m/sec - The straight-forward open tube and high Re number make it easy to clean by stand and C.I.P. (Clean In Place) technique. - Inserting scouring balls or roads to help clean membrane

24 4-4 Tubular module - Lowest surface area to volume ratio 2 πrl = = π r l ( 2 2 ) r high floor space requirements to install the equipment. packing density < 300 m2/m3 - High hold-up volume It limits the degree of concentration, especially when processing relatively small volumes - Highest in energy consumption Pressure drops (ΔP) for cm tubes will be approximately atm This combination of pressure drop (ΔP) and high flow rates (Q) makes it the highest in energy consumption. EQ = ΔP x Q

25 4-4 Tubular module - Monolithic membrane : i) special type of ceramic membrane. ii) A number of tubes have been introduced in a porous ceramic block. iii) The inner surfaces of these tubes are then covered by a skin layer, for instance by a sol-gel process.

26 Monolithic membrane

27 Monolithic membrane

28 4-5 Capillary Module - A large number of capillaries are assembled together in a module. The free ends of the fibers are potted with agents such as epoxy resins, polyurethanes or silicone rubber. - When porous ultra- or microfiltration membranes are employed, the capillaries mostly have a gradient in pore size across the membrane. - Packing density : 600-1,200 m 2 /m 3

29 4-5 Capillary Module -Two types inside-out better protection of skin layer (adv.) need clean feed (plugging) (disadv.) high pressure loss (disadv.) outside-in higher membrane area (adv.) one-tenth rule (adv.) Channeling (disadv.)

30 4-5 Capillary Module - Module arrangement : i) inside-out (skin : inside) ii) outside-in (skin : outside) The choice between i) and ii) is mainly based on the application where parameters such as 1) pressure, 2) pressure drop, 3) membrane type available,etc. are important.

31 4-6 Hollow fiber Module

32 4-6 Hollow fiber Module - Fibers : self-supporting structures Thickness : ~ 200 μm Outer diameters = 0.5 ~ 0.7 mm Inner (lumen) diameter = 0.3 ~ 0.5 mm Bundles : 50-3, individual fibers Sealing : epoxy resin Cartridge shell : clearsee-through polysulfone or translucent PVC - The Highest Packing Density : up to 30,000 m2/m3 - Hold up volume is low. - Pressure drop (ΔP) = 5-20 psig usually the inlet pressure (Pi) cannot exceed 25 psig.

33 4-6 Hollow fiber Module - Recommended operating velocity (inside-out) : m/sec (Re = 500-3,000) operate always in the laminar flow region. - Shear rates are very high ( γ w= 4,000-14,000 sec-1). - Two types like capillary module: Feed from outside-in (shell-side) or inside-out (lumen side) - Membrane replacement costs are high : Even if one single fiber out of 50-3,000 in bundle bursts, the entire cartridge has to be replaced. However, partial blocking of damaged fibers are possible. The small tube diameter (<500 μm) make the fibers somewhat susceptible - The small tube diameter (<500 μm) make the fibers somewhat susceptible to plugging at the cartridge inlet. need prefiltration

34 4-6 Hollow fiber Module *Hollow fiber : very susceptible to fouling and difficult to clean. : the cost of sophisticated pretreatment procedures should be taken into account (capital + operating costs). * Tubular : well suited for applications with a high fouling tendency because of its good process control and ease of membrane cleaning (for example, scouring ball) -AA big advantage : i) back-flushing capability ii) Lumen-flushing

35 Alternative direction of feed flow & Backflushing

36 Lumen flushing Recycle Permeate Lumen flushing Permeate in the shell side goes back to the lumen where permeate flow is shut off for brief periods during operation or recycle Partially cleaning the fibers during process

37 Principle of lumin flushing

38 4-7. Comparison of module configurations - The choice of module is based on : i) Economic consideration ii) Type of application

39 Membrane Modules Types: Flat Sheet Spiral Wound Tubular Capillary Hollow Fibre Contained (Pressure Vessel) Submerged (or Immersed)

40 Contained Module Membranes ( tubes, fibres, flat sheets) mounted in a pressure vessel. Transmembrane pressure provided by pumped p feed. Design determines fluid management.

41 Submerged Module Hollow fibres (vertical or horizontal) of flat sheets (vertical) immersed in atmospheric tank Permeate removed by suction pump or gravity Cake controlled by bubbling and/or backwash Applications: Water treatment and MBRs FEED PERMEATE ADVANTAGES: Avoids pressure vessel Reduces cost Ease of membrane replacement Simple scale up Lower energy consumption DISADVANTAGES: Di Driving i force < 1 atm Poor fluid management (AIR) Hollow fibres or flat sheet

42 Submerged Hollow Fibres 1989 [Yamamoto] 2000 [Zenon] First introduced in the late 1980s submerged hollow fibre MBRs are now a major technology

43 Example: Large Scale Submerged Hollow Fibre Water Treatment Plant The figure shows a 120 ML/d design with 6 20 ML/d chambers (similar concept to conventional sand filters). Filtered solids are removed from membrane by liquid backwash and external air-scour. Chemical cleaning is occasionally required. CMF-S plant, S10T p, (nominal 120 ML/day)

44 Commercial Hollow Fibre Modules MF / UF Contained Aquasource (inside-out) CleanSeas Koch Memcor Norit Asai-kasei(Outside-in) Submerged - Hyflux - Memcor - Mitsubishi - Zenon FEED PERMEATE (AIR) Hollow fibres