Membrane And Mixing

Membrane Technology

Introduction Membrane technology can be a very efficient and economical way of separating components that are suspended or dissolved in a liquid. The membrane is a physical barrier that allows certain compounds to pass through, depending on their physical and/or chemical properties. Membranes commonly consist of a porous support layer with a thin dense layer on top that forms the actual membrane. Application: water desalination and purification and cold sterilization

of beverages; recovery and fractionation of proteins from whey; clarification of fruit juice, wines and beer; removal of bacteria from water; effluent treatment for removal of heavy metals and organic materials; separation of

oil and water emulsions; and removal of volatile organic compounds from air.

Basic principle of membrane operation Transmembrane pressure is the major driving force for the movement of solution across a membrane. Because of its larger size, the solute is retained by the membrane; as the process continues the concentration of solute increases and this mass of solute is deposited on the membrane. Therefore there is a concentration gradient on the feed side of the membrane, with maximum concentration at the membrane surface. This build - up of solute cake or solute layer results in higher membrane resistance and thus decline in permeate flux across the membrane. The flow of liquid is tangential to the membrane surface, and this flow attempts to

reduce the thickness of the cake.

However, the increase in solute concentration may also increase

viscosity, which may affect transmembrane flux. Even if a pseudo - steady state is achieved, operation may not continue for long as there is a possibility of irreversible fouling of the membrane and eventual shutdown of the operation. Further cleaning and back - flushing of the membrane may result in restoration of the original flux. For solutes that are soluble, the concentration adjacent to the surface may reach the level of insolubility and precipitation of solute may result in higher membrane resistance and decline in permeate flux.

[ Note: The driving forces of membrane processes include concentration gradient, transmembrane pressure, chemical potential, osmotic pressure, electric field, magnetic field, and partial pressure.]

Membrane Modules/ Design Four membrane modules are generally used in industrial applications-

1. Tubular module 2. Hollow fiber module

3. Spiral wound module 4. Plate and frame module

Hollow fiber module A hollow fiber module consists of a large number of small – diameter fibers sealed at the ends with epoxy resin, polyurethanes and silicone rubber.

Membranes are asymmetric in nature, the feed side having lower pore size, and are self - supporting. They have very high specific area per unit volume compared with tubular membranes, up to 30000 m 2 · m − 3.

Tubular module Tubular membranes are located inside a tube. Tubular membranes have

a diameter of about 5 – 15 mm. Ceramic membranes are generally assembled in tubular modules. The advantages of tubular module are easy replacement of membrane and cleaning of membrane surface and no need

for any costly pretreatment of feed. Disadvantages of this module are high energy cost per unit of liquid treated, low surface area per unit volume, and necessity of high liquid flow rate to maintain turbulence in the module.

Spiral wound module Spiral wound module is adaptations of plate and frame module, where membrane sets are wrapped around a central line for collecting permeate. It has the advantages of compact structure with high pressure durability. It has the highest specific surface area among all the modules but feed needs to be

free of particles.

Spiral wound module

Plate and frame wound module Plate and frame modules were initially used for commercial purposes but their low specific area is unattractive. Plate and frame modules have been adapted into a conventional filter press. In this module two membranes

are sandwiched with the feed sides facing each other. A spacer is placed between the feed and permeate sides. To achieve the desired membrane area, a number of such sets are placed in parallel. Membrane replacement and

cleaning is easier and there is no need for costly pretreatment of feed.

Membrane materials Membrane materials include organic polymers like polysulfone, polyether sulfone, cellulose acetate, polyamides, and polyacrylonitrile, and inorganic material such as borosilicate glass, pyrolyzed carbon, and zirconia carbon.

Types of membrane process Four types of membrane process have found in industrial application-

1. Ultra filtration 2. Microfiltration 3. Reverse osmosis 4. Pevaporation

on the basis of molecular size of pores and transmembrane pressure.

Separation driven by partial pressure that depends on solubility & diffusivity of species.

Reverse Osmosis Reverse osmosis is essentially a pressure - driven membrane diffusion process for separating dissolved solvents. During the process, feed solution at high pressure is passed over the feed side of the membrane. The operating pressure is kept higher

than the osmotic pressure of the feed solution so that water will flow from the more concentrated solution to the more dilute solution through the membrane. In this process, normally solvent (e.g. water) flows across a semi permeable membrane from the dilute solution to the more concentrated solution until equilibrium is reached. Thus applying high pressure at the high - concentration side will cause this process to reverse. The attractive features of the process are that there is no requirement of phase change and it is a relatively low-energy requiring process.

Application: For desalination of sea water, demineralization of water and purification of beverages.

Ultrafiltration Ultrafiltration (UF) is the process of separating extremely small particles and dissolved molecules from fluids. The primary basis for separation is molecular size, although in all filtration applications, the

permeability of a filter medium can be affected by the chemical, molecular or electrostatic properties of the sample. Ultrafiltration membranes have a pore diameter of 0.01–0.1mm. Ultrafiltration membranes can also filter viruses. Permeability generally depends on pore size but other factors like electrostatic charge on membrane surface and solute also play a role. Ultra filtration also depends on applied pressure, nature of feed material, temperature of operation, feed concentration, and flow rate of feed stream across the membrane.

Application: Used for concentrating protein in whey, cheese making process and fractionation of whey protein.

Microfiltration Micro filtration (MF) is the process of removing particles or biological entities in the 0.025 μm to 10.0μm range from fluids by passage through a micro-porous medium such as a membrane filter. Pore diameter of

microfiltration membrane ranges from 0.05 to 10 μm. Cross – flow microfiltration is used for applications like removal of bacteria and micelle casein. The material properties of the membrane determine its efficiency.

Ceramic membranes are most advantageous in the dairy and food industry because of their strong mechanical resistance and wide pH tolerance.

Application: Dairy application include removal of micro-organism from milk to produce sterilized milk and others are in nutritional beverages, infant formulas.

Pevaporation Liquid mixtures can be separated on nonporous polymer membranes by partial evaporation. The procedure is called pervaporation because the substance crossing the membrane changes phase state.

The substance

encounters the membrane as a liquid and leaves it as a vapor, vacuum pressure on the permeate side causing the vapor to desorbs. The membrane used in pervaporation can be hydrophilic or organophilic depending on its composition and is generally nonporous.

Application: Pevaporation is used in the wine industry to produce more concentrated alcohol.

Mixing

Mixing Mixing may be defined as an operation in which two or more components are interspersed in space with one another. The aim is to achieve a uniform distribution of the components by means of flow. The efficiency of a mixing process depends on effective utilization of the energy used to generate the flow of the components.

Types of mixing system Three basic types of mixing system are utilized for mixing of foods1. Stationary vessel containing a moving stirrer, agitator, paddle i.e. an impeller or impeller assembly mounted on a rotating shaft .

2. Stationary vessel containing moving paddles, vanes, knives, ploughs, screw etc. These mixers have been developed for mixing higher-consistency materials- viscous liquids, dough's, pastes, fats etc. 3. Moving vessel containing moving and /or stationary paddles, vanes, knives, ploughs, screws etc. Mixer in this category are used for very high consistency mixes- dough's, pastes and plastic materials.

Three main types of impeller mixers are used for liquid mixing1. Paddle mixer. 2. Turbine mixer and 3. Propeller mixer.

Paddle mixer: This type of mixer consists of a flat blade attached to a rotating shaft, which is usually located centrally in the mixing vessel. The speed of rotation

is relatively low, in the range 20–150 rpm. The forms of paddle mixer include: (a) the gate agitator which is used for more viscous liquids

(b) the anchor agitator which rotates close to the wall of the vessel and helps to promote heat transfer and prevent fouling in jacketed vessels. (c) Counter-rotating agitators.

Turbine mixer A turbine mixer has four or more blades attached to the same shaft, which is usually located centrally in the mixing vessel. The blades are smaller than paddles and rotate at higher speeds, in the range 30–500 rpm. Simple vertical blades promote rotational and radial flow. Some vertical flow develops when the radial currents are deflected from the vessel walls. Swirling and vortexing are minimized with the use of baffles.

Propeller mixer This type of mixer consists of a relatively small impeller, similar in design to a marine propeller, which rotates at high speed, up to several thousand rpm. It develops strong longitudinal and rotational flow patterns.

Applications for impeller mixers: For preparing brines and syrups, preparing liquid sugar mixtures for sweet manufacture, making up fruit squashes,

blending oils in the manufacture of margarines and spreads, premixing emulsion ingredients.

Vortex and How it can be reduced If an impeller agitator is mounted on a vertical shaft located centrally in a mixing vessel, the liquid will flow in a circular path around the shaft. If laminar conditions prevail, then layers of liquid may form the contents of the vessel rotate and mixing will be inefficient. Under these conditions a vortex may form at the surface of the liquid. As the speed of rotation of the impeller increases this vortex deepens. When the vortex gets close to the impeller, the power imparted to the liquid drops and air is sucked into the liquid. This will greatly impair the mixing capability of the mixer. Rotational flow may cause any suspended particles in the liquid to separate out under the influence of centrifugal force. Rotational flow, and hence vortexing, may be reduced by locating the mixer off centre in the mixing vessel and/or by the use of baffles which located at the inner wall of the mixing vessel

Mixing of High Viscosity Liquids, Pastes and Plastic Solids During mixing of high viscosity liquids and pastes, the performance of mixer depends on direct contact between the mixing elements and the materials of the mix. This is because when the equipment mixing highly

viscous and paste like materials, it is not possible to create currents which will travel to all parts of the mixing vessel, as happens when mixing low viscosity liquids.

Thus,

the material must be brought to the mixing

elements or the elements must travel to all parts of the mixing vessel. For mixing high viscosity liquids and pastes usually paddle mixer, pan mixer and kneaders are used.

Pan mixer These are of two general types. In the stationary pan mixer the mixing elements move in a planetary path, visiting all parts of the stationary mixing pan. In the rotating pan type, the mixing vessel is mounted on a rotating turntable. The mixing elements also rotate, but in one position, and are located near the pan wall.

Kneader A common design of kneader consists of a horizontal trough with a saddle shaped bottom. Two heavy blades mounted on parallel, horizontal shafts rotate towards each other at the top of their cycle. The blades draw the mass of material down over the point of the saddle and then shear it between the blades and the wall and bottom of the trough. Mixing times are generally in the range 2–20 min.

Applications for pan mixers and kneaders: Application for pan and kneader include- dough and batter mixing in bread, cake and biscuit making, blending of butters, margarines

and cooking fats, preparation of processed cheeses and cheese spreads, manufacture of meat and fish pastes.

Mixing Dry, Particulate Solids

For mixing Dry, Particulate Solids usually tumbler mixer, horizontal screw and ribbon mixers, vertical screw mixers and fluidized bed mixers are used.

Tumbler mixer: These operate by tumbling the mass of solids inside a revolving vessel usually consist of hollow vessels, which rotate about horizontal axes. These vessels take various forms and some typical examples are-

Vertical screw mixer These consist of tall, cylindrical or cone-shaped vessels containing a single rotating screw, which elevates and circulates the particles. The screw may be located vertically at the centre of the vessel. Alternatively, it may be set at an angle to the vertical and made to rotate, passing close to the wall of the vessel.

Applications for screw, ribbon and tumbler mixer: Applications for screw, ribbon and tumbler mixer includepreparing cake and soup mixes, blending of grains prior to milling,

blending of flours and incorporation of additives into them.