9 synthesis of reaction separation system lec 9 heterogenous separation

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9 synthesis of reaction separation system lec 9 heterogenous separation

  1. 1. Lecture 9 : SYNTHESIS OF SEPARATION SYSTEM – HETEROGENOUS MIXTURE SEPARATION<br />Some Review<br />WHY SEPARATORS ARE NEEDED ?<br />- SEPARATION OF DESIRED PRODUCTS FROM BYPRODUCTS AND UNREACTED FEED UP TO THE REQUIRED SPECIFICATION.<br />- PURIFICATION OF FEEDS.<br />IS IT AVAILABLE AT ANY PLANT ?<br />.........<br />TYPES OF SEPARATOR<br />ADSORBER<br />ABSORBER<br />Extraction<br />Liquid-Liquid<br />Solid-Liquid<br />DISTILLATION<br />MEMBRANE<br />ETC...<br />STRIPPER<br />
  2. 2. WHERE ARE SEPARATOR LOCATED ?<br />Normaly, form the major part of any chemical plant.<br />REACTOR<br />reactor<br />SEPARATION <br />& RECYCLE <br />SYSTEM<br />HEAT EXCHANGER<br />NETWORK<br />UTILITIES<br />Prior and after reactor !<br />Short Exercise:<br />Separators are used for different purposes.<br />Discuss some of them.<br />
  3. 3. Separator System Synthesis – Heuristic Approach<br />During the conceptual design of a chemical process, among the related decisions that have to be made concerning separator system ;<br />Separator type<br />Sequencing of Separator<br />Operating Conditions<br />ABSORBER<br />DISTILLATION<br />TEMPERATURE<br />PRESSURE<br />CONCENTRATION<br />ADDITION OF EXTRA COMPS.<br />DISTILLATION<br />DISTILLATION<br />MEMBRANE<br />MEMBRANE<br />What type of<br />suitable separator ?<br />How are the <br />separators arranged ?<br />
  4. 4. HEURISTIC<br />SUGGESTED PROCEDURE….<br />1. Decide on type of separator that will be likely used for the required separation.<br />Off course, we have to start with gathering the related information !<br />What type of mixture ?<br />Heterogenous<br />Homogenous<br />Exploiting differences in certain<br />physical properties (eg. density,<br />molecular size, surface tension<br /> etc.) in order to perform separation.<br />Require addition or creation<br />of another phase to perform<br />separation.<br />eg. Distillation (liquid mixture)<br /> Extraction (solid, liquid mixture)<br /> Condensation (gas mixture)<br /> Absorption (gas mixture)<br /> Adsorption (gas mixture)<br />eg. Membrane Separation<br /> Filtration<br /> Gravity settler<br /> Centrifugal Separator<br /> Phase Separation<br /> Flotation<br />
  5. 5. SUGGESTED PROCEDURE (cont...)<br />2. Decide on the sequencing of the separator to achieve the process requirement.<br />1st Golden Rule : Perform the heterogenous mixture separation first or as soon as<br /> the mixture is formed during any stage of the process.<br />The rational : It is easiest to conduct !<br />The phase separation involves :<br />Vapour/Liquid<br />eg. flash drum<br />Liquid/Liquid (immiscible)<br />eg. phase separator<br />Solid/Liquid<br />eg. filtration, membrane separation, gravity settler<br />Solid/Vapour(gas)<br />Eg. Venturi Scrubber<br />Solid/Solid<br />eg. shaking, vibrating screen<br />
  6. 6. Gravity Settler Separators<br />a. Vapour Liquid Separation - Flash Drum<br />Liquid drops separated from vapour/gas flow based on the settling velocity (terminal velocity) of the liquid drop. It is a function of the droplets size. A general eqn. that can be used is;<br />vT = [ (4g d / 3 CD) (rP – rF / rF ) ] 0.5<br />VT – terminal velocity<br />d – particle diameter<br />CD – drag coefficient<br />For Re < 2 , CD = 24 / Re<br />For Re = 2 to 500, CD = 18.5 / Re 0.6<br />For Re = 500 to 200,000, CD = 0.44 <br />The maximum velocity of the vapour set within the vessel must be less than the settling velocity. <br />In the event where droplets particle is very small, the use of baffles of mesh pad could promote coalescence between the particles to form larger size thus easier to separate.<br />The method is not practical to separate particle diameter less than 10 micrometer. <br />b. Liquid Liquid Separation - Decanter<br />Separation of dispersed liquid phase from a continuous phase. The continuous phase velocity must be low enough to allow low density droplets to rise from bottom of the vessel to the interface and coalesce, and the high density droplets to settle down to the interface and coalesce. The velocity of the continuous phase must be lower than the terminal velocity for the particles calculated using equation above.<br />
  7. 7. c. Solid Liquid Separation - sedimentation<br />Solid particles settle to the lower part of the vessel as it travels with the liquid flow across the vessel. When the prime function of the equipment is to produce more concentrated slurry, it is called thickener while if the function is to remove the solid particles from the liquid, it is called clarifier.<br />vT = [ (4g d / 3 CD) (rP – rF / rF ) ] 0.5<br />VT – terminal velocity<br />d – particle diameter<br />CD – drag coefficient<br />For Re < 2 , CD = 24 / Re<br />For Re = 2 to 500, CD = 18.5 / Re 0.6<br />For Re = 500 to 200,000, CD = 0.44 <br />Similar equation could be used especially for the lower Re. The efficiency of particle collection could be reflected by the equation; <br />h = settling distance of the particles<br />H – height of the settling zone.<br />h = h / H<br />
  8. 8. Inertial and Centrifugal Separators <br />In the event when gravity settler is deemed to be too slow due to factor such as closeness in the density between the droplets or particles and the continuous phase, inertial or momentum separators could improve the efficiency by giving the momentum force in addition to the gravity force. The additional force provided is normally in the form of centrifugal force.<br />The design of the inertial separators normally follows a collection efficiency curve supplied by equipment manufacturers from sets of experiments conducted.<br />Cyclone (solid gas) and hydrocyclone (solid liquid or liquid liquid) are examples of equipment exploiting the principle.<br />Collection Efficiency<br />Particle Size (mm)<br />
  9. 9. Electrostatic Precipitation <br />Electrostatic precipitators are commonly used to separate particulate matter that is easily ionized from a gas stream. Electrostatic field is produced between wires or grids and collection plates by applying high voltage between the two. A corona is established around the negatively charged electrode. The corona ionizes the molecules of gases such as O2 or CO2 which in turn attach themselves to the particles and charging them at the same time. The opposite charge electrode (collection plates) will attract these particles and later dislodged them by mechanical forces.<br />The method is most effective when separating particles with high resistivity. The operating voltage typically range from 25 and 45 kV or more depending on the design and operating temperature.<br />The application is typically restricted to the separation of fine particles of solid or liquid from a large volume of gas. Again the design could be based on the collection efficiency curve as provided by manufacturer.<br />
  10. 10. Filtration <br />Suspended solid particles in a gas, vapour or liquid are removed by passing the mixture through a porous medium that retains the particles and passes the fluid (filtrate). The solid can be retained on the surface of the filter medium (cake filtration) or captured within the filter medium (depth filtration).<br />The filter media for cake filtration can be made of cloth, ceramic or even metal. There are various arrangement for the filter media such as plate & frame, bag, conveyor belt, rotating filter etc. Conventional filter media can be used up to temperature of 250 C. Higher temperatures require ceramic or metallic. <br />For the depth filtration, granular medium consisting of layers of particulate solid (eg stones, pebbles etc.) placed on a support grid is used as the filter medium. Downward flow of the mixtures causes the solid to be captured within the medium. Such arrangement is normally used for removing small amount of solids from large quantities of liquids. This filtration mechanism can removed particles with sizes down to 10 microns. <br />For smaller sizes particles down to 0.05 microns, microfiltration using polymer membrane is used. Two most arrangements used consist of spiral wound and hollow fiber.<br />
  11. 11. Scrubber <br />Scrubbing with liquid (usually water) can enhance the collection of particles when separating gas-solid mixtures. Packed bed, spray and venturi scrubber are the examples of the many possible designs.<br />gas<br />gas<br />gas<br />liquid<br />gas<br />Radial liquid Spray<br />Liquid spray<br />Good separation but at the expense<br />of pressure drop.<br />gas<br />gas<br />liquid<br />liquid<br />Again design is based on the collection efficiency curve produced by manufacturer.<br />
  12. 12. Flotation<br />Flotation is a gravity separation process that exploits the differences in the surface properties of particles. Gas bubbles are generated in a liquid and become attached to solid particles or immiscible liquid droplets, causing the particles or droplets to rise to the surface. This is used to separate mixtures of solid-solid particles after dispersion in a liquid or liquid-liquid mixtures of finely divided immiscible droplets. The liquid normally used is water and the particles of solid or immiscible liquid will attach to the gas bubbles if they are hydrophobic.<br />Gas bubbles are generated by means of dispersion, dissolution (changing of pressure from high to low in the vessel) and electrolysis.<br />Widely used in the mineral ore processing.<br />Drying<br />Removal of water moisture from a solids into a gas stream (normally air) by heat. Four commonly used type of dryer are tunnel dryers, rotary dryers, drum dryers and spray dryers. Another important class of dryers is the fluidised bed dryers. <br />Choice between dryers is normally made based on practicalitiessuch as material handling characteristics, product decomposition, product physical form etc.<br />Dryer efficiency is measured by heat of vaporisation / total heat consumed.<br />
  13. 13. Summary : Heterogenous Mixture Separation<br />Heterogenous mixture separation has to be conducted prior to attempting the separation of homogenous mixture<br />Need to know the type of mixture to be separated (gas-liquid, liquid-liquid or solid-liquid) before a suitable selection of the device could be made.<br />In case where natural phase separation aided by gravity fails, other means such as inertial or centrifugal forces, membrane barrier, electrostatic field etc. can be used to perform the separation but at higher capital and operation cost than gravity settlers.<br />

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