Flash distillation

Demonstration of flash distillation, using a mixture of Methanol and Water

1. The Demonstration Of Continuous Flash Distillation

2. ABSTRACT Distillation is a method of separating mixtures based on differences in volatilities of components in a boiling liquid mixture . Distillation is basically carried out in three ways: 1 ) Differential or simple distillation . 2 ).Rectification or fractionation . 3)Flash or equilibrium distillation. The Flash Distillation model is normally carried out either continuously or in batches. In this method, a liquid mixture is partially vaporized, the vapor and liquid are allowed to attain equilibrium and finally withdrawn separately . Material balance in a Flash distillation column is - [accumulation] = [in] - [out] + [generation] - [consumption ] The flash distillation is used on a large scale in petroleum industry in which petroleum sections are heated in pipe stills and the heated fluid flashed into overheated vapor and residual liquid streams

3. INTRODUCTION D istillation is a unit operation in which the constituents of a liquid mixture (solution) are separated using thermal energy. Basically, the difference in vapour pressures of different constituents at the same temperature is responsible for such a separation. The basic requirement for a separation of components by distillation is that the composition of the liquid with which it is in equilibrium- the vapour is always richer in the more volatile component than the liquid from which it is formed. If the vapour composition is the same as the liquid composition, distillation technique will not effect a separation. Equilibrium : Equilibrium refers to the absence of any tendency for a change to take place/ occur. It is a static condition in which the net transfer of material between the phases ceases for a given set of operating conditions.

4. Driving Force: When two phases, not at equilibrium, are brought into intimate contact, the phases will tend to approach equilibrium due to a tendency for a change to take place. The difference between the existing condition and the equilibrium condition is the driving force which causes a change . Equilibrium Stage: One class of mass transfer devices consists of a seires of stages through which phases flow, from one to the other, in a countercurrent fashion. A stage consists of two steps: Intimate contacting of two insoluble phases Mechanical separation of the phases.

5. THE METHODS OF DISTILLATION (BINARY SYSTEMS) The liquid mixture to be separated is heated to create a vapour. The vapours formed are condensed in a condenser and withdrawan as product. As there is no reflux, products of relatively low purities are obtained . A part of the condensed liquid is returned to the distillation still (as reflux) and the remaining part is withdrawn as product. In this method, the liquid and vapour are brought into intimate contact for a number of times and almost pure product can be achieved . This operation is called rectification or fractionation . Common methods used in distillation practice are: 1. Differential or simple distillation. 2. Flash or equilibrium distillation. 3. Rectification or fractionation

6. Differential or Simple Distillation: In this distillation technique, a known quantity of a liquid mixture is charged into a jacketed kettle or still. The jacket is provided for heating the mass in the still with the help of a heating media such as steam. The charge is boiled slowly, vapours formed are withdrawn and fed to a condenser where they are liquefied and collected in a receiver as a distillate . Rectification or Fractionation : The enrichment of the vapour stream as it passes through the column in contact with reflux is termed as rectification. Rectification is the separation of the component of a liquid mixture by successive distillation – successive vaporisation and condensation. With this separation technique, separations into almost pure components are possible.

7. Flash or Equilibrium Distillation: Flash distillation is normally carried out in a continuous manner. In this method, a liquid mixture is partially vapourised , the vapour and iquid are allowed to attain equilibrium and finally withdrawn separately . Feed is heated in a tubular heat exchanger. The hot mixture is then fed to a separator via a pressure reducing valve whereby the pressure is reduced and a vapour is formed at the expense of liquid adiabatically. The liquid is withdraw from the bottom of the separator and the equilibrium vapour leaves the separator from the top which is then liquefied in a condenser. heater Separator

8. A material balance of the more volatile component gives X f = f*y + (1-f)x …(i) f*y = -(1-f)x + x F … (ii) y = -(1-f)*x/f + x F /f … (iii) equation (iii) is the material balance/operating line for flash ditillation with a slope equal to –(1-f)/f and an intercept equal to x F /f .

9. The point of intersection of the operating line and the diagonal (x=y) is, y = -(1-f)*x/f + x F /f x = y Therefore x = -(1-f)*x/f + x F /f Xf = -x + xf + x F Therefore x = x F And y = x F   For f=1 , feed totally vaporised (feed 100 mole percent vaporised) Slope = -(1-f)/f = 0 And hence the operating line is parallel to x-axis through the point ( x f, x f ) on the diagonal. For f=0 , no feed is vaporised. Slope = ∞ A nd hence the operating line will be parallel to y-axis through the point ( x f ,x f ) on the diagonal .

10. Figure 1: Plotting the material balance line (operating line) for flash distillation on equilibrium diagram. Figure 2 : Determination of x and y for a given f

11. Modus Operandi for reactor design: To begin with, first we will make an tentative design of the whole setup and accordingly will order a platform for the setup.   Then we will order for the construction of cylindrical vessel and wrap the heating tape over its outer surface.   We will also bring 3 pumps for inlet flow , for distillate condenser and for bottom condenser respectively .   A flowmeter (like rotameter in this case) will be used for measuring the volume of inlet flow into the vessel.   The vessel is wrapped , first by glass wool and then by silver sheet.   Two condensers, distillate condenser and bottom condenser are also employed for collecting overhead and bottom product respectively.   All the connections are made with ½ inch RO pipe.

12. Chemistry Of The Process : we are going to flash the binary mixture of methanol and water.The quantity,cost and availability of the solvents should be considered,The solvent should be easily separable from the bottom product, and should not react chemically with the components or the mixture Methanol: Methanol, also known as methyl alcohol, is a chemical with the formula  C H 3 O H (often abbreviated MeOH ). Modern methanol is produced in a catalytic industrial process directly from carbon monoxide, carbon dioxide, and hydrogen . it is a polar liquid, and is used as an antifreeze, solvent, fuel, and as a denaturant for ethanol.It is also used for producing biodiesel via transesterification reaction. Properties: Physical State:  Liquid  Appearance:  clear, colorless   Odor :  alcohol-like -weak odor    Vapor Pressure:  128 mm Hg @ 20 deg C  Viscosity:  0.55 cP 20 deg C  Boiling Point:  64.7 deg C @ 760.00mm Hg  Freezing/Melting Point: -98 deg C  Autoignition Temperature:  464 deg C ( 867.20 deg F)  Flash Point:  11 deg C ( 51.80 deg F)  Explosion Limits, Lower: 6.0 vol %  Upper:  36.00 vol %  Solubility:  miscible  Specific Gravity/Density: .7910g/cm3  Molecular Formula: CH4O  Molecular Weight: 32.04

13. Chemical Stability :  Stable under normal temperatures and pressures.  Conditions to Avoid :  High temperatures, incompatible materials, ignition sources, oxidizers.  Appearance : clear, colorless . Flash Point: 11 deg C Storage:   Keep away from heat, sparks, and flame. Keep away from sources of ignition. Store in a cool, dry, well-ventilated area away from incompatible substances. Flammables-area. Keep containers tightly closed. Handling:   Wash thoroughly after handling. Remove contaminated clothing and wash before reuse Do not breathe dust, vapor , mist, or gas. Do not get in eyes, on skin, or on clothing . Avoid contact with heat, sparks and flame. Do not ingest or inhale. Use only in a chemical fume hood. Do not pressurize, cut, weld, braze, solder, drill, grind, or expose empty containers to heat, sparks or open flames. 

14. FLASH CALCULATION a liquid at a pressure equal to or greater than its bubble point pressure “flashes” or partially evaporates when the pressure is reduced,producing a two-phase system of vapour and liquid in equilibrium . We consider here only the P, T-flash, which refers to any calculation of the quantities and compositions of the vapour and liquid phases making up a two-phase system in equilibrium at known T, P , and overall composition .

15. Consider a system consisting 1 mole of non-reacting chemical species with an overall composition ,represented by the set of mole fractions .Let be the moles of liquid,with mole fractions ,and let be the moles of vapour,with mole fractions . The material balance equations are : Combining these equations to eliminate gives: Substituting and solving for yields: Since, therefore ,

16. Physical design : Storage Tank : ( Qty – 3) It is used to store the chemical (reactant). One tank is for storing input feed and the others 2 are for storing coolant . The tanks used in the project are of 5 liters capacity. Pumps : ( Qty – 3) It is to pump the feed in the tanks to the vessel.We are using ½ hp motor. Rota Meter : (Qty-1) It is a device used to measure the flow rate of the feed pumped by the pump from the storage tank. It is used to keep a check on the flow rate of the feed. The range of the Rota meter to be used 0-10 lph with brass nozzles. Cylindrical Vessel:(Qty-1) This is the main part of the equipment.It is used to flash the input stream.It is having dimensions about 0-2.5 metre length with dia of about 2 inch. Collecting tank : A collecting tank will be used collect the discharge from the tubes after the distillation . Condenser:(Qty-2) Two condensers are employed, one for distillate product and other one for bottom product.

17. Glass Wool Insulation: Glass Wool is wrapped over the surface area of the cylindrical vessel. Silver Sheet: The glass insulation is wrapped by silver sheet for protecting the former upon insulation. Platform: A 50 X 50 X 50 cm iron structure is used as a stand on which the reactor has been kept with a wooden back for attachments. Heating Tape: A heating tape is wrapped around the vessel inside glass wool insulation for heating the feed in the vessel. The heating tape can sustain upto a maximum temperature of about 250° C and is about 10 metres long. Connections : All the connections are made with ½ inch RO pipe. Rheostat(Qty-1 ): . A variable rheostat is used to maintain the appropriate temperature of the feed inside the vessel.

18. Continuous Flash Distillation Apparatus