distillation

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distillation

  1. 1. Distillation By : Govind N Manglani
  2. 2. Topics <ul><li>Types of Distillation </li></ul><ul><li>Action on an Ideal Plate </li></ul><ul><li>Mass Balance in a Distillation Column </li></ul><ul><li>Determination of Ideal Number of Plates – McCabe –Thiele Analysis </li></ul>
  3. 3. The methods of distillation <ul><li>Differential or batch distillation </li></ul><ul><li>Flash or equilibrium distillation </li></ul><ul><li>Continuous Rectification – Binary systems </li></ul>
  4. 4. Differential distillation <ul><li>The simplest examples of batch distillation at a single stage. </li></ul><ul><li>Starting with a still pot, initially full, heated at a constant rate. In this process the vapour formed on boiling the liquid is removed at once from the system. </li></ul><ul><li>Since this vapour is richer in the more volatile component, with this result the composition of the product progressively alters. </li></ul><ul><li>Thus, whilst the vapour formed over a short period is in equilibrium with the liquid </li></ul><ul><li>At the end of the process the liquid, which has been vaporized, is removed as the bottom product. </li></ul>
  5. 5. Differential Distillation <ul><li>Let S be the number of mols of material in the still and x be the mol fraction of component A. </li></ul><ul><li>Suppose an amount dS, containing a mol fraction y of A, be vaporised. </li></ul><ul><li>Then a material balance on component A gives: </li></ul><ul><li>ydS = d (Sx) </li></ul><ul><li> = S dx + x dS </li></ul>
  6. 6. Differential Distillation <ul><li>The integral on then right-hand side can be solved graphically if the equilibrium relationship between y and x is available. </li></ul><ul><li>Thus, if over the range concerned the equilibrium relationship is a straight line of the form y= m x + c </li></ul>
  7. 7. Differential distillation <ul><li>This process consists of only a single stage, a complete separation is impossible unless the relatively volatility is finite. Application is restricted to conditions where a preliminary separation is to be followed by a more rigorous distillation, where high purifies is not required, or where the mixture is very easily separated </li></ul>
  8. 8. Flash vaporisation or Equilibrium Distillation <ul><li>This method is frequently carried out as a continuos process. </li></ul><ul><li>Consist of vaporizing a definite fraction of liquid feed in such a way that the vapour evolved is in equilibrium with the residual liquid. </li></ul><ul><li>The feed is usually pumped through a fired heater and enters the still through a valve where the pressure is reduced. </li></ul><ul><li>The still is essentially a separator in which the liquid and vapour produced is reduced by reduction in pressure with sufficient time to reach equilibrium. The vapour is removed from the top of the separator and is then usually condensed, while the liquid leaves from the bottom. </li></ul>
  9. 9. Flash or Equilibrium Distillation <ul><li>It is used in petroleum refining, in which petroleum fractions are heated in pipe stills and the heated fluid flashed in to vapour and residual streams, each containing many components. </li></ul>
  10. 10. Continuous Distillation with Reflux <ul><li>Flash distillation is used most for separating components that boil at widely different temperatures. </li></ul><ul><li>It is not effective separating components of comparable volatility, which requires the use of distillation with reflux </li></ul>
  11. 11. Action on an Ideal Plate <ul><li>By definition a vapour leaving a plate are brought into equilibrium. </li></ul><ul><li>Assume that the plates are numbered serially from top down and that the plate under consideration is the nth plate from the bottom. </li></ul><ul><li>Then the immediately above plate n is plate n-1, and the immediately below is n+1. </li></ul>
  12. 12. <ul><li>Material Balance diagram for plate n </li></ul>Plate n Plate n+1 Plate n-1 V n ,y n L n-1 ,X n-1 V n-1 ,y n-1 L n-2 ,X n-2 V n+1 ,y n+1 L n ,x n
  13. 13. <ul><li>For example if two fluid enter plate n and two leave it, the liquid, L n-1 mol/h, from plate n-1 and the stream of vapour, V n+1 mol/h, from plate n+1 are brought into intimate contact. </li></ul><ul><li>A stream of vapour, V n mol/h, rises to plate n-1 and a stream of liquid, L n mol/h, descends to plate n+1 . </li></ul><ul><li>Since the vapour streams are the V phase, their considerations are denoted by y ; the liquid streams are the L phase and their concentrations are denoted by x . Then the concentrations of the streams entering and leaving the n th plate are as follows: </li></ul><ul><li>Vapour leaving plate, yn </li></ul><ul><li>Liquid leaving plate, xn </li></ul><ul><li>Vapour entering plate, yn+1 </li></ul><ul><li>Liquid entering plate, xn-1 </li></ul>
  14. 14. Boiling-Point Diagram showing rectification on ideal plate x n x n-1 y n+1 y n
  15. 15. Number of Plates Required in a Distillation Column <ul><ul><li>To develop a method for the design of distillation units to give the desired fractionation, it is necessary to determine the numbers of trays. </li></ul></ul><ul><ul><li>Before that the heat and material flows over the trays, the condenser and the reboiler must be established </li></ul></ul><ul><ul><li>Thermodynamic data is required to establish how much mass transfer is needed to establish equilibrium between the stream leaving each tray. </li></ul></ul><ul><ul><li>The diameter of the column will be dictated by the necessity to accommodate the desired flow rates, to operate within the available drop in pressure, while at the same time affecting the desired degree of mixing of the stream on each tray. </li></ul></ul>
  16. 16. Summary of the material balances for two components systems <ul><li>Let the process be analysed simply for a binary mixture of A and B as follows: </li></ul><ul><li>Let F be the number of mols per unit of feed of mol fraction x f of A. </li></ul><ul><li>D be the number of mols per unit time of vapour formed with y the mol fraction of A and </li></ul>
  17. 17. <ul><li>B be the number of mols per unit time of liquid with x the mol fraction of A. Then an overall mass balance gives: </li></ul><ul><li>F = D + B </li></ul><ul><li>Component A balance </li></ul><ul><li>Fx F = D x D + B x B </li></ul><ul><li>Eliminating B and D from these equations give the follow: </li></ul>
  18. 18. <ul><li>Net flow rates. Quantity D is the difference between the flow rates of the streams entering and leaving the top of the column. A material balance around the condenser and accumulator in the gives: </li></ul><ul><li>The difference between the flow rates of vapour and liquid anywhere in the upper section of the column is also equal to D. This surface includes the condenser and all plates above n+1. A total material balance around this control surface gives: </li></ul>
  19. 19. <ul><li>Similar material balances for A give the following equations: </li></ul><ul><li>Quantity D x D is the net flows rate of the component A upward in the upper section of the column. It too is constant throughout this part of the equipment. In the lower section of the column the net flow rates are also constant but, are in a downward direction. </li></ul>
  20. 20. <ul><li>The net flow of total material equals B; that of the component A is Bx B . The following equations apply: </li></ul>
  21. 21. Operating lines <ul><li>There are two sections in the column; there are also two operating lines, one for the rectifying section and other for the stripping section. </li></ul><ul><li>For the first section (rectifying section) the operation line is represented by: </li></ul>
  22. 22. <ul><li>Substituting for Dx D in the equation above and eliminating V n+1 </li></ul><ul><li>For the section below the feed plate, a material balance over control surface II gives </li></ul><ul><li>Rearranging this equation and taking into account that the slope is the ratio of liquid flow to the vapour flow, and also eliminating Vm+1 </li></ul>
  23. 23. Feed Line <ul><li>The conditions of the vapour rate or the liquid rate may change depending of the thermal condition of the feed. </li></ul><ul><li>It is related to the heat to vaporise one mole of feed divided by molar latent heat (q) </li></ul>
  24. 24. Various type of feed conditions <ul><li>Cold feed, q>1 </li></ul><ul><li>Feed at bubble point (saturated liquid), q=1 </li></ul><ul><li>Feed partially vapour, 0<q<1 </li></ul><ul><li>Feed at dew point (saturated vapour), q=0 </li></ul><ul><li>Feed superheated vapour q<0 </li></ul>
  25. 25. Feed Line
  26. 26. Feed Line <ul><li>Cold feed : It is assumed that the entire feed stream adds to the liquid flowing down the column. </li></ul><ul><li>Feed at bubble point: no condensation is required to heat the feed. </li></ul><ul><li>Feed partial vapour: the liquid portion of the feed becomes part of the L and the vapour portion becomes part of V </li></ul>
  27. 27. Feed Line <ul><li>Feed saturated vapour the entire feed becomes part of the V </li></ul><ul><li>Feed superheated: part of the liquid from the rectifying column is vaporized to cool the feed to a state of saturated vapour. </li></ul>
  28. 28. Feed Line Equation <ul><li>If x q = x F , and y q =x F then; </li></ul><ul><ul><li>The point of intersection of the two operating lines lies on the straight line of slope (q/q -1) and intercept (x F , y F ) </li></ul></ul>
  29. 29. Reflux Ratio <ul><li>The analysis of fractionating columns is facilitated by the use of a quantity called reflux ratio. </li></ul><ul><li>Two ratios are used, one is the ratio of the reflux to the overhead product and the other is the ratio of the reflux to the vapour. </li></ul><ul><li>Both ratios refer to quantities in the rectifying section. The equations for those ratios are </li></ul>
  30. 30. Reflux Ratio <ul><li>If the operation lines equations are divided D, the result is, for constant molar overflow, </li></ul><ul><li>This equation is an operation line of the rectifying section </li></ul>
  31. 31. Reflux Ratio <ul><li>The y intercept of this line is x D /(R D+1 ). </li></ul><ul><li>The concentration x D is set by the conditions of the design. </li></ul><ul><li>R D , the reflux ratio, is an operating variable that can be controlled at will by adjusting the split between reflux and overhead product or by changing the amount of vapour formed in the reboiler for a given flow rate of the overhead product. </li></ul>
  32. 32. Reflux Ratio <ul><li>A point at the upper end of the operating line can be obtained by setting x n equal to x D in the equation above. </li></ul>
  33. 33. Reflux Ratio <ul><li>A point at the upper end of the operating line can be obtained by setting x n equal to x D in the previous equation. </li></ul><ul><li>The operating line for the rectifying section then intersects the diagonal at point (x D , x D ) </li></ul>
  34. 34. Reflux Ratio <ul><li>The operation lines represented by those two equations are plotted with the equilibrium curve on the x-y diagram. </li></ul><ul><li>Those equations also show that unless Ln and Lm are constant, the operating lines are curved. </li></ul><ul><li>The lines can be plotted only if the change in these internal streams with concentration is known. </li></ul>
  35. 35. Reflux Ratio a e b x F Minimum Reflux Total (Maximum) Reflux
  36. 36. Influence of the Number of Reflux Ratio <ul><li>Any change in R will therefore modify the slope of the operation line as can be seen from the Figure </li></ul>b a f e d g
  37. 37. Maximum or Total Reflux <ul><li>If no product is withdrawn from the still (D=0), the column is said to operate under conditions of total reflux and, as seen from equation , the top operating line has its maximum slope of unity, and coincides with the line x=y. </li></ul>
  38. 38. <ul><li>The step become very close to the plate above, these conditions are known as minimum reflux and Rm denotes the reflux ratio. </li></ul><ul><li>Any small increase in R beyond Rm will give a workable system, though a large numbers of plate will be required. </li></ul>
  39. 39. Important deductions <ul><li>The minimum number of plates is required for a given separation at conditions of total reflux </li></ul><ul><li>There is a minimum reflux ratio below which it is impossible to obtain the desired enrichment, however many plates are used. </li></ul>
  40. 40. Calculation of Minimum Reflux Ratio R m <ul><li>Based on the previous figure, the slope of the line ad is given by </li></ul>
  41. 41. McCabe - Thiele <ul><li>Construction the operation lines: </li></ul><ul><li>Locate the feed line </li></ul><ul><li>Calculate the y-axis intercept x D /(R D + 1) of the rectifying line and plot that line through the intercept and the point </li></ul><ul><li>(x D , x D ) </li></ul><ul><li>Draw the stripping line through point (x B ,x B ) and the intersection of the rectifying line with the feed line. </li></ul>
  42. 42. Effect the feed condition on feed line <ul><ul><li>If the feed is a cold liquid, the feed line slopes will be upward and to the right; </li></ul></ul><ul><ul><li>if the feed is a saturated liquid, the line is vertical; </li></ul></ul><ul><ul><li>if the feed is a mixture of liquid and vapour, the lines slopes upward and to the left and the slope is the negative of the ratio of the liquid to the vapour; </li></ul></ul><ul><ul><li>if the feed is saturated vapour the line is horizontal and </li></ul></ul><ul><ul><li>if the feed is superheated vapour. The lines slope downward and to the left. </li></ul></ul>
  43. 43. McCabe - Thiele <ul><li>Feed Plate location </li></ul><ul><li>After the location of the feed plate the construction of the number of ideal trays is found by the usual step-by-step construction. </li></ul><ul><li>The process can begin at the top and also a total condenser is used. </li></ul>
  44. 44. McCabe - Thiele
  45. 45. END <ul><li>Summary </li></ul>

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