Distillation. The Continuous Column. Learning Outcomes. Recap - VLE for Meth H 2 O. Gavin Duffy School of Electrical Engineering DIT Kevin Street

Transcription

1 Distillation The Continuous Colun Gavin Duffy School of Electrical Engineering DIT Kevin Street Learning Outcoes After this lecture you should be able to.. Describe how continuous distillation works List the ajor coponents of a distillation colun Develop a atheatical odel for a continuous colun Recap - LE for Meth H 2 O Methanol Water LE Ya (Meth vapour) Xa (Meth liquid)

2 oil and Cool 4 ties Finish Methanol Water LE Ya (Meth vapour) Start Xa (Meth liquid) oiling a liquid with Xa of 0.2 produces a vapour with Ya of 0.57 oiling a liquid with Xa of 0.57 produces a vapour with Ya of 0.82 oiling a liquid with Xa of 0.8 produces a vapour with Ya of 0.93 oiling a liquid with Xa of 0.93 produces a vapour with Ya of 0.98 Alternatively use T--y Diagra Methanol Water LE (T--y) ubble Dew Tep C Xa, Ya (Meth) How to separate a binary iture Pot still oil the iture, condense the vapour and collect the distillate. Repeat the procedure until the desired purity is obtained. 2

3 Each still is a step on the -y curve.0 Methanol Water LE 4 Step off each stage using the =y line gives the sae result Ya (Meth vapour) Each step is an ideal stage in distillation 4 ideal stages to go fro 20% Meth to 95% Meth Xa (Meth liquid) Activity Count Stages LE Acetic Acid Acetic Anydride Ya Xa How any ideal stages are needed to take this syste fro a feed coposition of 0.2 to a distillate coposition of 0.95? Pros and Cons of the pot still? Cannot vaporise all of the iture Sall aount High purity Large aount Low purity Large aounts of energy required ery slow How can this be iproved? ery siple to ake Cheap iniu coponents Fleible can collect over tie before using 3

4 How to iprove the pot still? Reeber that boiling results in a change of coposition, and condensing also results in a change of coposition Therefore, cobine the two processes inside the colun to iprove the distillation process A distillation colun is designed to encourage vapour liquid contact Falling liquid eets rising vapour. oiling and condensing do not just occur in the reboiler and the condenser. They happen inside the colun also The Distillation Colun Distillation Colun Coponents Reboiler this heats the liquid Stripping Section MC is vapourised Rectifying Section LC is condensed Trays/Plates encourage vapour liquid contact Packing alternative to trays Condenser apour fro colun is cooled to liquid Reflu condensed vapour can be returned to colun Top product fro condenser otto product fro reboiler 4

5 Trays and Packing Liquid apour Encourage vapour liquid contact apour condenses to release LC Liquid uses this energy to boil and release MC Colun Mass alance F,, D, etc are flow rates in ol/s X f etc are ol fractions n trays in the rectifying section trays in the stripping section, L different Can also write for each coponent: F f,a = D d,a + b,a F f L n n+ + L b D d F = + D = L + D Activity Mass balance on Acetic Acid/Acetic Anhydride proble A iture of Acetic Acid and Acetic Anydride containing 40 ol % Acetic Acid is to be separated by distillation. The top product is to be 90 ol % Acetic Acid and the botto product 0 ol % Acetic Acid. The feed is heated to its boiling point. The vapour is condensed but not cooled and soe is returned at a reflu ratio of 3 kol/kol product. Carry out a ass balance on this colun 5

6 The Stripping Section L Liquid apour y Feed flows down colun to reboiler Reboiler heats liquid to P and vapour rises Liquid that does not vaporise is reoved as ottos apour rises and is forced into contact with falling liquid On the trays, soe liquid reboils and soe vapour condenses due to heat transfer between the phases - ore stages are created Reboiler L' L' + + = ' + + = ' L' y = b ' ottos + y + + ' b b Constant Molal Overflow The assuption of constant olal overflow is used to siplify the above equations. It eans that for every ole of vapour condensed, ole of liquid is vaporised. This does not happen in reality but it is an acceptable approiation. It is based on negligible heat of iing and heat loss and on constant olar enthalpies It eans that while the liquid and vapour copositions ay change the overall flowrate of each is constant through the colun, i.e. L n = L n+ and n = n+ Applying constant olal overflow Stripping Section y L' + = ' + ' b Applying constant olal overflow gives L' ' ' 6

7 Activity - Rectifying Section Develop the operating line for the rectifying section Condenser Reflu Dru Reflu n n+ y n+ L n n Distillate Top Product D d The Rectifying Section Condenser Condenser at the top of the colun cools the vapour, collected in the reflu dru A portion is returned to the colun as reflu Reainder is reoved as Distillate or Top Product Reflu Ratio = Reflu/Distillate Mass balance on flowrates gives apour = Liquid + Distillate n n+ y n+ n+ y Reflu Dru Reflu L n n n+ n+ y = L + D n+ n Distillate Top Product = L + D n n Ln = n+ d D n+ n+ D d d Applying constant olal overflow Rectifying Section Stripping Section y L n n+ = n + n+ D n+ d y L' + = ' + ' b Applying constant olal overflow gives L D y = + d L' ' ' 7

8 Activity Label this distillation colun! Reflu Soe condensed liquid is reoved fro the colun as distillate. Soe is returned. The reflu ratio is the ratio of liquid returned to the colun over the aount reoved R = L/D or L = DR Activity rewrite the operating line for the rectification section using the reflu ratio. Rectifying Operating Line The rectifying operating line is: L D y = + d Since L = RD and = RD+D, we get: RD D + RD+ D RD+ D R + d R+ R+ Copare this to y = + c it is a straight line d 8

9 Rectifying line on X-Y Diagra Slope = R/(R+) Y X d /(R+) d X Stripping Operating Line The boilup ratio is defined as the ratio of vapour returning to the colun to the bottos product flow: = / Therefore, the stripping operating line can be written as + Again of the for y = + c, another straight line Stripping line on X-Y Diagra Y 0.5 Slope = +/ X b X 9

10 Suary Operating lines The rectifying section (upper colun) The stripping section (lower colun) Or R + R+ R+ + L Note inus sign in stripping line d b Activity Operating lines A iture of Acetic Acid and Acetic Anydride containing 40 ol % Acetic Acid is to be separated by distillation. The top product is to be 90 ol % Acetic Acid and the botto product 0 ol % Acetic Acid. The feed is heated to its boiling point. The vapour is condensed but not cooled and soe is returned at a reflu ratio of 3 kol/kol product. Deterine the operating lines for the rectifying and stripping sections and draw the on an equilibriu curve. To help you: Start with the rectifying line it is easy just use the reflu ratio. Stipping line is harder we don t know the boilup rate needed. So Deterine and D fro an overall ass balance Use D and R to give L for rectifying section (L n ) Use L and D to give for rectifying section L for stripping section (L ) coes fro F and L n is the sae for both sections as feed enters as liquid Use L and and to give stripping operating line The intersection of the operating lines If the feed enters the colun as a liquid at its boiling point then the operating lines intersect at f. In this case: L = L n + F i.e. the liquid flow in the stripping section is the su of all of the feed and the liquid flow fro the top of the colun. All of this liquid will be at the bubble point. The feed ay not be liquid at its boiling point. It ight be at a lower teperature or at a higher teperature. There are five possible feed conditions. What happens to the operating lines if the feed is colder, i.e less than the boiling point? 0

11 Activity Feed Condition The feed to the colun can vary in for. It can be: Subcooled liquid ubble point liquid Partially vaporised feed Dew point vapour Superheated vapour Think, Pair, Share briefly (5 in) what this eans for the liquid and vapour flowrates in the stripping and rectifying sections of the colun. The q line This is used to show the feed condition on the -y diagra. It is obtained by writing the two operating lines at their intersection, i.e. at plate n and plate the feed plate An enthalpy balance on the feed plate is then carried out to give the following equation (see C&R ol 2, 4 th Ed. p449): L = L C q= p n + qf ( T T) o λ Where C p = specific heat capacity T o = boiling point of feed T = teperature of feed λ = latent heat of vaporisation +λ What is q? q = the enthalpy change needed to bring the feed to a dew point vapour divided by the enthalpy of vaporisation of the feed Defined as: q = Heat to vaporise ol of Feed Molar latent heat of vaporisation of the Feed If the feed is a iture of liquid and vapour then q is the fraction that is liquid C pl heat capacity of liquid For cold feed T cpl( Tb TF) b bubble point q= + T F Feed tep λ λ Latent heat of vapor n For superheated vapour C cp( TF Td) p heat capacity of vapou T q= d dew point λ T F Feed tep

12 The q line equation Using the above definition of q and a aterial balance over the whole colun we get the q line: q f yq = q q q The two points used to draw the q line are:. y f = f 2. The intersection point of the other two operating lines The q line and feed condition The feed condition can now be described by the q line: Subcooled liquid q > q line / ubble point liquid q = q line Partially vaporised feed 0 < q < q line \ Dew point vapour q = 0 q line --- Superheated vapour q < 0 q line / The q line and feed condition The slope of the q line depends on the feed condition 2