(1) Drying as a unit operation involves the removal of moisture from a material. Change in moisture content ofa material as a function of time provides information about the rate of drying of the material. (a)What is the difference between direct and indirect drying? What are the advantages and disadvantagesof each mode of drying? (6 marks)(b)Describe with illustration the performance of two varieties of dryers used in the chemical Industry. What are the variables in dryer design that will enhance dryer efficiency (8marks)(c)A wet solid is dried from 60 to 12 percent moisture in 30 ks. If the critical and equilibrium moisturecontents were 22 and 7 percent respectively, how long will it take to dry the solid to 9 percent moisture if the same drying conditions are applied?(11marks)
(2) Filtration involves the separation of solid particles from fluid through a filtering medium and ranges from simple straining to more complex operations. Depending on the objective of the filtration process, the fluid, solid or both may be useful outputs of the overall process(a)Describe the basic principles of filtration as applied in the chemical industry. Using a suitable example, explain with clear illustrationshow filtration technique can be used to separate solids from filtrates at industrial scale.(7marks)(b)By means of equations and illustrations, explain clearly the difference between constant rate filtrationand constant pressure differencefiltration.. When would either of these techniquesbe applied during a filtration process? (8marks)(c)An industrial slurry of density 2500kg/m3, per m3 of water and containing 300kg of calcium carbonate(precipitated) is filtered in a plate and frame press and takes 700 seconds to dismantle, clean and reassemble. Assuming the cake is incompressible and has a voidage of 0.35, calculate the volume of filtrate produced (10marks).
(3) Distillation is an essential operation used commonly for the separation of materials in process engineering.
(a)Describe the mechanism of distillation operation in a chemical processing environment showing clearly its significance as a separation technique.(6marks)
(b)Explain clearly (with diagrams) how you can use McCabe-Thiele graphical method to determine the number of equilibrium stages required to separate a mixture to top and bottom distillate streams. (6 marks)
(c)Explain the meaning of Azeotrope and their effect in distillation process. Describe how anAzeotropeof ethanol and water can be separated.(6marks)
(d)Data below shows Industrial Distillation of Methylene Chloride (MVC) and Ethylene Chloride You are required to plot the McCabe –Thiele diagram for the column showing equilibrium data for methylene chloride (MVC) and ethylene chloride, 45o line and also data for operating lines (Enriching and Stripping sections of the distillation column) all on the same plot. From the diagram determine the optimal feed stagefor the column(7marks)
Equilibrium DataOperating Line DataTop (Enriching data) Bottom (Strip data)XYYXYX0.000.000.420.07 0.170.270.100.2860.480.260.240.360.200.4810.600.380.320.40.300.6170.700.600.40.420.400.7190.780.780.50.440.500.7960.880.850,60,50.600.8570.910.900.680.590.700.800.900.9040.9420.9740.750.780.800.630.650.671.001.000,650.67
(4a) Typical gas absorption reactors come in the form of packed columns, plate towers, bubble columns and sieve plate reactors. By means of diagrams describe these four types of gasabsorption reactors highlighting theadvantages and disadvantages of each design for gas absorption process.(8marks)(4b) Describe the process of gas absorption in a named solvent within a platetower. Derive anexpression showing the relationship between the numberof transfer units, height of transferunits and the height of the tower for this process. (5marks)(4c) Describe briefly how Liquid-Liquid extraction can be made economical and also environmentally friendly? Using a typical example, describe the performance of non-environmentally friendly solvent and identify a suitable replacement alternative for this process.(5marks)(4d) Consider a situation where phenol is partitioned between water and benzene with a partition coefficient of 1.32 in favour of benzene. Suppose we have 2g of phenol dissolved in 250ml of water. How muchphenol will be extracted into 250ml of benzene with a single extractionand howmuch phenol is left in water?
