5.drying

DAY 2: CHEMICAL ENGINEERING –Drying

DRYING Review Questions 1. The vapor pressure exerted by the moisture contained in a wet solid depends upon the a) nature of the moisture b) temperature c) nature of the solid d) all a, b, and c 2. To remove all the moisture from a wet solid requires exposure to a) perfectly dry air b) highly humid air c) air at high temperature d) none of these 3. Milk is dried usually in a a) freeze dryer b) spray dryer c) tray dryer d) rotary dryer 4. Equilibrium-moisture curves of different solids a) are different b) are same c) depend on the humidity of the gas d) none of these 5. Detergent solution is dried to a powder in a a) spray dryer b) spouted bed dryer c) tunnel dryer d) pan open to atmosphere. 6. If moisture content of a solid on dry basis is X then the same on wet basis is a) X  X+1 b) X  1-X c) 1+X  X d) 1–X  X 7. In a) b) c) d)

paper industry, paper is dried in tunnel dryer heated cylinder dryer conveyor dryer festoon dryer

DAY 2: CHEMICAL ENGINEERING –Drying

8. Moisture content of a substance when at equilibrium with a given partial pressure of the vapor is called a) free moisture b) unbound moisture c) equilibrium moisture d) bound moisture 9. Calcium ammonium nitrate (a fertilizer) is dried in a a) rotary dryer b) vacuum dryer c) tunnel dryer d) none of these 10. Moisture in a substance exerting an equilibrium vapor pressure less than that of pure liquid at at the same temperature is a) bound moisture b) unbound moisture c) critical moisture d) none of these 11. Rotary dryer cannot handle a) free flowing materials b) dry materials c) sticky materials d) granular materials 12. Moisture in a solid exerting an equilibrium vapor pressure equal to that of the pure liquid at the same temperature is a) unbound moisture b) critical moisture c) free moisture d) bound moisture 13. Refractory bricks are usually dried in a a) tray dryer b) tunnel dryer c) conveyor dryer d) festoon dryer 14. Heat sensitive materials like certain pharmaceuticals and food stuff can be dried a) in indirect tray dryer b) in spray dryer c) by freeze drying d) none of these 15. Moisture contained by a substance in excess of the equilibrium moisture is called a) unbound moisture b) free moisture c) critical moisture d) bound moisture 16. Flights in a rotary dryer are provided to a) lift and shower the solids thus exposing it thoroughly to the drying action of the gas b) reduce the residence time of solid c) increase the residence time of the solid d) none of these

DAY 2: CHEMICAL ENGINEERING –Drying

17. In a) b) c) d)

the constant rate period of the rate of drying curve for batch drying cracks develop on the surface of the solid rate of drying decreases abruptly surface evaporation of unbound moisture occurs none of these

18. The falling rate period in the drying of a solid is characterized by a) increase in rate of drying b) increasing temperatures both on the surface and within the solid c) decreasing temperatures d) none of these 19. Dryer widely used in a textile industry is a) cylinder dryer b) conveyor dryer c) tunnel dryer d) festoon dryer 20. Sticky material can be dried in a a) tray dryer b) rotary dryer c) fluidized bed dryer d) none of these 21. Drying of a solid involves a) only heat transfer b) only mass transfer c) both heat and mass transfer d) none of these 22. A solid material shows case hardening properties while drying. Which of the following should be controlled to control the drying process? a) flow rate of inlet air b) relative humidity of outlet air c) humidity of inlet air d) temperature of the solid 23. Capacity of a rotary dryer depends on a) its r.p.m b) its inclination with ground surface c) both a and b d) neither a nor b 24. Air a) b) c) d)

at a particular humidity is heated in a furnace. The new dew point decreases increases depends on the extent of heating remains unchanged

25. The critical moisture content in case of drying indicates a) beginning of falling rate period b) beginning of constant rate period c) end of falling rate period d) none of these

DAY 2: CHEMICAL ENGINEERING –Drying

26. A slurry is to be dried to produce flaky solid. Which dryer would you recommend? a) spray dryer b) drum dryer c) rotary dryer d) dielectric dryer 27. For a) b) c) d) 28. All a) b) c) d)

continuous drying of granular or crystalline material, the dryer used is tunnel tray rotary none of these

moisture in a non-hygroscopic material is……moisture free equilibrium unbound bound

29. During constant rate period, the rate of drying decreases with a) decrease in air temperature b) increased air humidity c) both a and b d) neither a nor b 30. Which is the controlling factor for a drum dryer? a) diffusion b) heat transfer c) both a and b d) neither a nor b 31. Paper industry employs……..dryers a) cylinder b) rotary c) spray d) fluidized bed 32. Sand is dried in foundaries in a ……..dryer a) rotary b) fluidized bed c) vacuum d) spray 33. Free flowing granular materials can be best dried in a a) rotary dryer b) cylinder dryer c) drum dryer d) freeze dryer