INTRODUCTION

Solid-liquid mass transfer plays an important role in some industrial operations.The dissolution may

occur with or without chemical reaction.In case dissolution is accompanied by solid-liquid reaction ,it is

desirable to know the enhancement in the rate of mass transfer due to simultaneous reaction and

compare it with the enhancement predicted on the basis of the film and the boundary layer

models.Here the system under study is dissolution of benzoic acid in aqueous NaOH solution.The

dissolution of a solid in a solution accompanied with instantaneous chemical reaction can be expressed

as :

A + nBB => Product

Where A is the solid and B is the liquid phase reactant. Assuming the reaction to be instantaneous so

that Aand B don’t co-exist. The mechanism of solid dissolution involves :dissolution of A in liquid

followed by its reaction with species B diffusing from the bulk liquid phase at a reaction plane.If the

film model is applied to this situation, the enhancement factor, φ defined as the ratio of the solid liquid

mass transfer coefficient with reaction kr to the mass transfer coefficient without reaction k given by

And for boundary layer model it is :

OBJECTIVE :

1. To Study the dissolution of benzoic acid in aqueous NaOH solution.

2. To Compare the observed enhancement factor for mass transfer with those predicted by film and

boundary layer model.

3. Mass transfer with chemical reaction (solid - liquid system) : Dissolution of benzoic acid in aqueous

NaOH solution.

SETUP DESCRIPTION

The setup consists of a cylinder of benzoic acid mounted on a SS rod and with a D.C. motor. The

operational range of rotation is between 10 to 30 rpm. The cylinder is immersed in an aqueous solution

of sodium hydroxide of known concentration in a 500 ml vessel filled to two-third its capacity. The

position of the benzoic cylinder is so adjusted that the liquid level rises above the top surface by about

the top surface by about 3 cm. The dimensions of the benzoic acid may be fixed at diameter 2 or 3

cm,length 5 to 7 cm. the cylinder can be prepared by pouring molten benzoic acid in the mould of

desired dimensions with 4 to 5 mm SS rod located in the center of the mould in a vertical position .

PROCEDURE

1. Record the dimensions of the benzoic acid cylinder(o.d.,length and od of ss rod) and then fix it

in a vertical position with the d.c motor.

2. Fill the vessel with aqueous naoh solution of known concentration upto 2/3rd of its

volume.record the volume of aqueous NaOH solution added (v).

3. Start the water bath and fix the dissolution temperature,ambient upto 50 degree Celsius .

4. Fix the benzoic acid cylinder inside the vessel containing aqueous NaOH solution and start the

motor at a fixed rortional speed N rpm.

5. Run the experiment for 10 minutes.

6. Stop the motor and remove benzoic acid cylinder and measure its dimensions again.

7. Mix thoroughly the contents of the vessel and analyse it for unreacted NaOH concentration by

titration against standard HCl solution.

8. Measure the benzoic acid cylinder dimensions again.

9. Repeat steps 1 to 8 for different concentrations of aqueous NaOH solutions.

10. Repeat steps 1 to 8 using de-ionised water. This run may be carried for about 45 to 60 minutes

duration. During this period small samples should be withdrawn at regular intervals of 10

minutes and analyzed for dissolved benzoic acid by titration against 0.02kmol/m3 NaOH

solution.

11. CALCULATIONS12. CASE 1: TO FIND THE ENHANCEMENT FACTOR WHEN BENZOIC ACID IS DISSOLVED IN

1.5 L of 0.5N NaOH FOR A DURATION OF 10 MINUTES .

13. BEFORE EXPERIMENT

14. Outer diameter of the benzoic acid cylinder = 3.4 cm

15. Length of the benzoic acid cylinder = 4.9 cm

16. AFTER EXPERIMENT

17. Outer diameter of the benzoic acid cylinder = 2.75 cm

18. Length of the benzoic acid cylinder = 4.01 cm

19. Normality of HCL used for titration (N1) = 0.5 N

20. Volume of 0.5 N HCL used (V1) = 9.4 ml

21. Volume of NaOH taken (V2) = 10 ml

22. Let the normality of unused NaOH in dissolution be N2

23. N1V1 = N2V2

24. N2 =N1V1 /V2

25. =0.47 N

26. The concentration of unused NaOH = 0.47 N

27. The rate of dissolution of benzoic acid = R

28. R = N2*Volume of NaOH soln in tank

29. =1.175 *10^(-6) kmol/sec

30. Let the average dimensions of benzoic acid cylinder be

31. Dav = (3.4 + 2.75 )/2

32. = 3.075 cm

33. Lav = (4.9 + 4.01)/2

34. = 4.455 cm

35. Let S.S rod diameter d = 0.78 cm

36. Average surface area (AS)

37.

38. = 56.9*10(-4) m2

39. Specific rate of dissolution R'

40.

41. = 2.1* 10(-4) kmol/m2 - sec