No. 6. Reaction Rate Constant in CSTR Reaction. 1. Introduction

Transcription

1 No. 6. Reaction Rate Constant in CSTR Reaction 1. Introduction In the majority of industrial chemical processes, the reactor vessel in which the process takes place is the key item of equipment. The design of chemical reactors is therefore crucial to the success of the industrial operation. In general, the aim is to produce a specified product at a given rate using known reactants. Various types of reactor are used to achieve these objectives namely, continuous stirred tank reactors(cstrs), tubular(or plug flow) reactors, PBR, etc. At this experiment, we can measure the reaction rate constants at CSTR. 2. Objectives To find the reaction rate constant in a CSTR.

2 3. Prerequisites 3.1. Molecular weight of sodium hydroxide is 4. and ethyl acetate is Density of ethyl acetate is If this reaction is normal second order reaction, the reaction rate is (1) Here, = the amount of mole of at temporary time = volume of reactor = reaction and reverse reaction rate constant = molar concentration of = molar concentration of = molar concentration of = molar concentration of If the concentration of and is same and, we can ignore the reverse reaction. So equation (1) can be substituted to equation (2) (2)

3 4. Materials and apparatus 4.1. Materials Reagent in CSTR : 2 liter of.1m of sodium hydroxide and 2 liter of.1m of ethyl acetate Apparatus Pipette 2L flask * 2 Poly-gloves CEM stirred tank reactor-33 : computer program CEX Service Unit CEM MK Ⅱ (feed vessels & continuous stirred tank reactor)

4 5. Theory CSTR > The reaction: ie. second order overall, within the limits of concentration( ~.1M) and temperature(2 ~ 4 ) studied, can be considered equimolar and first order with respect to both sodium hydroxide and ethyl acetate. The reaction carried out in a Continuous Stirred Tank Reactor or Tubular Reactor eventually reaches steady state when a certain amount of conversion of the starting reagents has taken place. The steady state conditions will vary depending on concentration of reagents, flow rate, volume of reactor and temperature of reaction. To calculate the specific rate constant, k: The overall mass balance may be written as: ie. for a reactant in a reactor of volume V For the continuous reactor operating at steady state the volume may be assumed constant, (*) The steady state concentration of sodium hydroxide in the reactor ( ) may be used to calculate the specific rate constant ( ).

5 6. Experimental Procedures 1 Make up 2.L batches of.1m sodium hydroxide and.1m ethyl acetate. 2 Remove the lids of the reagent vessels and carefully fill with the reagents. Refit the lids. 3 Adjust the set point of the temperature controller to 3. 4 Collection of conductivity data will be until a steady state condition is reached in the reactor and this takes approximately 4 minutes. It is advisable to set the data collection period to, say, 5minutes. 5 Using the dial for each of the feed pumps, set the pump speed control to give 5ml/min flowrate. 6 Set the agitator speed controller to Switch on both feed pumps and the agitator motor, and begin taking readings. After a few minutes, the temperature sensor tip will be covered (about 25mm of liquid in reactor)-switch on the hot water circulator.

6 7. Raw data 7.1. CSTR Reactor Volume = 2. L Time (min) Conductivity What is the steady-state conductivity = 7.2. Conductivity and Concentration As experiment No 6., calculate the values from the following formulae: a F a b = a μ μ Fa + Fb Fa + Fb b = F b =, for =, for =.7[1+.284(T-294)], for T=294 =.195[1+.184(T-294)], for T=294 = (assumes = ) =, for =( - ), for =.195[1+.184(T-294)], if not =

7 For the values of each of the above, the spreadsheet can be used to calculate values of sodium hydroxide concentration( ) and sodium acetate concentration( ) and the degree of conversion( ) and ( ) for each of the samples of conductivity taken over the period of the experiment. These can be calculated and listed in columns alongside the readings of conductivity using the following equations. (for =) X a = a a1 a c1 X c = c (for =)

8 ** Nomenclature =total volume feed rate ( ) =volume feed rate of sodium hydroxide. ( ) =volume feed rate of ethyl acetate. ( ) =sodium hydroxide conc. in feed vessel ( ) =sodium hydroxide conc. in mixed feeds ( ) =sodium hydroxide conc. in reactor at time t ( ) =sodium hydroxide conc. in reactor after 8 time ( ) =ethyl acetate conc. ( ) =sodium acetate conc. ( ) =specific rate constant =reactor temperature (K) =volume of reactor ( ) =conversion of sodium hydroxide = a a a 1 =conversion of sodium acetate = c1 c c =conductivity (Siemens) =initial conductivity (Siemens) = conductivity (Siemens) at time t = conductivity (Siemens) at time.

9 8. Discussions 9. References 9.1. H. Scott Fogler, Elements of Chemical Reaction Engineering, 3rd ed, Prentice Hall, J. B. Butt, "Reaction Kinetics and Reactor Design," Prentice-Hall, Engelwood Cliffs, N. J., Relevance study Through above mass balance equation, drive k as a function of concentration(a).