Question

(a) Explain briefly in what circumstances you would require a Packed Bed Reactor (PBR) in a chemical process. How is a PBR different from an ordinary Plug Flow Reactor (PFR). Use sketches and design equations to justify your answers. [8 marks]

(b) The data in Table Q1 were obtained for the photochemical decay of liquid bromine dissolved in water when placed in sunlight:

TABLE Q1

Time (min)   10   20    30    40    50    60 Bromine (ppm) 2.45 1.74 1.23 0.88 0.62 0.44

(i) Using graphical means show that the reaction is first-order in bromine. [12 marks]

(ii) Calculate the reaction rate constant.
[5 marks]

Answer 1

a)

-Packed Bed Reactors are especially used to improve the contact between the reactants, since the packings provide a large amount of surface area. Also, the path taken by the reactants gives additional turbulence, improving the reaction.

-Packed bed reactors can also be used when the heat generated from the reaction needs to be stored, since if appropriate packing material is used, it can store heat.

-Packed bed reactors can be used for gas-solid or solid-liquid reactions.

PBR is different from Plug flow reactor as:

-A PFR does not contain any packing material, and is just a simple tube for the fluids to flow and react, while a PBR contains packing material.

-The surface area of contact is greatly enhanced in a PBR.

-The velocity profile in a PFR is that like a plug ( as in a turbulent flow profile) while in a PBR, it is complicated due to the packing.

-The pressure drop is great in a PBR due to the packing, as compared to a PFR.

-PBR are almost always used for a solid catalyst as the medium, while there is no such thing in a PFR.

-Design equation and construction.

PBR schematic: (http://www.essentialchemicalindustry.org/processes/chemical-reactors.html)

PFR schematic (http://www.informit.com/articles/article.aspx?p=2475565&seqNum=4)

(b) We change the concentrations from ppm to mol/L by multiplying 10^-6.

Also, a linear fit between log C and time would imply a first order reaction, with slope as the rate constant.

R square =1 implies a perfect linear fit.

Hence, its proved its a first order reaction.

Negative of slope = rate constant = 0.0343 min^-1