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Chemical Reaction Engineering

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Chemical Reaction Engineering

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    1. Chemical Reaction Engineering

    3. Design equations Batch: The conversion is a function of the time the reactants spend in the reactor. We are interested in determining how long to leave the reactants in the reactor to achieve a certain conversion X.

    4. Design equations CSTR: We are interested in determining the size of the reactor to achieve a certain conversion X.

    5. Design equations PFR: We are interested in determining the size of the reactor to achieve a certain conversion X.

    7. Isothermal reactor design Design procedure mole balance rate laws stoichiometry combination of the above three procedures and solve ODE obtain the volume/reaction time for the reactor

    10. Reactor design Batch: constant volume, well-mixed CSTR: constant volumetric flow rate

    11. Damkhler number ratio of the rate of reaction of A to the rate of convective transport of A at the entrance to the reactor estimation of the degree of conversion in a continuous reactor First order irreversible rxn: Second order irreversible rxn: Da = 0.1 ~ X = 10% ; Da = 10.0 ~ X = 90%

    12. Example, const.-V, batch, 2nd order rxn, isothermal

    17. Example, liquid phase CSTR, 1st order rxn, isothermal

    18. Example, liquid phase CSTR, 2nd order rxn, isothermal

    19. CSTRs in series, 1st order rxn, isothermal

    20. CSTRs in parallel, isothermal

    21. CSTRs in series constant flow rate conversion as a function of the number of tanks in series Two equal-sized CSTRs in series will give a higher conversion than two CSTRs in parallel of the same size when the reaction order is greater than zero.

    22. CSTRs in parallel constant conversion and rate of reaction in each tank The sum of the volume of the tanks equals the total volume of a single large CSTR. The conversion achieved in any one of the reactors in parallel is identical to what would be achieved if the reactant were fed in one stream to one large reactor of volume V. Considering the degree of mixing and the room required, a large tank might not be appropriate.

    27. PFR Gas-phase reactions are carried out primarily in tubular reactors where the flow is generally turbulent. Assuming no dispersion and there are no radial gradients in either temperature, velocity, or concentration. Should be aware of the change of the volume.

    28. PFR, 2nd order rxn, liquid phase, isothermal

    29. PFR, 2nd order rxn, gas phase, isothermal

    34. Pressure drop in reactors In gas-phase reactions, the concentration of the reacting species is proportional to the total pressure and proper accounting for the effects of pressure drop on the reaction system can be a key factor in the success or failure of the reactor operation (e.g. PBR). When accounting for the effects of pressure drop, the differential form of the mole balance must be used.

    35. PBR, 2nd order rxn, gas phase, isothermal

    36. Ergun equation Pressure drop in a porous bed:

    37. Ergun equation (cont.)

    38. Gas phase, PBR with pressure change

    39. Pressure drop in pipes without packing

    40. PBR, 2nd order rxn, gas phase, isothermal

    41. Spherical packed-bed reactors When small catalyst pellets are required, the pressure drop can be significant, and thus the conversion decreases. One type of reactor that minimises pressure drop and is also inexpensive to build is the spherical reactor, called an ultraformer. Spherical reactor: the cross-sectional area and the weight of catalyst are functions of the position. In addition to the higher conversion, the spherical reactor has the economic benefit of reducing the pumping and compression cost because of higher pressure at the exit.

    42. Mole balance and rate laws Concentration = f (conversion) We have done! There are a number of instances when it is much more convenient to work in terms of the number of moles (Ni) or molar flow rates (Fi) rather than conversion. Membrane reactors and multiple reactions taking place in the gas phase are two such cases where molar low rates rather than conversion are preferred. Concentration = f (molar flow rate)

    43. Algorithm - liquid phase Liquid phase For liquid-phase reactions in which there is no volume change, concentration is the preferred variable. We have only to specify the parameter values for the system (CA0, vo, etc.) and for the rate law to solve the coupled ODEs for either PFR, PBR, or batch reactors, or to solve the coupled algebraic equations for a CSTR.

    44. Liquid phase mole balance

    45. Algorithm - gas phase Gas phase For gas-phase reactions in which there is volume change, molar flow rate is the preferred variable. The total molar flow rate is given as the sum of the flow rate of the individual species. A mole balance on each species has to be specified.

    46. Gas phase mole balance

    48. Microreactors High surface area-to-volume ratio Reduce or eliminates heat and mass transfer resistances Shorter residence times & narrower residence time distributions Production of lab-on-a-chip, chemical sensors Assume PFR or in laminar flow

    49. Thermodynamically limited rxns Catalytic membrane reactors can be used to increase the yield of reactions that are highly reversible over the temperature range of interest. The membrane can either provide a barrier to certain components, while being permeable to others, prevent certain components such as particulates from contacting the catalyst, or contain reactive sites and be a catalyst in itself.

    50. Membrane reactors The membrane reactor is another technique for driving reversible reactions to the right in order to achieve very high conversion. These high conversions can be achieved by having one of the reaction products diffuse out of a semipermeable membrane surrounding the reacting mixtures. Two main types inert membrane reactor with catalyst pellets on the feed side (IMRCF) Catalytic membrane reactor (CMR)

    52. Startup of a CSTR Determine the time necessary to reach steady-state operation: Conversion does not have any meaning in the startup Use concentration rather than conversion as the variable in the balance equations.

    54. Semi-batch reactors When unwanted side reactions occur at high concentration of reactant B, or the reaction is highly exothermic. Examples of reactions: ammonolysis chlorination hydrolysis Reactive distillation: Carrying out the two operations, reaction and distallation in a single unit results in lower capital and operating costs. acetylation reaction esterfication reaction (remove water) A

    55. Semi-batch reactor Write the reactor equations in terms of concentration / numer of moles of each species Write the mass balance of the vessel Write the rate laws

    59. Recycle reactors They are used when the reaction is autocatalytic, or when it is necessary to maintain nearly isothermal operation of the reactor or to promote a certain selectivity. They are used extensively in bio-chemical operations. Two conversions: the overall conversion X0 and the conversion per pass Xs

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