Reactive Distillation Computations Including Heat Effects

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Consider a ternary mixture of components , , and with relative volatilities and . This mixture is subject to an exothermic equilibrium-limited chemical reaction with reaction rate , where the equilibrium constant .

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The mixture is fed into a reactive distillation column with 14 total plates. The feed stage location is stage number 5, the reactive stages are from stage 2 to stage 7, and the feed composition is equimolar in and (i.e., the feed is composed of 40 mole% , 40 mole% , and 20 mole% ). The feed flow rate is chosen as 100 kmol/hr.

Heat effects are included in the computation through the use of the negative dimensionless ratio, where and are the negative heat of reaction and the heat of vaporization, respectively (both expressed in kJ/kmol). One recovers the no-heat effects case by setting .

This Demonstration shows two profiles: (1) the composition versus plate number for components , , and (in red, blue, and green, respectively), with the reactive zone shown in light blue, and (2) a ternary diagram where the composition of versus the composition of is in mole%. In the second profile, the feed composition is shown by a magenta dot and the reactive stages (stages 2 to 7) are displayed in blue. The Demonstration also gives the nonconstant vapor and liquid flow rates in all stages. These flow rates obey the following relations, which are derived from global material and energy balances:

where ,

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You can set the values of the Damköhler number, , the reflux ratio, and the reboil ratio, where is the plate molar hold-up, is the feed flow rate, and the reaction rate constant. When , one recovers the case where no reaction is taking place. If is very large, the simulation represents a situation close to reaction equilibrium (i.e., ). The Damköhler number is the ratio of the characteristic residence time to the characteristic reaction time.

One snapshot shows a case where the reflux ratio is very high, a situation close to the total reflux operation. For such a situation, the bottom is almost pure and the distillate contains very little . The distillation flow is very small and the distillate stream can be recycled if desired. Thus, for this specific operation, one can produce , if this is the desired reaction product, and convert both reactants and with a single piece of equipment.

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Contributed by: Housam Binous, Selmi Manel, Ines Wada, and Ahmed Bellagi (March 2011)
Open content licensed under CC BY-NC-SA


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Details

For more information, see

M. F. Doherty and M. F. Malone, Conceptual Design of Distillation Systems, New York: McGraw–Hill, 2001.

S. Melles, J. Grievink, and S. M. Schrans, "Optimization of the Conceptual Design of Reactive Distillation Columns," Chemical Engineering Sciences, 55(11), 2000 pp. 2089–2097.



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