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Reactor Design and Safety Requirements

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Information about reactor design

Fluidized-bed reactors

·        Simplified modeling  

Bioreactors

Safety considerations

Simplified cost estimating 

When a reactor is either heated or cooled, cost it as the sum of the cost of a heat exchanger plus that of a pressure vessel.  For a plug flow reactor, calculate the area from the tube diameter, length and numbers.  For a fluidized bed reactor use the Q and Ts to calculate the area as for other heat exchangers.  For a stirred-tank reactor, add in the cost of stirring equipment. 

Modeling using HYSYS or UniSim

Warning about Aspen HYSYS 2006

Many of our students have encountered problems while modeling heterogeneous plug-flow reactors in HYSYS 2006 using the NRTL properties.  These problems may be peculiar to the academic version we have installed.   Since Aspen no long provides assistance to academic licensees, you’re on your own if you have similar problems.  I’d be interested in hearing the experience of other people.  Following are the problems our students have had, and how we have worked around them:

Types of reactors in HYSYS & UniSim:  Refer to the Reactor sections of the AspenHYSYSOperationsGuide.pdf documentation included with HYSYS and probably to be found at C:\Program Files\AspenTech\Aspen HYSYS 2004.2\Documentation\.  HYSYS has six types of idealized reactor models built in: conversion, equilibrium, Gibbs, yield shift, plug flow, and continuous stirred tank (CSTR).   The first four do not require reaction kinetics, but provide no information about the size of the reactor and, therefore, its cost.  The plug flow reactor and CSTR modules both produce size information, but require that at least the forward reaction kinetics be known. 

When entering the reaction kinetics in the basis environment, note that, in general, the reaction orders of the components are NOT the same as their stoichiometric coefficients.  Use the powers on the basis (concentration, mole fraction, etc.) for the actual reaction kinetics specified or taken from the literature.  For example if the rate is proportional to CA0.5 then the basis is concentration and the order for component A is 0.5.   If you are given only forward reaction kinetics and suspect that the reaction may be reversible, you can check using the method at calculation of equilibrium constants and reverse reaction kinetics.  (Derivation of expression for reverse reaction rate.)

The reactions and the percent conversion of a component must be given.  The conversion can be specified as a function of temperature.

The reactions must be specified and their equilibrium constants either given or calculated by HYSYS using the Gibbs free energy of the reaction at standard conditions.

The products are given, without specifying the reactions that would give those products.  HYSYS then calculates the equilibrium composition of the product stream by minimizing its Gibbs free energy, constrained by conservation of atomic species using the composition of the feed stream.  From the HYSYS documentation, “As with the Equilibrium Reactor, neither pure components nor the reaction mixture are assumed to behave ideally.”

From the HYSYS documentation, “The Yield Shift reactor unit operation supports efficient modeling of reactors by using data tables to perform shift calculations.  The operation can be used for complex reactors where no model is available, or where models that are too computationally expensive.”  This is beyond the scope of an introductory chemical engineering design course.

A packed bed catalyst will be inside the tubes, with a heat-transfer medium flowing co-current outside the tubes.  You should set only the inlet T and P of the process stream; HYSYS will calculate the outlet T and P.  (The outlet pressure must be less than the inlet pressure.)  Recommended Design Parameters: Ergun, Cooling by formula.  Design Heat Transfer: SS duty by formula, Heat medium (for an endothermic reaction; coolant for exothermic) given the wall heat transfer coefficient, Mole flow of heat-transfer medium, Heat capacity of the medium, and its Inlet T.  The heat-transfer flow and inlet T should be adjusted to give reasonable operation.  Usually the Tube side (catalyst) heat transfer will be specified.  If there is a hot spot near the beginning of an exothermic reactor, set the Reactions Overall to Re-init.  If the pressure drop is too large, increase the reactor volume without increasing the tube length (i.e., add more tubes to decrease the stream velocity).  Record the Q and the coolant temperatures in and out for use later.  See the reference to Ulrich and Vasudevan above for typical values of operating parameters.

·        Procedure for simulating cooling of a plug-flow reactor with a boiling liquid (Similar for heating with condensing steam)

·        How to make a reactor always isothermal, calculate yield, and generate a case study with two independent variables

 

Last revised February 5, 2013.  Please submit all questions, comments and suggestions to W.R. Wilcox

 

Disclaimer: The material on these pages is intended for instructional purposes by Clarkson University students only.  Neither Clarkson University nor Professor Wilcox is responsible for problems caused by using this information.

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