Exercises

Chemical reaction engineering

Questions and Answers

Supplement to my
online-course
(German)

german version

Introduction

The intention of this set of questions and answers is to provide a test for the students who visited the online-course chemical reaction engineering mentioned above (German). It should also be a catalogue of tests for all students of chemistry and chemical engineering in general. The further intention is to explain basic items when necessary. This will be realized by simple texts and visual information as for example graphs, images, computer aided and visualized calculations and results of simulations. For those who want to know how my simulations have been made: some general information.

What can you do ?

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Miscellaneous topics:

recommended links

*Important abbreviations and terms see: bottom (for thorough persons)

remarks of the author (for inquisitive persons)

look at a computer image of a plant (321.5 k, for persons interested in computer arts)

as soon as possible:
pdf-files: last pdf-version (xx.xx.07, z MB total) for download:
part1: all topics without blackboard sketches concerning heat balances (z MB)
part2: only blackboard sketches concerning heat balances (z MB)
(for persons who always need printed matters)

last date of changes: yy.yy.07

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* Let us go ahead!

Do you know........?

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start of questions

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* General Questions, kinetics, material balances, RTD

  1. Why is it useful to discuss chemical aspects of reaction engineering without dealing with the 'reactor design' in a very specific detail ? - a more introductory question!
    The chemical aspects....

  2. What are the characteristics of the basic types of ideal reactors?
    The basic types are?

  3. When you are exclusively regarding the efficiency of a reactor, without regarding temperature and catalytic effects, which quantity has to be optimized?
    The space time yield...

  4. How can an optimum of space time yield be reached?
    This depends on the ....

  5. Can you explain the statement that for simple reactions the optimal space time yield can be reached with choosing a maximum conversion and a not-back-mixed type of reactor, as for example the TFR ?
    There are two possibilities to explain it:...

  6. Do you know the plot 1/(r(U)) versus U and the possibilities for visualization of various aspects in this plot?
    If you start from the material balance of a reactor..

  7. Can you find out the necessity for concentration controlling by merely regarding the 1/r(U) - plot ? Yes or no?

  8. Can you give some examples for reactions where it is necessary to control concentration levels ( not merely optimization at a maximum conversion) when dealing with a reaction engineering optimization ? yes, of course, - or not ?

  9. In spite of the fact that the 'not back-mixed' reactor types are frequently the best choice for the optimization by control of concentration levels too, there are some rare cases where this statement is incorrect. Can you give examples ? Not very much, but:..

  10. Which information do we get from the the residence time distribution?
    We get information on characteristics and quality of macro-mixing (contacting pattern)!

  11. Which residence time information do you receive when using a delta-shaped Dirac pulse of the tracer, which when using a step-shaped function?
    The Dirac pulse...

  12. What is the usefulness of reactor combinations ? Two aspects:

  13. Do you know the 'area method' (to obtain conversion from RTD and reaction kinetics)?
    'Area method'

  14. Are you able to derive the 'functionality' of the RTD of a CSTR from it's material balance?
    The material balance reads:..

  15. Which possibilities exist generally for the reactor design (for simple reactions with known reaction kinetics)?
    List of possibilities: ..

  16. *Can you explain why the 'product outlet concentration' for a 2nd order reaction in any reactor type is higher for a segregated fluid than for a 'normal' fluid (molecular dispersed)
    2nd order segregated

  17. Can you explain the statement: 'Segregation inhibits backmixing'
    No ? ... but you should!!!

  18. Can you enlighten the confusion about TFR and Segregation ?
    The ideal TFR produces (by it's nature) a segregation effect, but....

  19. How important is this whole 'micro-mixing-stuff'?
    Fairly you have to admit.....

  20. Have you got clear for yourself with the terms macro-mixing, micro-mixing, macrofluid, microfluid and segregation ?
    'Overview'

  21. What do you know about the model of axial dispersion, is there a relation between the Bo-number and the number of vessels in a 'cascade' model ? 'Axial dispersion'

  22. How can you explain qualitatively that the degree of back-mixing drops down with a mounting number of vessels in a cascade and the properties approach to those of an ideal TFR ? help?

  23. For the modelling of which reactors is the Peclet-number (ax and rad) important ? - give an example for a simple application ! 'the Peclet number'

  24. Which laboratory reactor would you select for the measurement of the reaction kinetics of a 'simple isothermal homogeneous fluid reaction A + ... -> products' ?
    1. a CSTR
    2. a TFR
    3. a STR
    4. a differential loop reactor
    Give your arguments for 'Why and when' !
    see my proposal

  25. * When you classify reactions into three categories: very fast reactions, fast reactions and normal reactions, - you know the possibilities of Physical Chemistry for measuring reaction kinetics of very fast reactions and you also know the laboratory procedures of measuring 'normal' batch kinetics by following the concentration course with time. But do you know a possibility for measuring fast reactions with 'lifetimes' shorter than the response times of your 'normal' lab equipment, e.g. glass electrodes, potentiometric electrodes etc., - without skipping over to the 'extreme' physicochemical methods ? Take the neutralization reactions of some organic acid or base as an example, - their 'lifetime' is longer than that of inorganic acid base reactions, 'but fast enough' that you can not follow the concentration course with a glass electrode in a batch reactor. How would you measure the kinetics of such a reaction, when you are obliged to take a 'slow glass electrode' as sensor, i.e. 'you are not allowed' to apply a faster measuring device !
    you could give an answer, - or not?

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    * Questions concerning the heat balance:

  26. Can you write a (simple, no 'exotic' terms) 'general' heat balance for chemical reactors? Not ? - see...

  27. In which cases can which terms be omitted in the general heat balance ? You know that, or ?

  28. What makes the simultaneous solution of heat and material balance rather complicated ? that they are...

  29. What are the basic thermal operation modes of chemical reactors ? there are 3 modes:...

  30. What about 'adiabatic' in the strict physical sense and continuous reactors ? see here: ...

  31. What is the adiabatic temperature rise ? that is...

  32. Where does the heat of (an exothermal) reaction go to in an adiabatic STR and where in an adiabatic ideal CSTR ? Clear ? (Why is then the prompt answer so rare in colloquies ??) not ? - see here: ..

  33. Which possibility do you know for a simultaneous graphical visualization and solution of heat and material balances in adiabatic reactors ? a 'well-known' procedure !

  34. Which possibility do you know alternatively for the simultaneous solution of heat and material balances in adiabatic reactors ? the stepwise numerical solution:

  35. *What do you fundamentally know on the subject 'stability of thermal operations in continuous stirred tank reactors (CSTRs)' ? 'stability diagram' ; additionally there exists a special manner for the visualization in case of CSTR's

  36. Which forms of 'heat production curves' (as mentioned above) do you know ?
    The basic types of reactions are: endothermal, exothermal, reversible, irreversible, single step, multi step,....



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end of questions

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Summary / Table: general searching for explanations (alphabetical, with direct links to the 'matter')

click here

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Remarks of the author: All VEE applications are my own development, you may use them for your training as well as for applications in education. Errors may occur, please let me know when you found something. When 'investigating' you will take different paths, therefore take the browser's back to get to your previous file. For finding back to the 'main page' (faq1e.htm) I have added 'rescuing' back links. A general recommendation: when you encounter a link during reading a text, you should perhaps better finish the 'text environment' before 'linking on', because I believe that I have a bit a confusing manner in putting links in my texts, I apologize for that.

Ernst Bratz


author's advice at all events

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Recommended links to further (online-)simulations concerning reaction engineering (partially german and english, - very comprehensive/useful)

Freeware program KinDis v1.0 of Dr. G. Rexwinkel (NL), a program for the simulation of reactions in a continuous flow reactor, the degree of macromixing can be selected from plugflow to CSTR by an axial dispersion model http://www.chemspy.com/downloads.html

Virtual practical course for 'Technische Chemie'http://techni.tachemie.uni-leipzig.de/vipratech/index_ge.html

Recommended link for basic information on chemical reaction kinetics:
Softwareprojekt Physikalische Chemie/Prozessgrundlagen (Kinetik) Prof. Dr. Gerd Hradetzky
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Important abbreviations and terms:

STR = stirred tank reactor (Absatzweiser Rührkessel)

CSTR = continuous stirred tank reactor (Kontinuierlicher Durchflußrührkessel)

TFR = tubular flow reactor (Rohrreaktor)

RTD = residence time distribution (Verweilzeitspektrum)

U = conversion (Umsatz)

r = reaction rate (Reaktionsgeschwindigkeit)

back-mixed (rückvermischt)

stationary in space and time (raumzeitstationär)

not stationary in space and time (nicht raumzeitstationär)

conducting a process by controlling concentration levels (Konzentrationsführung)

conversion orientated optimization (umsatzorientierte Optimierung)

conducting a process isothermal, adiabatic, polytropic (isotherme, adiabatische, polytrope Reaktionsführung)

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