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 ?
- test your knowledge by questions and
answers
in medias res: shortcut to questions and
answers ? (needs only knowledge !)
- search for the explanation of important
terms
searching for explanations
(alphabetical, with direct links)
- Exercises: solve special tasks and problems (using
partially the results of computer simulations with Agilent-VEE).
- exercises
(elaborated with HP/Agilent-VEE)
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
* Let us go ahead!
Do you know........?
-
(X) General Questions, kinetics, material
balances, RTD (1-24)
-
(X) Questions concerning the heat balance
(25-35)
-
(X) explanations for (alphabetical
searching)
-
(X) how to solve tasks and
problems
start of questions
* General Questions, kinetics,
material balances, RTD
- 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....
- What are the characteristics of the basic types of
ideal reactors?
The basic types
are?
- 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...
- How can an optimum of space time yield be reached?
This depends on the
....
- 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:...
- 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..
- Can you find out the necessity for concentration
controlling by merely regarding the 1/r(U) - plot ? Yes or no?
- 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
?
- 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:..
- Which information do we get from the the residence
time distribution?
We get
information on characteristics and quality of macro-mixing (contacting
pattern)!
- 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...
- What is the usefulness of reactor combinations
? Two
aspects:
- Do you know the 'area method' (to obtain conversion
from RTD and reaction kinetics)?
'Area method'
- Are you able to derive the 'functionality' of the RTD
of a CSTR from it's material balance?
The material balance reads:..
- Which possibilities exist generally for the reactor
design (for simple reactions with known reaction kinetics)?
List of possibilities:
..
- *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
- Can you explain the statement: 'Segregation inhibits
backmixing'
No ? ... but
you should!!!
- Can you enlighten the confusion about TFR and
Segregation ?
The ideal TFR
produces (by it's nature) a segregation effect, but....
- How important is this whole
'micro-mixing-stuff'?
Fairly you
have to admit.....
- Have you got clear for yourself with the terms
macro-mixing, micro-mixing, macrofluid, microfluid and segregation
?
'Overview'
- 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'
- 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?
- For the modelling of which reactors is the
Peclet-number (ax and rad) important ? - give an example for a simple
application ! 'the Peclet
number'
- Which laboratory reactor would you select for the
measurement of the reaction kinetics of a 'simple isothermal homogeneous fluid
reaction A + ... -> products' ?
- a CSTR
- a TFR
- a STR
- a differential loop reactor
Give your arguments for 'Why and when' !
see my
proposal
- * 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?
* Questions concerning the heat
balance:
- Can you write a (simple, no 'exotic' terms) 'general'
heat balance for chemical reactors? Not ? - see...
- In which cases can which terms be omitted in the
general heat balance ? You know
that, or ?
- What makes the simultaneous solution of heat and
material balance rather complicated ? that they are...
- What are the basic thermal operation modes of
chemical reactors ? there are 3
modes:...
- What about 'adiabatic' in the strict physical sense
and continuous reactors ? see here: ...
- What is the adiabatic temperature rise ?
that is...
- 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: ..
- Which possibility do you know for a simultaneous
graphical visualization and solution of heat and material balances in adiabatic
reactors ? a 'well-known' procedure
!
- Which possibility do you know alternatively for the
simultaneous solution of heat and material balances in adiabatic reactors
? the stepwise numerical
solution:
- *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
- 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,....
end of questions
Summary / Table: general searching for explanations
(alphabetical, with direct links to the 'matter')
click here
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
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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