Experimental Design, Planning

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Experimental Design, Planning Preparation
EXPT_DES.ppt
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Context

• This is designed to help you get to grips with
  the planning preparation process of the
  experiments.
• You need to take the responsibility for following
  this advice through acting on it.
• It makes sense to follow a concrete example
  thus the leaching experiment will be discussed
  later (Please note this should not be
  interpreted as extra help for the groups doing
  leaching, but is merely providing a small
  supplement to the lab notes)

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KEY CONCEPTUAL STEPS some suggestions
You should follow these conceptual steps for all
the experiments Concrete example for leaching
will be presented shortly.

• Simplified diagram of process (capture
  essentials)
• What is the basic aim ?
• Reality Check - what are the essential features
  of the rig?
• Key parameters and their typical values and
  ranges
• (i) eg what range of flowrates, etc
• (ii) Variables which need to be measured?
  which are to
• be varied?
• - ie what will I measure, what will I
  change ?
• - Note not all variables are key

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KEY CONCEPTUAL STEPS some suggestions
(continued)

• Which key variables to vary first ?
• - typically keep the others constant
• - ie set up your plan of attack
• Trade-offs choose a plan of attack that will
  give you maximum benefit and use of the rig
  during your limited time there
• Calculations - have Excel spreadsheets etc ready
  to use during the actual lab sessions, so you can
  enter data and perform checking calcs during the
  run ie check data quality eg mass and energy
  balances etc

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Example Leaching Experiment

• Put some slurry feed in (slurry feedrate PVC
  solids dye water), put some water in (water
  flowrate), mix them up, allow the slurry to
  settle, extract the settled slurry (underflow
  rate PVC solids less dye water), allow the
  supernatant water dye to overflow.

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Simplified diagram of process (capture
essentials)
Slurry water dye PVC solids
Overflow water dye
Water
Mixer
Settler
Underflow water less dye PVC solids
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Basic Aim

• Maximize a profit function that accounts for
• the ease of processing the overflow (e.g., when
  extracting gold, etc., higher concentrations are
  easiest to process downstream of leaching), and
• the total amount of dye (analogous to gold, etc,
  in minerals processing) in the overflow, and
• the environmental burden of having dye in the
  unextracted (underflow) solids.

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Reality Check

• There are three mixer/settlers.
• They are operated countercurrently.
• This countercurrent configuration gives higher
  dye concentrations than a single mixer/settler
  unit.
• However, the basic idea of three mixer/settlers
  is the same as just one mixer/settler, so we can
  represent the three units here (schematically) as
  just one unit.

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Where Do I Start?

• You are responsible for finding out, from Mr
  Orellana and/or the laboratory briefing notes and
  /or the academic staff members (Professor Jim
  Petrie and/or Dr Gomes) and/or the tutor, what
  the likely feedrate of slurry is, what range of
  inlet water flowrates what range of underflow
  flowrates are possible.

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Second Steps

• You are responsible for finding out working
  out, from these sources, what variables need to
  be measured, what variables are to be varied.
• You must decide what considerations are required
  to set the slurry feedrate, the water flowrate
  the underflow rate.

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Do Not Panic

• I will take you through some of the thought
  processes now.
• The same types of thought processes apply for
  other experiments.

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• Measurements
• concentrations of dye slurry, to assess mass
  balances and work out costs profits
• flowrates to assess mass balances
• You can change
• slurry feedrate
• water flowrate (the main variable)
• mixer speeds
• underflow rates can also be changed.

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Not the Key Variables

• The mixing rate in the mixers is not really a key
  variable it just needs to be at an adequate rate
  to mix the water and the slurry.
• The underflow rate is more important, but again
  it is not the key variable all the solids must
  go through the underflow, the pipes must not
  block, as little dye as possible should go
  through it.

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Key Variables

• Water flowrate slurry feedrate.
• The ratio of these two rates is important the
  same overflow dye concentration will result from
• a slurry feedrate of 1 L/min, a water flowrate of
  0.1 L/min an underflow rate of 1 L/min as from
• a slurry feedrate of 2 L/min, a water flowrate of
  0.2 L/min an underflow rate of 2 L/min

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Focus on What You Can Do

• It is easiest to vary the water flowrate.
• Hence keep the slurry feedrate constant, vary the
  water flowrate.
• Now we will discuss how to decide on
• the underflow rate (same from each mixer settler)
• the slurry feedrate
• the water flowrate

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Example How Do I Decide on the Underflow Rate?

• Look at the equipment, use common sense.
• Slurry flows through the pipes.
• The total amount of solids flowing through the
  pipe is fixed at steady state to be the same as
  the amount of solids in the feed flow.
• A high underflow rate means a low underflow
  concentration vice versa.

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So What?

• A low underflow concentration means that a lot of
  water dye gets taken out with the underflow.
• This situation is undesirable because we want as
  much dye in the overflow, because the overflow is
  not a messy slurry, it is a pure liquid
• Pure liquids are easier to process than slurries.

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So We Want a High Underflow Concentration?

• Yes, this means a low underflow rate.
• However, a high underflow concentration is likely
  to block the pipes, resulting in the experiment
  stopping, so there is an upper limit to the
  concentration that can be used.
• This situation illustrates the common need to
  trade off one aspect against another.

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How Do I Resolve the Trade-off Here?

• Before the laboratory guess that the same
  concentration of solids in the underflow as in
  the feed slurry will probably not block the
  underflow pipes if it does not block the feed
  pipes.
• The feed slurry concentration has been set so as
  not to block the feed pipes.

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During the Experiment

• Adjust the underflow rate so that the underflow
  concentration (inversely proportional to the
  flowrate, by mass balance, since the same solids
  flowrate is involved) does not block the pipes.
• You will be able to see plugs of solid starting
  to form when you are approaching the point when
  concentrations are too high.
• Be prepared, during the experiment, to adjust the
  water flowrate to reflect any changes in the
  underflow rate.

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Example How Do I Decide on the Feed Flowrate?

• More trade-offs.
• A faster feedrate means a lower residence time,
  so the equipment will come to steady state more
  quickly, which is good because the experiment
  will be over quickly.

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Trade-off

• However, a faster feedrate means that you have
  less time to take all your measurements. It also
  means that the finite volume of slurry (100
  litres approximately, you are responsible for
  checking this value with Mr Orellana) will be
  used up more quickly.

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How to Resolve the Trade-off

• You should not choose a feedrate that results in
  the material being used up too quickly - if you
  get into this situation, it is an indication of
  your poor planning is likely to result in
  marking penalty.
• It is possible to select an appropriate feedrate
  we have not set up the experiments for you to
  fail.

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Example How Do I Decide on the Water Flowrate?

• This is where the chemical engineering science
  really starts.
• Mass balances on the total flowrates, the solids
  the dye can be done over each mixer/settler.
• Assume that the dye concentration in the water in
  the overflow is the same as that in the underflow
  (Why? Prof Petrie may ask you to explain).

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• Using mass balances, with
• a known slurry feedrate concentration
• a known underflow rate
• a particular water flowrate
• the assumption of equal dye concentrations in
  overflow and underflow
• then mass balances will tell you the dye
  concentration in the overflow, the overflow
  underflow rates
• so you should be able to determine the profit
  (which depends on the dye concentration in the
  overflow, the overflow underflow rates)

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Implementation

• On a spreadsheet, MS Excel.
• Start with one mixer/settler.
• When this calculation is satisfactory sensible,
  extend it to three units.
• Keep the slurry feedrate, underflow rate, feed
  concentration constant, vary the water flowrate,
  calculate the profit.

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So What?

• Plot profit as a function of water flowrate
  there should be a maximum point.
• This is the point of the calculation determining
  the water flowrate that results in the profit
  being maximized.

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Adjustments During the Experiment

• Have the spreadsheet on a PC somewhere nearby, in
  PC lab when you do the experiment so that you can
  (re) optimize water flowrate if the slurry
  feedrate changes during the time of the
  experiment, the feed concentration is different
  or you change the underflow rate.
• We expect you to adapt in this way to changing
  circumstances.

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Data Quality Checks

• Have another spreadsheet for mass balances checks
  (overall, solids, dye).
• Be prepared for data reconciliation adjust data
  within range of experimental uncertainties so
  that mass balances are satisfied.

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Example of Calculation

• One mixer/settler

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Slurry water 35 ppm dye 20 PVC
solids Total flowrate 1 L/min
Overflow water dye
Water, 0.1 L/min
Mixer
Settler
The values on this and the next few slides are
just to illustrate the type of calculations
required - students actually doing the leaching
experiment should confirm all their own values
Underflow water less dye 25 PVC solids
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Calculations

• Solids mass balance 20 solids in, total
  flowrate of 1 L/min in, 25 solids out, underflow
  rate 0.2 1 L/min / 0.25 0.8 L/min
• Total flowrate mass balance overflow rate out
  1 L/min slurry in 0.1 L/min water in - 0.8
  L/min underflow out 0.3 L/min

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Dye Concentration

• Dye balance 35 ppm in with 0.9 L/min of water (1
  L/min - 20 solids v/v 0.8 L/min 0.1 L/min
  water added) all this water flows out in
  underflow overflow dye concentration in
  overflow underflow 35 ppm in 0.8 L/min feed
  is diluted to 0.9 L/min water out, so
  concentration is 350.8/0.9 31 ppm.
• Flowrate of dye in overflow concentration
  overflow rate

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Other Experiments?

• I suggest that you use a process of analogy.
• For example, with the spiral heat exchanger,
  again, what can you vary, what do you need to
  measure, what measurements do you need to take
  and when, what are you going to do with these
  measurements?

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Conclusion

• This is typical of the thought processes required
  for good preparation.