Evaporation of a NaCl solution
This lab experiment was adapted from the Climbing Film Evaporator Instruction Manual UOP1b.
This text was prepared as a guide for using a specific equipment. In this case: Climbing Film Evaporator from Armfield. However, the instructions are so general, that these can be used with other equipments.
Please, follow safety instructions for the start-up and operation the evaporator and boiler.
1 Object of the experiment
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- Estimate the economy of an evaporator based on steam and solvent vapor condensates.
- Concentrate a $2.5%\,wt$ NaCl solution.
- Heat consumption in the condenser.
2 Procedure
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The stream of solvent vapor condensate $L$ along with the steam condensate $W$ are to be measured to estimate the evaporator's economy along with the final concentration of the thicken NaCl.
Since experimental data obtain from steady conditions in the evaporator are to be compared with theoretical estimations, thicken solution flow rate must be measured as well.
You will need to prepare enough NaCl solution for feeding the evaporator for about 1 hour of operation.
2.1 Steps to follow
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Please, follow the steps below to complete this task,
- make sure there is water in the feed tank L1,
- start-up the boiler so that steam at $1.5\, bar\, man$ is available,
- start-up the evaporator following the instructions for it and make sure a feed of $f=10\,L/hr$ is set in the fow meter F2 (see this post on how to use a rotameter). Flow rate $F$ can be adjusted with valve C8 (ball valve V8 should be completely open),
- once steam stream $W$ and $F$ are ready, open the steam valve C10 to throttle so that the steam chamber gets pressurized. Remember that $p_W$ is to be $1.5\, bar\, man$,
- check for steam pressure in Bourdon manometer P2 and evaporated water stream $L$ temperature in probe T7. At the beginning pressure and temperature will vary. Wait some minutes until the process in the evaporator stabilizes so that pressure and temperature remain constant (steady conditions),
- once the system achieves its steady conditions, record: steam $W$ pressure and vapor $L$ temperature,
- measure the $L$ flow rate in condensate tank L2 , the $S$ flow rate in the concentrate tank L3 and the steam condensate $C$ from the steam trap,
- record the data for about 1 hour.
2.2 Experimental data to be recorded
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For short, only flow rates at streams $L$, $C$ and $S$ are required to give the economy of the evaporator and other further comparisons with theoretical estimations. Absolute pressure $p_W$ and temperatures $T_6$ and $T_7$ are also recorded for check on steady conditions.
For the condenser calculations, temperature and flow rates will also be required.
You should create a spreadsheet to record the flow rate, pressure and temperature data. You should check that flow rate remains constant during the 1 hour operation of the experiment.
For the final concentration of the thickened NaCl solution you can built a caliration curve of solution density versus concentration. Please, visit this site for reference to these measurements. A digital scale will be needed.
2.3 Theoretical background
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The evaporator economy is simply defined as,
$Economy=\dfrac{Mass\, of\, solvent\, vapor}{Mass\, of\, steam}$ Eq. (01)
Being, both flow rates $L$ and $C$ in the same units, Eq. (01) becomes,
$Economy=\dfrac{L}{C}$ Eq. (02)
On the other hand, the steam consumption is defined as,
$Steam\consumption=Steam\, mass\, flow\, rate$ Eq. (03)
As you can see, the flow rates $L$ and $C$ are at all time required.
2.3.1 Mass and energy balances for estimations
Estimations or predictions of some parameters can be done and later compared with what was found experimentally. For this purpose, you will consider as inputs:
- the initial NaCl solution concentration $x_F$,
- the NaCl solution feed flow rate $F$,
- the absolute pressure of the heating media (steam), which is the one you fixed on the manometer,
- the temperature of $F$,
- the flow rate of steam $W$,
- the temperatures of the streams;
Based on the experimentally determined concentration $x_S$ of NaCl in $S$, make a mass balance using your known $F$ and $x_F$ and see if the experimental and theoretical flow rates of $S$ check.
2.4 How to perform the calculations
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Equations (02,03) need to be used with average flow rates. For validation, recall that single stage evaporators usually have an $economy$ smaller than 1. In other words $L<C$. As a final comment, you should notice that the density of the condensate is required to go from volumetric flow rate to mass flow rate.
Since, your calculations are to be performed in a spreadsheet, scientific notation is recommended for numbers smaller than $0.01$ and larger $1,000,000.00$. Traditional representation with two decimals can be used for all other numbers. You can use up to 4 decimal positions in scientific notation is required.
3 Your results
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For the sake of simplicity only two data are required from this experiment:
- the economy of the evaporator,
- the steam consumption,
- energy consumption in the condenser,
- a table with comparison of experimental and theoretical $L$ and $S$.
This is the end of the post. I hope you find it useful.
Other stuff of interest
- LE01 - AC and DC voltage measurement and continuity test
- LE 02 - Start and stop push button installation 24V DC
- LE 03 - Turn on/off an 24V DC pilot light with a push button
- LE 04 - Latch contact with encapsulated relay for turning on/off an AC bulb light
- LE 05 - Emergency stop button installation
- About PID controllers
- Ways to control a process
- About pilot lights
- Solving the Colebrook equation
- Example #01: single stage chemical evaporator
- Example #02: single stage process plant evaporator
- Example #03: single stage chemical evaporator
- Example #04: triple effect chemical evaporator
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Ildebrando.
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