Objective
To demonstrate the operation of the solid liquid extractor when configured for two stage continuous extraction in counter current flow, to determine the contribution of each stage in the extraction process and to compare the results with those obtained from the single stage exercise.
Method
By measuring the changes in conductivity and temperature of the streams of solvent at each stage of the process it is possible to find the percentage weight of extracted Potassium Bicarbonate (KHCO3) in the solvent streams. From this data the user can
determine the degree of extraction that is taking place at each stage.
Equipment Required
UOP4 MkII Solid Liquid Extraction Unit
Solid carrier - porous polymer pellets containing Potassium Bicarbonate (prepared in advance).
Stopwatch (not supplied)
Measuring cylinder, typically 250 ml (not supplied)
Optional Equipment
None
Theory/Background
When the system is configured as a two stage counter current flow extractor the fresh solid material and the fresh solvent are fed into the system from opposite ends of the process (refer to the flow diagram below). The solid material is moved through the system by the rotating cell and the solvent effectively flows through the material in the opposite direction being pumped to the sprinkler bars in the first and second stages.
Once the solid material has been processed it discharges into the extracted solids tank. The miscella containing the soluble component is collected in the final miscella tank.
In the counter current arrangement the most concentrated solids are processed by the least concentrated solvent and vice versa. This arrangement is used in industrial processing because it is generally more efficient than co-current operation. Similarly two-stage extraction is more efficient than single stage extraction.
Two stage continuous extraction is an open loop system and the rate of extraction will be dependent on variables such as the solvent temperature, the solvent flow rate, the solids feed rate, the amount of solute in the dry solid and the characteristics of the contact between the solid particles and the liquid.
Because the extraction is continuous the process will stabilise after an initial period however, transient changes as the process starts up and stabilises will be of interest as well as the readings when steady state is achieved.
The % weight of KHCO3 in the first miscella and final miscella can be determined
from measurements of temperature and conductivity of the appropriate miscella using the conversion graphs in Graphs of Concentration.
When choosing the operating parameters of an extraction process the main objectives are as follows:
a. To make the final miscella as concentrated as possible – to minimise the cost of further processing when recovering the soluble component from the
solvent.
b. To use the minimum amount of solvent – to reduce operating costs.
c. To lose the minimum amount of solvent – solvent retained in the solids after processing
d. To maximise product output – to process the required amount of product in the minimum time.
e. To extract the maximum percentage of the valuable soluble component from the feed – any soluble component remaining in the solids after processing is wasted.
f. To minimise the energy input to the process (operating at the lowest practicable temperature)– to reduce operating costs.
It is clear that all of these objectives are inter-related and that any attempt to
compare how a system performs with different combinations of settings would require a comprehensive series of tests to determine the individual effects of each
parameter. Such a test is suggested in the Project Work section.
The efficiency of the process can be assessed by considering the following parameters:
Concentration of the final miscella. Amount of soluble component extracted.
Amount of soluble component remaining in the solids after processing. Amount of solvent remaining in the solids after processing.
Energy required to perform the extraction.
Equipment Set Up
Configure the system for two stage continuous extraction as depicted in the flow diagram below. Water from the fresh solvent tank is pumped upwards through the first stage heater and sprinkled onto the surface of the solid material which has already been processed and moved beneath the sprinkler by the rotor. The first miscella draining from the bottom of the rotor is pumped upwards through the second stage heater and sprinkled onto the surface of the dry unprocessed solid material. The final miscella draining from the bottom of the rotor is collected in the final miscella tank. The flow of solvent and solid material is countercurrent.
Exercise D
Configuration for two stage continuous countercurrent extraction
Procedure
Start Up
Check that the UOP4 MkII is switched off, all switches are set to off and all rotary controls are set to minimum on the front panel of the console.
Fill the solvent tank with fresh water.
Fill collection trough one (right-hand side) to approximately 20mm depth by pouring water through the mesh at the base of the rotor.
Pour water into collection trough two (middle trough) until the water drains through the pipework into the final miscella tank.
Set the speed of pump one to maximum to purge air from the first stage of the process. When water drips from the first sprinkler bar stop the pump. Set the speed of pump two to maximum to purge air from the second stage of the process. When water drips from the second sprinkler bar stop the pump. After priming it may be necessary to top up the level of water in the first trough to return it to 20mm depth.
Note: the final trough will always run empty.
Once the system has been primed the two pumps can be switched on and adjusted to run at the desired solvent flow rate (typically setting 4.0 on the speed controls). The solvent stage system should now be taking liquid from the fresh solvent tank, cascading it through the two stages via the two pumps and troughs and then draining it into the final miscella tank.
After the system has been running for a while any imbalance in the stages will be indicated by a change in level in the first stage collection trough. If the level is falling reduce the speed of pump two. If the level is rising increase the speed of pump two.
Do not adjust the speed of pump one during the exercise. It will be necessary to check the level at regular intervals and to make small adjustments when required. Adjust the setpoint of controllers 1 and 2 to match the temperature indicated on the display (to avoid excessive overshoot when heating is required).
Start the rotor cell and adjust the speed control to give the required rotational speed (typically 5 on the speed control, approximately 2 revs per hour).
Put some untreated solid carrier in the hopper of the spiral material feeder.
Turn on the spiral material feeder and adjust the speed control to give the required depth of material in the cells of the rotor (typically 30 - 50% full).
When the solid material reaches the aperture in the base at the rear of the extractor it will be necessary to turn on the spray nozzle so that the material is washed into the extracted solids tank. Adjust the position of the nozzle and pressure regulator as required to prevent overspray.
Note: It may be necessary to refill the hopper with untreated solid carrier until the
system has stabilised and extraction can commence.
Allow the cells of the rotor to fill with untreated solid carrier. When the dry material passes beneath the sprinkler bars it may be necessary to make small adjustments to pump two to maintain the level in collection trough one.
Measure and record the temperature TS of the solvent direct from the fresh solvent tank indicated on the stage 1 temperature controller (T1) before the heater is switched on.
Once the system has settled down and the solvent stages are running in balance switch on heater ONE then adjust the set point on temperature controller ONE to 40oC. When T1 has stabilised switch on heater TWO then adjust the set point of temperature controller TWO to 40°C. Note that no heater should be switched on until there is a steady flow of liquid through the heater.
Monitor temperatures T1 and T2 (indicated on the appropriate temperature controller) and wait until the temperatures have stabilised. When the temperatures have
stabilised the extraction can commence as follows.
Extraction
Before commencing the extraction it will be necessary to organise recording of the required data. The following readings will be required to show the transient response as the system stabilises:
T1 Temperature of solvent entering the first cell of the rotor T2 Temperature of miscella entering the second cell of the rotor T4 Temperature of final miscella leaving the second cell
C1 Conductivity of solvent entering the first cell of the rotor (will remain constant)
Exercise D
C4 Conductivity of final miscella leaving the second cell
In manual operation it will be necessary to switch the multi function display on the console between C1, C2, C4 and T4 in order to take down these readings on a regular basis (typically once per minute) as the process progresses. T1 and T2 are indicated continuously on the appropriate temperature controllers.
Allow the hopper to empty of untreated solid carrier then fill the hopper with solid carrier containing the KHCO3 just as the untreated solid carrier runs out. Start the
stopwatch when the hopper is filled.
Note: It will be necessary to refill the hopper with treated solid carrier as the exercise
proceeds.
As the solvent extracts the Potassium Bicarbonate from solid carrier the outputs from conductivity probes C2 and C4 will rise and eventually stabilise. Once the system has reached a steady state the appropriate temperatures and the conductivities can be recorded.
Note: When the system has achieved steady state small cyclic variations will be
observed in the readings from conductivity probes C2 and C4 as the individual cells of the rotor present fresh solid material to the solvent stream. This effect can be observed more clearly when logging results continuously using the optional PC logging interface and software (Armfield product code UOP4MkII-303IFD).
Readings at steady state
Once the system has reached steady state the following measurements should be recorded:
Feeder speed setting NF %
Rotor speed setting NR %
Pump 1 speed setting N1 %
Pump 2 speed setting N2 %
Temperature of solvent at stage 1 T1 oC Temperature of solvent at stage 2 T2 oC
Temperature of final miscella T4 oC
Conductivity of solvent at stage 1 C1 mS Conductivity of solvent at stage 2 C2 mS Conductivity of final miscella C4 mS
Using a measuring cylinder and stopwatch collect a timed sample of the final miscella to determine the flowrate of solvent leaving the process (F4 litres/hr).
When shutting down the process switch off the heater before the solvent pump. If time permits the exercise can be repeated at different solvent flow rates, or solvent temperatures to investigate their effect on the rate of extraction and the efficiency of the process.
Similarly, the effect of the solid feed rate and/or the rotor speed can be investigated.
Presentation of Results and Analysis
From the measurements taken while the process was stabilising calculate the concentration of the KHCO3 in the first miscella and final miscella by converting the
readings of C2 –C1 with T2 and C4 – C1 with T4 into values of percentage weight KHCO3 (using the conversion graphs in Graphs of Concentration).
Plot the graph of percentage weight of extracted material in the first miscella and final miscella against time. Comment on the shape of the graphs as the process
stabilises. What is the contribution of each stage in the process?
Using the calibration graphs for the equipment where appropriate (refer to
Operational Procedures for details) convert the above measurements taken at steady state as follows:
Mass Flowrate of solid material (from NF using
calibration graph) MS kg/hr
Mass Flowrate of solvent entering the process
(from N1 using calibration graph) M1 kg/hr Mass Flowrate of final miscella (= F4 x density of
the solvent) M4 kg/hr
Loss of solvent in the process (= M1 – M4) ML kg/hr Concentration of KHCO3 in the solid (from
preparation data) ConcS %wt
Input of KHCO3 to the process (= MS x ConcS/100) MI kg/hr
Concentration of KHCO3 in the first miscella (from
C2 – C1 and T2 using graph) Conc2 kg/hr Concentration of KHCO3 in the final miscella (from
C4 – C1 and T4 using graph) Conc4 %wt
Output of KHCO3 from the process = M1 x
Conc4/100) MO kg/hr
Percentage of KHCO3 extracted from the solids (= 100 x
MO/MI) %
Energy input to stage 1 of the process (primarily heating of the solvent) (= 0.277 x M1 x (T1 – TS) x Cp) Note factor of 0.2777 for M1 in kg/hr and Cp in
Exercise D
kJ/kg/K
Energy input to stage 2 of the process (primarily heating of the solvent) (= 0.277 x M2 x (T2 – T4) x Cp)
Note factor of 0.2777 for M2 in kg/hr and Cp in kJ/kg/K. Note as temperature of fluid entering the second stage heater cannot be directly measured and T4 is taken to be equivalent temperature.
Q2 Watts
Total energy input to process (= Q1 + Q2) Qt Watts
Comment on the efficiency of the process by considering the magnitude of the calculated values obtained when the process has achieved steady state (the appropriate parameters are suggested in the Theory section above). Compare the results obtained with those for single stage extraction (Exercise C) and comment on the effect of the second stage in the process.
If results were taken under different operating conditions compare the results and comment on the effect of the changes on the efficiency of the process.