ABSTRACTThe liquid-liquid extraction process is one of the most
common technique to separate compounds based on their solution
preferences for two different immiscible liquids, usually water and
an organic solvent as in our experiment, a mixture of different
phases of materials (light and heavy phases) that have different
physical and chemical properties are to be separated by
liquid-liquid extraction method. Our focus in this experiment is to
examine how the change in flow rate can affect the whole extraction
process efficiency taking into account the variables that are
involved in this process including concentration of liquid,
temperature and pressure inside the column. We can see how great
impact the change in flow rate can make through the result.For this
experiment, two phases materials were used namely water as a heavy
phase material and hexane as a light phase material with the
presence of iodine as the solute dissolving in both solvents. From
the results obtained, as the flow rate of the heavy phase was
increased from 10 L/min to 20 L/min, the raffinate composition was
maintained at 0.4 0.5 g/L. Whereas the extract composition had a
slight fluctuation from 55.8 g/L to 52.9 g/L and finally 57.0
g/L.
Table of Contents
ABSTRACT11.0INTRODUCTION31.1Background of
Experiment31.2Objective of Experiment41.3Scope of
Experiment42.0LITERATURE STUDY43.0METHODOLOGY93.1Equipment and
Materials93.2Experimental Procedure94.0RESULTS AND
DISCUSSIONS114.1Experimental
Data114.2Discussion124.3Questions125.0CONCLUSION146.0RECOMMENDATIONS146.1Errors146.2Recommendations147.0REFERENCES158.0APPENDICES15
1.0 INTRODUCTION
1.1 Background of Experiment
Liquid/liquidextraction,alsoknownassolventextractionandpartitioning,isamethodto
separatecompoundsbasedontheirrelativesolubilityintwodifferent
immiscible liquids , usually waterand an organic solvent (hexane).
It is an extraction of a substance from
oneliquidphaseintoanotherliquidphase.Liquid/liquidextractionisabasictechnique
in chemicallaboratories, where it is performed using a separator
funnel. This type of process is commonly performed after a chemical
reaction aspart of the work-up.In other words, this is the
separation of a substance from a mixture by preferentially
dissolving that substance in a suitable solvent. By this process a
soluble compound is usually separated from an insoluble compound.
The basicprinciple behind extraction involvesthe contactingof
asolution withanothersolvent that is immiscible with the original.
The solvent is also soluble with a specific solute contained in the
solution. Two phases are formed after the addition of the solvent,
due to the differences in densities. The solvent is chosen so that
the solute in the solution has more affinity toward the added
solvent. Therefore mass transfer of the solute from the solution to
the solvent occurs. Further separation of the extracted solute and
the solvent will be necessary. However, these separation costs may
be desirable in contrast to distillation and other separation
processes for situations where extraction is applicable .A general
extraction column has two input stream and two output streams. The
input streams consist of a solution feed at the top containing the
solute to be extracted and a solvent feed at the bottom which
extracts the solute from the solution. The solvent containing the
extracted solute leaves the top of the column and is referred to as
the extract stream. The solution exits the bottom of the column
containing only small amounts of solute and is known as the
raffinate. Further separation of the output streams may be required
through other separationprocesses.
1.2 Objective of Experiment
The objective of this experiment is to observe the effect of
change in flow rate towards the performance of the extraction
process.
1.3 Scope of Experiment
In this experiment, iodine is separated from hexane by using
water as solvent. The flow rate changes factor is studied to obtain
the relationship between the efficiency of extraction process
liquids. Relations can be obtained by comparing the time for the
liquid-liquid extraction reached equilibrium at different liquid
flow rates.
2.0LITERATURE STUDYLiquid-Liquid Extraction is the process of
extracting a solute from a feed by use of a solvent to produce an
extract and a raffinate. In its simplest form, it may take the
guise of a single stage mixing and separation unit analogous to a
single stage flash in distillation. The choice of solvent is
critical in effecting a liquid-liquid extraction. Factors affecting
the choice are summarized below. It is usually necessary to
compromise in one area or another. As in distillation it is
frequently impossible to achieve the separation required by use of
a single stage unit, and a multistage unit is required. In
liquid-liquid extraction, two phases must be brought into contact
to permit transfer of material and then be separated. Extraction
equipment may be operated batch wise or continuous .The extract is
the layer of solvent plus extracted solute and the raffinate is the
layer from which solute has been removed. The extract may be
lighter or heavier than the raffinate, and so the extract may be
shown coming from top of the equipment in some cases and from the
bottom in others. The operation may of course be repeated if more
than one contact is required, but when the quantities involved are
large and several contacts are needed, continuous flow becomes
economical.
In dilute solutions at equilibrium, the concentration of the
solute in the two phases is called the distribution coefficient or
distribution constant K. K=Y/X Where the Y and X are the
concentrations of the solute in the extract and the raffinate
phases respectively. The distribution coefficient can also be given
as the weight fraction of the solute in the two phases in
equilibrium contact: K= y*/x, where y* is the weight fraction of
the solute in the extract and x is the weight fraction of the
solute in the raffinate .The rate at which a soluble component is
transferred from one solvent to another will be dependent, among
other things, on the area of the interface between the two
immiscible liquids. Therefore it is very advantageous for this
interface to be formed by droplets and films, the situation being
analogous to that existing in packed distillation columns. A
single-stage extractor can be represented as:
Figure 1: Single-stage ExtractorWhere; F = Feed quantity / rate,
mass R = Raffinate quantity / rate, mass S = Solvent quantity /
rate, mass E = Extract quantity / rate, mass
Xf, Xr, Ys, and Ye are the weight fractions of solute in the
feed, raffinate, solvent and extract, respectively. Partition
coefficient m is defined as the ratio of Ye to Xr at equilibrium
conditions. The flows and concentrations are represented in
solute-free basis as such are presentation leads to simplification
of equations. For example, for a 100 kg/hr feed containing 10%
weight acetic acid, F = 100-10 = 90 kg/hr, Xr = 0.1/ (1-0.1) =
0.111.The component mass balance can be represented as: F Xf + S Ys
= R Xr + E Ye.Assuming (i) immiscibility of feed and solvent and
(ii) the initial solvent is free of solute, i.e., F = R, S = E and
Ys = 0 and using the equilibrium relation of Ye = m Xr, this
equation simplifies to S = F/m (Xf /Xr 1) or reduction ratio, Xf
/Xr = 1+ m S/F.
The choice of Solvent is influenced by many factors some of
which are listed below:a) High Selectivity: The ability of a
solvent to extract a component or class of components in preference
to others. This factor will determine the number of extraction
stages required.b) Distribution or Partition Coefficient:The ratio
of the solubility of the solute in the solvent compared to the
feed. This factor will affect the selectivity and the amount of
solvent phase required.c) Density:The greater the density
difference between the feed and the solvent the easier it will be
to obtain phase separation.d) Viscosity:A high viscosity will
inhibit both mass transfer and separation of the phases. A low
viscosity (say less than 10 cP) is desirable.e) Interfacial
Tension: This affects the settling, coalescence and mass transfer
coefficient of a system. Coalescence and settling are generally
aided by high interfacial tension whilst mass transfer is
hindered.f) Volatility: The solvent is likely to need to be
separated from the solute and/or the feed. If this is to be done by
distillation the volatility should, where possible, be chosen to
allow this separation to be easily effected.g) Stability: The
solvent should be stable at process conditions in order to minimize
losses by degradation and generation of further impurities.h)
Corrosivity:If possible, there is a strong incentive to use a
component that is already in the process, such as a reactant feed
stream, as the solvent. This may avoid additional materials
handling, environmental and corrosion penalties later in the
process.i) Toxicity: The advantages of a non-toxic solvent are
self-evident in considering inherent process safety and capital
cost. Some solvents now appear on the "Environmental Red List" and
should be avoided.j) Cost:The extraction process may only be a
small part in the overall process and solvent losses should not
greatly affect process economics. No solvent is likely to meet all
the above criteria and the list is not claimed to be exhaustive. A
compromise will be necessary based on overall process economics.It
has already been indicated that it may require more than one stage
of liquid-liquid extraction in order to achieve the degree of
separation required. It is possible to achieve this by removing the
extract and making the raffinate the feed to another liquid-liquid
extraction unit using fresh solvent. This requires a considerable
amount of solvent to be used and as in distillation it is more
usual to employ equipment where a countercurrent flow of one phase
against the other occurs. In this experiment, a single stage
extraction is used. Below are the liquid-liquid extraction column
used in this experiment.
Figure 2: Liquid-liquid Extraction Column
3.0METHODOLOGY
3.1Equipment and Materials
1. Liquid-liquid extraction column 2. Conical flask3. Measuring
cylinder4. Burette5. Retort stand6. Stopwatch7. Water8. Iodine9.
Starch indicator10. Sodium thiosulphate solution (0.01M)
3.2Experimental Procedure
1. The iodine concentration in the liquid light phase is
determined using titration technique before the experiment began.2.
The feeding pump is turned on followed by the pulsating pump.3.
These parameters are set:Pulsating pump at 50% (10)Feeding pump for
heavy phase 50% (10) and light phase 75% (15)4. The system is let
to be operated for 15 minutes.5. The interface level was observed.
The level is made sure to be less than 20cm from the feeding point
of the light phase. When necessary, the balancing leg is used to
adjust the level. 6. When the interface level is stable, the timing
is started and the samples are collected every 5 minutes from the
raffinate through valve V17.7. The solute content (iodine) in the
sample is analysed using titration technique.8. The sample
collection process is repeated until the iodine content in the
sample is stable.9. When the iodine content is constant, sample
from valve V9 is collected by closing valve V7. The iodine content
in the sample is analysed also using titration technique.10. Steps
3 to 9 are repeated, but with the following flow rate: Heavy Phase
(15)Light phase (15) Heavy Phase (20)Light Phase (15)11. Titration
technique:a. 30 of sample is pipetted into the conical flask.b. A
few drops of starch indicator are added.c. The sample is titrated
using sodium thiosulphate solution (0.01M) gradually.d. The
titration is stopped once the blue-black colour turns clear.
4.0RESULTS AND DISCUSSIONS
4.1Experimental Data
Raffinate Composition (g/L)Time (min)
Heavy Phase Flow Rate:0.65
10 L/min0.610
0.415
Light Phase Flow Rate:0.420
15 L/min0.425
Extract Composition (g/L)Time (min)
55.85
Raffinate Composition (g/L)Time (min)
Heavy Phase Flow Rate:0.55
15 L/min0.510
Light Phase Flow Rate:0.515
15 L/minExtract Composition (g/L)Time (min)
52.95
Raffinate Composition (g/L)Time (min)
Heavy Phase Flow Rate:0.45
20 L/min0.410
Light Phase Flow Rate:0.415
15 L/minExtract Composition (g/L)Time (min)
57.05
4.2Discussion
Based on the result obtained, light phase flow rate was kept
constant throughout the experiment which is 15 L/min. Meanwhile,
heavy phase flow rate was varied at 10, 15 and 20 L/min. The
observation was made and recorded in order to know the effect of
change in the flow rate towards the performance of extraction
process at the time of five minutes. When the heavy phase flow rate
was set at 10 L/min, the extract composition was 55.8 g/L. The
raffinate composition also can be seen being constant at the times
of 20 minutes. However, when the heavy phase flow rate was
increased to 15 L/min which was the same flow rate as light phase
flow rate, the extract composition was lowered to 52.9 g/L.
Besides, the raffinate composition become constant at faster rate
compared to the first which was at the times of ten minutes. The
last column showed that the heavy phase flow rate was increased up
to 20 L/min and resulted extract composition at 57 g/L which was
the highest density above all the previous result. Hence, it can be
concluded that the higher the heavy phase flow rate, the higher the
extract composition. However, the heavy phase flow rate must be
kept higher than the light phase flow rate in order to have a
better performance of extraction process.
4.3Questions
1. State which component is the heavy phase, light phase,
solvent, solute and diluents.Heavy phase: n-hexaneLight phase:
waterSolvent: n-hexaneSolute: iodine
2. State the dispersed phase and the continuous phase in this
experiment.Dispersed phase: Light phase vaporContinuous phase:
Heavy phase liquid
3. What is the meaning of equilibrium contact?Introduction of
anew phaseto the system and allowing the components of the original
raw material to distribute themselves between the
phases.Equilibrium is reached when a component is so distributed
between the two streams that there is no tendency for its
concentration in either stream to change. Attainment of equilibrium
may take appreciable time, and only if this time is available will
effective equilibrium be reached. The opportunity to reach
equilibrium is provided in each stage, and so with one or more
stages the concentration of the transferred component changes
progressively from one stream to the other, providing the desired
separation.
4. Discuss the effect of flow rate towards the extraction
performance.The flow rate of heavy phase must be higher than the
flow rate of light phase so that the performance of extraction
process will be much better. 5. State others steps and measures to
be taken to increase the performance of this extraction process.a)
Has to add up the iodine so that the concentration of the iodine is
in the range.b) Take control of the pump wisely.c) Set the time
accurately and reduce human error.
6. Describe the importance of liquid-liquid extraction processes
in chemical engineering.The volatility of solution mixture
sometimes is very near or almost the same value with each other.
When this situation occurs, distillation is not a suitable method
to separate one substance from another as distillation process only
can be used if there is a big difference in volatility of solute
and the volatility of the mixture. Secondly, the solvent used in
the extraction process is not harmful. The advantages of non-toxic
solvent are self-evident in considering inherent process safety and
capital cost. Lastly, extraction process is higher efficient than
distillation. This is because the increase in the number of stages
will increase the efficiency of extraction process where more
solute will be extracted.
5.0CONCLUSION
From this experiment, we can conclude that there is a
relationship between the change of flowrate and the extraction
efficiency. We also ca observed the change of flowrate for the
heavy phase liquid in the liquid-liquid extraction will increase
the extraction efficiency as the raffinate composition to achive
extraction equilibrium will be less. However, the decrease in the
heavy phase flowrate will decrese the extraction efficiency as the
raffinate composition to achive extraction equlibrium will be
higher.
6.0 RECOMMENDATIONS
6.1 Errors
1.Parallax errors might occur when the extract or raffinate were
collected using the measuring cylinder and when taking the
titration reading.2.There might be some possibilities to have
impurities or dirt in the standard Natriumthiosulphate (0.01M)
used. Thus, this will give slight effects to the titration
process.3.The pumps may not be very efficient due to lack of
inspection that will cause errors of the flow rates.4.There is some
water that might appear in the apparatus such as measuring
cylinder, conical flask and burette were not clean properly. Thus,
the results of titration were affected.
6.2Recommendations
1.We have to add up the iodine so that the concentration of the
iodine is in the range.2.When taking the reading of the titration,
our eyes must be perpendicular with the burrete, so that we get a
correct measurement.3.We have to wear a proper PPE because the
gases emitting is hazardous.
7.0REFERENCES
1. Christie J.Geankoplis, Transport Processes and Unit
Operations, 3rd Edition, Prentice-Hall PTR, (1993).2. Mc Cabe, W.L,
Smith, J.C & Harriot, P., Unit Operations of Chemical
Engineering, 5th Edition, Mc Graw-Hill International, (1993).
8.0APPENDICES
Figure 3: Raffinate Column Figure 4: Extract Column Figure 5:
Titration Begin Figure 6: Titration Stop15