ABSTRACT The 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.
1
Table of Contents ABSTRACT.................................................................................................................. 1 1.0
INTRODUCTION................................................................................................. 3
1.1
Background of Experiment..................................................................................3
1.2
Objective of Experiment...................................................................................... 4
1.3
Scope of Experiment........................................................................................... 4
2.0
LITERATURE STUDY.......................................................................................... 4
3.0
METHODOLOGY................................................................................................ 9
3.1
Equipment and Materials.................................................................................... 9
3.2
Experimental Procedure...................................................................................... 9
4.0
RESULTS AND DISCUSSIONS............................................................................11
4.1
Experimental Data........................................................................................... 11
4.2
Discussion...................................................................................................... 12
4.3
Questions....................................................................................................... 12
5.0
CONCLUSION.................................................................................................. 14
6.0
RECOMMENDATIONS...................................................................................... 14
6.1
Errors........................................................................................................... 14
6.2
Recommendations............................................................................................ 14
7.0
REFERENCES................................................................................................... 15
8.0
APPENDICES.................................................................................................... 15
2
1.0
INTRODUCTION
1.1
Background of Experiment Liquid/liquid extraction, also known as solvent extraction and partitioning, is a method to
separate compounds based on their relative solubility in two different immiscible liquids
,
usually water and an organic solvent (hexane). It is an extraction of a substance from oneliquid phaseinto another liquid phase. Liquid/liquid extraction is a basic technique
in
chemical laboratories, where it is performed using a separator funnel. This type of process is commonly performed after a chemical reaction as part 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 basic principle behind extraction involves the contacting of a solution with another solvent 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 separation processes.
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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.
2.0
LITERATURE STUDY Liquid-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.
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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 Extractor Where; 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. 5
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. 6
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.
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Figure 2: Liquid-liquid Extraction Column
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3.0
METHODOLOGY
3.1
Equipment and Materials 1. Liquid-liquid extraction column 2. Conical flask 3. Measuring cylinder 4. Burette 5. Retort stand 6. Stopwatch 7. Water 8. Iodine 9. Starch indicator 10. Sodium thiosulphate solution (0.01M)
3.2
Experimental 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: 3 a. 30 cm of sample is pipetted into the conical flask. 9
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.
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4.0
RESULTS AND DISCUSSIONS
4.1
Experimental Data
Heavy Phase Flow Rate: 10 L/min Light Phase Flow Rate: 15 L/min
Heavy Phase Flow Rate: 15 L/min Light Phase Flow Rate: 15 L/min Heavy Phase Flow Rate: 20 L/min Light Phase Flow Rate: 15 L/min
4.2
Raffinate Composition (g/L) 0.6 0.6 0.4 0.4 0.4 Extract Composition (g/L) 55.8 Raffinate Composition (g/L) 0.5 0.5 0.5 Extract Composition (g/L) 52.9 Raffinate Composition (g/L) 0.4 0.4 0.4 Extract Composition (g/L) 57.0
Time (min) 5 10 15 20 25 Time (min) 5 Time (min) 5 10 15 Time (min) 5 Time (min) 5 10 15 Time (min) 5
Discussion 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. 11
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.
4.3
Questions 1. State which component is the heavy phase, light phase, solvent, solute and diluents. Heavy phase : n-hexane Light phase : water Solvent : n-hexane Solute : iodine 2. State the dispersed phase and the continuous phase in this experiment. Dispersed phase : Light phase vapor Continuous phase : Heavy phase liquid 3. What is the meaning of equilibrium contact? Introduction of a new phase to 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. 12
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.
5.0
CONCLUSION 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.
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6.1
RECOMMENDATIONS
6.2
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.
6.2
Recommendations
1.
We have to add up the iodine so that the concentration of the iodine is in the
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.
7.0
REFERENCES
range.
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).
8.0
APPENDICES 14
Figure 3: Raffinate Column
Figure 5: Titration Begin
Figure 4: Extract Column
Figure 6: Titration Stop
15
