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liquid liquid extraction

liquid liquid extraction We were asked to extract benzoic acid from a kerosene-benzoic acid mixture. This was to be done using the stirred liquid-liquid extraction column in the senior laboratory. Fresh water was used as the continuous phase in the extraction. We were asked to measure the benzoic acid concentrations of the feed, raffinate, and extract streams. These measurements were to be made at several different steady-states. The number of theoretical stages and the height of the theoretical stages (HETS) was also to be determined. Because the liquid-liquid extraction column had not been used in several years our assignment also included discussing any problems with the process and possible solutions. The discussion of equipment, results, and experimental procedure will be discussed in the following paper entitled "Liquid-Liquid Extraction.” Even though the process had not been used in several years, everything worked very well. The flow meter used to measure the water flow rate had a small leak which was easily remedied using a bucket. One of the storage tanks still had approximately 400 liters of kerosene with a benzoic acid concentration of 0.00208 (gm/mL). Using this as a feed for the process, concentrations of benzoic acid in the aqueous extract were found to be 0.00109 (gm/mL) and 0.000783 (gm/mL) for two separate steady state experimental runs. The flow rates of the kerosene feed for the two steady states were 0.361 (gal/min) and 0.157 (gal/min) respectively. The number of theoretical stages for a flow rates of 0.361 (gal/min) and 0.157 (gal/min) were determined to be 1.76 and 0.87 stages respectively. With the height of the column equal to 48.9 inches the HETS for the two steady state conditions were 27.71 inches and 56.42 inches respectively. Transferring the benzoic acid from the kerosene into the water was accomplished without any major problems. The equipment worked very well except for minor problems with the water flow meter. After working on the process we would recommend that it be used more often. We also believe the results obtained from our experiments are accurate and well within experimental error. 1. pH vs. BENZOIC ACID CONCENTRATION 3. EQULIBRIUM CURVE WITH OPERATING LINES 2. HEIGHT OF EQUIVALENT THEORETICAL STAGES Liquid-Liquid Extraction of Benzoic Acid The number of theoretical stages and the height of the equivalent theoretical stages were determined for a liquid-liquid extraction process in the senior laboratory. The process transferred benzoic acid form kerosene to water through a stirred liquid-liquid extraction column. The column was operated at two different steady state conditions to compare benzoic acid concentrations for various flow rates. From the experiment it was determined that the number of theoretical stages were 1.76 and 0.87 for flow rates of 0.361 (gal/min) and 0.157 (gal/min) respectively. The height of the equivalent theoretical stages for the above conditions were found to be 27.71 inches and 56.42 inches respectively, they were calculated using a column height of 48.9 inches. It is recommended that the experiment be repeated at different steady state conditions and at the same conditions as above to verify the results. It is also suggested that several different methods be used to determine the concentration of the benzoic acid in the kerosene. Liquid-liquid extraction is very common in industry and can be used to easily separate components that would otherwise be inseparable. This mass transfer operation takes advantage of the solubility of benzoic acid and two immiscible liquids. Good contact surface area, contact time, and the correct solvent choice are all very important for mass transfer. In this project liquid-liquid extraction was used to transfer benzoic acid from kerosene to water. Concentrations of the kerosene feed, extract, and raffinate were to be measured and used to find the number of theoretical stages and the height of equivalent theoretical stages (HETS) for the process. Concentration measurements will be made from samples of the feed, extract, and raffinate for two different steady state flow rates. Because the stirred column used in this experiment had not been used in several years this report will also include discussion of problems encountered in the experiment and possible solutions. Planning of the experiment will also be discussed. Careful planning was necessary because of the uncertainty of equipment, limited kerosene feed, and time constraints. A brief description of the apparatus and procedures used in this experiment will also be discussed. Liquid-liquid extraction is used to transfer one component between two immiscible liquids. This is possible because of the solubility of the component in both of the liquids. The component to be transferred is in solution with feed solvent. When this solvent comes into contact with extraction solvent the concentration gradient between the two solvents drives the component into the extraction solvent. If the two immiscible liquids were to have contact with each other for and infinite amount of time the three component system would reach equilibrium. In a liquid-liquid extraction column this would never happen because the column would need an infinite number of stages. Stages are used to determine the extent of mass transferred for a process. To determine the number of stages a graphical method found in Perry’s Chemical Engineering Handbook (6th edition) can be used. When designing a liquid-liquid extraction column it is necessary to determine the number of theoretical stages needed for a specific separation. It is also important to know the height of the equivalent theoretical stages. The graphical method requires an equilibrium curve for the three component system and an operating line which shows the actual separation achieved by the system. By stepping from the operating line to the equilibrium curve the number of theoretical stages can be determined. This method will be described in greater detail in the next section. The main objective of the experiment was to determine the number of theoretical stages and the height of the equivalent theoretical stages (HETS). Benzoic acid was to be transferred between kerosene and water and measurements of the concentration of benzoic acid in the feed, raffinate, and extract were to be measured. Because the process had not been used in several years we first looked the equipment over to determine how it worked. The basic process consisted of an extraction column, four storage tanks, and a pump. The column itself contained 10 stages with impellers between each stage. The impeller speed was set to 360 revolutions per minute. This impeller speed was chosen purely at random. The top of the column contains an inlet for the water and a outlet for the raffinate. Conversely the bottom of the column consisted of an outlet for the extract and an inlet for the kerosene feed. Both of the inlets came directly from pumps which pulled the kerosene feed and the water from there respective storage tanks. The outlets were piped directly to the raffinate and extract storage tanks. The piping was designed such that there is a place to take samples from each of the pipes entering and exiting storage tanks. After studying the equipment we determined the contents of the kerosene feed tank. The tank contained approximately 400 liters of a kerosene-benzoic acid solution with an unknown concentration. To determine the concentration we mixed a measured amount of the kerosene feed with a measured amount of tap water. This mixture was shaken until well mixed and allowed to settle. The procedure was repeated several times and for several different mixtures. The mixtures were considered to be in equilibrium and the acid concentration in the aqueous phase was measured. Rather than titrate the aqueous phase to determine concentration a chart was developed to correlate the aqueous pH value with the concentration. This was accomplished by taking the pH of known water-benzoic acid mixtures and preparing the graph below: From this graph it was easy to determine benzoic acid concentrations from the pH of a solution. Knowing the benzoic acid concentration of the aqueous phase in equilibrium with the kerosene feed the concentration of the acid in the kerosene could be determined from the following chart developed by Allerton (1943): Knowing the concentrations of the benzoic acid in both the aqueous and organic phases for the system at equilibrium. The concentration of the feed kerosene can be determined assuming the all of the benzoic acid in the system came from the kerosene feed exclusively. Knowing the concentration in the feed kerosene we began to run the process. First the water was used to fill the column to the top stage. The water entered from the top and exited from the bottom. For the process to be at steady state the water flow in the inlet and exit streams needed to be constant and equal. Because the only flow meter for the water was on the inlet we adjusted the water level in the tank using the inlet and outlet valves until it remained constant. With the water level constant we introduced the kerosene feed into the system. This was done slowly so that adjustments to the water flow could be made to keep the water level constant. Once the kerosene and the water were flowing with a constant water level the pH of the aqueous extract was measured at ten minute intervals until it too remained constant. When both the water level and the pH were constant for a ten minute period the process was considered to be at steady state. Samples of the extract and raffinate were taken to measure the benzoic acid concentration. For the aqueous phase the pH was taken and Figure 1 was used to determine the acid concentration. In the organic phase a measured amount of the raffinate was mixed with a measured amount of tap water and brought to equilibrium by mixing as was done for the feed kerosene. The aqueous phase was then measured for pH and the corresponding concentration. This concentration was used along with Figure 2 to determine the equilibrium concentration in the kerosene and then to determine the initial concentration in the raffinate. The concentrations of the extract and the raffinate could then be compared to the concentration in the feed kerosene to complete a mass balance. This process was repeated so that two steady state conditions were reached. Having collected all of the data necessary to measure the mass transferred, we began to finish the experiment by emptying the column and labeling the contents of the storage tanks. Following the procedure in Perry’s Handbook (6thedition) we drew operating lines next to the equilibrium curve using the data collected. These lines could then be used to determine the number of theoretical stages and the HETS. The following graph shows the operating lines drawn from experimental data and the equilibrium curve taken from Allerton (1943): As can be seen from Figure 3 it was difficult to determine the number of theoretical stages from the operating lines and equilibrium curve using a graphical method. Because of this difficulty we also used one of the Kremser equations from Perry’s Handbook to determine the number of theoretical stages. Where (m) is the slope of the equilibrium line, (e) is the extraction factor, (Xf) is the benzoic acid concentration in the feed kerosene, (Xr) is the benzoic acid concentration in the raffinate, (Ys) is the benzoic acid concentration in the extract, and (N) is the number of theoretical stages. With the number of stages determined and knowing the height of the experimental column, the following equation taken from Perry’s Handbook (6thedition) is used to calculate the HETS: Where (Zt) is the actual height of the experimental column, (N) is the number of theoretical stages, and (HETS) is the height of the equivalent theoretical stages. At steady state and a flowrate of 0.361 (gal/min) the number of theoretical stages is equal to 1.76 and the HETS is equal to 27.71 inches. For a kerosene flowrate of 0.157 (gal/min) the number of theoretical stages is equal to 0.87 and the HETS is equal to 56.42 inches. We consider both of the steady state conditions to have given good separations. We also believe the correlation between the pH of the aqueous benzoic acid solution and the actual acid concentration to be accurate. We recommend that the liquid-liquid extraction column in the senior laboratory be used more. The process is very interesting and appears to work well. Xf Concentration of benzoic acid in the feed kerosene (gm/mL) Xr Concentration of benzoic acid in the raffinate (gm/mL) Ys Concentration of benzoic acid in the extract (gm/mL) m slope of the equilibrium line dimensions N number of theoretical stages dimensions Zt actual height of experimental column (inches) HETS height of equivalent theoretical stages (inches) Robbins, L. A., “Liquid-Liquid Extraction”, in Perry’s Chemical Engineers’ Handbook, Sixth Ed., D. Green and R. H. Perry, McGraw Hill, New York, NY, p 51-1, (1984) Allerton, J., “Liquid Extraction in Perforated-Plate and Packed Towers.” American Institute of Chemical Engineers. 39:361-384 (1943). Bibliography:

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