Laboratory Course - UTRM - Ruhr
Transcrição
Laboratory Course - UTRM - Ruhr
RUHR-UNIVERSITÄT BOCHUM Lehrstuhl für Fluidverfahrenstechnik Laboratory Course Experiment: “Continuous Distillation” Summer Semester 2016 Advisors: Mrs. M.Sc. Iris Rieth Mrs. M.Sc. Carolin Stegehake Room: IC 3/099 IC 3/109 Phone Number: +49 (0)234 / 32 – 25 849 +49 (0)234 / 32 – 26 344 E-Mail: [email protected] [email protected] Laboratory: IDN 02 / 751 1 Inhaltsverzeichnis 1. Introduction ....................................................................................................................... 3 2. Operating Principle of a Continuous Distillation Column .................................................... 3 2.1. Forming a two-phase System...................................................................................... 3 2.2. Mass Transfer across the Phase Boundary................................................................. 4 2.3. Separation of the two phases ...................................................................................... 4 3. Design of a Continuous Distillation Column ....................................................................... 4 4. Thermodynamic Fundamentals ......................................................................................... 5 4.1. Boiling und Equilibrium Diagram of Binary Systems .................................................... 5 4.2. Classification of Real Systems .................................................................................... 7 4.2.1. Ideal Mixtures ....................................................................................................... 7 4.2.2. Mixtures with Negative Deviation .......................................................................... 8 4.2.3. Mixtures with Positive Deviation ........................................................................... 9 5. Equilibrium-Stage Operations ...........................................................................................10 5.1 Concept of Equilibrium-Stage Operation .....................................................................10 5.2 Packed Columns .........................................................................................................11 6. McCabe-Thiele Design Method ........................................................................................11 6.1 Determination of the Equilibrium-Stages by Using the McCabe-Thiele-Method ...........12 6.1.1. Operating Line for the Rectification Section .........................................................12 6.1.2. Operating Line for the Stripping Section ..................................................................14 6.1.3. The Intersection Line/Feed Line ..........................................................................16 6.1.4. Construction of the McCabe-Thiele Diagram .......................................................19 7. Instructions .......................................................................................................................20 7.1. Preparation of the Experimental Setup .......................................................................20 7.2. Operation of the Software ..........................................................................................20 7.3. Experimental Procedure.............................................................................................23 8. Task Formulation ..............................................................................................................23 9. Special Safety Instructions ...............................................................................................24 10. Short Questions ..............................................................................................................24 11. Appendix ........................................................................................................................25 12. Literature ........................................................................................................................32 2 1. Introduction Continuous distillation is used widely in the chemical process industries where large quantities of homogeneous liquids have to be distilled. This unit operation leads to high purity of products, even if the system is difficult to separate. [1] Continuous distillation is used when a simple distillation does not achieve a purity that is high enough. The task of the experiment in this laboratory is to investigate a continuous distillation of the system ethanol/water. 2. Operating Principle of a Continuous Distillation Column The principle of continuous distillation is the same as the principle of normal distillation. It consists of three steps: 1. Forming a two-phase system 2. Mass transfer across the phase boundary 3. Separation of the two phases 1. Step 2. Step 3. Step Figure 1: Principle of continuous distillation [7] 2.1. Forming a two-phase System To separate a mixture of substances into its components, a second phase is required. The separating component transfers into the second phase. The second phase is generated by partially boiling the initial phase – this means to supply heat in the vaporizer. Thereby, the mixture of substances is separated into a vapor and a liquid phase. 3 2.2. Mass Transfer across the Phase Boundary Mass transfer between the two phases is caused by disequilibrium of the substances – a difference in the intensive variables. Because of the different boiling temperatures of the pure substances, the thermodynamic properties can be influenced such that the components which have to be separated accumulate in different phases. This effect is enhanced by intensive contact of the two phases and a large surface caused by packing material. 2.3. Separation of the two phases Mass transfer between the liquid and the vapor phase leads to separation of the mixture into the vapor and the liquid phase. The composition of the two phases is different: the vapor phase mainly consists of light boilers and the liquid phase of high boilers. By condensing the vapor phase, there are two (liquid) output fractions in the distillation: the first one at the upper part of the column with light boilers of high purity (depending on the separation efficiency of the column) and the second one mainly with Condenser high boilers at the bottom. 3. Design of a Continuous Distillation Column Distinguished between the types of packing material, the Continuous Distillation Columns are called plate column, distillation tower or distillation column. [2] These designs can be operated as a batch or, Rectification section overhead Feed Reflux reflux divider overhead product Feed tray distillation. Continuous distillation columns consist of a bottom part with a reboiler, a separation column and a head part with a reflux Stripping section like in this laboratory, as a continuous Reboiler condenser and a “reflux divider”. The lower part of the column is called stripping section and the upper part is called rectification section. The feed section with a preheater is located in bottom bottoms product Figure 2: Continuous distillation column [3] 4 between the upper and the lower part. The schematic of a Continuous Distillation tower is presented in Figure 2. In the bottom part (sump) of the column, a section of the liquid is extracted and the rest of it is partially vaporized and supplied to the stripping section again. In the stripping section the reduction of the light boilers in the condensate phase takes place. In the rectification section the enrichment of the light boilers in the vapor phase takes place which is partly removed after condensation as the head product and partly recycled to the column. The reflux liquid gets in contact with the vapor and leads to mass transfer again. The ratio between reflux and overhead product is controlled by a reflux divider. With high reflux ratio, the residence time is increased. Residence time is the average amount of time that the substances spend in the column so that mass transfer takes place. The purity of the head product increases with increasing residence time. 4. Thermodynamic Fundamentals To analyze the separation efficiency of a Continuous Distillation Column it is important to understand some particular diagrams. The diagrams are based on theoretical principles. 4.1. Boiling und Equilibrium Diagram of Binary Systems In a mixture of a binary system there are specific intensive properties, e.g. composition of liquids xi, composition of vapor yi and boiling temperature Ti. It is common to refer the compositions of each phase to the light boiler i. The drawing of the related properties into a diagram results in the boiling point diagram and the vapor-liquid equilibrium diagram. The boiling point diagram (Figure 3) shows how the equilibrium compositions of the components in a liquid mixture vary with temperature at a fixed pressure. The boiling point diagram consists of the bubble-point curve (bubble-point is the temperature at which the liquid starts to boil) and the dew-point curve (dew point is the temperature at which the saturated vapor starts to condense). With the boiling point diagram it is possible to determine the composition of the liquid phase xi and the composition of the vapor phase yi at different temperatures at a fixed pressure. Besides, the boiling point and the dew point of a specific composition can be investigated. 5 vapor dew-point curve two-phase bubble-point curve liquid Figure 3: Boiling Point diagram [9] The vapor-liquid-equilibrium diagram (Figure 4) expresses the bubble-point and the dew-point of a binary mixture at a constant pressure. By using this diagram, the amount of equilibrium stages (theoretical trays) of the column can be determined. equilibrium line Figure 4: Vapor-Liquid-Equilibrium Diagram [9] 6 4.2. Classification of Real Systems 4.2.1. Ideal Mixtures Systems of ideal mixtures do not have any deviations from the ideal behavior. In this case Raoult’s law is applicable – the partial vapor pressure pi of component i of an ideal mixture of liquids is equal to the vapor pressure of the pure component 𝑝𝑖0 multiplied by its mole fraction 𝑥𝑖 in the mixture: [3] 𝑝𝑖 = 𝑝𝑖0 ∙ 𝑥𝑖 (1) Besides, Dalton’s law is applicable. It states that in a mixture of non-reacting gases, the total pressure exerted is equal to the sum of the partial pressures of the individual gases. [4] 𝑝𝐺𝑒𝑠 = ∑ 𝑝𝑖 (2) Moreover, the partial pressures of the components of the mixture can be calculated by the concentration of the component of the vapor phase and the total pressure: 𝑝𝑖 = 𝑦𝑖 ∙ 𝑝𝐺𝑒𝑠 (3) The linear behavior of the partial pressures for ideal systems is shown in the vapor pressure diagram (Figure 5, left). The bubble point diagram (Figure 5, middle) features the typical boiling point curve. This diagram shows the dependence of phase transitions of a mixture on temperature and composition. The vapor-liquid-equilibrium diagram (Figure 5, right) is typically determined experimentally but for ideal binary mixtures it can be calculated by knowing the separation factor α. The separation factor characterizes the separation behavior of an ideal binary system and describes the ratio of the ratios of the vapor-liquid-composition of the pure components A and B: 7 𝑦𝐴 𝑦𝐴 𝑝𝐴 ⁄𝑥𝐴 𝑥𝐴 ∙ 𝑝𝐺𝑒𝑠 ⁄𝑥𝐴 𝑝𝐴0 𝛼=𝑦 =𝑦 =𝑝 = 0 𝐵⁄ 𝐵⁄ 𝐵 𝑥𝐵 𝑝𝐵 𝑥𝐵 𝑥 ∙ 𝑝𝐺𝑒𝑠 𝐵 (4) The vapor-liquid-equilibrium curve for ideal mixtures is determined by Dalton’s law, Raoult’s law and the separation factor: 𝑦𝐴 = 𝛼 ∙ 𝑥𝐵 1 + 𝑥𝐴 ∙ (𝛼 − 1) (5) Many real component systems deviate from the ideal behavior. These systems are divided into two categories, depending on the trend of deviation. 4.2.2. Mixtures with Negative Deviation If the real partial pressures of the components in the mixture are lower than the (ideal) partial pressures calculated by Raoult’s law, the mixture is called “mixture with negative deviation” (Figure 5). This behavior results in a vapor pressure minimum in the vapor-pressure diagram and in a temperature maximum in the boiling point curve. The composition of the mixture in the extreme value is called azeotrope: the boiling temperature of the azeotrope of a mixture with temperature maximum is higher than the boiling temperature of the single components. For the continuous distillation it means in fact that the lighter boilers vaporize first (mainly). The temperature increases to the boiling point of the azeotrope and then the remaining mixture vaporizes as an azeotrope. Thereby the liquid and vapor phase have the same composition – the phases cannot be more purified by a simple distillation. 8 4.2.3. Mixtures with Positive Deviation For mixtures with positive deviation there are the same considerations applied as for the ones with negative deviation. The effects concerning maximum and minimum are reverse. The boiling temperature of the azeotrope is lower than the boiling temperature of the single components. The azeotrope vaporizes first (mainly). A complete separation of this mixture is not possible by using a simple distillation. vapor pressure diagram boiling point diagram equlibrium diagram ideal mixtures negative deviation positive deviation Figure 5: vapor pressure diagram, boiling point diagram, equilibrium diagram for the three classifications of real systems [9] 9 5. Equilibrium-Stage Operations The number of equilibrium-stages and the reflux ratio (equation 10) of a distillation column for a certain separation task are important characteristics to estimate investment and operation costs. 5.1 Concept of Equilibrium-Stage Operation The definition of equilibrium-stages was once established for columns that used trays as packing material. Mass transfer in tray columns takes place spatially separated on each tray and not steady over the whole column. The highest enrichment of a component on an ideally operated tray is characterized by the equilibrium-stage, if the following conditions are fulfilled: 1. Ideal mixing of the liquid 2. Ideal heat and mass transfer between vapor and liquid 3. Vapor is dry saturated – no liquid droplets are drawn along. condenser Reflux 3. Tray re-cooler 2. Tray distillate F4 1. Tray Figure 6: Vapor and liquid Hold-up on the different trays [3] The number of equilibrium stages is determined by using the McCabe-Thiele method with the required purity of the products as described in section 6. 10 5.2 Packed Columns Packing material is used to minimize pressure losses in case of high separation effects. To transfer the theory of equilibrium-stages to packed columns, the HETP-value (height equivalent to one theoretical plate) is defined [6]: 𝐻𝐸𝑇𝑃 = 𝑙𝑒𝑛𝑔𝑡ℎ 𝑜𝑓 𝑝𝑎𝑐𝑘𝑖𝑛𝑔 𝑚𝑎𝑡𝑒𝑟𝑖𝑎𝑙 𝑙𝑎𝑦𝑒𝑟 𝑛𝑢𝑚𝑏𝑒𝑟𝑠 𝑜𝑓 𝑒𝑞𝑢𝑖𝑙𝑖𝑏𝑟𝑖𝑢𝑚 − 𝑠𝑡𝑎𝑔𝑒𝑠 liquid vapor Figure 7: examples of packing material 6. McCabe-Thiele Design Method The McCabe-Thiele method is used to examine graphically the function of a distillation column for binary systems, such as the one in the laboratory (ethanol and water). This method is based on the vapor-liquid-equilibrium data and it assumes constant molar overflow which implies that: [5] - Molar heats of vaporization of the components are roughly the same - Heat effects (heats of solution, heat losses to and from column, etc.) are negligible - For every mole of vapor condensed, 1 mole of liquid is vaporized 11 (6) 6.1 Determination of the Equilibrium-Stages by Using the McCabe-ThieleMethod The design procedure is simple. Given the vapor-liquid-equilibrium diagram of the binary mixture, operating lines are drawn first. Operating lines define the mass balance relationships between the liquid and vapor phases in the column. There is one operating line for the bottom (stripping) section of the column, and one for the top (rectification or enriching) section of the column. The use of the constant molar overflow assumption also ensures the operating lines are straight lines. [6] 6.1.1. Operating Line for the Rectification Section The operating line for the Rectification Section is based on the following relations. The mass balance of the total flow and of the flow of the lighter boiler component lead to the following relations according to Figure 8: 𝐺̇ = 𝐿̇ + 𝐾̇ (7) 𝐺̇ ∙ 𝑦 = 𝐿̇ ∙ 𝑥 + K̇ ∙ 𝑥𝐾 (8) Figure 8: mass-balance rectification section [7] Solving equation (8) for 𝑦 results in equation (9): 𝑦= 𝐿̇ 𝐾̇ ∙ 𝑥 + ∙ 𝑥𝐾 𝐺̇ 𝐺̇ 12 (9) The reflux ratio 𝑣 – the ratio of the reflux flow 𝑅̇ to the distillate flow 𝐾̇ – leads to equation (10): 𝑣= 𝑅̇ 𝐾̇ (10) Substituting equation (7) in equation (9) and expanding the fractions results in equation (11): 𝐿̇⁄ 𝐾̇⁄ ̇ 𝐾 𝐾̇ 𝑦= ∙𝑥+ ∙ 𝑥𝐾 = ∙𝑥+ ∙ 𝑥𝐾 𝐿̇ + 𝐾̇ 𝐿̇ + 𝐾̇ 𝐿̇⁄ + 𝐾̇⁄ 𝐿̇⁄ + 𝐾̇⁄ 𝐾̇ 𝐾̇ 𝐾̇ 𝐾̇ 𝐿̇ 𝐾̇ (11) Substituting equation (10) in equation (11), in consideration of 𝐿̇ ≈ 𝑅̇ results in the operating line for the rectification section (12): 𝑦= 𝑣 1 ∙𝑥+ ∙𝑥 𝑣+1 𝑣+1 𝐾 (12) A complete reflux (𝐾̇ = 0) is expressed in equation (13) based on equation (8). 𝐺̇ ∙ 𝑦 = 𝐿̇ ∙ 𝑥 (13) Using the relation of the incoming amount of vapor to be equal to the outcoming amount of liquid, the operating line for complete reflux is according to equation (14): 𝑦=𝑥 (14) The operating line for complete reflux is equivalent to the diagonal line of the vaporliquid-equilibrium diagram. The operating line for the rectification section with a certain reflux ratio is constructed as follows. First the desired top product composition is located on the vapor-liquidequilibrium diagram, and a vertical line produced until it intersects the diagonal line. A line from this intersection point is drawn to the y-axis intercept according to equation (16), as shown in Figure 9. [6] 𝑥 = 𝑥𝐾 𝑦= 𝑥𝐾 𝑣+1 13 (15) (16) 6.1.2. Operating Line for the Stripping Section The operating line for the stripping section is constructed in a similar manner. equilibrium line rectification line Figure 9: rectification line in vapor-liquid-diagram The mass balance of the total flow and of the lighter component for the system boundary of the stripping section results in equations (17) and (18) according to Figure 10. 𝑆̇ is the extracted liquid in the bottom part of the distillation column. 𝐿̇∗ = 𝐺̇ ∗ + 𝑆̇ (17) 𝐿̇∗ ∙ 𝑥 ∗ = 𝐺̇ ∗ ∙ 𝑦 ∗ + 𝑆̇ ∙ 𝑥𝑆 (18) Figure 10: mass balance stripping section [7] 14 Analogous to the operating line for the refraction section results the operating line for the stripping section in equation (19): ∗ 𝑦 = 𝐿̇∗ 𝐿̇∗ − 𝑆̇ ∙ 𝑥∗ − 𝑆̇ 𝐿̇∗ − 𝑆̇ ∙ 𝑥𝑆 (19) The starting point for the construction is the desired bottom product composition (equation 20) on the x-axis. A vertical line is drawn from this point to the diagonal line, and a line of slope β according to equation (21) as illustrated in the diagram in Figure 11. 𝑥 = 𝑥𝑆 tan 𝛽 = stripping line 𝐿̇∗ 𝐿̇∗ − 𝑆̇ (20) (21) equilibrium line Figure 11: stripping line in vapor-liquid-diagram Depending on the state of the feed the slope of the operation line for the stripping section and the intersection point with the operation line for the rectification section changes. 15 6.1.3. The Intersection Line/Feed Line A separation of a mixture by continuous distillation is possible if the operation line for the rectification section and the stripping section intersect in a point below the vaporliquid-equilibrium line. The thermal state of the feed influences the liquid and the vapor flow in the feed-section of the column which results in the intersection point of the operating lines. [6] However, if the feed composition is saturated liquid, the vapor flow does not change but the liquid flow in the lower part of the column increases. If the feed composition is saturated vapor, the vapor flow increases. [5] Figure 12 shows the influence of the thermal state of the feed on the internal flows of the column. a) b) c) d) e) subcooled liquid e>1 saturated liquid e=1 mix of liquid and vapour 0<e<1 saturated vapour e=0 superheated vapour e<0 Figure 12: influence of the condition of feed on vapor and liquid flows in feed section [5] The condition of the feed can be deduced by the slope of the feed line or e-line. The e-line is that drawn between the intersection of the operating lines, and where the feed composition lies on the diagonal line. e is the caloric factor and is the ratio of the feed that flows downward the column as a liquid. (1-e) is the ratio of the feed that flows upward the column as vapor. [8] The heat balance around the feed section results in equation (22): ℎ𝐹′ − ℎ𝐹 𝑒 =1+ ∆ℎ𝑣 (22) ℎ𝐹 is the molar enthalpy of the feed, ℎ𝐹′ is the molar enthalpy of the boling liquid and ∆ℎ𝑣 the molar evaporation enthalpy. 16 The intersection line is determined by using the caloric factor and the heat balance around the feed section: 𝑦= 𝑒 1 ∙𝑥− ∙𝑥 𝑒−1 𝑒−1 𝐹 (23) Figure 13 shows the vapor-liquid-equilibrium diagram with the intersection line and the resulting intersection with the rectification line and stripping line. equilibrium line compound of vapor intersection line stripping line rectification line compound of liquid Figure 13: Intersection line in vapor-liquid-diagram [8] The intersection line intersects the diagonal line according to equation (24): 𝑥 = 𝑥𝐹 (24) The intersection of the intersection line with the x-axis results in equation (25): 𝑥= 𝑥𝐹 𝑒 17 (25) The intersection lines for the various feed conditions are shown in Figure 14. Table 1: Examples of various feed conditions [2] Example state of feed e-factor 1 superheated vapour <0 2 saturated vapour =0 3 mix of liquid and vapour 0<e<1 4 saturated liquid =1 5 subcooled liquid >1 Figure 14: Insection line in depency on thermical feed conditions (qualitative). [8] 18 6.1.4. Construction of the McCabe-Thiele Diagram The McCabe-Thiele method assumes the equilibrium between the liquid on a tray and the vapor above it. Using the constructed operation line for the used reflux ratio and the thermal state of the feed for a certain separation as described above, the required number of theoretical stages is graphically determined as a stage-construction, shown in Figure 15. The construction begins with a horizontal line from the intersection point of rectification line and diagonal line to the vapor-liquid-equilibrium line. A vertical line follows to the operation line. The construction of horizontal and vertical lines between operation line and vapor-liquid-equilibrium line repeated until the vertical line intersects the diagonal line. The resulting trays are equivalent to the necessary trays of a distillation column with respect to the number and composition of liquid and vapor phase. [6] compound of vapor equilibrium line rectification line compound of liquid Figure 15: Stage construction in McCabe-Thiele-Diagram 19 7. Instructions This chapter describes the procedure of the experimental setup. A flow chart and process picture of the distillation column are shown in the appendix. 7.1. Preparation of the Experimental Setup The following aspects have to be denoted to startup the distillation column: 1. Switch on the Refractrometer The weight proportions of the head and bottom product are determined by the refractive index. The refractive index is measured by a refractrometer. 2. Open the coolant valve The cooling water of the whole plant is run in a closed loop. The water tap is on the wall behind the distillation column. The coolant flow has to be high to ensure a sufficient cooling capacity. Inside of the stainless steel tray below the plant, there is a flow controller to prove the flow. 3. Switch on the Main Switch The power supply of the plant is ensured by the box on the left of the plant. The Main Switch has to be turned on. 4. Turn on the Computer Temperature and pump settings are controlled by a software on the computer. The computer is located on the left of the power box. After booting the computer, log in with the user name “user” and the password “user”. 7.2. Operation of the Software The software opens automatically after logging in on the computer. By clicking the left mouse button of the user interface, the process picture (Figure 22) shows up. Each required numerical value has to be confirmed by pressing “Enter”. Activated buttons are flashing green. 20 1. Voltage Supply By pressing the button „230 V” of the control panel (Figure 16), the voltage supply of the pumps and of the heaters are released. Not until the release occurred, heater and pumps can be started. The status of the emergency shut-off which is located on the power box is symbolized by a small triangle. Figure 16: Voltage Abbildung 1: supply Allgemein 2. Sump Temperature The sump of the column is heated by two quartz crystal heater. The desired temperature is entered by a left-click on “Feld 1” in the operating field “Regelung Sumpftemperatur” (Figure 17). By pressing the button “Temp. Sumpf” the heater of the sump is Figure 17: Sump Temperature turned on and the button flashes green. The automatic control of the sump temperature may have oscillations. Therefore, a manual control of the sump temperature is recommended. To activate the manual control, the button “Manuell” has to be clicked and the desired power in % has to be entered in “Feld 2” (Figure 17). The heating of the sump switches off automatically, if the filling level of the sump tank B01 is dropping below or above the minimum or maximum height. 3. Sump Pump To keep the filling of the sump tank constant during operation and to determine the concentration of the components in the sump, a sample has to be taken. The flow of the sump is controlled by the gear pump P02. The power of the pump is Figure 18: Sump entered in “Feld 3” (Figure 18). The pump is turned on by pump pressing the button “Start P02” so that the liquid is pumped into tank B04. 21 4. Feed Temperature The temperature of the feed is controlled by the preheater. The preheater is heated by a quartz crystal heating rod. The power is controlled, appropriate to the power of the sump pump, automatically or manually in the operating field “Regelung Vorheizer” (Figure 19). The automatic control is started by pressing the button “Start Temp.”. The desired temperature can be entered in “Feld 4” (figure 18). The Figure 19: Feed manual control is activated by pressing the button “Manuell”. Temperature The power of heating can be entered in “Feld 5”. The current temperature is shown in “Feld 4”. Note that the preheater is active just if the feed pump P01 is switched on! 5. Feed Pump The feed is supplied to the distillation column by a reciprocating pump. The unit of the pumping capacity is 1/min. The maximum pumping capacity is 180 1/min. By pressing the button “Start P01” (Figure 20), the pump is Figure 20: Feed started. The desired pumping capacity is entered in „Feld 6“ pump (Figure 20). 6. Reflux Separator The reflux separator works electrically and is controlled by the operation field „Rücklaufteiler“ (Figure 21). If the reflux separator is turned off, the condensate flows back into the column completely (reflux = infinity). Activating the Figure 21: reflux separator button “Automatik” turns on the automatic control of the reflux separator, which switches between supply pipe and return pipe. The value in “Feld 7” shows the time in which the condensate (as a product) flows from the column into the tank B03 (Figure 23). „Feld 8“ shows the time in which the condensate is fed back to the column. The example in Figure 21 expresses a reflux ratio of 𝑣 = 5⁄1. If the reflux separator is turned on, the button „Automatik“ flashes green. The 22 extraction of condensate with infinite reflux occurs with activating the button „Abnahme“. 7.3. Experimental Procedure After preparation of plant and refractrometer, as shown in chapter 7.1, it is possible to start the experiment. First, the feed has to be prepared by using a balance. The refractive index and the mass fractions of the solution have to be determined. Then the solution is filled into the column and the desired sump temperature has to be set. The sump heater cannot be turned on if the fluid level is too low or too high. In these cases the sump tank has to be filled or flushed. As soon as the vapor starts condensing, the supply feed can flow, the desired reflux ratio can be set and the reflux separator can be turned on. Depending on the parameters and the mass fractions of the feed, the distillation process is steady-state (equilibrium state) after 40-45 minutes. The samples of the head product are taken from tank B03. The samples of the sump product are taken from tank B04 (Figure 23). The refractive indices have to be determined directly after extraction because of the easily volatilizing ethanol. It is necessary to take three samples of each operating condition at intervals of seven minutes. Finally, pumps and heater are turned off. The coolant is not turned off until the column is cooled down. 8. Task Formulation The number of equilibrium stages is supposed to determine for the following operating conditions: Sump Temperature: 92- 93°C Feed Flow: 60 min-1 Reflux Ratios: (∞); (5:1) ; (5:2) ; (2:5) Feed Composition: 1. Mass fraction of Ethanol in Feed: - 30% Thermical State of Feed:: - Subcooled Liquid (23°C) 23 9. Special Safety Instructions During operation the plant gets hot. There is danger of burning! There is risk of shock because of the electrical parts of the plant. Avoid liquids in electronics! Ethanol is highly flammable – avoid ignition sources! Operate at atmospheric pressure! Negative pressure caused by vacuum pump P03 (Figure 23) may lead to implosion. The vacuum pump must not be used during the experiment. 10. Short Questions 1. Draw the schematic layout of a distillation column! 2. Explain the functional principle of a distillation column! 3. What diagram shows the composition of the vapor and liquid phase (at constant pressure)? 4. How are mixtures classified? 5. Which laws describe the behavior of these mixture? 6. What is an azeotrope? 7. What kind of column types do exist? 8. How are the different column types designed? 9. Explain the McCabe-Thiele method for the determination of the number of equilibrium stages by using the x,y-diagram! 10. Does the feed state have an influence on the distillation? 11. Which function does the reflux ratio in a distillation column have? 12. Which property is measured by a refractrometer? 24 11. Appendix Figure 22: Process picture of the used distillation column 25 Figure 23: Flow sheet of the used distillation column 26 boiling-point diagram Ethanol-Water 100°C bubble-point curve dew-point curve temperature 95°C 90°C 85°C 80°C 75°C 0% 10% 20% 30% 40% 50% mass percent Ethanol 27 60% 70% 80% 90% 100% refracting index Ethanol-Water 1,3650 1,3600 refracting index 1,3550 1,3500 1,3450 1,3400 1,3350 1,3300 0% 10% 20% 30% 40% 50% mass percent Ethanol 28 60% 70% 80% 90% 100% Table 2: refracting index at various mass percent ethanol at 25°C Mass percent Ethanol Refracting index at 25°C 000% 1,3324 010% 1,3389 020% 1,3458 030% 1,3518 040% 1,3560 050% 1,3594 060% 1,3614 070% 1,3629 080% 1,3631 085% 1,3629 090% 1,3623 095% 1,3611 100% 1,3591 Table 3: enthalpie temperature pressure [°C] [bar] 23,0 84,0 86,0 95,7 Enthalpie 30 mass% Ethanol in Water [kJ/kg] 0063,16 1 0310,53 0595,00 2198,92 29 Azeotropic point xETOH=95,6% equilibrium diagramm Ethanol-Water 1 0,9 Azeotropic point xETOH=95,6% 0,8 mass percent Ethanol in vapor 0,7 0,6 0,5 0,4 0,3 0,2 0,1 0 0 0,1 0,2 0,3 0,4 0,5 0,6 mass percent Ethanol in liquid 30 0,7 0,8 0,9 1 Table 4: vapor-liquid equilibrium for the system ethanol-water pressure [bar] temperature [°C] Mass percent ethanol in water liquid 1 vapor 99,6288479 0 0 96,7856255 0,025 0,2328148 94,4820698 0,050 0,3689599 92,5789585 0,075 0,4582788 90,9818473 0,100 0,5213259 89,6255695 0,125 0,5681531 88,4629583 0,150 0,6042613 87,4587360 0,175 0,6329184 86,5875692 0,200 0,6561343 85,8251803 0,225 0,6753756 85,1555691 0,250 0,6915564 84,5647358 0,275 0,7053575 84,0409024 0,300 0,7172811 83,5743468 0,325 0,7277075 83,1564579 0,350 0,7369322 82,7801245 0,375 0,7451902 82,4388467 0,400 0,7526721 82,1271245 0,425 0,7595370 81,8400134 0,450 0,7659200 81,5730689 0,475 0,7719388 81,3224578 0,500 0,7776987 81,0845689 0,525 0,7832957 80,8563466 0,550 0,7888201 80,6350688 0,575 0,7943585 80,4184022 0,600 0,7999961 80,2043466 0,625 0,8058186 79,9913466 0,650 0,8119140 79,7781799 0,675 0,8183741 79,5641799 0,700 0,8252969 79,3491799 0,725 0,8327880 31 79,1337354 0,750 0,8409632 78,9190687 0,775 0,8499508 78,7073465 0,800 0,8598947 78,5019576 0,825 0,8709582 78,3077353 0,850 0,8833278 78,1312909 0,875 0,8972188 77,9797353 0,900 0,9128930 77,8699575 0,925 0,9306173 77,8160686 0,950 0,9507564 77,8406798 0,975 0,9737210 77,9763464 1 1 12. 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