Lab report Essay Example for Free

Lab report EssayA molecular(a) public exposure experiment of propanone-air (redundant w/ last sentence. . concise)was conducted with the goal of determining the spreading coefficient of acetone into air. For this experiment, acetone was placed in a outpouring tub 3mm OD, 2mm ID . . (is that correct? ) NMR tube? e and was every last(predicate)owed to diffuse into non-diff employ air that was passed over the footrace tube. The air that passed over the tube was from natural circulation in the way of life and no air was forced over the top of the test tube.The dispersion occurred over a period of somewhat eight hours, with readings taken each hour. After analyzing the info collected from the per fashionance of this experiment, tThe airing coefficient was calculated to be 0. 098 + 0. 02 cm2/s at T = ?. After completing our calculations, oOur results were then compared using the Chapman-Enskog comparability as rise as the Fuller, Schettler, and Giddings method. The diffu sion coefficient calculated by the Chapman-Enskog was 0. 990 + 0. 001 cm2/s and the result of the Fuller, Schettler, and Giddings method was 0.104 + . 002 cm2/s. The books value found in Perrys Chemical Engineers Handbook was 0. 125 + 0. 00 cm2/s. (at T = ?. . . or extrapolated from? ) The agreement of our method with the other methods available for calculating the diffusion coefficient was very good (how good is very good. .. signifi give the axet discrepencies or not? ), and also agreed well with the literature value found. This conduct to a conclusion that this method of determining the diffusion coefficient of acetone into air can be aconsidered a reasonably reliable method. BACKGROUNDMolecular diffusion is the transfer or movement of individual molecules through a fluid by random molecular movements (Geankoplis 412, year of publication). In the diffusion process, the molecules of interest current from regions of high concentration to low concentration. Molecular diffusion can occur in both directions with the system. In the case of the diffusion tube experiment, however, acetone diffuses through non-diffusing air, which is passed over the top of the test tube containing the acetone. The air is all toldowed into the test tube, but does not diffuse into the acetone.Molecular diffusion of gases has been studied for many years. Molecular diffusion is a mass transport process Motivation for its study comes from the fact that chemical separation processes such as distillation, drying, ion exchange systems as well as many other processes dep wind up on molecular diffusion (Kirk-Othmer Vol 8, p 149(check format)). EXPERIMENTAL METHODS For the performance of this experiment, a junior-grade test tube was filled approximately a third full of acetoneBe specific. . how small, starting height, diam, etc.This test tube was then vertically placed in a 10mL graduated cylinder which contained small beads. The purpose of the beads was to look that the test tube rema ined vertical. This assembly was then placed on a digital scale. The amount of air movement provided by the ventilation system was assumed to be adequate so as to ensure that the concentration of the acetone at the top of the tube was zero. An initial acetone level in the test tube was taken, as well as the mass of the assembly and the temperature of the subject surrounding the assembly.After this initial data was taken, the area temperature and mass of the assembly were taken approximately every hour for the following eight hours. The final level of the acetone in the test tube was taken when the final temperature and mass reading were taken. DISCUSSION OF RESULTS From the data collected from the experiment, the diffusion coefficient was calculated using equation 6. 2-26 from Geankoplis (Equation 1) As the z value was only preserve at the beginning and the end of the experiment, the intermediate values of z had to be calculated.The following equation was used for the calculation of the intermediate z values (Equation 2) Thus, all values but DAB were known and could be plotted versus time to obtain a linear plot. By rearranging equation 1, it can be seen that the list of this plot will be equal to 1/ DAB (Equation 1. 1) The initial plot of data which includes all invests is shown below in Figure 1. This plot contains all records and has an R2 value of 0. 9478. From this plot the molecular diffusivity coefficient was determined to be 0. 108 + 0. 022 cm2/s. Figure 1 First plot of data in Equation 1The second point in the data (t=2700s) showed no diffusion occurred in the first 45 minutes, which seems unlikely (yes, good- sensitivity of balance, etc). If this point is taken as erroneous, the R2 value goes up to 0. 9639 (more substantial here will be the confidence interval on the slop. . . get that from Tools- Data Analyis-Regression menu in Excel or else in Polymath or TableCureve, etc) and the molecular diffusivity calculates out to be 0. 098 + 0. 021 c m2/s. The plot of the experimental data excluding the second point is presented below in Figure 2.Figure 2 Second plot of data in Equation 1. . forcing through zero point is good. . . looks to me like first FOUR points would give a lower Dab then the last 4. Problems with next 3 that lie below line? Anytihing suspicious happening here? To determine the time it takes for the system to reach steady state, the following equation can be used to calculate the fraction of steady state the system is at (Equation 3) By plotting the value of ((NA)t/(NA)t=? ) versus time, the curve in Figure 3 was generated which demonstrates the systems come near to steady state. Wow, great Cite source.(still wonder about SST conditions of 1st 4 pts though. . . Figure 3 Fraction of steady state versus time From this plot, it could be give tongue to that the system achieves steady state in 115 minutes however, there is strong evidence this may not be accurate. As mentioned earlier, the second point may be erroneous. This would change the path of the curve. In addition, data was not collected at a high enough frequency for this curve to be highly accurate at predicting the time to steady state. If in fact the second point is erroneous, the system could energise come to steady state well before 115 minutes.This time of 115 minutes at best, could be the upper bound (or lower bound according to Whitakers criteria in his bind (handout). . . not sure for the time it takes for the system to come to steady state. The scatter in the data can be attributed to various factors in the experiment. The scatter could be attributed to the changes in temperature, as the temperature did fluctuate slightly through the duration of the experiment Good. At what time did it stabilize?. The change in temperature would cause a change in the partial pressure of the acetone leading to further deviations.In addition, there was no measure of airflow past the tube. Changes in the airflow could also have contri buted to the scatter as it could effect the concentration of the acetone at the top of the test tube (Good ). The diffusion coefficient was also calculated using the Chapman Enskog equation, (Equation 4) and the Fuller, Schettler and Giddings method. (Equation 5) A literature value was also found for acetone at K(check Perrys), which was corrected to our experimental temperature using the correlation (Equation 6)The values obtained with these methods as well as those from the experimental data are presented in Table 2. Table 1 Values of molecular diffusivity coefficients found. ** ** A very good way to show this graphically in Excel would be to use a bar graph showing the values of Dab as height of a bar by method used, and error bars to easily demonstrate any overlap of uncertainty, discrepancy, etc. Example The Chapman Enskog method is accurate within 8% and the Fuller Schettler and Giddings value has a lower accuracy than the Chapman Enskog (Geankoplis 425).The Chapman Enskog va lue is less than 1% different than the experimental value and the Fuller Schettler and Giddings value only about 6% different. From this analysis, it seems these equations predicted the experimental value very well. These calculated values are about 20% lower than the literature value. This variance may come from the inconsistent temperature in the room or from pressure fluctuations in the room caused perhaps by the starting and stopping of the HVAC systems. For the derivation of Equation 1, several assumptions are make. Beginning with the general equation (Geankoplis 6.2-14) (Equation 7) One assumption was that because the case examined was a diffusing A (acetone) into non-diffusing B (air), the diffusion flux of air into the acetone (NB) was equal to zero. Another assumption made was that since the total pressure was low, the acetone gas diffusing into air was an ideal gas. This allowed for the term c to be replaced with its ideal gas equivalent, P/RT. Additionally, the air extre mely over the test tube was assumed to contain no water vapor. An average air velocity that was uniform was passing over the acetone containing test tube was also assumed.There are non-idealities that exist in the molecular diffusion of acetone into air. Some of these non-idealities are corrected for in the journal from downwind and Wilke. Acetone displays surface tension effects which, instead of having a perfectly horizontal liquid surface, give the liquid acetone a slightly downwards curved liquid level. Because of this curvature, the actual diffusion path length that the acetone travels is smaller than what the diffusion length would appear to be based on join liquid level or calculated liquid volume (Lee 2384).Along with a non ideal liquid surface, the air passing over the open end of the tube may cause some upheaval to exist in the top portion of the tube. With its existence, the turbulent area of the tube will cause a length to exist inside the tube where the concentration of acetone is zero. With the presence of this acetone vapor-free region, the diffusion length is again shorter than it would appear to be. To account for the non-idealities in the diffusion process, Lee and Wilke do not use the apparent diffusion path. Instead, they use an good average diffusion path which they give by(Equation 8) Where x is the effective average diffusion path, ? xs is the length of the curvature of the non-ideal liquid to account for the surface tension forces, ? xe is the length of the tube where the acetone vapor-free region exists due to turbulence that exists from the passage of the air, and ? x is the sum of ? xs and ? xe (Lee 2384). When this is substituted back into the diffusion equation, it becomes the following (Equation 9) Where Da is the apparent diffusion coefficient and D is the true diffusion coefficient based on the true diffusion path (Lee 2384).The way our experiment was setup, the driving force for the air across the test tube was natural air flow and did not employ forced air flow. Because of this, the length of the tube where the turbulence existed in the Lee and Wilke journal would most likely not have been present in our experiment. Also, the initial liquid acetone level selected in our experiment was such that the length of the curvature due to the surface tension forces on the acetone would have been negligible when compared to the apparent diffusion length of the tube.The initial height of the liquid in the tube for this experiment was chosen wisely. The reason for this is that with the initial level that was chosen, a sufficiently long diffusion path existed such that the non-idealities that were accounted for in the Lee and Wilke journal entry would have had a very insignificant impact on the results of our experiment. CONCLUSIONS AND RECOMMENDATIONS From the data collected an analyzed, it has been determined that the experimental procedure used here can determine the molecular diffusivity coefficient with some level of accuracy.For future experiments, some form of air flow regulation should be investigated. Something as simple as a room fan could be placed next to the scale to ensure a more constant air flow. Another increase in accuracy could be achieved by regulating the temperature with more consistency. If the experiment could be performed in a large insulated room, the temperature may not vary as much. Good job on Discussion, Conclusions, etc. . . to improve maybe expand to relate what YOU mean are the main uncertaintys that caused problems in your particular case and show evidence to support.