Column dispersion,accurate measurement

Many sea trials of dispersant chemicals to demonstrate the effectiveness of specific products or to elucidate the processes of oil dispersion into the water column have been described. Most tests have proved inconclusive, leading many to believe that dispersant chemicals are only marginally effective. Tests in a wave basin have been conducted to measure dispersant effectiveness under closely controlled conditions [261]. These tests show that dispersed oil plumes may be irregular and concentrated over small volumes, so extensive plume sampling was required to obtain accurate dispersant effectiveness measurements. In large-scale sea trials, dispersants have been shown effective, but only when sufficient sampling of the water column was done to detect small concentrated dispersed oil plumes and when it was known that the dispersant was applied primarily to the thick floating oil. 

The equations discussed in the previous chapter, that described the variance per unit length of a solute band after passing through an LC column, were all significantly different. It is, therefore, necessary to identify the specific equation that most accurately describes the dispersion that takes place, so that it can be employed with confidence in the design of optimized columns. The different equations were tested against an extensive set of accurately measured experimental data by Katz et at (1) and, in order to identify the most pertinent equation, their data and some of their conclusions will be considered in this chapter. 

Effectiveness of dispersants is difficult to determine as it is hard to accurately measure both the amount of oil in the water column and the oil remaining on the surface. While these are easier to measure in the laboratory, testing procedures vary greatly and may not always be representative of actual conditions. When testing in the lab, important factors influencing effectiveness, such as sea energy and salinity, must be taken into consideration. Results obtained from laboratory testing do not necessarily reflect what would take place in actual conditions, but should be viewed as a yardstick only. 

The dimensions of the exit tube from the detector are not critical for analytical separations but they can be for preparative chromatography if fractions are to be collected for subsequent tests or examination. The dispersion that occurs in the detector exit tube is more difficult to measure. Another sample valve can be connected to the detector exit and the mobile phase passed backwards through the detecting system. The same experiment is performed, the same measurements made and the same calculations carried out. The dispersion that occurs in the exit tube is normally considerably greater than that between the column and the detector. However, providing the dispersion is known, the preparative separation can be adjusted to accommodate the exit tube dispersion and allow an accurate collection of each solute band. 

The Van Deemter equation (1) was the first rate equation to be developed and this took place as long ago as 1956. However, it is only relatively recently that the equation has been validated by careful experimental measurement (2). As a result, the Van Deemter equation has been shown to be the most appropriate equation for the accurate prediction of dispersion in liquid chromatography columns, The Van Deemter equation is particularly pertinent at mobile phase velocities around the optimum velocity (a concept that will shortly be explained). Furthermore, as all LC columns should be operated at, or close to, the optimum velocity for maximum efficiency, the Van Deemter equation is particularly important in column design. Other rate equations that have been developed for liquid chromatography will be discussed in the next chapter and compared with the Van Deemter equation... 

A more accurate, manometric, technique has been used by Reith et al. (R10) and by Burgess and Calderbank (B32). The gas holdup in the dispersion is computed from measurements of the clear liquid height in the dispersion at successive manometer taps on the side of the froth-containing vessel. IfY( andjCf-i are the clear liquid heights recorded on the manometer at vertical positions Z, and Zf i above the column floor, then the gas holdup at the midlevel (/, i - 1) is given by... 

Since each calibration point is performed in a newly packed column, there is important data dispersion around the calibration line. In addition, the entire calibration procedure is rather time consuming. Therefore, another approach was developed, that provides more accurate results in a considerably shorter time. This recently published approach [32] employs a slightly modified version of the same FMC equipment. Instead of a flow-through arrangement enabling the measurement in steady state, the measurement is performed in a system with continuous circulation, as shown in Fig. 1. 

A simple computer program written in basic can be used to identify the optimum column length and other operating conditions. A copy of the program used in this study is shown in figure 10.11. The program is written in an extended manner to simplify explanation. Initially, the retention volume of both isomers need to be measured at three widely dispersed mobile phase compositions, and each at three widely dispersed column temperatures. The dead volume is taken as the retention volume of the solvent peak (the sample being made up in one pure component of the mobile phase). All measurements must be very accurate and made in duplicate. 

In the frame of alert systems, wind, waves and currents can scatter a few hours a large oil spill over a wide area within in the open sea. Moreover, while UV scanners capture the ultraviolet radiation reflected by the sea surface, false detection may occur due to the wind sheen. Both very low and very high wind speeds influence oil spill detection by radars. At high wind speed, even thick oil slicks are dispersed into the water column and oil caimot be detected at low wind speed it is not possible to distinguish between thick and thin oil sUcks false alarm or misinterpretation of SAR data can occur in the absence of accurate wind speed measurements. 

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