Sampling lines, constant pressure

The sampling of a suction lysimeter is initiated by applying a vacuum (approximately 40-50 cm of mercury) through the vacuum/pressure line with a hand pump or electric pump. The valve on the sampling line must be closed. A constant vacuum may be maintained on the lysimeter using an electric pump. The time required before collecting a sample from a lysimeter will depend on the method of vacuum application, the moisture content of the soil, and the soil type. 

The direction and flow rate of the test and hydraulic fluids are determined by nine three-way valves and six a1r-dr1ven hydraulic pumps that must be sequenced 1n the proper order. The position of the valves 1s determined by six air-driven actuators. Two of the pumps are miniaturized, air-driven, hydraulic pumps used for sample loading and pressurization. One of the remaining four pumps 1s a high-pressure, constant volume, positive displacement, piston metering pump to provide hydraulic pressure, and the other three are positive displacement syringe pumps for In-line addition of additives. 

The line EH represents constant partial water vapor pressure, as should exist in the system of Figure A2.1.1. The point E at T is the dew point for the sample with vapor pressure represented by the point H at T2. Any point... 

More importantly, the result of Booth et al. also suggests that only massive hydrate samples can survive the trip from the bottom of the ocean to ship deck. For example, if the massive MAT Guatemala 2 sample (topmost in Figure 7.7) were recovered at constant pressure, the temperature would need to rise more than 16°C before the sample reached the three-phase line, where dissociation would begin. This result is consistent not only with laboratory determinations for dispersed hydrates (Kumar et al., 2004 Pauli et al., 2005 Wright and Dallimore, 2005) but also shows the parallel of recovered core dissociation with radial dissociation due to depressurization in pipelines, modified for sediment content (Davies and Sloan, 2006). 

The stop-flow technique is the simplest and least expensive method of injection. The flow of the mobile phase is first stopped, either by turning an on—off valve in the line before the column (with constant-pressure systems), or by stopping the pump (with constant-flow systems). The column then returns rapidly to atmospheric pressure, and the sample can be injected directly on to the column with a normal low-pressure syringe. 

Complete each line in the following table for a given sample of gas, assuming constant pressure ... 

The extract is collected by depressurization on a column packed with a solid sorbent, in a vessel containing the appropriate solvent, in a collection device connected to a chromatograph, or on combined solid phase-solvent traps [92]. For extraction of volatile compounds, such solvents as acetone, CH2CI2, methanol, or liquid nitrogen are used. Silica gel columns are the most popular way of trapping solids. In this case, the selectivity of the process can be improved by selective elution of the sorbent [88, 92]. SFE can be conducted in a static mode in which sample and solvent are mixed and kept for a user-specified time at a constant pressure and temperature, or in a dynamic mode where the solvent flows through the sample in a continuous manner [56]. The extracted analytes can be collected into an off-line device or transferred to an on-line chromatographic system for direct analysis. 

Look at Figure 14-3, which includes a graph of volume versus temperature for a gas sample kept at a constant pressure. Note that the resulting plot is a straight line. Note also that you can predict the temperature at which the volume will reach a value of zero liters by extrapolating the line at temperatures below which values were actually measured. The temperature that corresponds to zero volume is —273.15°C, or 0 on the kelvin (K) temperature scale. This temperature is referred to as absolute zero, and it is the lowest possible theoretical temperature. Theoretically, at absolute zero, the kinetic energy of particles is zero, so all motion of gas particles at that point ceases. 

The change of volume with temperature, at constant pressure, is illustrated in Figure 12-5. From the table of typical data in Figure 12-5b, we see that volume (L, mL) increases as temperature (t,°C) increases, but the quantitative relationship is not yet obvious. These data are plotted in Figure 12-5c (line A), together with similar data for the same gas sample at different pressures (lines B and C). 

Figure 13-18 Some interpretations of phase diagrams, (a) The phase diagram of water. Phase relationships at various points in this diagram are described in the text, (b) Two paths by which a gas can be liquefied. (1) Below the critical temperature. Compressing the sample at constant temperature is represented by the vertical line WZ. Where this line crosses the vapor pressure curve AC, the gas liquefies at that set of conditions, two distinct phases, gas and liquid, are present in equilibrium with each other. These two phases have different properties, for example, different densities. Raising the pressure further results in a completely liquid sample at point Z. (2) Above the critical temperature. Suppose that we instead first warm the gas at constant pressure from W to X, a temperature above its critical temperamre. Then, holding the temperamre constant, we increase the pressure to point Y. Along this path, the sample increases smoothly in density, with no sharp transition between phases. From Y, we then decrease the temperature to reach final point Z, where the sample is clearly a liquid.

Let us clarify the nature of the fluid phases (liquid and gas) and of the critical point hy describing two different ways that a gas can he liquefied. A sample at point IVin the phase diagram of Figure 13-18b is in the vapor (gas) phase, below its critical temperature. Suppose we compress the sample at constant T from point IV to point Z. We can identify a definite pressure (the intersection of line IVZ with the vapor pressure curve AC) where the transition from gas to liquid takes place. If we go around the critical point by the path WXYZ, however, no such clear-cut transition takes place. By this second path, the density and other properties of the sample vary in a continuous manner there is no definite point at which we can say that the sample changes from gas to liquid. 

FIGURE 5.8 Variation of the volume of a gas sample with changing temperature, at constant pressure. Each line represents the variation at a certain pressure. The pressures increase from P to P4. All gases ultimately condense (become liquids) if they are cooled to sufficiently low temperatures the solid portions of the lines represent the temperature region above the condensation point. When these lines are extrapolated, or extended (the dashed portions), they all intersect at the point representing zero volume and a temperature of -273. 5°C. 

Redox experiments and ESR determination of Cu2+ were performed with a circulation all-glass apparatus equipped with a magnetically driven pump. The sample (0.2 to 1.0 g) was placed in a silica reactor equipped with a side ESR tube. All the samples before the redox cycles were treated in O2 at 773 K. The redox cycles consisted of (i) heating in He flow at 823 K for 2h, followed by evacuation at 773 K and heating in O2 at 773 K (ii) evacuation at RT followed by reduction with CO at 773 K (iii) evacuation at 773 K followed by a second treatment with O2 at 773 K. During the treatments (i) to (iii), the pressure of O2 or CO was monitored with a pressure transducer (MKS Baratron, sensitivity 1 Pa) until a nearly constant pressure was reached. All these measurements allowed the variation of the average oxidation number of copper to be followed. The acquisition or loss of electrons are expressed as e/Cu (number of electrons/total number of Cu atoms). At the end of treatments (i) to (iii), ESR spectra of Cu2+ species were recorded at RT. ESR measurements were carried out on a Varian E-9 spectrometer equipped with an on line computer. Absolute concentrations of... 

The heats of combustion of gaseous hydrocarbons are presently determined by using a constant pressure flame calorimeter (ref 8). However, calorimetric measurements cannot be made on-line and require information about the thermal properties of the combustion products of the test sample. The technique reported here, on the other hand, is direct, can be performed on-line, and requires no prior knowledge about the composition of the test sample. 

The heats of combustion of hydrocarbons are presently determined by using a constant volume bomb calorimeter for liquids and solids and a constant pressure flame calorimeter for gases. These measurements can be very accurate (< 1 percent), since they depend mainly on the bath temperature measurement. However, calorimetric measurements cannot be made on-line and require information about the thermal properties of the combustion products of the test sample. Tire technique reported here, on the other hand, is direct, can be performed on-line, and requires no prior knowledge about the exact composition of the test sample. (The only assumption made regarding the composition is that saturated hydrocarbons are the only combustibles present in the test samples). It thus appears that this new technique may be more useful for field operations where on-line measurements of the heats of combustion of the test gases are often needed. 

A spreadsheet to calculate the specific cake and medium resistances fi om constant-pressure filtration data if filtration time and volume filtrate are measured. Other required inputs are the total filtration pressure used, physical data on the slurry and the masses of wet and dry cake-or a sample of the cake. Cake moisture omtent is calculated from the last two inputs, and is needed for the calculation of mass of dry cake per unit volume filtrate and, hence the specific cake resistance from the gradient o the line of best fit between the time over filtrate volume plotted against filtrate volume. The example data were used in Figure 2.7, and the woriced exanqrle fidlowing that figure. Reference should be made to Equations (2.24) and (2.25). 

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