Adsorption pressure-programmed

Table V lists the independent variables and their respective ranges which were included in this study. Dependent variables can be defined in numerous ways depending upon the objectives of the particular experimental series being conducted. Typically, a pressure swing adsorption cycle program is input to the apparatus s computer control program, e.g., per Figure 8, and the cycle repeated under computer control until the experimental objective is achieved. For this work, 6-24 cycles were normally used with total run times up to 8 hours.

He) was introduced into the column, and the column pressure reached the adsorption pressure Pa. This process took about 5 seconds. Valve V-2 was then opened, and adsorption in the column from the inlet gas took place (adsorption step). During this period, the flow rate and concentration of CO, Gu and C/ were measured. (2) Valves V-1 and V-2 were closed, V-4 was opened, and the column was evacuated (desorption step). At the end of the desorption step, the pressure was below 13 Pa after 600 seconds. (3) As V-3 was also opened under evacuation, helium was supplied as a countercurrent puige to remove CO thoroughly (countercurrent purge step). (4) With V-3 closed and V-4 still open, the column was re-pressurized to the adsorption pressure with helium. The measurement conditions are summarized in Table 1, and the samples that were screened are listed in Table 2. The adsorption temperature was one parameter examined in this study, and a sequence controller was programmed for each set of conditions, so that steps (1) ( 4) were repeated over more than 4 hours. The total amount of desorbed gas was determined from the gas collected at the exit of the rotary flowm er. 

Temperature programming and particularly pressure programming provide an added dimension, along with the relative strengths of interactions of solute molecules with both the mobile fluid phase and the stationary phase, which contribute to the separation of compounds by SFC. Thus, SFC may prove to be more selective than HPLC in certain situations as the relative dominance of partition or adsorption interaction can be altered by pressure programming. Pressure is unquestionably the single most important parameter in SFC, and without pressure programming, some solutes would not be eluted. 

Undoubtedly, future research on SFC will expand its range of applications. While carbon dioxide remains the most widely used SFC eluent, investigations into the physico-chemical properties of other potential eluents should render more predictable separation results and increase the utility of SFC (e.g., 107). Applications to the isolation of natural products may prove uniquely suitable in difficult separations where pressure programming may complement relative adsorption to provide increased resolution. In this connection it is interesting to note a recent report on the use of liquid carbon dioxide as a solvent for TLC under sub-critical conditions for the separation of polycyclic aromatics (290). Resolution of the solute was good, but with a different order of elution compared to the use of hexane as mobile phase. 

A powerful technique in studying both adsorption and desorption rates is that of programmed desorption. The general procedure (see Refs. 36, 84) is to expose a clean metal filament or a surface to a known, low pressure of gas that flows steadily over it. The pressure may be quite low, for example, 10 mm Hg or less, so that even nonactivated adsorption can take some minutes for... 

The evolution of methylchlorosilanes between 450 and 600 K is consistent with the 550 - 600 K typical for the catalytic Rochow Process [3]. It is also reasonably consistent with the evolution of methylchlorosilanes at 500 - 750 K reported by Frank and Falconer for a temperature programmed reaction study of the monolayer remaining on a CuaSi surface after catalytic formation of methylchlorosilanes from CHaCl at higher pressures [5]. Both of these observations suggest that the monolayer formed by methyl and chlorine adsorption on pure CuaSi is similar to that present on active catalysts. For reference, methylchlorosilanes bond quite weakly to tiie surface and desorb at 180 - 220 K. It can thus be concluded that the rate-determining step in the evolution of methylchlorosilanes at 450 - 600 K is a surface reaction rather an product desorption. 

This study presents kinetic data obtained with a microreactor set-up both at atmospheric pressure and at high pressures up to 50 bar as a function of temperature and of the partial pressures from which power-law expressions and apparent activation energies are derived. An additional microreactor set-up equipped with a calibrated mass spectrometer was used for the isotopic exchange reaction (DER) N2 + N2 = 2 N2 and the transient kinetic experiments. The transient experiments comprised the temperature-programmed desorption (TPD) of N2 and H2. Furthermore, the interaction of N2 with Ru surfaces was monitored by means of temperature-programmed adsorption (TPA) using a dilute mixture of N2 in He. The kinetic data set is intended to serve as basis for a detailed microkinetic analysis of NH3 synthesis kinetics [10] following the concepts by Dumesic et al. [11]. 

Fig 3 shows the results of two temperature-programmed experiments. In the first (blank) experiment CH4 reacts with a "bare" FeZSM-5 zeolite, while in the second one it reacts with the zeolite after a-oxygen loading on its surface. Obviously, the bare surface is quite inert towards methane (Fig 3a) after reactor opening a weak CH4 adsorption occurs at room temperature. A slight heating results in a complete recovery of the CH4 pressure. 

Experimental values cited in the program. Estimated values bWater solubility in ppm at 25 C. Estimated from log Kow Vapor pressure in mmHg estimated by the modified Grain method dSoil adsorption coefficient in L/kg estimated by log Kow... 

In the Polybed version with ten adsorbent beds, three of them are in the adsorption phase at all times. Pressurization takes place in two steps, with an intermediate countercurrent purge by purified hydrogen, and a final cocurrem purge. Recompression also takes place in steps.The different sequences are programmed and automatically monitored. The yield in this case may reach 85 to 88 per cent for a feedstock containing 65 to 75 per cent volume hydrogen. 

Heats of adsorption can be experimentally measured by calorimetry, temperature programmed desorption (TPD), and adsorption isotherms taken at different temperatures. Calorimetry involves the direct measmement of temperature rise caused by the adsorption of a known amount of gas on to a well-characterized surface. TPD is the most coimnon method of determining the heats of adsorption. In this procedure, molecules are adsorbed on to a clean well-characterized substrate at a fixed temperature. The sample is then heated in a linear fashion while the pressure of the desorbing species is monitored with a mass spectrometer. The desorption rate E (t ) is given by... 

The computer simulation program which was available for miscible flood simulation is the Todd, Dietrich Qiase Multiflood Simulator (28). This simulator provides for seven components, of which the third is expected to be carbon dioxide and the seventh water. The third component is allowed to dissolve in the water in accordance with the partial pressure of the third component in the non-aqueous phase or pdiases. It is typically expected that the first two components will be gas components, while the fourth, fifth, and sixth will be oil components. There is provision for limited solubility of the sixth component in the non-aqueous liquid p ase, so that under specified conditions of mol fraction of other components (such as carbon dioxide) the solubility of the sixth component is reduced and some of that component may be precipitated or adsorbed in the pore space. It is possible to make the solubility of the sixth component a function of the amount of precipitated or adsorbed component six within each grid block of the mathematical model of the reservoir. This implies, conversely, a dependence of the amount adsorbed or precipitated on the concentration (mol fraction) of the sixth component in the liquid non-aqueous j ase, hence it is possible to use an adsorption isotherm to determine the degree of adsorption. 

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