Isothermal-temperature ramp

Fig. 13. Evaluation of kinetic parameters for the DOC model—NO oxidation (reaction R5 in Table II). Comparison of measured and simulated outlet NOx concentrations in the course of temperature ramp (2K/min) for two different space velocities (SV= 50,000 and 100,000 h-1). Lab experiment with isothermal monolith sample using synthetic gas mixture (100 ppm CO, 100 ppm C3H6, 500ppm NO, 8% 02, 8% C02, 8% H20, N2 balance).

At higher temperatures a significant amount of reaction may occur during the initial temperature ramp before approximate isothermal equilibrium has been attained. Some degree of correction for this is possible 16,17) by re-running the experiment on the reacted sample, under the same conditions, to obtain an estimate of the true baseline, as illustrated in Fig. 4. 

Curemeters are usually run under isothermal conditions to determine cure parameters at the temperature(s) of interest. Rosea and Vergnaud123 investigated the use of a temperature ramp as being a more efficient way of obtaining kinetic parameters. 

Figu re 11.13 Thermogram in a Calvet calorimeter showing the main reaction performed under isothermal conditions and the secondary reaction triggered during the temperature ramp (solid line and baseline). The dashed line is the temperature and the dash-point line the pressure. 

Radex Safety Calorimeter. The Radex calorimeter is a modular instrument that can simultaneously evaluate six different samples (size range 0.5 to 5 ml), or one substance under a variety of conditions. Each module is a separate entity with its own calibrated oven capable of being operated under an open, closed, or pressurized condition, with all temperature differences between the sample and the oven being stored in a microprocessor for further analysis. The Radex calorimeter is very versatile samples can be tested in either an isothermal or ramp mode. In the isothermal mode, each oven is heated to a preset temperature and held at that temperature throughout the experiment. In the ramp mode of operation, the oven is heated linearly to a preset temperature, or can be maintained at a given temperature for a predetermined time. The flexibility of oven function in the Radex calorimeter enables the user to determine the intrinsic stability of a chemical and to also compare the impact of such parameters as temperature, atmosphere, and impurities on the stability of a given substance. 

In the Applications categories of Table 2, except for those papers identified as presenting post-cure results, all of the papers involve curing. No differentiation was made as to isothermal or ramped cure, since both types of data would be of importance to any particular resin system. The heading Cure Rate and/or Catalyst Studies includes those papers in which explicit correlations between cure temperature or catalyst concentration are presented, whereas the heading General... 

Figure 31 F-3 High-speed chromatogram obtained with isothermal operation (30°C) for 37 s followed by a 35 C/min temperature ramp to 90°C. (Reprinted with permission from H. Smith and R. D. Sacks, Anal. Chem., 1998, 70, 4960. Copyright 1998 American Chemical Society.)...

In the following kinetic analysis of the cure of TGDDM with DDS at elevated temperatures, the equations are developed from both DSC and MR data (in some circumstances collected simultaneously (de Bakker et al, 1993)), and enable quantitative estimation of the reactive-group profile at any time during isothermal (and in some cases temperature-ramped) cure. 

In this the way the effect of the rate of temperature ramping may be taken into account in the total cure profile. In such studies the empirical E data may be adequate, but for detailed kinetic modelling it is generally recognized that isothermal experiments are appropriate, and even they may have limitations, as discussed later. 

The work of several authors (Peters et al, 1993, Halley et al, 1994) has demonstrated the use of non-isothermal dynamic sweep tests to examine the combined effects of shear rate and curing on the chemoviscosity of a highly filled epoxy resin simultaneously. These tests use a selected temperature ramp with repeated dynamic rate sweeps to investigate the effects on the chemoviscosity. The advantage of these tests is that the effects of shear rate and cure are not separated, which is similar to processing conditions. 

In collaborating with LLNL, R. Synovec s team at the University of Washington demonstrated very repeatable separations of alkanes and other analytes with the SWNTs grown in LLNL s 50-cm channels, under both isothermal and temperature ramped (as high as 60 °C s ) conditions, with H2 as carrier gas and 3 ms injection pulses [7]. The four-compound test mixtures were separated within... 

The first of the separation techniques to be used in process measurement was gas chromatography (GC) in 1954. The GC has always been a robust instrument and this aided its transfer to the process environment. The differences between laboratory GC and process GC instruments are important. With process GC, the sample is transferred directly from the process stream to the instrument. Instead of an inlet septum, process GC has a valve, which is critical for repetitively and reproducibly transferring a precise volume of sample into the volatiliser and thence into the carrier gas. This valve is also used to intermittently introduce a reference sample for calibration purposes. Instead of one column and a temperature ramp, the set up involves many columns under isothermal conditions. The more usual column types are open tubular, as these are efficient and analysis is more rapid than with packed columns. A pre-column is often used to trap unwanted contaminants, e.g. water, and it is backflushed while the rest of the sample is sent on to the analysis column. The universal detector - thermal conductivity detector (TCD)-is most often used in process GC but also popular are the FID, PID, ECD, FPD and of course MS. Process GC is used extensively in the petroleum industry, in environmental analysis of air and water samples" and in the chemical industry with the incorporation of sample extraction or preparation on-line. It is also applied for on-line monitoring of volatile products during fermentation processes" ... 

Although the TS mode of operation does not require isothermal or steady-state conditions, it is assumed that the reaction is at all times in steady state with respect to certain steps in the reaction. For example, in catalytic TS-PFR operation, it is taken that the adsorption/desorption steady state is achieved much mare rapidly than the time scale involved in the temperature scanning procedure. In the TS-CSTR we assume this, as well as the fact that complete mixing of reactor contents takes place on a time scale much shorter than the temperature ramping. Moreover, although there may be temperature differences and heat flows between various components of the reactor, of the catalyst, and of the reactants, these should not be flow-velocity-dependent, nor should there be any flow-velocity-dependent diffusion effects. 

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