Isothermal mode, measurement methods

The precise measurement of competitive adsorption isotherms not only of theoretical importance but may help the optimization of chromatographic processes in both analytical and preparative separation modes. The methods applied for the experimental determination of such isotherms have been recently reviewed [90], Frontal analysis using various flow rates can be successfully applied for the determination of competitive adsorption isotherms [91]. 

There is some merit to each of these methods, and both have been used. In isothermal mode, we estimate the change in the value of At]) with T for the reference electrode. In nonisothermal mode, we estimate the additional potential drop generated in the salt bridge, as a result of the thermal gradients. Neither can be measured directly. Subsequently, there is always some uncertainty in the value of the enthalpy of activation of electrode reactions. 

Many chemical reactions, such as polymer formation reactions, arc exothermic and readily monitored by DSC methods. Here, the determination of the rate of heat release, d/lldi, is used to determine the extent of reaction as a function of lime. Polymerization kinetics can be studied in both a temperature scanning and an isothermal mode. With some polymer systems, factors such as monomer volatility and viscosity can affect the measured kinetics. 

The temperature-modulated mode of operation has been well known for many decades in calorimetry [33], but became well established only during the 1990s, when commercial DSC was modified this way [34], The idea is to examine the behavior of the sample for periodic rather than for isothermal or constant-heating-rate temperature changes. In this way it is possible to obtain information on time-dependent processes within the sample that result in a time-dependent generalized (excess) heat capacity function or, equivalently, in a complex frequency-dependent quantity. Similar complex quantities (electric susceptibility, Young s modulus) are known from other dynamic (dielectric or mechanical) measurement methods. They are widely u.sed to investigate, say, relaxation processes of the material. 

The thermal stability of polymers can be tested by thermogravimetric analysis (TGA). In this method, a sample of a few milligrams is deposited in a scale pan mounted on a furnace. TGA measurements can be done in isothermal mode by maintaining a certain temperature for a fixed period of time or in temperature cycling mode. Most often the data are obtained with a constant... 

As mentioned above, titration methods have also been adapted to calorimeters whose working principle relies on the detection of a heat flow to or from the calorimetric vessel, as a result of the phenomenon under study [195-196,206], Heat flow calorimetry was discussed in chapter 9, where two general modes of operation were presented. In some instruments, the heat flow rate between the calorimetric vessel and a heat sink is measured by use of thermopiles. Others, such as the calorimeter in figure 11.1, are based on a power compensation mechanism that enables operation under isothermal conditions. 

Differential Scanning Calorimetry (DSC) This is by far the widest utilized technique to obtain the degree and reaction rate of cure as well as the specific heat of thermosetting resins. It is based on the measurement of the differential voltage (converted into heat flow) necessary to obtain the thermal equilibrium between a sample (resin) and an inert reference, both placed into a calorimeter [143,144], As a result, a thermogram, as shown in Figure 2.7, is obtained [145]. In this curve, the area under the whole curve represents the total heat of reaction, AHR, and the shadowed area represents the enthalpy at a specific time. From Equations 2.5 and 2.6, the degree and rate of cure can be calculated. The DSC can operate under isothermal or non-isothermal conditions [146]. In the former mode, two different methods can be used [1] ... 

The halogenated method employs a packed column of 1% SP-1000 on Carbopak-B (60-80 mesh) as its primary analytical column. The column is 8-ft X 0.1-in. i.d. It is operated at a helium flow rate of 40 mL/min under programmed temperature conditions of 45 °C isothermal for 3 min, then 8 °C/min to 220 °C, and then held at 220 °C for 15 min or until all compounds have eluted. An electrolytic conductivity detector operated in the halide-specific mode is used for measurement. 

Similar to studies reported by Litwinienko and co-workers discussed above, a recent report (Dunn, 2006b) demonstrated that non-isothermal (conventional) DSC, static mode P-DSC and dynamic mode P-DSC may be employed to study kinetics of the oxidation of SME. OT results obtained at ambient pressure for DSC and P = 2000kPa for P-DSC and with varying p = 1-20 °C/ min were analyzed by the Ozawa-Flynn-Wall method to calculate activation energies and rate constants. This work concluded that rates of the oxidation reaction could be calculated at any temperature based on accurate measurement of kinetic parameters from analysis of non-isothermal dynamic mode P-DSC scans. 

An earlier study (Stavinoha and Howell, 2000) examined the effects of TBHQ and a-tocopherol on oxidative stability of SME from four different sources by non-isothermal P-DSC in static (zero air-purge) mode. P-DSC curves were analyzed by measuring the OT where P = 2000 kPa, initial temperature = 25 °C, and (S = 5°C/min. Results for two of the SME samples showed that addition of 2000 ppm a-tocopherol increased OT by -20 °C while addition of 2000 ppm TBHQ increased OT by -30 °C. Addition of the same concentration of a-tocopherol and TBHQ to the other two SME samples increased OT by -30 °C and -40 °C, respectively. Interpretation of these results suggested TBHQ was more effective at increasing relative resistance to oxidation of SME than a-tocopherol, a conclusion that was in accordance with those by Mittelbach and Schober (2003) for the isothermal Rancimat method. 

The effects of antioxidants on OT of SME by non-isothermal (conventional) DSC, static mode P-DSC, and dynamic mode P-DSC were investigated by Dunn (2006a), which is summarized in Table 1.15. Results from all three methods consistently showed that treating SME with antioxidants TBHQ and a-tocopherol increased OT with respect to untreated SME. Statistical comparison of P-DSC results with those from isothermal analysis of OSI at 60°C was facilitated by calculation of the corresponding response factors (defined ratios of OT of the sample to that of methyl oleate, and of OSI of the sample to that of methyl oleate). Data for the sample and reference material (methyl oleate) were measured under the same experimental conditions. Results showed the highest degree of correlation (P = 0.79) between dynamic-mode P-DSC and isothermal OSI analyses. 

In situ Raman spectroscopy analysis of isothermal and nonisothermal oxidation of DWCNTs in air showed a decrease in the intensity of the D band starting around 370°C, followed by complete D band elimination at 440°C. The oxidation process produced the purest CNTs ever reported, which were free of amorphous carbon and highly defective tubes, while the removal of amorphous material was not accompanied by tube damage. In situ Raman measurements allowed us to determine the different contributions to the D band feature and show the relationship between D band, G band, and RBM Raman modes in the Raman spectra of DWCNTs upon heating. The described approach thus provides an efficient purification method for DWCNTs and SWCNTs, which is also selective to tube diameter and chirality. While oxidation of MWCNTs did not significantly decrease the D band intensity below 450°C, oxidation in air can be an effective route to control the number of... 

This chapter starts with an introduction to modeling of chromatographic separation processes, including discussion of different models for the column and plant peripherals. After a short explanation of numerical solution methods, the next main part is devoted to the consistent determination of the parameters for a suitable model, especially those for the isotherms. These are key issues towards achieving accurate simulation results. Methods of different complexity and experimental effort are presented that allow a variation of the desired accuracy on the one hand and the time needed on the other hand. Appropriate models are shown to simulate experimental data within the accuracy of measurement, which permits its use for further process design (Chapter 7). Finally, it is shown how this approach can be used to successfully simulate even complex chromatographic operation modes. 

The method is advantageously combined with the frontal analysis method, which also requires a concentration plateau and thus shares the disadvantage of high sample consumption if operated in open mode. As indicated in Fig. 6.24, the measurement procedure starts at maximum concentration. This concentration plateau is reduced step-by-step by diluting the solution. To reduce the amount of samples needed for the isotherm determination the experiments can be done in a closed loop arrangement (Fig. 6.17). It is also possible to automate this procedure. 

Blumel et al. [18] measured the isotherms of (+) and(-) hetrazipine (WEB170) on cellulose triacetate, using the perturbation method operated in closed-loop mode to reduce the amount of chemicals needed. The data were satisfactorily modeled with the Langmuir equation. The separation of the enantiomers of a-ionone... 

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