Heat flow rate measurement

Only the enthalpy of reaction is measurable in calorimeters by measuring the energetic change in the reaction broth. A review of the different types of calorimeters is available in [8-10] and in this Volume. Some types of calorimeters used for biotechnological applications will be briefly described. 

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The reaction calorimeter used in this study was a Mettler s RCl with a 1 L reactor vessel. Both heat flow and hydrogen uptake were measured. The heat flow rate measured under isothermal conditions is directly proportional to a summation of the rate of each reaction step weighted by heat of reaction A// of the corresponding step, i.e.,... 

Figure 6.18 Idealized differential heat flow rate measurement of a differential scanning calorimeter for a sample with sudden exothermic transition top) and a sample with endothermic phase transition (bottom).

It should be mentioned that every substance needs heat to change the temperature. Heating a sample linearly needs a heat flow rate, which is proportional to the heat capacity and the heating rate. Consequently, the steady-state heat flow rate measured in the absence of any reaction or transition is never zero in a scanning calorimeter. 

Let 0,(t) represent the heat flow rate measured by a calorimeter related to growth reaction. Thus, the heat produced between the first observation at time /i and theyth observation at tj is ... 

Reductive metabolism sets in as soon as the glucose uptake exceeds the oxidative capacity of S. cerevisiae. Ethanol is formed and biomass yield decreases. It is therefore of interest to control the feed rate function of a fed-batch experiment in order to avoid ethanol formation. The heat flow rate measurement is sensitive to the onset of the reductive metabolism since the reductive enthalpy yield on glucose is much lower than the oxidative yield (12 versus 190 kJ C-mof glucose). 

Randolph et al. [78] developed a control strategy based on the estimation of the respiratory quotient of S. cerevisiae from heat flow rate measurements and rcoj In fact, the oxygen consumption rate can be estimated from and the oxycaloric quotient, Qo, since almost all the heat released is due to the... 

Control based on RQ and heat flow rate measurements... 

Figure 33 Heat dissipation rate

From Figure 38 it is apparent that the heat flow rate was higher than the set-point during 1 h after the dilution rate increase so that there was no feed flow rate correction. Then, at t = 2.1 h, the controller activates because the heat flow rate became lower than the set-point. This is shown in Figure 38 where the feed rate decreased at this moment. Controller action confirms that the culture was forced to work at its highest specific Oj consumption rate, but at the same time ethanol production was extremely low (Figure 39). Moreover, the heat flow rate measured was almost equal to its set-point value towards the end of the experiment resulting in little modification of the flow rate. As a consequence, feed... 

The above experimenters have used the technique described to obtain flow rate measurements of the liquid wall-film at various mass velocities, tube dimensions, etc., and some typical results from Staniforth and Stevens (S7) are shown in Fig. 7. Also shown are the values of burn-out heat flux obtained at the four different mass velocities indicated. It can be seen that the liquid-film flow rate decreases steadily with increasing heat flux until at burn-out the flow rate becomes zero or very close to zero. We thus have confirmation of a burn-out mechanism in the annular flow regime which postulates a liquid film on the heated wall diminishing under the combined effects of evaporation, entrainment, and deposition until at burn-out, the film has become so thin that it breaks up into rivulets which cause dry spots and consequent overheating. 

The versatility and accuracy of the oxygen consumption method in heat release measurement was demonstrated. The critical measurements include flow rates and species concentrations. Some assumptions need to be invoked about (a) heat release per unit oxygen consumed and (b) chemical expansion factor, when flow rate into the system is not known. Errors in these assumptions are acceptable. As shown, the oxygen consumption method can be applied successfully in a fire endurance test to obtain heat release rates. Heat release rates can be useful for evaluating the performance of assemblies and can provide measures of heat contribution by the assemblies. The implementation of the heat release rate measurement in fire endurance testing depends on the design of the furnace. If the furnace has a stack or duct system in which gas flow and species concentrations can be measured, the calorimetry method is feasible. The information obtained can be useful in understanding the fire environment in which assemblies are tested. 

For flow rate measurements the volume or, more conveniently, the mass flow is suitable. In the first case a pressure- and temperature-dependent calibration is necessary if the gas does not show ideal behavior. This also applies for heat conductivity as the measured quantity often used in flow meters. Currently, real pressure- and temperature-independent measurement of a hydrogen mass flow of a hydrogenation remains problematic on the laboratory scale, at least for low substrate concentrations. 

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) was designed to obtain the enthalpy or the internal energy of those processes and also to measure temperature-dependent properties of substances, such as the heat capacity. This is done by monitoring the change of the difference between the heat flow rate or power to a sample (S) and to a reference material (R), A

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In practice, the true heating rates (dT/dt)ca and (dT/dt)cb are assumed to be equal to the programmed scan rate j3, and the true heat flow rate difference (heat flow rate difference, Ao, which reflects the intrinsic thermal asymmetry of the differential measuring system ... 

As indicated in Equation 8.3, qtot is not generally simply equal to the reaction heat-flow rate qReact (see Equation 8.4) but is affected by other physical or chemical processes which have heat changes, e.g. mixing or phase changes. As will be shown in Section 8.3, even for a simple reaction such as the hydrolysis of acetic anhydride, a significant heat of mixing occurs which must be taken into account. Furthermore, it should always be kept in mind that the qtot values determined by a reaction calorimeter also contain measurement errors such as base line drifts, time distortions or ambient temperature influences. 

Fig. 8.4 Heat flow rate (qtot) forthe hydrolysis of acetic anhydride measured at25,40 and 55°C. Reprinted in modified form with permission [18]. 

When the measured heat-flow rate (qtot) curve at 25°C is compared with the results obtained from the IR signal (see Fig. 8.5), it again becomes clear that the initial peak of the qtot curve, which is not visible in the IR signal, is not related to the chemical reaction but to the mixing. In this simple case, we have an excellent example of how the simultaneous... 

De Vos assumes that the rate of running costs of the plant, C, is made up of two parts. One part is the capital cost, which is assumed to be proportional to the investment and therefore proportional to the size of the plant. The other part consists of the fuel cost and is therefore proportional to the heat flow rate Qin- It is then assumed that Q x is an appropriate measure for the size of the plant and thus... 

The heat flow rate is measured by detecting the mass flow rate of the fuel gas and multiplying it by its heating value, which can be detected by Wobbe index sensors or calorimeters. Continuous and explosion-proof calorimeters are available for measurement of the heating value of any fuel gas (Section 3.2.4). Table 3.52 lists the compositions of various fuel gases. 

The heat flow rate (Q) of a gaseous fuel is calculated as the product of its volumetric flow rate at standard conditions (V0) and its calorific value (CV). The Wobbe index (WI) measures the ratio between the net CV and the square root of specific gravity (SG). With orifice-type flow sensors, the advantage of detecting the WI is that it eliminates the need to separately measure the specific gravity this is because the product of the WI and orifice pressure drop results in a constant times the heat flow rate (KxQ), without requiring a separate measurement of SG. 

Several effects can play a role in the temperature measurement accuracy. Due to the small channel length, the temperature difference between the channel outlet and inlet can be as small as the sensor sensitivity. Thermocouples can have a size comparable to the channel dimensions and where is measured the temperature is questionable. Moreover, the heat flow rate through the thermocouple itself can be not negligible. The importance of these effects must be appreciated. 

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