Microflow calorimeter design for heats of mixing

Raal, J. David; Webley, Paul A.

doi:10.1002/aic.690330409

Cited by 18 publications

(11 citation statements)

References 30 publications

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“…The microflow calorimeter is a sensitive and convenient instrument which greatly facilitates the accurate evaluation of excess enthalpies [21]. The values of the enthalpy of dilution of diglycine in aqueous xylitol solutions were determined with a 2277-204 measuring cylinder and a Thermometric 2277 thermal activity monitor (Thermometric, Sweden) at the temperature of 298.15 K. The 2277-204 measuring cylinder has a reference section which is connected in series with the outlet from the used sample side of the cylinder.…”

Section: Calorimetric Measurementsmentioning

confidence: 99%

Enthalpy of dilution and volumetric properties of N-glycylglycine in aqueous xylitol solutions at T=298.15K

Liu¹,

Wang²,

Li³

et al. 2011

The Journal of Chemical Thermodynamics

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Section: Calorimetric Measurementsmentioning

confidence: 99%

Enthalpy of dilution and volumetric properties of N-glycylglycine in aqueous xylitol solutions at T=298.15K

Liu¹,

Wang²,

Li³

et al. 2011

The Journal of Chemical Thermodynamics

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“…These derivations show that, depending on the magnitudes of V E and the ease of mixing factor F, equal pressure drops in the mixing and reference arms of a flow calorimeter will not in general ensure equal viscous dissipation, that is, the pressure drop in two geometrically identical mixing and reference sections may differ appreciably, if F>O. Equations 18, 17 and 12 are those given without detailed derivation in our earlier article (Raal and Webley, 1987).…”

Section: Case B: Difficult-to-mix Systemsmentioning

confidence: 96%

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mentioning

confidence: 99%

Separation of fluid frictional energy and heats‐of‐mixing in a flow calorimeter

Raal

1993

AIChE Journal

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In an earlier article (Raal and Webley, 1987), a heat-ofmixing flow calorimeter design was described, in which a key feature was the ability to separate the desired excess enthalpy from fluid frictional effects, unavoidable in a flow calorimeter.Thorough mixing of fluids along the flow path necessarily entails a pressure drop which becomes particularly serious if one or both fluids is viscous.In our earlier work, working equations were given for frictional energy loss in the measurement and reference arms of a flow calorimeter and for the respective temperature rises due to friction (Eqs. 6, 7, 9 and 10 of the earlier work). As a starting point, the macroscopic energy and entropy balances for an open system were given, and the derivational paths were briefly sketched. The derivation of Eqs. 6 to 10, however, is not obvious and requires a fair amount of detailed work. The derivations are given in detail below. Also, examples are given to illustrate the errors arising from fluid friction in a flow calorimeter.A flow calorimeter with mixing and reference modules in series is shown in Figure 1. Pure fluids A and B enter at exactly equal temperatures and pressures TI, pI. Mixing is complete at the mixer exit, with temperature rise to T2 and pressure drop top2. It is assumed that exactly sufficient electrical energy, Q, is added to compensate for the excess enthalpy of mixing, hE. The second and third terms in the summation in Eq. 1 are kinetic and potential energies whose differences are assumed negligible across each calorimeter section. KE and PE changes can in any case be easily eliminated by experimental design. The sequence of mixing and frictional energy loss in a flow calorimeter cannot be predicted, but two limiting cases can be defined. In the first, mixing is completed before frictional effects are manifested ("easy to mix systems"), and the pressure drop is due to the flow of the fully mixed solution. In the second case (difficult-to-mix fluids), the pure unmixed fluids experience a frictional pressure drop in cocurrent flow and a corresponding temperature rise to T2 before mixing. The pressure drop and temperature rise equations for these two cases are derived as follows: Case a: Easyto-Mix SystemsThe excess enthalpy is properly defined at TI, p , :hhE=hhAB-hlhA-m2hg (3) where subscript A B is a homogeneous mixture. tion:From Eq. 1 neglecting PE, KE changes for the mixing secThe enthalpy of the exit mixture at T2, p2 (hAB) may be related to its value at TI, pI by postulating a path in which the pressure is reduced top2 at constant temperature TI and the temperature increased to T2 at constant pressure p2:Assuming Cp# f ( T) , a [ = + ( $ ) I = volume expansiv-P itY#f(p). Combining Eqs. 4 and 5 , and dropping the subscript AB gives:

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“…In these experiments the experimental fluid was brought to a temperature a few degrees above or below that of the water bath by passage through a coiled tube immersed in another bath. The coiled tube was of sufficient length, as calculated by Raal and Webly7 to ensure a very close approach of the fluid temperature to the bath temperature at all flow rates studied. No electrical energy was supplied to the heater elements.…”

Section: Instrumentationmentioning

confidence: 99%

“…Quantification of this and other heat losses as a function of fluid flow rate and physical properties has been the subject of considerable effort by many investigators, and remains the principal problem in flow calorimentry for heat capacity measurement. (In flow calorimetry for excess enthalpy measurement, the principal problem becomes elimination or subtraction of frictional heating, again a function of fluid physical properties, from the measured mixing effect (Raal and Webley,7 Raal8).…”

Section: Introductionmentioning

confidence: 99%

Heat capacity measurement by flow calorimetry: An exact analysis

Hei

Raal

2008

AIChE Journal

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in Wiley InterScience (www.interscience.wiley.com).The principal unsolved problem in flow calorimetry for liquid heat capacity measurement accurate accounting for heat loss from the heater lead-in wires as a function of system properties is analyzed by exact procedures for a five-zone calorimeter model. Temperature distributions in the fluid, and bi-metal wire are obtained from solutions of the governing third-order ODE in the fluid temperature for realistic boundary conditions. Conductive heat losses at the fluid exit q HL /q˙are large (up to 20% of energy input), and physical property and flow rate dependent. A new correlating equation for (q HL /q˙) gives separately and explicitly, for the first time, its dependence on calorimeter characteristics, flow rates and fluid properties. Experiments on five pure liquids confirmed the predictions of the theoretical model and produced Cp values in close agreement with literature data. Fluid friction and small convection heat losses (U i A i (DT) lm ) were accounted for experimentally. 2008 American Institute of Chemical Engineers AIChE J, 55: 206-216, 2009 Keywords: heat capacity measurement, flow, exact analysis IntroductionDesign procedures for heat-transfer equipment generally require values for the Prandtl number Prð¼ Cpl=kÞ as a function of temperature for pure liquids and liquid mixtures. Calculation of the evaporation ratio by new techniques (for exact determination of limiting activity coefficients by differential ebulliometry), requires precise values of solvent Cp, and latent heat as functions of temperature (Raal et al. 1 ). Large collections of liquid heat capacity data (e.g., Zabransky et al. 2 and Domalski and Hearing 3 ) are available in the literature, but for many liquids reliable data will not be available or may be uncertain due to discrepancies in reported values. For mixtures no reliable prediction procedures in terms of pure component values appear to be available. Prediction procedures for pure liquids are proposed and evaluated by Poling et al. 4 based on group contribution methods or corresponding states procedures. These procedures can give considerable errors for certain groups of compounds or at high-reduced temperatures. A sophisticated C L P prediction procedure has recently been developed by Diedrichs and Gmehling 5 based on a modified group contribution (VTPR) Peng -Robinson EOS. C p is related, from its definition as an enthalpy derivative, to PVT properties and their derivatives through easily derived exact equations. Very good predictions were obtained for a substantial number of components.Heat capacities are now frequently measured with commercial instruments (e.g., TA. Instruments, Tian-Calvet calorimeter (Settaram)), based on differential scanning (DSC) techniques. Recent heat-capacity measurements on ionic liquids by Diedrichs and Gmehling 6 using DSC and the more rapid ''modulated-temperature DSC'' methods, showed differences of up to 5% for the two techniques. Fairly extensive calibration procedures are required. Flow calo...

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Microflow calorimeter design for heats of mixing

Cited by 18 publications

References 30 publications

Enthalpy of dilution and volumetric properties of N-glycylglycine in aqueous xylitol solutions at T=298.15K

Enthalpy of dilution and volumetric properties of N-glycylglycine in aqueous xylitol solutions at T=298.15K

Separation of fluid frictional energy and heats‐of‐mixing in a flow calorimeter

Heat capacity measurement by flow calorimetry: An exact analysis

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