A publication of CHEMICAL ENGINEERING TRANSACTIONS VOL. 31, 2013 The Italian Association of Chemical Engineering Online at: www.aidic.it/cet Guest Editors: Eddy De Rademaeker, Bruno Fabiano, Simberto Senni Buratti Copyright © 2013, AIDIC Servizi S.r.l., ISBN 978-88-95608-22-8; ISSN 1974-9791 Heat Exchange Elements in Sample Cells for Thermal Analysis Georg W. Suter*, Volker Stocks, Carl Gwerder, Manfred Eiche Swissi Process Safety GmbH, Schwarzwaldallee 215, CH-4002 Basle (Switzerland) [email protected] It is a basic assumption of micro calorimetric DTA and DSC tests that the temperature is homogeneous in the entire sample volume during the test. When using high heating rates or when investigating highly exothermic processes with high activation energy (highly energetic materials) this assumption may not be correct. This paper deals with the elimination of temperature inhomogeneity in DTA and DSC sample cells by using heat exchange elements inside the cell. With this principle the thermal kinetics can be recorded even for highly exothermic processes with high activation energy, without signal distortion. 1. Introduction In calorimetric measurements, in particular Differential Thermal Analysis and Differential Scanning Calorimetry, heat production rates are basically derived from measurements of temperature differences. Typically the difference between the sample temperature and some reference temperature (ΔT) is recorded, and – by using appropriate calibration factors – the heat production rate is derived from these values. Basic Assumption In isoperibolic, isothermal and dynamic testing modes of small scale (up to a few millilitres) testing equipment, it is assumed that a) the relevant temperature difference is small b) the temperature of the sample is homogeneous such that data are representative for behaviour of the entire sample at the reference temperature. The following effects have to be considered related to the ΔT values. Design of the instrument and the sample container. A high thermal resistance between the instrument and the sample (e.g. insulating layers, air gaps etc.) will increase the ΔT values, which means that even small heat production rates can precisely be detected. On the other hand, an instrument design optimized for low ΔT values will have a lower sensitivity than if higher ΔT values are achieved. Heating rate In dynamic test modes high heating rates increase ΔT values between that sample and the heating elements of the instrument, and – in case of exothermic processes in the sample – also between the sample and the reference temperature (Roduit, 2010) Temperature inhomogeneity Heating a sample, intrinsically is related to some temperature gradients inside the sample. The increase with increasing heating rate and are particularly relevant in dynamic testing modes. In addition, exothermic processes in the sample may lead to a non-homogeneous temperature distribution in the sample. This paper deals with the reduction of the temperature inhomogeneity sample cells for DTA and DSC tests by using heat exchange elements inside the cell. Heat Exchange Elements in Sample Cells for Thermal Analysis Georg Suter, Volker Stocks, Carl Gwerder, Manfred Eiche Swissi Process Safety GmbH, Basle, Switzerland swissi process safety gmbh Heat Production Rate Design of reactor and condensor AZT24 TMR q(T) Storage Stability Design of safety devices Classification (CLP, NFPA 704) swissi process safety gmbh q(T) from isothermal tests l swissi process safety gmbh DTA/DSC q(T) (see AKTS) swissi process safety gmbh Basic DTA Principle q(Tsample) = S x (Tsample – Tref) Tsample ≈ Tref q(Tref) = S x (Tsample – Tref) Tsample Tref Tcontrol swissi process safety gmbh Examples DSC swissi process safety gmbh Example RADEX, C80 Tsample Tsample Tref Tcontrol= Tref Tcontrol swissi process safety gmbh Question Tsample ≈ Tref ?? 28 200 Tref Temperature [°C] 160 24 140 T1(sample ) 120 DT1 20 16 12 100 8 80 4 60 0 40 -4 20 -8 ∆ T [K] 180 -12 0 25 50 75 100 125 Tref[°C] 150 175 200 swissi process safety gmbh Question «Tsample» = Tsample , really ?? 1cm 1cm 1cm swissi process safety gmbh Model: Heating from outside • Temperature is not homogeneous • Effect increases with – Increasing heating rate – Increasing sample dimension swissi process safety gmbh Model: Run Away t1 t2 t3 t4 t5 t6 t7 650 600 550 500 450 400 t1 t2 t3 t4 t5 t6 t7 350 300 250 200 150 -0.5 -0.3 -0.1 0.1 0.3 0.5 Radius (cm) swissi process safety gmbh Model: Run Away 650 600 550 500 t1 450 t2 t3 400 t4 350 t5 t6 300 t7 250 200 150 -0.5 -0.3 -0.1 0.1 0.3 0.5 Radius (cm) swissi process safety gmbh Critical Heat Production Rate: q(Tcrit ) = δc × λ ×R× T 2 crit ρ × r × Ea 2 Franck-Kamenetskii typical values for solids in a RADEX Cell λ T ρ Ea/R r 0.1 W K-1 m-1 400K 500 kg m-3 15000K 0.004 m qcrit = 373 W kg-1 = 0.37 W g-1 swissi process safety gmbh In Reality: 4K/min swissi process safety gmbh Improve T-Homogeneity swissi process safety gmbh Improve T-Homogeneity swissi process safety gmbh Avoid Run Away 650 226 600 224 550 222 500 t1 450 t2 400 t3 t2 218 t3 216 t4 350 -0.3 t4 300 t5 214 250 t6 212 t6 200 t7 210 t7 150 -0.5 t1 220 -0.1 t5 208 0.1 Radius (cm) 0.3 0.5 -0.5 -0.3 -0.1 0.1 0.3 0.5 Radius (cm) swissi process safety gmbh Example Na-Acetate (DSC) Here a thermal run-away occurs: The reaction gets much faster due to selfheating inside the cell Here the same effect is much smaller (see also y-scale!) swissi process safety gmbh Example: RADEX V8 Cell 4K/min swissi process safety gmbh One Step Further q(Tcrit ) = δc × λ ×R× T 2 crit ρ × r × Ea 2 Quadratic effect! swissi process safety gmbh Example C80 Cell swissi process safety gmbh Example: C80 Cells swissi process safety gmbh Example: C80 Cells, Liquid! swissi process safety gmbh Summary Heat Exchange Elements in Sample Cells for Differential Thermal Analysis • improve T-homogeneity • avoid Thermal Run-Away in Cells • are important in practice – for solid samples (low heat condcutivity, no convection) – for highly energetic processes with high activation energy – to collect data for modelling (e.g. AKTS) swissi process safety gmbh