Furnaces and Temperature Programmers

Resistance elements are perhaps the most widely used in furnace construction. Some resistance elements and their approximate temperature limits are given in Table 6.6. These temperature limits are, of course, dependent on the furnace design and insulation. 

In (a), a design described by Vassallo and Harden (29), the furnace is heated by a heater cartridge. It has provision for rapid cooling or for use below room temperature by passing a coolant through the cooling coils which surround the furnace. Sample and reference materials are placed in glass capillary tubes. 

In an attempt to control the atmosphere within the furnace and sample holder, various techniques have been employed. They include (l) hooding the furnace with a gaseous atmosphere (32-35) (2) vacuum furnaces 132. 33. 36-41) and (3) a dynamic gas Row atmosphere (32,33,42,43(. An elaborate high-vacuum system for DTA furnaces has been described by Wiedemann (30). 

Although most DTA instruments have only one furnace, to increase the number of samples that can be run each day several furnaces may be used in conjunction with the sample holder, amplifier, and recording system. In fact, an instrument that contains four different furnaces (44) has been described. 

The rate of temperature increase of the furnace is controlled by a tempera-lure programmer. This programmer should be capable of linear temperature 

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Even with the best furnace and temperature programmer design there is a difference between the furnace temperature T ) and the sample temperature T ). This temperature difference is influenced by... 

Accurately weigh 10 g of dry polymer into a platinum dish. Place the dish in a cold contamination-free muffle furnace and temperature programme as follows ... 

TG curves are recorded using a thermobalance. The principal elements of a thermobalance are an electronic microbalance, a furnace, a temperature programmer and an instrument for simultaneously recording the outputs from these devices. A thermobalance is illustrated schematically in Figure 4.2. 

The general conformation of thermal analysis (TA) apparatus, consisting of a physical property sensor, a controlled-atmosphere furnace, a temperature programmer and a recording device, is illustrated in Figure 2.1. The most common forms of TA are shown in Chapter 1 (Table 1.1)... 

Figure 6. Schematic diagram of a classical DTA apparatus a) Reference thermocouple b) Sample thermocouple c) Healing block d) AT amplifier e) Recorder or computer, logging Ts, AT, and time f) Furnace g) Temperature programmer, which may be linked to the computer h) Gas inlet 7s, sample temperature 7r, reference temperature AT=7 s-7 r...

All subsequent green coke operations were made in a second coker, which was fashioned from steel pipe approximately 18 cm in diameter and 25 cm in length. A metal plate was welded to one end and a metal collar was welded to the other end such that a steel lid could be bolted to the system. Typically, about 250 to 500 g of pitch were sealed imder nitrogen in the coker reactor and the system placed in a large temperature-programmable furnace. The heat treatment process was as follows. The temperature was raised 5°C/min to 350 °C and then l°C/min to 425°C and the temperature held at 425°C for 90 minutes. Finally the temperature was raised further at 3°C/min to between 500 and 600°C, and held there for 3 hours. The coker was cooled to room temperature and the material recovered to determine green coke yield. 

The construction of DTA apparatus is simple and consists of a furnace, differential thermocouple, temperature thermocouple, specimen holders, temperature programmer and recorder. The schematic of a typical DTA apparatus is shown in Figure 3.5. 

A TG apparatus consists of a precision balance (thermobalance), a furnace, sample holder assembly, temperature programmer and recorder and its schematic is shown in Figure 3.7. 

CO, reforming reaction was conducted at 500-750°C, reactants mole ratio of CH3 CO, He = 1 1 3, and space velocity = 20000-80000 1/kg/h. Methane oxidation was conducted at 150-550 °C using 1 % CH in air mixture (2 ml/min CH4 198 ml/min air) at space velocity = 60000 1/kg/h, and MIBK (4000 ppm in 150 ml/min air introduced by a syringe pump) combustion at 100-500°C and space velocity of 10000-30000 h 1. Catalytic reactions were conducted in a conventional flow reactor at atmospheric pressure. The catalyst sample, 0.1-0.3g was placed in the middle of a 0.5 inch I.D. quartz reactor and heated in a furnace controlled by a temperature programmer. Reaction products were analyzed by a gas chromatography (TCD/FID) equipped with Molecular Sieves 5A. Porapak Q, and 15m polar C BP 20 capillary column. 

In TA the mass loss versus increasing temperature of the sample is recorded. The basic instrumental requirements are simple a precision balance, a programmable furnace, and a recorder (Figure 1). Modem instruments, however, tend to be automated and include software for data reduction. In addition, provisions are made for surrounding the sample with an air, nitrogen, or an oxygen atmosphere. 

Much improved limits of detection can be obtained if either quartz or ceramic, flame-heated cells or graphite furnace atomisers are used (Szpunar et al., 1996). The effluent from the chromatograph is passed via a heated line either into the port provided for sample introduction (Radzuik et al., 1979) or the ports for the purge gases (De Jonghe et al., 1980). When the furnace atomiser is not coupled to a chromatograph, a temperature programme would be used to eliminate problems due to the matrix, but this cannot be used when coupled on-line and matrix interferences will occur. 

A modified Coleman Model 33 Carbon-Hydrogen Analyzer was used to control the oxygen flow rate, to pretreat the oxygen (anhydrous magnesium perchlorate and anhydrous lithium hydroxide) and to hold the combustion tube in position. The analyzers furnace was controlled by a Fisher Model 360 Linear Temperature Programme. The rate of temperature increase was variable from 0.5 to 25 C per minute. Normally, the temperature programmer... 

As shown in Fig. 4.8.1, a TG apparatus typically consists of a (1) recording balance, (2) furnace, (3) furnace temperature controller, (4) temperature programmer, (5) temperature detector, and (6) recorder. Most commercial models, e.g., Perkin-Elmer Model TGA-7, TA Instruments 2950 TGA, Mettler TC11ATG50, Seiko Instruments SSC 5200 TG-DTA 220, and Shimadzu TGA-50, consist of these basic components. 

Fig. 4.8.1. Schematic diagram of a TG apparatus, recording balance (/), furnace (2), furnace temperature (5), temperature programmer (4), temperature detector (5), and recorder (6)...

The temperature programme used was dry for 20 s (110°C), ramp 15 s for a 15s char (550°C) and ramp 9s for a 9s atomisation (2500°C). With determinations in chloride media, the ash or char stage is particularly critical in furnace atomisation, as hydrogen chloride may be formed which aids the removal of chloride. This might otherwise interfere by vaporising the analyte as the chloride before the atomisation temperature is reached. In more open rod-type systems this mechanism is not available, and chloride interferences may be more severe [4]. 

DSC works on a similar principle but has several advantages over DTA, not least of which being the ability to easily measure the energy associated with the transition as well as the temperature at which it occurs. There are two main forms of DSC. power compensation and heat fiux. Power compensation DSC involves the use of two furnaces (rather than the one used for DTA), one placed under the. sample and the other under the reference (Fig. 1B). The system operates on the basis of keeping the sample and reference at the same temperature. This therefore means that energy must be supplied to the reference in order to make it follow the predetermined temperature programme and... 

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