Dewar vessels

Phenylacetylene. Support a 5-litre glass Dewar flask in a wooden case. Equip the flask with a lid of clear Perspex, provided with suitable apertures for a mechanical stirrer, introducing solids (e.g., sodium) or hquids, a calibrated dip stick for measuring the volume of liquid in the Dewar vessel, a gas mlet tube and an ammonia inlet arrange for an electric light to shine downwards into the flask. 

The utihty stream gets started at operating temperature and flow rate. In the following experiments, the utihty stream is heated so as to initiate the reaction. The main and secondary process tines are fed with water at room temperature and with the same flow rate as one of the experiments. Once steady state is reached, operating parameters are recorded. Process tines are then fed with the reactants, hydrogen peroxide and sodium thiosulfate. At steady state, operating parameters are recorded, and a sample of a known mass of reactor products is introduced in the Dewar vessel. Temperature in the Dewar vessel is recorded until equilibrium is reached, that is, until the reaction ends. This calorimetric method is aimed at calculating the conversion rate at the product outlet and thus the conversion rate in the reactor. The latter is also determined by thermal balances between process inlet and outlet of the reactor. Finally, the reactor is rinsed with water. This procedure is repeated for each experiment... 

Two methods have been used to calculate the conversion rate in the reactor. They are based on thermal balances first between inlet and outlet of process and utility streams in the reactor and then between sampling and thermal equilibrium in the Dewar vessel. The former leads to the conversion rate obtained in the reactor, x and the latter gives the conversion rate downstream from the reactor outlet, 1 - X-... 

This approach consists in measuring the adiabatic temperature increase of a sample taken at the outlet of the reactor. Samphng is made in an adiabatic vessel (Dewar vessel) and temperature is recorded until the reaction ends, that is, until an equilibrium temperature is reached. The conversion rate is thus written as... 

Temperature difference in the reactor was less important than in the Dewar vessel because of the efhdent exchange with the utility stream. That is the reason error estimation in the reactor is higher than in the Dewar vessel. 

Mass of water equivalent to the mass of the Dewar vessel (kg) Mass of the sample (kg)... 

If, during the transfer, the pressure of boron trichloride should fall much below 15 mm., the Dewar vessel surrounding tube A should be lowered until the pressure rises to a satisfactory value and then replaced by another empty Dewar vessel at room temperature. It is also possible to achieve a relatively constant temperature by allowing a slow stream of compressed air to exit from a tube leading to the bottom of the Dewar vessel. 

AST Start 20°C, heat step 5°C, wait 24 hrs. lOOOg 55 c — Class Dewar vessel... 

FIGURE 2.21. Typical Temperature-Time Curves of Dewar Vessel Tests (after temperature equilibration between Dewar flask and oven has been reached). 

The UN deflagration test consists of a Dewar vessel with a volume of about 400 cm3. The vessel is filled with preheated material (standard temperature is 50°C if the stability of the substance permits), and the substance is initiated at the top of the vessel with a flame. The propagation of deflagration is recorded by temperature sensors that are located in the substance at preset distances. From the time required for passing two temperature sensors and from the known distance between them, the deflagration velocity can be calculated. 

Critical heat production rates (i.e., heat production rates that still do not lead to a runaway), are often determined by small scale experiments. However, the effect of scale-up on these rates, as discussed in [161], must be taken into account. An indication of the effect of scaling in an unstirred system is shown in Figure 3.2. In this figure, the heat production rate (logarithmic scale) is shown as a function of the reciprocal temperature. Point A in the figure represents critical conditions (equivalent heat generation and heat removal) obtained in a 200 cm3 Dewar vessel set-up. It can be calculated from the Frank-Kamenetskii theory on heat accumulation [157, 162] that the critical conditions are lowered by a factor of about 12 for a 200 liter insulated drum. These conditions are represented by... 

FIGURE 3.2. Relation between Critical Heat Production Rates of Small Scale and of Plant Scale (small scale = 200 cm3 Dewar vessel, point A) (large scale = 200 liter drum, line B)... 

For the more vigorous reactions, a twin-cell calorimeter was devised (188). It consisted of two nickel cylinders connected by a stainless steel needle valve and tubing and held rigidly to a metal top-plate. The cylinders and connections were immersed in a wide-necked Dewar vessel containing carbon tetrachloride which would react mildly with any BrF3 that escaped. Bromine trifluoride contained in one cylinder was transferred to the solid contained in the other cylinder by opening the valve and applying controlled suction. All measurements were made externally on probes in the carbon tetrachloride. 

This is followed by a specification for a new apparatus which includes the description of the pseudo-Dewar , reaction vessel [5]. This is a Dewar vessel, the Dewar-space of which can be filled with air or evacuated through a side-arm. Thus the contents can be cooled... 

Figure 8.1 Scheme of a Dewar vessel isoperibol reaction-solution calorimeter. A ampule containing the sample B ampule breaking system C calorimeter head D temperature sensor E stirrer F electrical resistance G Dewar vessel H plunger of the ampule breaking system I, J inlets K plug connecting the calibration resistance to the calibration circuit. 

The historical development of titration calorimetry has been addressed by Grime [197]. The technique is credited to have been born in 1913, when Bell and Cowell used an apparatus consisting of a 200 cm3 Dewar vessel, a platinum stirrer, a thermometer graduated to tenths of degrees, and a volumetric burette to determine the end point of the titration of citric acid with ammonia lfom a plot of the observed temperature change against the volume of ammonia added [208]. The capabilities of titration calorimetry have enormously evolved since then, and the accuracy limits of modern titration calorimeters are comparable to those obtained in conventional isoperibol (chapter 8) or heat-flow instruments (chapter 9) [195,198],... 

Figure 11.1 (a) Scheme of an isoperibol titration calorimetry apparatus A Dewar vessel B lid C stirrer D electrical resistance E thermistor F titrant delivery tube G O-ring seal, (b) Vessel for isothermal operation A stainless-steel, platinum, or tantalum cup B water-tight stainless steel container C heater D Peltier thermoelectric cooler E O-ring seal F heater and cooler leads. Adapted from [211],... 

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