Dewar vessel pressurization

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. 

A thermos bottle (Dewar vessel) has an evacuated space between its inner and outer walls to diminish the rate of transfer of thermal energy to or from the bottle s contents. For good insulation, the mean free path of the residual gas (air average molecular mass = 29) should be at least 10 times the distance between the inner and outer walls, which is about 1.0 cm. What should be the maximum residual gas pressure in the evacuated space if T = 300 K Take an average diameter of d = 3.1 X 10 °m for the molecules in the air. 

Bisbenzylidene-cyclopentanone 1 (1.04 g, 2.0 mmol) was evenly spread on the inner wall of a mirrored Dewar vessel (diameter 14 cm, height 20 cm) with some dichloromethane. After heating to 80 °C for 1 h the crystalline film was irradiated from within for 3-5 h at 30-35 °C with a high-pressure Hg-lamp (Hano-via 450 W) through a benzophenone filter (5% in benzene 5 mm 2 >380 run) and cooling with running water. XH NMR-analysis revealed 400 mg (38%) 3, 275 mg (26%) 2 (structure clarified in Ref. 2), 85 mg (8%) 4 and 190 mg (18%) unreacted 1. The separation of the products by preparative TLC at 200 g SiC>2 used dichloromethane. Recrystallization of the products was performed in 1,2-di-chloroethane, toluene, and methanol, respectively. 

A = Flask (6 cm dia.), B = Heating jacket, C = Connecting tube, D = Air condenser, E = Water condenser, F = Receiver, G = Capillary connecting tube, HJHi = Electrically heated aluminium cylinders, KJE = Dewar vessels containing solid COj, M - McLeod manometer, P = Vacuum pump, K = Valve, T = Buffer vessel, V = Pressure regulator, iii/ifj == Sampling vessels, T = Shaft, Z = Rotating table... 

A drawing of the cross section of a Dewar vessel is shown in Figure 2 [1]. It shows the basic elements of a high performance cryogenic storage vessel. A fill and drain line is provided at the bottom of the vessel to transfer fluid in and out of the tank. Liquid can be removed either by pressurization of the inner vessel with a pressurization gas or by a liquid pump. A vapor vent line is located near the top of the vessel to allow vapor formed from heat leak to escape. This line can also be used to introduce a pressurization gas. If pressurization is used to force liquid from the tank, a diffuser is provided to distribute the pressurization gas in the vapor space away from the surface of the cold liquid. This prevents the unwanted condensation of the warm pressurization gas by the cold liquid surface. 

Since many of the Group III addition compounds are volatile, easily dissociated compounds, the high-temperature bulb vapour pressure apparatus of Burg and Schlesinger has proved well suited to studies of dissociation equilibria. Direct calorimetric measurements have generally been made in simple Dewar-vessel calorimeters. ... 

This designation (under the main heading) indicates a liquefied gas at atmospheric pressure, e.g. in a Dewar vessel. 

Two vessels A and B are respectively containing an anhydrous solid and a liquid. The vessel A is initially isolated from the vessel B, in order to regenerate the solid under vacuum. On the other side, the liquid (inB) is set in a Dewar vessel containing liquid nitrogen. After liquid solidification, the Dewar vessel is replaced by a liquid bath at a constant temperature. It gives a constant vapour pressure... 

Most suppliers are now using a special fitting to be used in the shipment of Dewar vessels. Also, an automatic pressure relief valve and a manual valve prevent entry of moisture and air, which will form an ice plug. The hquid hehum fih (inner neck) should be reamed out before and after transfer and at least twice daily. Reaming should be performed with a hoUow copper rod, with a marker or stop to prevent damaging the bottom of the inner container. Some newer style Dewar vessels are equipped with a pressure rehef valve and pressure gauge for the inner vessel. 

Figure 2.3 Schematics of cryostats, (a) Cryostat for solid samples. Cl, coolant inlet TM and HC, wires (in socket) for temperature monitoring and heater control, respectively PR, pressure release valve N, vacuum valve VP, vacuum pump DV, Dewar vessel TC, thermocouple HT, heater OW, outer window CF, copper frame, (b) Cryostat for liquid samples. SI, sample Inlet Cl, coolant inlet CV, gas inlet valve PR, pressure release valve TM and HC, wires (in socket) for temperature monitoring and heater control, respectively W, vacuum valve VP, vacuum pump DV, Dewar vessel SC, sample chamber TC, thermocouple HT, heater IW, inner window OW, outer window.

The independence of k on pressure seans strange at first sight in fact it is well known that for manufacturing high quality Dewar vessels (thermoses), the presence of air in the space between the walls reduces the quality of the vessel (outflow of heat) the less pressure between the walls of the vessel, the higher the Dewar quality. 

Hazards in addition to those taken for hydrogen gas should be taken when handling or storing LH2. Therefore, any LH2 splashed onto the skin or into the eyes can cause frostbite burns or hypothermia. Vents and valves in storage vessels and dewars may be blocked by accumulation of ice formed from moisture in the air. Excessive pressure may then result in mechanical failure with jet release of hydrogen and potentially in BLEVE. 

Consequence of runaway reaction Temperature rise rates Gas evolution rates Adiabatic Dewar Adiabatic calorimetry Pressure ARC VSP/RSST RC1 pressure vessel... 

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