Vacuum Vaporization Start Temperature

Fig. 4-124 Vaporization Start Temperature 75 % of Vacuum Distillates (FVAl - FVA4) and Base Oils (Ml - MVII) versus Mean Molecular Weight...

The module was first charged with 40.8 ml of deionized water at room temperature. Subsequently the pump was started and the flow rafe was set to the desired value. Thereafter, the internal pressure of the chamber was reduced by means of a vacuum pump. At steady-state chamber conditions, the vapor partial pressure was calculated, using steam tables, assuming the temperature measured in the chamber is equal to the vapor saturation temperature at the computed vapor partial pressure. Thereafter, power was supplied to the diode and the data were recorded. 

Feed Slurry Temperature Temperature can be both an aid and a limitation. As temperature of the feed slurry is increased, the viscosity of the hquid phase is decreased, causing an increase in filtration rate and a decrease in cake moisture content. The limit to the benefits of increased temperature occurs when the vapor pressure of the hquid phase starts to materially reduce the allowable vacuum. If the hquid phase is permitted to flash within the filter internals, various undesired resiilts may ensue disruption in cake formation adjacent to the medium, scale deposit on the filter internals, a sharp rise in pressure drop within the filter drainage passages due to increased vapor flow, or decreased vacuum pump capacity. In most cases, the vacuum system should be designed so that the liquid phase does not boil. 

A very old gas-solid bromination of tyrosine (280) [97] has been revisited and it gave a quantitative yield for the reaction of rac-280 [22]. The doubly bromi-nated hydrobromide rac-281 is spectroscopically pure after removal of included gases at 50 °C in a vacuum. Quite spectacular is the specific and quantitative waste-free gas-solid tetrabromination of tetraphenylethylene (282), which shows some signs of autocatalysis and requires rotation of the flask around a horizontal axis at room temperature for 12 h as the reactant and product gases require mixing [60]. The isomer-free tetrabromide 283 is an attractive starting point for dendrimer syntheses and inclusion studies (Scheme 40). Also 4-bro-mo antipyrin hydrobromide is quantitatively obtained from antipyrin(hydro-bromide) and bromine vapor [22]. 

Currently, thermal reduction processes have replaced the electrolysis method. The starting material in these methods is limestone, which is calcined to produce calcium oxide. The latter is ground, mixed and compacted with aluminum, and reduced at temperatures between 1,000° to 1,200°C under vacuum. Calcium vapors formed in low yield under such thermodynamic conditions are transferred from the reactor and condensed in cool zones, thus shifting the equilibrium to allow formation of more calcium vapors. The reactions are as follows ... 

When does a liquid boil Clearly, boiling at constant pressure—say, atmospheric pressure—begins when we increase the temperature of a liquid or solution and the vapor pressure reaches a pressure of one atmosphere. Alternatively, the pressure over a liquid or solution at constant temperature must be reduced until it reaches the vapor pressure at that temperature (e.g., vacuum distillation). Yet it is well known that liquids can be superheated (and vapors supersaturated) without the occurrence of phase transfer. In fact, liquids must always be superheated to some degree for nucleation to begin and for boiling to start. That is, the temperature must be raised above the value at which the equilibrium vapor pressure equals the surrounding pressure over the liquid, or the pressure must be reduced below the vapor pressure value. As defined earlier, these differences are called the degree of superheat. When the liquid is superheated, it is metastable and will reach equilibrium only when it breaks up into two phases. 

A number of ceramic materials meet these specifications fairly well. Thus, synthetic mullite having a melting point of about 1835°C. and an expansion coefficient of 45 X 10-7 cm./cm./°C. between 20° and 1320°C., and synthetic zircon having a melting point of 1775°C. and an expansion coefficient of 42 X 10 7 between 20° and 1550°C. are suitable materials. However, both ceramics start to decompose under vacuum conditions above 1300°C. and show a not negligible vapor pressure of SiO and Si02 at considerably lower temperatures. In spite of this, mullite and zircon are better than fused quartz in the temperature range up to 1200°C. Moreover, they are cheaper and easier to be sealed to the vacuum system. 

Preparation of Hydrated Silicates. The hydrated silicate specimens used were all in the paste form—that is, mixtures of one of the calcium silicates with a limited amount of water to form a slurry, which sets and hardens as portland cement itself does. These pastes were prepared by the vacuum mixing procedure described by Powers, Copeland, Hayes, and Mann (23), adapted so that the temperature of the mix upon removal from the mixer was the temperature at which the specimen was to be hydrated. The 5° specimens were made by starting with an ice-water mixture the-50° specimens by starting with preheated water. A manostat was incorporated into the pumping system to prevent the pressure from dropping below the vapor pressure of water at the desired final temperature. This was especially important for the 50° mixes, to prevent excessive cooling. 

---