Water vapour tolerance

The maximum pressure of vapour that a vacuum pump, under ambient conditions of T— 20 °C, p = 1013mbar, can take in and transport is called the vapour tolerance of the pump. Since water vapour is the most commonly encountered vapour, pump manufacturers usually quote the water vapour tolerance for their pumps. The vapour tolerance can be obtained by rearrangement of Equation (3.3) ... 

Moisture is to be removed, in a drier, from compacted material with a microcrystalline structure. The drier, fitted with a heating jacket and a regulator to maintain a constant temperature, is charged with 100 kg product containing 30% water. The final product must have a residual moisture of 0.1% or less. Because of the nature of the product, its temperature should not exceed 50 °C and, to achieve the required dryness, a pressure of 2 mbar is required. It is proposed to carry out the process with a single-stage oil-sealed rotary pump (Seff = 100 m3 hr1, water vapour tolerance = 50 mbar at 40 °C and pult (with gas ballast) = 1 mbar). 

The water vapour tolerance of a pump is the maximum pressure of pure water vapour that the pump can take in and transport. Example 3.3 indicates how this is calculated. 

Table 5.1 Vapour tolerances of a modern rotary vane pump with a water vapour tolerance of 40mbar.

G enerally a value of40 - 60 mbar maximum is aimed at for the water vapour tolerance limit. This results from the fact that a condenser operated upstream of the vacuum pump and with normal circulating coohng water will reduce the partial pressure of the water vapour down to 40 mbar and up to 60 mbar. 

Presently, the method cannot tolerate high hydrocarbon concentrations in the flue gas for two reasons firstly, the mass balance considerations secondly, the water vapour measurements are violated. Consequently, a hydrocarbon sensor would be needed. 

In Example 6.6, the drying time (and the required Seff) could have been greatly reduced by increasing the process temperature. A water vapour pressure would have been generated, however, which would have considerably exceeded the tolerance of the pump. By the use of an appropriate vacuum-side condenser, this could be avoided. 

The durability of the canister material also poses a problem. At present, the favoured material is stainless steel. The conditions that the canister must tolerate include temperatures of up to 200 C, water vapour, water, and corrosion products from the immobilising solid. The time-scale of thousands of years increases the durability problem enormously. Additionally, the presence of other metallic elements, both inside and outside of the container, can lead to electrochemical corrosion. 

In addition to the stability of a material in the presence of water vapour, its effect on the CO2 adsorption process will also be significant. In some cases, evidence suggests that the presence of water may be beneficial to CO2 uptake in terms of increasing the capture capacity of immobilized amines, which are often tolerant to moisture." However, water can also act in competition with CO2 for adsorption sites and this can sometimes result in the degradation of the porous support Water sorption can therefore lead to significant decrease in CO2... 

The mother-liquors of the lye obtained from varec contain a tolerably large quantity of a singular and curious substance. It can easily be obtained. For this purpose it is sufficient to pour sulphuric acid upon the mother-liquid and to heat the whole in a retort connected with a receiver. The new substance which, on the addition of the sulphuric acid, is at once thrown down as a black powder is converted on heating into a vapour of a superb violet colour this vapour condenses in the tube of the retort and in the receiver in the form of brilliant crystalline plates, having a lustre equal to that of crystallized lead sulphide. On washing these plates with a little distilled water the substance is obtained in a state of purity. The wonderful colour of its vapour suffices to distinguish it from all other substances known up to the present time, and it has further remarkable properties which render its discovery of the greatest interest. 

A well-known phenomenon in inorganic salts is the salting-out effect. Adding sodium sulphate, ammonium sulphate or sodium chloride (common salt), for example, in portions to aqueous systems has the effect of driving out some of the volatile compounds into the gaseous phase, or into a solvent which is immiscible with water. Of the salts mentioned above, only common salt has any relevance to food. Additions of 5 to 15% to aqueous systems result in increases of head space concentration of ethyl acetate, isoamyl acetate and menthone up to 25% [10,32], This common salt concentration, however, is way above what is tolerated normally in foodstuffs. In foods with a normal salt content, the salt has virtually no effect on the vapour pressure of volatile compounds [9,10,32], The same is true for calcium chloride [8[, The possibility, that the salt content of the saliva has some effect on the vapour pressure cannot be ruled out however [32],... 

For cold-curing epoxides wide variations in adhesive material properties are possible, with different combinations of resin, hardener, filler, and the multitude of modifiers. Products which cure at ambient temperature cannot achieve the same performance as is obtained by curing at elevated temperature. For products cured at room temperature their TgS, at 40-50 °C initially, are relatively low and may be lowered even further by absorbed water, in liquid or vapour form. This may also be accompanied by a reduction in strength and modulus. Thus the use of materials with a slow and small water uptake is to be preferred, which implies a fairly highly cross-linked formulation. Such considerations do of course depend upon the performance and durability expectations in service. Whilst the environmental durability of joints can often be improved enormously by the surface pretreatment methods employed (see Chapters 3 and 4), the adhesive must be selected carefully to ensure long term durability in consideration of the modes and duration of loading, and the environmental conditions. Ideally the adhesive should be fairly tolerant of poor surface pretreatment procedures. 

Halide impurities are also disadvantageous for the functioning of MCFC. When halides react with molten carbonates it leads to the formation of alkali halides, CO2 and water. Due to the formation of alkali halides (LiCl/KCl/NaCl, etc.), the vapour pressure increases and the rate of electrolyte loss also increases. For tolerance purpose, the HCl level should be kept below 1 ppm in the fuel gas. The nitrogen compounds such as NH3 and HCN may also react with the electrolyte to form nitrate salts. 

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