Liquid Transfers Avoiding 2-Phase Flow

Abstract The transfer of cryogenic liquids like LNG down pipes is not as simple as pumping water. The difference is that cryogenic liquids are stored at their boiling points, whilst water is stored at ambient temperature, which is a long way from its boiling point at 100 °C. 

Pumping the boding liquid can easily lead to much vapour generation, and 2-phase flow. The result is that the pumped mass flow reduces to a minimum or to zero. 

The simple way to stop this transfer disaster is to use pressure sub-cooling of the liquid. 

2-Phase flow explained as a mixture of saturated vapour and liquid, with greatly reduced mass flow. 

Prevention with adequate sub-cooling by pressure application to counter NPSH of pump. 

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Pumped Liquid Transfer Avoiding 2-Phase Flow... 

For startup of column extractors, it generally is best to start from dilute-solute conditions to avoid unstable operation. For example, when starting a column in which the feed is the continuous phase, first fill the column with solute-lean feed liquid before starting the flow of solvent and actual feed. This way, the solvent quickly becomes dispersed and mass transfer approaches steady state from dilute conditions, promoting faster and more stable startup. 

Experimental and theoretical studies of mist cooling were conducted over three decades ago [2, 3]. These studies showed that the heat transfer performance of a single-phase flow can be significantly improved by adding mist into the primary air flow. A number of two-phase flow studies have been made for a variety of configurations [4, S]. Results show that latent heat of evaporation within the boundary layer is important and e formation of a thin liquid film over the surface will greatly enhance heat transfer rates. A constraint of the current study was to avoid the deposition of drops on the heated plate, therefore only evaporation of the liquid in the bound layer will be considered. 

Pohorecld [59] investigated the effectiveness of the interfadal area for mass transfer in two-phase flow in an MSR. He developed criteria to avoid saturation of the liquid wall film in the case of physical absorption or liquid of the film for absorption with instantaneous chemical reaction ... 

All the methods above require the correct gas flow pattern (plug flow, well back-mixed, or intermediate) to convert the transfer rate to a correct kLa value unless the degree of depletion of the gas phase is very low. This can be very important, as discussed in Chapter 11. Gas flow patterns can be determined from measurements of the gas residence time distribution using tracer gas (see Section 4-7.8). Two dynamic methods avoid this problem the double response method (Chapman et al., 1982), in which the dynamic responses of both liquid and gas phases are measured, and the initial response method (Gibilaro et al., 1985). 

High-performance cooling mechanisms such as forced convection liquid cooling and two-phase flow boiling heat transfer have good potential for use in PCs. Some of the key criteria for the selection of the cooling fluid are as follows the fluid must be non-corrosive, nonfreezing, environmentally safe, and an electrical nonconductor, that is, dielectric to avoid any short circuits. 

If the mass flow rate in Eq. (7.99) is not attained, a condition known as zero delivery results. In order to avert zero delivery, several techniques have been employed. Typically, vents are installed at intermediate points along the transfer line to release some of the trapped vapor, thereby increasing the mass flow rate into the line. Of course, a disadvantage of this method is that some of the cooling capacity is lost in the vented gas. Another technique that has been exploited is to install gas-liquid separators along the transfer line and allow most of the vapor formed in the line to escape continuously at these sites during transfer. Though two-phase flow cannot be avoided, this second technique will reduce the time for liquid delivery at the exit of the transfer line. 

A chapter on liquid handling then sets out the need to use sub-cooled liquid, to avoid 2-phase flow for trouble-free transfer. 

The HTE characteristics that apply for gas-phase reactions (i.e., measurement under nondiffusion-limited conditions, equal distribution of gas flows and temperature, avoidance of crosscontamination, etc.) also apply for catalytic reactions in the liquid-phase. In addition, in liquid phase reactions mass-transport phenomena of the reactants are a vital point, especially if one of the reactants is a gas. It is worth spending some time to reflect on the topic of mass transfer related to liquid-gas-phase reactions. As we discussed before, for gas-phase catalysis, a crucial point is the measurement of catalysts under conditions where mass transport is not limiting the reaction and yields true microkinetic data. As an additional factor for mass transport in liquid-gas-phase reactions, the rate of reaction gas saturation of the liquid can also determine the kinetics of the reaction [81], In order to avoid mass-transport limitations with regard to gas/liquid mass transport, the transfer rate of the gas into the liquid (saturation of the liquid with gas) must be higher than the consumption of the reactant gas by the reaction. Otherwise, it is not possible to obtain true kinetic data of the catalytic reaction, which allow a comparison of the different catalyst candidates on a microkinetic basis, as only the gas uptake of the liquid will govern the result of the experiment (see Figure 11.32a). In three-phase reactions (gas-liquid-solid), the transport of the reactants to the surface of the solid (and the transport from the resulting products from this surface) will also... 

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