Density difference, between liquid and gas

The extraction towers previously described are veiy similar to those used for gas-liquid contact, where density differences between liquid and gas are of the... 

Homogeneous (volumetric) void fraction Difference between liquid and gas density Angle, contact angle Density... 

Equations 15 and 16 develop Equation (14) for gas-to-condensed-phase transitions. It would also be desirable to develop Equation (14) for liquid-to-crystalline nucleation to assign physical meaning to An. However, development of Equation (14) is difficult both because d can be a large correction factor for liquid/solid interfaces and because the molecular density differences between liquids and solids are smaller than between gases and condensed phases (Kashchiev 1982). 

As we expect, as the temperature increases the surface tension decreases. This is because as a density gradient is introduced at the surface there is an extra entropy associated with the surface that reduces the free energy. As expected, at the critical point the surface tension goes to zero. At the critical point the difference between liquid and gas phases disappears, and the system is in a sense all interface. In fact, as one approaches the critical point square gradient theory predicts that the swface tension goes to zero according to a power law ... 

An interesting variation of the sink / float separation utilizing super- or near-critical liquid has been proposed [12], [13]. At the critical conditions of temperature and pressure the difference between liquid and gas phase disappears. Near - and super - critical liquids are compressible and their density can be varied by adjusting pressure. The advantages of some of the near - and super - critical fluids ( for example COj) are... 

Figure 26.2 For boiling, a useful order parameter is the difference between liquid and gas densities... 

In an airlift fermenter, mixing is accomplished without any mechanical agitation. An airlift fermenter is used for tissue culture, because the tissues are shear sensitive and normal mixing is not possible. With the airlift, because the shear levels are significantly lower than in stirred vessels, it is suitable for tissue culture. The gas is sparged only up to the part of the vessel cross section called the riser. Gas is held up, fluid density decreases causing liquid in the riser to move upwards and the bubble-free liquid to circulate through the down-comer. The liquid circulates in airlift reactors as a result of the density difference between riser and down-comer. 

This chapter treats reactions within a singie phase. Muitiphase CSTRs are treated in Chapter 11. For the singie-phase case, there is no essentiai difference between iiquid and gas reactors except for the equation of state. Density changes in iiquid systems tend to be smaii, and the density is usuaiiy assumed to be a iinear function of concentration. Liquid phase CSTRs can be hydrauiicaiiy fuii but frequentiy operate with a fixed ievei and have a free surface in contact with a vapor phase. Occasionaiiy they are mounted on ioad ceiis or use radiation-ievei detectors and operate with a fixed mass. 

The phase behavior of a SCF around the critical point can be demonstrated visually in an autoclave with a window, in which the meniscus between liquid and gas can be seen to disappear as the critical point is reached (Figure 4.2). Figure 4.2a shows a two-phase liquid-gas system with a clearly defined meniscus. When the temperature and pressure of the system increase, the difference between the densities of the two phases decreases and the meniscus is less well defined (Figure 4.2b). Finally, in Figure 4.2c, no meniscus is present as the system is now a single homogeneous SCF. 

The conventional configuration is to measure the level of the light liquid in the decanter, which floats on top of the heavy liquid. This is a liquid-gas interface, so the density difference is large and a level sensor, such as differential-pressure or displacement transmitters, can be effectively used. The liquid-liquid interface between the light and heavy liquid phases must also be measured. The density difference between these liquid phases is much smaller than between liquid and gas, so this measurement is more difficult. 

As mentioned earlier, the physical properties of a liquid mixture near a UCST have many similarities to those of a (liquid + gas) mixture at the critical point. For example, the coefficient of expansion and the compressibility of the mixture become infinite at the UCST. If one has a solution with a composition near that of the UCEP, at a temperature above the UCST, and cools it, critical opalescence occurs. This is followed, upon further cooling, by a cloudy mixture that does not settle into two phases because the densities of the two liquids are the same at the UCEP. Further cooling results in a density difference and separation into two phases occurs. Examples are known of systems in which the densities of the two phases change in such a way that at a temperature well below the UCST. the solutions connected by the tie-line again have the same density.bb When this occurs, one of the phases separates into a shapeless mass or blob that remains suspended in the second phase. The tie-lines connecting these phases have been called isopycnics (constant density). Isopycnics usually occur only at a specific temperature. Either heating or cooling the mixture results in density differences between the two equilibrium phases, and separation into layers occurs. 

For the sake of developing commercial reactors with high performance for direct synthesis of DME process, a novel circulating slurry bed reactor was developed. The reactor consists of a riser, down-comer, gas-liquid separator, gas distributor and specially designed internals for mass transfer and heat removal intensification [3], Due to density difference between the riser and down-comer, the slurry phase is eirculated in the reactor. A fairly good flow structure can be obtained and the heat and mass transfer can be intensified even at a relatively low superficial gas velocity. 

Fig.2 and Fig.3 show the typical liquid velocity and gas hold up distribution in the ALR. From the figures, one notices that the cyclohexane circulates in the ALR under the density difference between the riser and the downcomer. An apparent large vortex appears near the air sparger when the circulating liquid flows fi om the downcomer to the riser at the bottom. In the riser, liquid velocity near the draft-tube is much larger than that near the reactor wall, the latter moved somewhat downward. The gas holdup is nonuniform in the reactor, most gas exists in the riser while only a little appears in the dowmcomer. 

In the Fig.4, it can be seen that the gas hold-up in both riser and downcomer decreases with increasing the draft-tube horn-mouth diameter and approaches the maximum when the draft-tube hom-mouth diameter is 1.05m. However, due to the gas hold-up decreases more in the downcomer, the gas hold-up difference between the downcomer and the riser increases. Therefore, the apparent density difference between the riser and the downcomer enhances, causing higher liquid superficial velocity in the downcomer and in the riser With increasing the hom-mouth diameter. Fig.5 also shows that the existence of hom-mouth promotes the ability to separate gas from liquid and decreases the amount of gas entrained into the downcomer. 

In bubbling, the control of the bubble diameter is a little easier. In these methods bubbles are made at an orifice or a multitude of orifices. If there is only one orifice, of radius r, and if bubble formation is slow and undisturbed, the greatest possible bubble volume is 27rry/gp] y is the surface tension of the liquid, p the difference between the densities of liquid and gas (practically equal to the density of the liquid), and g is acceleration due to gravity. Every type of agitation lowers the real bubble size. On the other hand, if there are many orifices near enough to each other, the actual bubble may be much larger than predicted by the above expression. 

Another question that arises is the limiting size of the gas bubbles. As the bubble volume Vj, increases, the buoyancy force V gAp of the bubble increases (g is the acceleration of gravity and Ap is the density difference between the liquid and the gas). The bubble will tear away from the electrode surface as soon as this buoyancy force becomes larger than the forceretaining the bubbles. 

Due to the extreme density difference between gas and liquid (approximately 1 1.000), it must be expected that the gravitational acceleration g will exert big influence. One should actually write gAp but, since Ap = pi —po Pl, the dimensionless number would contain gAp/g pl/pl =... 

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