Joule cooling

Oxidation of n-alkanes is stroi y exothermic, the production of 1 kg biomass liberating 27,100 k Joules. Cooling costs to maintain temperatures at about 30°C are considerable. 

If high wellhead pressures are available over long periods, cooling can be achieved by expanding gas through a valve, a process known as Joule Thomson (JT) throttling. The valve is normally used in combination with a liquid gas separator and a heat exchanger, and inhibition measures must be taken to avoid hydrate formation. The whole process is often termed low temperature separation (LTS). 

Expansion from high to low pressures at room temperature cools most gases. Hydrogen is an exception in that it heats upon expansion at room temperature. Only below the inversion temperature, which is a function of pressure, does hydrogen cool upon expansion. Values of the Joule-Thorns on expansion coefficients for hydrogen have been tabulated up to 253 MPa (36,700 psi) (48), and the Joule-Thorns on inversion curve for i7n -hydrogen has been determined (49,50). 

Fouling factor, water side 0.0002 heating or cooling streams are shown at top of columns as C, D, F, G, etc. to convert British thermal units per hour-square foot-degrees Fahrenheit to joules per square meter-second-kelvins, multiply hy 5.6783 to convert hours per square foot-degree Fahrenheit-British thermal units to square meters per second-kelvin-joules, multiply hy 0.1761. 

To reduce the work of compression in this cycle a two-stage or dualpressure process may be usedwhereby the pressure is reduced by two successive isenthalpic expansions. Since the isothermal work of compression is approximately proportional to the logarithm of the pressure ratio, and the Joule-Tnomson cooling is roughly proportional to... 

Gas can be condensed by (a) mechanically refrigerating it, (b) compressing and expanding it, using turboexpanders, or, (c) pressure effects such as by Joule-Thomson cooling and overcoming the vapor pressure. The liquefaction of methane can involve all three of these effects. These effects can be separately evaluated to show the effectiveness of each in producing liquid. 

The diird curve, dial of die compression/Joule-Tliomson cooling, shows 58.2% efficiency. This is calculated based on die difference in die accounting shown in Table 3-2 and is die practical efficiency of diis effect. It would be 68%—that of die compressor—except for several losses, such as die warm end temperature difference and a small, not easily recoverable, portion of the flash loss. 

Figure 9-3 shows a typical cryogenic plant where the gas is cooled to -100°F to -150°F by expansion through a turbine or Joule-Thompson (J-T) valve. In this example liquids are separated from the iniei gas at 100 F and 1,000 psig. It is then dehydrated to less than I ppm water vapor to assure that hydrates will not form at the low temperatures encountered in the plant. Typically, a mole sieve dehydrator is used. 

Figure 11-58. Theoretical comparison of Joule-Thompson cooling effect with nitrogen vs. the use of a mechanical expander. (Used by permission Gibbs, C. W., (Ed.). Compressed Air and Gas Data, 1969. Ingersoll-Rand Co.)...

Joule and Kelvin proceeded differently. They found that w increased (i.ethe cooling diminished) with rise of temperature of the entering gas, and they assumed that in the equation representing w as a function of temperature, the absolute temperature could be replaced by the approximate value (273-7 -f 6). In this case the experimental results agreed with the formula ... 

Figure 3.6 shows that pj.r. is negative at high temperatures and pressures. Therefore, a gas heats up as it expands under these conditions. At lower temperatures, the gas continues to increase in temperature if the expansion occurs at high pressures. However, at lower pressures, the slope, and hence, Hj.t., becomes positive, and the gas cools upon expansion. Intermediate between these two effects is a pressure and temperature condition where //j.t. = 0. This temperature is known as the Joule-Thomson inversion temperature Tt. Its value depends upon the starting pressure and temperature (and the nature of the gas). The dashed line in Figure 3.6 gives this inversion temperature as a function of the initial pressure. Note that when Joule-Thomson inversion temperatures occur, they occur in pairs at each pressured... 

The Joule-Thomson expansion can be used to liquify gases. An expansion at pressure and temperature conditions inside the dashed line envelope where /o r < 0 cools the gas. This gas is used to precool the incoming gas so that the expansion occurs at still lower temperatures. Continuing this process eventually cools the gas until it liquifies. 

K to 500 K. A Joule-Thomson expansion in this range of pressure and temperature will cool the gas and can be used to liquify Ni.k Equations of state can be used to predict /zjt. and T. ... 

Many gases can be liquefied by making use of the Joule-Thomson effect, cooling... 

FIGURE 4.31 Cooling bv the Joule-Thomson effect can be visualized as a slowing of the molecules as they climb away from each other against the force of attraction between them. 

Figure 5 Ohm plots and the development of Joule heating of the same buffer system using different cooling methods.

The actual temperature of separation is determined by internal and external factors. The internal factor, as it was mentioned earlier, is the generated Joule heat. The external factor is the temperature control applied by the cooling system. A temperature increase decreases the viscosity of the electrolyte and increases the diffusion of the sample, resulting in zone broadening and a decrease in efficiency. 

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