Cold tower

Vapor Throi put, Over-all Plate Efficiency, % Plate Type Lb Mole/Hr of Dry H S Cold Tower Hot Tower... 

In the example illustrated in Figure 8 the isotope of interest is concentrated in the liquid phase. A feed solution is introduced at the top of a cold tower where it equilibrates in a multiplate column against a gas stream with which it undergoes an isotopic exchange reaction. The isotope concentration builds up to a maximum at the bottom of the cold tower. The hot tower serves as a refluxer for the cold tower. Its performance and function can be seen by focusing attention on the gas stream which is in a closed loop. At the top of the cold tower the gas stream composition is determined by equilibration against the cold feed liquid... 

Chemical reflux of a chemical exchange reaction accomplishes reflux by chemical inter-conversion of the two species. The conversion process supplies a countercurrent stream of enriched or depleted isotope of the appropriate isotopic composition. The use of a hot tower leads to a back transfer of enriched isotope from the enriching phase to the phase being depleted in the cold tower. The hot tower requires a number of plates comparable with that in the cold tower. The effective separation factor is, therefore. 

Lower cold tower temperatures would be preferable. Unfortunately, below 28°C, H2S forms solid hydrate. This limits the cold tower temperature to 30°C. The higher hot tower temperatures are preferred for better recovery. But operating hot tower beyond 130°C is uneconomical, as this leads to increase in hot tower diameter and steam consumption, offsetting the marginal gain in recovery. 

At 20 atm (2 MPa), the minimum safe cold tower temperature is around 30°C, below which hydrate formation occurs. The rapid increase in the condensation... 

Benedict, Pigford, and Levi have carried out mathematical analysis of the GS process. An exhaustive treatment of the process, including calculations for flow rates, dependence of composition on number of stages, effect of solubility and humidity on process analysis, temperature profile in cold towers, simultaneous heat and mass transfer in heat transfer section, concentration reversal in heat transfer section, corrosion, materials of construction, feed purification, and safety, etc. have been reviewed by Dave, Sadhukhan and Novaro. ... 

While high temperatures favor rapid mass transfer, low temperatures are desirable for higher a. NH3 freezes at —78°C and slow kinetics at lower temperatures suggests a lower limit of temperature of —40°C for the cold tower. 

Large hot and cold tower volumes due to large flows and complex heat exchanger systems are the major drawbacks of the bithermal process. 

Larger separation factors, better kinetics, lower cold tower temperature. 

As conversion of methylamine to hydrogen is difficult, only bithermal operation is possible. The cold tower temperature is fixed at 223°K where the rate is fast and a is 7.9, the highest known for any practical exchange reaction. The optimum hot tower temperature is limited to 313 K, a limit set by the decomposition of the catalysis. Sulzer, GmBH and Atomic Energy Canada Limited have given a flow sheet for 65tons/yr... 

By running liquid water countercurrent to recycled gaseous hydrogen sulfide through first a cold tower and then a hot tower, as shown schematically in Fig. 12.6, water enriched in deuterium may be withdrawn from the water leaving the cold tower. The principle of the process and process flow sheets are described in detail in Chap. 13. 

The D2 S reflux needed for the cold tower is provided by the hot tower. This operates at a... 

With a proper flow ratio of hydrogen sulfide to water, this lower separation factor makes possible transfer of deuterium from water to hydrogen sulfide in the hot tower and thus converts the HjS entering the hot tower into the DjS needed for refluxing the cold tower. 

Hydrogen sulfide is conserved by returning depleted Hj S from the top of the cold tower to the bottom of the hot. Heat is conserved by heat exchange between hot and cold liquid and between hot and cold vapor. In principle, no materials other than feed water are consumed in the dual-temperature system energy consumption can be reduced by efficient heat exchange, with a lower bound set by the minimum required by thermodynamics for the separation. 

For the cold tower, the overall deuterium material balance is... 

This is the equation for the operating line in the cold tower, which passes through the points (yp, Xp) and (yp, xp) and has the slope... 

It is thus possible to draw the McCabe-Thiele diagram with equilibrium lines established from the separation factors a and otf, and the operating lines established from specified values of feed, product, and waste compositions xp, xp, and Xy/ and assumed values of the gas-phase compositions yp and yp. The number of theoretical stages needed in the cold tower for a given set of conditions is then determined by the number of horizontal steps required to go from Xp to Xp the number of stages in the hot tower, from the number of steps to go from xjy to xp. For the separation example of Fig. 13.28, the number of stages in each tower is 16. 

The approach to equilibrium at the top of the cold tower equals that at the bottom of the hot ... 

The ratio of the slope of the equilibrium line to the slope of the operating line in the hot tower equals the ratio of the slope of the operating line to the slope of the equilibrium line in the cold tower ... 

Condensate from SC-1 and PC-1 is returned to the top of hot tower HT-1, and part of the condensate from SC-2 and PC-2 is returned to the top of HT-2A. The rest of the water condensed in PC-2 and SC-2 containing around 15 percent deuterium is withdrawn as plant product. Use of this stream for product instead of water from the bottom of cold tower CT-2B, which has about the same enrichment, is preferred because the condensate is cleaner. 

The dependence of a on temperature and pressure, as computed from Eq. (13.144), is shown in Fig. 13.33. In the cold tower an increase in pressure decreases a because it increases the concentration of Hi S in the liquid more than it decreases the concentration of Hj 0 in the vapor. In the hot tower, an increase in pressure increases a because it decreases the concentration of HiO in the vapor more than it inaeases the concentration of HjS in the liquid. 

Optimum operating conditions. Because the deuterium recovery increases with increasing ratio of a in the cold tower to a in the hot, it might be supposed that the optimum operating conditions would be the lowest possible cold tower temperature, the highest possible hot tower temjjerature, and low pressure. Other factors beside a must be considered, however. 

An increase in pressure above atmospheric leads to lower costs, despite the reduced spread in a s between the hot and cold tower, because of the greater mass flow rate of gas per unit area that can be taken through the towers at higher pressure. At a pressure of 300 psig, however, there is a discontinuous increase in the cost of equipment, because of the need to... 

The optimum temperature of the cold tower is as low as possible without risking formation of a third phase in addition to vapor and aqueous solution. Table 13.24 gives the temperatures at which solid hydrogen sulfide hydrate or liquid hydrogen sulfide form in the system HjS-HjO. At 300 psi, the minimum safe cold tower temperature is around 30°C. The rapid increase in condensation temperature above 300 psi is another reason for this being the optimum pressure. Before the first pilot plant for the GS process was operated, the possibility of hydrate formation was not recognized, and freeze-ups occuned until the cold tower temperature was raised above 30°C. 

Effect of hydrogen sulfide solubility and water volatility on analysis of process. The solubility of hydrogen sulfide and the volatility of water introduce changes in flow rates of gas and liquid and deuterium concentrations at the top and bottom of the hot and cold towers. Figure 13.34 illustrates the flow scheme and nomenclature to be used in working out these effects. 

Vapor flows up through the cold tower at a constant rate Gc until in leaving the tower the rate is reduced to Go owing to solution of some H2S in incoming feed water. The vapor flow rate to the hot tower is increased from Go to G by hydrogen sulfide from the stripper and by the water vapor needed to saturate the hydrogen sulfide at the temperature of the hot tower. Gf, remains constant in the hot tower. 

At the top of the cold tower, a deuterium balance on the streams above and below the point of HjS solution gives... 

A deuterium balance over the cold tower gives... 

The Kremser-type equation (13.120) for the streams at the top and bottom of the cold tower, converted to the notation of Fig. 13.34, leads to... 

A deuterium balance between the hot and cold towers, on streams flowing across CDEFGH in Fig. 13.34, gives... 

Under these assumptions, conditions in the cold tower will remain unchanged. Equation (13.122) still relates the product liquid assay xp, natural feed liquid assay xp, and cold-tower gas effluent assay yp. With ric = IS, G/F = 2.03, xp/xp = 4.0, and = 2.32, yp/xp = 0.4558. 

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