Cold Feed Distillation Tower

As the liquid falls, it becomes leaner and leaner in light ends, and richer and richer in heavy ends. At the bottom of the tower some of the liquid is circulated through a reboiler to add heat to the tower. As the gas goes up from tray to tray, more and more of the heavy ends get stripped out of the gas at each tray and the gas becomes richer and richer in the light 

Typically, the liquid out the bottom of the tower must meet a specified vapor pressure. The tower must be designed to maximize the molecules of intermediate components in the liquid without exceeding the vapor pressure specification. This is accomplished by driving the maximum number of molecules of methane and ethane out of the liquid and keeping a.s much of the heavier ends as possible from going out with the gas. 

Assume an initial split of components in the inlet that yields the desired vapor pressure. That is, assume a split of each component between the tower overhead (gas) and bottoms (liquid). There are various rules of thumb that can be used to estimate this split in order to give a desired vapor pressure. Once the split is made, both the assumed composition of the liquid and the assumed composition of the gas lue known. 

Calculate the temperature required at the base of the tower to develop this liquid. This is the temperature at the bubble point for the tower pressure and for the assumed outlet composition. Since the composition and pressure are known, the temperature at its bubble point can be calculated. 

Calculate the composition of the gas in equilibrium with the liquid. The composition, pressure, and temperature of the liquid are known, and the composition of the gas that is in equilibrium with this liquid can be calculated. 

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Figure 6-4 shows the cold feed distillation tower of Figure 6-3. The inlet stream enters the top of the tower. It is heated by the hot gases bubbling up through it as it falls from tray to tray through the downcomers, A flash occurs on each tray so that the liquid is in near-equilibrium with the gas above it at the tower pressure and the temperature of that particular tray.

Figure 6-5 shows a stabilizer with reflux. The well fluid is heated with the bottoms product and injected into the tower, below the top, where the temperature in the tower is equal to the temperature of the feed. This minimizes the amount of flashing. In the tower, the action is the same as in a cold-feed stabilizer or any other distillation tower. As the liquid falls... 

The GS enriching process is a counter-current gas-liquid extraction done at a pressure of 2000 kPa in a sieve tray tower with the upper half operating at 30 C and the lower at 130 C. ( 5) In the top half of the tower, feedwater extracts deuterium from the upflowing cold H2S, reaching a maximum at the centre of the tower. The recycled lean H2S entering the lower hot half of the tower strips deuterium from the water, which then leaves the system depleted in deuterium. A cascade of several stages is used to reach the desired feed concentration for the final water distillation or finishing unit. Transfer between cascades can be either by gas or liquid from the centre of the tower. 

At ordinary temperatures, the equilibrium favors the concentration of deuterium in the water, but at a temperature of around 130 C the equilibrium favors the concentration of deuterium in the hydrogen sulfide. The tower is therefore divided into two sections, the upper or cold section increasing the concentration of deuterium in the water, which is then used as feed for the lower or hot section, where the exchange leads to a further enrichment, this time in the hydrogen sulfide stream. The enriched gas from this section is then led to the second stage for further concentration. In a final stage, deuterium from the enriched gas is transferred to water, which is then fed to a vacuum distillation system for final enrichment to almost pure D2O (99.75%). 

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