Pressure purging

Vessels can be pressure-purged by adding inert gas under pressure. After this added gas is diffused throughout the vessel, it is vented to the atmosphere, usually down to atmospheric pressure. More than one pressure cycle may be necessary to reduce the oxidant content to the desired concentration. 

Because the vessel is pressurized with pure nitrogen, the number of moles of oxygen remains constant during pressurization, whereas the mole fraction decreases. During depressurization, the composition of the gas within the vessel remains constant, but the total number of moles is reduced. Thus the oxygen mole fraction remains unchanged. 

The relationship used for this purging process is identical to Equation 7-6, where nL is now the total moles at atmospheric pressure (low pressure) and nH is the total moles under pressure (high pressure). In this case, however, the initial concentration of oxidant in the vessel ( y0) is computed after the vessel is pressurized (the first pressurized state). The number of moles for this pressurized state is nH and the number of moles for the atmospheric case is nL. 

One practical advantage of pressure purging versus vacuum purging is the potential for cycle time reductions. The pressurization process is much more rapid compared to the relatively slow process of developing a vacuum. Also, the capacity of vacuum systems decreases significantly as the absolute vacuum is decreased. Pressure purging, however, uses more inert gas. Therefore the best purging process is selected based on cost and performance. 

Equation 7-6 is used to determine the number of cycles required. The initial mole fraction of oxygen y0 is now the concentration of oxygen at the end of the first pressurization cycle. The composition at the high-pressure condition is determined using the following equation  

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The cycles used to reduce the oxygen concentration to a target level are shown in Figure 7-2. In this case the vessel is initially at PL and is pressurized using a source of pure nitrogen at PH. The objective is to determine the number of pressure purge cycles required to reach the desired concentration. 

The number of purge cycles is j = 5.6. Thus six pressure purges are required, compared to four for the vacuum purge process. The quantity of nitrogen used for this inerting operation is determined using Equation 7-7 ... 

At this point the remaining cycles are identical to pressure purging and Equation 7-6 applies. However, the number of cycles j is the number of cycles after the initial pressurization. 

Vacuum and Pressure Purging with Impure Nitrogen... 

The equations developed for vacuum and pressure purging apply to the case of pure nitrogen only. Many of the nitrogen separation processes available today do not provide pure nitrogen they typically provide nitrogen in the 98%+ range. 

Assume that the nitrogen contains oxygen with a constant mole fraction of yoxy. For a pressure purging procedure the total moles of oxygen present at the end of the first pressurization is given by the moles initially present plus the moles included with the nitrogen. This amount is... 

Repeat Problem 7-7 using a combined vacuum and pressure purge. Use a vacuum of 20 mm Hg absolute and a pressure of 200 psig. 

When a pipeline is to be placed in service, the air in it shall be displaced. In order to avoid the creation of a combustible mixture, a slug of inert gas shall be introduced between the hydrogen and air. The hydrogen gas flow shall then be continued without interruption until all the air and inert gas have been removed from the facility. The vented gases shall be monitored and the vent closed before any substantial quantity of hydrogen gas is released to the atmosphere. Dead-ended legs of the pipeline system that cannot be swept by inert gas must be pressure purged. 

One way to combat this problem is with steam. As soon as the flow is interrupted, high-pressure purge steam is automatically opened into the heater tube inlets. The steam blows the residual liquid out of the tubes, and also helps remove heat from the tubes. 

In pressure-swing processes, it Is quite common to use a fraction of the less-adsorbed gas product as a low-pressure purge gas, as shown In Figure 2. Often the purge flow is in the opposite direction from the feed flow. Rules for the minimum fraction of less-adsorbed gas product for displacing... 

The vapor stream from the hot high-pressure separator is cooled stepwise to produce middle distillate and naphtha that are sent to fractionation. High-pressure purge of low-boiling hydrocarbon gases is minimized by a sponge oil circulation system. 

Step 6. Two pressures must be controlled in the column and in the gas loop. The most direct handle to control column pressure is by manipulating the vent stream from the decanter. We have three choices to control gas loop pressure purge flow, flow to the CO> removal system, and the fresh ethylene feed flow since fresh oxygen flow has been previously selected. Both the purge flow and the flow to the C02 removal system are small relative to the gas recycle flowrate. Any changes in either one would not have a large effect on gas loop pressure. Since ethylene composes a substantia] part of the gas recycle stream, pressure is a good indication of the ethylene inventory. So we choose the fresh ethylene feed flow to control gas recycle loop pressure. 

This study is on the development of high-purity isobutane production from isobutane-enriched stream by gaseous adsorption technology. Isobutane purification from Ci mixture, in which not only isobutane, but also n-butane and several kinds of Ct olefins in small or in trace are involved, is very difficult by a traditional distillation method because of their close relative volatilities between constituting components. The continuous layered 3-bed process in which was comprised of six steps as follows pressurization-1 by the cocurrent effluent gas from the other bed, pressurization-2 by isobutane (noduct, adsorption, cocurrent depressurization, countercurrent blowdown, and low pressure purge by isobutane product, was applied. From the experiment, isobutane product with over 99.9% purity and with the trace levels of olefin components could be obtained at ambient temperature. Silver impregnated cliq prefers to CMS for the removal of Ci olefins... 

As has been discussed in the foregoing sections, it is possible to combine cycles to take advantage of certain efficiencies. For example, in the TSA cycle, combining it with an inert gas purge, the time cycle is reduced. In the PSA cycle, it is possible to use a part of the adsorbed product as a low pressure purge to reduce energy requirements. 

Pressure Purging. The vessel is pressurized with an inert gas and then relieved outside. This procedure is repeated until the desired oxygen concentration is reached. The number of pressure purges, n, required to achieve the desired LOC can be calculated by using the following equation ... 

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