Differential pressure driving force

The flowsheet shown in Figure 10.15 does not show the plumbing required to run a realistic pressure-driven dynamic simulation. The key feature is that the pressure in the stripper must be greater than that in the main column so that vapor can flow from the top of the stripper back to the main column. Therefore in the simulation, a pump and a control valve are placed in the liquid sidestream. A control valve is also placed in the stripper overhead vapor line. All this plumbing is shown in Figure 10.16. In a real physical setup, it is usually possible to use elevation differences to provide the necessary differential pressure driving force to get the liquid to flow from the main column into the stripper at a higher pressure and avoid the use of a pump. 

The differential pressure driving force is then 20ft X 0.600 - 15ft X 0.061... 

The ethane is much lighter than the methyl chloride, so it accumulates in the condenser and acts essentially like an inert substance that blankets the condenser. The effect of the inert substance can be considered to reduce either (1) the bubblepoint temperature, thus reducing the differential temperature driving force and reducing heat transfer, or (2) the effective heat transfer area. Either effect is a reduction in heat transfer. So if the ethane is not vented off during the batch, the pressure cannot be controlled even with the chilled water valve wide open. 

Energy would be saved and a lower-temperature less-expensive heat soiuce could be used as pressure is lowered. However, there are competing effects that must be considered. Vapor density decreases as pressure is lowered, so the diameter of the column increases, which increases capital cost. In addition, the lower pressure means lower reflux-drum temperature, which decreases the heat-transfer differential temperamre-driving forces in the condenser. This results in more heat-transfer area being required, which increases capital cost. Therefore, an economic analysis is required to find the best balance between these effects. 

A reasonable differential temperature-driving force is about 20 K. If the AT is too small, the heat-transfer area of the condenser/reboiler heat exchanger becomes quite large. The pressure in the second column is adjusted to give a reflux drum temperature of 367 - - 20 = 387 K. The pressure in C2 is 5 atm. The base temperature in C2 at this pressure is 428 K, which will determine the pressure of the steam used in this reboiler. 

Figure 11.11 gives the flowsheet for the revised extractive distillation process in which the pressure in the methanol column is increased from 1 to 5 atm. This raises the reflux-drum temperature to 385 K with the 99.5 mol% methanol distillate. The base of the extractive column is 364 K, so there is a reasonable 21 K differential temperature driving force to... 

The high T] values above conflict with the common behef that distillation is always inherendy inefficient. This behef arises mainly because past distillation practices utilized such high driving forces for pressure drop, tedux ratio, and temperature differentials in teboilets and condensers. A teal example utilizing an ethane—ethylene sphtter follows, in which the relative number for the theoretical work of separation is 1.0, and that for the net work potential used before considering driving forces is 1.4. 

Refrigeration, like dilution, reduces the vapor pressure of the material being stored, reducing the driving force (pressure differential) for a leak to the outside environment. If possible, the hazardous material should be cooled to or below its atmospheric pressure boiling point. At this temperature, the rate of flow of a liquid leak will depend only on liquid head or pressure, with no contribution from vapor pressure. The flow through any hole in the vapor space will be small and will be limited to breathing and diffusion. 

The driving force for the separation is differential pressure. CO2 tends to diffuse quickly through membranes and thus can be removed from the bulk gas stream. The low pressure side of the membrane that is rich in CO2 is normally operated at 10 to 20% of the feed pressure. 

Three general test procedures used to measure the permeability of plastic films are the absolute pressure method, the isostatic method, and the quasi-isostatic method. The absolute pressure method (ASTM D 1434, Gas Transmission Rate of Plastic Film and Sheeting) is used when no gas other than the permeant in question is present. Between the two chambers a pressure differential provides the driving force for permeation. Here the change in pressure on the volume of the low-pressure chamber measures the permeation rate. 

Gas separation performances (H2/n-butane, n-hexane/2-2 dimethylbutane) have been measured using a sweep gas (countercurrent mode) in order to increase the permeation driving force (no differential pressure was used) permeate and retentate compositions (see Figure 2) were analysed using on line gas chromatography. 

Membranes act as a semipermeable barrier between two phases to create a separation by controlling the rate of movement of species across the membrane. The separation can involve two gas (vapor) phases, two liquid phases or a vapor and a liquid phase. The feed mixture is separated into a retentate, which is the part of the feed that does not pass through the membrane, and a permeate, which is that part of the feed that passes through the membrane. The driving force for separation using a membrane is partial pressure in the case of a gas or vapor and concentration in the case of a liquid. Differences in partial pressure and concentration across the membrane are usually created by the imposition of a pressure differential across the membrane. However, driving force for liquid separations can be also created by the use of a solvent on the permeate side of the membrane to create a concentration difference, or an electrical field when the solute is ionic. 

Bulk or forced flow of the Hagan-Poiseuille type does not in general contribute significantly to the mass transport process in porous catalysts. For fast reactions where there is a change in the number of moles on reaction, significant pressure differentials can arise between the interior and the exterior of the catalyst pellets. This phenomenon occurs because there is insufficient driving force for effective mass transfer by forced flow. Molecular diffusion occurs much more rapidly than forced flow in most porous catalysts. 

In most cases, the radon infiltrates from the soil into the building under the influence of a pressure differential caused by the structure itself. The amount of radon that results is dependent on the amount of radon in the soil gas, the ability of the soil gas to move through the soil, and the characteristics of the building that provides the driving force and entry pathways. 

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