Tray Gas-Pressure Drop

Typical tray pressure drop for flow of vapor in a tower is from 0.3 to 1.0 kPa/tray. Pressure drop (expressed as head loss) for a sieve tray is due to friction for vapor flow through the tray perforations, holdup of the liquid on the tray, and a loss due to surface tension  

The equivalent height of clear liquid holdup on a tray depends on weir height, hw, liquid and vapor densities and flow rates, and downcomer weir length, as given by the following empirical expression developed from experimental data (Bennett et al., 1983)  

As the gas emerges from the tray perforations, the bubbles must overcome surface tension. The pressure drop due to surface tension is given by the difference between the pressure inside the bubble and that of the liquid according to the theoretical relation 

Methods for estimating gas-pressure drop for bubble-cap trays and valve trays are discussed by Kister (1992). 

Example 4.7 Gas-Pressure Drop in a Sieve-Tray Ethanol Absorber 

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Estimate the tray gas-pressure drop for the ethanol absorber of Examples 4.4 and 4.6. Use a weir height hw of 50 mm. 

Tray diameter = 0.631 m Recommended tray spacing = 0.5 m Weir length = 0.458 m Weir location = 0.217 m from center of tray Gas-pressure drop = 358 Pa/tray... 

The plates may be any of several types, including sieve, bubble-cap, and valve trays. Valve trays constitute multiple self-adjusting orifices that provide nearly constant gas pressure drop over considerable ranges of variation in gas flow. The gas pressure drop that can be taken across a single plate is necessarily limited, so that units designed for high contacting power must use multiple plates. 

Dual-Flow Trays These are sieve trays with no downcomers (Fig. 14-27b). Liquid continuously weeps through the holes, hence their low efficiency. At peak loads they are typically 5 to 10 percent less efficient than sieve or valve trays, but as the gas rate is reduced, the efficiency gap rapidly widens, giving poor turndown. The absence of downcomers gives dual-flow trays more area, and therefore greater capacity, less entrainment, and less pressure drop, than conventional trays. Their pressure drop is further reduced by their large fractional hole area (typically 18 to 30 percent of the tower area). However, this low pressure drop also renders dual-flow trays prone to gas and liquid maldistribution. 

Weeping Weeping is liquid descending through the tray perforations, snort-circuiting the contact zone, which lowers tray efficiency. At the tray floor, liquid static head that acts to push liquid down the perforations is counteracted by the gas pressure drop that acts to hold the liquid on the tray. When the static head overcomes the gas pressure drop, weeping occurs. 

Unstable Systems Froths and Hollow Cone Atomizing Nozzles We usually think of interfacial contact as a steady-state system of raining droplets or rising bubbles, but some of the most efficient interfacial contactors take advantage of unstable interfacial geometry. The most common is the distillation tray which operates with a wild mix of bubbles, jets, films, and droplets. The mix is often described as froth. Gas pressure drop provides the energy to create the froth. 

Head of liquid which the gas must overcome as the gas passes through the next tray above. Assuming the same gas pressure drop over adjacent trays, this liquid head is equal to h,. 

Estimate the gas pressure drop across the tray, the percent of this pressure drop due to liquid head above the top of the bubble-cap slots, and the liquid head in the downcomer. 

For liquid-gas combinations which tend to foam excessively, high gas velocities may lead to a condition of priming, which is also an inoperative situation. Here, the foam persists throughout the space between trays, and a great deal of liquid is carried by the gas from one tray to the tray above. This is an exaggerated condition of entrainment. The liquid so carried recirculates between trays, and the added liquidhandling load increases the gas pressure drop sufficiently to lead to flooding. 

Because of their simplicity and low cost, sieve (perforated) trays are now the most important of tray devices. In the design of sieve trays, the diameter of the tower must be chosen to accommodate the flow rates, the details of the tray layout must be selected, estimates must be made of the gas-pressure drop and approach to flooding, and assurance against excessive weeping and entrainment must be established. 

Calculate head loss due to surface tension From Example 4.6, the surface tension is 70 dyne/cm = 0.07 N/m. Substituting in equation (4-42), ha = 0.0087 m = 0.87 cm. Calculate the total head loss/tray Substituting in equation (4-37) gives us hf = 8.45 cm of clear liquid/tray. For a liquid density of 986 kg/m3, this is equivalent to a gas-pressure drop of APG = 0.0845 x986 x9.8 = 817 Pa/tray. 

A conservative estimate of the total gas-pressure drop can be obtained assuming that the pressure drop through each tray remains constant at the value estimated for the bottom of the absorber, where the gas and liquid velocities are the highest. Therefore, the total gas-pressure drop is approximately APQ = 375 x6 = 2250 Pa. Assuming a mechanical efficiency of the motor-fan system Em = 60%, the power... 

In a column using trays, the pressure drop across the trayed section normally is about 0.15 psi for each actual tray [2]. This is about twice the usual kinetic energy in the inlet vapor. However, when trays are replaced with tower packings, the pressure drop is significantly reduced. If structured packings are installed, their pressure drop typically will be only 10% of that of the trays replaced. Thus, gas distribution must be critically reviewed whenever trays are replaced by packings. Further, if vapor... 

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