Tray column hydraulics

Packed fractional distillation columns run in the batch mode are often used for low-pressure drop vacuum separation. With a trayed column, the liquid holdup on the trays contributes directly to the hydraulic head required to pass through the column, and with twenty theoretical stages that static pressure drop is very high, e.g., as much as 100-200 mm Hg. 

The internal flow of liquid and vapor must be re-evaluated from the standpoint of column capacity, both in the design and performance studies of columns. The physical dimensions of a column can handle only limited ranges of vapor and liquid flow rates. The objective of this chapter is to evaluate the hydraulic aspects of fluid flow in trayed columns. The column performance is examined with regard to factors such as flooding, entrainment, pressure drop, mass transfer, and tray efficiency. 

The tray hydraulics model may be extended to include mass and heat transfer rates for calculating the liquid and vapor flow rates and compositions in trayed columns on the basis of a rate-based model. The objective is to more realistically represent the actual performance of the column by providing a basis for estimating a tray. For this approach to be practical, methods should be available for reliably predicting the mass and heat transfer rates. General rate-based models are also discussed in Chapter 15 for solving packed columns. 

An understanding of column hydraulics in both trayed and packed columns is essential for a complete performance analysis and design of such devices. The reader will find instructional coverage of these topics, as well as rate-based methods and tray efficiency, in subsequent chapters. 

These regimes appear to be an approximate function of the dimensionless flow parameter, introduced earlier in connection with tray and packed column hydraulics ... 

For example, in distillation we generally do not attempt to control a temperature or composition in the base of the column by manipulating reflux. There is typically a liquid hydraulic lag of 6 seconds per tray, so a change in reflux to the top of a 50-tray column does not change the liquid flow at the bottom of the column for about 5 minutes. The dynamic performance of this loop is poor, so we do not pair bottoms composition with reflux no matter what the RGA tells us to do. On the other hand, the vapor boilup affects all sections of the column quite quickly, so it can be paired with a controlled variable at the top of the column with no dynamic problem. 

The main consideration for introducing reflux or intermediate feed into a packed tower is adequately distributing the incoming stream to the packing. Unlike most tray columns, packed towers are sensitive to distribution. Maldistribution is detrimental to packing efficiency and turndown. The main devices that set the quality of distribution in a packed column are the top (or reflux) distributor, the intermediate feed distributor, the redistributor, and sometimes the vapor distributor. Adequate hydraulics in the inlet area is also important failure to achieve this can affect distributor performance and can also cause premature flooding. 

When column diameters are smaller than about 2V2 to 3 feet, a person cannot enter the column to install, inspect, and maintain the trays. Because of this limitation, a 2V2- to 3-ft trayed column is often installed, even if a smaller diameter is hydraulically sufficient. Alternatively, either packings or cartridge trays can be used, and column diameter reduced below 2V2 ft. 

One of the main advantages of the CS1 structure is that the constant reflux flow rate establishes steady liquid flow rates down through the trays of the column. Changing vapor rates can be achieved fairly rapidly (20-30 s). Changing liquid rates takes much longer because of the hydraulic lags introduced by weirs and baffles. The rule of thumb is about 3-6 s per tray. So in a 30-tray column, it will take 2-3 min for a change in reflux flow to work its way down to the base of the column. 

The numerical example used in this study has a large column (5.61 m in diameter) with large reboiler and condenser duties. Are these results applicable with smaller columns The answer is yes. All the vessel volumes and heat-transfer areas scale directly with flow rates, so the dynamics should be the same. The only exception is tray liquid hydraulics because weir lengths scale directly with column diameter, not cross-sectional area. So, liquid height of the weir is different for different capacities. However, the short-term pressure responses should not be affected by liquid flow rates. 

To understand the column hydraulics, the liquid and vapor responses can be derived using the basic tray relationships. If ... 

Betlem, B.H.L. (1998) Influence of tray hydraulics on tray column dynamics. Chemical Engineering Science, 53, 3991 003. 

Each tray also has a hydraulic equation [Eq. (3.79)]. We also need two equations representing the level controllers on the column base and reflux drum shown in... 

You may wonder why we would ever be satisfied with anything less than a very accurate integration. The ODEs that make up the mathematical models of most practical chemical engineering systems usually represent a mixture of fast dynamics and slow dynamics. For example, in a distillation column the liquid flow or hydraulic dynamic response occurs fairly rapidly, of the order of a few seconds per tray. The composition dynamics, the rate of change of hquid mole fractions on the trays, are usually much slower—minutes or even hours for columns with many trays. Systems with this mixture of fast and slow ODEs are called stiff systems. 

The digital simulation of a distillation column is fairly straightforward. The main complication is the large number of ODEs and algebraic equations that must be solved. We will illustrate the procedure first with the simplified binary distillation column for which we developed the equations in Chap. 3 (Sec. 3.11). Equimolal overflow, constant relative volatility, and theoretical plates have been assumed. There are two ODEs per tray (a total continuity equation and a light component continuity equation) and two algebraic equations per tray (a vapor-liquid phase equilibrium relationship and a liquid-hydraulic relationship). 

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