Flood trays calculation

If the downcomer clearance is too small, then liquid backs up in the downcomer, and the trays above flood. To calculate the height of liquid in the downcomer, due to liquid flowing through the downcomer clearance ... 

The vapor flood velocity calculation is one step in the tray design, or rating of existing columns. The column should be designed or operated such that the actual vapor velocity where /is the fraction of flood velocity, and typically should be between 0.70 and 0.85. Since is based on A - Aj, the actual vapor volumetric flow rate is... 

The flooding correlation assumes that (3, the ratio of the area of the holes, Ajio]g, to the active area of the tray, is equal to or greater than 0.1. If p < 0.1, then the flooding velocity calculated from Eq. (10 ... 

The preceding discussion on reflux assumes that the condenser is not limiting when the reflux is raised. For a severely limited condenser, an evaluation must first be made of the condenser heat transfer before analyzing the effect of a reflux increase with Smith-Brinkley. Likewise, a limiting reboiler or trays close to flood would have to be evaluated prior to Smith-Brinkley calculations. 

Another important consideration in tower design is tray downcomers size. At high ratios of liquid flow to vapor flow a proportionally greater area on the tray must be allotted to the downcomer channel opening. Downcomers are designed from basic hydraulic calculations. If the downcomer is inadequately sized and becomes filled with liquid, liquid level will build on the tray above. This unstable situation will propagate its way up to the tower and result in a flooded tower condition. Excessive entrainment can also lead to this same condition and, in fact, is usually the cause of flooding. 

The calculated entrainment values may be as good or better than measured values [183]. Figure 8-139 illustrates comparison of entrainment between bubble cap and sieve trays. Fair [183] concludes that for vacuum to moderate pressure applications, sieve trays are advantageous from an entrainment-flooding stand-point. 

For once-through natural circulation reboilers, the liquid backup height is calculated from the pressure balance equation. If this height, plus an allowance for froth, reaches the bottom tray level, flooding of the tower will occur. 

The method of calculation introduced in this chapter not only allows an exact determination of the column diameter for nonpulsed sieve tray columns, but also allows a good estimation of the diameters of pulsed and stirred extractors. For the latter, however, more exact specific equations exist for the flooding point, see for example [1,4]. 

Of course, any factor (dirt, polymers, gums, salts) that causes a reduction in the open area of the tray deck will also promote jet flooding. Indeed, most trays flood below their calculated flood point, because of these sorts of problems. Trays, like people, rarely perform quite up to expectations. 

It is normal to assume that the vapor leaving the top of a tower is at its dew point. That is, it is at equilibrium with the liquid on the top tray of the tower. Unfortunately, this assumption falls apart if the tower is flooding and liquid is being entrained overhead from the column, with the vapor. However, assuming a normal, nonflooded condition, we will guess that the tower-top temperature is 140°F. Using the vapor-pressure curves provided in Fig. 9.1, we would calculate as follows ... 

Two factors must be known for downcomer flood calculation the flow rate of liquid on the subject tray and the downcomer area. We will assume that liquid flow is given in gallons per minute at the hot flow-... 

The downcomer percent flood is a critical tray-loading factor. If it is over 90%, then tray flooding failure is likely. If it is below 20%, tray vapor blowthrough (downcomer side) is likely to happen. It is therefore important to keep the downcomer flood below 90% and above 20%. One more limiting downcomer flood value, the active area flood, is also important. The active area flood calculation is covered in the following section. 

The Vload factor is used to determine what is known as jet flood. Jet flood is simply the liquid jetting, causing liquid to recycle from one tray back to the tray above, from which the liquid passed. In some cases jetting can be so severe that it blocks the gas passage with pressure buildup. The following equations calculate Vload ... 

Data specific to tray type must be established next, but these inputs will be discussed later. The data inputted for the next six prompts are the same for all tray types and are primarily for tray efficiency calculations. If tray efficiency or tray liquid residence time values are not desired, these inputs may be skipped (i.e., remain as zero values). However, for bubble cap and sieve trays, the SURF TENS DYN/CM prompt is for active area tray flood calculation. This value should therefore be inputted. 

Tray active area flood is now calculated after all the preceding factors are calculated. The following equations have long been proven by published programs such as Chemcalc 13 [1,2]. 

Please note that HOLHA is the total hole area in ft2 on a single tray deck. It is used in Eq. (3.91) to calculate sieve tray jet flood and will be used to calculate sieve tray pressure drop as well. 

Downcomer backup flooding occurs when the backup of aerated liquid in the downcomer exceeds the available tray spacing. Downcomer backup can be calculated by adding the clear liquid height on the tray, the liquid backup caused by the tray pressure drop, and the liquid backup caused by the friction loss at the downcomer outlet. The downcomer backup is then divided by an aeration factor to give the aerated liquid backup. 

Fair s correlation (19, Fig, 6.10). The Fair flood has been the standard of the industry for entrainment flood prediction and was recommended by most designers (5,11,18,30-33). CSB is a function of the flow parameter Fu [Eq, (6.7)], tray spacing, surface tension, and fractional hole area. CSB is based on the net area AN, and is evaluated from Fig. 6.10, The flooding vapor velocity is calculated from... 

Tray area. Using an entrainment flooding correlation is the usual method of calculation, but short-cut methods (e.g., Ref. 10) are often adequate. However, to minimize trial and error, the author prefers to perform the preliminary estimate using the flooding correlation which will eventually be used, with some simplifying assumptions. In this example, the Kister and Haas correlation [Eq. (6.12)] will be used for the preliminary determination of tray area. Make the following simplifying assumptions ... 

The calculation of column diameter for distillation and absorption columns %h,6 is usually based on the flooding velocity, which, in turn, requires values of the flooding capacity factor, Cf- Fair s flooding-capacity plot for sieve trays [1] correlates the flooding capacity factor with a flow parameter Flv for each tray-spacing value, t, as shown in Figure 2. The flow parameter involves the liquid mass flow, LMi, and vapor mass flow, F v (both in lb/ s), as well as the densities of the two streams. 

The velocity of the continuous phase in the downcomer (or upcomer) Vjow, which sets the downcomer cross-sectional area, should be set at a value lower than the terminal velocity of some arbitrarily small droplet of dispersed phase, say, or in (0.08 or 0.16 cm) in diameter otherwise, recirculation of entrained dispersed phase around a tray will result in flooding. The terminal velocity of these small drops can be calculated by using Stokes law , = (gd Ap)/18 lc-... 

Flooding is much more sensitive to vapor rate than to liquid rate. Trays can, however, definitely be flooded by high liquid rates. Prediction of maximum liquid capacity is based on hydraulic calculation of liquid backup in the downcomers. For low-foaming systems, it is often assumed that liquid backup cannot be more than half of the downcomer height, calculated as clear liquid. For high vapor density, e.g., S.Olb/ft the limiting backup is even less. 

The actual process flow rates are important in nonequilibrium model simulations, whereas in most equilibrium stage simulations, a simulation with a feed flow rate of 1 unit is as meaningful as a simulation with a feed flow of 10, 100, or 573 units. In real columns the flow rates influence the mass transfer coefficients as well as the tray hydraulics. An inappropriate flow specification may mean the column will flood or, just as likely, dump all the liquid through the holes in the tray. Thus, it is important to ensure that the specified (or calculated) flows and tray or packing characteristics are consistent with the satisfactory operation of the column. 

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