Tray liquid

L - Liquid from bottom stripping tray = Liquid to top stripping tray (j)a = 1 - Ea, fraction not absorbed (jts = 1 - E5, fraction not stripped Vo = Vapor to bottom stripping tray... 

For most trays, liquid flows across an active area of the tray and then into a downcomer to the next tray below, etc. Inlet and/or outlet weirs control the liquid distribution across the tray. Vapor flows up the tower and passes through the tray active area, bubbling up through (and thus contacting) the liquid flowing across the tray. The vapor distribution... 

Sakata [180] evaluates the degree of mixing of the liquid as it flows across a tray and its effect on the tray efficiency, Figure 8-30. For plug flow the liquid flows across the tray with no mixing, while for partial or spot mixing as it flow s over the tray, an improved tray efficiency can be expected. For a completely mixed tray liquid, the point efficiency for a small element of the tray, Eog> tray efficiency, E V, are equal. 

Xjn = tray liquid mol fraction for start of calculations (most volatile component)... 

With current computer technology there are several commercial programs available (as well as personal and private) that perform tray-to-tray stepwise calculations up or dovm a column, using the latest vapor pressure, K-val-ues, and heat data for the components. This then provides an accurate analysis at each tray (liquid and vapor ancJy-sis) and also the heat duty of the bottoms reboiler and overhead total or partial condenser. 

Flexibility most flexible of tray designs for high and low vapor and liquid rates. Allows positive drain of liquid from tray. Liquid heads maintained by weirs. 

Figure 8-149. Correlation for aerated-tray-liquid pressure drop developed from published data for various valves. Note (j> = relative froth density. Reference numbers are from original article [201 ]. Used by permission, Klein, G. F., Chem. Eng. V. 89, No. 9 (1982), p. 81 all rights reserved.

Aerated tray, liquid pressure drop or equivalent clear liquid on tray, in. tray liquid Height of clear liquid on inlet side of tray, in. Height of clear liquid at overflow weir, in. 

Liquid leaving top tray Liquid leaving bottom tray... 

Theoretical trays, equimolal overflow, and constant relative volatilities are assumed. The total amount of material charged to the column is M q (moles). This material ean be fresh feed with composition Zj or a mixture of fresh feed and the slop cuts. The composition in the still pot at the begiiming of the batch is Xgoj. The composition in the still pot at any point in time is Xgj. The instantaneous holdup in the still pot is Mg. Tray liquid holdup and reflux drum holdup are assumed constant. The vapor boilup rate is constant at V (moles per hour). The reflux drum, eolumn trays, and still pot are all initially filled with material of eomposition Xg j. 

Ethyl acetate is a product of yeasts and a normal component of wine. Its level can be increased by Acetobacter contamination, although most wines showing excess volatile (acetic) acid do not necessarily contain excess ethyl ester initially. It is quite possible to obtain brandy of normal composition and quality by continuous distillation of newly fermented wine containing excess acetic acid, e.g., 0.1%. On the other hand, ethyl acetate can be formed in continuous columns, particularly if the distillation conditions provide for a relatively high ethanol concentration on the feed tray or immediately below. Since acetic acid is weakly yolatile in all mixtures of ethanol and water, it does not appreciably distill upward. Therefore there is no opportunity for acetic acid to combine wtih ethanol in tray liquids normally of high ethanol concentration. 

Guymon (21) reported the composition of tray liquids for brandy distilled in continuous column, respectively, at 130°, 170°, and 181° proof. The maximum level of fusel oil occurred on the tray nearest in proof to about 130°. This is the second tray below the product tray for the customary 170° proof of distillation of the product. 

Later we measured the concentrations of n-propyl, isobutyl, and the combined isoamyl and active amyl alcohols (3-methyl-l-butyl alcohol and 2-methyl-l-butyl alcohol) in distillation tray liquids using a gas chromatographic method with n-butyl alcohol as the internal standard. The distribution of the higher alcohols in the 14-tray concentrating section of a 12-inch pilot column during a run in which the product from tray 7 was 169° proof is shown in the upper portion of Figure 4. The... 

Figure 4. Distribution of higher alcohols (above) and ethyl esters of three fatty acids (below) in tray liquids of concentrating section producing brandy at 169° proof from tray No. 7...

The lower portion of Figure 4 shows the distribution of three ethyl esters in the same set of tray liquid samples. These high boiling esters tend to concentrate at slightly lower proof and tray levels than the higher alcohols, but they all overlap. Consequently, a fusel oil or low oils layer drawn from a column will include both higher alcohols and these fatty acid esters. 

A question to be resolved in predicting efficiency concerns the liquid-flow pattern. It is usual practice to assume that the vapour is fully mixed, but there is a diversity of treatments of the liquid phase. The two limiting cases are completely-mixed-liquid and plug-flow-liquid. Achieved efficiencies on well designed trays usually fall between these cases. The assumption of a well-mixed tray liquid is only valid for the smallest trays (pilot scale). 

Adjustable weirs (Fig. 14-22 ) are used to provide additional flexibility. They are uncommon with conventional trays, but are used with some proprietary trays. Swept-back weirs (Fig. 14-22b) are used to extend the effective length of side weirs, either to help balance liquid flows to nonsymmetric tray passes or/and to reduce the tray liquid loads. Picket fence weirs (Fig. 14-22c) are used to shorten the effective length of a weir, either to help balance multipass trays liquid flows (they are used in center and off-center weirs) or to raise tray liquid load and prevent drying in low-liquid-load services. To be effective, the pickets need to be tall, typically around 300 to 400 mm (12 to 16 in) above the top of the weir. An excellent discussion of weir picketing practices was provided by Summers and Sloley (Hydroc. Proc., p. 67, January 2007). 

Flooding is an excessive accumulation of liquid inside a column. Flood symptoms include a rapid rise in pressure drop (the accumulating liquid increases the liquid head on the trays), liquid carryover from the column top, reduction in bottom flow rate (the accumulating liquid does not reach the tower bottom), and instability (accumulation is non-steady-state). This liquid accumulation is generally induced by one of the following mechanisms. 

FIG. 14-32 Pressure-drop contributions for trays, hd = pressure drop through cap or sieve, equivalent height of tray liquid hw = height of weir how = weir crest hhg = hydraulic gradient hda = loss under downcomer. 

Entrainment (Fig. 14-33) is liquid transported by the gas to the tray above. As the lower tray liquid is richer with the less-volatile components, entrainment counteracts the mass-transfer process, reducing tray efficiency. At times entrainment may transport nonvolatile impurities upward to contaminate the tower overhead product, or damage rotating machinery located in the path of the overhead gas. 

Vacuum systems. Packing pressure drop is much lower than that of trays because the packing open area approaches the tower cross-sectional area, while the tray s open area is only 8 to 15 percent of the tower cross-sectional area. Also, the tray liquid head, which incurs substantial pressure drop (typically about 50 mm of the liquid per tray), is absent in packing. Typically, tray pressure drop is of the order of 10 mbar per theoretical stage, compared to 3 to 4 mbar per theoretical stage with random packings and about one-half of that with structured packings. 

Side downcomer dimension, in XX.X Number of tray liquid passes XX.X... 

Tray key data for calculations. Please notice in Fig. 3.6 that three key data inputs are required tower diameter, side downcomer dimension, and number of tray liquid passes. 

CDCAREA = center downcomer area for 2- or 3-pass tray, ft2 CDOC = FPL factor for 3-pass tray, ft FPL = flow path length of cross-tray liquid flow from downcomer inlet to downcomer outlet, in H3 and Hs = span factors of 4-pass tray for FPL calculation... 

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. 

Hl = tray liquid height above tray deck surface, in... 

---