Fouled trays

C-NMR. Table 2 shows C-NMR data [7] for liquid samples from the bottoms of the atmospheric and vacuum towers, and for side streams from and above the most seriously fouled vacuum tower trays. The data show the vacuum bottom to contain 12 % more aromatics than the atmospheric bottom, while the amount of paraffins is 6 % lower. This indicates that the vacuum residue has a greater "solubilizing power" than the atmospheric tower residue, which possibly accounts for the decreased amount of microscopic particulates in the vacuum bottom. In contrast, the side stream from the severely fouled trays contains only 25 % aromatics and 47 % aliphatics, suggesting a relatively low "solubilizing power" for the local reflux. 

Tower deposition appears stratified, indicating localized fouling. One possible explanation is the preferential precipitation of the heavy ends asphaltenes facilitated by the low solubilizing power" of the local reflux. The reduced fouling effects observed for the higher trays support this hypothesis. Side streams fi-om the less fouled trays contain considerably more aromatics and naphthenes than those drawn from the more fouled trays. 

FIGURE 19-1 Fouled trays indicated by pressure-drop proiile... 

Glycol carry-over Plugged downcomers High liquid level Excessive vapor velocity Fouled trays... 

Vertical once-through One theoretical tray Simple piping and compact Not easily fouled Less cost than kettle Difficult maintenance High skirt height No control of circulation Moderate controllability... 

Horizontal natl. circ. Ease of maintenance Lower skirt height than vertical Less pressure drop than vertical Longer tubes possible Less cost than kettle No theoretical tray Extra space and piping as compared to vertical Fouls easier as compared to vertical Accumulation of higher boiling point components in feed line, i.e., temperature may be slightly higher than tower bottom... 

The actual number of trays needed for a particular separation duty depends on the efficiency of the plate, and the packings if they are used. Thus, any factors that cause a decrease in tray efficiency will also change the performance of the colunm. Tray efficiencies are affected by such factors as fouling, wear and tear and corrosion, and the rates at which these occur depends on the properties of the liquids being processed. Thus the proper materials of construction must be selected for tray construction. 

Application all services except extremely coking, polymer formation or other high fouling conditions. Use for extremely low flow conditions where tray must remain wet and maintain a vapor seal. 

T Round valve retained by a fixed holddown and a sharp-edged hole in the tray floor for all services, including fouling, slurry and corrosive applications. 

The A, Aq, T and Tq valves can be supplied either with a flat periphery for tightest shutoff against liquid weepage at turndown rates or with a three-dimpled periphery to minimize contact with the tray deck for fouling or corrosive conditions. 

These contribute to the uniform distribution of liquid as it enters the tray from the downcomer. There are about as many tray designs without weirs as with them. The downcomer without inlet weir tends to maintain uniform liquid distribution itself. The tray design with recessed seal pan ensures against vapor backflow into the downcomer, but this is seldom necessary. It is not recommended for fluids that are dirty or tend to foul surfaces. The inlet weir is objectionable for the same reason. 

The major factors governing the proper design far clearance under the downcomer (see Figure 8-63), and the distance between the bottom of the downcomer and the tray it is emptying onto are [190] (a) downcomer sealing, (b) downcomer pressure drop, and (c) fouling and/or corrosive nature of the fluids. TTie smaller the clearance, the more stable will be the tray start-up due to the greater restriction to vapor flow into and up the empty liquid downcomer. 

Packed towers are used in some distillation operations in preference to plate towers. Usually the selection requires an understanding of the fouling characteristics of fluids of the system. These towers have been used even in polymer forming operations. However, other contacting devices can be cleaned easier. For some processes the packed tower is much more effective as well as cheaper than a tray tower. 

Some types of scrubbers that can handle fouling streams, and baffle trays that can handle fouling and solids-containing streams. 

The idea is best explained with an example. Suppose the base level in a distillation column is normally held by bottoms product withdrawal as shown in Fig. 8.4a. A temperature in the stripping section is held by steam to the rcboiler. Situations can arise where the base level continues to drop even with the bottoms flow at zero (vapor boilup is greater than the liquid rate from tray 1). if no corrective action is taken, the reboiler may boil dry (which could foul the tubes) and the bottoms pump could lose suction. 

Should the liquid level in the bottom of the tower rise to the reboiler vapor return nozzle, the tower will certainly flood, but the reboiler heat duty will continue. Unfortunately, reboiler shell-side fouling may also lead to tray flooding. This happens because the fouling can cause a pressure-drop buildup on the shell side of the reboiler. 

To solve this problem, Jim recommended to his boss that the softened water be preheated by using it on one side of a heat exchanger to cool off a waste foul liquor stream which was flowing to the effluent treatment pond. This heated the softened water from 120 to 160°F. The 50-psig steam demand was reduced by 25 percent. This reduced steam flow prevented the trays from flooding. 

In spite of their apparent complexity of construction in comparison with sieve trays, they usually are less expensive than sieve trays because of their larger holes and thicker plates which need less support. They are more subject to fouling and defer to sieves for such services. 

---