Page 167 - Applied Process Design For Chemical And Petrochemical Plants Volume II

156 Applied Process Design for Chemical and Petrochemical Plants

changed and the performance re-evaluated to adjust in mance requires a mechanical interpretation of the rela-
the direction of optimum performance. tionship of the tray components as they operate under a
Pressed steel caps of 12 to 14 US. Standard gage are the given set of conditions. This evaluation includes the deter-
most frequently used, although cast iron caps are used in mination of:
some services such as corrosive chlorinated hydrocarbons,
drying with sulfuric acid, etc. Alloy pressed caps maintain 1. Tray pressure drop
the light weight desirable for tray construction, yet fre- a. Slot opening
quently serve quite well in corrosive conditions. Special b. Static and dynamic slot seals
caps of porcelain, glass, and plastic are also available to fill c. Liquid height over weir
specific applications. The heavier caps require heavier d. Liquid gradient across tray
trays or more supports in the lighter trays. The use of 2. Downcomer conditions
hold-down bars on caps is not recommended for the aver- a. Liquid height
age installation; instead, individual bolts and nuts are pre b. Liquid residence time
ferred. Some wedge type holding mechanisms are satisfac- c. Liquid throw over weir into downcomer
tory as long as they will not vibrate loose.
3. Vapor distribution
4. Entrainment
Slots
5. Tray efficiency
The slots are the working part of the cap, i.e., the point
where the bubbling action is initiated. Slots are usually The evaluation is made in terms of pressure drops (sta-
rectangular or trapezoidal in shape, either one giving tic and friction) through the tray system. Figures 8-63 and
good performance. A single comparison [5] indicates the 8-66 diagrammatically present the tray action.
rectangular slots give slightly greater capacity than the An understanding of the action of the bubble cap tray is
trapezoidal, while the trapezoidal slots give slightly better important to good design judgment in deciding upon the
performance at low vapor rates (flexibility). This study acceptance of a particular design. The passage of vapor
shows that triangular slots are too limited in capacity, through the caps and liquid across the tray is complicated
although they would be the better performers at low vapor by fluid actions associated with the mechanical configura-
rate. Generally, the capacity range offered by the rectan- tion and with the relative velocities of the fluids at various
gular and trapezoidal slots is preferred. points on the tray. The quantitative considerations will be
given in more detail in later paragraphs. However, the
Slot sizes qualitative interpretation is extremely valuable. The fol-
lowing descriptions are presented for this purpose.
Width: %-%in., %in. recommended rectangular %in. x
%-in. to %-in. x %-in., %win. x %in. recommend- Tray Capacity Related to Vapor-Liquid Loads
ed trapezoidal
Height: %-in. to lxin., Win. to 1Min. recommended Figure 8-101 presents a generalized representation of
the form useful for specific tray capacity analysis. Instead of
Shroud Ring plotting actual vapor load versus liquid load, a similar form
of plot will result if actual vapor load per cap (here the cap
This is recommended to give structural strength to the row relative to inlet or outlet of tray is significant) versus
prongs or ends of the cap. The face of the ring may rest the liquid load per inch or foot of outlet weir length.
directly on the tray floor or it is recommended to have Although each plot must be for a specific system of con-
three short legs of %-in. for clean service. For all materials ditions, Figures 8-102 and 8-103 are extremely valuable in
the skirt clearance is often used at ?4 to 1-in., and for dirty analyzing the action of a bubble tray.
service with suspended tarry materials it is used as high as For Figure 8-103 Bolles points out that the cap loads for
1% in. These legs allow fouling or sediment to be washed inlet and outlet rows will be essentially balanced or “lined
out of the tray, and also allow emergency cap action under out” when the shaded areas are equal.
extremely overloaded conditions-at lower efficiencies. From Figure 8-101, the region of satisfactory tray opera-
tion is bounded by performance irregularities. Here all
Tray Performam-Bubble Caps the caps are flowing vapor; the bubbling action is accept-
able from an efficiency standpoint; entrainment is within
A bubble cap tray must operate in dynamic balance, and design limits; there is no dumping (or back flow) of liquid
the closer all conditions are to optimum, the better the down the risers, and no undesirable vapor jetting around
performance for a given capacity. Evaluation of perfor- the caps.