472
The Chemistry and Technology of Petroleum
and liquid are being contacted—the contacting area and (2) the part where vapor and liquid are
separated, after having been contacted—the downcomer area.
Classification of trays is based on (1) the type of plate used in the contacting area; (2) the type
and number of downcomers making up the downcomer area; (3) the direction and path of the liquid flowing across the contacting area of the tray; (4) the vapor (gas) flow direction through the
(orifices in) the plate; and (5) the presence of baffles, packing, or other additions to the contacting
area to improve the separation performance of the tray.
Common plate types, for use in the contacting area, are the following:
1. Bubble cap tray in which caps are mounted over risers fixed on the plate (Figure 17.6). The
caps come in a wide variety of sizes and shapes, round, square, and rectangular (tunnel).
2. Sieve trays come with different hole shapes (round, square, triangular, rectangular (slots),
and star), various hole sizes (from about 2 mm to about 25 mm), and several punch patterns
(triangular, square, rectangular).
3. The valve tray that also is available in a variety of valve shapes (round, square, rectangular,
triangular), valve sizes, valve weights (light and heavy), orifice sizes, and either as fixed or
floating valves.
Trays usually have one or more downcomers. The type and number of downcomers used mainly
depends on the amount of downcomer area required to handle the liquid flow. Single pass trays are
trays with one downcomer delivering the liquid from the next higher tray, a single bubbling area
across which the liquid passes to contact the vapor and one downcomer for the liquid to the next
lower tray.
Trays with multiple downcomers and hence multiple liquid passes can have a number of layout
geometries. The downcomers may extend, in parallel, from wall to wall. The downcomers may be
rotated 90° (or 180°) on successive trays. The downcomer layout pattern determines the liquid flow
path arrangement and liquid flow direction in the contacting area of the trays.
Giving a preferential direction to the vapor flowing through the orifices in the plate will induce
the liquid to flow in the same direction. In this way, liquid flow rate and flow direction, as well as
liquid height, can be manipulated. The presence of baffles, screen mesh or demister mats, loose or
restrained dumped packing, and/or the addition of other devices in the contacting area can be beneficial for improving the contacting performance of the tray, viz. its separation efficiency.
The most important parameter of a tray is its separation performance and four parameters are
of importance in the design and operation of a tray column: (1) the level of the tray efficiency,
in the normal operating range; (2) the vapor rate at the upper limit, that is, the maximum vapor
load; (3) the vapor rate at the lower limit, that is, the minimum vapor load; and (4) the tray pressure drop.
The separation performance of a tray is the basis of the performance of the column as a whole.
The primary function of, for instance, a distillation column is the separation of a feed stream in
(at least) one top product stream and one bottom product stream. The quality of the separation
performed by a column can be judged from the purity of the top and bottom product streams. The
specification of the impurity levels in the top and bottom streams and the degree of recovery of
pure products set the targets for a successful operation of a distillation column. It is evident that tray
efficiency is influenced by (1) the specific component under consideration (this holds specially for
multi-component systems in which the efficiency can be different for each component, because of
different diffusivities, diffusion interactions, and different stripping factors, and (2) the vapor flow
rate; usually increasing the flow rate increases the effective mass transfer rate, while it decreases
the contact time, at the same time. These counteracting effects lead to a roughly constant efficiency
value, for a tray in its normal operating range. Upon approaching the lower operating limit, a tray
starts weeping and loses efficiency.
The Chemistry and Technology of Petroleum
and liquid are being contacted—the contacting area and (2) the part where vapor and liquid are
separated, after having been contacted—the downcomer area.
Classification of trays is based on (1) the type of plate used in the contacting area; (2) the type
and number of downcomers making up the downcomer area; (3) the direction and path of the liquid flowing across the contacting area of the tray; (4) the vapor (gas) flow direction through the
(orifices in) the plate; and (5) the presence of baffles, packing, or other additions to the contacting
area to improve the separation performance of the tray.
Common plate types, for use in the contacting area, are the following:
1. Bubble cap tray in which caps are mounted over risers fixed on the plate (Figure 17.6). The
caps come in a wide variety of sizes and shapes, round, square, and rectangular (tunnel).
2. Sieve trays come with different hole shapes (round, square, triangular, rectangular (slots),
and star), various hole sizes (from about 2 mm to about 25 mm), and several punch patterns
(triangular, square, rectangular).
3. The valve tray that also is available in a variety of valve shapes (round, square, rectangular,
triangular), valve sizes, valve weights (light and heavy), orifice sizes, and either as fixed or
floating valves.
Trays usually have one or more downcomers. The type and number of downcomers used mainly
depends on the amount of downcomer area required to handle the liquid flow. Single pass trays are
trays with one downcomer delivering the liquid from the next higher tray, a single bubbling area
across which the liquid passes to contact the vapor and one downcomer for the liquid to the next
lower tray.
Trays with multiple downcomers and hence multiple liquid passes can have a number of layout
geometries. The downcomers may extend, in parallel, from wall to wall. The downcomers may be
rotated 90° (or 180°) on successive trays. The downcomer layout pattern determines the liquid flow
path arrangement and liquid flow direction in the contacting area of the trays.
Giving a preferential direction to the vapor flowing through the orifices in the plate will induce
the liquid to flow in the same direction. In this way, liquid flow rate and flow direction, as well as
liquid height, can be manipulated. The presence of baffles, screen mesh or demister mats, loose or
restrained dumped packing, and/or the addition of other devices in the contacting area can be beneficial for improving the contacting performance of the tray, viz. its separation efficiency.
The most important parameter of a tray is its separation performance and four parameters are
of importance in the design and operation of a tray column: (1) the level of the tray efficiency,
in the normal operating range; (2) the vapor rate at the upper limit, that is, the maximum vapor
load; (3) the vapor rate at the lower limit, that is, the minimum vapor load; and (4) the tray pressure drop.
The separation performance of a tray is the basis of the performance of the column as a whole.
The primary function of, for instance, a distillation column is the separation of a feed stream in
(at least) one top product stream and one bottom product stream. The quality of the separation
performed by a column can be judged from the purity of the top and bottom product streams. The
specification of the impurity levels in the top and bottom streams and the degree of recovery of
pure products set the targets for a successful operation of a distillation column. It is evident that tray
efficiency is influenced by (1) the specific component under consideration (this holds specially for
multi-component systems in which the efficiency can be different for each component, because of
different diffusivities, diffusion interactions, and different stripping factors, and (2) the vapor flow
rate; usually increasing the flow rate increases the effective mass transfer rate, while it decreases
the contact time, at the same time. These counteracting effects lead to a roughly constant efficiency
value, for a tray in its normal operating range. Upon approaching the lower operating limit, a tray
starts weeping and loses efficiency.