Simplified flow diagram of the experimental column at left, and the column itself equipped with new instruments, pumps, piping, and sight glasses
I
EARL MANNING, Jr., STANLEY MARPLE, and G. P. HINDS, Jr. Houston Research Laboratory, Shell Oil Co., Houston 1, Tex.
A Plant-Scale Unit ‘for Distillation Tray Research Data obtained from this experimental unit predict closely the performance of plant columns
As and liquid streams pass through the contacting zone of a tray in VAPOR
a distillation tower, each flow has a large effect on the hydraulics of the other stream. This interaction has not been satisfactorily described mathematically for most tray designs used on a large scale. Best tray designs must therefore be determined by large scale experiments. T o develop tray designs for vapor-liquid contacting towers, the Shell Oil Co. has constructed a large hydraulic test unit, consisting of a column, 5 feet in diameter, with distillation trays for testing the hydraulic behavior and mass-transfer efficiency. Simultaneous hydraulic and separating tests under actual distillation conditions are of great advantage in tray design development. The Physical Layout
A tower, 5 feet in diameter and about 32 feet high, was removed from refinery service and used exclusively for experimental studies. Instruments, pumps, and most of the piping were replaced
with new equipment. Nineteen sight glasses (5-inch diameter windows) were placed in the vessel wall so that tray action could be observed visually and photographically. Angle rings for tray support were installed a t 6-inch intervals inside the vessel to facilitate tray spacing studies. Thermowells, sample connections, and taps for hydraulic studies were made available a t each tray location. (There are 172 openings in the vessel wall.) The column can be entered through any of three 18-inch manways or by removing the flanged head. The unit is equipped with two heat-exchanger type steam reboilers and has four vertically mounted condensers with 828 square feet of surface in each. The reflux accumulator has a capacity of 30 barrels. Three 400-barrel tanks provide sufficient storage for test mixtures. The column design is intended to provide steady operation, rapid equilibra-” tion, minimum test system requirement, wide variation in loading rates, ease of sampling, .and convenience in obtaining accurate test data.
Operations
A flow diagram for the experimental distillation unit shows a 5-foot diameter column-the capacity of which requires considerable tankage for continuous oncethrough operation. Therefore, recycle (product) lines were installed to permit operation with a constant inventory of about 40 to 50 barrels of test mixture. For total reflux operation no product is withdrawn and all overhead material is pumped back as reflux. When the column is operated as a rectifying section, the material that would normally be overhead product is pumped to the bottom of the column. Here a stream rich in the light component is mixed with bottom tray liquid overflow rich in the heavy component; thus, the mixture in the bottom of the column is similar to that on a feed tray. In stripping operation, a stream corresponding to the bottom product is drawn from the bottom of the column and is then recycled to the top tray with the reflux. In this case the top tray simulates a feed tray. VOL. 49, NO. 12
DECEMBER 1957
2051
1 0 ; 0
P
i
Figure point
1.
Figure-2.
Determination
of
flood
,R,.-.-” --mr Efficiency calculation
45
---A
Figure 3. Layout of the round cap crossflow tray
The normal level of liquid in the bottom of the column does not have sufficient head to cause natural convection through the reboilers. Therefore a 600-gallon-per-minute pump takes suction from the bottom of the column and discharges through the reboilers and vapor-return line back into the column. T o get sufficient suction head for pumping the hot bottoms liquid, the pump is set in a pit some 5 feet below ground level. Efficiency runs are made by setting the recycle rate at the proper value to correspond to the desired vapor-liquid ratio (the recycle rate is zero at total reflux operation). The heat input is then adjusted to give the desired degree of loading. After conditions appear to be steady, based on temperatures, pressures, and flows, the column is allowed about 1 hour additional to reach equilibrium before samples are taken. Two sets of samples are withdrawn, 30 minutes apart, to see if the column compositions have become steady in the expected time interval since line-out. Capacity, or flood point, determinations are made in the same way except that at every 15- to 30-minute interval the heat input is increased by a few per cent. The pressure drop across the column will gradually increase with load. At the flood point (by definition as used in this work) a small increase in heat input gives a sharp rise in pressure drop, which normally causes a significant break in the plot of pressure drop us. some function of load, as shown in Figure 1. Sampling the column is one of the easier parts of the operation. As relatively large quantities of liquid are held up on the tray a t most operating conditions, a small continuous liquid drawoff from a tray would have an insignificant effect on compositions a t that point. Therefore, small copper tubing lines were run from each of the several sampling points over the entire column to a conveniently and centrally located sample bench where sample bottles can be quickly filled from the continuously flowing streams. Hot streams are sampled through coolers to avoid flashing. For bubble-cap trays, tray efficiencies are based on downcomer samples. Initially this column was operated on a fullv continuous basis. As operating techniques were developed, the time re-
quired for bringup and shutdown was reduced considerably. The column is filled with inert gas on shutdown. For some periods the column has been operated only on the day shift, the period of useful operation being about 6 hours. Column Control
Adequate column control is one of the most essential features in an experimental unit. Therefore the column was provided with more instruments than those usually installed on a plant column. Although the heat input (via steam reboilers) is set by a manually operated valve, the steam supply pressure is automatically controlled and has satisfactorily regulated the heat input. The steam flow to each reboiler is recorded. During operation at pressures above atmospheric, the pressure is held at the desired level by either of two methods: 1. By controlling the pressure of an inert gas bleed from the overhead accumulator, the cooling water flow being high. A small inert gas flow into the accumulator is maintained to assist in this control. 2. By regulating the flow of reflux to the column with a pressure recorder controller. In this case a liquid level is maintained in the vertically mounted condensers, the level being set by the rate of reflux withdrawal. Thus with constant water temperature and flow, the liquid level in the condenser establishes the useful heat transfer surface and consequently the condensing temperature and pressure.
The flow of the product recycle stream is independently set by a flow recorder controller. The quantity of the recycle varies over a \$ride range as vapor to liquid flow ratio is changed. In a single day it may vary from 1000 to 45,000 barrels per day. T o get reliable readings the orifice size must be changed at frequent intervals. By using Daniel senior orifice fittings, the proper size orifice can be installed in a very short time without interrupting the flow through the line. The reflux flow, when not tied in with pressure control, is set by a level controller located on the reflux accumulator. The reflux flow is also recorded. ,4 multipoint temperature indicator is used to obtain temperatures at desired points (tops, reflux, bottoms, trays, and cooling water) Test Mixture
In /Sw.
pg- WPOA DENSITY
P L- LlWlO E N S I N I bM.-42 U.$ GALLONS
LIQUID LOAD, bti/24irs
Figure 4.
2052
Tray capacity,
5 Ib. per square inch gage
INDUSTRIAL AND ENGINEERING CHEMISTRY
Several different test mixtures have been used in the experimental columnbinary hydrocarbon mixtures, multicomponent mixtures, systems to simulate those in a particular plant where efficiency or capacity problems arose, and others. However, most of the experimental work was done with the binary system consisting of iso-octane (2,2,4trimethylpentane) and toluene because of the following advantages:
D I S T I L L A T I O N T R A Y RESEARCH The components are available in tank car quantities as nitration-grade toluene and as a standard reference fuel for octane number determinations. About three or four tank cars of each component have been used to date. Samples can be quickly and accurately analyzed by refractive index. A difierence in composition of 0.001 mole fraction is equivalent to a change of one in the fourth decimal place of the refractive index. The relative volatility of these two components is known accurately and is in a range that permits accurate efficiency determinations to be made over 1 to 15 theoretical stages. Calculations
Column loading calculations are made on the basis of the metered flows with correction for composition and temperature a t a given point within the tower. For example, the vapor flow entering the top tray may be calculated by enthalpy and material balances around the top section of the column and including the top tray. Thermodynamic properties may be obtained from the tables issued by API Project 44 (2) and are convenient for calculating with automatic computing equipment, thus greatly reducing the manpower required for data work-up. Efficiency calculations for binary systems were also made on automatic equipment by adapting well known graphical techniques in the following manner. Using the equilibrium data (I), plots were made of AL, the change in liquid composition for a theoretical tray, us. X I , the composition of the upper sample of a pair from adjacent or nearby trays. The data were also used in tabular form, and a separate curve or table was needed for each column pressure used. At total reflux, the liquid tray efficiency is
( X I - X,)measuped ALn
L
in which X1-X2 is the measured difference in composition and n is the number of actual trays between liquid samples. This method is not accurate for intervals much over one theoretical tray and is implemented by averaging efficiencies for a number of sample pairs. For tray efficiencies during rectifying or stripping operation, the following procedure is used : The column internal flows (corrected for reflux subcooling) and the product Composition are used to compute an operating line or internal material balance equation of the usual type. For a given liquid sample composition, the horizontal distance AX0 between the operating line and the diagonal Y = X is calculated (Figure 2). The quantity AXn for an ideal tray is computed by looking up the quantity
AL for total reflux separation on the same horizontal line as in previous step and subtracting the quantity AX0 from it. Values of AL can be tabulated from the tables for total reflux calculation, thus eliminating errors owing to graphical construction. The tray efficiency is found by dividing (XI - X2),the difference between successive samples, by AXB, and by the number of actual trays, n, between the samples. For either rectifying or stripping calculations, AXE is calculated using the upper sample of each pair. For accuracy, tray efficiencies should not be calculated for sample sets for which AXE is markedly uncertain due to analytical or sampling error. Or, too close an approach to the intersection of the equilibrium and operating curves (Figure 2) must be avoided. Results
The tray design column has been used in a number of tray studies, including some development work on Shell Turbogrid trays. The studies of several runs on a cross-flow bubble-cap tray are shown in Figure 3. The weir height was l'/* inches and the downcomer skirt clearance was 11/, inches. No tray inlet weir was employed, and bypassing of liquid around the contact zone was prevented by vertical baffles, about 8 inches high, laid perpendicular to the ends of the overflow weir. Each of the 23 caps had 33 slots for a total of 276 square inches of slot area (exclusive of the '/,-inch clearance between cap teeth and tray), with riser and reversal areas equal to slot area. The cap slots were 11/4 inches high, tapering from a/16 inch width a t the top to 13/82 inch a t the bottom. The tray spacing was 24 inches. Total column cross section is 2830 square inches and the downspout area was 231 square inches. The downspout was a projection downward in the area marked downcomeropening area (Figure 3). The use of cold reflux produced an unstable operation of the top tray. T o avoid unrealistic capacity measurements, the top tray was spaced 48 inches from the tray below it and the overflow weir was set flush with the top tray deck. Six trays were installed in the column. The down pipe from the bottom tray was 36 inches long and was fitted with an overflow box a t the lower end to provide a liquid seal. Detailed test data are given in Table I. The capacity curve for the tray (Figure 4)shows vapor handling capacity plotted as
as a function of liquid load.
The
I
0
100
50
% CAPAaM
Figure 5. reflux
Downcomer backup at total
U, is the superficial column vapor velocity and p~ and p L are the vapor and liquid densities, respectively. As liquid load increases, the column capacity decreases-caused by increases in liquid flow which the downcomers must handle and in drop entrainment a t high liquid level on the trays. A further understanding of the flooding characteristics of the column can be gained by a study of local tray pressure differences. The rapid increase in equivalent liquid head in the downcomers as flooding is approached is shown in Figure 5 . The equivalent liquid head in the downcomer between trays 4 and 5 a t the flood point varied from 10 inches of water a t high liquid rate to 18.9 inches of water a t low liquid rates (Table I). These limiting backups are probably slightly less than those for the top tray (tray 1) which has somewhat higher volumetric liquid flow. The downcomer head is balanced by three major resistances-vapor-pressure drop between trays, liquid level at the tray entrance, and liquid friction a t the tray entrance. If disengagement of liquid-vapor mixture in the downcomer is poor, these resistances may cause the downcomer to be overloaded a t much lower column loads than for the well settled downcomer contents. The I
I
I
I
1
Figure 6. Efficiency of crossflow bubble-cap tray VOL. 49, NO. 12
DECEMBER 1957
2053
Table Run Number Top pressure, lb./sq. in. gage Top temperature, O F. Vapor/liquid mole ratio Vapor rate, lb.-moles/24 hr. Liquid rate at top Lb.-moles/24 hr. Gal./min. Vapor velocity, in./sec., superficial Allowable, % (S)Q Slot velocity, in./sec. Tray liquid head, in. of water Entrance, tray 4 Exit, tray 4 Center, tray 1 Center, tray 6 Effective head, in. of water Downcomer 6-5 Downcomer 5-4 Pressure drops, in. of water over-all (6 trays) Tray 1 to tray 5 (5 trays) Under downcomer 5-4 skirt Average Murphree l i a i d plate efficiency, yo 2,2,4-Trimethylpentane, %m. Top product Tray 4, liquidb Capacity, % Average specific gravity, downcomer 5-4, dzz 0-4 inches
I,
Performance of Round-Cap Cross-Flow Tray Flooding Runs Total Reflux FRlO FR9 FR6 FR7 2 1 3 10 5.0 236 1.00 1600
FR8
4.8 232 1.00 9150
5.0
5.0
5.0
5.0
5.0
1.00 4206
5.1 235 1.00 6965
0.226 8920
0.521 11,145
1.00 12,740
1.88 13,430
4.08 14,135
1600 20.9 4.69 18 44.1
4206 55.7 12.2 48 115
6965 91.4 20.3 79 191
9150 120 26.9 104 253
39,580 457 26.4 95 248
21,220 2 50 32.8 120 308
12,740 159 37.2 141 351
2.5 2.8 1.5 1.7
2.7 2.9 1.6 1.8
2.9 2.8 1.7 2.1
3.8 ‘2 .9 1.9 2.0
3.7 3.1 4.3 3.0
3.8 3.1 4.0 2.9
1.9
3.0
3.5 4.5 11.2
5.0 6.0 17.0
6.5 10.1
8.7 12.6
19.8 1.3 42
... 4.4 ... ... 61 59.6 45.8 13
... ... ... ... ...
4-8 8-12 12-16 16-20 20-24 a
5.1 234
...
...
7.2
...
... 0.6
77
82
0.6 78
63.6 50.1 34
61.6 46.3 56
62.4 48.2 74
.
I
.
... ... ... ... ... ...
... ... ... ...
...
. I .
e . .
... ...
...
... ... ...
7,160 83.1 39.2 142 370
3460 40.1 41.4 150 390
4.4 4.4 3.0 3.2
4.0 3.9 2.8 2.5
4.4 4.0 2.6 3.2
14.4 15.8
8.7 13.7
15.6 18.9
27.5 1.4
33.0 1.4
33.5 0.8
38.5 0.9
46
57
... ...
45
... ...
42
...
... ...
0.83 0.61 0.49 0.43 0.26 0.09
0.79 0.60 0.58 0.45 0.30 0.03
0.67 0.68 0.63 0.48 0.51 0.30
0.73 0.70 0.66 0.41 0.42 0.32
0.74 0.71 0.73 0.52 0.40 0.37
... ... ...
. a .
...
... ...
... ...
... ...
... ...
Assuming 20 dynes/cm. interfacial tension. Additional samples available for efficiency calculation.
equivalent specific gravity of the downcomer fluid changes from bottom to top of the downcomer a t flooding loads (Table I). At low liquid rates, settling is more nearly complete in the top of the downcomer, and the downcomer continues to handle liquid at a high tray pressure drop and a correspondingly high vapor load. At higher liquid rates, disengagement in the downcomer is much poorer, so that the maximum available downcomer head is small, and the tower floods a t a lower vapor load. The results indicate that tower capacity is limited by the ability of the downcomer to handle the mixed-phase flow. Liquid head gradient from tray entrance to overflow weir was determined visually a t low loads, and by a manometer at higher loads. For this tray no substantial hydraulic gradient was found to exist. However, the range of liquid loads tested is exceeded in some types of commercial operation. I n all cases, the equivalent settled tray liquid depth was more than the weir height. The usual calculation for head over the weir was not made as visual observation showed that much liquid was thrown or pumped over the weir in a dispersed state. The over-all pressure drop along the capacity curve increases as liquid load decreases-a result of the higher limiting vapor rates as liquid rate is reduced. At the lowest liquid load, the flooding pressure drop was 75y0greater than the
2054
flooding pressure drop a t the highest liquid rate. The average Murphree liquid plate efficiency for total reflux operation is shown in Figure 6. Visual observations indicate that a t low loads, efficiency improves with increasing load due to better dispersion of vapor and liquid streams as vapor velocity and liquid hold-up increase. At high loads, however, tray efficiency falls off because of vapor channeling and/or entrainment. Visual observation above 80% of capacity is very difficult as the entire tray space is filled with dispersed flying liquid. Under these circumstances, some entrainment is certainly occurring. However, entrainment was not measured directly during these runs and did not appear severe below 7595 of capacity. Nomencllature = change in liquid mole fraction AL for more volatile component due to one equilibrium distillation tray at total reflux = number of actual trays effectn ing the separation measured = nominal column vapor velocity, Uc inches per second, based on total column cross section V / L = vapor-liquid flow ratio through a column section X = mole fraction of light component in liquid phase XI = mole fraction of light component on upper tray of a pair of samples
INDUSTRIAL AND ENGINEERING CHEMISTRY
Xz
= mole fraction of light com-
ponent on lower tray of a pair of samples XDlst = composition of overhead product Y = mole fraction of light component in vapor phase AXE = change in liquid molal composition for one theoretical tray at any vapor-liquid ratio AXo = horizontal distance between diagonal and operating equations (Figure 2) or the equivalent in stripping operation pa = vapor density pL = liquid density Acknowledgment
A number of people, J. W. Askins, Edward Gelus, T. H. Green, M. E. Klecka, C. M. Jones, F. A. Olson, and Dan Urbanek, participated in the design and experimental work. Literature Cited
(1) Gelus, E., Marple, S., Miller, M. E., IND.ENG.CHEM.41, 1757 (1949). ( 2 ) Rossini, F. D., others, “Selected Values of Physical and Thermodynamic Properties of Hydrocarbons and Related Compounds,” Am. Petroleum Inst. Research Project 44, Pittsburgh, Pa. (1953). ( 3 ) Souders, hl., Jr., Brown, G. G., IND. ENG.CHEhl. 26, 98 (1934). RECEIVED for review IYovember 29, 1956 ACCEPTED March 25, 1957
