COMMERCIAL TRAY DESIGN METHODS AND SOFTWARE –

FROM PENCILS TO PENTIUMS

By

Daniel R. Summers, P.E. SULZER CHEMTECH

Presented at the AIChE Spring Meeting Distillation Honors: Dale Nutter

Paper 100d April 26, 2004

New Orleans, Louisiana

Copyright © 2004 by Sulzer Chemtech USA, Inc.

Unpublished

Abstract As popularly known, Dale Nutter was involved early in his career with putting together a design manual on Float Valves that dates back to 1959. This manual was subsequently made into several electronic (computer) format programs in the early 1960's by Dale Nutter and others. In 1988, these programs were put together into a "Baler" spreadsheet by Dave Perry with Dale’s help and called the Electronic Design Manual or EDM. In 1995, the EDM was rewritten (with Dale's help and guidance) in Visual Basic (VB) and included both structured and random packings. Finally, in 1999 after Sulzer purchased Nutter Engineering, the EDM program was recompiled and became what is popularly known today as SulTray. Popular tray design concepts will be discussed in this paper. Additionally, state-of-the-art tray design and tray rating features that are currently incorporated into this "child" of Dale Nutter will be presented. The features discussed will include "Initial Design" and "Performance Diagrams."

Background Prior to the successful test of the Float Valve at FRI in 1964(1) Nutter Engineering developed a design procedure for engineers to arrive at a design for a distillation tray. This procedure was written down and put into a Design Manual(2) format with easy to follow steps to arrive at a good tray design. The manual was written to allow an engineer to do the calculations in a logical manner using slide rule and pencil with a minimum of effort. By today’s standards this would be extremely tedious. Dale Nutter was instrumental in developing the correlations and graphs used in this manual. As can be seen from Figure 1 which is a page from this manual, an engineer need only follow the steps shown, fill in the blanks and arrive at a logical design. By 1970, the manual was updated and streamlined because computers were starting to be used for these tedious calculations. Figure 2 shows a rating sheet from 1967 and 1970 side by side and the one difference you will note is that the rating sheet on the right is printed by Computer printer instead of by hand. In 1962 Dale Nutter was already using a computer to generate the tray hydraulics that was then hand written onto the customer rating sheets. It wasn’t until 1970 however, that the computer was used to generate these sheets directly. An example of the simple type input used in the early 1960’s is shown in Figure 3. In 1988 Nutter Engineering provided to the world, the first customer direct-interactive computer program for the PC to determine tray capacity and pressure drop. This was called EDM which stands for Electronic Design Manual. In 1995 this spreadsheet based program was converted to VB (Visual Basic) and made windows based. The program was subsequently renamed SulTray in 1999 and multi-tray input capability was added.

Capacity There is one primary and very basic need for a computational method for the design (or rating) of a distillation tray set. This basic need is to know exactly how close that tray set is to maximum capacity. All other hydraulics parameters and calculations are secondary. These secondary calculations include, but are not limited to, pressure drop, minimum capacity, entrainment, weeping, tray holdup, spray regime, froth gradient and predicted tray efficiency. This paper will deal with the primary computational need for determining the proximity to maximum capacity. Tray maximum capacity can be limited in several ways. However all limitations result in the tray set filling with liquid and exhibiting very high pressure drop. There are 4 common causes for a column to attain maximum capacity or “flooding.” These are:

1. Fluidization 2. Jet Flooding 3. Downcomer Choking 4. Downcomer Backup

Two of these flood mechanisms have to deal with vapor capacity and two with liquid capacity. We will focus on the vapor capacity issues in this paper to provide a reasonable limit to its scope. The Fluidization capacity, or system limit, deals with the fact that there is a particular vapor rate, such that even with no internals in the tower, liquid droplets will always be blown upward. When this vapor rate is reached, the tower will flood. This of course assumes that gravity is the only force used to separate the liquid away from the vapor. When the buoyancy sheer force of the up-flowing vapor exceeds the gravitational pull downwards on the liquid in the tower, all the liquid will be forced to buildup in the tower and eventually go overhead. This concept is dealt with quite nicely by FRI in Topical Report 125(3) and does not need to be elaborated here. The jet flooding capacity has to deal with massive entrainment of liquid upwards from one tray to the next. Some people refer to this as the mode of operation when the froth height on the tray equals or approaches the tray spacing height. All jet flooding correlations are empirical(4) because no one has yet found a way to look at maximum vapor capacity from a purely fundamental perspective. When examining laboratory jet flood capacity data, and I have to emphasize jet flood data, not liquid capacity limited data, one typically sees the typical plot shown as Figure 4. The question then arises, “How does one take this data and turn it into a computational method or correlation that a person or computer could use?” The first issue that must be addressed before this can be attempted is to deal with the very basic question of how exactly do you define jet flood? This issue has always troubled design engineers and researchers alike. Dave Perry addressed this issue by determining a term called Useful Capacity(5). Through many observations, he determined that over the normal operating range of a tray, there is a

relatively constant tray efficiency. In addition, he observed that just prior to physical flood of that tray, the efficiency starts to be adversely effected. This onset of lower than normal tray efficiency was defined as Useful Capacity. He also observed that this point was almost always near to the loadings where 10% entrainment occurred. Therefore he defined the Useful Capacity to be the loads that represent 10% liquid entrainment. This was something that was easily measurable in the lab and understandable to the design engineers. If one looks again at Figure 4, you can observe two distinct phenomena. The tray’s vapor capacity is affected not only by liquid load but more importantly by tray spacing. Tray spacing appears to have the larger effect while liquid load, expressed here in terms of weir loading, has a more limited effect. Vapor capacity typically varies by the square root of the tray spacing ratio. However, we at Sulzer Chemtech have noticed that a larger value of this power is more appropriate for tray spacings less than 24” (610 mm) and a smaller value of this power should be used for tray spacings greater than 24”. As far as liquid load is concerned and its effect on vapor capacity, the effect appears to be linear (from observations of Figure 4) and of a uniform nature. Based on Figure 4 and an unbelievable number of curves like this for different pressures, operating conditions and styles of trays, one can see a trend in the maximum capacity. By extrapolating these “families” of curves out to very low liquid loads, a plot can be made for a particular tray style (i.e. BDH valve tray). Then if one adds in actual operating data from highly loaded towers, one can see this trend if plotted correctly. In the early days, Nutter Engineering used a capacity correlation that used liquid surface tension divided by vapor density to correlate jet flood capacities. This was the first time surface tension was noticed in the industry as being important to determining tray capacity. This surface tension divided by vapor density relationship worked good until one had to deal with higher pressure water based systems. These towers (typically amine towers) seemed to have some really good capacities compared to other systems with the same surface tension over vapor density, see Figure 5. When this data was re-correlated simply against surface tension, there was a much better fit, see Figure 6. From Figure 6 one could draw a straight line through the data points and arrive at a correlation, but a better fit has a slight downwards trend to it at low surface tensions. See Figure 7. If you then combine this figure with the aforementioned liquid loading and tray spacing effect, one arrives at a capacity correlation for trays. Keep in mind that all data used in determination of this correlation avoided the spray regime(6). This capacity correlation can be expressed as:

Useful Capacity = 100 * Cf / (Max Cf * SysFac) (Eq. 1) Where,

Max Cf = (TS / 24)N * (F’ - (0.0033 * WL)) (Eq. 2) TS = Tray Spacing, inches

σ = Surface Tension, d/cm WL = Weir Loading, GPM/in. F’ from Figure 7 Cf = (V-Load/Af)

V-Load = CFSV * (ρV/(ρL-ρV))0.5 CFSV = Vapor Rate, ft3/sec ρV = Vapor Density, lb/ft3

ρL = Liquid Density, lb/ft3

Af = Tray Free Area (available vapor cross sectional area, well above tray deck), ft2

Sysfac = Tray de-rating system factor for foaming systems One thing to note is that the capacity is correlated against Tray Free Area not the traditional bubbling area. This is because we got a superior fit to the capacity data using free area provided a limitation was put on it. The limitation was to maximize the free area at 15% over the traditional bubbling area. Note that at Sulzer Chemtech we have chosen to use 95% of the best fit curve in Figure 7 to be used in Equation 1, see Figure 8. Also note that “Percent of Flood”, is a popular and industry accepted form of expressing the vapor capacity of a tray. Sulzer Chemtech has decided to adopt the percent of flood as a measure of vapor capacity to be provided to customers for its proprietary trays. This primary correlation, along with many others that have to deal with the secondary issues (as mentioned above) were correlated and combined into the current design and rating tool called SulTray. This tool can only be run on Pentium type computers because only they have enough computing power to accommodate the “Windows” that are open and provide the speed needed to give timely answers. With the advent of computers, many short-cut methods were abandoned and many iterative techniques are used to find solutions to correlations. One example of this is the Colwell(7) equation for determining the head of liquid and relative froth density on a distillation tray. Without a computer, this sort of calculation would not even be attempted for multi-pass trays.

Initial Design With the computer comes the ability to iterate and find an optimum solution quickly. Many times the designer of a distillation tray would like to have a quick tray layout and a rough diameter determination to see an approximate tower size. This capability has been added to SulTray in the form of an “Initial Design” Button. The “Initial Design” button enables the user to obtain an approximate tray design based on a given set of loads. The user of SulTray is provided with a rough diameter, an appropriate number of passes and a rough tray geometric layout for a set of given hydraulic parameters. For a given tray type the “design” will use the given Tray Spacing to estimate major tray geometry based on a few hydraulic parameter assumptions. These assumptions are: 1. Percent of Flood of 78% (+/- 4 percentage points)

2. Downcomer Top(entrance) Velocity of 0.30 ft/sec (0.091 m/sec) 3. Maximum Weir Loading = 12 gpm/in(29.8L/m•sec) 4. Dry Tray Pressure Drop = 1.5 inches Water (38 mm Water) for Fixed

openings 5. Dry Tray Pressure Drop = 2.0 inches Water (50 mm Water) for Movable

Valves The “Initial Design” Button actually determines the Tower Diameter, Number of Tray Passes, Downcomer Top Width, Downcomer Bottom Width, Downcomer Clearance, Tray Open Area and Outlet Weir Height through an elaborate iterative procedure. The resulting values calculated for these terms are “raw” values and are NOT rounded to any nearest english or metric value. The downcomer sizes may very well result in dimensions that violate current design practice. In addition, there are other limitations built into the design procedures. The minimum allowable diameter for a two-pass tray is 60 inches (1524mm) and the minimum allowable diameter for a 4-pass tray is 120 inches (3048mm). No “Initial Design” will be allowed for a tray diameter less than 24 inches (610 mm). In addition, a 3-Pass design is never attempted. The internal design methodology is to calculate the total top downcomer area based on the given maximum downcomer velocity. Next, the bubbling area is adjusted based on the actual weir loading. Using these areas, a diameter is chosen. Then 1, 2 and 4 pass tray designs are examined to see which one provides a design with the least number of downcomers and satisfies the maximum weir loading criteria established above. Downcomer Clearance is checked next. If the existing downcomer clearance results in a Downcomer Head Loss that is greater than 1.5 inches (38 mm) of Liquid, then the clearance is raised. Outlet weir heights on the same tray panel are adjusted so that the liquid on any tray panel has a positive seal between the outlet weir and downcomer clearance of 0.5 inches (13 mm). “Initial Design” 4-pass designs will apply an Equal Bubbling Area design philosophy that uses 2.0 inch (51mm) outlet weir heights and a center downcomer that has a tall “picket

fence” type outlet weir. The effective outlet weir length on each side of the center downcomer will match one side downcomer weir length. This ensures that the V/L ratio on each tray panel will be as close to a value of 1.0 as possible. The equal Bubbling Area philosophy was chosen because of its simplicity. The tray open area is determined using the dry tray pressure drop. This value is sufficiently large enough to enable approximately a 3 to 1 turndown ratio for fixed opening devices and a 4 to 1 turndown ratio for movable valve units. Smaller values for dry tray pressure drop result in too large of open areas on trays.

Performance Diagrams For many companies it is important to know what the potential capacity of their equipment under ever-changing or even extreme conditions. The Performance Diagram is intended to provide the user with that ability to generate a graph of all potential operating points for a given tray geometry. The theory behind such a diagram is that once a particular tray geometry is established (diameter, tray type, number of openings, downcomer size etc.) then a window of potential operation can be established. If plotted on a volumetric basis, this window of potential operation is then independent of the operating system employed. In other words, this diagram can represent any set of compounds and any operating pressure. In reality this is true, provided the surface tension is not too different from the original design conditions. This is the one physical property that can change the shape of a performance diagram. The Performance Diagram is a 2-dimensional plot using liquid volumetric loads as the x-axis and vapor volumetric loads as the y-axis. On these axes, several lines are drawn that will ultimately form the “window” portion of the diagram. For example, a constant % of (jet) Flood line is plotted by the program (as well as several other lines) to define the upper vapor load limit of the window. The value of this maximum (or limiting % of jet Flood) line is chosen by the user. What the computer program does is actual execute the complete SulTray program 400 times (a matrix of 20 different liquid and 20 different vapor loads) and records all the significant hydraulic parameters. Then an interpolation routine is run to find particular hydraulic parameters (75% Jet Flood or 80% Downcomer Backup for example). The Performance Diagram is ultimately plotted on an Excel spreadsheet. This common format was chosen so that the results can be viewed, and adjusted easily by the user to fit their particular needs, see Figure 9.

The following lines typically form the Performance Diagram operating “window” boundaries; · % Flood, which typically forms the upper boundary · Spray Transition, which is the left side boundary · GPM/in (max. weir loading), which will form the right side boundary · and Dry Tray Pressure Drop, which forms the lower boundary. In addition, there are other lines that are important and sometimes influence the operating window’s boundaries. These are: · % Downcomer Backup · % Downcomer Velocity

Conclusions We have taken the reader from the early design years of tray design where pencils and slide rules were dominant to today’s Pentium type computers that can do multiple calculations simultaneously. The future of tray design will see further advancements in correlation and computational methods. The day will come when the column diameter, number of trays, tray type, number of passes, open area, etc. will all be automatically specified by computer methods.

References (1) Yanagi, T., Keller, G. J., “Nutter Type B Float Valve Trays”, FRI Topical

Report No. 31, July 20, 1964 (2) “Float Valve Bubble Trays Design and Test Data”, Nutter Engineering

Company 1959 (3) Fitz Jr., C. W., “Ultimate Capacity of Fractionators”, FRI Topical Report No.

125, June 15, 1997 (4) Lockett, M.J., Distillation Tray Fundamentals, Cambridge University Press

(1986), pp 88 (5) Perry, D.,”Predict a Tray’s True Upper Operating Limit”, Chemical

Engineering Progress, May 1995, pp 83-88 (6) Lockett, M.J., Distillation Tray Fundamentals, Cambridge University Press

(1986), pp 34-38 (7) Colwell, C.J., “Clear Liquid Height and Froth Density on Sieve Trays”,

Industrial Engineering Chemistry, Process Design and Development, Vol. 20, No. 2 (1981) pp 298-307

Figure 1

Figure 2

Figure 3

Tray Capacity@ 10% Entrainment 4’ Test Column - Air/Water

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30" TS24" TS18" TS30" TS24" TS18" TS

Liquid Load, gpm/in.

Cf,

Ft/s

ec

Figure 4

Tray Capacity vs. Surface Tension/Vapor DensityTS=24”

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AmineAir-WaterAir-IsoparDepropC6C6C4C4

Cf,(

CFS

/Af)*√(ρ V

/(ρL-ρ V

))

Surface Tension/Vapor Density, dynes - ft3/cm - lbFigure 5

Tray Capacity vs. Surface TensionTS=24”

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MitsuiAmineAir-WaterAir-IsoparDepropC6C6C4C4

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))

Surface Tension, dynes/cmFigure 6

Tray Capacity vs. Surface TensionTS=24”

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AmineAir-WaterAir-IsoparDepropC6C6C4C4Best Fit

Cf,(

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/Af)*√(ρ V

/(ρL-ρ V

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Surface Tension, dynes/cmFigure 7

Tray Capacity vs. Surface TensionTS=24”

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AmineAir-WaterAir-IsoparDepropC6C6C4C495%

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Surface Tension, dynes/cmFigure 8

95% of best fit

Performance Diagram

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80% Backup 82% Flood 0.45" Dry Drop 75% D-Vel 2.75 Spray 10 GPM/in Operation

V-Lo

ad, C

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Rho

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Figure 9L-Load, GPM