Good Practices in Tray Design

8/3/2019 Good Practices in Tray Design

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Good Practices in Tray Design

Good Practices in Tray Design is quite simply, keeping oneself out of trouble.

However, once one has determined the products a distillation or absorption tower is to produce, then goodEngineering practice in tray design is minimizing the cost of a new tower or maximizing capacity of an oldone. There are important practices, other than capacity, that also need to be considered:

1. Minimize Pressure Drop in Vacuum or Temperature sensitive services2. Improve tower performance though higher tray efficiency3. Reduce downtime with anti-fouling devices4. Eliminate fatigue stress cracking in "harmonic" service5. Maximize operating flexibility for seasonal or other market conditions6. Pick the correct material of construction to avoid corrosion7. Miscellaneous levelness, fabrication tolerances, install parts correctly

Listed below are detailed explanations of the above:

CapacityThe maximum capacity of a distillation tower can be defined as the point at which the liquid cannot be

pulled downward by the force of gravity in sufficient quantity to satisfy the desired heat and mass balance.

In other words, if left unchecked, the liquid phase will buildup in the tower until the vessel is completely

liquid filled. Tray design practices must be employed to prevent this phenomena from happening prior towhen the desired purities and production rates are achieved. Many different mechanisms can cause

premature capacity limitations. There are quite a number of these mechanisms that can be attributed to

operational error or inattention. This is not the subject of this discussion. However there are still quite afew that can be avoided by employing good tray design practices. I will discuss the major causes of

capacity limitations due to poor design practices and suggest what areas need attention to avoid theseproblems during design. These are:

1. Sufficient cross sectional area for vapor traffic incorporatedDeck area, free area, area above tray at transitions

2. Sufficient cross sectional area for liquid traffic incorporatedDowncomer Area, Clearance or spout area, Splash baffles, trusses in the way

3. Sufficient Tray Spacing used for liquid to disengage from the vaporAvoid entrainment, watch out for foaming

4. Sufficient Tray Spacing used for vapor to disengage from the liquidAvoid downcomer backup, watch out for foaming

5. Spray regime has been avoidedAvoid low weir loadings, insufficient open area, too large of openings

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Emulsion regime has been avoidedAvoid high liquid inlet velocities especially with small openings at low delta density7. Vapor cross flow channeling avoided

Avoid long flow path lengths with moderate liquid rates and small tray spacings

8. Sufficient deck open area incorporated to avoid pressure drop limitationsDo not choke off downcomer capacity with high tray pressure drop, avoid plugging withtoo small of openings


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Pressure Drop

Pressure Drop is the second most important hydraulic parameter next to capacity and many times the two

are inseparable. Generally once the capacity of a tower has been determined, the pressure drop can beexamined. But for low pressure (generally at vacuum) service, the pressure drop across the trays has a

significant effect on the top pressure and the subsequent vapor density. A high pressure drop can reduce

the tower head pressure to levels well below the desired value and greatly reduce the vapor density. Thislower vapor density at the top of the tower, increases the vapor volumetric flow rate resulting in lesscapacity or larger tower diameters than expected.

A designer has a choice of two sets of liquid and vapor loads to represent a particular trays hydraulics

conditions. Experience says to use the Vapor Loading TO the tray and the Liquid Loading FROM the tray.However, for very low pressure service, especially at the very top of the tower, the vapor rate will increase

in the froth above the tray. A good practice to follow, to be conservative, is to also examine the vapor

loads FROM the top tray to ensure that the top tray is adequately sized.

Another aspect of pressure drop is the dry tray pressure drop. Sometimes this parameter is overlooked for

its value to hydraulic parameters other that the overall tray pressure drop. The dry tray pressure drop can

tell the designer several things; turndown, spray fluidization, and tray stability. To assist in determining

turndown, it is generally believed that too low a dry tray pressure drop (less than 0.25 inches of water pertray) will be insufficient to maintain the liquid on the tray unless it is a bubble cap tray. Too much open

area on the tray or insufficient vapor traffic will enable the liquid to weep through the tray deck resulting in

loss of tray efficiency. The point at which the tray performance falls below the minimum acceptable level

is defined as the turndown point. It is possible to efficiently operate a distillation tray while weeping. FRIhas noted good efficient operating trays at weeping values of 30% or more. However, there is a point at

which the weeping is so great that the desired separation can no longer be achieved.

Spray Fluidization is a phenomenon where the vapor velocity/momentum is sufficiently high to blow the

liquid off the tray. Too little open area or high velocities through the open area can blow moderate

amounts of liquid off the tray. In the laboratory a tray in the spray regime is observed to have a vapor

space above the tray with droplets dancing in it. This is different from a more normal froth regime wherethe froth on the tray is a continuous liquid phase with vapor bubbles passing through it. Too high of a dry

tray pressure drop can contribute to the onset of spray fluidization. Spray fluidization may not adverselyaffect the tray efficiency, but almost ALL tray correlations were developed for tray operating in the froth

regime.

Tray instability is a phenomenon where the dry tray pressure drop is insufficient for the vapor to self

distribute uniformly across the tray bubbling area. It can be shown theoretically that a tray will beintrinsically stable if the ratio of the dry tray pressure drop divided by the liquid head on the tray is greater

than 1.0. This ratio is sometimes referred to as a stability factor. What this means is that if a tray has a

misaligned feed or is upset, that the vapor will naturally seek to distribute itself uniformly on the next tray

above, provide the stability factor is greater than 1.0. In actual practice, depending on the type tray and

levelness of that tray, it has been demonstrated that the stability factor can be as low as 0.5 and still providea stable vapor distribution.

The accuracy of pressure drop calculations will many times come into question. This will occur quite

frequently when this parameter is of a critical nature; temperature sensitive products, vacuum service, whentrying to avoid using a higher temperature heat source, and others. The Sulzer Chemtech pressure drop

correlations are as accurate as the data used to generate them. The pressure drop correlations were recently

refit to a more modern correlation that covers a wide range of operating conditions and therefore modelsthe real world quite well. The accuracy of this new model is with 15%, see attached figure.


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Tray Efficiency

Many things influence tray Efficiency. The first and foremost is the type device employed for the service.

Next is the system itself including the pressure, L/V ratio, relative volatility, and physical properties.

Finally there are design practices that can enhance performance.

The choice of device is important from the viewpoint of capacity, but many times a higher capacity device

will inherently have a lower level of performance. We have plotted tray efficiency vs. relative tray capacityfor various types of devices. Generally, the higher capacity devices exhibit a lower tray efficiency. The

reason for this is that the contact time between the liquid and the vapor is, quite simply, greatly reduced at

high throughput.

There are certain rules of thumb in distillation that apply to tray efficiency behavior. Some of these are:a. Increased pressure increases tray efficiency

b. Higher Liquid rates decrease tray efficiencyc. Increased viscosity decreases tray efficiencyd. Increased Relative Volatility decreases tray efficiency

There are practices that can be taken by the designer to enhance tray efficiency. Increased flow path lengthwill increase the contact time for vapor/liquid contact, therefore increase tray efficiency. Any device that

can enable plug flow of the liquid flowing across the tray will increase tray efficiency by eliminating

backmixing. There are push type devices that can be added to distillation trays that will assist in


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eliminating the gradient on long flow path length trays. These devices improve tray efficiency by allowing

the vapor to have a uniform distribution and eliminates premature entrainment due to vapor cross-flow

channeling. Increased tray spacing will decrease entrainment and gain tray efficiency at highly loaded

conditions. Higher outlet weir heights will increase the contact time between the liquid and the vapor

thereby increasing tray efficiency. High outlet weirs should be avoided though because the additional trayefficiency achieved with outlet weirs much above 2 is very limited. Finally, there are certain tray designs

that enable the liquid to flow the same parallel flow direction on adjoining trays. These designs will

improve tray efficiency by enabling the mass transfer driving force to be uniform across the flow pathlengths of adjacent trays, thereby maximizing tray efficiency.

Anti-Fouling Design

There are certain types of trays that are inherently resistant to fouling. One of them is the Dualflow tray

and another is the V-Grid tray. The Dualflow tray is well known for its anti-fouling capabilities but it haslimited operating flexibility and typically low tray efficiency. The SVG V-grid tray is also well

documented on its ability to accommodate a fouling environment. The large raised opening on the tray

deck in combination with the pushing action provided by the V-Grid valve unit eliminates stagnation on

the tray and keeps it clear of the foulant. When used in combination with a high dry tray pressure drop and

a high downcomer velocity, the tray can accommodate some of the most hostile environments. The SVGand even the MVG tray has been used repeatedly in such notoriously fouling services as:

Beer mash towersDepropanizers

Butadiene Service


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Fatigue Stress Cracking

There is a phenomenon that occurs on fixed opening devices that is quite unusual yet very destructive.

Several sets of V-Grid and Sieve trays have exhibited a behavior of harmonic vibrations resulting in

catastrophic failure of the tray decks. This phenomenon:

Occurs at Low Loadings or turndown

Affects towers in the 10 to 20 ft. diameter range mostResult in severely cracked & broken tray decks

Oscillate or Hum at 10-15 Cycles/second

Will occur regardless of tray strength.

To avoid this phenomenon the designer needs to keep the dry tray pressure drop sufficiently high to get the

tray out of the harmonic range of operation. In other words, design for a high dry tray pressure drop.

Operating Flexibility

Operating Flexibility or turndown can be accommodated in a tray design by using tray deck devices and

downcomer designs that enable such behavior. Large tray spacings, high dry tray pressure drop, the use of

float valve devices, radius tipped downcomer bottoms and low outlet weir heights all contribute toimproved operating range flexibility.

As mentioned above in the Pressure Drop section, once the maximum capacity has been established,

turndown will determine the operating rage of the trays. Turndown can be defined as the loading

conditions at which the tray either becomes unstable or weeps more than 30% of the liquid to the tray. Ifeither of these phenomena occur, then tray performance should be adversely affected to a significant extent.

Several different type trays typical potential turndown capabilities are listed below based on a 24 inch tray

spacing.


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Most often downcomer backup determines the maximum turndown possible. A minimum amount of

pressure drop must be built into a tray design to prevent weeping at low loads. At design loads this

pressure drop then cannot result in downcomer backup that exceeds the tray spacing.

Materials of Construction

The choice of materials of construction can have a profound effect on the performance of a unit if corrosion

sets in. The engineer is constantly striving to produce an economical design with the least expensive

materials. However, there are minimum specifications on the types of materials to be used in commonservices to ensure minimal corrosion or stress cracking. Some of these are:

Hydrocarbons (no H2S) Temp >40 Deg. C Carbon Steel (A-569)Hydrocarbons (no H2S) Temp 30 to 40 Deg. C Killed Carbon Steel

Hydrocarbons (no H2S) Temp 100 to -30 Deg. C 3 Nickel Steel (SA-203)

Hydrocarbons (no H2S) Temp


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panel, outlet weir and downcomer curtain be level as well. This means that fabrication tolerances must be

sufficiently tight to allow the installer the flexibility to meet the installation levelness tolerances.

Manufacturing tolerances are typically the installed tray tolerance for that reason. It is also very

important that the parts be installed correctly. There have been numerous cases where tray decks have been

installed backwards or even upside-down! We have personally observed towers where manways wereforgotten, downcomer curtains left resting on the deck below, material left in towers (gloves, rags, lunch

boxes, tools, ladders, and excess hardware), and my favorite, a withdraw nozzle that was on the opposite

side of the tower from the chimney tray withdraw sump. At the design level, die stamping the partsshowing which side is up and which direction a tray panel should face is extremely helpful for the installer

who is working in a typically very dark environment. The installation drawings should have installation

instructions listed clearly, in a stepwise manner, what needs to be done inside the tower. Small isometric

sketches of the tray deck are most helpful to the installer to instruct the least experienced person on theircrew what to look for. Minimizing welding and cutting in the tower also is good design practice to reduce

installation time and cost. For example, in larger towers, piping networks should be flanged wherever

possible and care should be taken to ensure that all the pieces can fit through the vessel manhole. However,

many times the flange on a tee has to be welded on in the tower simply because it cannot fit through thevessel manhole attached.

I realize that this is a microcosm of the vast amount of information available and applied in actualdistillation tray design practice. There are so many other topics not covered here but can be useful to a

more experienced tray designer. These topics are 2 and 4 pass design optimization, Dualflow and BubbleCap tray design, high capacity tray design, and finally, ultra high capacity tray design.

I trust this information is helpful and will aid the designer in determining the optimum design for their

trayed tower.

Daniel R. SummersManager Chemicals & Gas Applications

Tulsa, OK

918-447-7654

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Good Practices in Tray Design