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MODERN TROUGH DISTRIBUTOR DESIGN TO IMPROVE PLANT

MAINTENANCE AND OPERATION

Daniel Norton, P.Eng. & Rene Dijkstra, P. Eng.

Presented by:

Tyler Caviglia, P.Eng.

CHEMETICS INC.

Suite 200 – 2930 Virtual Way

Vancouver, BC, Canada

Presented at the

American Institute of Chemical Engineers

Central Florida Section

4798 S. Florida Avenue, #253

Clearwater Conference

June 7 – 8, 2013

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TABLE OF CONTENTS

ABSTRACT ........................................................................................................................ 4

INTRODUCTION .............................................................................................................. 4

DESIGN METHOD ............................................................................................................ 4

SIMPLIFIED HEADER ..................................................................................................... 6

INTERNAL FEATURES ................................................................................................... 8

CALMING PLATES AND DEFLECTORS ...................................................................... 9

INTEGRATED SCREENS ............................................................................................... 10

INTERNAL BAFFLES .................................................................................................... 10

AIR GAP BETWEEN EXIT ORIFICES AND DOWNCOMERS .................................. 11

COMPARISON BETWEEN WEIRS AND ORIFICES .................................................. 12

SUMMARY ...................................................................................................................... 12

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LIST OF FIGURES

Figure 1 Early CFD model of Prototype Distributor. ...................................................... 5

Figure 2: Scale Model Testing Assembly. ....................................................................... 5

Figure 3: Full-Scale Model of Chemetics’ New Trough Distributor

during Water Testing. ....................................................................................... 6

Figure 4: Traditional Trough Distributor, Top, and New Iso-Flux™ Trough

Distributor, Bottom (downcomers to packing omitted for clarity). .................. 7

Figure 5: Iso-Flux™ Trough Assembly with Calming Plates, Deflectors, Screens,

Acid Inlet, and Downcomers (baffles in lower trough not visible). ................. 8

Figure 6: Internals of Assembled Iso-Flux™ Trough Distributor

(acid inlet removed for clarity) ......................................................................... 9

Figure 7: Side View of Trough Showing Air Gap between Exit Orifices

and Downcomers. ........................................................................................... 11

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ABSTRACT

Strong acid distributors are a critical component of any acid plant. Chemetics®

has modernized its distributor design to address common challenges of traditional units.

Through the use of unique, patent pending features, Chemetics’ new Iso-Flux™ Trough

Distributor is insensitive to the acid inlet velocity. This allows the distributor to produce a

highly uniform acid distribution with only a single acid inlet per trough, regardless of the

trough’s size. The simplified acid feed header reduces the overall cost, lead time, and

installation time while improving maintenance access and performance. In addition, the

internal geometry makes the distributor highly resistant to sedimentation.

This paper discusses the details of the distributor design including the function

and importance of its features, as well as the distributor’s advantages relative to existing

technology.

INTRODUCTION

Chemetics’ former trough distributor design evolved from weir style cast iron

trough distributors. This design relied upon introducing acid into the troughs at a low

velocity to maintain a calm surface and uniform distribution. With proper design and

clean acid, these distributors were capable of achieving a very uniform flow distribution.

However, the design of the acid feed header was complex, and the trough often struggled

in plants operating with dirty acid. After reviewing the cost and performance of the

original distributor, the areas identified as most in need of improvement were the overall

cost, lead time, installation time, maintenance access, and the sedimentation resistance of

the distributor. By using a combination of traditional design methods, Computational

Fluid Dynamics (CFD), scale model testing, and full-scale testing, Chemetics was able to

improve upon the original design. Chemetics’ new Iso-Flux™ trough distributor has a

simplified feed header with only a single inlet per trough, unique internal geometry, and

an air gap to separate the flow control orifices from the downcomers. All of our trough

distributors are manufactured by Chemetics equipment division near Toronto, Ontario,

Canada using SARAMET®

high silicon stainless steel. Manufacturing the distributors in-

house using our patented metal allows us to have full control over the design and quality

of our equipment.

DESIGN METHOD

Chemetics’ Iso-Flux™ trough distributor was designed using a combination of

traditional and modern design techniques. First, weaknesses in traditional designs were

identified and new and potential features were discussed. Next, an economic analysis was

used to identify possible areas to reduce the cost. Conceptual designs were investigated

using a combination of analytical calculations, CFD, and scale model testing.

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Figure 1: Early CFD model of Prototype Distributor.

Figure 1 shows an early CFD model of the prototype Iso-Flux™ model. In this

figure, the surface level distribution is shown as the interface in the upper third of the

distributor. It was initially necessary to simulate an existing distributor so that we could

validate our computer model. All CFD models were validated using the known

performance of previous existing distributors, and the results were used to refine the CFD

model. CFD was used primarily to gain insight into the internal workings of the

distributor and to quickly test variations of early designs. Once a promising design was

identified, it was necessary to carry out physical testing to confirm the real world

performance of the design and to validate the results of the CFD models.

Figure 2: Scale Model Testing Assembly.

Figure 2 shows our scale-model testing assembly. It was necessary to build and

test a scale model of our previous distributor alongside the Iso-Flux™ prototypes so that

any side effects from the scaling-down of the trough distributor could be accounted for.

The trough used was one side of a ¼ scale model of a distributor for a 30 ft diameter

tower. Only one side of the trough was used because each trough is symmetrical about its

centerline. The scale model was particularly useful for understanding the physical

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phenomena that CFD has difficulty simulating, such as surface turbulence. It was also

useful for testing small design variations, and understanding the importance of each

feature.

Figure 3: Full-Scale Model of Chemetics’ New Trough Distributor during Water

Testing.

After the scale model testing and further design refinement, a full-scale prototype

was built and tested. Figure 3 shows the full-scale prototype during water testing. The

water testing was done to verify that the exit orifices had equal flow rates, that the

distributor could withstand significant amounts of sedimentation, and to confirm that the

performance was in agreement with our earlier simulations. Finally, the full-scale

prototype was installed into an acid tower to confirm the testing results. The distributor

shown in Figure 3 is now installed and has been operating problem free since early 2010.

By using these design and testing methods, we were able to make and verify substantial

improvements in the distributor design.

SIMPLIFIED HEADER

The most significant change in Chemetics’ new Iso-Flux™ trough distributor is

the simplified header. Figure 4 highlights the difference in headers between the previous

distributor design and the new Iso-Flux™ trough distributor. The new distributor requires

only a single inlet per trough, regardless of the tower size. In comparison, a large

distributor of Chemetics’ previous design could require over 10 inlets per trough.

Previously, a low acid inlet velocity was necessary to maintain a calm surface and

uniform acid distribution. The new design is able to provide a uniform acid distribution

and calm surface regardless of the inlet velocity. Using SARAMET®

allows for an

increase in the acid velocity within the trough without damaging the equipment.

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Figure 4: Traditional Trough Distributor, Top, and New Iso-Flux™ Trough

Distributor, Bottom (downcomers to packing omitted for clarity).

The new header is also simpler to install when compared to traditional headers.

Typically the new header can be supported from either the troughs themselves, or from

the tower at each end. With no side branches to support, it isn’t necessary to hang the

header from the candle plate, which in turn simplifies the design of the candle plate. In

addition, maintenance access is improved as there’s only a single pipe crossing the tower.

The troughs in the Iso-Flux™ distributor have also been stiffened, and no longer

require hangers. This allows the troughs to be supported directly by the corbel or by clips

attached to the tower walls in an alloy tower, which simplifies their installation. It also

often allows the Iso-Flux™ trough distributor to directly replace existing pipe distributors

by raising the height of the troughs.

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INTERNAL FEATURES

Chemetics’ new Iso-Flux™ trough distributor uses a combination of unique

internal features that work together to control the acid, distribute it evenly throughout the

trough, and prevent sedimentation from affecting the acid distribution. Figure 5 shows a

typical assembly of a trough, including the internal features. The calming plates,

deflectors, and screens separate the upper and lower sections of the trough while baffles

are located in the lower section of the trough. The subassemblies of calming plates,

deflectors, and screens are identical throughout the tower to simplify their manufacturing

and installation.

Figure 5: Iso-Flux™ Trough Assembly with Calming Plates, Deflectors, Screens,

Acid Inlet, and Downcomers (baffles in lower trough not visible).

Each calming plate assembly is bolted onto the distributor with the same bolts as

the downcomers, which simplifies the installation and minimizes the number of required

parts. The downcomers are welded to plates which are bolted onto the bottom of the

trough. This allows the distributor to fit through a manway into the tower, and also for the

downcomers to be replaced if necessary.

Calming Plate

Screen

Deflector

Acid Inlet Downcomer

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Figure 6: Internals of Assembled Iso-Flux™ Trough Distributor (acid inlet

removed for clarity)

CALMING PLATES AND DEFLECTORS

The calming plates and deflectors are primarily responsible for distributing the

acid along the length of the trough while maintaining a calm surface. The calming plates

separate each trough into an upper and lower section, as shown in Figure 6. The acid is

introduced to the trough in the lower section. It then travels along the lower section of the

trough, and is distributed into the upper section through regularly spaced openings. These

essentially transform the lower portion of the trough into an internal, integrated header.

The acid enters the upper section of the trough up through the openings in the calming

plates. The deflectors redirect the acid so that it travels horizontally in a fan around each

opening counter current to the flow in the lower section. This helps to maintain a calm,

uniform distribution along the length of each trough. In addition, the deflectors use the

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moving acid to stir the upper half of the trough, which prevents fine suspended solids

from falling out of suspension and accumulating around the exit orifices.

During full-scale testing, we tested the effectiveness of the deflectors to prevent

the accumulation of suspended solids. Plastic injection molding pellets were used because

of their comparable settling velocity to packing chips in acid. These pellets had a

diameter of approximately 4 mm, and were irregular in shape. They were poured directly

into the upper half of the distributor in multiple locations, and approximately 600 mL of

pellets were directly poured onto the calmest area in the trough. All the pellets were

removed in less than 10 seconds, validating the effectiveness of the design in preventing

sedimentation.

INTEGRATED SCREENS

Integrated screens, as shown in Figure 5, are located in each opening in the

calming plates. These screens are sized to prevent large pieces of packing chips, bricks,

and other debris from reaching the upper half of the trough where they could block the

exit orifices. During the sedimentation testing, significant quantities of solids were

introduced into the inlet of the distributor prior to start-up. The results showed that large

suspended solids tend to accumulate at the end of the troughs, where they do not affect

the acid distribution. The accumulated solids can be easily cleaned out during shutdowns

by removing the calming plates at the end of the trough.

Afterwards, we investigated the effects of clogging the screens in the calming

plate openings, even though we did not see substantial plugging in these screens during

testing. This was done by adding a significant quantity of solids to the liquid inlet, then

covering each opening with a piece of sheet metal and holding it in place with multiple

flanges. The combined represents a worst case scenario compared to the conditions seen

by real world acid plants. Surprisingly, the distributor still performed well, with a nearly

equal flow distribution through the exit orifices. The primary consequence was a

noticeable increase in surface turbulence. The integrated screens, in combination with the

deflectors effectively prevent suspended solids from degrading the distribution quality of

the distributor.

INTERNAL BAFFLES

Internal baffles are used to improve the distribution along the length of each

trough, and to prevent the acid from flowing preferentially along the bottom wall. When

the acid enters the distributor, it flows preferentially along the bottom of the distributor

which can cause poor liquid distribution near the inlet. Baffles in the inlet region detach

the acid flow from the bottom of the trough so that the flow is evenly distributed

throughout the lower section. In addition, baffles are located near the openings in the

calming plates to redirect a portion of the acid into the upper trough. The use of these

baffles greatly improves the acid distribution along the length of the trough.

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AIR GAP BETWEEN EXIT ORIFICES AND DOWNCOMERS

The Iso-Flux™ trough distributor uses an air gap to separate the exit orifices in

the trough from the downcomers, as shown in Figure 7. This air gap separates the flow

metering system (exit orifices) from the liquid distribution system (downcomers). By

doing so, each distribution point has its flow rate controlled only by the liquid level in the

upper trough and the size of the orifices. Resistance in the downcomers does not affect

the driving force in the metering system.

Figure 7: Side View of Trough Showing Air Gap between Exit Orifices and

Downcomers.

Occasionally, the air gap was a “leak path” in the former trough distributor

design. Packing chips could obstruct the exit orifices in the trough, which would re-direct

the acid horizontally. This would cause some of the acid to miss the downcomer, which

in turn would cause it to spray sideways into the gas stream. Although the majority of the

acid would fall to the packing, some droplets could be entrained in the gas stream where

they would be captured by the mist eliminators. In addition, if the acid was not entering

the downcomers then the distributor would not have a uniform distribution throughout the

tower. In order to assure that the acid always enters the downcomers, three major

improvements have been made in the new Iso-Flux™ distributor design. First, the

combination of screens and deflectors prevents sedimentation from blocking the exit

orifices. Second, the height of the air gap has been decreased. Finally, the downcomers

are designed to be oversized with respect to the exit orifices, so that even if the acid

leaves the exit orifice traveling somewhat horizontally, it will still enter the downcomer.

The downcomers serve two purposes: First, they transfer the acid away from the

trough so that it can be distributed onto the packing in a uniform pattern. Second, they

allow for each distribution point to have its own independent static head within the

downcomer tube. This is important, as the packing tends to block the exits of the

downcomers. In experiments, Chemetics found that the average downcomer had its open

area blocked by 15%, while 1 in 10 of the downcomers was more than 40% blocked. In

addition, deviations in downcomer length and height can affect the flow rate through the

downcomer. With the Iso-Flux™ design, each downcomer receives an identical amount

of flow set by an exit orifice located above. As the amount of blockage at the exit of each

downcomer varies, the height of the acid within each downcomer adapts so that the flow

rate remains constant. These downcomers are designed to handle at least an 80%

blockage without any change in the flow rate per distribution point. Most existing trough

distributor designs do not have an air gap or other means to separate the flow control

Trough

Air Gap

Downcomer

Exit Orifice

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orifices from the downcomers. This causes the flow distribution to be directly and

significantly effected by the conditions at the downcomer exit, which causes a less

predictable, more random, and lower quality distribution throughout the tower.

COMPARISON BETWEEN WEIRS AND ORIFICES

There are two main methods of flow control in acid distributors: weirs and

orifices. Each method has its strengths and weaknesses.

With weirs, a small change in surface height can cause a large change in flow

rate. This allows a weir based distributor to have a large turn-down ratio, but any

variations in height between the liquid surface and the bottom of the weirs can cause

large variations in the flow rate between distribution points. Because of this, weir based

distributors require tight manufacturing tolerances, careful leveling, a very calm liquid

surface, and a uniform liquid level throughout the distributor to maintain a uniform

distribution. Even the sag in the middle of a simply supported trough can cause a

substantial variation in the flow rates between distribution points.

In comparison, orifices are not as sensitive to surface height variations. This limits

turndown, but improves the uniformity of the distribution. In addition, they are simpler to

manufacture, and are always at the same elevation within the trough. In an orifice based

distributor, changes in the acid surface height can be used to diagnose flow issues. An

increase in surface height can be a sign that the orifices may be clogged, or that more

flow is going to the trough than expected. In a weir based distributor, the change in

surface height would be less obvious.

In general, weirs are less susceptible to clogging from suspended solids than

orifices. However, Chemetics’ new Iso-Flux™ trough distributor’s screens and deflectors

prevent suspended solids from blocking the orifices. Chemetics chose to use orifices over

weirs because they are less sensitive to surface height variations, easier to manufacture,

and useful for diagnosing flow problems. Although weirs do not have the same

sedimentation challenges as orifices, they make it more difficult to diagnose any flow

problems, are more expensive, and are more likely to have varying flow rates. It was

determined that we could more reliably address the sedimentation issues of orifices than

the distribution issues of weirs.

SUMMARY

Chemetics was able to simplify and improve upon its former trough distributor

design. The new Iso-Flux™ Trough Distributor has a simplified header, unique internal

geometry, and maintains the best features of Chemetics’ original trough distributor. The

simplified header reduces the overall cost, lead time, and installation time while

improving maintenance access. The unique internal geometry makes the Iso-Flux™

distributor highly resistant to sedimentation while providing a uniform acid distribution

throughout the tower. An air gap between the exit orifices and the downcomers ensures

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that each distribution point has the same flow rate. The combination of these features

makes Chemetics’ new Iso-Flux™ trough distributor a reliable, affordable, high quality

distributor well suited for the conditions found in real world acid plants.