High capacity distillation revamps - DigitalRefining

High capacity distillation revamps 

A review of fundamental tray design principles used to increase column capacity. 

Understanding how high-capacity trays work assists in selection and design 

Daryl Hanson and Edward Hartman Process Consulting Services Inc 

In the past 10 years the mass transfer industry 

has developed a number of tray designs to 

increase column capacity by 10–30 per cent 

over well-designed conventional trays. While 

some of these trays, such as UOP’s and Shell’s 

have been in service for more than 20 years, 

several have only been commercialised in the 

past five. High capacity tray designs employ 

several different mechanical features to increase 

capacity. 

Maximising tray hydraulic capacity always 

reduces tray efficiency and/or operating flexibility. 

Balancing capacity, efficiency, and operating 

flexibility is the challenge facing the engineer 

performing a revamp. Some designs have inherently 

more efficiency due to the longer distance 

the liquid travels across the tray. Others have 

extremely high capacity, lower efficiency and 

little turndown. Which high capacity tray is 

needed depends on the specific process 

objectives. 

High capacity trays increase column vapour 

and liquid capacity by increasing active area, 

increasing downcomer capacity, decreasing weir 

loading and reducing hydraulic gradient. 

Trays flood when the liquid and/or the vapour 

rate exceed the capacity of the particular design. 

In some cases an optimised conventional tray 

will debottleneck the column. In those instances 

where it is necessary to use high capacity trays, 

one or all of the four principles to increase 

vapour and liquid handling may be needed. 

Often, tray capacity, column efficiency, and operating 

flexibility will need to be balanced to meet 

revamp objectives. Two case studies, described 

later, highlight this balance. 

Tray capacity 

Trays flood because the active area and/or the 

Figure 1 Conventional two-pass tray 

downcomers are not capable of handling the 

operating vapour and liquid rates. Figure 1 shows 

a conventional two-pass tray where liquid flows 

down the column through downcomers, while 

vapour flows up through the active area. 

Incipient flooding begins when liquid is entrained 

with the rising vapour and reaches the tray 

above. If the downcomers have the capacity to 

handle both the entrained liquid plus the normal 

liquid flow, then fractionation will suffer but the 

column will be operable. 

Once the downcomer capacity is exceeded, the 

tray vapour space begins to fill with liquid. At 

this point the tray will hydraulically flood, 

common symptoms being periodic loss of level 

below the flooded trays followed by “dumping” 

of the accumulated liquid causing high levels. 

www.digitalrefining.com/article/1000325 PTQ Q4 2001 1

Figure 2 Jet (vapour) flooding 

When trays flood the column pressure drop 

increases. 

Trays fractionate by mixing the liquid and 

vapour phases together on and above the active 

area. The mixed phases separate by Stoke’s Law 

with the liquid settling onto the tray deck or into 

the top of the downcomer, while liquid-free 

vapour rises up the column through the next 

tray. 

Jet flooding occurs when the localised velocity 

through the tray deck is high enough to entrain 

liquid to the tray above (Figure 2). Jet flooding 

is common in low and moderate pressure services 

such as atmospheric crude, FCC main 

fractionator, or naphtha splitters. These low to 

moderate pressure systems have high vapour 

rates and relatively low liquid rates in the fractionating 

sections. 

Tray downcomers must have the capacity to 

handle the liquid flowing from one tray to the 

next without backing liquid onto the tray deck. A 

downcomer can flood by two distinct mechanisms. 

Downcomer choke flood occurs when the 

rate of liquid entering the top of the downcomer 

is too high to allow entrained vapour to 

disengage. 

The downcomer chokes with liquid and vapour, 

and when it does the liquid level on the tray deck 

builds up and the column floods. On a simple 

basis, downcomer backup flood occurs when the 

liquid and froth level in the downcomer exceeds 

the tray spacing plus the weir height. Liquid fills 

the normal vapour space above the tray deck, 

causing flooding. 

Downcomer backup is impacted on by several 

tray design parameters with the pressure loss 

components shown below: 

• Dry tray pressure drop – vapour induced loss 

• Weir height – set by tray designer 

• Weir crest (height of liquid flowing over the 

weir) – gpm/inch of weir 

• Liquid gradient 

• Pressure loss under the downcomer or through 

dynamic seal – velocity 

Downcomer backup determines the maximum 

and minimum operable liquid rates through a 

high capacity tray, assuming choke is not occurring. 

There must be enough liquid height in the 

downcomer to overcome the pressure drop of the 

various components. 

Total liquid height in a downcomer results 

from the dry tray pressure-drop (vapour induced 

pressure drop through the sieve holes or valves), 

weir height, height of liquid flowing over the 

weir (gpm/inch of weir), liquid gradient and 

pressure loss caused by liquid flowing under the 

downcomer or through the dynamic seal. 

Liquid level on the tray deck is not uniform 

and this reduces tray capacity. The liquid gradient 

is the driving force needed to push the liquid 

across the tray from inlet to outlet. This gradient 

causes non-uniform vapour flow rate through the 

active area with the variability depending on the 

magnitude of the gradient. Liquid gradient 

causes high vapour velocity near the outlet weirs, 

which results in localised flooding of the tray 

active area. 

High capacity tray revamps need to balance 

hydraulic capacity, turndown, and column efficiency 

to meet fractionation objectives. Each 

factor impacting on tray capacity should be 

reviewed independently. In practice, the designer 

concurrently evaluates all the factors involved, 

otherwise an unwanted limit can be created. 

Increasing active area 

Tray active area is the part of the tray where the 

vapour flows through the valve or sieve holes. 

Increasing vapour rate requires more active area 

assuming the downcomer is not restricting 

capacity. For instance, increasing tray active area 

by 15 per cent will increase the vapour handling 

2 PTQ Q4 2001 www.digitalrefining.com/article/1000325

Figure 3 Downcomer flooding: choke and backup 

capacity by about 15 per cent, all other factors 

being equal. Conventional trays have a top downcomer, 

bottom downcomer and active areas. 

Reducing one or both of the downcomer areas 

can be used to increase active area. 

Increasing active area, assuming the top downcomer 

area cannot be changed, requires less 

bottom downcomer area. Conventional trays 

have a solid plate at the bottom of the downcomer 

(inlet panel). High capacity designs either 

heavily slope the downcomer from top to bottom 

or they truncate the downcomer above the tray 

deck. While both increase active area, the heavily 

sloped downcomers maintain the downcomer 

height. This maximises liquid handling 

flexibility. 

However, as the number of tray passes 

increases, the heavily sloped tray inlet panel will 

take up a significant amount of bottom downcomer 

area. Alternatively, the downcomer can be 

truncated above the tray deck. This eliminates 

the tray inlet panel (solid plate) and maximises 

active area for any given top downcomer area. 

Most high capacity trays will either heavily slope 

or truncate the downcomer to increase active 

area. One version of Koch-Glitsch’s Superfrac 

tray has very heavily sloped downcomers, while 

other high capacity trays use truncated downcomers 

to maximise capacity. Ultimate capacity 

and operating flexibility will determine which 

design should be implemented. 

Increasing active area, while maximising 

downcomer height improves liquid handling 

flexibility. UOP’s MD tray was the first commercial 

application of a truncated downcomer. This 

concept totally eliminates the bottom downcomer 

area and maximises active area. 

Subsequently Shell developed the HiFi and 

Calming Section trays. Shell trays are designed 

and manufactured by Sulzer Chemtech. 

Other tray vendors, including Saint-Gobain 

NorPro and ACS, have applied truncated downcomers 

to maximise active area. Increasing active 

area lowers vapour velocity, which decreases the 

spray or froth height on the tray. This unloads 

the trays, allowing vapour rate to be increased. 

Increasing downcomer capacity 

Maximum tray liquid rate is limited by downcomer 

choke or backup flood. Once either of 

these limits is reached, the tray begins to fill with 

liquid. Both potential downcomer flooding mechanisms 

must be assessed when revamping a 

tower (Figure 3). 

Downcomer choke occurs because the top 

downcomer area is not large enough to concurrently 

separate the vapour and allow liquid to 

flow into the downcomer. Vapour is always 

entrained with the liquid entering the downcomers, 

the quantity being a function of the 

difference between the vapour and liquid densities, 

weir loading, and the liquid surface tension. 

High-pressure distillation, such as depropanisers 

and deethanisers, requires large downcomers 

due to the system properties. The top area must 

be large enough for the entrained vapour to flow 


Shell HiFi tray Koch-Glitsch Superfrac tray 

out of the downcomer while liquid is flowing into 

the downcomer. Often, the various hydraulic 

calculation programs do not address downcomer 

choke flood. 

Downcomer flooding by backing up causes the 

liquid or froth height in the downcomer to overflow 

the tray weir. Backup flood can be caused 

by any of the components affecting liquid height 

in the downcomer, including dry tray pressure 

drop, weir height, height of liquid over the weir, 

gradient, and head loss under the downcomer or 

through the dynamic seal. Increasing the active 

area, increasing hole area on the tray, or using 

lower pressure drop valves or sieve decks reduces 

dry tray pressure drop and lowers downcomer 

backup. 

Maximum hole area as a percentage of active 

area for small movable valves and sieve holes is 

approximately 15 per cent and 13 per cent 

respectively, to maintain vapour rate flexibility. 

Higher hole area will further reduce pressure 

drop and increase vapour capacity; however, it 

causes weeping and loss of efficiency when the 

vapour rate is reduced. Using open area above 

these guidelines has caused instability and low 

tray efficiency. 

Truncated downcomers use slots or holes in 

the bottom downcomer plates to seal the downcomers 

and prevent vapour flow up the 

downcomers. The downcomer slot area and 

downcomer height largely determine the liquid 

handling flexibility. Downcomer height is tray 

spacing minus the height of the truncated downcomer 

above the tray floor. Increasing the 

downcomer clearance on a heavily sloped tray or 

the slot area in the truncated downcomer will 

lower backup. These also reduce liquid turndown 

because the downcomer can unseal. 

Once a downcomer unseals, vapour flows up 

the downcomer and tray capacity and efficiency 

is reduced. Trays have been known to enter into 

an unstable dual-flow mode. 

Decreasing weir loading 

Weir loading is a measure of the amount of 

liquid flowing over a unit of weir length (gpm/ 

inch of weir). Decreasing the weir loading 

increases vapour capacity, decreases the height 

of liquid over the weir (weir crest), increases the 

downcomer capacity (gpm/ft 2 liquid entering the 

top downcomer), and reduces liquid gradient 

across the tray. 

Maximum tray vapour capacity occurs at 

approximately 3–4gpm/in of weir, assuming all 

other parameters being equal. Tray capacity will 

increase by 10 per cent when weir loading is 

reduced from 8 to 3gpm/in of weir. Reducing 

weir loading requires more weir length. 

Conventional and many high capacity trays use 

one-, two-, four- – and very rarely six-pass – 

trays to increase weir length. The maximum 

number of passes is set by the column diameter, 

liquid rate and vapour rate. 

Minimum column diameter for a two-, four-, 

and six-pass tray is approximately 5, 11 and 14ft, 

respectively. Once column diameter is at or 

above the minimum, the number of passes can 

be selected based on the required weir length 

needed to control the weir loading. Increasing 

the weir length (more tray passes) decreases the 

flow path length (FPL). Once the FPL drops 

below about 16in it is not possible to inspect the 

trays by removing the tray panels. Decreasing 

the FPL also reduces tray efficiency and may 


equire higher vapour and liquid rates to meet 

the desired fractionation. 

Some high capacity tray designs, such as the 

Shell HiFi, allow the number of downcomers to 

be customised to the specific liquid rate so that 

optimum weir loading is achieved. This maximises 

tray capacity. At other times, the number of 

downcomers can be selected to maximise tray 

efficiency, while meeting the column capacity 

objectives. 

Reducing weir loading has several other benefits. 

It reduces the liquid height flowing over the 

weir. This decreases the liquid level on the tray 

and decreases downcomer backup. Reducing 

weir loading also decreases the downcomer top 

area required to disengage vapour and liquid in 

the top of the downcomer. The liquid flowing 

over the weir travels less horizontal distance, 

which reduces the required downcomer top area 

and allows more active area. 

Reducing hydraulic gradient 

Liquid gradient provides the driving force to 

push liquid across the tray’s flow path length. 

Increasing flow path length will increase the 

liquid gradient. Conventional valves restrict 

liquid flow and increase hydraulic gradient when 

compared to a sieve tray. Reducing hydraulic 

gradient results in more uniform vapour flow 

rate through the active area and higher tray 

capacity. Minimising liquid gradient can increase 

tray capacity by 6–8 per cent. 

Liquid gradient influences vapour flow through 

the tray deck because the total pressure drop 

across any part of a tray must be constant. Total 

tray pressure drop includes dry tray (flow 

through the valves or sieve holes) and liquid 

pressure drop. Conventional and high capacity 

cross-flow trays have the highest liquid level and 

liquid density where liquid enters the active area. 

Liquid head at the tray inlet is the highest, and 

is lowest at the outlet weir. 

Vapour flow is highest where liquid head is the 

lowest because more vapour- induced pressure 

drop is required to meet the constant total pressure 

loss criteria across each section of the tray. 

Special design features such as directional 

valves, slots, and bubble promoters can be used 

to minimise liquid gradient and aerate the liquid 

on the tray to a more uniform density. Using 

vapour to push the liquid across the tray reduces 

liquid gradient. The Koch-Glitsch Superfrac tray 

NorproTriton tray 

uses directional devices to minimise gradient 

and maximise capacity. In recent years, several 

tray vendors have designed their valves to push 

the liquid across the tray including Saint-Gobain 

Norpro’s Triton and ACS Max Flow. 

Efficiency and capacity 

Column efficiency, capacity, and operating flexibility 

must be balanced when applying high 

capacity trays. Improving column efficiency can 

reduce the required vapour and liquid rate for a 

given separation requirement. The specific distillation 

system will determine whether efficiency 

has a meaningful impact on vapour and liquid 

load. 

The number of trays and the efficiency of each 

tray set the column efficiency. Fractionation 

requirements and column efficiency determine 

the tray vapour and liquid loads. Revamps need 

to balance the number of trays, tray efficiency, 

tray capacity and liquid rate flexibility 

Vessel height and tray spacing determine the 

number of trays that will fit in an existing 

column. Process simulation with the appropriate 

VLE and transport property methods are 

required to determine the reflux ratio and 

column efficiency relationship, as well as the 

tray-by-tray liquid and vapour loads. Some 

columns have extreme vapour and liquid rate 

variability; therefore, tray spacing in the bottom 

of the column can be increased, while reducing 

tray spacing elsewhere. Lowering the tray spacing 

reduces tray capacity by the square root of 

the tray spacing ratio. For instance, a tray on 

12in spacing will have approximately 70 per cent 

of the capacity of a tray on 24in. Therefore, the 


optimum number of trays is a balance between 

column efficiency and tray capacity. Lower tray 

spacing also reduces liquid rate flexibility. 

Tray efficiency is a complex function of the 

distillation system properties (key components, 

relative volatility, viscosity, surface tension, etc) 

and the tray design. For a given distillation 

system, tray efficiency decreases as flow path 

length is reduced. Heavily sloping or truncating 

the downcomer can maximise FPL and increase 

efficiency. Directional valve or bubble promoters 

allow longer flow path lengths while minimising 

liquid gradient. 

Liquid flow patterns can be improved through 

special design features such as the side-downcomer 

layout or directional valves on the 

Koch-Glitsch Superfrac tray. These help minimise 

stagnant zones and improve tray efficiency. 

Operability and flexibility 

Some of the high capacity tray design features, 

such as the truncated downcomer, influence 

operability and flexibility. The area under the 

truncated downcomer uses valves, directional 

valves, or bubble promoters to increase vapour 

capacity and reduce liquid gradient. 

Truncated downcomers must be sealed so that 

liquid flows through the downcomers, while 

vapour flows through the active area. If a significant 

amount of vapour flows up the downcomer, 

liquid is prevented from flowing down. Unsealed 

downcomers can cause poor operating stability, 

reduce tray capacity, and lower tray efficiency. 

Most conventional trays have an outlet weir 

height equal to or greater than the downcomer 

clearance. This creates a positive seal and 

prevents vapour flow up the downcomer. All 

high capacity and some conventional trays use a 

dynamic seal. The liquid rate flowing under the 

downcomers must be high enough to develop the 

seal. Truncated downcomers have a plate at the 

bottom with holes or slots. A minimum liquid 

rate is needed to create the seal and prevent 

vapour passage. 

Maximum liquid rate occurs once the downcomer 

fills with froth and backs liquid onto the 

tray above. Truncated or dynamically sealed 

downcomers can only operate between these two 

limits. This is important during the review of 

start-up procedures and when establishing the 

liquid rates required for stable operation. 

When the downcomer unseals, vapour flows 

up the downcomer. Once the vapour flow is high 

enough to prevent liquid flow into the downcomers 

or the pressure drop through the valve or 

sieve holes gets low enough, then liquid flows 

through the tray deck. Poor vapour-liquid mixing 

occurs and tray efficiency decreases. High capacity 

trays can operate as dual-flow trays under 

some conditions. Vapour and liquid alternately 

flow through the same hole. Tray efficiency loss 

of 30–40 per cent has been observed in a xylene 

splitter when the downcomers unseal. 

Another potential problem area is the distance 

from the bottom of the truncated downcomer to 

the tray deck. It must be high enough to prevent 

the froth on the tray from reaching it, while 

providing sufficient cross-sectional area between 

the tray deck and edge of the truncated downcomer 

for vapour to escape. If the downcomer is 

too close to the tray deck, then the vapour flow 

area is choked. Thus, the active area under the 

downcomer is ineffective, which lowers overall 

tray capacity. 

If excess height is used, the column liquid 

capacity is reduced because the downcomer 

height is very short. 

High sieve or valve hole area in the active area 

also controls the tray operating stability. High 

tray hole areas increase vapour capacity, but 

lower the vapour rate flexibility. As a rule, the 

higher the capacity through a given column 

diameter, the less flexibility is available. In fact, 

extreme tray designs have approached pointoperation 

and will only operate over a very 

limited range of vapour and liquid rates. They 

have no turn down capability. 

Case Study 1: Column efficiency 

Revamping a C 3 splitter with improved efficiency, 

high capacity trays increased column feed rate 

by 7 per cent, while maintaining fractionation. 

The column in this case study was operating at 

two distinct constraints: condenser and tray 

capacity limits. C 3 splitter reflux ratio and tray 

loading will decrease as column pressure is 

reduced because the propylene/propane relative 

volatility increases. 

Concurrently, low pressure reduces the propylene 

stream condensing temperature, which can 

lower the condenser capacity through reduced 

cooling water and propylene stream temperature 

differences. 

In this example, during low pressure operation 


the reflux ratio was lowest, but the 

condenser capacity limited unit feed 

rate. Increasing tower pressure alleviated 

the condensing limitation but 

increased the tray loads and flooded 

the column. Prior to the revamp, the 

column had high capacity trays, 

however, they had relatively low 

efficiency. 

C 3 splitter fractionation is more 

complex than many refinery services 

because of the impact of operating 

pressure and column efficiency on tray 

loading. Many times, when there is a 

bottleneck in column capacity, high 

capacity trays are specified without 

looking at the big picture and formulating 

the best revamp strategy. 

Quantifying the existing column and 

auxiliary (condenser) equipment limitations 

is critical before any revamp. 

Column efficiency evaluation showed 

current operation at the inflection point 

of the reflux (or reboil, reflux ratio) 

versus stage curve. Understanding fractionation 

efficiency sensitivity was 

essential to finding the best solution. 

Figure 4 is a curve for the separation, 

which analyses the reflux ratio versus 

stage curves. There are three distinct 

zones of operation shown on Figure 4 

and described in Table 1. They are 

represented by Zones A, B and C. 

For this splitter, the existing operation was on 

the borderline of Zones A and B. The most costeffective 

revamp strategy was to increase tray 

efficiency to stay within the condenser constraint. 

The existing splitter had 100 high capacity trays. 

The trays were set on 15in tray spacing 

throughout the column. The single phase feed 

entered the tower onto tray 45. The condenser 

used cooling tower water. The existing trays were 

a high capacity and low efficiency design. 

Column peripheral equipment and utility limitations 

prevented increasing feed rate with the 

low efficiency trays. The existing cooling water 

condensers were limited to 104 million Btu/hr 

due to cooling water availability. 

Increasing the cooling water supply would have 

required another cell to be added to the existing 

cooling tower. This required major capital investment 

and it could not be justified. Hence the 

Figure 4 Column efficiency vs reflux ratio curve 

Zone Sensitivity to efficiency Result 

A High Tower benefits from addition of stages or 

increased tray efficiency 

B Moderate Tower may not reap benefits of additional 

stages 

C Low Revamp to higher tray spacing is beneficial 

to gain capacity. Revamp with low 

efficiency trays is a good strategy 

Table 1 

Fractionation efficiency sensitivity 

condenser limit was a significant factor in the 

revamp. When operating in Zone A and B, 

increasing column capacity without increasing 

condenser capacity requires more column 

efficiency. 

Increasing column operating pressure raises 

the overhead vapour temperature and increases 

condensing capacity by raising propylene 

condensing temperature. However, this increase 

also raises tray vapour and liquid rates due to 

decreasing relative volatility as pressure 

increases. Both the condenser and tray capacities 

were limiting the unit feed rate. Column feed 

rate was a balance between the limitation 

imposed by the trays and the overhead condenser 

system. 

The revamp strategy was to increase the 

number of fractionation stages by using higher 

efficiency trays. The new tray design used a 

longer flow path length and design features to 


Figure 5 Increasing liquid handling flexibility 

minimise liquid gradient. Since the tower was 

operating in the Zone A part of the curve, additional 

efficiency reduced the required reflux 

ratio. Lower reflux ratio translated into greater 

throughput at existing condenser limitations. 

After the revamp the feed rate increased by 7 per 

cent. 

Post revamp operating tray efficiency increased 

and the reflux ratio decreased from 10.4:10.6 to 

9.2:9.4. Further analysis of product and intermediate 

tower samples indicates the tray efficiency 

is 88–92 per cent. This is an efficiency increase 

of 15 percentage points over the low efficiency 

tray. This revamp illustrates the strategy of 

revamping a tower for additional column efficiency 

when operating in Zone A or B of the 

stage versus reflux curve. 

Case Study 2: Improved flexibility 

In the previous case study, increasing column 

efficiency was the correct strategy given the 

constraints. In this case, an FCC depropaniser 

was limited by liquid handling flexibility. The 

column was operating in the middle of Zone C of 

Figure 4; therefore, reflux ratio is not sensitive 

to the number of trays and the tray efficiency. 

The column had 56 trays on 16in tray spacing. 

The design used short 12in deep truncated downcomers, 

which set the maximum and minimum 

liquid rate. Reboiler fouling limited available 

heat input as run-length progressed, therefore 

reflux ratio decreased. This reduced the liquid 

rates inside the column. Due to the short spacing 

of the internals, the liquid flexibility on the trays 

was small and the tray downcomers unsealed. 

This caused the trays to operate in a dual-flow 

manner with liquid and vapour flow through the 

downcomers and active area. 

A revamp strategy was formulated to increase 

operating flexibility. The 56 trays were analysed 

and it was confirmed that unsealing of the downcomers 

occurred as the reboiler fouled. A simple 

fix would be to retrofit the existing truncated 

downcomer dynamic seals with a lower slot area 

to increase liquid head loss and increase downcomer 

backup. This would eliminate the 

unsealing to meet the minimum rates, but then 

the design would not meet the maximum liquid 

rate. 

The problem was operating flexibility due to 

short tray spacing. Short tray spacing, in 

conjunction with truncated downcomers, yields 

low liquid rate flexibility. 

The revamp strategy focused on understanding 

the reflux versus column efficiency curve for this 

system. A key question was: “Does this tower 

need 56 trays for the required separation?” 

A revamp strategy was formulated based on 

future operating conditions of 115 per cent of 

current rates, while improving product separation 

due to higher reflux ratio. Operating range 

was specified as 105 per cent of design (115 per 


cent of current) with a turndown to 50 per cent 

of rates. 

The answer to this question comes directly 

from the reflux ratio versus stage (column efficiency) 

curve. The tower had 56 trays and was 

operating in the Zone C region of the curve; high 

tray count (more efficiency) was not a benefit. 

Therefore, a revamp strategy to decrease the 

number of trays was developed. 

The optimal revamp strategy was to reduce the 

number of trays from 56 to 44 new trays (Figure 

5). The tray spacing was increased to 20in. This 

yielded a tray vapour capacity increase of 12 per 

cent (square root of 20/16) with the additional 

benefits of improved liquid rate flexibility due to 

increased downcomer height. The downcomer 

height is a function of tray spacing and the 

distance of the truncated downcomer from the 

tray deck. 

As the truncated height was kept essentially 

constant, the downcomer plus weir height was 

increased by over 4in or approximately 25 per 

cent. 

By moving the operating point from Zone C to 

a Zone B operation, fewer trays are required 

without materially affecting the reboiler duty, 

condenser duty or reflux requirements. The key 

to using this curve is to understand the separation 

and the relative volatility of the specific 

system. 

Small errors in relative volatility in a low relative 

volatility system, such as a C 3 splitter, will 

result in large reflux ratio errors and major 

equipment design problems. With high relative 

volatility systems, like an FCC depropaniser, 

small relative volatility errors have much less 

impact. This revamp illustrates the strategy of 

revamping a tower for additional tray capacity 

when operating in Zone C of the column efficiency 

curve. Column capacity and operating 

flexibility were driven by tray hydraulics, not 

column efficiency. 

Links 

More articles from: Process Consulting services 

More articles from the following categories: 

Mass Transfer & separation 

Revamps, shutdowns and Turnarounds

High capacity distillation revamps - DigitalRefining

Create successful ePaper yourself

Delete template?

Save as template?