85 replies to this topic

[NAP]

Is there anyone in this forum that I could converse with who has actually done a CDU simulation inside-out recently or considers himself having CDU experties?

 

I am a sr. process engineer workign for a licensor/engineering company and I am currently tasked with simulating a CDU for a new and upcoming refinery.

 

Despite of numerous guidelines, manuals, and checks present in my company, I want to have a fresh perspective on some of the design features of CDU before I submit my design to ensure that I havent missed anything.

 

I am looking for a specific and clear discussion of various design aspects of my CDU simulation.

 

I am not looking for theoretical answers or guesses or speculative solutions.

 

 

[PingPong]

What exactly do you need to know?

[NAP]

I have fixed my flash zone temperature based on two things.

 

1. The condenser pressure fixed by naphtha dew point (on air coolers) and then adding the pressure drop across all the stages gives me pressure in flash zone. At the flash zone pressure (slightly above atm.), I looked at the gas oil bubble point as GO is the heaviest side draw from the column.

 

2. I have the product specs such as Kero D86 90% temp and GO D86 90% temp to which I need to design CDU. So I looked at the TBP/ASTM D86 curve of the crude I charecterized and considered the D86 90% gas oil temp to be near the flash zone temp

 

Anything else I need to think in deciding flash zone temp?

[gegio1960]

NAP,

it's a very long way.... good luck!

The first thing I could suggest is to increase the FZ temp to provide 3-5% overflash.

The second... is to find someone with specific experience in your company ;-)

Regards

[NAP]

I have fixed the overflash at 5%...

 

Its ironic, I have experience in lot of other complex licensed units but when I am doing a CDU..I am sweating like a pig..

 

If its a long way..then its the one I am most definitely willing to take...and so should every process engineer. Without a CDU design..no one really is a process engineer...

 

All the so called "experts" in my company..they are text-book experts (in this case "company design manual experts"). They dont really want to get their hands wet..lol

[gegio1960]

I like and share your approach.

I perfectly understand and know the reference to the the text-book experts that don't want to get their hands wet.

I know hundreds of process engineers... no more than ten can design a CDU... seven of them have been my pupils ;-)))

I'm available to help you on specific points but I've to only rely on my old brain... neither simulator nor company design manual available,

regards,

gegio

[NAP]

Thats just perfect...an old seasoned brain is what I am looking for.

 

Simulators are just garbage in garbage out....i dont want my brain to be like a simulator...

 

My first question...if I have product specs that I need to design my CDU...what is the significance of TBP cut points?

 

Arent the TBP cut points just supposed to give a preliminary indication as to how crude is going to be split in CDU tower?

[gegio1960]

mmm.... simulators are precious provided that you don't put garbage in ;-)

nowadays, you can't develop a CDU w/o a (good) simulation.

to answer your question: you're right, the TBP cut points give only a guess of the quantities of the various cuts you can obtain.

[NAP]

The CDU processing capacity is 200000 bpd. My target kero rate is 20000 bpd and GO rate is 60000 bpd.

 

I believe this BPD targets are at standard conditions, right?

 

My first run of the simulation is based on specification of cut point distribution in the column.

Naphtha - 165 C

Kero - 220 C

Gas Oil - 360 C

 

The cut point specs are based on target product specs which are

Kero ASTM D86 90% T of 230 C

GO ASTM D86 90% T of 370 C

 

My simulation is converging. However, I am not able to get the total quantity of the Kero and GO cuts from the column when I am trying to meet quality.

 

What do I change in the simulation to slip more naphtha in kero and more AR in Gas oil?

[gegio1960]

The TBP cut points are sound.

The product quantities depends by the crude quality/TBP and the final specifications of the products.

Briefly speaking:

- you can put more naphtha in the kero up to the Flash point limit (after the stripping, if any)

- you can put more residue in the GO up to the Cloud point limit (for instance)

 

Contributions from other members are more than welcome, of course ;-)))

[NAP]

My simulation right now is very preliminary.

 

A crude oil charectrisation, a basic refluxed absorber tower as CDU, no side strippers and no pump arounds.

 

Side strippers are used only to do final quality corrections in the side draws and pump arounds are used to do effective energy management which I can do later once I get my CDU skeleton firm.

 

Since my converged simulation is based on specs of column cut points, how do I check in the simulation that my overflash is maintained at 5%?

[PingPong]

All the so called "experts" in my company..they are text-book experts (in this case "company design manual experts"). They dont really want to get their hands wet.

But surely your company must have designed a CDU before, otherwise the client would not have awarded the project to you? Normally clients demand a company experience list, not only of the past projects but also of the key personal that will be doing the work they are paying for.

 

Designing a CDU column is not really different from any other fractionator with at least one sidestripper and at least one pumparound. In fact: distillation principles are the same in all distillation columns, with or without strippers or PA's.

Sidestrippers only serve to remove lights from the sidedraws so as to meet flash point specs.

Pumparounds only serve to provide high level heat for feed preheat, or reboiling other distillation columns. You must realise though that they also affect the V/L ratios in the column and consequently affect fractionation (separation). A pumparound reduces the V/L ratio above its locating and consequently it requires more theoretical stages to achive the same separation. You should realise that, if you first make a design without pumparounds, adding pumparounds requires that you also add theoretical stages.

 

 

 

Since my converged simulation is based on specs of column cut points, how do I check in the simulation that my overflash is maintained at 5%?

Problem with Overflash is that there are two definitions floating around:

Originally Overflash is defined as the amount of extra vapor in the column feed, on top of the distillatie products.

However: most people nowadays call the liquid from the last tray immediately above the flashzone the Overflash. That liquid however also contains the Stripout from the bottoms section (where the residue is steam stripped).

I suspect that you and gegio use the latter definition as 5 % would be rather high if using first definition.

 

What matters is the amount of liquid from the last wash tray immediately above the flash zone, in relation to the hydrocarbon vapors from the flash zone. This dirty wash oil quantity should be about 10 wt% of the total hydrocarbon vapors (so without steam quantity) entering the wash section trays

 

 

But let's first start at the very beginning:

1) Which simulator are you using?

2) What crude are we talking about? Do you have a good crude assay?

3) Did you cut the whole crude into narrow boiling pseudofractions, and use that as input to the simulation? Or what do you use as input to the simulation?

4) What is a list of ALL the required product specifications? Surely there are more than just those two 90 % ASTM specs?

5) Is this a new CDU column in an existing refinery, or is this for a completely new refinery?

If it is in an existing refinery then I would expect that the client has requirements for the TBP cutpoints so as to suit existing downstream process units.

5) How many products streams are you producing in the CDU? For Example: Light Unstabilised Naphtha, Heavy Naphtha, Light Kerosene, Heavy Kerosene, Light Gasoil, Heavy Gasoil, Atmospheric Residue? Or what do you produce?

6) What are the (approximate) TBP cutpoints between all those fractions? In other words: what exactly does the client want to produce?

7) What pressure and temperature do you use in the reflux drum?

8) What pressure do you use in the flash zone?

 

EDIT: note that text about overflash is modified.

Edited by PingPong, 16 September 2013 - 10:51 AM.

[NAP]

Overflash def. - Fraction of excess crude vapourized..in other words, the total amount of vapors generated in excess to that required for the fractionation.

 

 

1) Which simulator are you using? HYSYS

2) What crude are we talking about? Do you have a good crude assay? Light Arabian. Yes, I have an assay.

3) Did you cut the whole crude into narrow boiling pseudofractions, and use that as input to the simulation? Or what do you use as input to the simulation? Using the TBP vs Vol%, light end fractions and bulk properties, I charecterized the crude and installed it in the simulation.

4) What is a list of ALL the required product specifications? Surely there are more than just those two 90 % ASTM specs?

Naphtha - ASTM D86 90% T 155 C max (spec for the aromatics complex)

Naphtha 95% - Kero 5% gap = 5 c

Kero - ASTM D86 90% T 230 C max

           Flash point 38 C min

Atmospheric Gas Oil - ASTM D86 90% T 370 C max

           Flash point 55 C min

 

5) Is this a new CDU column in an existing refinery, or is this for a completely new refinery? Im workign on CDU in a feasibility study for a completely new refinery.

If it is in an existing refinery then I would expect that the client has requirements for the TBP cutpoints so as to suit existing downstream process units. Client has given product specs as mentioned above.

5) How many products streams are you producing in the CDU? For Example: Light Unstabilised Naphtha, Heavy Naphtha, Light Kerosene, Heavy Kerosene, Light Gasoil, Heavy Gasoil, Atmospheric Residue? Or what do you produce?

LPG going to LPG Merox, Naphtha going to Naphtha & Aromatics Complex, Kero to KHDS, GO to GOHDS and AR to RHDS or VDU (not decided)

6) What are the (approximate) TBP cutpoints between all those fractions? In other words: what exactly does the client want to produce? No cut point requirements from client but only product specs. We are deciding the cut points to meet the specs as mentioned above

7) What pressure and temperature do you use in the reflux drum? Based on naphtha dew point temp., condenser pressure 0.7 barg and temp 58 C (air coolers).

8) What pressure do you use in the flash zone? Flash zone pressure = condenser @ 0.7 barg + 0.5 bar delta P across the column = 1.2 barg

Edited by NAP, 16 September 2013 - 07:28 PM.

[PingPong]

That definition of Overflash is very vague and does not make clear wether it includes Stripout or not.

 

I suggest we call the Dirty Washoil the Overflash as most people nowadays do. In that case the Overflash should be about 10 wt% on hydrocarbon vapors from flash zone entering the wash section, as I already stated in my previous message.

 

I have no experience with Hysys, I use Pro/II (Provision), so I cannot help you with simulator related questions.

 

Im workign on CDU in a feasibility study for a completely new refinery.

That was my point really. The CDU should do what the refinery wants, not the other way around. Normally first a refinery planning study is done, usually using Linear Programming (LP), to determine all (approximate) TBP cut points, stream quantities and their properties/specifications, process unit capacities, product blending, et cetera. I have done that several times in the past. Only after that refinery study is finalised and approved by client one can fix the Basis Of Design (BOD) for the CDU so it will properly serve all downstream facilities. TBP cutpoints and CDU product quantities are usually (roughly) set in that BOD.

 

This study seems to be rather vague, and as it stands now I get the impression that the CDU will only make a few products: unstabilised full range naphtha (FRN) in the reflux drum, only one Kero sidedraw, only one gasoil (GO) sidedraw and an atmospheric residue (AR). That makes it a rather simple CDU design. You better make sure that this is really what the client wants.

[NAP]

My current simulation is converging when I have specified 3 cut points as my column specs in HYSYS and naphtha rate as the 4th spec. But in this face, I fail to get the requared BPSD out of the draw.

 

Kero draw (on the 17th tray in a 45 tray column) is supposed to be 20000 bpd for processing in KHDS but I am getting only 13000 bpsd.

 

Gas Oil draw (on the 26th tray in a 45 tray column) is supposed to 60000 bpsd for processing in GOHDS but I am getting only 39000 bpsd.

 

If I run the simulation with alternative spec such as quantity of Kero to match 20000 bpsd, the ASTM specs go out of the window and I still cant meet GO draw rate.

 

I have fixed the feed at 39th tray with  6 trays for AR stripping zone in the column.

[NAP]

That definition of Overflash is very vague and does not make clear wether it includes Stripout or not.

 

I suggest we call the Dirty Washoil the Overflash as most people nowadays do. In that case the Overflash should be about 10 wt% on hydrocarbon vapors from flash zone entering the wash section, as I already stated in my previous message.

 

I have no experience with Hysys, I use Pro/II (Provision), so I cannot help you with simulator related questions.

 

 

 

Im workign on CDU in a feasibility study for a completely new refinery.

That was my point really. The CDU should do what the refinery wants, not the other way around. Normally first a refinery planning study is done, usually using Linear Programming (LP), to determine all (approximate) TBP cut points, stream quantities and their properties/specifications, process unit capacities, product blending, et cetera. I have done that several times in the past. Only after that refinery study is finalised and approved by client one can fix the Basis Of Design (BOD) for the CDU so it will properly serve all downstream facilities. TBP cutpoints and CDU product quantities are usually (roughly) set in that BOD.

 

This study seems to be rather vague, and as it stands now I get the impression that the CDU will only make a few products: unstabilised full range naphtha (FRN) in the reflux drum, only one Kero sidedraw, only one gasoil (GO) sidedraw and an atmospheric residue (AR). That makes it a rather simple CDU design. You better make sure that this is really what the client wants.

 

Yes, the client has looked at the TBP cut points and accepted the cut points. Draw rates are suggested by us for further processing in KHDS and GOHDS which are also accepted by the client.

 

The base case of the refinery is petrochemical production so naphtha quantity is to be maximized and AR to be minimized. Naphtha will be stabilized further, split and then fed to isomerization and reforming units.

Edited by NAP, 17 September 2013 - 03:28 AM.

[PingPong]

No two Arab Light crude assays are the same, and I have no insight in yours, but I would expect following product yields, based on the cut points you mentioned in message #9:

 

Naphtha and lighter (165 minus): 22 vol%

Kero (165 - 220): 10 vol%

Gasoil (220 - 360): 26 vol%

AR (360 plus): 42 vol%

 

With 200,000 BPSD crude those cutpoints should yield approximately 20,000 BPSD Kero and 52,000 BPSD Gasoil.

 

I am not familiar with specifying cut points in Hysys, but obviously something went wrong, as the simulation only gave 13,000 and 39,000 BPSD. Most likely something is wrong with the crude feed in the simulation. You either made an input typing error, or the way Hysys handles TBP curve input is lousy.

[NAP]

One of the problems is free water.

 

Based on crude assay kero is 10% and Gas oil is 25% as you correctly mentioned above. However, the assay is based on dry crude.

 

We have to account for the free water which is roughly 0.5% volume (assumed in absence of any data as yet).

 

Once I take the crude cut yields (based on dry assay) and add another stream of water to make feed stream, the % yield goes down a little bit

 

I re-ran the simulation to draw kero @ 20000 bpsd but for that I had to increase the cut point to 230 C and the product spec gets screwed up, lol.

 

Anyway...how does one decide which tray to draw of the side products from?

[PingPong]

The problem is not caused by a little water.

 

Anybody can calculate what the Kero or GO yield is going to be for certain given cutpoints: it simply follows from the assay.

 

If the simulator result is different from a simple hand calculation, then something went wrong regarding the crude feed in the simulation. Either you made an input error, or Hysys is inaccurate in handling TBP curve input.

 

I used to cut the crude into pseudofractions myself, and enter all those, plus the light ends, as a feed into the simulation. Of course it is much easier for a user to enter as little as possible and have the illusion that Hysys knows the rest, but in reality one has no idea what Hysys really does. Making pseudofraction by myself is more work, but then I know for sure that the feed breakdown is correct. Moreover I can correct for variation in UOP-K for the different fractions, which has impact on enthalpy values and vapor pressures (equilibrium values), so it is more accurate. 

[NAP]

The problem is not caused by a little water.

 

Anybody can calculate what the Kero or GO yield is going to be for certain given cutpoints: it simply follows from the assay.

 

If the simulator result is different from a simple hand calculation, then something went wrong regarding the crude feed in the simulation. Either you made an input error, or Hysys is inaccurate in handling TBP curve input.

 

I used to cut the crude into pseudofractions myself, and enter all those, plus the light ends, as a feed into the simulation. Of course it is much easier for a user to enter as little as possible and have the illusion that Hysys knows the rest, but in reality one has no idea what Hysys really does. Making pseudofraction by myself is more work, but then I know for sure that the feed breakdown is correct. Moreover I can correct for variation in UOP-K for the different fractions, which has impact on enthalpy values and vapor pressures (equilibrium values), so it is more accurate. 

 What do you mean by cutting the crude in pseudofractions yourself?

 

The crude assay gives a list of about 30 fractions at various boiling point in increasing order of boiling point from IBP to FBP. For each fraction, the assay provides information such as API gravity, UOPK, viscosity etc.

 

Do you mean that you take each of these fractions, create a pseudocomponent by using individual fraction's properties and then blend these pseudocomponents according to the vol% yield (of fraction of total crude) given in the assay data to create a crude stream to be used in simulation?

 

I understand that by simply providing a TBP vs Vol% curve of the crude and light end fractions, we ask the simulator to approximate the intermediate fractions based on equations which may not necessarily match with the fractions given in assay which has come out from a testing lab experiment.

[Padmakar Katre]

Hello,

Looking your reply in red color, I have doubt on couple of parameters,

1. Gas Oil Flash point - 55 deg C looks very low (usual it is ~ > 70 deg C) based on Kero product T90 D86 recovery.

2. Flash zone pressure is calculated based on column delta P of 0.5 kg/cm2, absolutely fine no issues, but you have to add column ovhd circuit (line + air cooler) delta P also hence your flash zone pressure will be somewhat higher than 1.2 kg/cm2g. This parameter is critical in entire simulation as it decides heater COT, % Over Flash etc.

 

In past I have worked on revamps of Crude Units (CDU-VDU) and used AL assay in blends with other crude. As far as I understand AL is one of the completely defined assays, it has gravity, molecular weight data for the entire boiling range and i.e. really helpful in crude characterization exercise. I have few suggestions based on my experience,

1. I have used Pro-II, Hysys and Aspen Plus and found Pro-II results in close match with test run data. Here if you are using RefSys and have crude manager license it will further help in better crude characterization.

2. Before switching to simulator have preliminary estimation of approx yields of each cut based on TBP (actual yields will be different with given quality specs). This will give an indication of required % vaporization at flash zone conditions. (Total distillate yield + 3-5 % overflash).

3. Once you have characterized crude in Hysys Oil Manager and exported the stream in Simulation environment, before flow-sheeting have sketch hand drawn ready with preliminary P,T conditions written.

4. I want to know here the CDU configuration, with Preflash drum or a pre-fractionator? P, T conditions of preflash drum will help in optimization of hot-preheat and heater duties.

5. Always have preliminary estimate of heater transfer line delta P based on which you can decide heat outlet temperature (COT) to the extent the Crude Flash Zone Temperature does not exceed 371 deg C.

6. In Hysys use a reflux absorber template for a fractionator column, and keep on adding pump arounds and side strippers one by one. I have faced convergence issues with Hysys, if you make the complete main fractionator and then run, it will make you cry.

7. As a starting, give specifications of product rates based on preliminary estimates in step-2 above .

8. Once converged, optimize for product quality. Some rules are as below for optimizations,

 - Estimate the D86 T10% points for all cuts as Flash Points (Chevron Close Cup method) say for Kero product D86 T10 point to be XX deg C to achieve Flash point of > 38 deg C.

 -Based on deviations from the D86 T10 temperatures adjust Side Stripper duties (stripping steam or reboiler duties) front end distillation is sensitive to the changes made in side stripper duties.

- You can alter the yields to match the end point distillations such as D86 T90 points

- Based on the water dew point + margin to be the Top pump around return temperature (tray 1 temperature)

- Get the preliminary estimates for stripping steams (usually lb/hr of stripping steam as a function of product yields)

 

I presume you are referring standard configurations for main fractionator column i.e. number of trays in each section (mass transfer, heat transfer). Refinery distillation handbook by Watkins will give you an idea to start with i.e. as a minimum number of  actual trays required in each section. Please note that these are actual trays, you have to convert those in theoretical trays. The recommendations by Watkins are based on those CDU fractionator columns in early 50-60s and now a days usually I have seen more trays than min recommended by Watkins (here you can get the reference from your company's past projects).

 

I hope this helps you. Please don't hesitate to ask any kind of help you need or still any confusion persists.

 

P.S.-There are lot of discussions available, just search in forums and I am sure you will get lot of useful search results.

[NAP]

Padmakar,

 

1. Gas Oil Flash point - 55 deg C looks very low (usual it is ~ > 70 deg C) based on Kero product T90 D86 recovery. Its a minimum value of spec. My actual flash point is 70 C

2. Flash zone pressure is calculated based on column delta P of 0.5 kg/cm2, absolutely fine no issues, but you have to add column ovhd circuit (line + air cooler) delta P also hence your flash zone pressure will be somewhat higher than 1.2 kg/cm2g. This parameter is critical in entire simulation as it decides heater COT, % Over Flash etc. I will take a look at this again.

 

In past I have worked on revamps of Crude Units (CDU-VDU) and used AL assay in blends with other crude. As far as I understand AL is one of the completely defined assays, it has gravity, molecular weight data for the entire boiling range and i.e. really helpful in crude characterization exercise. Unfortunately, the assay available with me does not have gravity and mol. wt. curves. I am charecterizing the crude based on TBP, light end % and API gravity for each point on the TBP curve.

 

I have few suggestions based on my experience,

1. I have used Pro-II, Hysys and Aspen Plus and found Pro-II results in close match with test run data. Here if you are using RefSys and have crude manager license it will further help in better crude characterization. I am using HYSYS standard package.

2. Before switching to simulator have preliminary estimation of approx yields of each cut based on TBP (actual yields will be different with given quality specs). This will give an indication of required % vaporization at flash zone conditions. (Total distillate yield + 3-5 % overflash). Upto GO yield, % vapourization required is 52% and my crude is flashed to 56% at heater outlet which is currently at 4% overflash and I need to increase this to 5%.

3. Once you have characterized crude in Hysys Oil Manager and exported the stream in Simulation environment, before flow-sheeting have sketch hand drawn ready with preliminary P,T conditions written. Done

4. I want to know here the CDU configuration, with Preflash drum or a pre-fractionator? P, T conditions of preflash drum will help in optimization of hot-preheat and heater duties. Preflash column with roughly 5% crude flashed in the overhead as vapour.

5. Always have preliminary estimate of heater transfer line delta P based on which you can decide heat outlet temperature (COT) to the extent the Crude Flash Zone Temperature does not exceed 371 deg C. Currently crude flash zone temp is 358 C. It will increase somewhat as I will increase condenser pressure to account for aircooler and line delta P and increase the overflash from 4% to 5%

6. In Hysys use a reflux absorber template for a fractionator column, and keep on adding pump arounds and side strippers one by one. I have faced convergence issues with Hysys, if you make the complete main fractionator and then run, it will make you cry. I am currently at a stage of refluxed absorber without any pump around or side stripper.

7. As a starting, give specifications of product rates based on preliminary estimates in step-2 above. I am doing with a product cut point spec. but I can try this approach as well!

8. Once converged, optimize for product quality. Some rules are as below for optimizations,

 - Estimate the D86 T10% points for all cuts as Flash Points (Chevron Close Cup method) say for Kero product D86 T10 point to be XX deg C to achieve Flash point of > 38 deg C.

 -Based on deviations from the D86 T10 temperatures adjust Side Stripper duties (stripping steam or reboiler duties) front end distillation is sensitive to the changes made in side stripper duties.

- You can alter the yields to match the end point distillations such as D86 T90 points

- Based on the water dew point + margin to be the Top pump around return temperature (tray 1 temperature)

- Get the preliminary estimates for stripping steams (usually lb/hr of stripping steam as a function of product yields)

 

I presume you are referring standard configurations for main fractionator column i.e. number of trays in each section (mass transfer, heat transfer). Refinery distillation handbook by Watkins will give you an idea to start with i.e. as a minimum number of  actual trays required in each section. Please note that these are actual trays, you have to convert those in theoretical trays. The recommendations by Watkins are based on those CDU fractionator columns in early 50-60s and now a days usually I have seen more trays than min recommended by Watkins (here you can get the reference from your company's past projects). I started with actual trays of 45 based on guidelines in design manual in my company. 39 trays above flash zone and 6 trays below in the stripping zone.

 

My draw off trays are kind of selected randomly with an objective of converging the simulation. Is there is a logical choice of selecting number of draw of tray for each product?

[NAP]

Padmakar,

 

I am keeping the overflash at 5%.

 

In HYSYS, the heater outlet stream is vapourized @ 57% where as the crude vapourization required for distillate seperation as per TBP curve is ~52%.

 

Is this an accurate way of ensuring 5% overflash in column?

[PingPong]

It seems to me that you are using the first definition of Overflash (see my message #12). In that definition 5 % Overflash is extremely high.

 

As I have advised already twice before: Dirty Washoil flowrate should be about 10 wt% on hydrocarbon vapors from flash zone.

 

Moreover I is not clear to me how you get to the 52 % (is that liquidvolume% or weight% ? ) crude vapourization required for distillate separation as per TBP curve.

[Padmakar Katre]

Hi,

Consider the addition of net vaporization in stripping section too. Over-flash is ratio of volumetric flow of last tray in fractionation section (very first tray above flash zone) to the feed volumetric flow (both at STP conditions).

 

e.g. feed is 100 m3/h to column, distillate (RCO minus material) yield is 50m3/h, net vaporization in stripping section is 10 m3/h so net vapors to be = 50 + 3-4 m3/h over-flash = 53 m3/h total vapors leaving flash zone.

 

Generate the flash at the heater outlet stream for heater outlet Temp and Flash Zone Pressure. Adjust the Net flashed vapors to be ~ 53-10 = 43 m3/h by manipulating heater outlet Temp (here I presume Flash Zone Pressure is fixed).

 

Please note that above numbers are indicative only, you have do iterations to get the required overfalsh.

 

If you are comfortable with the steps below,

- say Tray #36 is last tray (above flash zone)

- Use a spreadsheet and get the estimated # 36 liquid flow inside spreadsheet thru import option

- Import the feed flow in SM3/h

- Get the ratio of these two variables i.e. actual overflash

- Use an "Adjust" operation. For this manipulated/adjusted variable heater outlet temperature, target variable is the spreadsheet cell where you have calculated over-flash and you will get the required overflash

 

This exercise should be the final step of optimization.