#  > Petroleum Industry Zone >  > Oil And Gas Process >  >  >  Common mistakes in process design/simulation

## mohamedellejmi

Guys,

through my carreer, I had to mentor some new process engineers and I noticed that most of them make almost the same mistakes... Here are some:

1- When designing scrubbers or separators, people take the Standard Flow for Actual flow and they calculate huge volumes
rule of thumb : Actual volume =~ Standard volume / pressure (barg)

2- When dealing with gas compressors (especially reciprocating), people have the tendence to believe that compressor discharge pressure is set by the compressor it self. Which is totally wrong!
Discharge pressure is the backpressure exerced by downstream equipment.
(In case the downstream equipment is a control or a globe valve, the discharge pressure changes with flowrate, which is controllable on the compressor it self)

3- When using HYSYS for designing distillation columns, people have the tendency to use short-cut column and to inject the resulting tray number, reflux ratio, feed position, reboiler temperature,... into the column model and run it...!!!
They spend a week trying to converge the column... Waste of time!!!
I recommend you to take only the number of trays out of the short-cut method + 2 to 5 trays and take the feed position.
Then, you should specify the light key content in the bottom product and the heavy key content in the overhead product, and if necessary, specify one of the product's flowrate based on the inlet composition.


This will converge immediately... then it's up to you to OPTIMIZE!

4- Some people rely on HYSYS to determine the H2S content of a stream... WRONG! HYSYS doesn't deal correctly with H2S solubility in water and in HC. Be Careful!


Feed us with your experience guys... Tell us about the common mistakes you know!See More: Common mistakes in process design/simulation

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## mkhattaby

Thanks alot , I personalyy experienced similar mistakes with my direct reporters .

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## subramanian.R

Its really a valuable information . Pls everyone share ur experience . This error almost all process will make ,its for sure.....

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## mohamedellejmi

Another mistake I came across yesterday...

When designing a process where gas Chillers/Coolers are used, and when calculating the energy consumption, people have the tendency to use the Chiller duty (say 1MW) as an energy consumption.

However:

- if the chiller is a propane refrigeration unit, the duty to be considered is the energy absorbed by the Propane compressor's driver + lube oil pumps (if electrical) + condenser fans.

- if the chiller is just an Air Cooler, the duty to be considered is only the energy absorbed by fans.

- if the chiller is just a sea-water/gas heat exhanger, the duty to be considered is only the energy absorbed by the sea-water pump.


Guys, we need your experience... come on!

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## uetian98

EOS Selection Criteria from myside.
Hope this helps.

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## talha_sangi

from your question i understand that u are talking about simple coolers/heatexchangers, in which we only enter process condition of one side.
if yes,
then the calculated duty given by heaters/cooler is only for process fluid whose condition is given in the heater by you. it doesnot tell you that how much flow requirement and how do you add or remove that heat from process fluid side.
this is only susggestion.

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## Ahmed El-said

In simulation
A mix of therrmodynamics (shifts) that isn't compatible.
Can give occurence to sudden temperature jumps which looks strange
Thermodynamics selection. E.g. when H2S is involved one needs SOUR (ProII) or similar to get closer to reality. Seen one used a straight SRK in which there was 10 wt% H2S in the feed. Thermodunamic methods are correlations (only) and are never perfect, but are the best methods available.
Water solubility in HC fractions is another area of errors. These are nearly always misleading. Solubility curves are always advisable to use (if they exist).
Running distillation columns below weeping points. The column will converge in the calculation but in real life, "it swings like...."
Not accounting for fouling properly and not retrofit area excess into fouling.
Not doing an ASB (AS Built) simulation to retrofit actual equipment to behaviour. Should be standard procedure but is seldom done.

In Design
Not doing a proper job on startup/Shutdown cases which can turn out to be governing cases for parts of the unit. 
Not retrofit the hydralic pressure drop profile causing control valves to operate at low openings.

A few of the things that comes to mind

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## aria

Dear all,
 This is a wonderful topic, it would be great if we could create a wiki like page to create a document for this topic i.e "Common mistake in process design/simulation". I am sure it will be one of those page with several thousands hits and will be helpful to students and practicing engineers.

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## aria

Dear all,
 This is a wonderful topic, it would be great if we could create a wiki like page to create a document for this topic i.e "Common mistake in process design/simulation". I am sure it will be one of those page with several thousands hits and will be helpful to students and practicing engineers.

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## omli

people must make the difference between design and optimization, at the begining we try to design the unit or a part of process system, own calculations and effort are needed, it is different from optimizing an existing case, here we must enter all equipements and installations properties and run in dynamic mode when modifying conditions , but when designing we start in steady state after objectif is fixed , we optimize, we must put in our mind that hysys and any other simulator is only a tool which can be used correctly only when good practice of engineering fundaentals and states of arts 
sorry my english is bad hope mssage will be received

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## omli

for exemple if i need to make a simulation in which there is a column, i must have a minimum informations about trays calculations, relation between column hight and the number of trays, tray spacing, fouling, relation between reflux and head product purty, heat exchangers.....

when we talk about co2 removal we must know that it is recomended at hight pressure and low temperature ((because it an exothermal reaction) and the regeneratio must be at high temp (reversibility of the rection) and low pressure ( henry law) without forgeting corrosion effect and......

conclusion : the simulation come latter after a good database is made

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## alexcv

Thanks

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## Qalander

Thanks Dear uetian98 for fairly good resource
Best regards


QalanderSee More: Common mistakes in process design/simulation

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## anwarahmad

There is also common mistakes in oil characterisation. Check out which approach is the most suitable to choose when characterising the crude in simulation. **[link Point to another website Only the registered members can access]
*link*




<![CDATA[[Only Registered And Activated Users Can See Links]

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## ommolina

Thanks a lot! Do someone use ansys cfx actually to simulate separation process in a gunbarrel?

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## jotar

A commun mistake in crude heater simulation is not to consider the small thermal -----ing of large molecules, producing gas and other species.

Jotar

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## jamy862004

very nice & useful data.. thank you!

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## mkhurram79

wonderful
Thanks----------

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## dattatraykolte

In my Simulation model whenever i i run the case it run smoothly but when i try to generate report from Report manager 
i just stuck & Hang

Please tell me the reason...why it is happening also is there any other method to generate report for the given streams & specified properties only

1- Also what do u think about Lee-kesler Method for density calculation for LNG 
is it right?

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## Chris VP

Dear all Im a new member here ang need to update myself with the simulation stuff.
Really appreciate the inputs given here.

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## mbc.engg

Nice topic for sharing experience

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## anwarahmad

Welcome! Enjoy this forum and just shoot up your curious here to get the some promising answers.

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## saverr

Dear friends, 
Thanks for sharing. These tips are very useful.

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## zubair1950

In case of smulating thermosyphon reboiler be carefeul as hysys inside column flowsheet indicates very high temperature at the last column tray even so that the reboiler return temperature is lower than the inlet .This inlet temperature to reboiler is misleading .Solution is to take the stream out o fthe flow sheet and install reboiler there, on coming out it will start showing right temperatures

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## m2009

thanks

See More: Common mistakes in process design/simulation

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## haih5

This is a successful topic. It was total out of my knowleadge until i work with Hysys during a half year. Thank you very much my juniors.

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## radenbagus

nice sharing, just continue this topic. its helpfull.. thx alot

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## Sherif

When designing a process where gas Chillers/Coolers are used, and when calculating the energy consumption, people have the tendency to use the Chiller duty (say 1MW) as an energy consumption.

However:

*- if the chiller is a propane refrigeration unit, the duty to be considered is the energy absorbed by the Propane compressor's driver + lube oil pumps (if electrical) + condenser fans.*
Why would we consider the summation of all these duties and not the duty exerted inside the chiller itself(i.e, energy absorbed by propane from HC or the fluid flowing on the other side of the exchanger?

- if the chiller is just an Air Cooler, the duty to be considered is only the energy absorbed by fans.

- if the chiller is just a sea-water/gas heat exhanger, the duty to be considered is only the energy absorbed by the sea-water pump.


Guys, we need your experience... come on![/QUOTE]

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## khodaei

Thanks

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## suzy

Hi 

Anyone help me?


How can i convert unit from Nm3 to kg for natural gas? Q =2310 Nm3/h sp. gr = 0.6351

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## Spenta

Hi

Many thanks for the very useful common mistakes. Points well taken. Warm regards and keep up the good work.

Mano Spenta [spenta@ii.net]

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## charlesakay

How to use alphabets as components in Aspen Plus

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## ashrath25

Dear all 

you have shared ur good experience. i woud like to know the process design calculation for the indirect water bath heater. if any one of you have this please share..

thank you

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## ahmadfallahi

thanks alot

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## zinokabyl

really appreciated thankx

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## SLB

Hi friends
Hope everything goes well with you
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Cheers.

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## ftheba

Invaluable information. Much appreciated. Thanks

See More: Common mistakes in process design/simulation

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## sathish_che

hi

I had problem in the pipe phase software. when I simulating the natural gas at 715 psi and 66 deg C  in the transmission line, my velocity is gradually reducing , normally as per ideal law of gas the actual volume of gas should be increase due to reduction in pressure loss. when I doing the same model with natural gas at 300 psi and 66 deg c the velocity is gradually reducing in the line due to friction loss. I am not able understand this behavior in the software, ca any one explain the solution to overcome this situation.

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## sathish_che

hi

I had problem in the pipe phase software. when I simulating the natural gas at 715 psi and 66 deg C  in the transmission line, my velocity is gradually reducing , normally as per ideal law of gas the actual volume of gas should be increase due to reduction in pressure loss. when I doing the same model with natural gas at 300 psi and 66 deg c the velocity is gradually reducing in the line due to friction loss. I am not able understand this behavior in the software, ca any one explain the solution to overcome this situation.

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## jotar

Remember that any expansion of a gas normally entails a temperature reduction, at least heat transfer with the surroundings is fast enough to hold constant temperature.

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## gad480

Much appreciated. Thanks

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## wyb

The conversion from SCFM to ACFM can be expressed as

ACFM = SCFM [Pstd / (Pact - Psat Φ)](Tact / Tstd)         (1)

where

ACFM = Actual Cubic Feet per Minute

SCFM = Standard Cubic Feet per Minute

Pstd = Standard absolute air pressure (psia)

Pact = absolute pressure at the actual level (psia)

Psat = Saturation pressure at the actual temperature (psi)

Φ = Actual relative humidity

Tact = Actual ambient air temperature (oR)

Tstd = Standard temperature (oR)


Correct  ?

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## gtaa

Actual to standard volumes can be calculated using the Combined Gas Law:

P1.V1/z1.T1 = P2.V2/z2.T2

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## mobek

what does actual & standard conditions have to do with humidity?

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## abdullooh1234

In my exper., gas compt until c7+ is enough for eq. Sizing but for flow assurance & compre. Will  give another concern

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## metaltribe

Yes, Agree., 
special for Air as fluid since AIr has humidity factor that will affect the ACFM.





> The conversion from SCFM to ACFM can be expressed as
> 
> ACFM = SCFM [Pstd / (Pact - Psat Φ)](Tact / Tstd)         (1)
> 
> where
> 
> ACFM = Actual Cubic Feet per Minute
> 
> SCFM = Standard Cubic Feet per Minute
> ...

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## tarmanto

Thanks bro mohamedellejmi for these useful information..

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## mohammadkhalid

Thanks for the info! I would like to share one more common mistake in simulation i.e Over specifying the simulation. It is the hardest one to identify.. Do not make the full flow sheet at once and then start converging unit operation one by one until you are quite familiar with the whole process. Instead of that make flow sheet bit by bit and make it converge step by step and proceed.

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