7 replies to this topic

[lemon29]

Hi everyone,

I am a chemical engineering student and have to design a methanol-acetic acid plant from natural gas, which means first we have to produce methanol from natural gas and later on produce acetic acid using the produced methanol through methanol carbonylation. I have one question whether is it cheaper to buy CO for the methanol carbonylation or to recycle co from the syngas.

thank you in advance
your replies might be really helpful.

[DB Shah]

Dear lemon29,
Plants you are referring will be a big complex in itself. To give you a brief outline of your project-

Methanol
Requires H2/CO/CO2 for methanol production which you can get from SMR using NG.

Acetic Acid
Basic raw material is CO & Methanol.

Major issues if you produce in house CO-
Cost of production - either SMR base or Gasification.
Management of H2 (you will be in excess of H2, all the H2 will not be consumed by Methanol plant & may have to burn as fuel)

Hence I think you can consider buying CO from external source. You may consider CO rate at ~ 400 $/MT.

I am attaching file which will give you idea about various routes you can go for.

Some important factors & parameters useful for your mass balance-
NG/MT methanol ~ 950 SM3/MT (CH4)
H2/CO/CO2 at SMR outlet without CO2 recycle = ~75% / 12% / 10% (By mole)
H2/CO/CO2 at SMR outlet with CO2 recycle = ~65% / 21% / 7% (By mole)
H2/CO/CO2 at Gasifier outlet = ~50% / 42% / 6% (By mole)
Methanol per MT of Acetic Acid = 0.54 MT/MT
CO per MT of Acetic Acid = 0.545 MT/MT

Acetic Acid plant will have exotic material of construction like Zr & Hastalloy

Attached Files

•  Meth-AA.xlsx   25.09KB   161 downloads

[lemon29]

Dear DB Shah

Thanks a lot for your reply. It really helps me in getting a clearer view of the plant. I just want to clarify some things
1)my plant capacity is decided to produce 700000 metric ton methanol per year and 250000 metric ton acetic acid per year. Do u think it is feasible if i use a part of the syngas formed in the reformer and process it to separate out carbon dioxide, carbon monoxide and hydrogen and use that CO as feed stock for the acetic acid production. This means i will not have 2 SMR in in the plant.
2) for the methanol synthesis, there are 2 reactions that can take place
CO + 2H2 → CH3OH
CO2 + 3H2 → CH3OH + H2O
Which of this is more favoured?

Thanks

[DB Shah]

The reason for 2 SMR is that CO outlet of SMR is only 12% (without CO2 RC). Recovery of CO with inlet 12% will not be feasible.

If you RC CO2 & get only H2+CO rich gas with too little CO2. In methanol process you have a concept of R value = (H2-CO2/(CO+CO2). For optimum conversion R value = 2.0.
At R value = 2.0 & CO/CO2 = ~ 1 the crude has ~80% Methanol(mass) & rest water+impurities.

If syn gas is only H2+CO2 (no CO) then the crude you get is 65% (mass) of methanol and rest water + other impurities. This should not be a problem.

However if you go for CO2 RC & end up with CO rich gas without/very less CO2, catalyst sintering is an issue. Higher CO is not advisable.

In this case your route will be
Single SMR (CO2 RC) --> H2+CO rich gas

Part of H2+CO to CO unit to generate CO for AA

Install CO2 recovery from Flue gas of SMR and use CO2 for Methanol systhesis
Balance gas from SMR + Purge gas of CO unit + CO2 from flue gas -->Methanol.

As an option for CO2 generation, you may pass part of RG (H2+CO) in CO shift converter.

[lemon29]

thank you sir. Your explanation was very helpful.
Is it possible for you to give me your email address as i want to email to you the block diagram that i have done to see whether it is feasible or not. I can't seem to upload it here.
thanks

[DB Shah]

you can mail me at divyang_shah@yahoo.com

[DB Shah]

I checked ur block dia in word file. I will like you to make the block diagram in excel, start with the basis of 700000 MTY AA production & with the factors I mentioned above work out the CO/methanol/RG/CO2 flow and mail me excel file. You will get better insight of the options if you workout in excel sheet. (I can forward my working to you but that defeats the purpose of learning).
Work out like this- AA prodn as basis, CO & Methanol required, assume CO recovery in cold box or membrane unit (~85%-90%), calculate CO required at CO unit inlet and hence total gas at CO unit inlet. From total gas to CO unit inlet deduct the CO for AA and you have balance gas for Methanol production, check this flow and analysis of this gas.

[johnybupa]

hello sir, i am also a chemical engineer and for my design project, i have to design a plant to manufacture acetic acid
i have some questions if you don't mind me asking please
do you know the conversion of methanol ? i will be using the Cativa route in the manufacture of acetic acid, CO is produced using steam reforming of natural gas and the H2 is used in the same site for ammonia production.
do you know the composition of the off-gas from cativa route and the recycle stream ?

i would be grateful if you could help me
many thanks