5 replies to this topic

[rmel711]

Good day. This is my first time to post, and luckily I have found this community. Im a chemical engineer and works as a process engineer in a petrochemical plant here in the Philippines. As of now, we have 2 VIE's in the plant's OSBL, and nitrogen is being supplied thru truck deliveries. I am making a feasibility study regarding Nitrogen Generation Plant. This is my first job and I have a lot of queries that I am having trouble finding the answers.

I have established data for the nitrogen plant based on our actual consumption.

It is proposed to have nitrogen generation plant with Cryogenic Air Separation process at 1000 Nm3 per hour production rate at 99.999% purity.

I have read about PSA's and membrane separation, but I found out that at 1000 Nm3/hr capacity, cryogenic air separation can give the desired purity.

1) i have computed for the work of the compressor with the given discharge is 1000Nm3/hr, Cp/Cv is 1.4 (for standard air), 810.6 kPa... while the suction is at 30 deg C, and 101.325 kPa.

the computed discharge temp is 275.87 deg C
suction is 4416.36 m3/hr
Work is 353.03 kW

i assumed that the compressor has an 80% efficiency, so 353kW / 0.8 will give me 441 kW

is it safe to assume that the design capacity of my compressor is 500kW or just 450kW? what kind of compressor should it be?
also i have read from heuristics that the discharge temp should be 121 to 149 deg C only, i got 275.87 deg C using (T2/T1) = (P2/P1)^(n-1/n)


2) I have no idea how to size up my mole sieves column, cold box, rectification column.
for continuous users, GAN from the rectification column will be used... I will also include 2 15000gal VIE's to serve as storage for my LIN and be used in case of shutdown.


Im having problem with how to size these vessels, and I need it for the capital investment estimation on my feasib. I just want to know if im on the right track. Thanks.

[Art Montemayor]

Rich:

First let me congratulate you. I started my career in industrial gases, operating and managing air separation plants. So I share your experience and can relate to what you are going through. My comments are as follows:

• Do not resort to using acronyms without first identifying them. I don’t have the slightest idea of what you mean by “VIEs”. I guess OSBL means “outside battery limits”. Is that correct?
• 1,000 Nm3 per hour of gaseous nitrogen at 99.999% purity (volumetric) is a lot of very pure nitrogen and requires a special process. I presume you are already discussing this tentative proposal with major suppliers such as L’Aire Liquide, Air Products, Linde, etc. this is what you should be doing because they will furnish you with all the required information you require for your feasibility study. They can – and will – provide you with their estimates of power and air consumptions as well as the type of process(es) used and the space, consumptions, and weights involved.
• You don’t identify how you calculated your compressor power consumption and why you picked 810 kPa as the discharge. You should leave that to the experts that I mention above. The same advice applies to the use of PSA units, storage requirements, etc.
• You cannot be expected to generate an accurate (or credible) feasibility study for this size of investment if you are only a recent young grad. That does not make good sense. This has nothing to do with your lack of experience, but rather goes against what we experienced engineers know and do as a matter of common, everyday decisions.

I hope this experience is of some help to you.

[ankur2061]

rmel711,

As Art Montemayor has suggested the design of a Cryogenic Air Separation Unit should be left to a technology licemsor such as Praxair, Air 'Liquide or Linde.

However for familiarizing yourself with the process of Air Separation the following two links might prove useful:

http://en.wikipedia..../Air_separation

http://upetd.up.ac.z.../02chapter2.pdf

Hope this helps.

Regards,
Ankur

[rmel711]

to Sir Art,

sir, good morning! thank you for your comments. all of these are noted.
you are right with the OSBL. VIE refers to vacuum insulated evaporators, the vessel where LIN (Liquid nitrogen) is being stored.

I computed for the work of the compressor using

W= ((n*P1*V1)/(n-1)) * [((P2/P1)^((n-1)/n))-1]
I have read from a literature that the working pressure in the distillation column is 8 bars.

I was actually thinking that I can make the feasibility study just by researching alone. Thank you sir for your guidance.


SIr Ankur,

I have read the documents. I have now a clear idea regarding the distillation process of cryogenic separation.


THANK YOU FOR YOUR RESPONSE.

regards,
Richmond Rommel Mercado

[kkala]

1. N2 gas of such a high purity is supposed for special purposes, e.g. polymerization reactor purging or other uses per wikipedia http://en.wikipedia...._nitrogen_plant '> http://en.wikipedia...._nitrogen_plant . Said link reports practically minimum size of cryogenic plants = 250 Nm3/h, and presents elementary flow sheet and data.
Moreover proper size of cryogenic plants is 5000 - 10000 Nm3/h of N2 (or higher), according to Grasys, http://www.grasys.co...genic/nitrogen/ '> http://www.grasys.co...genic/nitrogen/ .
A N2 plant of 1000 Nm3/h (purity 99.999%) may have rather high operating (and capital) cost. This will be investigated in the feasibility study, yet relevant suppliers could probably give a rough answer now.
2. In last decade local refineries have installed two PSA nitrogen units there. One is still under construction. Produced nitrogen gas (min N2=99.5%, maxO2=0.5%, dew point -40 oC) will be also used for purging hydrocarbon pipelines, to my information. Supply N2 gas pressure is 8-9 barg, design 11 barg.
3. Capacity determination of liquid nitrogen storage does not seem to be an easy task, since several realistic scenarios have to be taken into consideration. In the said local refinery case, this is based on the assumption that there is no N2 supply for three consecutive days, so the needs of the factory are covered by the liquid storage (normal operation, plus peaks that can "reasonably" happen in the three days). But this base has to be individualized. You may not dismantle the existing truck unloading facility of liquid N2, when the new N2 unit is installed.
Assuming that 1000 Nm3/h N2 covers also peaks (which may not), your daily demand is about 1000/22.4*28*24= 30000 kg of N2, corresponding to 30000/690=44 m3/d of liquid N2. Assumed storage of 2x15000 gal could be enough for about (80%*2*15000*3.785/1000)/44=2 days (supposed max filling 80%).
Existing evaporators are expected to have total capacity higher than 1000/22.4*28=1250 kg/h, to cover instantaneous peak N2 demands.
4. You have to ask budget quotations from Nitrogen generator suppliers, as previous posts suggest. This is usual in feasibility studies. You had better address to those that you will send requisitions later, in case that Nitrogen Plant is feasible; elaborate data as above and state all requirements clearly. Inform the responders about the conclusions of the feasibility study later.
5. Hopefully some of above points could be useful.

[rmel711]

hi.

 

good day.

 

I just want to share what happened.

 

All of my queries before I used them to understand the Nitrogen processes. Our plant is a PP plant here in the Philippines with a 160000 MT capacity.

 

I did evaluation in the suppliers, LINDE, AIR LIQUIDE, and INGASCO.

 

All of them offering cryogenic distillation processes.

 

All your support helped me perform the evaluation well and understand the processes well.

 

Thank you. If you have more concerns, I think I can help.