4 replies to this topic

[Molefe]

I'm final chemical Engineering student, in our design, I'm designing the ammonia separation section of the plant. will cool and condense the ammonia through a well designed train of heat ex-changers. I've considered cooling the reactor product gases by heating the recycle stream. I was able to get the product stream from 433 ^oC to 149 ^oC. My main concern: is it feasible to use cooling water to take this stream from 149 ^oC to 

55 ^oC and begin to use a refrigerant-NH3 in two stages to chill the mixture to the operating temperature of the knock out drum which is operating at -31.5 ^oC. 

 

My aim is to recover as much ammonia as possible, I must produce 300 000 ton/yr of anhydrous ammonia with these specs: maximum water content (0.5%) and 2% maximum uncondanseble gases (H2,N2,Ar, etc). I will attch my preliminary PFD

Attached Files

•  PFD-Ammonia Separation.docx   146.17KB   47 downloads

[Art Montemayor]

Molefe:

 

You show good effort and a good Visio drawing.

 

However, your effort is exactly why I am against students using Visio drawings to communicate.  Visio is essentially useless when seeking comments, revisions, improvements, and making notations.  That is why I recommend all students to use Excel.  Many engineering students on our Forum continue to disagree; that is their priviledge.  I speak as a Senior Project Manager and that is what I demand of those engineers working under me - as do countless other senior engineers in industry who recognize the importance of using the proper tools for the specific job at hand.  You must employ good communicating tools, and that means tools that are available to all your readers in order to deliver your message efficiently and accurately.

 

You, as an engineering student HAVE to be fully competent in working with Excel - because it gives you the three (3) basic tools all engineers require to do their work and communicate:

1. generate engineering calculations and present tabular data;
2. generate graphical charts and curves as well as regressions;
3. generate drawings and sketches for communicating ideas and concepts.

Visio can do only one of the above functions - and even then, it doesn't allow for interfacing with others.  Excel does ALL THREE - and allows interfacing.

 

Please study the attached workbook and make you own comments and remarks on the related worksheet.  Include or attach any additional sketches or work on that sheet or another sheet and title the workbook with a revision number.  THAT is the way professional engineers work and if you are entering the engineering world soon, that is what will be expected of your work quality and product.

 

I offer this advice with 53 years of experience in the engineering ranks and hope that it helps you out.

 

 Molefes Ammonia Haber Process PFD Rev0.xlsx   145.53KB   53 downloads

[Molefe]

Thank you sir for the advice,

 

I have downloaded the excel sheet and seen the comments and I will make necessary changes for my final PFD. does my heat ex changer train look sensible? I was trying to attach my stream table but had some difficulties because I've linked my excel spreadsheet with Aspen simulator and its is currently down. My concern: Is it feasible after performing heat integration by heating the recycle, to use cooling water  to cool this stream from 149 ^oC to 

55 ^oC and begin to use a refrigerant-NH3 in two stages to chill the mixture to the operating temperature of the knock out drum which is operating at -31.5 ^oC or I can do it in one go by taking it straight from 55 ^oC to -31.5 ^oC (using more refrigerant, huge heat ex-changer=expensive)?

 

I know it would make it easier if I had my stream table with temperatures and pressures, but i cannot post it at the moment.

 

Thank you for your help.

[Art Montemayor]

Molefe:

 

Your PFD looks sensible.  However, it isn’t practical in my opinion.  If you have a stream at 149 ^oC you want to avoid using cooling water to cool down such a highly hot stream.  What is the normal thing to consider is to employ this excessive heat in a productive way rather than create a utility stream that will require wasting the excessive heat.  For example, you could use this heat to pre-heat some streams upstream or downstream.  You might also consider generating low pressure steam with this heat.  This low pressure steam could be of use in the process or elsewhere.

 

The practical point to consider here is that introducing cooling water into such a high temperature stream might leave a lot of fouling due to the dissolved solids in the water.  And it will take a lot of water and a large exchanger to cool down this gas mixture.  You want to avoid boiling out the cooling water within the first sections of the exchanger, so you would have to raise the pressure of the cooling water.  Cooling with an intermediate fluid – such as an oil heating medium would be better, but as usual will involve more capital expense (a usual engineering trade off in these cases).  Once the hot stream gets to approximately 125 ^oC, you can cool it down with cooling water.  From 40 ^oC you can refrigerate the gas stream with ammonia.

 

I still don’t know exactly where you are within the Haber process because your PFD lacks definition and stream values.

[Molefe]

Thank you Sir,

 

I have attempted to avoid adding utility streams as much as possible, however at this point I can not do heat recovery with other areas of the plant , I've to do my own heat recovery/integration, I even considered generating low pressure steam and estimated the amount that I could obtain from selling it, because it's stated in the design brief that it could be sold to an adjacent plant (needed at  15 bar-abs) but I was trying to use cooling water cause its readily available at (25 ^oC,10 bar), even though I would have to use about 540 tonnes/hr Cooling water to take the hot (149^oC) stream to 75^oC, the required area is 950 m² ​(according to the heuristics A_max=1000 m² ).

 

This is my Process description of the plant (Summary): 

 

Synthesis gas is delivered to the battery limits and enters the shift converter systems (high and low temperature) at 33bar and 270oC.Carbon monoxide is catalytically converted to CO2 and H2O, the water formed is subsequently separated from the synthesis gas. This process stream is forwarded to the CO2 removal unit, where the carbon dioxide is absorbed using solvent with a subsequent regeneration and recovery producing almost pure CO2 stream.

 

The synthesis gas after CO2 removal enters the methanation unit where residual CO and CO2 are catalytically converted to methane and water with a subsequent water removal. The purified synthesis gas is combined with unreacted gases from the product recovery and fed to the converter in the proportion 2.9:1 of H2:N2 and the product mixture sent to product purification. This last stage chills the temperature of the mixture down under refrigeration, ammonia condenses down as liquid and collected. The unreacted gases are sent back to synthesis loop after being purged to reduce accumulation of impurities.