3 replies to this topic

[Jopeth]

Hi there!

Name's Jopeth, a 5th year ChE student from the Philippines. I'm into plant design and I'm new to these unit operations: Ion exchange and Chromatography. On Ion exchange, my purpose of using this is to separate Na2SO4 from the sugar solution (xylose, arabinose, galactose, mannose, cellulose and xylan to be specific). I don't know how to create a mass balance around nit as it is unsteady state (efficiency is lowered at a time and accumulation follows, same as Chromatography to concenterate xylose from the solution. I'm planning to use dowex ion exchange resins. I've found a software but it's used only in water demineralization purposes. Our plant design is production of Xylitol from corn stover.

Attached herewith is the process flow diagram of our plant.

Any help will be greatly appreciated. thanks!

p.s. Ion exchange for the removal of Na2SO4 is labeled as salt removal.  242198_1886341331333_1625410695_1824142_7060452_o.jpg   190.54KB   24 downloads

[kkala]

...I'm into plant design and I'm new to these unit operations: Ion exchange and Chromatography. On Ion exchange, my purpose of using this is to separate Na2SO4 from the sugar solution (xylose, arabinose, galactose, mannose, cellulose and xylan to be specific). I don't know how to create a mass balance around it as it is unsteady state (efficiency is lowered at a time and accumulation follows, same as Chromatography to concenterate xylose from the solution. I'm planning to use dowex ion exchange resins. I've found a software but it's used only in water demineralization purposes. Our plant design is production of Xylitol from corn stover. Attached herewith is the process flow diagram of our plant. Any help will be greatly appreciated... p.s. Ion exchange for the removal of Na2SO4 is labeled as salt removal

.
In the past I experienced a similar problem running a pilot wet scrubber to treat ~7000 m3 of phosphoric fertilizer reactor gases (containing NH3 and Fluorides). We could not find any other way out but running the installation as steadily as possibly. For instance, water flow from network was not steady; we placed overflowing barrels onto upper floor and supplied water from there. Ammonia content in the gas varied ~10 fold; we started the scrubber just after changing ammonia feed drum, since broad NH3 variation occurred during the change.
Substances are diluted into the liquid hold up of each vessel or evaporator. Suppose a gas concentration A suddenly changes to A'. Gas phase out will get A' quite rapidly, but what about liquid out? It will need time to respond, restoring its concentration from B to B'. Indeed, liquid concentrations depend not on the gas concentration of the moment, but rather on previous gas concentration "history". So making a balance (considering substances in gas and liquid) can be imprecise.
Consequently such inconsistencies could be smoothed out through following.
1. Try to have as steady conditions as possible. This is easy said - hardly done, but worth of tries. Observe operations carefully to trace conditions under which disturbances happen, then try to limit variations. Instrumentation and control improvements may be necessary.
2. Try to limit liquid hold up of the vessels to the extend possible. But these might have limited possibility in modern industrial practice. Less hold up in a vessel may cause another disturbance.
3. Assuming you make the balances on continuous flows by samples and measurements, try to make the stable measurements as long as possible, hoping that natural variations will somehow be smoothed.
I believe No 1 is the most effective and most difficult of the three. But it is a trial and see procedure, requiring patience and open mind. No 2 concerns the case when gases and liquid coexist, which does not happen in all steps of the Xylitol unit.
Besides the unit has some evaporators. The multistage evaporators never obtain steady state, according to (fatty?) words of one colleague. In fact we needed hours to obtain a somehow steady state in a single stage phosphoric acid evaporator.

Edited by kkala, 30 June 2011 - 11:54 PM.

[Jopeth]

it's quite hard for me to imagine that! but the thing is it can't be a steady state due to the accumulation occurring in the resins. i can't even know how many theoretical stages would be required on the process. But will do sir!

[kkala]

You have limited time as a student; to point out the problem (the specific one, being a bit different to the general picture of the post) is still success. I remember the presentation of our graduate works, when two colleagues did not manage to hydrogenate benzene, but realized that the vapor mixture found a way to bypass the catalyst of the lab reactor. A teacher noted that the finding was more worthy than hydrogenetion itself and the work was graded as excellent.
Try to have conditions as steady as possible and give an interpretation of the results.
Hope of good success.