Acetylene can be produced from the reaction of CaC2 and H2SO4:
CaC2(s) + H2O ---> C2H2 (g) + Ca(OH)2
Then, in the next step H2SO4 is used for the purification of acetylene to around 99%.
I know if we produce acetylene according to the conventional method, using CaC2 and H2O, then we can purify the acetylene using different methods. There are different impurities in the produced gas. Moreover the N2 exists in the produced gas.
May I know your suggestion for better purification of gas?
Three columns were used for the acid wash step. In first column the weak acid is used for the washing at 45 C, in second column stronger acid is used at 45 C, and in the 3rd column NaOH is used for purification. During this procedure we are facing some problems as mentioned bellow. I need some suggestion to identify the following issues?
1- The gelatinous brown material, in the top of the column that used for acid washing the gas.
I think this was formed by the reaction of 98% H2SO4 at 40 to 60 degree C with acetylene.
Under these conditions, I think the acetylene will polymerize forming tar like products.
I am looking to confirm this theory- so is this jelly a mixture of polymerized acetylene and H2SO4? How can one prevent it?
Is it correct to say that polymerization of C2H2 means longer chain hydrocarbons?
As this is occurring in the presence of H2SO4, I guess there will be C4, C6, C8 compounds with SOx fragments. is it correct?
What kine of characterization techniques can be used to confirm the existence of these products? FTIR, XPS, GC/MS are adequate for this purpose?
2- The hard crystals
These form in, and at the exit of, a tower containing 10% NaOH.
I suspect they are the result of reaction of H2SO4 with NaOH. This crystal might be Na2SO4?
I really appreciate your cooperation and guides.
Best regards,
Zahra
Dear Zahra,
though I am not any expert in acetylene chemistry and purrification, I think you are right when guessing the gellatinous product on a top of the second column might be acetylene polymerization products. The reaction starts via addition of H+ on one of the two pee bonds of acetylene as follows: C2H2 + H+ = CH2=CH+ carbcation which starts polymerization of acetylene. According to the reaction conditions linear by-products containing -SO3H, or aromatic - benzene and styrene /at temperatures over 4OO oC/ may be formed.
Maybe HPLC/MS, CG/MS or PY-GC/MS could give you information concerning the "jelly". H1 NMR, FTIR or even UV spectroscopy can give you also intersting data, problem is quantification of the by-products
I am of the opinion, that reduced washing temperature of the produced acetylene with H2SO4 and higher number of columns might supress formation of "polymeric unsaturated moieties".
As for the crystals arising in the final alkali washing tower, they are posibly a mixture of Na2SO4 and H2C=CH-/CH=CH/n-CH=CH-SO3- Na+ salts.
Best regards,
R.S.
Dear Zahra,
I believe all your presumptions to be true, I would suggest to use lower temperatures and weaker acids/bases, that should reduce the amount of polymerized material. You might also try to switch something between the acid and base wash, just a zeolite or condenser or something in order to reduce the amount of acid you are carrying with the gas. If you just want to see what you polymer is made of, without exactly quantifying it, a normal ESI-MS should be enough.
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