Carbon Formation in Reformer

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  • Author
  • #96572
    Mark Brouwer
      • Expert

      Dear Gentleman,
      carbon formation on catalyst is
      1- Already coming solid carbon particles with gas.
      2- very low S/C ratio minimum ratio is 2.6-2.9 for safe operation lower than that will create problem for HTS as well as LTS.
      3- Temperature lower than 750 C is also problematic and your plant D/S will be unstable.

      So feed steam and air by keeping in mind the S/C and H/N required and vendor recommended.

        • Participant

        1. It is not recommended to comedown below 2.24 mol/mol steam to carbon ratio or specified by your process licenser to avoid catastrophic carbon formation in reformer tubes even in very minimum time of operation. So, Good margin of S/C ratio to be ensured always to ensure safe operation of reformer.
        2. As your plan is to utilize margin in secondary reformer & managing CH4% slip, eventually your shift conversion will badly effected with very low steam to Gas ratio’s (At u/s of shift converters). The CO% slippage from the LTS will have major effect on plant operations & production cost.
        3. Due to Low GAS to AIR ratio’s in secondary reformer, the temperature generated in combustion zone will be very high (considering very low steam to reformer & enriching Air), your existing refractory/catalyst may not withstand hence secondary reformer Airgun & the gap between gun & catalyst may be redesigned suitably.
        4. There will be very change in Synloop H2/N2 ratios (very low H2/N2 with high Air flow to Sec. reforming), your Ammonia converter may not perform good. Generally, the PROCESS AIR to sec. reformer will be varied to maintain the Synloop H2/N2 ratio not for additional reforming. Please not,it is not alone for reforming purpose.
        5. To maintain the loop H2/N2 ratios (2.7 to 3), adequate purge has to be taken from synloop & your purge gas recovery unit should allow this change. This is again a energy loss.
        6. Finally, if you operate at very low S/C ratio’s, any small disturbance in STEAM network will result in less steam flow to reforming, leads to severe carbon formation in reformer tubes.

          • Participant

          Unfortunately, metal dusting (carburization) becomes a problem at low S/C ratios. Unless your plant’s metallurgy is suitable, you can rapidly destroy downstream equipment.

            • Participant

            Dear My friends

            Let me explain a little bit regrading the topic:

            possible carbon forming reactions are :

            2 CO ———- C + CO2 ( Boudouard dismutation principle)
            CO +H2 ———– H2O +C (CO reduction)
            CH4 ———– C+ 2H2 (Methane cracking)
            Carbon formed by methane cracking should always be removed by reaction with steam or carbon dioxide provided that the rate of carbon removal is fatster than the rate of formation.
            This is normally true at temperature bellow 650 C provide in active catalyst.
            To prevent carbon formation,the reformer must not be run bellow the minimum calculated steam to carbon ratio.
            It is evidence that determining the tempetarure is depond on equilibrium constant which is included partial pressures of hydrogen and methane in reformer outlet.

            So,please check the reformer outlet average analysis to determine the equilibrium constant and then find your safe and relevant temperature to keep the reformer in safe margin based on convensional curves.

            Best Regards

              • Participant

              I have been working at the most updated Ammonia producing plant where S/C ratio is operated as low as 2.6 and Primary reformer outlet temperature is as high a 800 C and we don’t face any carbon formation at these conditions. SO Thermodynamically speaking you are safe upto 800 C with your S/C ratio as low as 2.6.

                • Participant

                Dear Ahsan Sarfraz Bhai,
                Sir I want to decrease the firing rate of the furnace (Decrease the Manifold Temp) and also the S/C which definitely increase the CH4 slippage. I also want to increase the CH4 slippage (up to acceptable limit) from the furnace so as to increase load to Secondary Reformer (As it has the margin to do so). Let suppose the CH4 slippage at this time is say 11%, and I want to increase say 14% by decreasing the S/C and firing(Manifold). So what are the bottom manifold temp and S/C so as to avoid carbon formation ??????
                Can I decrease the manifold temp to 650C at S/C of 3.5 ?????????
                I want to know the minimum allowable manifold temp at S/C 3.5.

                  • Participant

                  with high slc ratio no worry from carbon formation
                  keep it higher than 3.8

                  M Ahsan Sarfraz
                    • Expert

                    Dear Sohail,
                    The subject matters only prevails when there is limited steam environment. That is why steam to carbon or Low steam flow protections are there.
                    usually it occurs during abnormalities if the interlock are by passed are malfunctioned. This are all general comments. What particular you are interested to know ? Please elaborate a bit more if this doesn’t satisfy you.

                      • Participant

                      Dear All,
                      As we know that the carbon formation temp is ~450C. Can anyone share the carbon formation temp (In Primary Reformer) at a S/C of 2.8,2.9,3.1,…………..,3.8 ???????????
                      We maintain the manifold temp of about 780C at S/C of 3.6. At these conditions what should be the conditions (Temp) for carbon formation ??????????????

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