NH3/CO2 Feed Ratio

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  • #97112
    Mark Brouwer
    Keymaster
      • Expert

      Dear Gentleman,

      for reaction of ammonia and CO2 to get urea need 2:1 and excess than this will increase the loop pressure only.But R &D data about urea shows that with increase of N/C ratio the CO2 conversion to form urea increases but the load on down stream of plant also increases which increases the utility consumption.

      #97107
      niranjanmurthyp
      Participant
        • Participant

        Dear nick
        Minimum N/C ration required in feed 2, because of the stoichiometric reaction 2moles of NH3 reacts with 1mole of CO2 to form 1mole of urea.
        But N/C ratio in the synthesis system has to be more than 2,
        1. Because to increase the convertion rate the quantity of the excess reactent should be more.
        2. Ratio required in the reactor depends on the synthesis pressure.
        3. Ratio required in the condenser to condence will be higher as you increase the synthesis pressure.

        #96936
        muhammad-adnan-hanif
        Participant
          • Expert
          quote nick102286:

          Where do you like to have your NH3/CO2 feed ratio set at and why? One theory I’ve heard is that by running more on the NH3 side it increases the density of the solution which creates better travel through the sieve bends (less plugging). Has anyone else heard of this or has other reasons for running the ratio where you choose for it to be? Our plant is Stamicarbon design, final product is prills.

          Dear friend,

          NH3/CO2 ratio is a design factor of urea synthesis reactor volume accordingly, its optimum range is also fixed by licencer during design phase and it may vary from plant to plant of same licencer. e.g for our old plant it is 3.4-3.8 and for new one it is 3.2 – 3.3

          Conversion increased with increase in NH3/CO2 ratio upto a certain level but after that its effect becomes negligible and it causes extra heat load on downstream sections.

          #96927
          Mohamed Moustafa
          Participant
            • Participant

            Dear friends
            As long as we are discussing about synthesis section N/C ratio there is an important factor should be kept in mind which is LP steam production as we all know we need the condensation reaction to take place at the highest possible temperature to make sure that we are close as possible to top ridge line, Also checking temperature differance over the conditioning cooling water of HP scrubber to make sure that we are not over loading HP scrubber.
            Meanwhile monitoring the synthesis section pressure and reactor bottom temperature are important factors to check the N/C ratio in synthesis section.

            Best regards,

            #96866
            norozi1972
            Participant
              • Expert

              DEAR FRIEND
              My reply is about N/C ratio in the synthesis and we know that in stamicarbon CO2 stripping process the N/C feed ratio up to overflow of reactor is 3/1 but after that must be decrease to 2/1.
              khpc-iran
              norozi

              #96864
              Mark Brouwer
              Keymaster
                • Expert

                Dear Mark,
                I do agree with you for the feed ratio, as it is constant almost on 2 for all the processes, So it seems that we have to discuss about N/C ratio in the synthesis that is different for every process.

                #96863
                Mark Brouwer
                Keymaster
                  • Expert

                  Lets distinguish between
                  N/C ratio in the synthesis
                  and
                  N/C feed ratio

                  N/C ratio is the synthesis are typically between 3 and 4 depending on the technology

                  N/C feed ratio should be close to 2 as the overall urea formation reaction requires 2 mol NH3 and 1 mol CO2.

                  I do not see the relation between N/C ratio of the feed and the sieve beds in the crystallisation section… as these are so far separated from eachother it is hard to imagine any direct relation.

                  #96862
                  norozi1972
                  Participant
                    • Expert

                    dear friend
                    The following urea reaction shows that the CO2 is a limit compound in the reaction and for better interaction between NH3&CO2 in the synthesis , the amount of NH3 must be higher than CO2.Also The biuret content is a serious concern because it is often toxic to the very plants that are to be fertilized and if the amount of ammonia in the synthesis be lower than expected value the reaction tends to produce more biuret because the synthesis situation(higher tem. & more resident time)are two major factor for producing biuret in the urea plant’s synthesis .If the ammonia be more than expected value the overall temperature of synthesis will be go down and finally the pressure of synth . can not be under control and as MR MOHAMADIAN mentioned before ,the recovery of ammonia in the LP section will be take more energy .
                    Urea reaction: 2 NH3 + CO2 ↔ (NH2)2CO + H2O
                    Biuret formation: 2 NH2CONH2 → H2NCONHCONH2 + NH3
                    khpc-iran
                    norozi

                    #96861
                    Mark Brouwer
                    Keymaster
                      • Expert

                      Every urea process has its own N/C ratio and as per the design we have to work in somewhere near to the design figure in order to be able to manage the recirculation section to recover unconverted ammonia and send it back to the synthesis section, So for example in Stamicarbon if you work in higher N/C ratio(3.4 for instance) you cannot recover that excess ammonia and therefore you will face with higher waste and higher ammonia consumption.

                      #92714
                      nick102286
                      Participant
                        • Participant

                        Where do you like to have your NH3/CO2 feed ratio set at and why? One theory I’ve heard is that by running more on the NH3 side it increases the density of the solution which creates better travel through the sieve bends (less plugging). Has anyone else heard of this or has other reasons for running the ratio where you choose for it to be? Our plant is Stamicarbon design, final product is prills.

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