Ammonia Converter: First bed pre-reduced or oxidised ?

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  • #117748
    Rama Raghava Kumar Kotti
    Participant
      • Expert

      Dear Mr. Safieh Karimi,
      Using oxidized catalysts in all beds, including the first bed, is not a commercially viable option due to several significant drawbacks:
      1. Production Loss: Pre-reduced catalysts in the first bed significantly enhance the rate of reaction, thus facilitating early ammonia production. The use of oxidized catalysts would slow down this process, leading to substantial production delays and inefficiencies. This delay directly translates to economic losses that outweigh any initial cost savings from using oxidized catalysts.
      2. Environmental Concerns: The reduction process of oxidized catalysts generates a considerable amount of water. The increased volume poses additional operational and environmental management burdens. This is particularly relevant in regions with stringent environmental regulations where efficient water management is crucial.
      3. Operational Efficiency: Overall efficiency of the process will low
      3. Commercial Viability: It is essential to consider that pre-reduced catalysts in the first bed have been widely adopted in the industry due to their proven benefits.

      Numerous commercial plants have demonstrated the advantages of this approach, achieving higher efficiency and reduced start-up times. Conversely, there is limited commercial precedent for using oxidized catalysts in all beds, which raises concerns about the reliability and efficiency of this strategy.

      I would be interested to understand the specific motivations behind considering oxidized catalysts for all beds and whether there are any unique operational contexts or constraints that might drive such exploration. However, from a commercial and operational standpoint, the use of pre-reduced catalysts in the first bed remains the optimal choice.

      #117379
      Emmanuel Ogoh
      Participant
        • Participant

        At my plant in Nigeria, we used oxidized catalysts for all beds until 2 years ago when we charged pre-reduced catalyst into the 1st bed and other beds oxidized. The catalyst reduction time, decreased water formation, and early declaration of ammonia production compensated for the extra cost incurred in the purchase of the pre-reduced catalyst.

        #115764
        safieh karimi
        Participant
          • Participant

          Dear Mr. Baboo
          Thank you for your valuable explanation. Do you know a commercial plant using an oxide catalyst for the first bed?

          #115682
          pbaboo
          Participant
            • Expert

            Yes, in recently studies First bed must have contained pre reduced catalyst while others two is in oxidized state. In the ammonia converter The catalyst usually used magnetite state(Fe3O4), generally Pre-reduced catalyst loaded in the first catalyst bed for increasing the reaction rate of catalyst reduction. The reaction first starts in first bed; all the bed contains pre reduced catalyst then reaction of reduction is fast in all beds. A hydrogen/nitrogen mixture with an approximate ratio of 3 to 1 should be used. Activation with normal ammonia synthesis gas at a pressure which is not too low means that the ammonia synthesis reaction will take place somewhat before the activation is completed, which presents two important advantages as following During catalyst activation oxygen is removed from the catalyst by reaction with H2 to produce water. The heat produced makes it possible to increase the gas circulation rate above the rate that can be supported by the start-up heater alone, and this means that the last part of the activation process will take less time than would otherwise have been the case. Also, a gradual transition from activation to normal operation is achieved. The co-condensation of the ammonia produced and the water, i.e. water released during catalyst activation, results in a more efficient removal of water from the synthesis gas before the gas is recycled back to the synthesis converter. This is especially important because water is poisonous to the activated catalyst. With pre reduced catalyst, about 20 litres of water will be formed per tonne of catalyst (with unreduced catalyst, about 280 litres of water per tonne), these figure for 2200 TD ammonia plants. With 3.8 tons of water 21.7 m3 of pre reduced catalyst in the first bed, 25.3 m3 in the second bed and 38 m3 in third bed in, approximately will be formed during the activation process. During reduction of the first bed the ammonia concentration is to be expected in the range of 0 – 90%. When the reduction of the second and third starts, the ammonia product will be above 95%, since the first bed has gained activity, and there is a maximum allowable reduction water content outlet the converter (3000 ppm). For unreduced catalyst in bed 2 and bed 3, the attached heating rates in period 3 and 4 is allowed to be increased 2-4°C, max 5°C thus decreasing the reduction period correspondingly. At the end of the reduction the ammonia concentration will be >99.5%. The activation procedure is divided into five periods as tabulated attached. The minimum circulation rate will therefore be achieved when the inlet temperature of the first catalyst bed is at its maximum, approximately 415°C, and the ammonia synthesis reaction is about to start. The circulation rate should at this point in time correspond to a space velocity of at least 1000 Nm3 /h per m3 of catalyst, preferably higher.

            #115677
            Mark Brouwer
            Keymaster
              • Expert

              From our Expert Venkat Pattabathula:
              “They can use oxidised catalysts in all the beds, which might be a low-cost option. There were many examples in the past, and nowadays, folks install pre-reduced catalysts at least in bed # 1 since it could easily be justified due to reduced time for catalyst reduction & production of less water during the reduction process. I don’t see much justification for installing oxidised catalysts in Bed#1. The benefits of a high-cost pre-reduced catalyst that enables early NH3 production would outweigh the additional cost of purchasing pre-reduced catalysts for Bed#1. Hence, having all oxidised catalysts in NH3 synthesis converters is relatively uncommon. As I stated before, an additional benefit would be reduced quantities of ammoniacal water during reduction, which, otherwise, costs to handle & dispose it off if there is no onsite consumer.”

              #115667
              Mark Brouwer
              Keymaster
                • Expert

                Our ammonia converter is axial radial, Casale technology.
                The first one we use pre-reduced catalyst for bed 1, and oxidised for bed 2 and 3.
                My question is ok to used oxidised type for all beds?
                Is there any commercial reference for this?

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