Change in Urea Product Colour

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  • #98431
    akbar-ali
    Participant
      • Participant

      Dear Gentlemen,

      The intent of this post is to share our problem related to Blocking In and its impact on product quality, at one of unit ( 1800 MTD STAC Tech with Granulation) we are facing a frequent problem regarding off specs product due to its dull white color, this is a common thing now whenever urea solution kept under Blocking In either for 24 Hrs. or even less than 24 Hrs..

      During recent SD ( April 29, 2014), urea solution was kept about 25Hrs. for blocking In, after start up, more than two days production was remained off specs due to dull white color. Regular analysis for Fe and Ni in final product were carried out and found that Fe content was very high ( up to 5 PPM against normal <0.1 PPM) ), but in all sample Ni was < 0.1PPM which is acceptable and normal for us.

      We checked the synthesis parameter before Blocking In and during Blocking In as below.

      O2 to stripper as per analyzer was 0.66% Vol.
      Reactor Temperature during Blocking In came down to 140C.
      Synthesis Pressure came down to 54Kg/cm2.
      N/C was operating 2.98 to 3.0.
      CO2 feed was cut first as CO2 compressor was tripped.
      NH3 Feed cut after 3 Minutes.
      When product become normal, Fe in final product was analyzed and found normal <0.1 PPM.

      The material of Reactor Liner, down comer and trays is BC.01

      We are investigating this incidents and seeking your experience and technical assistance concerning this problem.

      Addition to above we are looking for following information from your side.

      A. What is the explanation for, if active corrosion starts, then it will not stop untill SD the plant and complete drain and you will have Ni in final product..
      B. On what basis we can ignore the chances is active corrosion start, if the exact O2 content were 0.66 vol % before blocking In.
      C. If only Fe is reprting with dull white product, what are the sources of this Fe.
      E. What are the chances of Fe in the product from rectifying column packing, tubes etc.
      F. How we can avoid dull product with Fe after every start up following with Bloking In.

      Best Regards

      #98056
      nimesh7
      Participant
        • Participant

        Dear Friends,
        Colour change of Urea prills is due to two main reason
        (1) due to corrision
        (2) due to oil contamination….. as earlier mentioned by my friends
        I want to discuss about oil contamination……..
        when oil seals of ammonia compressor or carbon dioxide compressor is leaking oil comes in system..
        we can know the presense of oil in system by viewing in glass window of MP Urea holder & in LP holder…if there is any foaming means oil is coming in the system….
        If oil coming with liquid ammonia we can remove it by draining ammonia receiver tank for some time..when you drain it..oil lumps will come out…..you have to drain ammonia receiver tank regularly till oil leakage problem is not solved… this will be helpful..

        #98050
        rengelv
        Participant
          • Participant

          Now the tendency is to decrease to normal value (0.02 ppm) after stopped urea liquid tank consuption, therefore it is like you said: there is not an active corrosion phenomena.

          Thanks a lot for your reply

          #98049
          Mark Brouwer
          Keymaster
            • Expert

            In case you would have a damage of passive layer and as a result active corrosion, this active corrosion will not stop even if you increase the oxygen content.
            This in case you would have active corrosion somewhere you would see an increasing trend of your nickel level.
            You see this ?

            There is no normal limit, every plant has its typical norma nickel level depending on used materials etc.

            #98048
            rengelv
            Participant
              • Participant

              Dear Sirs.

              Now I take advantage from this round table to ask you about the normal limits on nickel content in urea for a reliable plant operation.

              Last few days we faced a plant shut down with a delay of 03 days for recuperation. Yesterday after startup we noted a yellow color in urea grains, we checked the nickel content, It is 4.44 ppm a great difference against the normal average is 0.02 ppm. We have stopped feeding liquid urea from the tank.

              We would like your opinion or advises about that high Nickel from point of view of the pasivation layer damage or equipment corrosion.

              We hope you can help us.

              Best Regards

              Victor Rengel
              Process Engineer
              FertiNitro

              #96705
              Mark Brouwer
              Keymaster
                • Expert

                Dear Gentlemen,

                As per my information , the product colour can change

                1- High oil contents in product.
                2- High iron contents.
                3- Carry over of CO2 system absorbent to synthesis loop

                Please confirm the passivated air range between 0.6—0.8% in CO2 which is dependent upon the N/C ratio and the material of construction of HP synthesis system.During colour change measure the oil, Fe and Ni contents in the product and different sample points.
                During the hold up time whether in reactor or urea solution tank , the product colour always change in start up.

                #96702
                Mark Brouwer
                Keymaster
                  • Expert

                  Impurity contamination from recycle tank or UF tank (in case of use) could be another cause.

                  #96701
                  akbar-ali
                  Participant
                    • Participant

                    Thanks for your Inputs.

                    We faced this problem during a startup after Holdup Urea solution in the reactor for 48Hrs. Just before Shut Down we encounter a problem of increased H2 Slippage up to 1.4% (for 15 Days) with CO2 from Ammonia side due to some problem in ammonia plant, the Delta T across H2 Reactor approached up to 68C.
                    We are suspecting that during this period of time Passivation Air Flow Control could not be managed accordingly and can be seen from the trends of O2 % at downstream of H2 reactor and consequently made a cause of change in product color due to active corrosion establishment.
                    After 3 Days of Operation we Resumed the Original Colour.

                    Furthermore to counter check reliability O2 Analyzer, what should be the air flow against the delta T across Convertor and CO2 Flow at Suction can be calculated with the help of simple formula?

                    Air Flow = Suction Flow of CO2 (0.000321*ΔT + 0.01936)
                    Air Flow = KG/Hr
                    CO2 Flow = Kg/Hr

                    With Best Regards

                    #96700
                    Mark Brouwer
                    Keymaster
                      • Expert

                      As mark said, It is mostly the corrosion product you will see after a stoppage or load reduction. check for Iron nickel in product.

                      #96699
                      malik-sohail
                      Participant
                        • Participant

                        Dear Akbar Ali Sb,
                        Yes the possible causes of product color change are well stated by Mr. Mark, among them I believe that during plant operation the causing factors are;
                        1. High oil content from Pumps, compressor
                        2. Abrupt change in levels (Lower side) of Recovery and concentration sections
                        The indication is from the level glass of final separator.Foaming in first stage evaporator (Can be seen by their level glass).
                        ➡ Some time when you import ammonia from ammonia storage then there is chance to have high oil content.

                        #96697
                        m-farooq
                        Participant
                          • Expert

                          Dear Akbar,
                          is this color change is permanent phenomena with the urea product?
                          or did you observe during foaming , excess ammonia at concentrator and rectifinfg column?
                          regards

                          #96694
                          processengineer
                          Participant
                            • Participant

                            Methanol presence in CO2 from ammonia plant also causes color change; which can be observed with new LTS catalyst.

                            #96691
                            nasir410
                            Participant
                              • Expert

                              Oil and corrosion products are the most common causes of prills colour change.
                              Another reason can be if urea solution stay enough time in the Urea melt storage tank. When we recycle this urea with fresh urea melt, this results in colour change.
                              For corrosion products samples can be collected at the exit of Reactor, Stripper, MPD, Lpd etc. for exact location.

                              #96690
                              Mark Brouwer
                              Keymaster
                                • Expert

                                I know two causes:
                                1. oil
                                2. corrosion product (iron-oxides).

                                Please check Nickel content trend in the prills and compare with your normal level.
                                and/or check any high oil consumption somewhere. Or a low level in a tank could take some oil to the downstream sections

                                Anybody know of more causes ?

                                #92677
                                akbar-ali
                                Participant
                                  • Participant

                                  Dears;

                                  what are the possible causes for change in product Colour like Dull White?

                                  Best Regards

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