Hi LIMa Long time I was not here so it is good to start with you. Yes this true the Monel will corrode at temp. 150 c and when oxygen is presence. So Upgrading to the C-276 hastalloy will make the corrosion rate 10 times lower the Monel corrosion rate. In regard to that I recommend to do the following: •Reduce the amount of the oxygen as soon as possible. •Use the cathodic protection that will consume the amount of the oxygen and the system will pass the Monel material free of oxygen. •Apply coating why Not? Mark Mor

Luis You have done a farily good analysis on limitiation in use of Hastelloy C 276 or Hastelloy C 2000. Hastelloy can stand upto 70 o C with C.R at < 10 mils per year. At your design temperature (150 o C) both will perform nearly same or even more C.R than Monel. This may not help you. Monel can stand upto your design temperature - 150 o C - better than Hastelloy for all concentrations. Under conditions of aeration - likley to be present as you have noted in your first mail -due to operating parameter controls not likely to be strictly maintained : i.e., for higher concentration of HF at 90% or above and for your design temperature Monel is susceptible for acceleraed or higher corrosion. Solution for this problem :- you have diagnozed correctly : Limiting aeration (i.e, Oxygen concentration should be avoided) and limiting the water content will be the best to improve life of Monel tubes in service. If the process operating conditions do not control (say your process people insist that they still operate within design process conditions of aeration and water entry in the system) i believe that you may have to live with short tube run life with Monel tubes. For this, a change in tube metallurgy may not be an ideal solution. You may have to do further diagnosis : Why this affects only II Pass in your exchanger ? Is it because there is more drastic temperature variation or is it possible in II Pass temperature exceeds design conditions (>150 o C)? Even a change of 5-10 oC upwards in operating temperature exceeding the design process temperature of 150 o C can drastically alter the C.R in your Monel tubes. This probably could be a contributing factor for the varation in tube life you get from 4 months to 19 months life with Monel tube usage. Have you looked into the data of operating temperature controls in II pass - specifically? Also diagnosis from : Where from moisture or water collection of 1% comes in a closed system of HF circulation ? Have you looked in process control parameters in HF Alkylattion unit in your closed system circulation? If aeration (Oxygen entry in the system) is not controllable, from process operation controls, tube metallurgy for higher concentration of HF may not fully help you for your design temperature operation conditions. Trust this is of help to you. C.V.Srinivasan Nishi Engineers Pvt Ltd India Oct 26,2005 E-mail:nishi@vsnl.com >Tank you for the empathy Mark Mor!. >I don’t know if you work in a refinery or if you know the >Alky HF process. If so I would like to know how do you deal >with the subject posted in my previous posting? >About Hastelloy materials, versus Monel-SB 163 UNS 04400, >the experience and literature says that: >Monel 400 is used extensively in HF alkylation units and in >the manufacture and handling of HF. It has excellent >resistance to liquid HF over the entire concentration range >in the absence of oxygen to at least 150ºC. >C-276 Hastelloy can be used with caution in low (2%) HF >concentrations at 70ºC, in these conditions corrosion rates >can be as lower as 9 mils per year. >C-2000 Hastelloy is good in 30% HF concentrations at >temperatures around 30ºC, in these conditions corrosion >rates can be as lower as 4 mils per year. >For the above statements in the HF alkylation process is not >clear to have an alternative material to Monel. >It is quite clear that to avoid Monel corrosion to keep less >than 1% of water in HF, maintaining acid purity above 88% >and avoid oxygen contamination seems to be the good >operating windows practice. >I would like to know how others deal with process operating >windows and if have experienced a similar situation to the >one I describe. >Regards >Luis