2022 02 Baboo Material Technology for Fertilizers Industries Part 6
Over the past two decades, the ammonia and urea industry have witnessed spectacular metallurgical developments for process equipment. For example, stainless steels, modified with special materials, can improve high temperature creep rupture resistance. Using duplex stainless steels and modern corrosion abatement techniques are other methods that improve plant-operating performance. Materials plays very important role in any industry. Selection of material is vital at design stage itself. Wrong selection of material may lead to catastrophic failures and outage of plants & even loss of human lives; right selection of material leads to long life of plant. Over the past two decades, the ammonia and urea industry have witnessed spectacular metallurgical developments for process equipment. For example, stainless steels, modified with special materials, can improve high temperature creep rupture resistance. Using duplex stainless steels and modern corrosion abatement techniques are other methods that improve plant-operating performance. The actual reactor has been constructed using a variety of materials over the years that can resist ammonium carbamate corrosion. In the latest plants specialty duplex steels, such as Sandvik’s Safurex, have greatly improved the resistance to ammonium carbamate corrosion. Fertilizer plants employ various corrosive, hazardous and abrasive fluids and chemicals. The temperatures involved range from cryogenic (-33 oC) in ammonia storage to reactor pressure 1000 oC in reformer. The pressure is as high as 175- 350 kg/cm2 in ammonia converter and in urea plant reactor pressure 150-250 kg/cm2. Once equipment has been selected, the materials for its construction must be established. Although a process engineer is not expected to be knowledgeable as a metallurgist, the engineer should have a general idea of what materials are compatible with the process. Therefore, this topic presents some general guidelines in the selection of material for process equipment. In all ammonia and urea plants worldwide the problem of severe erosion and corrosion of high pressure vessels have been a common phenomenon. Duplex steels have also been adopted for piping, valves, etc. Since the advent of industrial urea processes based on the direct reaction between ammonia and carbon dioxide, designers and urea plant owners have had to deal with the problems of corrosion. Although pure urea solutions are not very corrosive, ammonium carbamate (an intermediate in urea synthesis) is a highly corrosive liquid. For example, carbon steel in contact with carbamate solutions will corrode at a rate of more than 900 mm/year. A corrosion- resistant layer made of a special type of stainless steel is therefore required to protect carbon steel pressure vessel walls in the urea synthesis section. In this paper all material used in ammonia and urea fertilizers covered.