Tag: Ureaknowhow

In Part 3 of this series on stripper efficiency issues, we continue the discussion on some some of the many causes of lower stripper efficiency. Here we discuss liquid divider fouling and bad installation of liquid dividers.

Part 1 of this series on stripper efficiency issues provided a brief history of the CO2 stripping process and discussed how the invention of the HP CO2 scrubber back in the 1960s revolutionised urea technology. In part 2 we take a look at how high liquid load can affect stripper efficiency.

In 1967, Stamicarbon revolutionised the urea production process with the invention of the HP CO2 stripper by Mr Petrus JC Kaasenbrood. At a time when there was an energy crisis in many parts of the world, the HP CO 2 stripper led to three main benefits:

Biuret is a chemical compound with the chemical formula HN(CONH 2 ) 2 , also known as carbamylurea. It is a commonly occurring undesirable impurity in urea-based fertilisers, as biuret is toxic to plants. Biuret is formed from urea, according to the following overall reaction:

Mark Brouwer and Jo Eijkenboom of ureaknowhow.com examine the major shifts in global urea production. They also discuss the future of the urea industry and, in particular, how the sector is being affected by the increasing focus on low-carbon ammonia production.

Every urea plant is also a water plant as the overall reaction starts with two molecules of ammonia and one molecule of carbon dioxide resulting in one molecule of urea and one molecule of water. Further water is added to the process via the steam ejectors in the evaporation section. All this water, which is contaminated with ammonia, carbon dioxide and urea plus possibly other contaminants like formaldehyde, methanol, oil, etc is collected in an ammonia water tank and then sent to a wastewater treatment section. The purpose of the wastewater treatment section is to reduce the ammonia, carbon dioxide and urea levels to acceptable levels. First the wastewater is treated in a first desorber column, where LP steam is used to strip off the ammonia and carbon dioxide, reducing the ammonia content from approx. 6-8 wt-% ammonia to approx. 1 wt-% ammonia. Nothing happens with urea in the first desorber as temperatures are too low to hydrolyse the urea back to ammonia and carbon dioxide. This takes place in the next step, in the hydrolyser, which can be quite a large counter current column operating with MP steam at approx. 23 bar (Stamicarbon design) or a horizontal deep hydrolyser operating at approx. 33 bar (Saipem design, refer to figure). Downstream of the hydrolyser there is another desorber column to strip off the remaining ammonia and carbon dioxide. Nowadays, boiler feed water quality can be realised by modern wastewater treatment. But at higher plant loads the operating margin in the wastewater treatment can become too small leading to higher ammonia and urea levels during certain operating conditions… n

Today some 75% of all urea plants worldwide operate a prilling tower as the solidification and finishing technology. A prilling tower is a large hollow concrete tower in which concentrated urea melt is sprayed from the top via a rotating bucket or static shower heads. The urea melt droplets cool and solidify while falling down some 70-100 m. The heat is removed by ambient air flowing upwards either as a natural draft due to the temperature increase or forced by means of air blowers.

It is common knowledge that wet CO 2 is corrosive to carbon steel and “dry” CO 2 is not corrosive to carbon steel. So typically engineers and contractors choose carbon steel for dry CO 2 conditions and stainless steels for wet CO 2 conditions. This can be seen in the CO 2 feed section of almost every older urea plant. But is it true that carbon steel is always the right choice for dry CO 2 and is dry CO 2 really dry under all circumstances?

Rohit Khurana and Umesh Jainker of KBR presented a technical paper on this topic during the 2013 Asian Nitrogen + Syngas Conference. It can be found in the UreaKnowHow.com E-Library with the title: ”Replacing ammonia plant catalyst with maximum efficiency and lowest cost”. The paper addresses the importance of de-dusting catalyst beds before commissioning and the serious impacts on the plant if not performed thoroughly. Many ammonia plants have faced problems related to the plugging of exchangers, pipe choking, pressure drop increase of the downstream catalyst beds and separators or foaming in the CO 2 removal section which could be caused by the presence of catalyst dust. Most of these problems have led to either decrease in the efficiency of the plant or operation at lower throughputs. The paper presents the critical steps and procedures for proper dedusting of the catalyst beds before commissioning. In addition, the foaming problem in the CO 2 removal section associated with catalyst dust is discussed signifying the importance of cleaning the CO 2 removal system and solution. The role of filters in the CO 2 removal section was also emphasised.

Avoiding chloride contamination is critical in urea plants, not only from the process and utility side but also from the atmospheric side. The applied stainless steels in urea plants are susceptible to detrimental failure modes when chlorides and moisture are present. This UreaKnowHow.com Round Table discussion provides several examples of failures and important prevention measures relating to chloride stress corrosion cracking risks in urea plants.