Skip to main content

Nitrogen+Syngas 382 Mar-Apr 2023

31 March 2023

Plant Manager+

Problem No. 66 Urea storage for bagged urea

Storing urea under hot and humid ambient conditions can be a chall enge. Several quality parameters of the urea product itself like moisture, temperature and particle size distribution are critical. Fluctuations of these parameters over time are also important and can lead to caking issues and complaints by clients. Off-spec product means big losses in revenue and results in a troublesome stream that has to be handled separately. Learning from each other’s experiences is vital to minimise and avoid these problems.

Amir Tafazol from Shiraz Petrochemical Company in Iran kicks off this interesting Round Table discussion: We want to build a storage facility for bagged urea. Are there any standards or conditions that need to be considered? Is it necessary to consider an air conditioning (AC) system? The location of the storage is in a port which has humid weather.

Mark Brouwer of UreaKnowHow.com in the Netherlands asks a question for clarification: Is it for 50 kg bags (more closed) or 1000 kg big bags (more open)?

Amir replies: The storage is for big bags.

Mark asks some further questions: I assume your concern is caking? Caking is determined by the product quality (moisture content, hardness and smoothness of granules, dust-content, etc.) and on the local-specific conditions, such as the ambient conditions (relative humidity and temperature and their fluctuations). Basic philosophies for a urea warehouse is to keep it as airtight as possible (keep doors closed) and apply thermal insulation. This will minimise any relative humidity and temperature fluctuations. I assume the product temperature is already ambient when it reaches the port and that the big bags are properly closed. Does anyone have experience of whether AC is required in hot and humid climatic conditions? Does anyone use special big bags? Are you talking about prills or granules? What is the moisture content?

Amir responds: Yes, my concern is about caking. As you mentioned, we know the basic philosophies for a urea warehouse but I am wondering if it is for bulk storage only and whether it is also essential for bagged product? We have experience of some temporary general warehouses which are not airtight and do not have an AC system, and we had any problems of caking in these warehouses. But now we want to build a standard bagged product warehouse for our company at the port, so it is better to consider the essentials in the design. Furthermore, you assumed right, the product temperature is already ambient when it reaches the port after couple of days and the moisture content is decreased. We produce both prilled and granulated urea. The moisture content of fresh prills is max. 0.3 wt-% and max 0.5 wt-% for fresh granules.

Mark replies again: You state you have seen no caking issues of either product in the available warehouses at the port which are not airtight and have no AC. In that case why would you expect problems in a new airtight warehouse?

Prem Baboo, Ex NFL India and Dangote Fertilizers Nigeria joins the discussion: In hot countries there is no need for air conditioning in a urea storage warehouse, the critical relative humidity (CRH) of urea is high (70 to 75% at 30°C) compared to ammonium nitrate and calcium ammonium nitrate (CAN). In the rainy season all doors of the silo (urea storage) should be closed to withstand the hot and humid conditions in those countries. Only ammonium nitrate requires an air conditioning system.

The relative humidity can be controlled in the prilling system by operating the position of the louvers in the bottom and top of the prilling tower, these can be throttled in the rainy season to control the humidity. The relative humidity can also be controlled by means of a desiccant-type dehumidifier.

Up to 0.3 wt-% moisture content of urea is ideal to avoid physical quality problems like caking. In the summer season, urea must be cooled to below 60°C (even better 45-50°C) because the trapped moisture in the hot urea can condense when it is stored or bagged resulting in caking. Cooling of urea in the summer season can be done by means of SOLEX coolers.

The moisture content can be controlled by process parameters like vacuum pressure, free ammonia content, etc.

Prem refers to the following articles with more background information:

  • 2002 Orphanides Urea caking problems, how to avoid them: To the knowledge of the author there are not enough precise reports explaining the phenomenon of urea caking when stored in bulk (in onshore storage or in hatches of ocean-going vessels). In the literature, many studies and investigations report on the effect of various parameters with regard to caking (CRH, particle size and hardness, abrasion and impact resistance, treatment and conditioning, storage pressure and temperature, urea moisture, moisture absorption, etc.), but only few explain the mechanism of how caking spreads from a relatively limited initial area to the whole pile, often observed in even very good quality urea, stored under more or less correct conditions in bulk. The basic phenomenon governing the spread of caking in a heap of urea in bulk, so-called humidity migration, is where initially a relative thin layer only has absorbed humidity, (due to the hatch cover being left open during a period of high atmospheric relative humidity), or where a relatively small amount of warm urea (above 55°C) with a higher residual moisture content is present in a heap of relatively cold urea with low residual moisture content. If this water pick-up, or this inclusion happened, and urea is subject to temperature cycling, which usually is the case during storage, or loading – shipping – unloading, then caking of the whole holding of the hatch may result. This caking may be severe, resulting in hard to break lumps, or less severe (easy disintegration of lumps), depending on the quality of the urea and the number of cycles.
  • 2018 06 Baboo Caking of urea in summer season: In India the ambient temperature reaches 46-48°C in peak summer. The prill temperature goes even higher i.e., in our plants, the line-1prill temperature is 70-72°C and has reached 78°C at higher plant load with recovery of urea solution. The prill temperature of line2prills is 64-66°C without bulk flow cooling. The caking of urea fertilizer has been investigated in a warehouse of large scale in National Fertilizers plant. In this article the caking tendency of urea was investigated for a silo and urea bags. The main cause of caking is the growth of crystal bonds on the contact point of prills. The caking tendency increases with moisture, the ammonia content causes water to mitigate through the pile, which increases the caking tendency. The effect of plant load, temperature, relative humidity, and storage time on the formation of a bridge between these particles was analysed. The objective was to describe the geometrical changes in the contact region and to measure the strength of the resulting inter particle bridge. Urea particles are used which are known for creating solid bridges under well-defined climatic conditions. The measurements indicate that, unlike isotropic materials, the bridge between two particles has higher shear than tensile strength. Moreover, the strengthening of the bridges with storage time is very inhomogeneous. The effect of load, temperature, relative humidity, and storage are the main objective of the study.
  • 2018 04 Baboo NFL urea product quality: This article includes many studies and investigations reporting on the effect of various parameters on prill quality like crushing strength, size distribution, abrasion and impact resistance, humidity factors urea moisture absorption, vacuum studies etc. The nitrogen, moisture, prill strength and biuret contents and the size distribution of prilled urea are important factors determining urea quality. High temperature of prilled product is common in most urea plants in India. In some plants in India the temperature of prills reaches 80°C on hot summer days at high load. At our plant the prill temperature is also 65-70°C. This result in poor strength, dust formation, and an increase in caking tendency. Granulated urea has a definite advantage over prilled urea but has been more expensive to produce until recently. We have installed a bulk flow cooler for lines 1 and 2. A modification was also carried out on urea line-1 in March 2018 to solve a foaming problem in the waste water section (distillation tower). This modification is now proven and beneficial.

This series of discussions is compiled from a selection of round table topics discussed on the UreaKnowHow.com website. UreaKnowHow.com promotes the exchange of technical information to improve the performance and safety of urea plants. A wide range of round table discussions take place in the field of process design, operations, mechanical issues, maintenance, inspection, safety, environmental concerns, and product quality for urea, ammonia, nitric acid and other fertilizers.

Latest in Outlook & Reviews

Protectionism casts a shadow over the new year

The start of a new year is a traditional time to take stock of the previous 12 months and look ahead to the next. In this regard, CRU’s most recent annual client survey, conducted at the end of December last year, makes interesting reading as to your own concerns for 2025 and beyond. There were numerous responses across commodity and financial sectors, and broadly based worldwide, if slightly skewed towards Europe and North America, but across all of these the key worry for the coming year clearly emerged as trade tariffs and protectionism. This is perhaps unsurprising, given incoming US president Donald Trump’s avowed intent to impose blanket 20% tariffs on all goods entering the US, and up to 60% on China. While most clients did not think tariffs would rise as much as some of Trump’s rhetoric might suggest, most expect rises of 5-10% across the board, and Asian businesses are most concerned. CRU’s most recent position paper on US tariffs highlights some of the internal political and legal challenges in implementing these, but does acknowledge that some rises will be inevitable, and may well produce the kind of reciprocal measures last seen in the previous Trump administration’s trade war with China and the EU in 2018.

Tariff uncertainties cloud the picture

Nitrogen+Syngas went to press just a few days before Donald Trump’s swearing-in as the next president of the United States. While it is sometimes difficult to sort the truth from the hyperbole in his public pronouncements, nevertheless, if taken at face value, they would seem to indicate that we may be in for a turbulent four years in commodity markets in particular. While he is an avowed military non-interventionist, on the economic policy side he has emerged as a firm believer in the power of tariffs to alter markets in the favour of the US, and has promised 20% tariffs on all goods entering the US, potentially rising to 25% for Canada and Mexico, and 60% for his particular bugbear, China, sparking a scramble for wholesalers to stock up in the last few weeks of the Biden presidency. Trump previously raised tariffs on Chinese goods entering the US to 20% during his first term, and the Biden administration made no attempt to reverse this, and even added some additional ones, for example 20% on Russian and Moroccan phosphate imports.