Kimre has recently achieved the lowest ever urea dust emissions using its SXF™ Semi-Cross Flow Scrubber technology at a urea granulation plant. The challenge for tougher emission control of urea dust was presented with the development of new urea production in the United States. With recent focus on the health impacts of fine airborne particulate, the permitted emission limit values negotiated between plant sites and local environmental authorities were lower than any previously demonstrated limits.
Urea dust is characterized as “particulate matter”, one of six criteria air pollutants regulated by the US Environmental Protection Agency (EPA) which established requirements for PM10 (particulate matter with aerodynamic diameter of 10 micrometers or less) and PM2.5 (particulate matter with aerodynamic diameter of 2.5 micrometers or less). PM10 and PM2.5 have come under increased scrutiny due to recent studies showing detrimental health impacts of the penetration of these fine particles into the human lung. Requirements of less than 10 mg dust per Nm3 gas on a dry basis were desired for an industry that had previously required no less than 20 mg/Nm3 urea dust.
Opacity regulations came along as new, separate enforceable emission requirements to particulate matter. With limits as low as zero percent, also known as “invisible stack” conditions, it is the most stringent air permit requirement in the industry.
Ammonia, classified as a precursor to PM2.5 which then becomes a regulated pollutant, is another source of emissions from urea granulation plants. It is not particularly difficult to deal with ammonia since the removal is enhanced by an acidic reagent liquid. In most previously constructed plants, however, there was no permitted requirement on ammonia emissions in the air exhausted from the plant.
In urea granulation, Kimre has partnered with licensor Uhde Fertilizer Technology to design highly practical, including the most efficient, granulator and cooler scrubber units for new and retrofit installations.
The SXF™ Semi-Cross Flow Scrubber combines conditioning spray and downstream mesh pads that operate as KON-TANE® Scrubber and Tower Packing and B-GON® Mist Eliminator stages. (see Figure 1).
Kimre’s uniquely interlaced monofilament structure (referred as “mesh”) is an original invention from 1973 which revolutionized mass transfer operations. It is highly effective in the collection of dust and mist via impaction and interception. Combinations of different styles of mesh allow for customized efficiency removal for any set of gas, solid, and liquid properties.
The term “semi-cross flow” refers to contactors in which the liquid (usually aqueous solutions or slurries) is sprayed co-currently with the gas on the upstream side of the mesh pad, washing the dust off the media. The gas flow is horizontal and the liquid runs down the pad, crossing the gas flow and discharges at the bottom. An added benefit of the horizontal orientation of the scrubber is providing easy access to internal components which, in addition to removable mesh pads, allow for total system flexibility. It is a convenient feature for quickly changing the pad design to increase or reduce the removal efficiency and adapting to changes in the normal operating and design conditions.
Another advantageous design feature is the ability to treat multiple gas contaminants in one common vessel. Through the use of weirs that isolate individual sections of the scrubber, it is possible to separate concentrations and/or chemistries. Each stage in a series of stages can operate as an independent scrubbing system, with its own liquid-to-gas ratio, in a manner similar to that of a distillation tower.
In case of large variations in design mode specifications (i.e. variations in gas, liquid flows, inlet load of contaminants, lack of particle size distribution) and/or superior particle removal requirements, Kimre’s AEROSEP® Multi-Stage Aerosol Separation System can be added to a typical SXF™ scrubber arrangement. The AEROSEP® provides high efficiency removal of urea dust smaller than one micron, reducing particulate emissions, particularly PM2.5, to the lowest possible levels.
The first retrofit installations to come online were two granulator scrubbers handling off gases from drum granulators. Each of the scrubbers replaced three and four impingement-type scrubbers and were designed to improve the collection of urea dust. An SXF™ Semi-Cross Flow scrubber with three KON-TANE® stages and one B-GON® stage was designed to scrub off the particulates from the gas stream with recirculated water and minimum amount of fresh water. The urea was recovered from the scrubber as a 40-45 % wt. solution and recycled back to the process. Recent test results showed that the scrubbers outperformed the air permit requirements.
|Table 1: Urea Granulation Retrofit Installation Air Permit Requirements and Results|
|Air Permit Requirements||Test Results|
|Granulator Scrubber 1|
|Total Particulate||20.05 lb/h||8.23 lb/h (equivalent to 12.6 mg/Nm3 dry)|
|Opacity||< 20 %||1.2 %|
|Granulator Scrubber 2|
|Total Particulate||15.2 lb/h||8.74 lb/h (equivalent to 10.2 mg/Nm3 dry)|
After successful pilot test research with ThyssenKrupp Industrial Solutions for urea and ammonia abatement were carried out, a granulator and cooler scrubbing system was installed at the new CF Industries Donaldsonville plant site in Louisiana, US. The scrubbers were designed to recover urea as a 40-45% wt. solution and ammonia as an ammonium nitrate solution of 10 % wt. concentration solution while meeting particulate and opacity requirements. The cooler scrubber was a simple SXF™ Semi-Cross Flow scrubber with two stages designed to scrub off urea dust and send all liquid effluent to the granulator scrubber which was designed to scrub off urea dust and ammonia in one common casing. The first two stages of the granulator were KON-TANE® stages dedicated to scrubbing off urea, followed by two other KON-TANE® stages dedicated for ammonia absorption with a mild nitric acid solution. The two liquid effluents from the granulator scrubber were recycled back to the process with minimal use of fresh water.
Recent test results show that the scrubbers outperformed the air permit requirements achieving the lowest ever emission values. An added benefit to cleaner air is the near full product recovery.
|Table 2: CF Industries Urea Granulation Installation Air Permit Requirements and Results|
|Air Permit Requirements||Test Results|
|Particulate Matter (filterable)||29.67 lb/h||1.63 lb/h (equivalent to 0.87 mg/Nm3 dry)|
|Opacity||< 20 %||3.8 %|
With 3.3 million m3/h already online and 1.5 million m3/h coming online in the next 9 months, Kimre expects to continue meeting and exceeding the ever-changing stringent environmental challenges imposed in the US and throughout the world.
Kimre is also engaged in continuing pilot tests and optimizing the technology for customized designs to meet specific requirements at new and revamp sites. In addition, we are developing improvements to provide more cost-effective operations and maintenance for Kimre scrubbers.