Controlling the Fertilizer Manufacturing Process with the On-Line Image Analysis

The major ingredients present in synthetically manufactured fertilizers include various types of compounds containing potassium (K), nitrogen (N), and potassium (K). These fertilizers are used to supply nutrients for  half the world’s food consumption.

Examples of the many synthetic fertilizers include KCl (potassium chloride) and DAP (di-ammonium phosphate). While most fertilizer manufacturing processes produce solid “round” granules in the 1 to 6 mm range or the 2 to 4 mm range, the initial feedstock to the process is in liquid form, known as the “Melt.” A commonly used Drum Granulation process is shown in Figure 1.

The Process

Drum Granulation

The Melt enters through the front end of the drum granulator which rotates, and with the help of lifters present along the inside of the drum, it gently throws the granules as they grow in size. The exit stream contains granules that range from 0.1 to 10 mm in size. The stream is screened for granules at 4 mm, with +4 mm granules going to a large silo from which they pass through a roll crusher and then back into the drum granulator.

The -4 mm stream is screened for granules at 2 mm, with the -2 mm stream going to an undersize silo and then back to the drum granulator. The resultant product is the 4 x 2-mm stream which makes up about 1/3 of the entire process stream at any given time.

Flow diagram of typical drum granulation fertilizer manufacture. Optional sampling points to bring sample to an online Microtrac PartAn for measuring particle sizes and shapes are shown. There are several variations of this basic process for synthetic compound fertilizers. Some descriptions follow: move to main text below.

Figure 1. Flow diagram of typical drum granulation fertilizer manufacture. Optional sampling points to bring sample to an online Microtrac PartAn for measuring particle sizes and shapes are shown. There are several variations of this basic process for synthetic compound fertilizers. Some descriptions follow: move to main text below.

Pan Granulation replaces a pan (saucer-shaped) granulator for the drum granulator.

Prilling introduces a liquid mixer in which the undersize silo stream dissolves before returning to the granulation process, which is a prilling tower in this case. The prilling tower, a tall vertical tower, is fed with granules by a rotating funnel-shaped container with holes in it. The funnel is then fed with the Melt and the dissolved undersize stream. A column of air rises up through the prilling tower, causing the melt and stream to fall to the bottom as near dry granules, after which they are passed through the 4 mm screen. The +4 mm stream goes directly to the roll crusher and subsequently to the undersize silo.

The +4 mm and the -2 mm streams are recycled by nozzle spraying to a mixer for dissolution in oil. These dissolved streams, together with the liquid Melt, are fed to sets of nozzles close to the top of a tall tower. These nozzles spray the combined feed streams in an upward direction, and the droplets then fall through a column of air which is sent up through the tower similar to the prilling process.

In Fluid Bed processing, a fluid bed unit operation is used instead of the drum granulator for producing the granules that are subsequently screened.

The lesser-used processes are Spheroidization, Crystallization and Pug Milling, which can also replace the drum granulator and create a narrowly sized stream of “round” granules.

The Problem

Coating the granules with a thin wax layer is a very important finishing step in all these processes and this controls the leaching rate once the fertilizer has been applied to crop fields. It is for this reason that the granules need to be  a narrow size distribution and as round and consistently smooth as possible. Particles with irregular shapes are more like fused globules than round granules; they are likely to break at the narrower points, exposing surfaces without coating, and can be difficult to coat uniformly over the whole surface.

The Solution

Image Analysis, a particle characterization technique, provides additional information and therefore it is increasingly being used over other traditional methods. Usually, only a single particle size is measured - either the particle volume distribution, such as electrical sensing zone (ESZ), or the equivalent spherical diameter (ESD) distribution, such as sieve analysis, sedimentation, and laser diffraction. On the other hand, Image Analysis offers up to 24 different shape and size morphological parameters for each particle measured. This technology can provide roundness, size, and surface roughness data relating to the product, enabling all these properties to be measured and controlled with a single analyzer for applications like optimizing control of fertilizer manufacturing.

On-Line Analysis

Online Image Analyzers have been used in fertilizer plants worldwide since the initial installation in 1988. The Microtrac PartAn analyzer, which provides a prolonged operation without failure, is shown in the following figure.

Online image analyzer in a fertilizer plant, working continuously since 1995 without service

Figure 2. Online image analyzer in a fertilizer plant, working continuously since 1995 without service

On-line version of PartAn being installed Dec 2011

Figure 2a. On-line version of PartAn being installed Dec 2011

The advantages of on-line control of a process versus lab control are shown in the graphs below. The top graph shows the lengthy time taken between sampling and analysis when using lab control. One process fluctuation takes 2 hours, depending on a change in some control set point (average value) from data of the last analysis. In this case, the set point is indicated at 80%. In a fertilizer process, one set point could be the % of product within 2.5 and 3.5 mm, that is, the narrower the distribution the better. Alternatively, one set point might also be the % of product with a “roundness” factor greater than 0.88 (1.0 being a perfect sphere), or a Convexity value (surface roughness (1.0 being perfectly smooth) greater than 0.92.

Sampling and measurement are almost continuously automated with on-line control. This means that the on-line process variables, which may need to be changed to bring it back under control, (moving back toward set point), are modified more often than in lab control, and the set point can be increased to acquire more product within specification, which in this case is 90 vs 80%, thus optimizing the fertilizer’s performance in the field. The turn-around time for sending new on-line control signals automatically back to the process in fertilizer production is in fact in the order of about 5 minutes.

  1. On-line sampling frequency of PartAn detects the fluctuations
  2. Lab sampling frequency of every 2 hours does not detect the fluctuations

Graphs of process fluctuations illustrating the advantages of on-line control over lab control.

Figure 3. Graphs of process fluctuations illustrating the advantages of on-line control over lab control.

An example of the PartAn Trend (fluctuations over time) presentation. Parameters for presentation are user-selectable in the PartAn software. These parameter results can be used by the operator to make the proper process adjustments when necessary to bring the process under tighter control, or they can be acted on automatically by a computer equipped with PID and Programmable Logic control algorithms for adjusting the process variables in a feedback loop.

Figure 4. An example of the PartAn Trend (fluctuations over time) presentation. Parameters for presentation are user-selectable in the PartAn software. These parameter results can be used by the operator to make the proper process adjustments when necessary to bring the process under tighter control, or they can be acted on automatically by a computer equipped with PID and Programmable Logic control algorithms for adjusting the process variables in a feedback loop.

Large fluctuations can be handled by process equipment. Increased production can be a cause of less fluctuation because the Melt feed rate is an adjustable process variable; the higher it can be run, due to faster control response and lower fluctuations, the higher the production rate will become. (1)

The use of PartAn can reduce the unexpected down-time in the process (longer up-time), as on-line control will bring the process from start-up to steady state operation much faster than that of lab control. (2)

Based on the increased revenue obtained from these production increases, one can calculate the payback period for the equipment investment (3).

Research and Development

The PartAn can also be used as a lab instrument for research and development purposes. With the development of new and better fertilizers from new compounds and/or processes, the ability to generate the same amount of information about the product makes sense as it would later be controlled on-line. During the development phase, the various morphological parameters measured would be made available in the history of how the product performed, with regard to these parameters. This information can then be directed to take control actions as on-line analysis would indicate in the process.

The Technology

One of the newer technologies being used for particle characterization is Image Analysis. This technique is simple, logical, and easy; providing about 24 more morphological (shapes and sizes) parameters compared to traditional particle size analyzers. It features a combination of optics, a strobe light, optics, and a digital camera, which records images of passing particles. If the pixel size is known, and based on the number and relative locations of the pixels, all the parameters can be easily calculated. Some of the size parameters measured are the various equivalent diameters, widths and lengths, as well as maximum and minimum distances.

Aspect ratio, and various surface roughness and roundness parameters are some of the shape parameters. Some of the data forms available for each parameter are summary data and both volume and number distributions. Shown below is a file of images of all particles measured, and both the 3D and 2D parameters are listed at the upper right side of any particle selected. Of all the image analyzers, only the PartAn can calculate the full 3D results based on a patented particle tracking technique.

Particle images. Each row contains different orientations of one particle, which allows the 3-D calculations.

Figure 5. Particle images. Each row contains different orientations of one particle, which allows the 3-D calculations.

A Scatter Diagram (shown below in blue) provides a view of where each particle is located, relative to the two distribution parameters in the red graphs. In this example, diameter (size) in the top graph and roundness (shape) - the two most important features for fertilizer - are plotted to the right of the Scatter Diagram.

Scatter Diagram giving the relative location of each particle for size and roundness, and distributions of those parameters along with their Summary data. Any two of the 24 parameters can be plotted here.

Figure 6. Scatter Diagram giving the relative location of each particle for size and roundness, and distributions of those parameters along with their Summary data. Any two of the 24 parameters can be plotted here.

Typical distribution graphs and tabular data can be displayed in up to six overlays of different parameters for one analysis, or the same parameter for six different historical samples, as illustrated below.

Overlays of six different parameters and as differential and cumulative distributions for one sample.

Figure 7. Overlays of six different parameters and as differential and cumulative distributions for one sample.

Conclusion

Good fertilizer quality can be ensured through important parameters such as shape and size. Round, smooth particles and narrow size distributions are vital for even leaching rate of the nutrients to crops. Image analyzers can easily measure these two parameters.

Since 1988, on-line image analyzers have been used in fertilizer plants worldwide to provide stringent and rapid automatic control of these parameters. For new and better fertilizers, lab image analyzers can provide the historical shape and size data through the development process. This data can be used to plan the production control schemes for innovative products. Thanks to the short payback time for on-line installation, unforeseen downtime can be easily reduced. The only image analyzer that gives full 3D size and shape information is the Microtrac PartAn.

The Microtrac PartAn, laboratory version.

Figure 8. The Microtrac PartAn, laboratory version.

Footnotes

1. Benefit: Increased production - If production can be increased by 1% for a 50-ton/hour production facility, and up-time in the process is 90%, then the increase in production is 0.5ton/hour * 24 * 365 * 0.9 = 3942 tons/ year, or $1,971,000 / yr.

2. Benefit: Increased production time (up-time) - Assuming a conservative increase of 1% that will result in about 90 hours longer production time per year. At 50-ton/hour that will give 50 ton/hour * 90 hours * $500/ton = $2,250,000 / yr. Combined, these two benefits of on-line analysis represent about a 2% increase in production and revenue.

3. Payback time for PartAn on-line installation: Based on the 2% incremental revenue, if the price of the PartAn on-line installation with sampler is $ 150,000 and the price of 1 ton of fertilizer is $500, then the payback period will be about 14 days.(3)

This information has been sourced, reviewed and adapted from materials provided by Microtrac.

For more information on this source, please visit Microtrac.

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