T o understand the technological advancements that have been achieved with sulfur granulation over the last five years, it is important to first understand the basics of granulation, as it applies

to the global sulfur industry.Granulation in the sulfur industry is generally defined as

the building up of a small seed particle by coating it continuously, layer by layer, until the desired size is attained. Seed particles are typically 1 mm or less, and the desired size of the final product is typically 2 - 6 mm.

In order to build a granule of sulfur, there are two primary processes required. The first is a process for generating the seed that provides a base to spray sulfur on to. This process is different depending on whether the seeds are created inside the granulation drum (internal seed generation) or outside the granulation drum (external seed generation). The second is the enlargement process of the seed to make it into a full size granule by continuously coating it over and over. Sulfur granulation is performed in a rotating drum that uses flights to carry the sulfur seeds up to the top of the drum and drop them in front of nozzles spraying liquid sulfur (often referred to as sulfur curtains). If all of the granules can reach the desired size with a single trip through the drum, the process is referred to as a single pass process. If more than one trip is required, using recycle conveyors, the process is referred to as a multi-pass process.

There are also supporting systems associated with every granulation technology that allow the primary processes to take place. The supporting systems involve the following functions:n Water sprays in the drum for evaporative cooling.n Air flow through the drum for removing evaporated

water from inside the drum.n Capture of sulfur particulate from the air flow

to meet air emissions requirements.n Handling of captured sulfur

particulate.

THE EVOLUTION OF GRANULATIONTHE EVOLUTION OF GRANULATIONCasey Metheral, Sandvik Process Systems, Canada, examines the world of sulfur granulation and how, inevitably, technological advancements have changed the industry for the better.

However, not all sulfur granulation processes are created equally. The technology has gone through several evolutionary steps since its beginnings in the late 1970s. This article will not touch on all of them, only the major ones, namely multi-pass to single pass, and internal seed generation to external seed generation.

Historical multi-pass with internal seed generationMulti-pass refers to the use of a screen and recycle conveyors to move undersize product from the discharge of the drum back to the front of the system for further enlargement. In other words, a particular granule may pass through the drum more than once. This is used when an acceptable product size cannot be achieved in a single pass. Historical multi-pass sulfur granulation was achieved with internal seed generation.

Historical single pass with internal seed generationSingle pass refers to the absence of a recycle conveyor to move undersize product at the discharge of the drum back to the front of the system for further enlargement. This is only possible when an acceptable product size can be achieved in a single pass. The advantages of a single pass system include a reduction in equipment and a simplification of the process. Historical single pass sulfur granulation was achieved with internal seed generation.

Modern single pass with external seed generationModern single pass evolved from the advantages of historical single and multi-pass systems. The evolution drew upon the benefits of minimal

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Figure 1. RS-1500TM – modern single pass granulation system.

equipment found in the historical single pass system, and the wet scrubbing system found in the historical multi-pass system. Combining these benefits with the introduction of external seed generation has led to further advancements.

Modern single pass with external seed generation has created improvements in the following categories:

n Operational availability. n Forming capacity. n Maximum sulfur spray temperatures. n Sulfur/water filtration requirements. n Sulfur/water pressure requirements. n Energy requirements. n Equipment footprint. n Effluent particulate capture and management.

Operational availabilityDaily shutdowns have been the benchmark in sulfur granulation for decades. It became standard practice that operators would shut down for one to two hours on a daily basis to clean out buildups in the drum flights and downstream air ducting. Buildups in granulation systems are a result of sulfur mist generated at the fringes of the main liquid sulfur sprays. This inadvertent mist may not come into contact with a sulfur granule. Instead, it makes contact with a flight or the wall of a duct, where it solidifies, creating a buildup. The amount of mist generated, and therefore the rate of buildup, is a function of sulfur spray pressures and air flow velocity. With the RS-1500TM using low sulfur spray pressures and a larger diameter drum to keep air velocities low, the rate of buildup inside the drum and in the ducting has been greatly reduced. Not only does this mean longer run times, it also means less labour for general housekeeping. Testing on the RS-1500 has pushed a single run to 80 hours without stopping.

n Historical uptime: 22 - 23 hours out of 24 = 91.6 - 95.8% n RS-1500 uptime: 80 hours out of 81 = 98.8%

Forming capacityThe capacity of any sulfur forming system is directly related to its ability to remove heat from the sulfur that it is solidifying. In sulfur granulation, this is a function of the capacity of the air system that pulls air through the drum. The diameter of the drum is important because there are upper limits on the air velocity inside the drum. The higher the volume of air that can be pulled through the drum, the more capacity there is to evaporate water for cooling purposes. The RS-1500 uses the largest sulfur drum diameter in the industry, which allows for the highest capacity of air flow in a single unit. Together, these allow for the highest volume of water evaporation inside a sulfur granulation drum, which provides the highest capacity of sulfur solidification.

Note that the capacity figures in Table 1 are based on similar conditions for each technology (ambient conditions of 50˚C and 100% relative humidity, 130˚C sulfur supply and 85˚C product discharge temperature).

Maximum sulfur spray temperatureIn historical technologies, a high sulfur temperature creates problems with seed generation. Since the seed generation

Table 1. Granulation process comparison

Units Historical multi-pass

Historical single pass

RS1500

Operational availability

% 95.8 95.8 98.8

Sulfur forming capacity

Tpd per unit

1200 345 1700

Maximum sulfur spray temperaturein drum

°C 140 140 159

Sulfur filtration requirement

Mesh size (US standard)

140 140 20

Water filtration requirement

Mesh size (US standard)

500 500 50

Required sulfur supply pressure

kPag 2150 1130 800

Required water supply pressure

kPag 2000 900 500

Energy requirements

Connected kW/tpd capacity

0.174 0.119 0.138

Sulfur particulate emissions

mg/m3 of effluent

50 50 31

Figure 2. Sulfur granules.

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Figure 4. Negligible buildup in drum after 80 continuous hours of run time.

on these technologies occurs inside the drum, this creates a limitation for the entire sulfur spray system. The external seed generation system used on the RS-1500 avoids this issue altogether, which means the only limit on the sulfur spray temperatures inside the drum is the point where the high viscosity of high temperature sulfur makes it difficult to pump efficiently (>159˚C).

Sulfur/water filtration requirementsThe RS-1500 makes use of larger orifice sulfur and water nozzles than previous technologies. This means the filters can be designed for much larger particulate, which is far less demanding. Granulation systems with smaller nozzles need filters that remove smaller particulate. This requires more frequent element cleaning or replacement, which results in increased labour and higher operating costs. The probability of a small nozzle plugging is also much higher, which impacts availability, as a plugged nozzle can easily lead to a process upset. The lower filtration requirements of the RS-1500 mean lower operating costs for the filtration system, lower pressure requirements for the upstream pumping systems, and higher system availability due to the low probability of a plugged nozzle.

Sulfur/water pressure requirements Pressure requirements are directly related to power consumption. If they exceed certain thresholds, it also means more expensive piping, valves and instruments to meet higher pressure ratings.

Historical multi-pass sulfur granulation systems have required high pressure sulfur and water sprays in order to properly create seed particles at the front end of the drum. Historical single pass sulfur granulation systems have lowered these spray pressure requirements. The RS-1500 has dropped them further, to the lowest levels ever.

Energy requirements Generally speaking, the more a process can be simplified by removing non-essential moving components (rotating, vibrating, conveying), the lower the power consumption will be. The power requirements of the RS-1500 benefit from both a simple process design (minimal components), as well as the economies of scale associated with high production capacity. In other words, one large component can do the job more efficiently than multiple smaller components that add up to the same capacity.

Historical single pass technology actually has slightly lower energy consumption requirements than the RS-1500. This is due to the replacement of the wet scrubbing system with a heated cyclone. A wet scrubbing system has additional rotating equipment for pumping water that a heated cyclone does not have, but, as described later, lower power requirements get traded for higher emissions. The RS-1500 does not make this compromise, choosing slightly more power consumption for much lower emissions.

Equipment footprintA smaller footprint means lower civil costs, smaller shelters/buildings, shorter tie-in lengths (electrical home run cables, sulfur/utility piping, and collector conveyors).

Figure 3. Sulfur granulation drum.

Sulfur curtain

Sulfur spray

Water spray

Drum flight

Figure 5. Elevation view and plan view comparison.

Smaller skids also allow for pre-assembly and pre-wiring to dramatically reduce the installation time of equipment in the field. It should be noted that, while the footprint of a historical single pass unit looks smaller than that of a modern single pass unit, approximately five of these units are required to meet the same capacity as an RS-1500 (Figure 5).

Effluent particulate capture and managementRemoving sulfur particulates from the airstream in any granulation technology is essential to meeting most environmental regulations.

n Historical multi-pass: § Capture technology: wet scrubber. § Fines management: dewatering system to extract sulfur sludge. Sludge is melted, filtered and injected back into the process.

n Historical single pass: § Capture technology: steam jacketed cyclone. § Fines management: liquid sulfur drains from cyclone into a sulfur pump tank for injection back into the process.

n RS-1500: § Capture technology: wet scrubber. § Fines management: fines are recycled into the seed generation system to be used as-is, in the process (no remelting required).

The sulfur particulate emissions from a sulfur granulator are a function of the spray conditions inside the drum, as well as the particulate capture technology. For sulfur particulate, wet scrubbing technologies outperform heated cyclones due to the generation of sulfur vapour inside a heated cyclone. Combining wet scrubbing technology with low sulfur spray pressures, the RS-1500 has reduced the emissions down to levels never before seen from a sulfur granulation system. The high sulfur spray pressures of historical technologies create increased particulate loading on the effluent treatment systems. This has made it very difficult, and in some cases impossible, for these technologies to meet regulatory emissions requirements.

Note that the measurements are according to EPA ‘Method 5’ total particulate.

Extended drum life and reduced drum maintenanceSulfur granulation technologies rely on a rotating drum that supports very heavy loads. These systems need to be properly designed and maintained to maximise their lifespan. Historical systems rely on a sloped drum to move granules from one end to the other, which creates additional maintenance and accelerated wear and tear on the system.

n Historical: sloped drum used to direct granules towards discharge end. § High loads on thrust rollers and wheels. § Routine alignments required.

n RS-1500: horizontal drum with advancing flights used to direct granules towards discharge end. § Minimal loads on thrust rollers and wheels. § Routine alignments not required (Figure 6).

ConclusionThe development of the RS-1500 in 2010 and 2011, followed by the testing and refinements on the technology that continue today, has pushed the limits of granulating sulfur. Those limits will continue to be pushed to meet the industry's increasingly stringent demands.

AcknowledgementsThe following professionals also contributed to this article: Bill Martin, Cory McConnell and Tom Smith, Sandvik, and Les Lang, Sulfuric Group, Inc.

Figure 6. Horizontal drum (zero slope).

Sandvik and sulfur processing/handlingThe RS-1500™ is among the most recent additions to the Sandvik portfolio, complementing the company’s already extensive range of sulfur solidification and handling equipment.

In terms of solidification, the company is best known for its Rotoform pastillation system, suitable for small to mid-size capacity processing requirements. First used for

sulfur solidification in the early 1980s, the Rofoform offers exceptional product uniformity and environmentally friendly operation, and is the world’s most widely used process for the production of premium quality sulphur pastilles.

The company also supplies equipment for sulfur degassing, molten truck and rail car loading, block pouring, remelting, upstream handling, downstream storage/reclamation, as well as bulk loading solutions for truck, rail and ships.

Sandvik Process SystemsDivision of Sandvik Materials Technology Deutschland GmbHSalierstr. 35, 70736 Fellbach, Germany

Phone +49 711 5105-0 · Fax +49 711 5105-152E-mail: info.spsde@sandvik.comwww.processsystems.sandvik.com