Reprinted from HydrocarbonEnginEEring | October 2011

building

blocks

Although sulfur blocks may not always be the first thing to come to mind when asked to consider the critical components of a hydrocarbon processing operation, they

nonetheless play a critical role in both the sulfur supply chain and the operational plan for dealing with sour feedstock.

Sulfur blocking traditionally consists of pouring sulfur into large, monolithic blocks for storage. In most cases, moveable slipforms or other temporary forms are used to shape the block, although concrete blocks and even dirt berms have been used to form the walls of a sulfur block. It is interesting to ask where and why the idea of building these gargantuan blocks derived, and whether or not this clearly low tech practice will continue to play a role further into the 21st century.

origins of blockingIt is true that some sort of sulfur blocking has most likely been used in emergency situations for as long as molten sulfur has been handled either as a byproduct from smelting or as a feedstock into sulfuric acid plants. However, blocking first became a common practice in sulfur handling when the Frasch method took hold along the Gulf Coast of the USA. With the drive and entrepreneurial spirit of people like Herman Frasch himself, as well as others like the Brady brothers, a large industry grew from non-existence to dominance of global sulfur production in a relatively short period of time.

Mining, excavation and smeltingHowever, this type of production possessed distinct differences from the other sulfur sources in existence at the time, namely

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Reprinted from October 2011 | HydrocarbonEnginEEring

traditional mining and excavation of shallow sulfur deposits and ore smelting. In traditional mining, the production rate is obviously very simple to control. A mine can easily change the number of workers or adjust the size or number of pieces of equipment in operation in order to adjust the production rate. In addition, the incremental cost per ton is generally flat, so the margin does not change substantially based on the mine’s production rate. Also, the mine can easily shut down and start up with very low cost. due to this adaptability, a sulfur miner possesses the ability to produce sulfur in accordance with the needs of the customers. there is no need to maintain more inventory than is required to meet customer commitments.

Smelting byproduct sulfur production more closely mirrors the sour oil and gas processing sulfur production that can be seen today. It is purely a byproduct of processing a more valuable commodity, like copper. As a result of being tied to another, predictable process, the production tends to be steady and is known in advance. A smelter knows how much sulfur will be produced well before actual production. therefore, arrangements can be made for storage, transportation and sale.

The Frasch methodthe Frasch method shares none of these characteristics of control and predictability, especially in the early days of the process. Frasch mining generally involves sending a well bore into a subterranean deposit of sulfur. the well usually consists of three concentric pipes. One for injecting superheated water to heat and melt the sulfur; one for injecting compressed air to aid the sulfur flow; and one allows the molten sulfur to flow to the surface for filtering, storage, and ultimately shipment to a customer.

this process requires a large amount of energy to heat and pump the water and air, as well as taking a substantial amount of time for a well to reach a stable production rate upon start up. Once

a well begins flowing, the well will typically be produced at as high a rate as possible for as long as possible.

Although experience in the industry has allowed engineers to make reasonable predictions about production rates, this was a fine art, not a mass balance calculation or exercise in stoichiometry. In addition, the flow rates did not necessarily follow a predictable pattern over the life of the well. As a well became more mature, the area of sulfur potentially exposed to hot water might be higher, resulting in higher flow rates, assuming a sufficient volume of hot water was available. However, there were countless other factors that could have the opposite effect.

the bottom line is that Frasch mining had neither the control of traditional sulfur mining nor the predictability of a byproduct of smelting. therefore, Frasch producers desperately needed a way to absorb large and fluctuating inventories. Given the obvious costs of molten tanks, they chose to pour the sulfur into forms and build solid blocks. Since sulfur is almost completely insoluble in water and not terribly reactive, it could be stored outside with little or no protection from rain or weather. As a result, these blocks became larger and soon appeared virtually identical to the blocks seen today.

A Frasch mine operator would pump the produced sulfur from the ground and onto a block. Usually, the molten sulfur would go through some sort of settling and/or filtration system to remove some of the impurities that would rise up to the surface along with the sulfur. Once it was poured onto the block, it would stay there until needed for shipment to a customer. Although the molten sulfur market grew steadily over time, when Frasch sulfur dominated the market, most international trade was in crushed bulk form. Essentially, the sulfur was simply broken off the block and loaded into rail cars, trucks, barges or ships.

contemporary blockingdespite the similarity between the blocks of the early 1900s and today, the methods change just as with any technology. methods for both building the blocks and recovery of sulfur off the blocks have adapted over time to higher standards of safety and environmental responsibility. the starkest contrasts between the early days of blocking and today lie in recovery of sulfur from the blocks.

take a moment to consider what a young man from East texas might do when faced with the following problem: he has one monolithic block of sulfur approximately 40 ft high, 200 ft wide and 500 ft long. He estimates it contains 200 000 t of sulfur that has been pumped out of nearby Frasch mines over the last few weeks. In a week he will need half of that sulfur to be in broken up chunks that he can load into rail cars. When faced with the conundrum of finding the cheapest and fastest way of breaking big, solid things into tiny pieces, the obvious answer is dynamite.

Figure 1. Pouring towers, Kaybob, Alberta, Canada.

Figure 2. Sulfur blocking operation at the Syncrude refinery, Alberta, Canada.

Reprinted from HydrocarbonEnginEEring | October 2011

Although other, less spectacular means of demolishing a block were also employed in times past, the fact that dynamite was used extensively illuminates the drastically different safety culture existing in the first half of the 20th century. the days when worker injuries were a known cost of doing business are distant memories of the industrial past. the advancement of worker safety continues to drive most of the industry developments in sulfur blocking and recovery.

In the contemporary industrial landscape, sulfur blocks are still very common and new blocking facilities continue to be built. the basis for the earlier explanation of block development rested exclusively on the unique nature of Frasch sulfur production. now that byproduct sulfur from sour oil and gas facilities dominates the global supply, the need for blocking initially appears questionable. the production is very predictable, because a refiner generally knows how much sulfur the plant will produce even before the upstream well pumps it out of the ground.

the answer lies in the fact that the reason for blocks today is not the same as it was for the initial emergence of blocking. When Frasch sulfur dominated, most sulfur spent time on a block somewhere between the well and end use. today, only a very small percentage of sulfur produced in the world will ever spend time as a yellow monument to industrialisation. In modern oil and gas processing, sulfur blocking generally serves only as an alternative to either going to a customer as liquid or being formed into granules or prills for international shipment.

Reasons for blockingthree general reasons for blocking exist in today’s sulfur market. the first reason is a case where locations are geographically isolated from both consumers and a port for export. transportation may be impossible because of insufficient infrastructure, or simply uneconomical based on the price of sulfur and distance. locations such as northern areas of Alberta, Canada and facilities in kazakhstan are good examples of locations where transportation costs can exceed the market price for sulfur.

the second reason is to provide a back up solution when equipment for forming or exporting fails. When downtime due to failure of a shiploader, forming unit, conveyor, or any other piece of equipment exceeds the allowance considered for molten storage capacity, a blocking facility sitting idle for years suddenly becomes extremely important.

lastly, blocks are very effective for absorbing inventory during severe market disruptions. Whether related to weather, armed conflict or economic conditions, as proven in late 2008 and early 2009, there is no guarantee a buyer will appear for every ton of sulfur available in the world. refineries and gas plants around the world from the middle of the US to Saudi Arabia, Qatar and the UAE wisely incorporate blocking into their plants to insure that a facility will not shut down due to an inability to offload their sulfur production.

Changing methodsSince a clear justification still exists for new and existing blocking, a number of changes have occurred to differentiate a modern sulfur block from the old days of dynamite and dumping on the ground. For recovery of sulfur from a block, the most common method remains demolition by a trackhoe or other heavy equipment, based on the size of the block. the broken up sulfur is either remelted or shipped to a consumer as crushed bulk sulfur.

Although the technology is over 30 years old, the most ‘modern’ technological development in block recovery is what is commonly known as the Ellithorpe melter. this melter consists of what is essentially a large, steam heated vertical plate mounted to a slow moving tracked frame. It could be described as a bulldozer with an oversize, steam heated blade. the tracked frame steadily pushes the

plate into the block melting the sulfur. the sulfur drains down to the bottom of the plate where a trough collects it and it drains to a larger collection sump or tank. Although proven effective in commercial use primarily in Canada, it has never seen widespread use. this is most likely due to a higher capital cost and much higher complexity than mechanical demolition.

New equipmentChanges also occurred in the equipment used to pour sulfur onto blocks. In many early Frasch sulfur facilities, an operator directed a high pressure stream of molten sulfur through a nozzle much like a fire monitor. It was sprayed across the block and the sulfur was directed toward the shallow areas on the block. this method was effective, but obviously dangerous, and it required the constant attention of an operator. Frasch mining concerns soon figured out that sulfur would flow to fill the surface of a block without being sprayed at high pressure. they began to replace the high pressure nozzles with cantilevered pipe that would direct the sulfur onto the block. these towers were virtually identical to most of the pouring towers seen today.

most modern pouring towers consist of a tower with either one or two arms hanging down. normally, each arm includes two swivel joints, which allow the operator to raise and lower the arm as well as swivel from side to side. the operator will raise the arm as the block grows, keeping it high enough to not touch the block, but low enough to minimise the distance the sulfur falls to the block. this reduces the tendency of the sulfur to melt a deep pool of molten sulfur directly underneath the discharge. the operator will also use a chain or cable hanging from the pouring arm to pull it from side to side. this assists in getting the sulfur to flow where desired on the block, and also helps to prevent deep pools of molten sulfur caused by pouring in one location for a long period of time.

Figure 4. Enersul ProPivot block pouring system.

Figure 3. Forms on block at Syncrude refinery, Alberta, Canada.

Reprinted from October 2011 | HydrocarbonEnginEEring

Technology in actionAlthough variations of this basic design continue to be found at most blocking sites around the world, there are some newer developments that continue to improve both the safety and economics of sulfur block pouring. Facilities in kazakhstan and Qatar have installed equipment adapted from agricultural irrigation equipment. Essentially, they consist of a short centre pivot sprinkler section that connects to a tower via a jacketed flex hose. the advantages of such a system lie in improved sulfur spreading across the block and improved automation, reducing the amount of time operators must be on the block. the effective length of the pouring arm is greatly increased, as it does not have to hang from a tower, but rests on its own wheels on the block. this allows the arm to spread the sulfur across a much larger area. In addition, the arm moves under its own power, so it can move constantly without an operator physically pulling the arm from side to side.

As one would expect, there are some drawbacks to this design. First, the jacketed flex hose is somewhat prone to failure. pouring arm diameter is usually on the high side of what is practical for flex hose construction. At 6 in. and higher sizes, these hoses become very heavy and stiff. Furthermore, they will fail if subjected to too much stress, even just bearing their own weight. In addition, the fixed, pivoting end of the arm can become stuck in the sulfur, causing the arm to bend and sometimes fail. the low speed and high torque of the electric drives on the moving, discharge end of the arm prevents those wheels from freezing into the sulfur. However, the wheels next to the fixed end barely move, and they are not driven by power. As a result, they can freeze up in the sulfur. the outlet end wheels continue to drive, bending the arm.

A new developmentOne of the latest developments in block pouring is the Enersul propivot. this technology combines the best attributes of traditional block pouring arms with the benefits of the centre pivot design previously described. the arm is attached to the tower with the same swivel joint arrangement as a traditional tower, so there is no need for a costly or troublesome flex hose. However, rather than hanging by a cable, which limits the practical length of the arm and requires additional structure to hang the cable, the arm rests on wheels at the discharge end. An electric drive turns these wheels, giving all the benefits of improved block integrity and reduced operator interaction, but without the drawbacks of earlier powered pivot designs.

Sulfur block operators innovate other parts of the operation as well. Improved and more specialised personal protective equipment, fall arrest systems, and fixed access ladders have drastically reduced the hazards block workers face. In addition, modern operators install runoff control and geotextile barriers between the block and the underlying soil to mitigate the risk of acidification of soil in the area. these and other adaptations help bring the sulfur block up to the same standards of safety and environmental responsibility now expected in all areas of the oil and gas industry.

conclusiondespite clear improvements, it is unlikely that any foreseeable technological advancement will promote sulfur up the hierarchy of hydrocarbon processing from the rank of a byproduct like petcoke to the hallowed status of jet fuel. However, it nevertheless remains important to continue to pursue improvements in the safety and effectiveness of sulfur operations, even on the often overlooked sulfur block.