C. Bahubali, Wood India Engineering & Projects Pvt. Ltd., Chennai, India
The transportation of large quantities of liquid sulfur is costly and often impractical since it requires a continuous supply of heat energy to maintain its liquid form. One of the most commonly accepted industrial solutions is transporting the sulfur in solid form. This article highlights the challenges and best practices of handling liquid and solid sulfur.
CHALLENGES WITH LIQUID SULFUR
More than 10 allotropes of sulfur have been established;1 however, only two or three allotropes are considered by the industry for plant design purposes.
The stable form of sulfur is orthorhombic (α-sulfur). At 95.3°C, α-sulfur converts into monoclinic (β-sulfur), which melts at 119.6°C and undergoes polymerization at 160°C,1 resulting in a steep increase in its viscosity up to 93155cP.2
The optimum temperature for the transportation and handling of liquid sulfur is between 125°C and 155°C. This is achieved by carefully maintaining jacketing steam/heat tracing/heating coil operating conditions. Increases in the jacketing saturated steam pressure or temperature will increase the liquid sulfur temperature; conversely, the low steam temperature will cause a delay in startup and the solidification of the sulfur. FIG. 1 illustrates the critical physical properties of sulfur.
Good engineering practices. Sulfur solidification occurs on the tank roof; therefore, sulfur acts as an insulator and contributes to further cooling of the surfaces. As the temperature continues to fall, traces of condensed water formed by the oxidation of hydrogen sulfide (H2S) will react with the solid sulfur and iron in the tank walls, creating the ideal environment for the formation of iron oxide (Fe2O3) and iron sulfide (FeS) that further accelerate corrosion. Therefore, temperature maintenance in the roof, shell and bulk is extremely important for trouble-free operation.
Additionally, the requirement for protective coating on the inner surface can be reviewed. In molten sulfur storage tanks, the roof and shell temperature should be > 125°C to prevent sulfur solidification, corrosion and pyrophoric FeS formation. The temperature gradient between the wall and the center of the tank (bulk liquid) may vary with respect to tank diameter. Careful computational fluid dynamics (CFD) analysis shall be carried out as part of any design by the vendor.
TABLE 1 illustrates the critical points of good engineering practices of the sulfur handling industry.
Jacketing or heat load estimation. The proper supply of heat to sulfur piping and tanks is critical for successful plant operation and product quality.
Sulfur piping heat loads must be estimated for cold startup conditions since normal consumption will be less. During design, the remelt heat duty must be checked to minimize plant downtime.
The self-draining (ensuring the draining of liquid sulfur during shutdown or emergency shutdown and maintenance without external force, e.g., such as sloped piping) facility of the plant will reduce the remelt heat load requirement. The complete sulfur melting process would require approximately 145 kJ/kg. Typically, the cold startup heat load will be the governing load and Eqs. 1 and 2 can be used for an initial estimation of the low-pressure steam requirement. An additional margin should be considered for flanges, spacers and pipe mountings.
Governing load (cold startup) = Heating the inner and outer pipes + radiation loss to atmosphere
Cold startup warming-up load (Cw) = [(Wi + Wo) × (T1 – T2) × Cpipe ] × L [H × t ] (1)
where,
Wi = Weight of inner pipe, kg/m
Wo = Weight of outer pipe, kg/m
T1 = Steam temperature, °C
T2 = Average pipe temperature, °C
Cpipe = 0.114 kcal/kg
H = Latent heat of steam at final temperature, kcal/kg
t = Time of warming up of pipeline, hr
L = Total length, m
Radiation loss to atmosphere (CR) = [A × U × (T1 – T2) × E] / H × L (2)
Governing load (kg/hr) = Cw + CR
CR = Steam/condensate load, kg/hr.m
A = External area of pipe, m2
U = 14.6 kcal/m2°C
T2 = Lowest air temperature,
E = Insulation efficiency [i.e., 75% = (0.75)]
H = Latent heat of steam, kcal/kg
L = total length, m
For liquid sulfur storage tank roof heating, the shell heating and coil load must be estimated properly to maintain the optimum liquid sulfur temperature. The supplied heat should compensate the heat loss from the roof, shell, ground and vapor. The CFD model of the liquid sulfur tank must be checked for detailed heat loss estimation:
Heat load for liquid sulfur tank = sum of heat loss (dry shell + wetted wall + ground + heat loss to vapor + roof)
It should be noted that the steam consumption during normal operation will be radiation loss; however, steam will be consumed until the liquid sulfur reaches thermal equilibrium temperature with steam. Therefore, the steam consumption during normal operation will be the sum of radiation loss and heat load to achieve thermal equilibrium with liquid sulfur.
Jacketing steam pressure and temperature. The flashpoint of liquid sulfur can vary from 168°C–188°C, depending on the purity of the sulfur. Impurities such as hydrocarbons present in the sulfur will reduce the flashpoint. Pure sulfur will have a lower flashpoint of 188°C. Using the jacketed steam/heat tracing system above this flashpoint will add additional risk. Optimum steam pressure should be made available at the farthest user point since steam temperature depends on steam pressure.
During plant operation, it is often misunderstood that sulfur flow also ensures jacketing steam flow, as well. However, this may be an incorrect interpretation—during normal operation, jacketing steam is not required for liquid sulfur to flow, as insulation will conserve the energy in the jacketed pipe. Therefore, frequent inspection of steam trap performance is always recommended, and it should be ensured that the traps remove condensate and entrained air. Calculating the steam pressure loss in the jackets and jump-over lines is more complex; rather than hydraulics-based pressure settings, a conservative approach will help in better design.
The dryness of the steam should be maintained. Steam traps play a critical role in heating economics and safety: steam traps to remove condensates should be standard for better heat transfer. Condensate carryover into the molten sulfur tank will result in over-pressurization during snuffing or sealing operations. Therefore, it is recommended to have steam with a little higher superheat than saturated temperature.
Transient analysis. When a valve is closed suddenly at the end of the pipeline and a pressure wave propagates in the pipeline due to the momentum change, a phenomenon of hydraulic shock—often called surge or hammering—occurs. This can result in noise, vibration and pipeline rupture. In general, the hammer effect decreases with an increase of volume (i.e., with longer pipeline) and slightly decreases with travel time, and is dependent on fluid compressibility. Higher compressibility will result in lower oscillation amplitude.
To avoid surge pressure, the valve closure time for a fail close (FC) valve or emergency shutdown (ESD) valves should be higher than the time taken by a transient pressure wave to travel back and forth along the pipeline. Pressure wave travel time can be estimated by the formula given in Eq. 3:
t = 2L × C (3)
t = Wave travel time, sec
C = Speed of sound in liquid sulfur (1,300 m/sec at 119°C–1,390 m/sec at 155°C)
L = Length of the pipeline, m
When the pump trips or the upstream valve is closed, the fluid downstream will attempt to continue flowing; this ultimately creates a vacuum and leads to pipe implosion and collapse. In such a scenario, the pipeline/piping should be designed for full vacuum condition rather than providing air valves that may result in sulfur solidification.
Subject to owner approval, ASME B31.3 allows a maximum surge pressure value of 33% above (no more than 10 hr at any one time and no more than 100 hr/yr) the maximum allowable operating pressure.4 ASME B31.4 criteria for transient overpressure allows a maximum surge pressure of 10% above the internal design pressure.5 This credit can be utilized during design and operation.
Relief design. Normally, relief valves are not used to protect the liquid sulfur system equipment; it is preferred to use rupture discs. The relief system should be routed to a remelt pit or vessel, and the entire relief system should be heat traced. A pool fire scenario is not considered, since solidification will occur upon the release or leak of liquid sulfur to the atmosphere—normally, a liquid sulfur system will be operated well below its flashpoint. A remelt pit or vessel should be sized for the maximum liquid sulfur drain load (shutdown case or ESD case)/relief load (from rupture disc).
Handling the sulfur plug. During the process of solidification of liquid sulfur, monoclinic allotrope will be changed to orthorhombic form, which will cause shrinkage in the solid form and form voids that are subsequently filled with more liquid sulfur.
When sulfur is remelted, it requires more space to expand since liquid occupies more space than the same mass of solid. If the solid sulfur is in a confined space, then serious overpressure can cause pipe rupture/leak. This is normally encountered in the plant during startup or a startup after an ESD. It is important to ensure that all ESD valves and isolation valves are kept open before remelt operation begins.
It is noteworthy to mention here that heating in the middle of the plug should be avoided. Solid sulfur in direct contact with a heat source will melt immediately and will not readily transfer the heat to the mass since the thermal conductivity is very low. It is always better to avoid solidification by properly maintaining the temperature of the jacketing steam/heat tracing system and ensuring complete drainage during shutdown. For a steam jacketing steam system, plant operations should:
Fire and explosion hazard. Sulfur fires are uncommon in plant operations (TABLE 2). Degassed sulfur will reduce the fire hazard potential. Proper sizing of the rim and center vents will ensure the disposal of stripped H2S and will reduce H2S accumulation in the tank, thereby decreasing the risk of fire and explosion. All of these vents should be heat traced to avoid any blockage.
Air inbreathing due to pump-out condition may expose pyrophoric scales to oxidizing atmosphere, which may cause fire. TABLE 3 shows the fire risk potential of sulfur-handling plants.
The heat of combustion of sulfur is approximately 300 kJ/mol and the flame temperature will be > 1,000°C with an explosion pressure of 7 bar, which will damage the integrity of structures, equipment and piping. Additionally, the rate of increase in pressure is faster than the flame propagation speed.
A sulfur dust explosion begins with blast pressure, and the flame appears 0.1 sec–0.2 sec later.6 Under normal temperature and pressure, the initial rate of flame propagation is 2 m/sec–3 m/sec. Due to burning inflation, the pressure rises rapidly, which accelerates the flame front propagation speed up to 300 m/sec. Furthermore, the secondary explosion happens during the sulfur dust explosion. Sulfur fire will generate a large quantity of SO2, which is lethal.
Aspects to be considered in a firefighting system design are summarized in TABLE 3.
Static electricity hazard. As one of the best electric insulating liquids known (its conductivity at 115°C is 100 pico-Siemens/m) and with a high dielectric constant, sulfur can easily generate enough static electricity to cause a spark ignition.7
Due to the poor conductivity of molten sulfur, static electricity will build up where free fall is allowed. An extension of the inlet feed lines to the tank/vessel bottom must be considered to minimize free fall and agitation. Wherever possible, the system should be designed for bottom filling rather than top filling (flash filling).
The linear velocity in the pipe entering the tank should be kept below 1 m/sec or pipe ID in meters, whichever is less, until the pipe inlet (slow start) is submerged to minimize static generation. It is believed that a higher sweep air velocity (> 1 m/sec) at the surface of liquid sulfur will also generate static electricity. Proper computational fluid dynamic (CFD) analysis should be carried out during the design phase.
As per NFPA 655,8 to avoid static electricity hazards, all sulfur lines, vessels and tanks shall be bonded and grounded with resistance of less than 1.0 × 106 ohms to ground. Static resistant coatings and dust control measures should also be considered as design tools. Providing static eliminators, static collectors, static neutralizers (needle, tubes, brushes, strings, etc.) for conveyors, belts and optimum velocity should be maintained.
Humidification limits static generation and affects product quality, as well. Spark promotors, such as the accumulation of H2S on the vapor space, should be avoided by venting/diluting with inserts. Conveyor belts should be inspected for slipping or jamming to reduce the chance of the generation of static electricity and ensure proper lubrication, which helps the dissipation of static electricity but sometimes results in pitting on bearing surfaces. Periodic inspection on conveyors improves the safety and availability of the plant.
Presence of H2S. The presence of sulfur dust in a H2S environment, such as tanks/ pit/vessels, along with an air sweep/breath system will increase the risk of fire and explosion.
Pyrophoric fire hazard. Pyrophoric iron sulfide can form from the impurities of the molten sulfur being stored.
Precaution should be exercised during maintenance/opening to the atmosphere. An incorrect temperature of the tank roof or shell will cause a wet condition inside the tank/vessel, resulting in the formation of more pyrophoric deposits and scales. Using a higher grade material (e.g., stainless steel) for an uncovered liquid sulfur area will reduce the possibility of formation of pyrophoric scales; this does incur additional cost.
Preventing an oxidizing atmosphere (nitrogen/inert blanketing) also reduces the pyrophoric fire risk. A Low-Low level setting of the pit of a tank should be above the heating coil to prevent exposing the heating coil to air, which will result in fire from pyrophoric iron sulfide.
Sulfur dust. The flammability range of sulfur dust is 30 g/m3–1,400 g/m3.9 The ignition temperatures for a sulfur dust cloud and dust layer are 190°C and 220°C, respectively. A sulfur dust cloud has a low minimum ignition energy of 15 mJ, which increases the dust explosion potential. The proper design and installation of a dust suppression system, dust collectors and wet scrubbers, as well as practicing good housekeeping procedures, will minimize the dust explosion potential.
Proper earthing and bonding of the conveying system and silos will reduce static electricity hazards. Better design of the dust control and removal system and good housekeeping practices will limit the extent of the classified area. It is always better to conduct dust dispersion analysis to identify the potential equipment, buildings and structures that will accumulate the dispersed dusts.
Monitoring air quality on a continuous basis can provide better input for safe operation. The process of identifying high attrition points and reviewing alternate methods to suppress dust generation should be carried out as part of any design review. The identification of the sulfur dust deflagration hazard area should be as per U.S. National Fire Protection Association (NFPA) 655.8
Detection. Sulfur fire can be detected visually by its yellow plume. Temperature transmitters placed in the vapor space will alert the operator of the rate of temperature rise.
A 2°F/min–5°F/min rise is an indication of fire inside a confined space like a pit and tanks. Then, sealing steam can be opened to prevent air ingress and further acceleration of the fire.
A temperature indicator shall be placed in the vapor space; an immersed temperature indicator will not provide the surface temperature or vapor temperature. SO2 detectors placed inside the confined place can indicate a sulfur fire, as SO2 is produced as a product of sulfur fire. Upon the detection of SO2, air flow (sweep air) can be stopped to extinguish the fire inside the tank or pit.
Early warning signals. H2S detectors placed in the confined space will alert operations about any risk of fire [an alarm set at 25% of LEL (i.e., 0.75 vol% at operating temperature)]. Sweep air flow monitors (flow transmitters) will provide an early warning signal about the fire since low flow may be due to plugging or positive pressure (due to fire) inside the tank or pit. It is wise to carefully monitor the sweep air flow, vapor space temperature, SO2 detectors and H2S detectors to safeguard the assets from sulfur fire. Note: the sealing steam or snuffing steam temperature should not exceed the flashpoint of the sulfur.
Hazardous area classification. Sulfur dust area classification should be carried out as per NFPA 655.8 All sources of release must be considered, including the duration of release during startup, normal operation, shutdown, maintenance, regeneration and other operating scenarios.
Three basic grades of release are classified in TABLE 4. Continuous grade is defined as a release that is continuous or expected to occur for long periods. Primary grade is defined as a release that can be expected to occur periodically or occasionally during normal operation. Secondary grade is defined as a release that is not expected to occur during normal operation and, if it does, is likely to do so only infrequently and for short periods.
The relationship between the frequency or probability of leak and the classification of zone/division are provided in TABLES 4–6.10,11,7
TABLE 5, which is based on rule of thumb on horizontal surfaces, can be used in hazardous area classification. NFPA 655,8 Table A.4.4.2 offers guidance for area electrical classification, which is summarized in TABLE 6.
The temperature class of the electrical components and instruments for the sulfur dust area should be carefully analyzed. The auto ignition temperature (AIT) of sulfur dust is 220°C. As per International Electrotechnical Commission (IEC) 61241/60079, the maximum permitted surface temperature must be reduced with respect to dust layer thickness. The maximum permissible surface temperature is determined using the Eqs. 1 and 2; the lowest value of these equations should be used for engineering.
Tmax = 2/3(TAIT) = 146°C
Tmax = TAIT − 75 K = 145°C (5-mm thick sulfur dust layer)
During temperature rating, it should be noted that sulfur will become liquid above its melting point of 120°C. Therefore, the maximum surface temperature requirement for sulfur dust layers is not applicable. However, the applicability of the temperature rating for sulfur dust prescribed by IEC 61241/60079, which holds true for combustible dust, should be reviewed in detail.
Housekeeping. Housekeeping requirements are highlighted in NFPA 655. For safe solids-handling plants, housekeeping plays a critical role in the prevention of dust explosion. Dust accumulation in remote areas, on top of structures and in inaccessible areas must be removed periodically by suitable methods. Scheduled and unscheduled housekeeping tools, intervals and threshold accumulation are outlined in NFPA 6558 and NFPA 49911.
Waste management. A remelt pit should be designed to handle the dust collected through the dust collection system, any spillages, off-spec product, etc., and recycle it back to the liquid sulfur system. Dust scrubbers/wet scrubbers can be used to limit dust emissions to atmosphere during the liquid sulfur granulation process. The industry-accepted sulfur dust emissions concentration is < 40 mg/m3.
Artificial intelligence (AI) and digital twins. As industry is transforming from automation to an autonomous regime, AI-powered digital twins enable and improve the availability of the asset, optimize process parameters and reduce downtime. However, the issue of data sovereignty maintains prime strategic importance. With data being produced from simulations, design data, plant operations, vendor data and a plethora of other sources, finding the right balance between internal control and external access must be carefully defined. Interaction between digital twins in upstream units (sulfur recovery units, sulfur storage units), utility units (DM water, cooling water, instrument air, etc.) and downstream units (granulated sulfur storage, railcar loading units, ship loading units, etc.) must be carefully evaluated and control/access defined and restricted.
AI enables operators to maximize the utilization of plant assets and develop better preventive maintenance strategies to minimize plant downtime and improve plant availability. All day-to-day activities, such as equipment monitoring, process variable monitoring, emissions control, personnel management, maintenance plan development, spare management, product/raw material storage and movement, job safety analysis, work permit issuance, real-time data analysis, trend analysis, etc., can be carried out with or without minimal human intervention.
Takeaway. The safe, efficient and economical transportation of liquid/molten sulfur presents unique challenges that can be addressed by adopting good engineering practices, carrying out meticulous design and engineering in line with guidance available in NFPA 655 and NFPA 499, and applying principles of IEC 61241/60079. HP
ACKNOWLEDGEMENT
The author acknowledges the encouragement, support and guidance of Ramalingam Muralitharan, Senior Manager – Process, Wood India Engineering & Projects Pvt. Ltd.
LITERATURE CITED
CHANDRAGUPTHAN BAHUBALI is a Chief Process Engineer at Wood India Engineering & Projects Pvt. Ltd., Chennai, India (previously Amec Foster Wheeler India Pvt. Ltd.), and has 20 yr of post-graduate experience in oil and gas projects. He has also executed numerous technically challenging complex projects for global energy giants. Bahubali holds an MS degree in refining and petrochemical engineering from the University of Petroleum & Energy Studies and a BS degree in chemical engineering from Madras University, India. He has successfully completed many AIChE certificate courses in process safety, as well as many international certification courses and training from Harvard and the London Business School. Bahubali has authored numerous papers on flow assurance, gas hydrates, fixed-bed reactors, retrofit engineering and economics. The author can be reached at c.bahubali@woodplc.com and b.chandragupthan@gmail.com.