Suncor's Optimization Of A Sulfur Recovery Facility
Suncor Incorporated Resources Group acquired the 7-22 Progress Gas Plant, located in Spirit River, Alberta, Canada, in 1996. Due to the extremely competitive nature of the natural gas business, all gas processors, including Suncor, are obsessed with reducing operating costs. As part of Suncor's cost reduction program, a close examination of their sulfur recovery unit by Suncor and Gas Technology Products resulted in significant cost savings by optimizing the method of operation.
The 7-22 Progress Gas Plant processes 20 MMSCFD of natural gas at approximately 950 psig. As shown in Figure 1, the processing train consists of an MDEA amine unit for removing CO2 and H2S, a glycol dehydrator and an Autocirculation LO-CAT®® unit for removing the H2S in the amine acid gas prior to exhausting to the atmosphere.
The LO-CAT® process is a proprietary process, which converts H2S to elemental sulfur by employing a patented, environmentally safe, multichelate, iron catalyst. The overall reaction (Rx 1) of the process is the modified Claus reaction; however, the mechanism for achieving this reaction is much different than that which occurs in the Claus process.
The LO-CAT® process is a liquid phase, ambient temperature process, which is in contrast to a Claus unit, which is a gas phase, elevated temperature process. The LO-CAT® reactions can be separated into the Absorption Section and the Regeneration Section. In the Absorption Section, the H2S is absorbed (Rx 2) from the gas stream into the circulating aqueous (aq.) LO-CAT® solution. Once absorbed, the H2S ionizes (Rx 3) into hydrogen and hydrosulfide ions, and the hydrosulfide ions then react (Rx 4) with ferric ions to form elemental, solid sulfur and ferrous ions.
The ferrous ions are not capable of being reduced any further; consequently, to have a continuous process they must be oxidized back to the ferric state. This is accomplished by sparging air through the effluent solution from the Absorption Section of the process. Oxygen is absorbed (Rx 5) into the solution, which then oxidizes the ferrous ions (Rx 6) back to the ferric state.
Adding reactions 2 through 6 yields reaction 1 with all components in the gaseous phase with exception of the sulfur, which is in the solid phase.
The LO-CAT® unit employed at the Suncor plant was installed in 1990 and is of the patented Autocirculation design, which is shown in Figure 2. In this configuration, the Absorption and Regeneration Sections of the process are located within the same vessel. The acid gas from the amine unit is sparged into a centerwell (Absorber), which is nothing more than an open-ended pipe located within a cone-bottom tank. The Absorption reactions (Rx 2 – 4) occur within the centerwell. The regeneration air is sparged into the solution outside of the centerwell where the regeneration reactions (Rx 5 & 6) occur. Since there is considerably more regeneration air than acid gas, the solution outside of the centerwell is more aerated (less dense) than the solution inside of the centerwell. Consequently, this density difference creates a natural liquid circulation from the Regeneration Section of the vessel to the Absorption Section. Thus liquid circulation is established without the need of pumps.
Sulfur settles out of solution into the cone section of the Autocirculation Vessel where it is removed as a 10 wt% to 15 wt% slurry. The sulfur slurry is heated to approximately 120o C by heat exchange with hot oil, and the solid sulfur is melted. The molten sulfur and LO-CAT® solution are then separated, with the solution being returned to the process. The melter system operates under approximately 3.5 barg pressure to prevent the aqueous solution from boiling. The molten sulfur is allowed to solidify, and is then hauled to a landfill. Due to the remoteness of the plant and the relatively small amount of sulfur being produced (2.2 MTPD), it is more economical to dispose of the sulfur at the local landfill than to sell it to a remote user. Trucking the sulfur to a remote user will cost more than what the sulfur is worth.
Even though water is produced in the process, a small amount of makeup water is still required for the LO-CAT® unit to compensate for the moisture consumed in saturating the regeneration air. The plant originally used well water for this purpose; however, the plant personnel discovered that some of the problems they were experiencing in the unit, such as occasional foaming, disappeared when they switched to deionized water. Obviously, the well water contained surface active components which were conducive to foaming. In general, LO-CAT® units only require water of potable quality.
An Autocirculation LO-CAT® unit is an extremely simple system to operate. In essence, the unit consists of a cone-bottom tank through which air and acid gas are sparged through an aqueous solution of chelated iron. A small slip stream of concentrated sulfur slurry is pumped through a hot oil heat exchanger with molten sulfur being withdrawn from the system, and solution being returned to the cone-bottom tank. Operators' attention is only required to test the solution for pH and Redox potential once or twice a day.
Operating Costs Associated with the LO-CAT® Unit.
There are two major sources of operating costs associated with a liquid redox process such as LO-CAT®. They being the costs related to chemical makeup and those related to power consumption. The major components of the power costs for the Suncor unit involved two, rotary lobe blowers (65 KW/each) used for blowing air into the process, and the major components of the chemical makeup costs are associated with iron and chelate makeup's and caustic addition to the unit.
Chelates are water soluble, organic compounds which hold the iron in solution. By employing the proper chelate, the solubility of iron in water can be increased from a few parts per million to the 5 wt% range. The chelates will, over time, oxidize by a free radical mechanism and require replacement. The rate at which they oxidize can be controlled by utilizing a stabilizing agent, which acts as a free radical scavenger.
The LO-CAT® process utilizes a proprietary means of reducing chelate degradation. Within the process, a small portion of the H2S is deliberately converted to thiosulfate ions (S2O3=), which are excellent free radical scavengers. In fact, many years of research by Gas Technology Products have failed to result in a better free radical scavenger. By being able to generate thiosulfate within the process, chelate replacement costs can be held to a minimum with minimal effort.
Iron is lost from the system by physically removing solution from the unit. For LO-CAT® systems with sulfur melters, the major means of iron lost is by the blowdown of solution from the unit to control solution specific gravity.
The specific gravity of a LO-CAT® solution is dependent upon the amount of salts, which are dissolved in the solution. Since LO-CAT® operates at relatively low iron concentrations, the chelated iron is not a major contributor to the density of the solution. This is in sharp contrast to other redox processes. In a LO-CAT® unit, the major contributors to solution density are carbonate/bicarbonate salts and thiosulfate/sulfate salts. Carbonate/bicarbonate salts are formed when gas streams containing CO2 are being processed. Depending on the CO2 partial pressure and the desired pH of the solution, CO2 will develop equilibrium concentrations of carbonate and bicarbonate as follows:
The equilibrium constants for each reaction are pH dependent, increasing with increasing pH. Although the Suncor LO-CAT® unit is processing an acid gas stream containing a high concentration of CO2, the system is operating at atmospheric pressure, which yields a relatively low CO2 partial pressure. Consequently, the carbonate/bicarbonate concentrations in the solutions are also low.
As previously discussed, thiosulfate formation is desirable as a means of controlling chelate degradation. However, if solutions become too oxidizing (high oxidation/reduction potential), thiosulfate can be oxidized to sulfate, which has no beneficial value in the process. The formation of thiosulfate and sulfate occur as follows:
It is important to note that the above reactions (Rx 7 – 11) all produce acidic products (H+). Consequently, to maintain the solution in the slightly alkaline range, which is required to promote good absorption of H2S, caustic in some form (KOH, NaOH or NH3) must be added to the solution. The thiosulfate/sulfate salts also contribute to increasing the specific gravity of the solution.
If the concentrations of dissolved salts are allowed to increase unabated, a point will eventually be reached when one or both of two things will occur. First, the solution will become less capable of absorbing oxygen and H2S, which will affect the ability of the process to remove H2S from a gas stream, and second, the solution will eventually become saturated and salts will actually start precipitating from the solution resulting in plugging problems. To prevent this from happening, a small blowdown stream is taken. For systems, which have a filter for removing sulfur from the unit, a sulfur cake is produced, which usually contains sufficient solution to compensate for salt formation; however, in a system, which produces molten sulfur such as Suncor's, a liquid blowdown stream is usually required. These blowdown streams represent a lost of iron from the process, which require makeup iron to be added.
Unless there is a hazardous or toxic component in the gas stream being treated, which is soluble in aqueous solutions, the blowdown liquid will be merely water containing dissolved salts; however, it will have a chemical and biological oxygen demand due to the chelates. Consequently, there is a cost associated with treating the blowdown stream. In Suncor's case the blowdown is hauled away for remote treatment since there are no onsite wastewater treatment facilities.
Prior to Suncor's purchase of the Spirit River facility, little attention was given to the LO-CAT® unit. In general, over injecting chemicals into a LO-CAT® unit will not result in operating problems, just higher than required operating costs. Consequently, in remote gas plants such as Spirit River, it is sometimes difficult to convince operators to change operating conditions if the unit is not presenting any operating problems. However, this is not Suncor's operating philosophy.
Upon taking ownership of the facility, Suncor contacted Gas Technology Products and stated that the operating costs associated with the LO-CAT® unit were not cost effective and something had to be done about chemical consumption, power consumption and the disposal cost of blowdown liquid or the unit would be shutdown. At this point, Suncor and Gas Technology Products started working closely together to optimize the system.
The Suncor LO-CAT® unit is operating at close to the design conditions of 123.4 Nm3/Hr of acid gas containing 57.5% H2S (2.19 MT/day of sulfur). As suspected, an initial examination of the unit and a review of historical operating data show that the chemical addition rates were much higher than the original design rates, and that the operators were maintaining a lower solution specific gravity than required by maintaining a higher than normal blowdown rate. For example, the iron concentration was being maintained at more than twice the design level while the chelate concentration was more than three times the design level. In addition, the system was designed to operate with one air blower in operation; however, the unit was being operated with both air blowers in operation. These conditions resulted in a very positive, solution oxidation-reduction potential (redox potential), which promoted the formation of sulfate rather than thiosulfate resulting in a relatively high chelate oxidation rate. Consequently, this highly oxidized solution state resulted in high salt formation and thus a high blowdown rate to maintain the less than design solution specific gravity. Consequently, the high blowdown rate resulted in a high iron replacement rate and higher than required solution disposal costs. The unit was also experiencing symptoms such as occasional sparger plugging, which indicated that the solution was close to saturation.
The obvious remedy to Suncor's problems was to
get the solution chemistry back to a normal state. This was accomplished by
discontinuing the chemical addition of the makeup chelate and the makeup iron.
In addition, one air blower was shutoff, thus immediately reducing power
consumption by 50%. During this period, GTP monitored the solution chemistry on
a biweekly basis. Over a relatively short period of time, the iron, chelate, and
stabilizer concentrations and the solution specific gravity were bought into
line. At this time, the chemical addition and blowdown rates were
re-established, but at rates corresponding to actual processing conditions of
the unit. When the unit was operating at design conditions, power consumption,
chemical costs and disposal costs were reduced by approximately 50%, thus
achieving operating costs, which were in line with Suncor's requirements. In
addition, problems with sparger plugging were greatly reduced. All of these
reductions were achieved while maintaining an H2S
removal efficiency of greater than 99.99+%. Table 1 summarizes the before and
after operating costs.
It is important to note that these cost savings were obtained by simply turning pumps and blowers off. No daily detailed chemical analyzes were required, just simple redox potential, pH and specific gravity readings. And although it took years to get the LO-CAT® solution in a state of near saturation, the situation was remedied in a matter of weeks.
Although dramatic reductions in chemical costs were achieved at the Suncor facility, even more chemical savings can be realized in more modern LO-CAT® units due to the ability to more accurately control thiosulfate concentrations.
A determined commitment to teamwork by Suncor and Gas Technology Products has resulted in a significant reduction in plant operating costs in the LO-CAT® unit while maintaining high H2S removal efficiency and ease of operation. As summarized by the plant operation staff– "Chemical consumption and cost savings are important to Suncor. The ultimate is to remain emission free while running as economical as possible. This process can run emission free which will in turn guarantee the safety of the plant operators and our environment."