Sulfur Plant Management in a Petroleum Refinery
Debopam Chaudhuri
Process Design Engineer
Fluor Daniel India Pvt Ltd

Srinivasa Oruganti
Department Manager - Process
Fluor Daniel India Pvt Ltd

The Crude slate is gradually widening. While at the same time the Sulfur specs and emission norms around the world are getting more stringent. In spite of the thrust for renewable and other alternate energy sources there will be a steady growth in fossil fuel in the coming decades. Hence there is an inherent need to have a closer look at the Sulfur Recovery Process in existing and new Refineries. Technology developments are being done at Licensors' end, but Refiners are also looking for optimization in design with adequate flexibility in operation.

Typically, while the refinery configuration gets finalized based on the available crude slates and the market demands of the products, a few important decisions need to be made regarding the Sulfur Recovery Unit. These are linked mainly with respect to SRU Plant capacity, configuration, technology and product management. This paper will highlight and discuss these issues; and provide means and methods in making these decisions early in the project lifecycle.


The environmental effects of burning hydrocarbon fuels have led most world governments and environmental bodies to impose stringent specifications on the sulfur content of hydrocarbon fuels. And these emission norms are gradually getting more and more stringent, requiring tighter specifications for hydrocarbon products. The chart on the right clearly demonstrates how in the last two decades the sulfur specifications have become tighter, while at the same time the crude slate is gradually shifting towards high sulfur crudes.

This creates additional challenges to design and improve the treatment process of various distillates but also develop and implement better means and methods to recover that sulfur. Hence the proper design of a Sulfur Recovery Unit (SRU) becomes vital from the very initial stage of a project. From the very inception of the project, when the blueprint of the refinery is being prepared based on the available crude slates and the planned products as per the market demand, a few key decisions need to be made with respect to the design of the SRU. This holds good for Refineries planning to undergo a revamp either to cater to a wider crude slate or to meet the newer emission norms. An early and proper judgment of these important parameters provides a smoother path of engineering and beyond in the lifecycle of the project. This paper intends to discuss these parameters and provides guidelines to find answers to these questions.



























The Sulfur Recovery Process:

The following simplified flowchart demonstrates how sulfur travels in a Refinery starting from the crude oil till getting recovered as liquid sulfur .



























The sulfur compounds present in the crude ultimately gets displaced from the hydrocarbon phase as H2S and that is captured by an amine solvent in the amine treatment units or gets dissolved in the water. The amine circulates in the Refinery in a closed circuit, capturing the H2S from the hydrocarbon phase in various amine treaters. This rich amine(rich in H2S) is then regenerated in the Amine Regeneration Unit (ARU) to liberate the H2S gas and generating lean amine (lean in H2S) to be circulated back to the various amine treaters. Similarly, the sour water generated from the various process units in the refinery is treated in the Sour Water Stripping Unit (SWSU) to liberate the H2S gas. The combined stream of the H2S rich gases from ARU and SWSU is then sent to the Sulfur Recovery Unit (SRU).

Most SRUs at present utilize the modified Claus Process for recovering sulfur . In the modified Claus process the H2S is burnt with sub-stoichiometric amount of air to generate SO2, which in turn reacts with the unconverted H2S to produce elemental sulfur. It is imperative to maintain the ratio of H2S and SO2 at 2:1 for maximum recovery of sulfur. The governing equations are :

3H2S + O2 = 2H2S + SO2 2H2S + SO2 = 2H2O + 2S

The reaction first takes place in a thermal reactor (Claus Furnace) followed by a series of (typically two or three) catalytic reactors. The unrecovered sulfur in the Claus tail gas may be further processed to achieve a very high degree of overall sulfur recovery. There are alternate processes available to recover the last bits of sulfur from the tail gas. And ideally, the final stream of gas after the tail gas treatment containing ppm levels of H2S is finally burnt in an Incinerator furnace.

The sulfur produced in the Claus section of the SRU is cooled and liquefied in Sulfur Condensers, and collected as liquid sulfur in pits. This product sulfur needs to be degassed to remove dissolved H2S before it can be sent as the final product. Beyond this simplified definition and description of the sulfur recovery process, there are numerous variations where a wrong decision made will lead to an un-optimized sulfur recovery process. This may ultimately lead to a bottleneck in the design of the entire refinery.

The Technical Considerations:

This section will discuss about the most defining parameters of the Sulfur Recovery Unit (SRU) and will assist in achieving the most optimized solution .














































































1. The Plant Capacity

This is one of the major decisions that need to be taken at the very beginning of any project; the plant capacity of the SRU. Once the overall mass balance of refinery is being set up based on all the available crude slates, the sulfur balance of the refinery may easily be calculated from the overall material balance of the sulfur. For that, typically, the worst feed(crude oil feed with the highest amount of sulfur) is selected, and a material balance of the sulfur is prepared based on the allowable product specification of sulfur. The overall mass balance provides with the ball park figure for the Sulfur Recovery Unit plant capacity.

The above statements hold good even for a revamp project initiated by a change in crude slate and/or meeting tighter product specifications. Thus, a new hydrotreating facility or a conversion process for heavies to light and middle distillate initiates the requirement of increase in the sulfur handling capabilities of the existing Refinery. In that case the sulfur balance happens over a smaller section of the complete refinery, and the determination of the plant capacity depends not only on this sulfur balance but also the plant capacity and configuration of the existing sulfur recovery units.

A typical refinery example is provided in the table here. This considers a 9MMTPA refinery processing a mix of high sulfur crude and Low sulfur crude and generating a mixture of fuels and hydrocarbons for downstream processes. A mix of high sulfur crude and low sulfur crude in a ratio of 80-20 is considered for sulfur balance as the worst feed with respect to the sulfur content.

Based on the actual mathematical sulfur recovery capacity, the real capacity of the Sulfur Recovery may be selected by adding a margin. The margin typically is 10% or 15% over the calculated plant capacity. So for the selected plant capacity for this example will typically be 660 TPD, rather selecting a 600 TPD plant.

The train capacity though is intrinsically related to the train configuration of the SRU as has been defined in the next section.

2. Train Configuration

Sulfur Recovery Units in a Refinery almost always is designed in multiple train configurations. The main reason behind this is to have operation flexibility not only for SRU operation, but also for the entire Refinery .

For a large and complex refinery with multiple crude distillation and downstream processing units, the SRU train configuration will in most cases be defined by the shutdown groups that is by the concept of the "mini-refinery". Once the overall sulfur plant capacity has been determined, the sulfur plant capacity for the different shutdown groups is also calculated. And for each of the shutdown blocks, the sulfur plant capacity is compared, and that defines the single train capacity of the SRU. Based on the catalyst replacement cycle, the process units will need a planned maintenance shut down once every 3-4 years. The train configuration allows individual train of SRU to undergo planned shutdown along with the turnover of the "mini refinery" for catalyst change over, maintenance, etc.

The train configuration of the SRU also takes into account the impact on the operating capacity of the refinery when one of the SRU train becomes unavailable due to operational upsets. To have a maximum operation capacity in many large refineries, the SRU trains are designed with a lot of operational margin by having spare trains and designing as 3 X 50% or 4 X 33% train configuration. A thorough economic evaluation is performed before installing such spare capacities in SRU trains by considering the availability factor of the SRU train and the economic loss of the refinery due to operation at reduced through-puts. And as a part of optimization, installing common Claus and Incinerator Air Blowers, or common tail gas treating units for two Claus Units may be considered before sacrificing the spare train concept.

















Hence, for the specific example provided in Table 1 in the previous section, the desired plant configuration would be to select either a 3 X 330 TPD or 4 X 220 TPD of SRU considering maximum operating margins. But if the decision is made based on economic considerations of installed costs, rather than operating flexibilities, then the preferred configuration of the sulfur plant would become either 2 X 330 TPD or 3 X 220 TPD. The plant configuration on whether to have 2 trains or 3 trains would mainly be defined by the "mini refinery" concept.

For the said example above though, the data is for a single crude train, hence having a single train of SRU with the required capacity will have no operation flexibility, while designing it for a 2 X 660 TPD configuration will attract a lot of unnecessary installation cost. Thus the prudent option for this scenario would to select a 2 X 400 TPD sulfur plant configuration which will allow for a refinery operation at 70% capacity even with one SRU train operating with optimum additional installation cost.

For revamp projects, the easy solution is to add one or more SRU train in line with the existing capacity of the SRU trains. But in most occasions, the new SRU train configuration is defined by the available plot space and the overall refinery operational flexibility. Though in many cases, modifying to oxygen enrichment technology may provide an even simpler and cheaper option for revamp; as has been discussed in the following section .

3.Technology Selection

The alternate option of Oxygen Enrichment process is gaining importance for retrofitting and revamps sulfur plants as it provides the easiest way to increase the sulfur plant capacity as is evident from the table here which shows how oxygen enrichment allows increment in acid gas handling capability of an SRU theoretically. Oxygen enriched operation reduces the amount of nitrogen entering the process and hence allows acid gas to replace it while keeping the total through put of gases same; and hence the pressure drop in the system similar. This drastically increases the plant capacity as the extent of oxygen enrichment increases. For revamp units, shifting from a standard straight through process to low-level oxygen enrichment technology(oxygen < 28%) will significantly increase the plant trough put (10-30%) with hardly any other modification in the entire plant. Utilizing medium level (28-45%) or high level (> 45%) oxygen will increase the plant capacity quite drastically (up to 75% for medium and up to 150% for high); but it also will lead to considerable modifications in the existing plant (new burner designs, and additional equipments). But in any case the capital investment will be a very small percentage of installing a new train of SRU (around 20%). The requirement of managing the oxygen as a feed to SRU would of course be the additional constraint for this change.

Oxygen enrichment is also a viable option for new installations especially for cases where the plot space becomes very limited and where operational flexibility requires multiple trains with installed spare capacity. As oxygen enriched plants may be easily operated with air only, thus it provides a higher turndown operation with air only operation; and hence higher operational flexibility.

Available Processes: COPE by Air products and GAA-Fluor; Oxy Claus Process by Air Liquide; SURE single combustion, SURE double combustion and SUREMAX by Worley Parsons.

4.The Target Recovery with Tail Gas Treatment for SRU

Typically all medium to mega refinery complexes target a recovery of more than 99% of the sulfur in SRU to meet the stringent SOX emission norms. The straight through Claus Process, even with three catalytic reactors can at max achieve a sulfur recovery of 95-97.5%. So at present the tail gas treatment has become inevitable in all present Refinery SRU, irrespective of the plant capacity. Three main methods of tail gas treatment presently available on a commercial scale are:
  • Sub Dew Point Process - Thistypically utilizes an alumina based catalytic reactor for the conversion of sulfur compounds, H2S and SO2 to sulfur. The reaction happens below the sulfur dew point temperature which increases the yield of the sulfur. The catalysts are typically regenerable upon heating. Available Processes: CBA (Cold Bed Adsorption) from Amoco, MCRC from Delta Hudson, Sulfreen from Lurgi, Clinsulf SDP from Linde, Clauspol from Prosernat.
  • Selective or Direct Oxidation Process - This utilizes a direct and selectiveconversion of the residual H2S in the Claus off gas using a special catalyst(alpha alumina or silica extrudate) in an oxygen rich environment.
  • Available Processes: SuperClaus from Jacobs, BSR Selectox & BSR/ HI-Activity from Worley Parson, Clinsulf DO from Linde.
  • Catalytic Reduction and Absorption Process (amine based TGT) - This utilizes a two step process of first catalytic reduction of the entire sulfur to H2S followed by the absorption of the H2S thus produced with an amine typically in MDEA or a variant of it. The H2S is finally liberated from the amine and recycled back to the feed. This process has the capacity to capture the maximum amount of sulfur from the feedgas .


















Available Processes: SCOT from Shell, RAR from KTI, BSR/MDEA from Worley Parson, HCR from Siritec Nigi, Clintox from Linde.

The relative cost and sulfur recovery achieved in the various processes of the tail gas treatment is shown the table here. This will provide the preliminary guidelines in selecting the tail gas treatment method to strike a balance between the extent of sulfur recovery and the overall economics. But most refineries at present are forced to opt for the Catalytic Reduction and Absorption process to meet the required sulfur recovery (>99.5%) due to the amount of sulfur handled by them.

Very old refineries initially installed the sulfur recovery plants without the tail gas treatment facilities, thus it was having the Claus section only , and the entire tail gases were being incinerated. This was achievable mainly because the sulfur emission specifications were more lenient and this allowed more SOX to pass to the atmosphere. For such units, the best way to improve the sulfur recovery and meet the emission specs is to install an appropriate tail gas treatment unit; ideally an amine based TGTU. These TGT units are flexible to operate and may operate with even very low acid gas turn down flows. Hence for revamp units of such scenarios, it is also possible to have common tail gas treatment units for multiple Claus plants, which has less impact on additional plot space required. Alternate methodologies for revamp units include changing the type of amine used for H2S absorption, or even increasing the concentration of MDEA or the amine used in the unit. This though has only a marginal impact in increasing the overall recovery of sulfur from the Sulfur Plant.

Conclusions

For all Refiners, while the project set up is being done the attention provided to the proper design of an SRU is not always adequate. This is for the obvious reasons that the more important process units naturally demand deeper insights and more concerted efforts from all parties. But if neglected the bottlenecks in SRU design may lead to hindrances in the overall refinery performance and operation.

A proper balance of the sulfur across the refinery followed by selection the train configuration and operational capacity margins set the path for unhindered refinery and SRU design and operation. For revamp projects as well, technology selection, and possible pathways for increasing the target recoveries will provide ways to avoid adding new trains of SRU and hence saving significant project costs.

Sulfur plant management in a Petroleum Refinery is an important subject and is gradually gaining importance; and a few simple yet key decisions when taken early in the life cycle of the project goes a long way towards a smoother execution of the project and better operability of the refinery.

(The conclusions presented in this article are solely those of the author/s, and cannot be ascribed to Fluor Corporation and/or any of its subsidiaries.)