Jun-2022

Digital twin optimises FCC operations for real separator behaviour

Digital twins based on process simulation models are invaluable for overcoming limitations in the design or operations phases to optimise plant profitability.

Rodolfo Tellez-Schmill, KBC (A Yokogawa Company)
Tom Ralston and Wim Moyson, MySep

Article Summary

Separation processes play a crucial role in the oil and gas upstream, midstream, and downstream sectors. For example, in refineries, the overhead vessels and the crude distillation and vacuum distillation units separate hydrocarbon reflux from sour water and overhead vapours. In many cases, excessive liquid carry-over influences product quality or yield. To assure appropriate product yields, treatment and conversion units employ two- and three-phase separation processes critical to proper operation of such units.

Other important separators include the compressor suction scrubbers or knock-out (K.O.) drums. The proper design of these devices is necessary to ensure the operational integrity of the compressors. Common concerns include excessive carry-over and large droplet size in the suction gas. Often, droplet size criteria are met while the flow rate of liquid carry-over is excessive or vice versa.

In most oil companies and engineering, procurement and construction (EPC) contractors, separation expertise is either limited or absent. Therefore, in the design phase of new facilities, oil companies and EPC contractors rely heavily on vendors for vessel sizing and/or vessel performance estimates. Additionally, the industry traditionally lacks unified separation vessel design practices. As a result, the design and sizing of vessels rely on various traditional in-house standards, rules-of-thumb, and disparate ‘spreadsheet tools’. Frequently, in-house tools provide criteria based sizing of vessels and internals, but cannot predict carry-over performance. This means engineers can produce designs without appreciating the extent to which the separation equipment will fulfil its purpose. On the one hand, vessels may be oversized. On the other hand, they may have insufficient capacity to handle off-design conditions or process upgrades.

Software tools
Due to the complexity of many oil and gas process operations, insightful engineering teams are keenly aware that an appropriate digital twin is vital to achieving key business objectives, including:
- Improve profitability with assured ROI by increasing operating margins while reducing expenses
- Better facility management, production planning, and decision making from a holistic view of facility performance
- Meet and enhance unit production targets through continuous process unit monitoring
- Identify system bottlenecks and major operational risks
- Devise possible debottlenecking strategies with corrective actions

Process simulation provides a powerful platform for designing, monitoring, and optimising refinery and petrochemical operations. Recent developments in simulation technology have improved the accuracy and user-friendliness of these tools. The Petro-SIM process simulator is well suited to building digital twins because it provides meaningful data regarding the efficiency and effectiveness of plant operations at an asset level.

In addition, MySep software is adept at simulating the performance of separation equipment. For design, the software guides engineers to follow sound practices to assure performance. For evaluating existing equipment, it brings to bear proprietary incremental modelling to predictions of carry-over. KBC and MySep have partnered to combine the strength of Petro-SIM’s process simulation with MySep’s rigorous separator modelling. The combination of these tools help operators mitigate risks and optimise operations to ensure the following:
- Efficiency: They provide a complete detailed representation of the plant to assess the interactions between various units and asset groups
- Accuracy: They use rigorous thermodynamic packages and research-validated ratings. The model predictions can be used with confidence, even when extrapolating them to new conditions and feedstocks
- Better decision-making: Petro-SIM’s time series function enables process engineers to run a series of steady-state simulations and observe the long-term impact on operations
- Cost savings: Engineering, operations, training, planning, and capital improvement projects use one model. All key stakeholders use the same technology to streamline work processes

Case study
In a refinery FCC unit, reactor products enter the main fractionation column (MFC). The side stripper on the MFC produces heavy naphtha and light cycle oil. Then, light gasoline and light hydrocarbons in the MFC overhead stream are routed to the gas concentration unit (GCU). Due to the low pressure of the MFC, the overhead stream produces gas that contains a significant concentration of heavy hydrocarbons, whilst the overhead liquid product contains light hydrocarbons. The resulting vapour stream is sent to the GCU with a wet gas compressor for high-pressure recontacting and separation.

Poor separator design and inappropriate selection of internals can cause excessive liquid carry-over. This liquid carry-over propagates through the process, affecting downstream equipment. Ultimately, it can lead to progressive degrading of compressor performance and premature machine failure. Unplanned shutdowns due to equipment failures are associated with significant revenue losses. Shutdown of an FCC unit may incur operational losses of up to $1.5 million per day. Loss-risk of such a magnitude can be mitigated with a moderate investment in a high- fidelity digital twin capable of simulating all key equipment.

Figure 1 presents a basic flow diagram of the MFC and GCU systems in the FCC unit. Gas from the GCU is compressed and combined with primary absorber bottoms and stripper overhead gas. This combined stream is then cooled and sent to the high-pressure receiver. Gas from this separator is routed to the primary absorber.

Based on economic analysis and production planning, the operator modified the production targets of the FCC unit. The plan included an increase of the throughput by 15% (Case A), which is 5% above the design capacity. Additionally, more propane and light product would be produced, reflecting a weakening market for naphtha whilst the market for petrochemicals was seen to be strengthening. The strategy involved increasing the ZSM-5 catalyst addition to the existing inventory (Case B) and increasing the riser outlet temperature to 540áµ’C (1004áµ’F) to increase conversion (Case C). Rating calculations were required for all equipment, including separators around the MFC.

This case study investigated the performance of the following four separators in the FCC unit:
1. The condensate receiver of the main fractionation tower
2. Two compressor K.O. drums in the gas plant
3. The high-pressure separator

To accommodate the increase in gas production from the MFC, the operator added a third compression train, identical in size and capacity to the two existing ones.
This case study was conducted to determine the adequacy of the existing equipment, particularly the overhead condensate receiver, both compressor suction K.O. drums, and the HP separator. A key operational requirement involved limiting both the maximum droplet size and excessive volume of entrained liquid. These efforts helped prevent cumulative damage to costly rotating equipment and minimised the risk of an unplanned shutdown.