S. Donson, Integrated Global Services, Rayleigh, England, UK; and A. KOROBEINIKOV, Integrated Global Services, Brno, Czech Republic
Inefficient heat transfer in the convection section of a process heater is readily evident by an increase in stack temperature beyond design parameters. The timing and urgency to address this situation are related to not only the degree of overheating in the stack, but also product demand, economics and fuel costs.
This article will discuss the author’s company’s solutiona approach to restoring heat transfer efficiency of fired heater furnaces and the results achieved at an Egyptian oil refinery.
Inspection and technical evaluation. For many projects that involve the company’s solutiona, a site visit is made to inspect the convection section and collect current operating data so an evaluation can be conducted. The evaluation determines the baseline operating conditions and estimates the potential project scope and expected benefits.
The evaluation is based on calculating the external fouling resistance factor for each bundle in the convection section based on the process data. The resistance factors will be relieved by cleaning and a further evaluation determines the post-cleaning performance, based on a constant processing duty, for example. The cleaning technologya allows for > 90% of all the tube and fin surface area to be treated, unlike other more conventional methods that reach only 20%–40% of the tube surfaces, depending on configuration.
Project planning and execution approach. To perform an effective cleaning using a unique remotely operated vehicle (ROV) and protect the existing equipment from water exposure, the following five steps are considered for each project (FIG. 1):
The ROV is designed to fully clean convection bank coils by penetrating its lance deep between tube rows. The technologya removes more than 90% of fouling from all convection bundles. No refractory is damaged since the ROVs are programmed to direct a high-pressure medium to the tubes only. All activities typically can be completed in 72 hr–120 hr (three to five 12-hr shifts).
Post-project benefit analysis. In many cases, the stack temperature is determined to be a key performance indicator (KPI) to identify and quantify the benefits after cleaning. To illustrate the benefits, a recent project referred to as Project X is reviewed here. On completion of the project, a sizeable benefit of 40°C (104°F) in stack temperature reduction was achieved (FIGS. 6–8). The plant also reported an average increase in overall fuel efficiency from 89.5% to 91.5%, leading to 2 MW less heat loss to the stack under the same operating capacity. The customer reported a payback period of less than 4 mos.
Results and new data 2 yr after cleaning. A balanced heat distribution between sections is crucial in coking-sensitive services. Maintaining a clean convection section surface not only helps to save fuel and increase steam generation, but also positively influences the steam cracking process. The achieved benefit on Project X was 16,000 MWh/yr in fuel savings and a reduction of 2,500 tpy of carbon dioxide (CO2) with a fuel mixture of methane (CH4)/hydrogen (H2).
After almost 2 yr, the client reported a slight elevation in stack temperature of 10°C (50°F) and a stable efficiency increase of 1.5%. The next cleaning is anticipated to take place after 6 yr to reinstate the efficiency. To ensure the long-lasting benefits, the author’s company also recommends a combination of the ROV cleaninga with proprietary ceramic refractory coatingsb to protect and encapsulate the ceramic fiber and stop refractory deterioration and new fouling formation on the outside surface of the convection tubes.
Example of ROV cleaninga: A steam cracker furnace. The author’s company was provided with furnace configuration and process parameters for each bank of the convection section of this ethylene furnace. The objective was to keep the process duty—and thus, the coil outlet temperature—the same and then compare scenarios before and after.
For this scenario, constant flowrates for process streams are assumed, but it is also possible to consider all the factors and heat balance changes for the entire system (TABLE 1).
Example of ROV cleaninga: A CCR platforming heater. In configurations where a steam generator is located only in the convection section (e.g., platforming heaters), it is crucial to adjust flowrates and reflect changes in the absorbed duty for each bundle that may result from cleaning. Specifically, a steam drum should be included in the model to respond accurately to all changes in temperatures/pressures of inlet/outlet streams.
The convection section detailed in TABLE 2 reflects the most widespread design with the following bundles (from top to bottom): economizer, upper steam generation, steam superheating and lower steam generation (example under the same firing rate).
CASE STUDY—REVITALIZING THE PERFORMANCE OF A REFINERY’S H2 GENERATION UNIT
An Egyptian oil refinery in operation since 1999 has faced challenges with its H2 generation unit (HGU) since 2005. Issues such as hot spots on catalyst tubes, aging reformer tubes and outlet systems, and reduced H2 demand have led to the unit operating at a reduced capacity. To address these bottlenecks and evaluate the unit's status, a comprehensive assessment and debottleneck study was conducted by the original equipment manufacturer (OEM).
Key study findings. One significant finding revealed the underperformance of the convection coils, which hindered the unit from achieving its desired efficiency. Throughout more than 20 yr of operation, the convection coils—primarily consisting of finned tubes—suffered from increased fouling due to inadequate inspection and cleaning practices. The study recommended a potential solution of inspecting and robotically cleaning the external surface of the finned tubes to overcome this issue.
Project overview. On the advice of the OEM, the author’s company was contracted to perform a convection section performance recovery servicea at a H2 production unit at the refinery (FIG. 9). The project involved increasing the size of six existing access doors in the convection section and the robotic de-fouling of convection coils.
The project commenced on March 29, 2023 and was completed on April 3, 2023. The original planned scope of work, which included de-fouling and door installation, remained unchanged throughout the project.
Safety. Safety is a primary concern for the author’s company, and a robust safety program was implemented to ensure a safe working environment for all personnel involved. The company maintains a zero-incident safety philosophy and actively promotes a culture of safety among its employees. Daily toolbox talks, safety observations, unit walk-downs and job safety audits were conducted to mitigate potential hazards and maintain safety standards throughout the project.
The company has a strong safety track record, with a total recordable incident rate (TRIR) of 0.0 in 2022, well below the industry average. The company adheres to U.S. Occupational Safety and Health Administration (OSHA) best practices and local safety regulations to ensure compliance and maintain a safe work environment.
Quality. Stringent quality control standards are followed to meet customer requirements. The project was executed in accordance with the company’s quality control standards, and a quality control package (QCP) was agreed upon before the start of work.
Achieved results. A performance test run was then conducted to evaluate the unit's condition after the cleaning process (FIGS. 10 and 11). The unit's capacity was successfully raised to 100% on April 16 and maintained for 24 hr. The test procedure for evaluating the unit's performance after cleaning was based on the HPU's latest probation test, ensuring consistency and comparability. Data from various sources, including distributed control system (DCS) data, laboratory analysis results, outside field/local data and electrical data were collected during the test to accurately assess the unit's performance.
Test parameters. The performance test for the HGU lasted 24 hr. The main feedstock for the unit was natural gas, supplemented by a small portion of recycled H2. The composition of the natural gas feed, as well as mass flowrates for different streams, were recorded. The laboratory analysis results showed changes in the feed composition and products during the probation test.
Unit operating parameters. Various operating parameters of the unit—such as temperatures, pressures, steam-to-carbon ratio and steam drum pressure—were monitored during the probation test. The unit's performance was compared to previous tests, revealing improved performance in terms of duty recovered by the convection section and a reduction in stack temperature.
Probation test evaluation. The evaluation of the probation test results indicated that the cleaning of the convection section had led to a 14% increase in duty recovered from flue gases compared to the previous test in December 2021. The reformer inlet temperature also increased from 437°C to 501°C, contributing to improved fuel efficiency. The stack temperature reduction of 22°C and increased efficiency resulted in cost savings of approximately $220,000/yr.
Case study conclusions. The successful cleaning of the convection section in the refinery's HGU unit marked a significant step towards restoring the unit's performance. The removal of fouling from the finned tubes facilitated enhanced heat absorption and a reduction in stack temperature, thereby improving overall thermal efficiency. This achievement represents a noteworthy milestone in the refinery's ongoing expansion project and contributes to its long-term operational success.
The author’s company also provided technical recommendations for future maintenance and improvement, including the application of refractory coatingb to prevent refractory fouling. The conclusion of the project underscores the collaborative efforts between the oil refinery and the company, as well as the positive working relationship among all team members.
Takeaways. Project execution excellence (as well as certainty in terms of % clean surface) and the ability to rigorously evaluate the future performance of the heater system provides the company with an opportunity to offer unique services to clients. This combined approach reflects the increased demand for thoroughly conducted feasibility studies, even for small projects. Moreover, it is essential to quantify the influence (if any) of all company products on fired heater performance. HP
NOTES
a Integrated Global Services (IGS’s) Tube Tech
b Cetek ceramic refractory coatings
SCOTT DONSON is VP – International Business Development and Technical Operations, for Integrated Global Services (IGS). He has a wealth of experience within the petrochemical industry, with more than 25 yr at Tube Tech and 30 yr in the industry in total. With an abundance of technical knowledge, Donson is an expert in heat exchanger cleaning services, representing an industry-leading fouling removal contractor, Tube Tech Industrial, an IGS solution.
ANTON KOROBEINIKOV is a Senior Process Engineer with Integrated Global Services—focused on the Cetek, Tube Tech and Hot-tek Product lines—and is responsible for the process modeling, data analysis and achieved benefit verification. He holds BSc and MSc degrees from Gubkin University, both in chemical engineering. Korobeinikov began his career as a plant operator and then moved to another company to start his process engineer career path in vertical integrated and service companies.