Ethylene furnaces: A guide to optimize convection section performance

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Fouling in petrochemical manufacturing plants, particularly in ethylene plants, has become a primary concern due to its negative effects on plant throughput, energy requirements, environmental implications and product quality.

Process fouling within heat transfer equipment accounts for significant economic losses, costing industrialized nations up to 0.25% of their Gross National Product (GNP).

Fouling in the convection sections of an ethylene plant exacerbates GHG emissions and poses serious safety hazards. The energy intensity of an ethylene cracker exacerbates this problem, with an increase of 50°C above design stack temperatures resulting in an efficiency loss of 1.5-2.0%. Considering a plant section with five operational furnaces, it is estimated that a robotic clean could result in a cumulative saving of $5.44M over a three-year period.

Temperature control and the cracking process

To achieve the correct cracking of hydrocarbons, ethylene furnaces maintain a temperature of approximately 2000°F (1093°C). A narrow band of temperature range is vital for successful cracking. The reactions involved are endothermic, demanding a substantial amount of energy. However, due to the exceptionally fast cracking process, the light feed must be exposed to this intense heat for only milliseconds, allowing conversion to ethylene and propylene without promoting coke formation.

The role of the convection section

The convection section serves as the vital pre-heat mechanism in ethylene furnaces, ensuring the correct temperature range is achieved. Deviating from this range can lead to inefficiency and limited conversion if the temperature is too cold or excessive coke formation if it becomes too hot. Maintaining an optimal flow rate is equally important to prevent overheating the product and generating coke.

Consequences of fouled convection sections

When fouling occurs in the convection section, heat transfer becomes limited, resulting in an inefficient pre-heat process. To compensate, additional firing and fuel consumption are required to reach the desired temperature. This has a knock-on effect and overheats the radiant section, leading to potential complications. The energy intensity of an ethylene cracker is such that a 50°C increase above design stack temperatures, as witnessed in a fouled furnace convection bank can result in an efficiency loss of 1.5-2.0%.

The solution: Automated rover system

The use of pre-programmed robotics offers a safer step-change improvement over more mature manual lancing systems. The use of customized robots allows 90-95% surface area cleanliness of convection banks between every tube row, regardless of fouling levels with cleaning standards verified using real-time digital video capture.

IGS Tube Tech is the only service globally offering such robotic cleaning systems. Its robots use proprietary lance and nozzle technologies which achieve very close tube surface contact in situ.

The versatile robots are used with air and high-pressure water. While very high pressures are involved e.g. up to 1,000 bar (15,000 psi), the total volume of water is much lower than standard hydroblasting.

There is minimal risk of refractory damage as the rotary jet on the lance is specifically angled towards the finned tubes. The robot has sensors which are programmed to stop prior to the refractory wall. The absorption rate of water during Tube Tech robotic cleaning service has been examined in a collaborative report based on trials around the world.

Case study: Tube Tech reinstates heat transfer efficiency of an ethylene furnace

Furnace description

A cracking furnace functionally is divided into three zones: radiant section, convection section and transfer line exchangers (TLEs). TLEs are placed vertically, with cracked gas in the inner pipe (tube side) and boiler water/steam in the outer pipe (shell side). TLEs are waste heat boilers (WHB) placed at the outlet of the radiant coils to rapidly reduce the cracked gas temperature to mitigate secondary reactions by producing saturated very high-pressure (VHP) steam.

Radiant section

The firebox is a rectangular, refractory-lined volume within which the radiant coils are suspended vertically, firing from the bottom. The radiant coil, in this case, is short residence time (SRT), favouring selectivity towards ethylene production. Two fireboxes have a common convection section, so the performance of the convection section influences two radiant chambers at the same time.

Convection section

The convection section is located above two fireboxes. The services of the different heat transfer banks shown in figure 1 include:

  • Mixed feed reheater (MFPH-I);
  • Economizer (ECO);
  • Cold high-pressure steam superheater (CHPSSH);
  • Hot high-pressure steam superheater (HHPSSH);
  • Mixed feed preheater (MFPH-II).

Case description

Over the last six years in operation before cleaning, the stack temperature rose from 190°C to 230°C, equalling 2.0% losses in fuel efficiency.

The purpose of this project was to reinstate the overall fuel efficiency, save fuel and cut emissions. Unfortunately, windows in the convection section were not designed to provide access to any type of fouling removal equipment.

Solution

To perform effective heat transfer restoration using a unique remotely operated vehicle (ROV) fouling removal technology and protect the existing equipment from water exposure, the following three steps have been implemented:

  1. New access openings were created to enable placing an ROV on top of each convection bundle.
  2. The radiant box has been isolated.
  3. All effluent and debris after fouling removal have been collected and displaced to the ground level in sediment containers.

Key benefits

The ROV technology entirely removed fouling from the convection bank coils. The system removed 90%+ of fouling from all convection bundles.

No refractory was damaged since ROVs are programmed to direct a high-pressure medium to the tubes only. All activities were done in 4×12-hour shifts.

A 40°C drop in stack temperature was achieved. The plant reported a two percentage points increase in overall fuel and a payback period of fewer than four months.

Acknowledgments and new data two years after cleaning

Heat distribution between c/s and r/s is crucial in coking-sensitive services. Keeping the convection section surface clean helps save fuel and increases steam generation and positively influences the steam cracking process. The achieved benefit was cca. 16 000 MWh/year fuel savings and 2500 tons annual CO2 reduction (fuel is CH4/H2 mixture). Due to higher crossover temperature (radiant inlet temperature), the residence time will be slightly less, with an additional opportunity to make more ethylene due to enhanced selectivity.

After almost 2 years, the client reported a slight elevation in stack temperature of 10°C and a stable efficiency increase of 1.5 percentage points. The next fouling removal project is anticipated after six years to reinstate the efficiency again.

To make obtained benefit long-lasting, IGS also recommends a combination of the Tube Tech ROV cleaning with Cetek’s proprietary ceramic refractory coatings to protect and encapsulate the ceramic fiber and stop refractory deterioration and new fouling formation on the outside surface of the convection tubes.

Commercial impact of the project

The plant reported a two percentage points increase in overall fuel efficiency from 89.5 to 91.5 on average leading to 2 MW fewer combustion losses. The plant has reported a payback period of fewer than four months. A 40 °C drop in stack temperature was achieved. The plant reported a two percentage points increase in overall fuel and a payback period of fewer than four months.

Final outcome

Optimizing the performance of ethylene furnaces is critical for industries aiming to achieve maximum efficiency in ethylene and propylene production. By maintaining the correct temperature range, monitoring flow rates and mitigating fouling issues in the convection section, facilities can ensure smooth operations, enhance safety and minimize downtime, ultimately elevating their overall production output.

For more information, visit: tubetech.com.

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