March 2019

Special Focus: Petrochemical Technology

Triple-lane layout for enhanced cracking coil performance

Ethylene is a precursor to many chemicals, fibers and plastics that are used in daily life.

Ethylene is a precursor to many chemicals, fibers and plastics that are used in daily life. At most petrochemical sites, the ethylene plant is the mother unit that feeds a number of downstream units. Nearly all ethylene is produced in specially designed furnaces by thermally cracking ethane and longer-chain hydrocarbons.

Cracking furnaces are complex equipment and consume a large portion of the capital and operating costs of an ethylene plant. They also require significant operator attention and maintenance. It is essential to have a well-running furnace section to ensure smooth and profitable operations at the plant and site. Frequently shutting down furnaces for decoking or maintenance costs money, disrupts downstream sections and prevents the plant from meeting its production targets.

This article describes the development of a proprietary triple-lane radiant coil layout^a that can be used in cracking furnaces to obtain markedly longer furnace run lengths, higher capacities and better yields. This concept was first implemented in 2012 at a Middle East cracker as part of a furnace modernization project with the aim of increasing the capacity, reliability and availability of existing furnaces. Since then, this concept has been applied at two other projects. Key results that demonstrate the performance and benefits of the technology are provided here.

Cracking furnace design and operation

Ethylene plants, also referred to as steam crackers, consist of five main sections:

• Furnace section
• Quench section
• Cracked gas compression
• Recovery and purification
• Refrigeration.

The furnace section is where the feed is cracked at high temperature to convert it to ethylene, propylene and other co-products. Modern plants typically have 5–10 cracking furnaces. These cracking furnaces are an intricate assembly of many individual pieces of equipment and must be custom designed to match the feed quality, cracking severity, run length and other parameters.

Key considerations

Cracking, or pyrolysis, is a complex, non-catalytic phenomenon that involves numerous reactions, intermediate species and final products. In its most basic form, it may be thought of as dehydrogenation and carbon-carbon bond rupture. The reactions are endothermic, so heat is needed to initiate and sustain them. The pyrolysis product is a mixture of components that includes:

• Hydrogen
• Methane
• Monoolefins (e.g., ethylene, propylene, butenes)
• Diolefins (e.g., butadiene, propadiene, isoprene)
• Acetylenes (e.g., acetylene, methylacetylene, vinylacetylene)
• Naphthenes (e.g., cyclopentene, methyl cyclohexane)
• Aromatics (benzene, toluene, xylenes)
• Tars.

Three main parameters determine pyrolysis yields and selectivity to ethylene and propylene:

1. Cracked gas temperature—This refers to the coil outlet temperature (COT) and the gas temperature profile as it flows through the cracking coil. Each feedstock is associated with an optimum COT that would provide the best conversion and yields. This is a function of
   the feed characteristics and required products and usually lies between 800°C (1,472°F) and 880°C (1,616°F).
2. Cracked gas pressure—Low hydrocarbon partial pressures favor selectivity toward ethylene. For this reason, the coil operating pressure is kept as low as practically possible. To obtain still lower hydrocarbon pressures, a controlled amount of process steam, known as dilution steam, is co-fed with the hydrocarbons. This not only lowers the hydrocarbon partial pressure, but also reduces coke deposition.
3. Residence time—A multitude of side reactions can consume the formed ethylene and degrade it to lower-value, long-chain hydrocarbons. These reactions can be kept to a minimum by designing the coil to have a very short residence time and by quickly cooling the gas after it leaves the coil.

Coking and furnace run length are two other key considerations in furnace design and operation. Coke is an undesirable byproduct of pyrolysis that accumulates on the inside of the radiant coil. It is a matrix of severely dehydrogenated long-chain molecules that are formed by complex interactions between the gas phase components and the tube wall material. Coking rates vary with location, and they generally increase from coil inlet to outlet and peak wherever the wall temperature is higher.

Coke forms a barrier for heat transfer and increases coil pressure drop by reducing the cross-sectional area left for gas flow. These two phenomena lead to higher tube wall temperatures and a higher gas pressure profile. While one accelerates coking rates, the other leads to a drop in the yield of desired products. This can be remedied by periodically taking the furnaces offline and cleaning them. Cleaning consists of decoking the radiant coil by blowing a mixture of steam and air (controlled coke burn) or steam only (coke gasification).

Run length is the number of days that a furnace can operate between consecutive decokes. It depends on cracking conditions and often varies between 35 d–75 d. Furnaces that coke heavily will have shorter run lengths and will need to be decoked more frequently. This has several impacts:

• Lower furnace availability for cracking
• Lower ethylene production
• Higher energy cost and increased emissions
• Shorter coil life
• Increased load on operators and maintenance staff.

Furnace designs can be optimized to increase capacity, run length and yield/selectivity. The design changes needed to achieve one of these goals usually come at the expense of others. For example:

• A furnace can be designed for a long run length by selecting a coil with a high surface area. However, this is likely to increase the residence time, which is unfavorable for olefins yields. Conversely, a short residence time coil is likely to have less surface area—and, therefore, an inferior run length—but it will give a higher yield.
• One way to reduce total plant CAPEX is to select fewer, larger furnaces rather than numerous, smaller furnaces. However, at a certain point the maximum practical furnace dimensions are reached. If more capacity is pushed into the furnace when it is at its limit, run lengths become shorter and the availability will decrease.

Furnace description

Cracking furnaces can be functionally and physically divided into three zones: radiant section, convection section and transfer line exchangers (TLEs) (FIG. 1):

• The convection section recovers waste heat from the flue gas leaving the firebox. It is made up of several banks of tubes for preheating hydrocarbon feed, dilution steam, boiler feedwater and superheating the saturated very-high-pressure (VHP) steam produced in the TLEs. Alternatively, combustion air can also be preheated (not shown in FIG. 1). Depending on the design, the cooled flue gas is vented to atmosphere by a fan or natural draft.
• The radiant section comprises the cracking coils and the firebox, including the burners.
• TLEs are waste heat boilers placed at the outlet of the radiant coils to rapidly cool the cracked gas and kill secondary reactions. They use the recovered heat to produce saturated VHP steam.

 

Convection section. The convection section is usually located above the firebox at a small offset to it. The tubes are laid horizontally with the flue gas in vertical crossflow between them; the tubes themselves may be bare or finned. The services of the different heat transfer banks shown in FIG. 1 include:

• Feed preheater (FPH)
• Economizer (ECO)
• Dilution steam superheater (DSSH)
• Mixed heater 1, feed + dilution steam (HTC-1)
• Steam superheater 1 (HPSSH-I)
• HP steam superheater 2 (HPSSH-II)
• Mixed heater 2, feed + dilution steam (HTC-2).

Radiant section. The firebox is a rectangular, refractory-lined volume within which the radiant coils are suspended vertically, and burners are placed in the bottom and/or in the side walls. The coils are arranged in one or two lanes. In a single-lane layout, the coils are arranged on the firebox center line. In a dual-lane layout, they are on both sides of the center line. Each coil consists of one or more pipes that are connected to each other in a series parallel arrangement.

FIG. 2 shows examples of coil layouts that are used in cracking furnaces. Example A in this figure has four passes in series, with one pipe in each pass. Example B depicts a two-pass coil with two parallel pipes in the inlet pass and a third larger pipe in the outlet pass. Example C amalgamates some features of A and B: it is a four-pass coil with two parallel passages in the first two passes. Example D is a two-pass coil with one pipe in each pass; it is commonly referred to as a U coil. The number and type of coils and their geometric details, such as diameter, length, wall thickness, etc., are intrinsic features of the furnace design. Burners are located along the box length and on both sides of the coil lane(s). They may be located in the firebox floor or in the side walls.

 
Within a coil, cracking starts once the gas mixture crosses threshold temperature and proceeds as the gas flows through the coil. The cracked gas leaving the radiant coils is cooled in the TLEs. This is done to halt the cracking reactions when the gas is cooled below a threshold temperature.

TLEs. TLEs may be double-pipe exchangers or conventional multi-tube exchangers. Both types are placed vertically, with cracked gas in the inner pipe (tube side) and boiler water/steam in the outer pipe (shell side). The shell is connected to an overhead steam drum by downcomers and risers. The cooled cracked gas from the TLEs is combined and sent to the furnace outlet and then to the cracked gas header.

Evolution of single-lane and dual-lane layouts

Prior to the 1970s, cracking furnaces were usually equipped with horizontal coils that had several passes and residence times of a few seconds. The furnaces were also small, with capacities of a few thousand metric tpy of ethylene. FIG. 3 depicts how the radiant section would look when viewed from the top and from the end walls.

 
The 1970s saw rising feedstock and energy costs, but also booming demand for ethylene derivatives. This led to sweeping changes in the ethylene industry:

• Larger plants
• Fewer and larger cracking furnaces
• Greater emphasis on yields and energy efficiency.

The relatively short horizontal coils were phased out, and taller vertical coils took their place. This allowed larger furnaces with taller fireboxes and shorter residence times. The increased box height allowed more heat absorption in the radiant section and better box efficiency. FIG. 4 shows two examples of a radiant section with vertical coils: The one on the left is an inline coil and the one on the right is a staggered coil. Staggered coils require less box length but have higher peak tube wall temperatures and, as a result, more coking and shorter run lengths.

 

The next new trend was split coils. These permitted an increase in coil surface area without a proportional rise in coil volume. In other words, the heat absorption by the process could be increased without incurring too much penalty on yields. On another front, the market saw several new radiant coil materials with higher allowable temperatures. These could be used to increase heat flux and pare down residence time to the millisecond range.

Ethylene plants continued to grow larger and larger, and the market became increasingly competitive during the last decades of the 20th century and in the first decades of this century. Producers wanted even larger furnaces to keep capital and operating costs down. FIG. 5 demonstrates this trend.

 

The application of process intensification resulted in the dual-lane layout shown in FIG. 6. This is a better configuration than the staggered layout, as shadowing is less pronounced and the peak to average differential is smaller, which enabled designers to obtain a higher furnace capacity from the same firebox volume. Due to the success of the dual-lane layout, a detailed study was undertaken to develop the next-generation radiant coil, a triple-lane layout. The advantages of a triple-lane layout over contemporary layouts can be explained from first principles. The analysis has been based on a U-coil in dual lanes, but the conclusions can be extended to any other existing layout.

  

A refresher on heat radiation

Radiation is the principal mode of heat transfer in the firebox of a cracking furnace. The refractory sidewalls and burner flames are discrete heat sources. However, they can be idealized as a single, composite radiating plane with a uniform temperature. This plane occupies the length and height of the box. Therefore, a firebox would have two hot radiating planes along its length, one on each side of the box. The heat transfer rate from the hot radiating plane to a tube wall can be calculated using Eq. 1:

Q₁₂ = σ × A₂ × F₁₂ × (T₁⁴ – T₂⁴)     (1)

where

Q₁₂ = Net heat absorbed by a tube

A₂ = Tube surface area

T₁ = Temperature of the hot radiating plane

T₂ = Tube wall temperature

F₁₂ = View factor from the radiating plane to the tube wall

σ = Stefan-Boltzmann constant.

The “view factor” is a term that embodies the spatial relationship between the radiant and absorbing surfaces and their emissivities. The view factor is:

• 1 when a radiant beam strikes an absorbing surface normally and both surfaces are ideal black bodies (i.e., their emissivities are 1)
• 0 when the beam strikes a surface tangentially
• Between 0 and 1 when the impinging angle is between 0° and 90°, and when the surfaces are not black bodies.

The heat flux and temperature of the absorbing surface go hand in hand with the view factor: both reach a maximum at locations where the view factor peaks and come down as the factor falls off. Such a direct relationship exists between the first two parameters and the view factor that the latter may be considered a proxy for them. This forms the basis for subsequent paragraphs where the different coil layouts are evaluated by comparing their view factors.

FIG. 7 illustrates the effect of shielding. Since radiation occurs in straight lines, a body that is in a shadow created by another object does not receive any heat directly from the primary heat source. It will, however, receive radiation from the gas volume that surrounds it and with which it has a direct line of sight.

 

Heat flux and wall temperature in a dual-lane layout with U coils

In a dual-lane layout, both lanes have the same number of inlet tubes and outlet tubes and the same degree of exposure to the radiating planes. Each row receives radiation from the radiating plane that is adjacent to it and from the plane on the other side of the box. The space between the two rows is called the inter-lane space and is cooler than the radiating plane. FIG. 8 is a top view of a dual-lane layout showing the radiation impingement on a tube from both radiating planes. It shows how one side of the tube gets the full brunt of radiation, while the other side is partly shielded by the tubes in the adjacent row. As a result, the side facing the inter-lane space has a smaller view factor and a lower tube metal temperature (TMT) than the side facing the refractory. This is shown in FIG. 9.

 

 

A dual-lane furnace fitted with U coils is used to illustrate this phenomenon. In such a coil, the inlet pass and the outlet pass have the same orientation to the radiating plane and the other tubes, so they have the same view factors. FIG. 10 shows view factors computed around the tube circumference for such a coil: It is 1 at the point directly facing the burners/refractory wall, and approximately 0.75 on the opposite side. The wall temperature and heat flux will have the same asymmetric trend as the view factor due to the close relationship between them. These results imply that the tube surface area is not optimally used. The portion that is facing the refractory wall must transfer more heat (therefore, it is hotter), while the area facing the inter-lane space must transfer less (cooler). It must be recognized that the coking rate and run length are related to the peak wall temperature. An incentive exists to reduce this peak value. The challenge of developing a new layout that would improve the circumferential heat distribution and thereby shave off the peak was undertaken. A description of the resulting triple-lane layout follows.

 

Advantages of triple-lane layout

The triple-lane concept turns the problem on its head: rather than trying to reduce shielding and its effects, it uses them to reduce peak wall temperatures.

FIG. 11A shows U coils arranged in dual lanes. The red circles are the inlet tubes and the yellow circles are the outlet tubes. The furnace run length is effectively determined by the peak wall temperature in the outlet tubes on the side facing the refractory wall. The team recognized that the peak wall temperatures could be lowered if the outlet tubes could be repositioned so as not to face the radiating plane. However, this is not possible when there are only two lanes. It does become feasible if the number of lanes is increased to at least three and the outlet tubes are placed in the central lane. FIG. 11B illustrates this configuration.

 
In this new arrangement, the outlet tubes are surrounded by the relatively cooler inter-lane gas. As a result, the circumferential variations in the view factor, wall temperature and heat flux of these tubes become fewer, while the overall view factor is still high. FIGS. 12A and 12B are a comparison of view factors in a two-lane and a triple-lane U coil layout. The change from dual lane to triple lane has a moderating effect on the view factors of the hottest, most critical part of the coil, namely the outlet tubes. They adopt a nearly circular profile and the maximum value is only about 0.75, compared to 1 in the dual lane.

 

As a result, the outlet pass has a significantly lower peak wall temperature and the circumferential profile becomes more uniform. This reduces coking and pressure drop in this pass. The total heat duty of the outlet pass decreases while the inlet pass increases. The heat fluxes change along the length of the coil, as shown in FIG. 13. The higher heat flux in the inlet tubes has no adverse effects. In fact, shifting a part of the duty to the less hot inlet tubes is advantageous, as it lowers the maximum wall temperature in the outlet pass.

 

A design exercise was performed to test this hypothesis. Two furnaces were designed for the same feed quality/rate, cracking severity and other operating conditions: one had a U coil in a dual-lane (base case) arrangement, and the other had the same coils in a triple-lane arrangement:

• The base case was designed for a run length of 60 d.
• In the triple-lane case, three alternative designs were made for longer run length, higher capacity and better selectivity.
• In all cases, the furnace end-of-run criterion for tube wall temperature was fixed at 1,080°C (1,976°F).

TABLE 1 summarizes the results.

• The annual ethylene output is a good overall indicator of the improvement achieved, as it factors into the roles played by feed capacity, yield and furnace availability. All the triple-lane cases have a higher annual ethylene output than the base case, but for different reasons. In the longer run length case, the responsible factor is the higher furnace availability due to the fewer number of decokes per year, while in the higher-capacity case it is due to the larger feedrate.
• The exact improvement that can be obtained will depend on how it was originally designed and how it is operating. In general, the poorer the present operation, the greater the relative improvement that can be achieved.

 

Implementation of triple-lane concept in major project

This section describes the successful implementation of the triple-lane concept in a furnace revamp project executed in 2011–2013. This project was delivered to a global olefins manufacturer for its plant in the Middle East. This unit has 10 furnaces cracking an external liquid feed. All units were revamped. The original furnaces were designed to crack gas condensate and were equipped with one-pass cracking coils arranged in a single lane.

The plant had been experiencing severe problems with the coils for many years due to:

• Very short coil run lengths
• Frequent blockage and rupture due to coke.

This had major consequences on site operations:

• The average coil lifespan was approximately 1.5 yr, far less than the industry standard of about 6 yr. As the plant had more than 1,500 radiant coils, this meant that many coil repairs and replacements were required every year. The result:
  • High furnace downtime for inspection and repair
  • Abnormally high spending on coil materials
  • Heavy workload for inspection and maintenance crew.
• Furnace availability and reliability were poor due to the short run lengths and the high inspection and maintenance frequency. The annual ethylene production output suffered.

The client embarked on a furnace modernization project to solve the existing deficiencies and improve the plant’s capability beyond its original capacity. Its objectives were ambitious:

• Modify the furnaces to reduce coking and extend run lengths
• Improve furnace availability and reliability to bring them on par with industry standards
• Increase capacity by at least 25%.

Furthermore, the client insisted that the longer run lengths would need to be realized at the increased capacity.

A detailed two-phase study was conducted. The first phase consisted of computer simulations of the present operation. The second phase consisted of selecting the best revamping option. Both phases relied on the extensive use of a proprietary software for steam cracking yield prediction and complete furnace simulation of gas or liquid feedstocks.^b

After studying various revamp options, it was apparent that a U coil arranged in three lanes offered the best solution. This coil type, which has been applied in many liquid cracking furnaces, would be better than other alternatives because:

• It would allow more tubes to be placed in the existing firebox, resulting in higher gas flow area (i.e., capacity) and more surface area (lower heat flux and lower tube wall temperatures).
• It would shield the outlet pass from peak temperatures/heat flux and reduce coking.

The site works for this revamp project were staggered over a period of 1 yr, with one or two furnaces being modified at any given time. This avoided the need for a lengthy plant shutdown and allowed the client to more quickly realize the benefits of the upgrades.

TABLE 2 summarizes the pre-revamp and post-revamp performance of the furnaces. The performance data for both situations are based on actual operational data, rather than design data. For reasons of confidentiality, the changes are shown on a relative basis rather than absolute values.

The differences in furnace geometry and performance before and after the revamp project include:

• The 50% rise in surface area during the revamp was more than enough to compensate for the 26% growth in feed capacity. As a result, the revamp design had a lower average heat flux over the coil, despite an increase in total firing.
• The main advantage of the triple-lane design is its lower peak wall temperatures in the outlet pass. Together with the drop in average heat flux, this was responsible for lower coking rates and longer run length.
• The 31% rise in ethylene product rate is greater than the feed capacity increase. The delta is mainly due to the better furnace availability and the higher average coil selectivity over the furnace run length (average between furnace start of run and furnace end of run).

Takeaway

Radiant coil designs have evolved over the past few decades to meet greater industry demands. This journey has been characterized by numerous small steps, as well as several vital, groundbreaking innovations. Key milestones include the introduction of vertical coils, inline dual-lane coils, staggered coils and dual-lane coils.

The triple-lane concept is the latest development in this lineage. It has several merits over preceding and competing developments:

• Reduces peak tube metal temperatures
• Is a simple and elegant way to improve run length, capacity and selectivity
• Can be applied in revamps and new furnaces
• Is suitable for all cracker feeds, from ethane to VGO.

The first plant has been operating successfully for more than 6 yr, and another for 3 yr. In addition, a third project is in the execution phase. HP

NOTES

  ^a Triple-Lane Coil Layout is a proprietary technology of TechnipFMC

  ^b SPYRO is TechnipFMC’s proprietary software for steam cracking yield prediction and complete furnace simulation of gas or liquid feedstocks.

The Authors

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