AIChE Paper Number 88b

ADVANCED MATERIALS FOR RADIANT COILS

Dr. Dietlinde Jakobi

Schmidt + Clemens GmbH + Co. KG 51789 LINDLAR/GERMANY

Dr. Jörg Weigandt

Schmidt + Clemens GmbH + Co. KG 51789 LINDLAR/GERMANY

Prepared for Presentation at the 2010 Spring National Meeting

San Antonio, Texas, March, 22 – 25, 2010

AIChE and EPC shall not be responsible for statements or opinions contained in papers or printed in its publications

ADVANCED MATERIALS FOR RADIANT COILS Dr. Dietlinde Jakobi

Schmidt + Clemens GmbH + Co. KG, 51789 LINDLAR/GERMANY Dr. Jörg Weigandt

Schmidt + Clemens GmbH + Co. KG, 51789 LINDLAR/GERMANY

Abstract

Because radiant coils operate under severe conditions of carburization, oxidation and creep, tube material selection plays an important role.

The Schmidt + Clemens bench-scale unit allows simulation of the possibility to simulate high temperature corrosion of alloys in petrochemical plants: samples are examined at steam cracking conditions, i.e. high metal temperatures/ lower gas temperatures, low residence times in order to provide catalytic and pyrolytic coke formation (similar to the coke formed in the radiant coils) and a gas mixture which corresponds to the composition within the outlet tube of a radiant coil.

This new and unique high temperature corrosion test rig allows S+C to improve and develop centrifugal cast materials for radiant coils in a very fast and focused way. Therefore, the tube materials can be "tailor- made” in accordance with the customer's needs, the specific coil design, operating conditions and feed contaminants.

Exemplary test results of a new class of the alumina forming materials– the "HT-Alloys” family and also the results of field applications will be presented: Despite the excellent combination of a high carburization & oxidation resistance and excellent creep strength the alloy Centralloy® HT E was further refined for steam cracker applications by optimizing the oxide layer stability and reducing the catalytic coke formation.

Introduction

Thermal cracking of hydrocarbons in the presence of steam is the main process for the production of the lower olefins, ethylene and propylene.

It proceeds in pyrolysis heaters by feeding the hydrocarbon/steam mixture into tubular reactor coils which are fired by bottom and/or wall burners in a firebox. Such radiant coils are exposed to very severe conditions: high metal temperatures and alternating corrosive atmospheres combined with cyclic tensile stresses; therefore coil materials exhibit a limited and non-predictable lifespan.

In the course of the cracking process, tube metal temperatures increase as a consequence of coke formation which develops on the inner tube surface. Coke is deposited mainly according to two different mechanisms: catalytic coke (filamentous coke) and pyrolytic coke (1,2,3) Catalytic coke is formed at catalytic active sites on the coil material surface: for instance an active metal particle may be lifted from the tube surface and acts as a catalyst for coke growth to form thin filaments. This type of coke is only formed in the first hours or days after the hydrocarbons are added to the pre-oxidized radiant coils (during the furnace start-up), but it is very important, as it causes a fast initial decrease of the heat transfer when deposited on the surface. Pyrolytic coke is formed as a function of the feed composition and the operational parameters (e.g. cracking severity). It is continuously formed during the thermal cracking of hydrocarbons. The insulation effect of this growing coke layer leads also to a gradually increasing pressure drop, which impacts on the product selectivity (ethylene and propylene). The reduced heat transfer is compensated by a gradual increase in heat output of the burners, resulting in an increased tube metal temperature. When the maximum allowable operational parameters are reached (e.g. upper limit in tube metal temperature), a so called "decoking” operation is performed by using an air/steam mixture: the coke is burned off in an exothermic reaction. This time consuming process results in a significant loss of production.

Preventing or retarding coke formation and prolonging the cracking run length as well as the lifetime of tubes by any reasonable measures have an enormous economic advantage. From the reasons mentioned above it is apparent that the metal quality and the pretreatment of the radiant coils during the furnace start-up (formation of a dense, continuous, defect-free oxide layer) is extremely important.

Material Development for Radiant Coils

The upper temperature limit for radiant coils is determined by the coil material properties: highly alloyed centrifugally cast materials are required with adequate creep strength and structural stability at high operating temperatures. Additionally, these alloys must possess excellent oxidation, carburization and coking resistance (4,5,6).

The maximum tube metal temperature of alloys presently used in ethylene cracking furnaces is 1150°C and the aim is to achieve even higher operating temperatures. Therefore, Schmidt + Clemens has developed a new class of alumina forming materials – the Centralloy® HT Alloy Family.

Aluminum additions above a certain level lead to the formation of a thermodynamically very stable alumina layer – this is much more stable than the chromium oxides formed on the surface of standard materials. Under typical cracking conditions, the chromia layer converts to carbides if heated to temperatures above 1050°C and as a consequence the shielding and protective effects of chromium oxides are completely lost in due time (7). Alumina on the other hand does not convert to carbides and keeps its protective properties for the lifetime of the base material.

The dense, strongly adherent aluminum oxide layer formed on the surface of the Centralloy® HT alloys therefore offers excellent protection against attack by oxidation and carburization at temperatures beyond 1150°C. Moreover, it significantly reduces the formation of catalytic coke in carbon rich steam cracking atmospheres. All Centralloy® HT modifications exhibit an excellent structural stability, a very good creep resistance and high temperature creep rupture strength. The alumina forming material Centralloy® HT possesses - compared to standard chromia forming alloys - superior resistance to cyclic oxidation at high temperatures (1150°C) and has therefore the ability to be rehealed in service during decoking. Figure 1 presents the typical weight loss of metal samples (oxide loss at the surface) as a function of the number of cycles for standard chromia forming materials as well as for the alumina forming Centralloy® HT material at 1150°C testing temperature. The standard alloys suffer after a few cycles from high weight loss (spallation of oxide at the surface) and the Centralloy® HT alloys reflect a parabolic aluminum oxide scale growth rate and no further change in weight over 250 cycles (no spallation).

The state-of-the-art alloys for ethylene coils are shown in Table 1. Depending on the furnace design and the specific operating parameters, the coil consists of one or several materials: lower grade alloys (e.g. 25Cr/20Ni to 25Cr/35Ni alloys) in the colder passes and higher grade alloys (e.g. 35Cr/45Ni and aluminum containing “HT” alloys) in the hotter passes, as these areas of the coil suffer more from corrosive attack and coking.

The Centralloy® HT materials used for the hottest (outlet) passes of the ethylene cracking coils are produced as spun cast tubes, but also as induction

bent tubular components or statically cast fittings (return bends, elbows, y-pieces).

The welding procedures are available for matching but also for all dissimilar weld joints to standard austenitic materials. The welding of aged materials has been studied on tube sections removed from different field applications after several years in service and therefore also repair welding procedures can be supplied on request.

New Strategies for Radiant Coil Material Development: Tailor – Made Materials

Conventionally, tube material development is carried out by testing different alloy compositions separately with respect to specific alloy properties: e.g. creep rupture tests are performed in order to evaluate the creep rate and creep strength (time to rupture), pack carburization or gas phase carburization tests are performed to simulate the carbon diffusion into the alloys matrix and cyclical oxidation tests are carried out to evaluate the stability of oxide layers and it’s adherence to the base material (8,9). But it is difficult to simulate coke formation and the interaction with the tube material on a laboratory scale in the same way as it occurs in commercial cracking tubes (10,11,12,13).

Therefore, a new approach was used to build an unique test rig dedicated to simulate and investigate the gas-surface interaction at the radiant coil outlet of a steam cracker where the tube material is exposed to the most severe conditions regarding temperature and reactive gas. The effect of different operating conditions can be studied in this test rig: e. g. the pre-conditioning of the alloys, the start-up of new and aged coils, as well as the cracking of hydrocarbons and the resulting coke formation as well as subsequent decoking. All of these factors may be studied separately or consecutively in this way. Also, several gases can be mixed in a wide range of compositions and reactive elements and/or feed impurities can be added to the process gas. Other key features of the actual process parameters in an industrial cracking furnace have been implemented in the test rig, e.g. the gas-residence time and the temperature of the gas combined with a typically higher sample temperature. This setup therefore effectively simulates the relevant process steps of a steam cracker in a very realistic way (14).

The advantage of this testing procedure is that samples can be taken off from the rig at any stage of a simulated cracker operation, e. g. after the start-up of the furnace (pre-oxidation) or after a certain number of cracking/decoking cycles, in order to study the coking behavior and the relevant chemical and structural changes of the tube material.

The test rig therefore allows therefore a structured examination to qualify modified or newly developed alloys for radiant coils. Furthermore it enables

testing of new or optimized process conditions in terms of their benefits in achieving better cracking furnace performance or better resistance of the tube materials against coking, corrosion and carburization (14).

As a result of the alumina forming material family Centralloy® HT could be further refined by optimizing the diffusivity of oxide forming elements in the matrix. It could be demonstrated that the faster diffusion of oxide forming elements stabilizes the initially formed oxide scale (grown during the furnace start-up) with respect to catalytic coking and, during the normal cracking process, with respect to high-temperature corrosion resistance. It has also been shown that the aluminum content needs to be well above a certain level (3%) in order to form a uniform and dense protective alumina scale.

Different Centralloy® HT alloy modifications were tested in the S+C test rig under cyclic operating conditions corresponding to those in an ethylene cracking furnace. Some modifications turned out to have a very different resistance to the oxidizing and carburizing atmosphere: e.g. the alloy modification Centralloy® HT A suffered after only three cycles from detrimental changes.

Considerable amounts of Al2O3 formed at interfaces and grew into the near-surface areas of the base material (inner oxidation at phase-, grain- and subgrain boundaries). Only a small surface fraction remained covered by alumina and non-uniformly grown chromia nodules were present on the surface. Small parts of the matrix are encapsulated by oxides. These "islands” are depleted in Al and Cr which increases their Ni-fraction and thus, potential catalytic sites for coke formation have been created at the surface (as Ni is highly catalytically active). After 10 cycles the situation became worse (Figure 2). The inner oxidation penetrated deeper, the chromia nodules were more pronounced and the quantity of Ni-rich metal islands in the surface increased.

In both experiments (3, 10 cycles) of alloy Centralloy® HT A carburization was visible in the subsurface area. This produced a higher density of small carbides in the matrix. The carburization mainly occurred due to the lack of a dense, continuous protecting oxide layer. The matrix of the base material itself actually should at least be as resistant as alloy Centralloy® HT E against carburization which besides the existence of a protecting oxide as diffusion barrier is also controlled by the solubility and by the diffusivity of carbon in the matrix material.

After 10 cycles the alloy Centralloy® HT E maintained a dense alumina layer covering the entire sample surface (Figure 3). Normally, carbide precipitation within the alloy's matrix provides an excellent creep strength, but if present at the tube surface, these carbides might interrupt the continuous oxide scale or lead to the formation of a different oxide layer composition which can contain catalytically active sites. Therefore, it is absolutely essential to form initially (during the start-up of a new coil) a dense and catalytically non-active oxide layer. After exposure of the alloy Centralloy® HT E to 10 cracking & decoking cycles the carbides were still covered by alumina, others were

encapsulated by alumina. No spalling, cracking of the oxide layer and no inner oxidation of the base material could be observed. Ni-rich metallic particles were not observed on the surface of this material Centralloy® HT E.

The structure of the oxide layer corresponds to an -alumina type of oxide. This continuous and dense oxide layer protected the base material of the alloy Centralloy® HT E from carburization.

Figure 2 and Figure 3 show the extent of the Al-depletion in the base material near the oxide layer, analyzed by EPMA. The alloy Centralloy® HT E with the continuous alumina layer is not depleted in aluminum, which in the long term is an important prerequisite for the material to maintain or to regenerate a stable alumina layer. Due to the considerable degree of inner oxidation alloy Centralloy® HT A exhibits a near-surface zone in which Al is depleted. In the area with inner oxidation the aluminum concentration tends to zero (Local fluctuations in the profiles are caused by small carbide precipitates).

This means that after several cycles in operation, the protective alumina scale formed initially on the surface of the alloy modification Centralloy® HT A can not be regenerated and the surface contains catalytically active sites (high in Ni content) while the state of the art modification Centralloy® HT E is still perfectly protected by an alumina scale and does not suffer from any aluminum depletion.

In addition to the above mentioned development of new alloys and refining of existing materials, the test rig enables S+C to provide process recommendations in order to achieve a better cracking furnace performance: e.g. longer run lengths (due to reduced catalytic coke formation), shorter decoking times, increased conversion rate or increased furnace capacity or longer cracking coil life.

The excellent behavior of alumina forming S+C Centralloy® HT materials has been confirmed in a large number of field applications with the first generations of the Centralloy® HT material (e.g. alloy Centralloy® 60HT E) as well as the optimized state of the art alloy Centralloy® HT E.

Status and Experience with Alumina Forming Alloys - Centralloy® HT Family

The first coils in Centralloy® HT material have been installed in Petrochemical Industry furnaces in 2003 followed by a wide range of partial and full furnace applications both in gas as well as in naphtha cracking furnaces:

- Kellogg Millisecond (gas cracker) - Technip / KTI (gas cracker) - Several Linde Pyrocrack 1-1 (naphtha cracker) - Linde Pyrocrack 1-1 (gas cracker) - Several Linde Pyrocrack 4-2 (naphtha cracker) - Linde Pyrocrack 4-2 (gas cracker) - Linde LSCC 4-2 (naphtha cracker) - Several Linde Pyrocrack 2-2 (naphtha cracker) - Several Linde Pyrocrack 2-2 (gas cracker) - Two Lummus SRT® V (naphtha cracker) - Lummus SRT® V (gas cracker) - Several KTI multiple pass (naphtha cracker) - KTI 4 pass (gas cracker) - Stone & Webster 24W (gas cracker)

The benefits of the application of improved Centralloy® HT furnace tubes have been realized in different ways: by increasing the run length, by utilizing the higher temperature stability of the alumina forming material to allow an increase in furnace severity and thus e.g. increasing the ethylene conversion rate (with maintained run length), by increasing the furnace capacity or simply by increasing the tube/coil life.

All these applications with Centralloy® HT materials enabled S+C to collect furnace data and experience for more than 7 years up to now.

One of the first modifications of the alumina forming Centralloy® HT alloy

family has been applied in October 2003 in a Linde Pyrocrack 1-1 furnace of the Braskem UNIB-RS plant. Tubes, bent tubes as well as fittings in Centralloy® 60HT E material have been installed in this furnace operated with a rather high cracking severity. With this new Centralloy® 60HT E material, the runtime of the furnace was significantly increased under the same furnace settings as before.

Exemplary field test results: • Short Residence Time Gas Cracker The Centralloy® HT material has been installed in 2005 in one coil (consisting of 24 tubes) of a Linde Pyrocrack 1-1 furnace, all other coils of this furnace have been equipped with the standard chromia forming 35Cr/45Ni-alloy (Centralloy® ET 45 Micro).

The performance of this new material was evaluated by comparing the choking pressure ratio (CPR) of the coil in the Centralloy® HT material to the CPR of the coils equipped in the 35Cr/45Ni material. In these furnaces EOR (End of Run) is generally determined by the CPR. The CPR increase for the coils equipped with the standard material (Centralloy® ET 45 Micro) has been evaluated in comparison with the Centralloy® HT material: the average CPR increase in time for the Centralloy® HT material was less than half of the CPR increase in time for the standard material Centralloy® ET 45 Micro (35Cr/45Ni-alloy). It can therefore be concluded that coke lay-down in the Centralloy® HT coil is much less owing to the formation of a continuous and stable alumina layer in that coil as compared to the other coils. It has also been demonstrated that the material possesses an excellent thermal stability.

On the basis of these results the customer may increase the ethylene yield (higher severity) of a furnace fully equipped with the alumina forming Centralloy® HT material in case the run length is kept constant (owing to the lower amount of coke formation). If on the other hand the run length should be more important the run length of a furnace equipped with this alumina forming material could be more than doubled.

Run length prolongation in this case gives savings of around 150,000 € per year from less decokings alone, while at the same time the increase of furnace availability amounts to approx. 6,000 tons (~6%) of ethylene per year (being an equivalent to 1,2 Mio € per year). • Naphtha Cracker In 2004, a Linde Pyrocrack 1-1 furnace was recoiled with tubes made of the alumina forming material Centralloy® HT replacing the standard material Centralloy® ET 45 Micro (35Cr/45Ni). Process data of this furnace were compared by statistical means (S+C- RCLP multiple regression analysis program) with similar Linde Pyrocrack 1-1 furnaces equipped with the 35Cr/45Ni material (operated under the same process conditions).

The furnace equipped with Centralloy® HT material consistently showed a more than double run length compared to the furnaces with Centralloy® ET 45 Micro (35Cr/45Ni) coils over more than 5 years. Moreover, an increase of the overall furnace availability was experienced owing to less decokings and less maintenance work. As a result the production of ethylene equivalents could be

increased by 2,700 tons per year (a yearly production increase of 6%) in this recoiled furnace alone resulting in an average profit of 540.000 € per year.

Gas Cracker: Metallurgical evaluations preformed after 3 ½ years in

operation Several tube sections have been removed by an end-user from different Linde

Pyrocrack 2-2 furnaces which have been operated with gas feed over 3 ½ years at the same furnace settings.

Metallurgical evaluations have been performed in order to compare the behavior of the standard materials 25Cr/35Ni (Centralloy® 4852 Micro) and 35Cr/45Ni (Centralloy® ET 45 Micro) with the performance of the Centralloy® HT alloy exposed to the same conditions in the hottest outlet passes of these cracking furnaces.

Figure 4 shows the corrosive attack which occurred on the inner tube surface of the three different materials:

The investigations revealed that the lowest alloyed material, the 25cr/35Ni alloy (Centralloy® 4852 Micro), was strongly carburized (carbon contents up to max. 4%), strong corrosive attack occurred on the inner tube surface leading to spallation of the protective oxide layer. The chromium content in the sub-surface area was reduced below a critical level due to formation of chromium carbides (carburization); therefore the protective oxide scale could not be regenerated during the normal de-coking process. The high amount of catalytically active sites on the inner surface (mainly Ni particles) caused a faster catalytic coke formation which shortened the furnace run-length. This higher number of de-coking cycles and the carburization of the tube material caused a significant reduction of the creep strength (creep voids have been detected in the alloys matrix) leading to the reduction of the tube life.

The highest grade standard material 35Cr/45Ni, the Centralloy® ET 45 Micro, was also carburized (max. carbon content 1.6%), inner oxidation occurred as well as spallation of the oxide layer, but no creep damage has been detected in the tube sections removed from service.

Exposed to the same strongly reducing conditions in the outlet passes of the Linde Pyrocrack 2-2 furnace, the Centralloy® HT material showed after 3 ½ years in operation still a continuous protective alumina layer, the material was not carburized and the furnace consistently showed higher (double) run-lengths.

These results confirmed the superior stability of the alumina layer formed on the inner surface of radiant coils equipped with the material Centralloy® HT.

Conclusions

Radiant coils for cracking furnaces operate under severe cyclic conditions

of carburization, oxidation and creep, and therefore material selection plays an important role. The unique S+C high temperature corrosion test rig offers the ability to improve and develop spun cast materials for radiant coils and to test furnace specific operational settings in a very fast and well focused way. Tube materials can be "tailor-made” depending on customer needs, the specific coil design, operating conditions and feed contaminants.

This test rig has been operated over several years with a large number of results regarding material composition, surface quality, material pre-treatment during the furnace start-up with new coils or after de-coking and material deterioration during the normal cracking process.

As a result the alumina forming family of Centralloy® HT-alloys has been further refined by optimizing the diffusivity of oxide forming elements in the matrix. By this means the initially formed oxide layer was stabilized with respect to catalytic coking and to high-temperature corrosion resistance. Also, the tendency to suffer severe inner oxidation has been reduced drastically.

Therefore, the state of the art material, the Centralloy® HT E, provides excellent creep strength, excellent carburization and oxidation resistance and higher temperature stability together with a substantially reduced coking rate compared to conventional alloys.

The Centralloy® HT materials have been in operation since the year 2003 in a number of different furnace designs in gas as well as in naphtha crackers. The benefits have been realized in different ways: by longer (e.g. doubled) run-lengths or shorter decoke times (when operational parameters are maintained) – resulting in increased furnace availability, increased conversion rates or increased furnace capacity or simply in an increased coil life.

-10

0

10

0 50 100 150 200 250

Number of Cycles

We

ight

Cha

nge

, mg/

cm²

Centralloy® G 4852 Micro Centralloy® ET 45 Micro Centralloy® HT E

Tables

Table 1: State of the art alloys for pyrolysis furnace coils S+C Grade

Centralloy®

C

(%)

Cr

(%)

Ni

(%)

Nb

(%)

Fe

(%)

Al

(%)

Additions, other

G 4848 0.4 25 20 - bal. - -

G 4868 0.5 30 30 - bal. - -

G 4868 Micro 0.5 30 30 1 bal. - MAE, RE

G 4852 0.4 25 35 1 bal. - -

G 4852 Micro 0.4 25 35 1 bal. - MAE, RE

ET 45 Micro 0.4 35 45 1 bal. - MAE, RE

HT Alloys 0.4 24-33 bal. 1 15-25 3.0-6.0 W, MAE, RE

RE: Addition of reactive elements; MAE: Addition of micro-alloying elements

Figures

Figure 1: Cyclic oxidation of metal samples, comparison: chromia forming standard alloys and alumina forming alloy Centralloy® HT E (45 min at 1150°C,

15 min at room temperature in air)

0,0

0,2

0,4

0,6

0,8

1,0

1,2

0 20 40 60 80 100depth [µm]

norm

alis

ed c

once

ntra

tion

Figure 2: (a), (b) Alloy Centralloy® HT A: oxide layer and structure at the inner surface of a tube after 10 coking / decoking cycles

(c) Normalized concentration profile of aluminum near the inner tube wall after 10 coking / decoking cycles at 950 °C

M23C6

inner oxidation

(a)

Cr2O3

(b)

Al2O3

Ni-rich

(c) Centralloy® HT A

Aluminum profile

Carbide precipitates

0,0

0,2

0,4

0,6

0,8

1,0

1,2

0 20 40 60 80 100depth [µm]

norm

alis

ed c

once

ntra

tion

Figure 3: (a), (b) Alloy Centralloy® HT E: oxide layer and structure at the inner

surface of a tube after 10 coking / decoking cycles at 950 °C (c) Normalized concentration profile of aluminum near the inner tube wall after

10 coking / decoking cycles at 950°C

Al2O3 scale

(a)

M23C6 covered by Al2O3

(b)

(c)

Aluminum profile

Carbide precipitates

Centralloy® HT E

(a) (b)

(c)

Figure 4: Metallurgical evaluations after 3 ½ years in operation in the outlet passes of a Linde Pyrocrack 2-2 gas cracker: (a) alloy Centralloy® 4852 Micro,

(b) alloy Centralloy® ET 45 Micro and (c) alloy Centralloy® HT.

carbides/carbonitrides

Ni-plating

continuous alumina layer

strong carburization

strong inner oxidation/spallation

creep voids

carburization

chromium depletion

inner oxidation

References

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9. S. B. Parks, C.M. Schillmöller (1997), Use these alloys to improve ethylene production, Stainless Steel World 9, 27-35

10. G. F. Froment, K.M. Sundaram (1979), "Kinetics of Coke Deposition in the Thermal Cracking of Propane”, Chemical Eng. Science 34, 635-644

11. L. Albright, J. Marek (1988), "Coke Formation during Pyrolysis:Roles of Residence Time, Reactor Geometry, and Time in Operation”, Industrial & Engineering Chemistry Research 27, 743-751

12. G. Zimmermann, W. Zychlinski, B. Ondruschka, H. Woerde, P. van den Oosterkamp (1997), "Studies in a new ring cracking reactor with radial temperature gradient”, Chem.-Ing.-Tech. 69, 662-666

13. G. Zimmermann, W. Zychlinski, H. Woerde, P. van den Oosterkamp (1998), "New developments in fouling inhibition of cracking coils and TLE's”, 10th Ethylene Producers Conference, 189-203

14. D. Jakobi, C. van de Moesdijk, P. Karduck, A. von Richthofen (2009), "Tailor – Made Materials for High Temperature Applications: New Strategies for Radiant Coil Material Development”, NACE Corrosion 2009, Paper No. 09155.