Niobium Stabilized Alloys In Steam Hydrocarbon Reforming

Introduction

The chemical industry has used hydrocarbons for long time, fats, oils, molasses and coal are few examples used in the past. The period between finding of petroleum and production of stainless steel in 1920, underscores the need for better material to meet with the pressure and higher temperature, requirement of the process. In the past refining was done in vessels made of 300 series stainless steels. The typical service temperatures were up to 1800^oF at pressure up to 580 psi. Higher pressure and temperature were addressed by use of nickel and Ni-Cr-Fe solid solution alloys. In early stages several experimental alloys of nickel were developed, causing frequent shutdowns and catastrophic failures. This period is responsible for knowledge of alloy phases that were detrimental to their properties at higher temperatures. The discovery and knowledge about M₂₃C₆ in 1940s and of Σ phase in 1950s is some of the notables of these experiments.

The processing of petroleum promised innumerable possibilities as demand for cost efficiency and competitiveness increased. Increased capabilities and efficient processes, were designed in the refining process, and called “thermal cracking” of hydrocarbons. The catalytic reforming is often used in the production of, Methanol, Hydrogen, Oxidizing and reducing gases, Plastics, Polymers, Drugs, Synthetic rubber, Chemical fertilizers, Ammonia etc. In the production of Ammonia and nitrogenous fertilizers the reforming is very important tool. The process consumes energy; hence efficiency of the reformer has major role in the efficiency of any fertilizer plant.

Catalytic Reforming

The process of catalytic reforming is designed to upgrade the quality of petroleum byproducts. This process takes place in tubes that are suspended in a furnace, where varieties of reactions occur in the presence of some catalytic agent. The tubes are fed with hydrocarbon and steam mix. The catalyst synthesizes ammonia by chemically combining Hydrogen and Nitrogen under pressure. The catalytic reaction of steam and hydrocarbons mixture, at an elevated temperature supplies Hydrogen for the reaction.

C_nH_m+ nH₂O ⇌ nCO + (m/2 + n)H₂ ……………..(1) Reforming reaction.

 

CO + H₂O ⇌ CO₂ + H₂ ……………………………(2)

 

Since the number of moles of the product exceeds, the number of moles of reactants, this is an endothermic reaction. The endothermic reforming requires energy input in the form of fuel firing, like, natural gas or naphtha. All these demand careful design and operation of Reformer tubes.

The combustion and high temperature cause axial stress in the body of the tube, which develops longitudinal creep. The internal pressure in the tube creates hoop stress that develops circumferential creep. Both kinds of stress reduce the life of the reformer tube, leading to its failure. This creates a need for special material, that has best possible creep strength and it could meet the service conditions in these furnaces. These service conditions demand that tube material have following properties:

1. High temperature resistance under internal pressure,
2. High Creep resistance properties,
3. Resistance to attack from furnace contaminates.

These properties aim to achieve high creep rupture strength property.

Important aspects to consider in a Reformer system

The following six aspects are considered in design of efficient Reformer system.

• Reformer tubes – Material of construction       

 

8. Reformers are centrifugal cast Fe-Cr-Ni tubes. A typical tube is machined and has wall thickness of 12 mm to 19 mm and internal diameter of 88.90 mm to 100.98 mm. The wall thickness is kept as low as possible to reduce the weight and the risk of developing longitudinal creep stresses. The schematic arrangement of reforming plant is given in figure –1 and figure – 2 shows the tubes in actual furnace.

Figure: 2

 

The material of construction of reformer affects the throughput and the energy consumption in a fertilizer plant. Conventionally, HK 40™, HP35™, HP45™ or IN 519™ or equivalent alloys were used.

Physics of grain structure

Irrespective of the composition the material, the cast structure of these alloys varies according to the cooling rates–with in a controlled range-these alloy castings may have following common grain structures.

1. Columnar gain adjacent to skin of the cast tubes, and
2. Equiaxed structure

 

The austenitic cast materials tend to have more columnar structure than the ferrite structure. In a controlled cooling they often range from 90% to 100% columnar structure, this property of austenite is the key factor in the choice of conventional or new niobium stabilized alloy variants.

These alloys have austenitic structure; hence on solidification they develop large columnar grains. The foundry practice to cool the outer surface of the mould to arrest austenite structure forms the nuclei, it helps in the formation of the columnar gain by increasing the cooling rate in the direction of heat travel. At the solidification the isotherm move nearly equal to the growth rate of the macrostructure. The columnar growth is obtained in dendrite as well as austenitic alloys. The dendrite or eutectic fronts grow at the speed (Vs) and the isotherm speeds (Vm) are directly related. This is but only one factor in the promotion of columnar structure. The other variables are the three under cooling,

Δ Tc = Temperature at which Columnar starts to form

Δ Tn = Temperature at which Nuclei are formed

Δ Te = Temperature at which Equiaxed solidification starts.

Under ideal conditions the 100% columnar growth can be predicted using following equation:

G > A* No^1/3 {1-(Δ Tn / Δ Tc)³} Δ Tc

Where,

No = Density of grain at Tn

A = Constant

A higher columnar structure is one of the important reasons for selecting a centrifugal cast material over wrought material for steam hydrocarbon reformer tubes. The other factors that would impact the formation of columnar structure are the superheat temperature, mould rotation speed and density of the fluid material.

The Carbon Niobium Ratio

Carbon as an austenite former and Niobium as ferrite former, influence the creep rupture properties of austenitic alloys. Their influence on creep properties is both positive and negative, depending on the specific combination of the two elements in the alloy. Several combinations of these two elements have been experimented. The results were compared to find how they affect the creep rupture properties of an austenitic alloy.

At a given level of carbon, increasing the niobium content increases the creep rupture life to maximum. However this increase in creep rupture strength is limited by stoichiometric composition ratio of carbon and niobium, highest creep strength is obtained at this ratio. The creep strength of these alloys is basically the result of the precipitation of Nb₄C₃ from solution. The undisolved niobium carbide tends to control the rupture strength.

The observation of microstructure suggests that the dislocations of undisolved niobium carbide cause low creep ductility. Such dislocations nucleate a dense precipitation around the particles, which cause added strengthening. But very large amount of undisolved niobium carbide causes reduction in the rupture life. This is ascribed to the formation of eutectic during solidification, causing the undisolved particles to enlarge, thus increasing the numbers of dislocation centers and reducing the localized precipitation strengthening. Statical analysis has determined that at any given solution-treatment temperature, the solubility product is given as:

[Nb} [C] = ks

The amount of niobium present in the undisolved niobium carbide, Nb_NbC is equated to 7.75C_NbC. The Nb or C, at a solution treatment temperature can be expressed as total Nb_T or C_T less that is present as undisolved niobium carbide. A quadratic equation can be derived using the solubility product,

7.75 (C_NbC) – {7.75 C_T + Nb_T} C_NbC + Nb_T C_T – ks = 0

The equation will give the amount of carbon in undesolved niobium carbide. Using solubility relationship amount of undisolved niobium can be determined. It is suggested that at 700^oC NbC available for precipitation and undisolved NbC_u (Undisolved) were significant at 0.1% and following relation were derived.

1          Log_e rupture life = 2.44 +7.66 NbC_ppt + 1.24 NbCu

(±18.98)                       (±2.20)             (±0.47)

2        % elongation = 30.06 – 48.34 NbC_ppt

(±18.98)                         (±14.99)

3        % reduction in area = 73.16 – 99.03 NbC_ppt

(± 30.66)                              (±24.22)

NbC_ppt = Niobium Carbide available for precipitation during testing.

These findings lead to several experiments to develop alloys with niobium in stoichiometric composition with carbon (Nb₄C₃ composition). Alloy In-519™ is one such alloy. The trend to use micro-alloys has proven more useful to the industry. Other similar alloys have been developed to achieve higher creep resistance. Typical composition of modified stabilized alloy HPNb™ and IN 519™ is given in the table below.

Elements	HPNb™Percent Composition	IN 519™ Percent Composition
Carbon	0.45	0.25 – 0.35
Chromium,	25	24
Nickel,	35	24
Niobium	1.25- 1.5	1.5
Titanium	0.1 –0.3	Nil
Silicon	1.00	1.00
Manganese (Max.)	1.00	1.00
Iron	Balance	Balance

 

These materials have highly stable carbide, increased creep strength, higher durability and oxidation resistance as compared to the conventional materials. The advantages of using these microalloys are:

1. Possibility of operation at higher temperature & pressure,
2. Reduced reformer wall thickness,
3. Increased quantity of catalyst packing in the same space – this aspect has been utilized advantageously, for increasing the capacity and reducing the energy consumption of existing Reformers.

2. Reforming catalyst

The selection of the right catalyst is essential for the efficient operation. The following factors affect the performance of a catalyst.

• Chemical composition of the catalyst – Typically, metallic Nickel dispersed over some support material is used. Common used support material are α-alumina, calcium aluminates and magnesia α-alumina spinel-that are crystal system with oxide anions arranged in a cubic-close-packed lattice and cations occupying some or all-octahedral and tetrahedral sites in the lattice.

• Calcium aluminates are generally used for naphtha reforming. Magnesium aluminate, as support material has the advantage of higher surface area. High temperature calcined, MgO free material is required to prevent, hydrolyzing at temperatures below 572^oF.
• Geometry of the catalyst – The main mechanism of heat transfer from the inner tube wall to the gas, is through convection. Hence, the efficiency depends on the gas distribution over the catalyst bed. The catalyst with a better geometrical shape, results in lower tube skin temperature.

3. Operating Conditions

Conventionally, the reformers were operated at pressures of about 3 MPa, because the reformer tube material could not withstand higher pressures. The use of micro-alloyed reformer tubes allow for relatively higher operating pressure of up to 4 MPa. The reformer reaction results in increase in volume of the gases, this increase in reformer pressure allows a significant saving in compression energy. Another advantage of increasing the reformer pressure is that it allows higher heat of condensation of the recovered surplus steam. However, the elevation of reformer pressure tends to shift the equilibrium towards the left, additional firing is required to bring back to process equilibrium.

4. Steam to Carbon ratio

The steam to carbon ration is an important parameter, affecting the performance of reformer. The maintenance of ratio is key to the prevention of excessive deposition of carbon on the catalyst, shift conversion of carbon monoxide and reduce carburization damage to the tube material. Early designs were based on higher steam to carbon ratio, typically 4.0 to 4.5. With the development of superior catalysts, which are active at lower S/C ratio, it is possible to maintain a ratio of 2.7 to 3.0. The lower ratio has the advantages of:

• Pressure drop in the front end of ammonia plant
• Reduction in mass flow inside the reformer tube, resulting in reduction of firing, for the endothermic reaction.

In an ammonia plant these advantages result in an overall reduction in energy consumption by about 0.2 GCal/MT of ammonia.

5. Furnace design

The furnaces are either top or side fired. In the top-fired furnace, the process gases and flow gases have co-current flow. They are characterized by high heat flux and are generally preferred for high capacities. In this design the burners are limited and positioned at one level, this results in very limited possibility of adjustment in heat input.

The side fire furnace has multiple burners located at different levels. This allows for flexibility to use different burners in order to achieve uniform heating. This gives uniform tube-skin temperature and better heat control. The limitation of this design is capacity, which is overcome by providing multiple chambers.

The efficiency in dispersion of heat is also improved by lining the furnace walls with ceramic fiber refectories, replacing most of the conventional firebricks. This helps keep the outside temperature below 572^oF.

6. Installation of Pre-reformer

The installation of a prereformer, upstream of the primary reformer is a common design practice, particularly, in the naphtha based ammonia plants. The pre-reformer process breaks down naphtha into methane, carbon monoxide and hydrogen at about 932^oF. This allows for the primary reformer to function purely as a gas reformer. Other advantages of pre-reformer are:

• Allows flexibility in feedstock including LPG, Naphtha with higher boiling point and Kerosene.
• Primary reformer can act as a pure natural gas reformer.
• Act as Sulphur guard to the catalyst in the primary reformer.
• Extend the life of the primary reformer catalyst.

To avail the flexibility of pre-reformers in reduction of Steam to Carbon ratio (S/C) in the primary reformers, they have been designed and installed in many fertilizer plants. This change has generated substantial energy saving of up to 0.4 Gcal/MT of ammonia.

References:

1. Metallurgy and weldability of Steam Hydrocarbons Reforming Equipment by Ramesh Singh Thesis paper to TWI-2000.
2. Effect of Niobium Carbide on the Creep rupture properties of Austenitic Steels: By S.R. Keown and Dr. F.B Pickering ASM Hand Book “Service conditions and requirements in the chemical industry” page. 138-143.
3. “Welding and Metallurgy of 20Cr-32Ni-Nb and HP 45 Castings” By Prof. B.M Patchett Dept. of Chemical &Material Engineering University of Alberta & R.W. Skwarok of Syncrud Canada Ltd. Research Department. Proceedings of conference by the metallurgical society of CIM 1998.
4. “Creep rupture properties of tubes for High temperature Steam Power plants” By Chitty, A and Duval, D – Proceedings of joint International Conference on Creep, The institute of Mechanical Engineers, London 1963.