Operation of hydrofluoric (HF) acid alkylation units using primarily carbon steel construction material has been the norm for over half a century in oil refining industry, with few problems associated to the use of this material for many years due to nonuniform corrosion phenomenon, leading to plant incidents; the potential cause for this problem was attributed to Residual elements (RE), Cr, Ni and Cu in carbon steel.  To dispel doubts about this RE The International standards developed the “Materials Specification for HF Alkylation Plant”, defining test and corrosion rate measurement procedures for evaluating materials in HF environments and writing a Carbon Steel materials specification with respect to the levels of residual elements and carbon in steels for HF alkylation equipment.

In distillation tower the various components of crude oil are separated (fractioning) into different components (fractions) of increasing molecular weight, moreover impurities are removed from the oil.

The products resulting from this either serve as fuel or as raw material for a great number of products (process industry, for instance DOW, BOREALIS), either as lubricant or as raw material for road construction, roofing etc.)

Schematic presentation of a distillation tower.

In petrol production, where light products such as butane, butene and propylene react with isobutane, that’s when alkylate product take place in the Alkylation Process.

MATERIALS suitable for HF alkylation

The corrosive environment can be composed by 2 types of acids:

1- Oxidising acids: concentrated nitric acid (HNO3) and sulphuric acid (H2SO4) – that emit oxygen to the environment;

2- Reducing acids: hydrochloric acid (HCl), and hydrofluoric acid (HF) – that take away oxygen from the environment

For this reason, only 2 materials are suitable for application in an alkylation process under HF:

– Carbon steel ‘HF-N’ grade,

– Nickel-copper: grade N04400 (‘Alloy 400’).

All other materials, including stainless steel and nickel super alloys are not resistant under HF conditions; because they are corrosion resistant materials thanks to the passive chromium oxide film (Cr2O3) that protects the underlying material from acids corrosive environment. HF is a reducing acid that takes away oxygen from its environment. In case of stainless steel or a nickel super alloy this means that the oxygen is taken away from the chromium oxide while forming water (H2O) and that the passivating oxide film is fully dissolved:

Cr2O3 + HF à 2CrF3 + H2O

Contrary to Cr2O3 the chromium fluoride film formed (CrF3) does not seal off, so that the exposed material can easily be corroded as a result of the penetrating HF-acid.

Nickel-copper does not build up a passivating oxide film; nickel-copper is simply resistant in hydrofluoric acid. This is why this material is used in HF-alkylation at high temperatures; HF-N carbon steel is used at slightly lower temperatures.

Hydrofluoric acid (HF) is made by making the mineral fluorite react with concentrated sulphuric acid, thus resulting in hydrofluoric acid and gypsum:

CaF2 + H2SO4 → 2 HF + CaSO4

Carbon steel

Carbon steel has no passivating oxide film, but for HF there is however oxygen to take away.

Carbon steel as a matter of fact has a rust film (iron oxide) from which the oxygen may be taken away by HF while forming water:

Fe2O3 + 6HF à 2FeF3 + 3H2O

The iron(III)fluoride layer does seal off and further corrosion does not take place; carbon steel has become passive in an HF environment.

RE

It appears that some residual elements (RE) in carbon steel have a negative influence on the degree of sealing off of the iron fluoride layer formed. These elements are nickel (Ni), copper (Cu) and chromium (Cr). They mix in the layer and disrupt the sealing effect. It has been proven that nickel (Ni) and copper (Cu) are quite disruptive, but that carbon (C) undoes the harmful effect of chromium (Cr).

NACE

In 2003 NACE published an extensive study “Specification for Carbon Steel Materials for Hydrofluoric Acid Alkylation Units”, in which the following conclusions are drawn and the following recommendations are made:

For basic material the optimum specification should be:

– C > 0.18%

and

– Cu + Ni < 0.15%

For welding material (where the carbon content is lower) the optimum specification should be:

– Cu + Ni + Cr < 0.15%

ASTM

A number of ASTM-specifications contain supplementary requirements that have adopted NACE

for carbon steel for HF-service:

1- ASTM A516 (carbon steel plate for pressure vessels at moderate and lower temperatures)

2- ASTM A106 (seamless carbon steel pipe for high-temperature applications)

3- ASTM A333 (pipe for low-temperature applications)

4- ASTM A960 (normal requirements for pipe fittings)

5- ASTM A961 (general requirements for flanges, fittings and valves)

SUMMARIZING the requirements of the ASTM A333 for carbon steel products for concentrated hydrofluoridric acid service:

  • Minimum C based on heat analysis shall be 0.18 wt%
  • Maximum section thickness less than or equal to 1 in. CE max 0.43
  • Maximum section thickness above to 1 in. CE max 0.45
  • Carbon equivalent CE (S2.6) = C + Mn/6 + (Cr + Mo + V)/5 + (Ni+ Cu)/15
  • Maximum Vanadium (see S2.5) = 0.15 wt%
  • Maximum Niobium (see S2.5) = 0.15 wt%
  • Maximum Vanadium plus Niobium ≤ 0.03% (V and Nb are required to produce a fine grain structure are also detrimental to the stability of the iron oxide film
  • Maximum composition based on heat analysis of Ni + Cu shall be 0.15 wt%
  • Normalization (S2.8) to improve resistance to corrosion.

In addition to product marking of the specification, an “HF” stamp is provided on each component:

If the material still corrodes slightly, it must have a fine structure for optimum resistance to corrosion, because the corrosion prefers to move along the grain boundaries (where the atoms are least adhesive to each other). In order to keep the corrosion path as lengthy as possible we need to have a fine structure. In a fine structure the grains (crystals) are very small and so there are a great number of grain boundaries and as a result the corrosion path is quite lengthy (and the product will last longer without collapsing).

Here we see that the intergranular corrosion path (red line) through a metal with grain size 1 is much shorter than through a metal with grain size 8.

A corrosive product with a fine structure therefore “lasts longer” than a coarse-grained product.

The effect of the presence or absence of many residual elements nickel, copper and chromium is demonstrated by the illustration below in which two elements have been welded together (on the right ‘normal’ carbon steel and on the left ‘HF-N’ carbon steel), and subsequently have been exposed to hydrofluoric acid. The material on the right has simply been eroded by HF, while the material on the left has remained more or less intact, as has the weld in the middle (special HFwelding metal).

Fittinox can provide the most comprehensive line of fittings and flanges for HF Alkylation service: a full range of low-temp carbon steel components for use with hydrofluoric acid (HF) as a catalyst in converting isobutane and low-molecular-weight alkenes into alkylates (a high quality gasoline component).

Our products include:

• MSS SP-97 reinforced outlets

• B16.11 Forged Steel Fittings

• B16.9 Butt-Weld Fittings

• B16.5 Flanges

According ASTM A960 S78 , ASTM A961 S62 , ASTM A333 S2 , SHELL MESC SPE 74/004 with in-house forging, machining, heat treatment, and testing capabilities, all backed by a dedicated engineering department for design and product support

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