Stainless Steel Grade for Sulfuric acid Manufacturing

Scope

This article describes the general metallic alloys used in sulfuric acid manufacturing and their performance. We start by austenitic stainless steel used in older plants (before 1980) then we present the silicon containing stainless steel which makes the backbone of all new sulfuric production equipment.

Introduction

Sulfuric acid is one the most produced chemicals world-wide and it’s viewed as good index to, economic activity, as the process develops and production on the industrial scale increase, the span life of manufacturing equipment is very limited by the corrosion rate. Effort deployed to overcome this problem through years of experience and engineering innovation settled on silicate SiO2 rich surface as the optimum solution to minimize corrosion.

Sulfuric acid Specifications

In a Sulfuric acid Manufacturing plant by sulfur burning we deals the below specification of sulfuric acid

Non-Stainless Steel Materials

Here we present the set of materials used to resist sulfuric acid corrosion in manufacturing plant we exclude austenitic stainless steel

Ductile cast iron A combination of optimum cost and corrosion resistance justify the wide uses of ductile cast iron in acid manufacturing plant(before the eighties). For the below sulfuric acid specification cast are used as base material of construction for:

? Piping

? Acid distributor

? Immersed static pump parts

Cast iron pipe repair is costly & time consuming and even unfeasible sometimes, so one a flawed pipe segment (Elbow, T pipe) detected the economic solution is replace it with identical spare part.

Carbon steel Carbon steel founded in all equipment, dealing with cold concentrated sulfuric acid . For the below sulfuric acid specification cast are used as base material of construction for:

? Storage tank

? Piping for Cold acid

Acid proof brick In sulfuric acid manufacturing the packaged tower is the chemical reactor inside it H2SO4 is produced.The combination of high temperature & high velocity made the silicate based brick the optimum choice to resist corrosion.

Plastic material? they found their uses in:

? Teflon (PTFE) Gasket in sulfuric acid piping

? Lining : Packaged tower, some specific piping

Austenitic Stainless Steel (3XXSteels)

The below list of stainless steel widely used in sulfuric acid manufacturing equipment.Advances in steel manufacturing technology decrease the cost of 3XX steels and made them a justifiable choice for material selection in sulfuric acid medium.By the eighties austenitic stainless steel uses rise and many equipment replace older with new one manufactured from the 3XX steels

?? Shell & acid cooler (304L, 316 L, 904 L)

? Acid distributor :310 MoLN (UNS S31050)

Mechanical Properties

The family of steels described in Table 1&2 is relatively easy to deform (rolling) and weld this was of great importance for repair & maintenance tasks. The suffice ”L” like 304 L is to indicate that this alloy contains very low carbon< 0.02, higher carbon % austenitic stainless steel are sensitive to chromium depletion when welded.

Corrosion rate

Table 3 summaries the 3XX family corrosion rate for normal condition in sulfuric acid plant manufacturing

The 304 over-perform1 theoretically all other members of this family, however in practice the 316 is the best option due the presence of some Halides (Fluorides & chlorides). The sources of halides impurities are marine environment (chlorides) and the stack gas from the phosphoric acid reactor (Fluorides)

Equipment manufacturing materials

Acid cooler for: [H2SO4] at 150F (68 C), 95.25% All surfaces in contact with acid are polarized to a passivization potential (anodic protection). Shell and tube acid cooler where acid flow through the shell and sea water in the tubes.

? The shell is made of the 316 L stainless steel with Anodic protection

? The tube are made of 904 L: Pitting corrosion due to chlorides

Acid cooler for : [H2SO4] at 240 F (115 C), 99.25% All surfaces in contact with acid are polarized to a passivization potential (anodic protection). Shell and tube acid cooler where acid flow through the shell and sea water in the tubes.

? The shell is made of the 304 L stainless steel with Anodic protection

? The tube are made of 904 L: Pitting corrosion due to chlorides

The Absorption tower (packaged tower) A vertical column: the outer shell made of carbon steel lined internally with

? A layer acid-resistant rubber

? A multi-layer of acid proof brick

? The mist scrubber & the structure above the acid distributor are made of the 316L

Pumping system

? Piping: ductile cast iron

? Pumps

–?? Impeller :high performance alloy

–?? Shaft : high performance alloy (+Teflon (PTFE) lined)

–?? Column: Cast iron

–?? All immersed parts ,apart from column, are of high performance super-alloy due to a combination of high speed & temperature

? Pumping tank, carbon steel cylindrical shell lined with silicate based brick

Tanks? For [H2SO4] > 98.5%and T ≤ 40C

? Tanks are generally made up of carbon steel

Silicon Containing Austenitic Stainless Steel

It’s well known that silicate SiO2has excellent corrosion-resistance to sulfuric acid at high temper- ature & different concentration, so exploit this character in the steel alloy lead to the development of silicon grade austentic stainless steel. A minimum of 5% silicon by weight added to any kind of 3XX prove to enhance its corrosion resistance. The most used alloy chemical composition is given in table 5.

All the listed above alloys has good performances in hot sulfuric acid, this I think due to the formation of silicate rich layer (silicon oxidesSiO2) which the main defense line. This is the same defense line used in the absorption tower.

The absorption tower line internally with multi-layer of silicon-based brick 4 which contain about 70 % SiO2

Mechanical Properties

These alloys have a medium yield strength and high tensile strength. However, when heat treated (σy = 80ksi) and σt = 110 Ksi causes this affect its weldability. Those steel generally cannot be welded by shielded metal arc welding process (SMAW), the GTAW or GMAW is required, rolling & machining is costlier that the 3XX steels those disadvantages are outweighed by the corrosion-resistance performance of those steels.

Corrosion rate

For a sulfuric acid manufacturing plant in normal condition, we deal primary with:

? Absorption tower acid (produced) : [H2SO4] at 115 C, 99.25%

? Drying tower acid: [H2SO4] at 68 C, 96%

From Table 7 it’s clear on the most severe conditions 5% silicon steel supersede any other alloy.

Equipment manufacturing materials

Acid cooler: [H2SO4] at 150 F (68 C), 95.25%

? The shell & tubes are made of the Silicon grade austenitic stainless steel with or without Anodic protection

? When the cooling fluid is sea water the tube are alloyed with other element that improve pitting resistance (High PERN)

Acid cooler : [H2SO4] 240 F (115C), 99.25%??????? Shell and tube acid cooler where acid flow through the shell and sea water in the tubes

? The shell & tubes are made of the Silicon grade austenitic stainless steel with or without Anodic protection

? When the cooling fluid is sea water the tube are alloyed with other element that improve pitting resistance (High PERN6)

Packaged tower???? The Absorption & drying tower a vertical metallic column

? Shell manufactured from Silicon grade austenitic stainless steel

? No internal lining (Light weight)

? Package are supported on metallic

? The mist scrubber & the structure above the acid distributor silicon grade austenitic stainless steel

? Tower manufactured from silicon-containing steel had one half the weight of brick lined tower

Pumping system

? Piping: Silicon grade austenitic stainless steel

? Pump

–?? Impeller :high performance alloy

–?? Shaft : high performance alloy and Teflon (PTFE) lined

–?? column: Cast iron

–?? All immersed parts ,apart from column, are of high performance super-alloy due to combination of high speed & temperature

Pumping? tank???? carbon steel cylindrical shell lined with silicate based brick

Halides Impurities

Fluorine, chlorine impurities presence detrimental to corrosion-resistance performance, and those two impurities are unpreventable in phosphate based fertilizer plant and marine environment.

In fertilizer plant always the unit of sulfuric acid and phosphoric acid are nearby. The stack gas of thephosphoric acid unit is rich in fluoride compound (HF ...) and they find their way to the sul- furic acid unit downstream gas (The combustion air is sucked from atmosphere), For many other reason fertilizer plants preferred localization near-shores and there chlorides are unavoidable.

I mentioned those two issues because we had and experience when 5% silicon steel failed dramat- ically:

? For less than three months a New acid acid cooler failed completely

? Packaged tower: multitude leaks and unscheduled shutdown

Never rely solely on original equipment manufacturer claims, take your time and evaluate the specific circumstances specific to your own applications.

Material Selection

For sulfuric acid manufacturing by the contact process we could base our selection on the alloy performance which is function of:

? Purchasing cost

? Corrosion rate

? Cost of repair & manufacturing (rolling, cutting, welding...)

5% silicon austentic stainless steel is the backbone of all newly (after 2000) constructed sulfuric acid plant,however, older plant still rely on the 3XX & cast iron alloy, the choice of replacing all older equipment with new one (manufactured from silicon containing stainless steel) is costly. Only critical equipment (Acid distributor, acid cooler) justify the higher cost of replacement.

One of the purchasing cost riser of the steel is alloying element (Molybdenum, copper, chromium, nickel), eliminating or minimize the weight percentage could affect drastically the alloy cost which was the case for silicongrade austenitic stainless steel (no Copper, molybdenum limited % of chromium and nickel) and the low cost of silicon.

Tips for corrosion prevention and assessment

Good engineering practices for corrosion prevention and assessment (sulfuric acid manufacturing equipment)

Good engineering practices for corrosion prevention and assessment (sulfuric acid manufacturing equipment)

? Material corrosion resistance performance

? Prevent acid stagnation

? Minimize the flow speed whenever possible

? The Cost-effectiveness over life span

? Easy repair with minimum shutdown duration

? Evaluation of impurities particulates types and quantity

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