TECHNOLOGY · LICENSED BY ARCET
Sulphuric Acid Plants
A sulphuric acid plant is a steam plant that happens to make acid. Designed around its energy balance it carries a large part of the complex it sits in. Designed around the acid alone, it is a tower that has to be maintained.
OUR APPROACH
The acid is the product. The steam is the economics.
Sulphuric acid is the only unit on a fertiliser site that is a net energy producer. Every step of the process gives off heat, and the plant’s return depends far more on what happens to that heat than on the acid itself.
Burning sulphur releases heat. Converting sulphur dioxide to sulphur trioxide releases heat. Absorbing the trioxide into acid releases heat again. A plant that turns the first of those into high-pressure steam and sends the rest to cooling water is a normal plant. A plant that also recovers the low-grade heat is a different investment case, and the decision has to be made at the flowsheet stage because it cannot be retrofitted cheaply afterwards.
So we begin these projects from the steam and power balance of the site around the plant, not from the acid tonnage. If the complex has somewhere useful to put the steam — a phosphoric acid unit, an evaporation train, a turbine — the answer looks one way. If it does not, it looks another.
THE PLANT
Four sections, and each one decides something
The flowsheet is well established and has been for a century. What is not established is how any particular plant should be configured, and that is decided section by section.
Sulphur handling and burning
Melting, filtration and combustion in dried air. Drying the air is not a detail: moisture carried into the process becomes acid mist and corrosion downstream, and it shows up as maintenance for the life of the plant.
Conversion
Sulphur dioxide to trioxide across several catalyst beds with cooling between them, because the reaction is limited by equilibrium at high temperature and by reaction rate at low temperature. The catalyst is not the same in every bed, and specifying it as though it were is a common and expensive simplification.
Absorption
Acid strength, circulation temperature and distribution across the tower decide both the absorption efficiency and how long the tower lasts. Materials selection and anodic protection belong to this section, and so does the mist eliminator that keeps the stack clean.
Heat recovery
Waste heat boilers, superheaters and economisers on the hot side; recovery of absorption heat as usable steam on the cold side. This is the single largest lever on the plant’s return, and the one most often left out to save capital at the design stage.
DECISIONS
What a project actually has to choose
Four questions set the shape of the plant. They are commercial questions with engineering answers, which is why they are better settled at the start than argued about during detailed design.
We put them on the table in the first round of work, with the energy balance and the cost consequence attached to each, so the project chooses knowingly rather than inheriting a configuration from whichever reference plant was nearest to hand.
Where the acid feeds a phosphate complex on the same site, these questions stop being about the acid plant at all and become questions about the complex. That is the case we see most often, and it is the one where the integration is worth the most.
Single or double absorption
Double absorption reaches the conversion that modern permits require. Single absorption is simpler and cheaper, and in most jurisdictions it is no longer an option. The permit decides this, not the process engineer.
How much heat to recover
Recovering low-grade absorption heat costs capital and buys steam for the life of the plant. Whether that trade is worth making depends on what the steam is worth on your site, which is a number the project has and the process licensor does not.
Captive or merchant acid
Acid made to feed a phosphoric acid unit next door is specified differently from acid made to be sold and shipped. Storage, strength, cooling and dilution all follow from that one decision.
Feedstock
Elemental sulphur, smelter off-gas and spent acid regeneration are three different plants that happen to make the same product. Gas cleaning, turndown and availability change completely between them.
EXISTING PLANTS
Most of this work is done on plants already running
Sulphuric acid plants are long-lived, and most of the ones operating today were designed under an older emissions limit and an older view of what energy was worth. Both have changed.
The usual reasons to reopen the design are a tightened stack limit, a capacity increase the rest of the site now needs, catalyst performance that has drifted, or a steam balance that no longer matches the complex around it. In many plants the available improvement in heat recovery is larger than anything the acid side can offer for the same money.
Emissions compliance
Routes to a tighter stack limit, evaluated against the pressure drop and space the plant can physically accept.
Capacity increase
Debottlenecking the converter, the towers and the gas circuit, working from the plant’s own operating data rather than its nameplate.
Energy retrofit
Adding or upgrading heat recovery where the steam has somewhere to go, and saying plainly when it does not.
Diagnosis
Rebuilding the flowsheet, heat balance and material balance from plant data to find the cause of a recurring problem rather than its symptoms.
NEXT STEP
Four numbers are usually enough to start
Target acid capacity, the feedstock you have, what a tonne of steam is worth on your site, and the stack limit you have to meet. From those we can come back with a configuration, an energy balance and an honest view of what the plant will cost to build and to run.
RELATED
On the same site
MAP & DAP plants
The usual destination for captive sulphuric acid, through phosphoric acid. Ammoniation and granulation, where the recycle loop sets both product quality and power consumption.
Fertiliser & acid processes
How the acids, the phosphates and the compounds on one site fit together — and why the savings are usually in the connections rather than in any single unit.