In continuous contact-process sulfuric acid production, sulfur is combusted, sulfur dioxide is oxidized over promoted vanadium pentoxide catalyst, and sulfur trioxide is absorbed in circulating strong acid. The product is supplied as 93 wt%, 98 wt%, and oleum grades; oleum free sulfur trioxide content ranges from 20 wt% to 65 wt%. A 98 wt% acid stream has a density of approximately 1.84 g/cm³ at 20°C, and commercial shipments are analyzed against ASTM E223-16 for total acidity, Baumé gravity, and nonvolatile residue. Transport classification follows 49 CFR §172.101: sulfuric acid with more than 51 wt% acid is UN1830, while oleum is UN1831. Because acid production is continuous and storage is finite, the availability of the acid plant—not downstream demand—is the governing constraint on delivery during maintenance and turnaround events.

Acid Plant Maintenance and Turnarounds: What They Mean for Delivery

At the operational level, a planned turnaround removes a production block from supply for a defined interval. A contact acid plant rated at 2,000 t/d and offline for a 14-day turnaround withdraws approximately 28,000 t of acid from delivery availability; if the same unit is down for 21 days, the volume reaches 42,000 t. Inventory at terminals and plant tanks can cover short interruptions, but a full converter catalyst replacement or waste heat boiler repair cannot be covered by typical terminal storage of 5,000–15,000 t per site. During the outage, producers commonly reduce monthly nominations under contract proration clauses and divert available acid to water treatment, alkylation, and pH-control consumers. Published operator presentations indicate that planned turnarounds for large sulfur-burning plants are commonly scheduled every 24–48 months with durations of 14–28 days; discovery work such as waste heat boiler tube leaks or acid cooler shell corrosion is the primary source of schedule extension and delivery shortfall.

How Do Turnaround Schedules Translate into Sulfuric Acid Delivery Allocations?

Production planning around acid plant turnarounds uses the difference between nameplate capacity and allocated deliverable supply. A unit with 2,000 t/d nameplate and 92% service factor yields 1,840 t/d annualized; a 21-day turnaround removes approximately 38,640 t of acid if output would otherwise have been 92% of nameplate. Contract nominations are typically prorated by the ratio of available production to total contracted volumes across the outage window. Spot buyers face greater exposure because term contracts receive priority under most supply agreements. Rail logistics add another constraint: turnaround recovery often coincides with high demand in alkylation units and water treatment, when railcar cycle times can extend from 7 days to 14 days. Terminal storage, marine barge scheduling, and truck loading windows are adjusted to draw down inventory before the outage and rebuild afterward.

Carbon steel remains acceptable for storage and piping of sulfuric acid at concentrations above 88 wt% and ambient temperatures up to approximately 30°C because the iron sulfate film passivates the surface. At 98 wt%, shell-and-tube acid coolers with anodic protection use 316L stainless steel tubes, with acid outlet temperatures controlled below 110°C to avoid activating corrosion. Dilute acid below 50 wt% is substantially more aggressive to carbon steel and requires PTFE-lined pipe, alloy 20, or high-purity fiberglass-reinforced plastic. Maintenance inspections focus on acid cooler tube-to-tubesheet welds, acid pump mechanical seals, and tower distributor corrosion. Sulfuric acid with chloride above 50 mg/kg can destabilize protective films; nitric acid contamination accelerates corrosion even at low mg/kg levels. Carbon steel storage tanks for concentrated acid are vented; if dilution occurs, iron dissolution generates hydrogen and corrosion accelerates. Published data for this specific configuration is limited, but materials selection follows NACE SP0294-2006 for ambient storage tanks and ASME B31.3 for process piping.

Critical Inspection Scopes and Unit-Specific Downtime Drivers

Turnaround inspection scopes in a contact acid plant are organized by gas path, acid path, and utilities. In the gas path, the main air blower, sulfur furnace, waste heat boiler, hot gas filter, converter, and gas-gas heat exchangers are opened or borescope-inspected. Catalyst beds are screened and topped up; dust and scale are removed; pressure drop across each bed is measured against baseline. A converter with four beds may show total pressure drop of 150–300 mbar in clean service, rising to 400 mbar or more as dust accumulates. Sulfur dioxide conversion efficiency at end of run can fall from above 99.5% to below 98.0%, increasing stack sulfur dioxide and reducing acid output per tonne of sulfur. In the acid path, drying tower acid at 93–96 wt% is checked for ceramic packing breakage, distributor plugging, and mist eliminator fouling; absorption tower acid at 98.5–99 wt% is checked for cross-flow channeling and product draw points. Acid cooler inspections include eddy-current testing of tubes, pressure testing of anodic protection circuits, and replacement of anodes at intervals of 36–60 months. Each of these scopes can be planned, but discovery work such as waste heat boiler tube leaks or acid cooler shell corrosion remains the main source of schedule extension and delivery shortfall.

When Drying and Absorption Towers Are Opened for Maintenance

If a drying tower is opened during turnaround, the acid plant’s moisture-control chain is interrupted. Drying tower outlet moisture is maintained below 100 mg/m³ in many plants; higher moisture accelerates converter catalyst degradation and increases acid mist. After tower internals are reinstalled, circulation must be re-established at 93–96 wt% with the acid cooler on-line before the blower starts. Absorption tower work similarly requires re-establishing 98.5–99 wt% acid circulation and inspecting mist eliminators to keep stack opacity and acid mist below permit limits. Startup after turnaround often produces off-spec acid during the first hours to days; this material is directed to off-spec tanks and slowly re-blended into the product stream. Delivery impact of startup is therefore not limited to the shutdown window but extends through the re-commissioning period until ASTM E223-16 total acidity and nonvolatile residue targets are met.

Outage scheduling in the sulfuric acid market is often coordinated with downstream alkylation units and metals leaching operations to minimize simultaneous demand spikes. Terminal operators draw down inventory to minimum working levels before the outage; minimum working inventory for a 5,000 t terminal may be 1,000 t due to pump suction and tank heel constraints. Railcars are pre-positioned at the plant and customer sidings, but under turnaround recovery, railcar dwell times can extend from 7 days to 14 days. Delivery lot sizes are adjusted: tank truck quantities of 20–25 t, railcars of 90–100 t, and barges of 1,500–2,500 t depending on river draft. Operational limits define delivery: no producer can supply acid that is not produced, and storage cannot substitute for a converter catalyst replacement or acid cooler retubing. Contract force majeure clauses may apply to unplanned outages, while planned turnarounds are normally excluded from force majeure because they are foreseeable maintenance.