SULPHURIC ACID REGENERATION

1. Design Basis

The Sulphuric Acid Regeneration (SAR) plant reconcentrates spent sulphuric acid received from refineries and chemical manufacturers. Two parallel, identical process trains each handle half the total feed, merging downstream at the main blower and sharing common gas-conditioning, drying, conversion, and absorption equipment.

2. Process Overview & PFD

The SAR process comprises eleven unit operations in series. Spent acid is preheated and atomised into a refractory-lined spray furnace where H₂SO₄ decomposes to SO₂, H₂O, and O₂ while organic impurities combust. Hot furnace gas passes through a waste-heat boiler, a direct-contact quench tower, an indirect shell-and-tube gas cooler, and a wet electrostatic precipitator. The two trains merge at the main blower, which delivers combined gas to the drying tower, the four-bed catalytic converter, and finally the absorption tower.

Process Route Summary

Cumulative Process Flow Diagram

Figure 2.1 — SAR Plant Cumulative PFD (all units to scale)

3. Unit Equipment Specifications

3.1 Storage and Mixing

3.2 Spray Furnace (per train)

3.3 Waste-Heat Boiler (per train)

3.4 Quench Tower (per train)

3.5 Indirect Gas Cooler (per train)

3.6 Wet Electrostatic Precipitator (per train)

3.7 Main Blower (plant-wide — both trains merged)

3.8 Drying Tower

3.9 Catalytic Converter

Per-Bed Data

Figure 3.1 — Converter bed temperatures (left) and per-bed SO₂ conversion increment (right)

3.10 Absorption Tower

4. Economic Methodology

The profitability analysis follows the framework of Seider & Lewin, Product and Process Design Principles — Chapter 17 (approximate profitability measures) and Chapter 27 (improvement-step case study). Capital cost estimates are ROM (±30%) derived from the Seider Chapter 16 bare-module method, updated from CEPCI 2001 = 394 to CEPCI 2024 ≈ 800 (factor 2.03). All monetary values are in 2024 U.S. dollars.

Metric Definitions

Commodity Price Basis (2024)

Capital Cost Parameters

5. Profitability by Improvement Step (Table 27.3)

Following the Seider Chapter 27 case-study methodology, the SAR design is developed through five successive improvement steps starting from a minimal feasible concept. Rows highlighted in gold represent steps where the plant achieves positive Venture Profit (VP > 0), i.e., where after-tax return exceeds the 20% minimum ROI hurdle.

VP = (1−40%)·GP − 20%·TCI. Basis: 330 operating days/yr. Capital: ROM ±30%, Seider Ch. 16 bare-module method.

Venture Profit Progression

Figure 5.1 — VP by design step. Bars highlighted with orange outline indicate profitable steps (VP > 0).

EP, GP & VP Chart

Figure 5.2 — EP, GP, and VP for all five improvement steps.

TCI by Step

Figure 5.3 — TCI across improvement steps. Step 3 (air-ratio optimisation) achieves the largest capital reduction.

6. Annual Cost Sheet — Step 4 (Current Baseline)

The following cost sheet corresponds to the fully optimised design (Step 4): 551 kST/yr H₂SO₄, 502 kST/yr HP steam (45 bar), and 199.9 kST/yr LP steam (3 bar) recovered from absorption-acid cooling.

Figure 6.1 — Step 4 annual production cost breakdown by category.

7. Capital Cost Breakdown — Step 4

Bare-module costs are order-of-magnitude estimates (ROM, ±30%) based on Seider Chapter 16 Guthrie correlations updated to CEPCI 2024 ≈ 800 (factor 2.03 from 2001 base). Bare-module costs include direct and indirect costs for each piece of equipment in place and ready to operate.

8. Key Performance Indicators — Final Design

Design Insight: Air-Ratio Optimisation

The most impactful single design decision is targeting 10% SO₂ dry at the converter inlet (O₂/SO₂ = 1.57). Relative to a 7% SO₂ design, this reduces gas volumetric flow by approximately 30%, directly downsizing the ESP, main blower, drying tower, and absorption tower. The TCI reduction of approximately $22 MM swings Venture Profit from −$2.76 MM/yr (Step 2) to +$3.98 MM/yr (Step 3) — the largest single VP improvement across all five steps. This mirrors the role of synthesis-loop pressure reduction in the Seider ammonia case study.

Profitability Assessment

At $150/MT H₂SO₄, the fully optimised SAR plant achieves a Venture Profit of $+4.37 MM/yr, corresponding to an after-tax ROI of 23.7% and a payback period of 2.51 years. The project modestly exceeds the 20% minimum ROI threshold. The WHB steam credit ($9.1 MM/yr) and LP steam recovery ($2.2 MM/yr) together contribute $11.3 MM/yr of ancillary revenue, without which the plant would fall below the profitability threshold.