The IBR (Indian Boilers Regulation) steam boiler has been replaced with a Thermal Oil Heater (TOH). Rice husk combustion at ≤700°C heats a synthetic heat transfer fluid (Therminol 55 or Dowtherm A) to 180–220°C at near-atmospheric pressure. The hot oil circuit distributes heat to all process consumers via shell-and-tube heat exchangers — leach reactors, evaporator first effect, spray dryer air heaters, and causticisation reactor.
| Parameter | IBR Boiler | Thermal Oil | Winner |
|---|---|---|---|
| CAPEX | ₹160L | ₹45–55L | TOH saves ₹105–115L |
| Operating pressure | 6–8 bar(g) | 0.5–1.5 bar(g) | TOH — far simpler, safer piping |
| IBR regulatory | CF required, 8–10 weeks | Not applicable | TOH saves 8–10 weeks on critical path |
| Boiler attendant | IBR-licensed, ₹5–6L/yr extra | Not required | TOH saves ₹5–6L/yr OPEX |
| Water treatment | Feedwater softener required | Not required | TOH saves ₹10–15L CAPEX + ₹5L/yr |
| Heat delivery temperature | 170–175°C (steam) | Up to 220°C (oil) | TOH — more flexible, better for spray dryer |
| Temperature control | Good (steam pressure control) | Better (oil flow control, more stable) | TOH — less thermal shock to PP linings |
| Energy efficiency | 82% boiler efficiency (already in design) | 85–90% TOH efficiency (slight improvement) | TOH slight edge |
| MVR evaporator compatibility | Steam supplies first effect directly | TOH heats first effect via S&T HX — same result | Equal — MVR works the same way |
| CO₂ availability | Flue gas from furnace (unchanged) | Flue gas from furnace (unchanged) | Identical — CO₂ capture unaffected |
| Piping complexity | High-pressure steam piping, steam traps, condensate returns | Low-pressure oil piping, no traps, no condensate | TOH — less maintenance, fewer failure points |
⚠️ Design note: The evaporator's first effect still uses steam — but this steam is generated internally from the process liquor boiling, not from an external steam generator. The TOH heats the first effect shell via oil HX, causing process liquor to boil and generate its own vapor that the MVR compressor reuses. No external steam generator is needed at any point.
PP lining on SS316 shell requires: (a) periodic inspection for PP liner disbonding, (b) replacement every 5–8 years, (c) risk of NaOH permeation behind liner causing shell corrosion. Glass-lined mild steel (borosilicate glass on CS shell) is the standard for alkaline service up to 150°C and 6 bar — well within leaching conditions of 90°C, atmospheric.
- Glass lining: 20+ year service life vs PP liner 5–8 years before replacement
- No liner disbonding risk — glass is chemically bonded, not mechanically attached
- CS shell is 30–40% cheaper than SS316 shell
- Standard glass-lining vendors in India: GMM Pfaudler (Vadodara), HLE Glascoat (Vadodara)
- Standard sizes: 10,000L glass-lined reactor is a catalogue item — shorter lead time
- Temperature limit: 150°C (well above our 90°C leach) — no constraint
- Risk: glass can chip if mechanical impact; care during cleaning needed
In continuous precipitation, Na₂SiO₃ feed and CO₂ gas meet in a recirculation loop reactor — a compact vessel with a high-shear agitator, continuous feed in and product slurry out. pH is controlled in real-time by CO₂ flow rate. This is how Evonik, PPG, and major HDS silica producers operate. Key benefit: each unit of reactor volume produces 2–3× the throughput of batch operation.
- Continuous operation: no batch cycle time lost (loading, heating, unloading = 30–60 min/batch wasted)
- Smaller reactor volume needed: 3×5KL instead of 3×10KL for same throughput → saves vessel cost
- Better grade control: pH at every second is controlled, not just at endpoint
- HDS grade: CTAB ≥175 mg/g is achieved more reliably with continuous pH profiles
- For different grades, adjust CO₂ feed rate and residence time — same equipment
- Capital cost of continuous loop reactor (5KL with high-shear agitator): ₹25–35L each → 3 units ₹75–105L vs current ₹120L batch
CaO + H₂O → Ca(OH)₂ releases 63.7 kJ/mol. At 13.364 MT/day CaO (238,400 mol/day), this is 15.2 GJ/day of free heat currently sent to cooling water at 168 L/min. Installing a plate heat exchanger on the slaker discharge to recover heat into the causticisation reactor feed saves TOH fuel equivalent and eliminates most of the cooling water requirement.
- Plate HX on slaker discharge: recover 10–12 GJ/day to pre-heat causticisation reactor feed (no additional heating needed for this stage)
- Spray dryer exhaust recuperator: rotary or plate HX, recover 6–7 GJ/day to pre-heat incoming spray dryer air
- Net TOH load reduction: ~15–18 GJ/day = ~13–15% less fuel required from furnace
- This directly reduces rice husk consumption or increases thermal surplus further
- Cooling water requirement for slaker drops from 168 L/min to ~40–50 L/min
A draft-tube causticiser uses internal circulation to create a distinct upflow/downflow pattern. Ca(OH)₂ slurry is fed into the draft tube where it rises rapidly in turbulent contact with Na₂CO₃ — then overflows and recirculates. This intensifies contact without extra residence time. Literature reports CE of 86–90% vs 78–84% for simple agitated tanks at equivalent conditions.
- CE improvement 82% → 86%: ₹0.47 Cr/yr × 4 = ₹1.88 Cr/yr NaOH saving
- Finer CaCO₃ crystals: draft-tube flow shear produces d50 0.5–0.8 µm vs 1.5–2.5 µm for agitated tank
- Finer crystals = directly usable for sealant-grade PCC (₹26/kg) without separate classification
- CAPEX: draft-tube causticiser ₹50–65L vs standard agitated tank ₹30L → incremental ₹20–35L
- Vendors: ANDRITZ, Sulzer (India), or fabricated locally from design drawing
- Payback at ₹1.88 Cr/yr NaOH saving alone: ~2 months
With a multi-stage hydrocyclone bank, the 20.272 MT/day PCC can be fractionated into three distinct grades simultaneously — without additional process steps. Each fraction goes to its own stearic coating step (or skipped for coatings grade). The upgrade from coatings-only (₹8–15/kg) to a mixed premium portfolio is the highest-revenue unlock at lowest CAPEX.
| PCC Fraction | d50 | Volume (Ph1A) | Price | Annual Revenue |
|---|---|---|---|---|
| Coatings (bulk, no coating) | 2.0 µm | 40% = 8.11 MT/day | ₹10/kg avg | ₹2.68 Cr/yr |
| Sealant (stearic coated) | 0.7 µm | 40% = 8.11 MT/day | ₹26/kg | ₹6.96 Cr/yr |
| Plastics (OCC coated) | 0.5 µm | 20% = 4.05 MT/day | ₹42/kg | ₹5.62 Cr/yr |
| Total with classification | Mixed | 20.27 MT/day | — | ₹15.26 Cr/yr |
| Coatings only (no classification) | ~2 µm | 20.27 MT/day | ₹10/kg | ₹6.69 Cr/yr |
| Revenue uplift from two-stage classification: | +₹8.57 Cr/yr | |||
- Two-stage hydrocyclone bank: ₹15–20L incremental over current single-stage design (₹80L)
- Stearic acid coating tank for sealant grade: ₹8–12L additional (small, simple)
- OCC (Oleic acid or stearic acid-based surface treatment) coating for plastics grade: ₹8–12L
- Total incremental CAPEX: ₹30–45L for potentially +₹8 Cr/yr revenue
- Payback: <1 month — this is the highest ROI item in the entire plant
FSSAI E551 requires precipitated silica with Pb ≤1 ppm, Cd ≤1 ppm, As ≤3 ppm. RHA from agricultural sources typically contains these metals at 2–5 ppm from soil uptake. A dilute HCl wash (0.5% HCl solution, 15–20 minutes contact in filter press between water washes) strips heavy metals from the silica surface. The silica retains its structure and BET surface area — only surface-bound metal ions are removed.
- No new equipment: wash is done inside existing filter press as an extra wash sequence
- Capital: acid dosing system + PP storage tank = ₹5–10L only
- HCl consumption: ~0.5 kg HCl per MT silica = ~6.6 kg/day = ~₹2,178/day = ₹0.72L/yr (negligible)
- Effect: FSSAI-quality material produced from commissioning day — material can be stockpiled or sold at standard grade while FSSAI approval is pending
- FSSAI inspection at Month 12 sees documented quality from day 1 → stronger application
- If FSSAI comes through at Month 18 instead of Month 30: one additional year of dental revenue = +₹4–6 Cr
The triple-effect evaporator was specified to minimise steam consumption (important when steam is from an IBR boiler with fuel cost). With TOH providing heat from free biomass combustion, the incremental thermal efficiency gain from triple vs double effect is less economically significant. A double-effect MVR reduces CAPEX and mechanical complexity. However, verify the thermal balance still shows sufficient surplus before confirming this change.
- Triple-effect: ₹150L (as in DPR v15, including MVR) → Double-effect MVR: ₹100–120L
- Thermal consumption increase (double vs triple): ~15–20 GJ/day more TOH heat needed
- At 83 GJ/day thermal surplus: still 63–68 GJ/day surplus after Phase 1A — Phase 1B and Phase 2 anchor preserved
- Requires recalculation of full energy balance before committing
- Simpler equipment: fewer effects = fewer failure points, easier maintenance
Desilicated RHA retains amorphous carbon and residual silica — ideal composition for a soil amendment / biochar blend registered under FCO (Fertiliser Control Order). Market price ₹3,000–5,000/MT vs current ₹900/MT for brick kilns. Requires: simple acid wash (0.5% HCl), drying (can use spray dryer exhaust air), granulation, and FCO registration.
- Revenue upside: 944 MT/yr × (₹3,000 − ₹900) = +₹0.20 Cr/yr minimum
- FCO registration: ~₹2–5L one-time, 6-month process
- No new major equipment — uses existing acid and drying capability
- Consider deferring to Phase 1B when operational bandwidth is available
The CEMS system already monitors SO₂ and NOₓ for environmental compliance. But to use flue gas CO₂ for food-grade silica precipitation, the gas must be cleaned of sulfur compounds (SO₂ ≤ 10 ppm at silica contact point). A simple two-stage system: cyclone to remove fly ash → small packed NaOH scrubber to absorb SO₂ → blower to precipitation reactor. This is essential for FSSAI E551 compliance.
- SO₂ in RH flue gas: typically 50–200 ppm (from sulfur in rice husk ~0.1–0.2%)
- NaOH scrubber with caustic recirculation removes SO₂ to <10 ppm
- Already have NaOH on site — the scrubber caustic can be recovered back to process
- Cyclone separates fly ash before the scrubber (protects packing material)
- This circuit is very small relative to total plant scale
- Critical for dental grade — cannot get FSSAI E551 approval with sulfate contamination
6 × 5KL reactors in two banks of 3 offers: (a) planned maintenance on one bank without production loss, (b) Phase 1B activation is simply starting bank 2 with Phase 1B RHA feed, (c) easier to match feed rate to available RHA. The 5KL size is more standard for glass-lined reactors — better availability and lower unit cost.
- 6 × 5KL glass-lined: ~₹90–100L vs 3 × 10KL SS316+PP: ₹120L → saves ₹20–30L
- Phase 1A uses Bank 1 (3 × 5KL); Phase 1B activates Bank 2 (already installed)
- Redundancy: any one reactor offline = only 33% throughput loss, not 100%
- Requires piping designed for both banks — adds ₹5–8L in pipework
- Net saving vs current design: ₹12–22L
A portion of the Na₂SiO₃ stream can be diverted to a silica gel precipitation circuit using dilute H₂SO₄. The resulting silica gel is aged, washed, and dried differently (not spray dried — dried in trays or fluid bed). This uses the thermal surplus and existing chemical infrastructure. Silica gel for desiccant applications (₹80–100/kg) and chromatography media (₹150–300/kg) are high-value niches requiring small volumes.
- Silica gel line: 1–2 MT/day diversion from PS stream
- Additional equipment: acid addition tank, aging vessel, tray dryer or fluid bed dryer
- CAPEX for silica gel side-line: ₹25–40L
- Revenue: 1 MT/day × ₹80,000/MT × 330 days = ₹2.64 Cr/yr at desiccant grade
- Consider in Phase 2 once Phase 1A/1B operations are stable
After Phase 1A + 1B, 74 GJ/day of thermal surplus is available in the thermal oil circuit. An ORC turbine (uses refrigerant-like working fluid in a Rankine cycle at low temperatures) converts this heat to electricity at 10–12% efficiency. At 74 GJ/day × 10% = 7.4 GJ/day electrical = ~308 kW continuous. Net of plant consumption, ~200–300 kW can be exported. At ₹7/kWh: ₹1.2–1.6 Cr/yr export revenue.
- ORC unit 200–500 kW: ₹80–150L CAPEX (imported, 18–24 month lead time)
- Requires TSSPDCL net metering / energy banking agreement for grid export
- ORC works synergistically with thermal oil heater — draws from same oil circuit
- Consider in Phase 2 business plan — substantial revenue for Phase 2+ operations
- This is already mentioned as an upside item in DPR v15 sensitivity analysis
| # | Optimisation | Status | CAPEX Delta (₹L) | OPEX Delta (₹ Cr/yr) | Revenue Delta (₹ Cr/yr) |
|---|---|---|---|---|---|
| 1 | Thermal Oil Heater replaces IBR boiler | Confirmed | −105 to −120 | −0.10 to −0.11 | — |
| 2 | Glass-lined CS leach reactors (×6 at 5KL) | Recommended | −12 to −22 | −0.02 to −0.03 | — |
| 3 | Continuous loop precipitation reactor | Recommended | −10 to −20 | — | +BET quality |
| 4 | Slaking exotherm + spray dryer WHR | Recommended | +8 to +12 | −0.12 to −0.18 | — |
| 5 | Draft-tube causticiser | Recommended | +20 to +35 | −0.94 to −1.88 | +PCC grade premium |
| 6 | Multi-stage PCC hydrocyclone classification | Recommended | +30 to +45 | — | +₹8.57 Cr/yr |
| 7 | In-filter acid wash (dental grade Y2) | Recommended | +5 to +10 | +0.01 | +₹4–6 Cr (1yr earlier) |
| 8 | Double-effect MVR (vs triple-effect) | Consider | −30 to −50 | +0.10 (more heat) | — |
| 9 | Residue → soil amendment FCO | Consider | +2 to +5 | — | +₹0.20 Cr/yr |
| 10 | Flue gas cleaning for CO₂ (bag filter + NaOH scrubber) | Consider | +8 to +15 | — | Quality gate for dental |
| 11 | Modular dual-bank leach (already in Opt 2) | Consider | Included in Opt 2 | — | — |
| 12 | Silica gel side line (Phase 2) | Phase 2 | +25 to +40 | — | +₹2.64 Cr/yr |
| 13 | ORC electricity generation (Phase 2) | Phase 2 | +80 to +150 | — | +₹1.2–2.0 Cr/yr |
| CONFIRMED + RECOMMENDED TOTAL (1–7) | −64 to −90L net | −1.05 to −2.10 Cr/yr | +₹12–15 Cr/yr | ||