How to Calculate Direct Air Capture Solvent Replacement Rate
Solvent attrition is a hidden but material cost driver for liquid-based direct air capture (DAC) plants. Hydroxide, amine, and hybrid solvent systems all degrade with each adsorption–regeneration cycle, while mechanical slip and maintenance activities remove additional mass from circulation. Without a defensible estimate of the replacement rate, procurement teams either under-order critical chemicals or tie up working capital in excess inventory. This walkthrough delivers a rigorous, auditable approach to express solvent make-up in kilograms per day and per tonne of CO₂ delivered.
We begin by defining the solvent balance and framing the calculation. We then document variables, units, and data sources required for accuracy before deriving the deterministic equations that underpin the embedded calculator. A step-by-step workflow explains how to assemble field data, reconcile it with lab assays, and convert it into operational metrics. Validation guidance shows how to benchmark results alongside regeneration energy analytics from the regeneration energy walkthrough and productivity metrics from the sorbent productivity guide. Finally, we flag limitations, escalation triggers, and reporting best practices so finance, engineering, and compliance stakeholders remain aligned.
Definition and operating boundary
Solvent replacement rate captures the mass of make-up solvent required to keep a DAC system operating at steady-state capture performance. It combines chemical degradation, thermal breakdown, mechanical entrainment, and losses during maintenance into a single kg/day figure. We normalise that rate by net CO₂ delivered (tonnes/day) to express the chemical intensity of capture, an input often requested by offtake partners, credit verifiers, and insurers evaluating plant reliability.
This method focuses on loop-style solvent systems—vertical contactor columns, rotating packed beds, or modular boxes that circulate solvent between capture and regeneration trains. Adsorbent pellets or structured contactors used in solid sorbent DAC fall outside the scope because their attrition mechanisms differ substantially. Nonetheless, the structure of the calculation mirrors the balance-of-plant thinking applied in the direct air capture credit stack calculator, helping you connect chemical OPEX with revenue modelling.
Variables, symbols, and units
Use consistent SI units and document measurement intervals carefully. Key variables include:
- Minv – Total solvent inventory in circulation, including storage tanks and surge vessels (tonnes). Confirm with calibrated level transmitters and density measurements.
- δ – Fraction of solvent irreversibly degraded per adsorption–regeneration cycle (dimensionless). Express in percent per cycle (0.18% = 0.0018).
- C – Effective cycles completed per day (cycles/day). Multiply the number of regeneration events by the portion of the fleet they cover.
- Lmech – Mechanical or handling loss not captured in degradation (kg/day). Includes filters, leaks, sampling, and slip into product gas.
- RCO₂ – Net CO₂ captured and delivered after downtime (tonnes/day).
- Ṁmake-up – Solvent make-up requirement (kg/day).
- Isolv – Solvent intensity per tonne of CO₂ (kg solvent per tonne CO₂).
Degradation fractions originate from lab assays measuring amine oxidation, carbonate depletion, or contaminant buildup. Align lab cadence with operational shifts—monthly is typical during ramp, quarterly at steady state. Mechanical loss should come from mass balance reconciliations: what enters the plant, what leaves with CO₂, what is drained for maintenance, and what remains in inventory.
Equations connecting degradation to make-up
The solvent balance follows a simple structure:
Daily degradation loss: Ṁdeg = Minv × δ × C × 1,000 (kg/day)
Total make-up requirement: Ṁmake-up = Ṁdeg + Lmech
Solvent intensity: Isolv = Ṁmake-up ÷ (RCO₂ × 1,000)
Multiply inventory in tonnes by 1,000 to convert to kilograms before combining with δ. If lab assays report degradation in parts per million (ppm), convert to a fraction of the total mass (for example, 1,800 ppm ≈ 0.18%). When downtime reduces RCO₂, intensity climbs—even if Ṁmake-up stays constant—highlighting the operational incentive to minimise outages.
Step-by-step workflow
1. Establish the solvent inventory baseline
Measure tank levels, temperature-correct densities, and piping hold-up to determine Minv. Capture data at the same point in the cycle (for example, immediately after regeneration) to avoid oscillations. Reconcile the total against purchase records to catch unrecorded drains.
2. Quantify degradation per cycle
Use lab assays that track solvent active concentration, inhibitor depletion, or impurity build-up. Compare consecutive samples and express loss as percent per cycle. Adjust for partial rejuvenation—some plants reclaim solvent offline, temporarily lowering degradation rates.
3. Determine effective cycles per day
Count regeneration events, multiply by the fraction of contactor trains they cover, and average over the analysis window. For modular boxes, treat each regeneration as one cycle even if multiple boxes share infrastructure. Document bypass or maintenance outages that alter cycle coverage.
4. Capture mechanical and handling losses
Review maintenance logs, filter swaps, and CO₂ product analyses to quantify Lmech. Convert occasional drains into kg/day by dividing by the interval between events. When slip contaminates the CO₂ stream, incorporate lab results from liquefaction condensate or adsorption beds to estimate mass carryover.
5. Measure net CO₂ delivery
Pull RCO₂ from custody-transfer meters or verified monitoring, reporting, and verification (MRV) datasets. Align the measurement window with the solvent sampling window so the ratio remains meaningful. Exclude downtime hours where capture halts entirely.
6. Execute the calculation
Apply the equations to produce Ṁmake-up and Isolv. Record inputs, units, and timestamps in a version-controlled workbook or notebook so future audits can reproduce the result exactly.
Validation and governance
Validation means reconciling calculated make-up against procurement and inventory records. Sum actual deliveries and drains over the analysis window, compare to calculated make-up, and investigate variances above 10%. Cross-check solvent intensity with metrics from the sorbent productivity workflow; large divergences may signal inconsistent CO₂ throughput data. When presenting to finance or offtakers, convert kg/day to cost per tonne using vendor pricing and integrate it with revenue analytics produced by the credit stack calculator.
Establish governance by setting tolerance bands and review cadences. Many operators refresh inputs monthly during ramp, quarterly at steady state, and after major process changes. Archive every assumption—lab methods, flow meter calibration certificates, downtime logs—to satisfy MRV and lender requirements.
Limitations and escalation triggers
The calculation assumes degradation is proportional to cycles and that mechanical losses are independent. In reality, high contaminant loads or elevated regeneration temperatures can accelerate breakdown non-linearly. If calculated intensity jumps unexpectedly, re-run lab assays with expanded analytics (e.g., metals, organics) and inspect heater controls. Likewise, step changes in Lmech may indicate leaks or poor reclaim performance.
Transient operations—cold starts, emergency shutdowns, solvent reactivation campaigns—do not follow steady-state behaviour. Model those episodes separately using dynamic mass balances or process simulation, then splice the results into quarterly reports so stakeholders understand exceptional consumption.
Embed: Direct air capture solvent replacement calculator
Provide solvent inventory, degradation per cycle, cycle cadence, capture throughput, and optional handling losses to compute daily make-up requirements and solvent intensity per tonne of CO₂ delivered.