Where does the control request travel?
The control path starts with the H2S source measurement, passes through the flow or ratio controller, reaches the caustic valve or pump, and ends at the packed tower. The verification path runs from the tower outlet analyzer and liquid sample back to the controller. Follow the signal through every hop before changing the chemical ratio.
| Hop | Required information | Typical stopping point | Proof before proceeding |
|---|---|---|---|
| H2S source to transmitter | Gas flow, temperature, pressure, and complete composition | Blocked sample line, wrong range, or wet/dry basis mismatch | Compare indicated flow and composition with an independent sample or reference |
| Transmitter to controller | Signal scaling, tag mapping, update timing, and bad-quality handling | Scaling or communications error | Inject or observe a known input and confirm the controller receives the same value |
| Controller to caustic actuator | Ratio setting, output limits, valve action, and pump capacity | Valve stiction, reversed action, or saturated output | Stroke the final element and confirm actual NaOH flow follows the command |
| Tower to verification instruments | Outlet H2S and liquid composition | Analyzer lag, contaminated sample, or unrepresentative sample point | Reconcile analyzer response with a fresh laboratory sample |
No controller address, network port, scan period, or signal range is specified for this installation. Read those values from the controller configuration, I/O module setup, network diagnostics, and instrument data sheets. Layer one first: inspect wiring, tubing, sample conditioning, valve travel, pump rotation, liquid distribution, and packing condition before troubleshooting the protocol.
What reaction must the feed system control?
The net conversion is NaOH + H2S → NaHS + H2O. The notation NaSH used in some reaction descriptions refers to the same sodium hydrosulfide composition commonly written NaHS. On a molar basis, the net reaction requires one mole of NaOH for each mole of absorbed H2S.
The reaction can proceed through sodium sulfide:
2NaOH + H2S → Na2S + 2H2O
Na2S + H2S → 2NaHSIf H2S delivery or absorption stops after the first reaction, the liquid can contain sodium sulfide and excess caustic instead of the intended NaHS-rich product. A correct inlet feed ratio alone does not prove conversion: gas bypassing, poor wetting, inadequate contact, or a rapidly changing H2S load can leave the absorber chemically off target.
Calculate the initial ratio from molar flow, not volumetric flow alone. Convert gas flow using its measured temperature, pressure, and composition; convert caustic solution flow using its verified NaOH concentration and density. The commissioning check is a calculated molar balance showing NaOH addition versus H2S entering and H2S leaving.
Which measurements distinguish overdosing from poor absorption?
| Observation | Likely mechanism | Deciding check | Correction |
|---|---|---|---|
| High residual NaOH with low outlet H2S | Caustic feed exceeds the absorbed H2S load | Recalculate the molar ratio from current laboratory concentration and gas composition | Reduce the ratio setting gradually while monitoring breakthrough |
| High residual NaOH with high outlet H2S | Contact or measurement failure rather than simple underfeed | Inspect distributor coverage, packing, circulation, analyzer sample path, and final-element response | Restore mass transfer or measurement integrity before changing the ratio |
| NaOH consumption higher than the H2S balance predicts | Other acidic gas components consume caustic | Obtain a complete gas analysis, including CO2 | Include competing acid gases in the reagent balance or separate them upstream |
| Liquid composition varies while inlet flow appears steady | Unmeasured H2S concentration changes, sampling lag, or poor mixing | Trend inlet composition, outlet H2S, caustic flow, and liquid results on one time base | Repair the measurement path or add composition compensation |
CO2 matters because caustic absorbs it without producing NaHS. Other acidic compounds can also consume NaOH or contaminate the liquid. A single pH measurement cannot close the balance among free caustic, sodium sulfide, and sodium hydrosulfide; use an analytical method selected for those species. The check is agreement between the laboratory result, gas-side sulfur balance, and recorded reagent addition.
How should the NaOH and H2S feeds be commissioned?
- Record H2S gas quantity per day, temperature, pressure, and full composition. Mark whether each gas value uses an actual, standard, wet, or dry basis.
- Verify the NaOH solution concentration and density from a representative sample. Do not calculate molar delivery from pump speed alone.
- Inspect the packed tower, liquid distributor, recirculation path, demister, sample system, and outlet gas path. Confirm that gas cannot bypass the wetted packing.
- Prove every measurement and actuator from sensor to control display and from controller output to actual reagent flow. Match signal scaling, engineering units, tag mapping, address, port, and update timing to the installed configuration.
- Calculate the initial caustic demand from
1 mol NaOH / 1 mol H2S absorbed, then account separately for other measured acidic components. Apply the site-defined operating margin rather than inventing one. - Introduce H2S and caustic under the approved process-safety procedure. Trend inlet H2S load, caustic molar flow, outlet H2S, controller output, and liquid analysis.
- Adjust the ratio in small controlled increments. Allow the actual vessel residence time, sample transport time, and analyzer response time to pass before judging each change; obtain those timings from the installed equipment and test response.
- Stop reducing caustic when the product reaches its required composition without unacceptable H2S breakthrough. Record that operating point across the expected load range.
The step is complete when a commanded feed change produces the expected measured flow and the subsequent outlet and liquid responses occur in the correct sequence.
When is NaHS production the wrong removal route?
A sellable stream requires more than H2S removal. The receiving paper mill or other buyer must accept the NaHS concentration, residual caustic, sodium sulfide, water content, and contaminants. Caustic used on refinery streams may also contain sodium mercaptide. Caustic from naphtha, jet fuel, or light kerosene service may contain sodium phenolate, sodium cresolate, or sodium naphthenate; these contaminants can make sale or disposal difficult.
| Route | Best fit indicated by the process data | Product or residue | Primary decision |
|---|---|---|---|
| NaOH packed-tower absorption | A controlled H2S feed and an identified outlet for qualified NaHS solution | NaHS solution with composition dependent on feed balance and contaminants | Buyer specification versus reagent and purification cost |
| Amine recovery followed by catalytic sulfur conversion | Large recoverable H2S loads; refinery examples operate at 50 to 600 tons per day | Elemental sulfur | Capital and operating economics at the measured plant load |
| Aqueous ferrous-iron treatment | Relatively small gas streams where a saleable chemical is not the objective | Small, dense precipitates that readily settle | Reagent, solids handling, and disposal requirements |
Build the economic comparison from measured daily H2S mass, gas conditions, complete composition, reagent consumption, product acceptance, residue handling, and local emissions requirements. The check is a written mass balance and buyer-approved product specification for the selected route.
How is the complete process verified?
Run the tower at representative minimum, normal, and maximum H2S loads. For each condition, reconcile inlet sulfur, outlet sulfur, liquid sulfur, and NaOH addition over a sampling interval long enough to include vessel and analyzer response. Investigate any material imbalance before declaring the ratio valid.
Challenge one element at a time: alter the measured H2S load, confirm the controller calculates the corresponding caustic request, verify the actuator delivers that flow, and observe the expected outlet and liquid response. Test bad-signal behavior and the approved shutdown action without assigning unstated trip values. Release product only after laboratory results meet the buyer's specification and repeated operating points show stable residual caustic and acceptable outlet H2S.
FAQ
Why does excess NaOH remain in an NaHS scrubber?
The delivered NaOH molar flow exceeds the H2S actually absorbed, or the gas bypasses effective liquid contact. Recalculate against absorbed H2S and inspect packing wetting before reducing the feed.
Why does sodium sulfide appear instead of NaHS?
The first reaction can form Na2S; additional H2S converts it to NaHS. Insufficient H2S absorption leaves the conversion incomplete.
Why does CO2 increase caustic consumption?
CO2 is an acidic gas that also reacts with NaOH but produces no NaHS. Include measured CO2 and other acidic components when reconciling caustic demand.
Why does spent caustic fail a buyer's specification?
Residual NaOH, sodium sulfide, sodium mercaptide, phenolate, cresolate, or naphthenate can make the stream unsuitable. Test the actual liquid against the buyer's written acceptance limits before treating it as a product.
How do I verify the NaOH-to-H2S control ratio?
Challenge the H2S load, trace the measurement through the controller to actual caustic flow, then reconcile inlet gas, outlet gas, liquid analysis, and NaOH addition after all process and analyzer response times have elapsed.