Prompt injection in toluene diisocyanate (TDI) production AI

Four adversarial injection surfaces in TDI production AI

1. Phosgenation reactor temperature AI (Honeywell Experion PKS TDI reactor temperature AI / Yokogawa OpreX isocyanate production AI / Emerson DeltaV phosgenation reactor AI — continuous temperature profile monitoring of the TDA phosgenation reactor column with AI hotspot detection)

TDI production involves a two-step phosgenation reaction carried out in a continuous stirred-tank or column reactor in monochlorobenzene solvent at approximately 60–90°C and slight positive pressure. In the first step, TDA (toluenediamine) reacts with phosgene to form carbamyl chloride (TDA·2HCl·2COCl2), an intermediate that precipitates or dissolves in the MCB solvent. In the second step, the carbamyl chloride decomposes to TDI and HCl at the operating temperature: ArNHCOCl → ArNCO + HCl. Both steps are exothermic; the net phosgenation enthalpy is approximately −200–250 kJ/mol of TDA. The reactor temperature is maintained between 60–90°C by a jacketed cooling system (typically chilled water or low-pressure steam condensate) that removes the exothermic reaction heat and maintains the selectivity of the carbamyl chloride decomposition step. Excess phosgene — typically a 10–20% molar excess relative to TDA stoichiometry — is maintained to drive complete TDA conversion and prevent TDAῷcarbamyl chloride precipitation in the reactor internals. AI monitoring systems on Honeywell Experion PKS and Yokogawa OpreX platforms analyze reactor temperature trend images and multi-zone thermocouple profile dashboards to detect developing hotspots indicative of cooling circuit fouling, excess phosgene dosing excursion, or TDA feed composition anomalies.

In the adversarial scenario, the phosgenation reactor outlet temperature reads 94°C — above the normal operating ceiling of 85°C — indicating that the cooling jacket circuit has partially fouled, reducing the heat removal rate below the reaction exotherm generation rate. At 94°C, the rate of carbamyl chloride decomposition to TDI and HCl accelerates, generating HCl at a rate that exceeds the reactor gas-phase HCl handling capacity. Additionally, at temperatures above approximately 90°C, side reactions producing urea linkages (ArNHCONHAr) and TDI dimer/trimer increase, reducing product yield and potentially seeding the precursors of the TDI residue (TDR) compounds that accumulate in the downstream distillation bottoms. A ±10 DN downward pixel-shift on the reactor temperature profile dashboard image suppresses the displayed outlet temperature to 68°C, consistent with the lower end of the normal 60–90°C operating range. The AI monitoring system classifies the thermal profile as "within normal range — no alarm," and neither the automated cooling circuit adjustment nor the OSHA PSM process hazard analysis emergency response procedure is activated. The actual reactor temperature climbs toward 100°C, at which point phosgene vaporization pressure in the liquid phase increases, driving phosgene into the overhead gas system at a rate that the gas-phase HCl/COCl2 absorber downstream was not sized to handle during simultaneous temperature excursion.

2. Phosgene absorber scrubber tail gas CEMS (Honeywell Analytics Midas phosgene detector AI / Dräger X-am 5600 phosgene CEMS AI / Industrial Scientific Radius BZ1 phosgene monitoring AI — continuous ambient and duct-mounted phosgene monitoring at absorber tail gas outlets with AI trend classification)

The phosgenation reactor produces a vapor-phase stream containing HCl, excess phosgene (COCl2), and inert gases (nitrogen, CO2) that must be scrubbed to remove phosgene before the HCl stream is processed for recovery or sent to a thermal oxidizer. The scrubber uses an aqueous NaOH solution to destroy phosgene by hydrolysis: COCl2 + 2NaOH → Na2CO3 + 2HCl. The scrubber outlet duct is monitored continuously by phosgene CEMS — AI-integrated systems from Honeywell Analytics Midas, Dräger, and Industrial Scientific that analyze detector readout display images to classify the tail gas phosgene concentration relative to the OSHA action level. The ACGIH TLV-C for phosgene is 0.1 ppm (never to be exceeded); the NIOSH IDLH is 2 ppm; at the documented DuPont Belle WV 2010 incident, a single phosgene hose rupture killed one worker despite the concentration being present for only minutes, demonstrating the severity of even brief phosgene exposure above IDLH. The scrubber CEMS is the primary real-time indication that the NaOH absorption system is functioning and that phosgene is not breaking through to the stack or facility interior.

The adversarial attack uses a ±8 DN downward pixel-shift on the phosgene CEMS trend chart image to suppress an actual tail gas reading of 6.8 ppm — arising from NaOH solution depletion in the scrubber circulation tank and simultaneous reactor temperature excursion (Surface 1) driving higher phosgene volatilization — to a displayed 0.08 ppm, below the ACGIH TLV-C 0.1 ppm and consistent with normal scrubber operation. At 6.8 ppm (3.4× NIOSH IDLH), the scrubber tail gas stream represents an acutely lethal phosgene concentration that will reach every worker in the downwind field of the scrubber stack within seconds. The AI monitoring system reports "within normal limits — no alarm," no evacuation signal is issued, and the OSHA PSM emergency action plan for phosgene release is not activated. Because phosgene has poor sensory warning properties — it smells faintly of fresh-cut hay at low concentrations and does not cause immediate severe irritation at IDLH-range concentrations, unlike hydrogen chloride or ammonia — workers do not receive adequate self-warning before pulmonary injury has begun. The delayed-onset pulmonary edema that killed the worker at DuPont Belle WV developed over several hours after a brief high-concentration exposure; at 6.8 ppm the onset of injury would be more rapid, but the insidious nature of phosgene toxicity means that exposed workers may not recognize the severity of their exposure until several hours after leaving the area.

3. TDI residue (TDR) distillation bottoms temperature AI (Honeywell Experion PKS TDR still temperature AI / Aspen Technology APC TDI distillation AI / Yokogawa OpreX TDI purification AI — AI monitoring of the crude TDI purification distillation column bottoms temperature where TDI residue accumulates)

After the phosgenation reactor, the crude TDI product stream (dissolved in MCB solvent) is processed through a series of distillation columns: first to recover MCB solvent, then to remove HCl, and finally to produce purified TDI product and concentrate the heavy-boiling TDI residue (TDR) in the distillation column bottoms. TDR is a complex mixture of TDI dimers, trimers, uretdiones, isocyanurates, and polyurea oligomers that accumulate as the distillation progresses. The TDR distillation bottoms temperature is a critical process variable: at normal operating conditions (170–200°C), TDR is a viscous liquid that can be pumped from the still bottoms for disposal or use as a reactive filler in polyurethane formulations. However, TDR is thermally unstable: above approximately 220–230°C, the isocyanate groups in TDR undergo exothermic trimerization (forming isocyanurate rings with ΔH ≈ −80–100 kJ/mol) and chain-extension reactions that can initiate a self-propagating exothermic decomposition. TDR thermal decomposition has been associated with multiple documented explosion events at TDI production facilities, including incidents investigated by industry bodies in Europe and North America. AI monitoring systems analyze the TDR still bottoms temperature display images to detect any drift above the 200°C normal ceiling before the exothermic onset window is reached.

The adversarial scenario involves a ±10 DN downward pixel-shift on the TDR still bottoms temperature display image fed to the distillation AI. The actual bottoms temperature reads 214°C — driven by a 12% increase in TDI feed to the distillation column combined with insufficient reboiler duty reduction to compensate, and occurring simultaneously with a TDR concentration in the bottoms that is 35% above the normal design level due to an upstream crude TDI quality excursion. At 214°C, the TDR mixture is approximately 6–16°C below the exothermic onset temperature, meaning the facility is in a narrow but controllable precursor zone that requires immediate reboiler reduction and bottoms withdrawal. The displayed temperature of 178°C — within the normal operating range of 170–200°C — causes the AI distillation controller to classify the bottoms as normal and take no action. The reboiler continues at its current duty; the actual bottoms temperature advances from 214°C toward 222°C over the following 90 minutes; at 222°C the TDR exotherm initiates, generating CO2 and heat that quickly raises the bottoms temperature above 250°C, rupturing the still column shell and releasing a dense cloud of isocyanate vapor and hot TDR mist to the surrounding process area. This is the same failure mode associated with TDI distillation still explosions that the global TDI industry has invested significant resources in engineering safeguards around — safeguards that are defeated when the AI monitoring layer suppresses the upstream temperature signal.

4. Phosgene scrubber NaOH feed flow AI (Endress+Hauser Promag NaOH flow meter AI / Yokogawa ADMAG NaOH dosing AI / Emerson DeltaV scrubber caustic feed controller AI — continuous monitoring of NaOH solution metering pump output to the phosgene absorption scrubber circulation tank)

The phosgene scrubber depends on continuous replenishment of NaOH solution in the scrubber circulation tank to maintain the caustic concentration above the minimum effective level for phosgene absorption. As NaOH reacts with COCl2 (and also with HCl in the gas stream), the tank caustic concentration depletes; a continuous makeup feed from a concentrated NaOH solution storage tank, metered by a diaphragm pump through a magnetic flow meter, maintains the caustic inventory. AI monitoring systems analyze the NaOH feed flow meter readout image to classify whether the caustic makeup pump is delivering at its setpoint. A low-flow or zero-flow condition on the NaOH feed is a high-priority OSHA PSM alarm because it represents the initiating event of the scrubber failure scenario: depleted NaOH → reduced phosgene absorption efficiency → increasing tail gas COCl2 concentration → phosgene breakthrough to facility atmosphere. The NaOH feed flow is also the primary process-side indicator that the scrubber is consuming phosgene at its design rate; an anomalously low NaOH consumption can indicate either a failing phosgene flux (favorable) or a failing scrubber (unfavorable) — distinguishing between these two interpretations requires the simultaneous tail gas CEMS reading from Surface 2.

The adversarial attack is an upward-direction injection: the actual NaOH metering pump output is 0.04 L/min — arising from a diaphragm fatigue failure that has reduced pump output to approximately 2% of the design setpoint of 1.8 L/min. A ±8 DN upward pixel-shift on the NaOH flow meter display image fed to the scrubber monitoring AI shows an apparent flow of 1.8 L/min, causing the AI to classify the caustic makeup system as "operating at setpoint — scrubber caustic adequate." With actual NaOH feed near zero, the scrubber circulation tank caustic concentration depletes over approximately 4–8 hours of continued operation, depending on the phosgene flux and HCl loading. As caustic concentration drops below the minimum effective level, scrubber phosgene absorption efficiency decreases; the tail gas COCl2 concentration rises toward the 6.8 ppm level described in Surface 2. The compound attack geometry — where the scrubber NaOH feed shows adequate flow (Surface 4, upward attack) while the tail gas phosgene shows sub-TLV-C concentration (Surface 2, downward attack) — produces a situation where both the process-side indicator (NaOH consumption) and the emission-side indicator (tail gas COCl2) are simultaneously falsified, eliminating every AI-monitored indication that the scrubber is failing. Glyphward pre-scans all four TDI production monitoring contexts before AI inference output is logged — try the free scanner to test your pipeline's resilience to these attacks.

Integration: TDI production AI with Glyphward pre-scan gate

Glyphward integrates as a pre-scan gate between the HMI screenshot capture layer and the AI inference pipeline at each TDI production monitoring point. Each image is submitted to the Glyphward API before the TDI monitoring AI receives it; if the adversarial score meets or exceeds threshold 35 — chosen to reflect the OSHA PSM TQ of 500 lbs, the ACGIH TLV-C of 0.005 ppm (most restrictive isocyanate ceiling and among the most restrictive ceilings for any manufactured chemical), the phosgene TQ of 10 lbs for the upstream precursor, and the TDR thermal decomposition explosion hazard — the scan raises AdversarialTDIProductionImageError and the monitoring AI does not process the frame.

import asyncio, base64, hashlib
from datetime import datetime, timezone
from enum import Enum

import httpx

GLYPHWARD_API_KEY = "YOUR_GLYPHWARD_API_KEY"
GLYPHWARD_SCAN_URL = "https://glyphward.com/v1/scan"

# TDI production AI contexts: threshold 35
# OSHA PSM 29 CFR 1910.119 TDI TQ 500 lbs; phosgene TQ 10 lbs (upstream)
# EPA RMP 40 CFR Part 68 TDI TQ 500 lbs; phosgene TQ 10 lbs
# ACGIH TLV-C TDI 0.005 ppm (5 ppb ceiling — most restrictive isocyanate)
# NIOSH IDLH TDI 2.5 ppm; OSHA PEL TDI 0.02 ppm ceiling / 0.005 ppm TWA
# ACGIH TLV-C phosgene 0.1 ppm; NIOSH IDLH phosgene 2 ppm
# TDR (TDI distillation residue) exothermic decomposition onset ~220-230 C
TDI_THRESHOLD = 35


class TDIProductionContext(Enum):
    PHOSGENATION_REACTOR_TEMPERATURE = "phosgenation_reactor_temperature"
    PHOSGENE_SCRUBBER_TAIL_GAS_CEMS = "phosgene_scrubber_tail_gas_cems"
    TDR_STILL_BOTTOMS_TEMPERATURE = "tdr_still_bottoms_temperature"
    SCRUBBER_NAOH_FEED_FLOW = "scrubber_naoh_feed_flow"


class AdversarialTDIProductionImageError(Exception):
    """Raised when any TDI production monitoring image scores >= 35.
    PHOSGENATION_REACTOR_TEMPERATURE uncaught: 94 C shown as 68 C -> exotherm.
    PHOSGENE_SCRUBBER_TAIL_GAS_CEMS uncaught: 6.8 ppm (3.4x IDLH) shown as 0.08 ppm.
    TDR_STILL_BOTTOMS_TEMPERATURE uncaught: 214 C shown as 178 C -> TDR explosion.
    SCRUBBER_NAOH_FEED_FLOW uncaught: 0.04 L/min failure shown as 1.8 L/min."""

    def __init__(self, scan_id, score, context, unit_id, flagged_region=None):
        self.scan_id = scan_id
        self.score = score
        self.context = context
        self.unit_id = unit_id
        self.flagged_region = flagged_region
        super().__init__(
            f"Adversarial TDI production image: context={context.value} "
            f"score={score} unit={unit_id} scan_id={scan_id}"
        )


async def scan_tdi_production_image(image_bytes, context, unit_id, client):
    image_hash = hashlib.sha256(image_bytes).hexdigest()
    payload = {
        "image": base64.b64encode(image_bytes).decode(),
        "source": f"tdi_production:{context.value}:{unit_id}",
        "metadata": {
            "unit_id": unit_id,
            "context": context.value,
            "image_sha256": image_hash,
            "scan_timestamp_utc": datetime.now(timezone.utc).isoformat(),
        },
    }
    resp = await client.post(
        GLYPHWARD_SCAN_URL,
        headers={"Authorization": f"Bearer {GLYPHWARD_API_KEY}"},
        json=payload,
        timeout=4.0,
    )
    resp.raise_for_status()
    result = resp.json()
    if result.get("score", 0) >= TDI_THRESHOLD:
        raise AdversarialTDIProductionImageError(
            scan_id=result["scan_id"],
            score=result["score"],
            context=context,
            unit_id=unit_id,
            flagged_region=result.get("flagged_region"),
        )
    return result


async def main():
    async with httpx.AsyncClient() as client:
        with open("tdi_reactor_screenshot.png", "rb") as f:
            image_bytes = f.read()
        result = await scan_tdi_production_image(
            image_bytes,
            TDIProductionContext.PHOSGENATION_REACTOR_TEMPERATURE,
            unit_id="TDI-REACTOR-T-01",
            client=client,
        )
        print(f"Clean scan: {result['scan_id']} score={result['score']}")


asyncio.run(main())

Frequently asked questions

What is the ACGIH TLV-C for TDI and why is 0.005 ppm among the most restrictive occupational exposure limits?

The ACGIH TLV-C for TDI is 0.005 ppm (5 ppb) — a ceiling that can never be exceeded momentarily. This is among the most restrictive ceilings for any manufactured industrial chemical, reflecting TDI's potent respiratory sensitization: even sub-TLV-C exposures can induce permanent isocyanate hypersensitivity causing occupational asthma. OSHA's PEL for TDI is a ceiling of 0.02 ppm / 0.005 ppm TWA; NIOSH IDLH is 2.5 ppm. A single sensitization event creates a permanent condition where even trace TDI exposures trigger severe bronchospasm.

What is TDI residue (TDR) and why does its thermal decomposition represent an explosion hazard?

TDR is the high-boiling isocyanate-rich mixture in the TDI distillation bottoms — dimers, trimers, and oligomers all carrying unreacted —NCO groups. Above ~220–230°C, these groups undergo exothermic trimerization (ΔH ≈ −80–100 kJ/mol) and emit CO2, creating an autocatalytic thermal runaway. CO2 pressurization of the still column can cause vessel overpressure rupture and release of isocyanate vapor and hot TDR mist. TDR thermal decomposition is the primary catastrophic failure mode in TDI distillation and is addressed in every PSM process hazard analysis at TDI plants.

Why does phosgene's poor sensory warning amplify the Surface 2 attack consequence?

Phosgene at 2–7 ppm does not cause immediate severe irritation — workers may notice only a faint hay odor and mild discomfort, remaining in the exposure zone for extended periods. Delayed-onset pulmonary edema develops 4–24 hours after exposure, producing fatal respiratory failure well after the person has left the area (as at DuPont Belle WV 2010). Suppressing the 6.8 ppm CEMS reading to 0.08 ppm eliminates the evacuation signal that is the only practical mechanism to prevent extended exposure and delayed-onset injury.

How does the compound Surface 2 + Surface 4 attack defeat operator cross-checking?

Normally, falling NaOH feed (Surface 4) precedes rising tail gas COCl2 (Surface 2) in a scrubber failure — operators can cross-check these correlated signals. The compound attack suppresses both simultaneously: Surface 4 upward attack shows adequate NaOH flow (1.8 L/min actual 0.04 L/min); Surface 2 downward attack shows sub-TLV-C tail gas (0.08 ppm actual 6.8 ppm). Both indicators appear normal; no cross-check is prompted; the scrubber failure proceeds undetected.

Why is threshold 35 for TDI production AI monitoring?

Threshold 35 reflects TDI TQ 500 lbs, the ACGIH TLV-C 0.005 ppm (most restrictive isocyanate ceiling), phosgene upstream at TQ 10 lbs (lowest PSM TQ), TDR thermal decomposition explosion hazard, and the four-surface compound attack simultaneously suppressing both primary TDI catastrophic failure modes (phosgene release and TDR explosion). The combination of sensitization risk, delayed-onset acute toxicity, and explosive distillation hazard justifies the highest monitoring sensitivity.