Practical Ratio Control: Black Liquor Oxidation with Yokogawa CENTUM VP and ABB 800xA
The Core Problem: Oxygen-to-Liquor Ratio Drift
Ratio control maintains a fixed proportion between two process variables. In black liquor oxidation, operators must add pure oxygen to black liquor at a precise ratio. The goal: reduce H2S and mercaptan emissions while recovering sulfur values. If the ratio drifts, either the oxidation stays incomplete or oxygen wastes excessively.
Yokogawa CENTUM VP handles this loop with its Advanced Process Controller (APC) function. ABB 800xA uses its Control Builder to implement the same logic in a redundant AC 800M controller. Both platforms require the same three components: a wild flow transmitter, a captive flow transmitter, and a multiplier block that drives the setpoint.
Step 1: Configure the Differential Pressure Flow Transmitters
Both black liquor and oxygen lines use orifice plate meters with differential pressure transmitters. In a CENTUM VP system, connect the FT-101 (black liquor) and FT-102 (oxygen) to the AAI143 analog input card. Enable the square root extraction function. This converts the raw 4-20 mA signal to a linear flow value.
- Orifice plate: 4-inch ANSI 600# with 2.5-inch bore
- DP transmitter range: 0–200 inH2O
- Flow range: 0–1500 GPM (black liquor), 0–300 SCFM (oxygen)
- Calibration: Apply 4.00 mA (zero) and 20.00 mA (span) with a HART communicator
In ABB 800xA, use the AI810 analog input card in the AC 800M controller. Configure the square root function in the Control Builder. The CI853 serial communication module connects the HART devices for remote diagnostics.
Step 2: Build the Multiplier and Ratio Setpoint Logic
The multiplier block is the heart of ratio control. It takes the wild flow signal and multiplies it by the ratio coefficient R. The result becomes the captive flow setpoint.
Formula: SPO2 = Fliquor × R
Where R is typically 0.15–0.25 SCFM O₂ per GPM black liquor. Calculate R from the stoichiometric oxygen demand. In the black liquor oxidation process, approximately 0.18 SCFM O₂ per GPM liquor achieves 85% oxidation efficiency at 180°F and 15 psig.
Configure a manual controller (HC) in parallel with the automatic ratio controller. Operators use HC to set a fixed oxygen flow during startup. Once the loop stabilizes, switch to AUTO and let the ratio controller track the liquor flow.
Set the ratio coefficient alarm: HI = 0.30, LO = 0.10. If R drifts beyond these limits, trigger an operator alarm in the HMI trend display.
Step 3: Tune the Captive Flow PID Controller
The oxygen control valve (FCV-102) must respond quickly to setpoint changes. Use the auto-tune function in CENTUM VP's PID block. Set the controller mode to PI. Typical tuning parameters:
- Proportional band: 50–80%
- Integral time: 3–8 seconds
- Derivative: 0 (disable for flow loops)
- Setpoint rate limit: 5% per second (prevent valve slam)
Verify valve performance with a step test. Step the setpoint by 10% and record the response time. Acceptable criteria: rise time under 3 seconds, overshoot below 5%, settling time under 15 seconds.
In ABB 800xA, download the PID parameters to the AC 800M controller via the Control Builder. Use the Online Change function to adjust parameters without stopping the process.
Step 4: Diagnose and Correct Ratio Drift Faults
Ratio drift faults fall into three categories.
- Wild flow transmitter drifts low: The controller underestimates the liquor flow, so oxygen falls short. The oxidation reaction becomes incomplete. H2S readings rise in the vent stack.
- Wild flow transmitter drifts high: The controller demands excess oxygen. Oxygen consumption rises by 15–20%. The oxygen pressure regulator opens more frequently, causing wear on the FCV-102 seat.
- Multiplier coefficient drifts due to a corrupted register: This causes a sudden setpoint jump. The oxygen valve slams open or shuts tight. Detect this by monitoring the rate of change of the setpoint signal. If dSP/dt exceeds 20% per second, trigger an interlock that holds the valve at its last known position.
Check the transmitter grounding. A floating ground on the 4-20 mA loop causes random signal noise. Use a process calibrator to inject a 12.00 mA signal at the analog input terminal. Verify the DCS reads 50% of the flow range.
Conclusion and Action Advice
Ratio control in black liquor oxidation requires three steps. First, configure the orifice plate DP transmitters with square root extraction on both the AAI143 (CENTUM VP) and AI810 (ABB 800xA) cards. Second, build the multiplier block with a locked coefficient range of 0.10–0.30 and a manual controller for startup. Third, tune the captive flow PID with PB=65%, Ti=5s, and a 5%/sec rate limit on setpoint changes.
Schedule a monthly calibration verification of both flow transmitters. Use a HART communicator to check the DP transmitter output against a reference manometer. If the error exceeds 1% of span, perform a zero and span adjustment. Document the results in the maintenance log.
When ratio drift occurs, trace the fault through the transmitter, multiplier, and valve. A process calibrator at the analog input terminals confirms whether the fault is in the field or the control system. Keep the manual controller ready at all times during oxidation startup.
