HCL synthesis units are an important part of many chemical operations worldwide, supplying a reliable and cost-effective source of HCL and HCL gas for a wide range of applications.
Production of HCL results from a strong exothermic reaction of hydrogen and chlorine. In all cases, it is essential to design-in safety. It is critical that safety is designed into the burner, tower design and control system. Burner safety management protocols need to be compliant with the National Fire Protection Association (NFPA) and National Electric Code (NEC) rules and standards, and instrumentation must be adequate to manage the entire process. This includes burner operation, monitoring upstream feed gas conditions and synthesis unit operation and ensuring final product quality. In addition, there must be systems in place to assure that critical utilities are available to deal with special case scenarios and possible emergency conditions.
NFPA rules and regulations should be considered the minimum requirements. The owner must take additional steps to make sure the systems are safe and meet OSHA, EPA, state, and local requirements. The NFPA’s minimum requirements are summarized as such:
- Use of an appropriate flame sensing device meeting industry, insurance, and jurisdiction requirements. Flame sensors are generally UV and UV/IR devices
- Flame observation components must have adequate operation and positioned properly to observe pilot and main burner operation.
- Fuel supply and the required oxygen supply must be monitored to be within the safe operating ranges for the synthesis unit
- Logic solving devices must meet specific reliability requirements and have the appropriate approvals
- Fuel and oxidizer shut off devices must have appropriate approvals, leakage rates, and actuation times and reliability.
- The synthesis unit furnace/burner system must have high temperature protection devices. This generally implies the burner cooling systems must be integrated into the Burner Safety Management (BSM) system
- There can be requirements for certain sensors and devices to be hard wired and their indications monitored outside a Distributed Control System (DCS) and/or Programmable Logic Controller (PLC) management system.
- Specific purge volumes and timing must be clearly indicated for starting and stopping the burner.
Generally, State and local fire marshal laws have the final say on safety requirements. Insurance companies will also have specific safety and design requirements in addition to OSHA and the EPA having specific hazard review requirements. EPA emission permit limits may require additional safety system configurations.
Process Determined Safety Requirements
CL2/H2 burners have a very specific startup, transition to low fire and shutdown sequence. Normal operation requires management of the burner and the absorption plant to operate within feasible and safe windows.
Feed gas supplies must be monitored for temperature, pressure, and composition. Metering of the feed gas must then be compensated for variations in temperature, pressure and composition. The resulting combustion products need to remain in the reducing region, not pass back and forth between oxidizing/reducing regions. Excess H2 must then be managed in order to prevent too much HCl flowing into the tail tower. The BSM system must be connected to monitor external process variables and utilities such as:
- Cooling water supply
- Absorption water supply
- Purge gas supply
- H2/Total oxidizer ratio
- System interlocks not directly related to the burner
- Downstream systems and destination interlocks
Feed Gas Metering Requirements
H2 supply
Wet H2 gas supplies must be conditioned to arrive at the meter loop at a non-condensing temperature and pressure. Gas analyzers for H2O and N2 contents should be employed. The supply pressure for H2 must be greater than 3.0 psig to allow accurate metering and mixing with Cl2 at the burner tip and the flow rate of H2 must be compensated for temperature, pressure and molecular weight/composition. Keeping in mind that even small concentrations of H2O and inerts change the average molecular weight of flowing H2 gas. All final compensated flows should be reported in volumetric and not weight units.
Cl2 supply
Wet primary gas must be conditioned to arrive in a non-condensing condition at the meter loop. Primary gas should be analyzed at the supply point with a UV device or by regular lab samples. Cl2 content can be estimated by measuring temperature and pressure off the final stage. O2 and CO2 are commonly measured using LED laser devices. O2 can also be measured using a paramagnetic sensor, while CO2 is sometimes measured using an IR device.
Cl2 delivery pressure must be at least 3.0 psig and the flow rate must be compensated for temperature, pressure and molecular weight/composition. All final compensated flows should be reported in volumetric and not weight units.
Operating H2/O2 ratio
The DCS must compute the H2, Cl2 and O2 component flows using the compensated gas flows and estimated compositions. A configured ratio loop meters the H2 based on the oxygen flow including the startup combustion air. The minimum H2/O2 ratio should be between 1.05 and 1.15 under normal circumstances and the shutdown limit should be <1.05 for a specific short time period. Control values must be properly sized to provide H2/O2 control over a wide range. Generally, split-range valves are required for low pressure Cl2 supplies.
Primary flow elements should be venturi tubes for low pressure supply gases and should utilize dual flow element. Pressure transmitters should utilize sealed leads and be absolute transmitters. Select primary transmitters that have SIL2 reliability ratings and select UL labeled fuel shut off valves intended for H2 and pilot fuel service.
All DCS metering, compensation and composition calculations must be kept in a secure password protected area of the DCS and any operator inputs required must be checked for out-of-range values.
Normal Operation and Supervision
Quality Control
The DCS computes desired Cl2 flow for a given capacity and ramps the unit to this set point. The H2 flow is ratioed to the total oxidizer flow. The deionized water flow to the tails tower is ratioed to the Cl2 flow and trimmed by the density signal on the product HCl setpoint. Cooling water tracks the synthesis unit heat load under temperature control and the HCl product is transferred to storage under level control.
DCS supervision
DCS continuously scans an interlock truth table looking for exception, alarm points, and violated interlocks. The BSM has independent interlocks that are monitored by the DCS. However, the BSM has ultimate authority over the burner operation, its safety control and commands.
The DCS manages the basic control loops, logic for startup, shutdown and normal operation of the synthesis unit. The DCS manages and copes with external variables not suited to be managed by the BSM system. The DCS can communicate with the BSM regarding remote start/stop and transmitted interlock bits.
Special Situations
Stack Fires
Stack fires are caused by lightning strikes or static electricity accumulation. Stack fires must be detected by UV/IR devices. In most cases stack fires cannot be extinguished using N2 only. The more effective method to extinguish is to use a steam atomized spray nozzle aimed into the H2 flame plume. Shutting down the synthesis unit is not recommended in that it will cause an abrupt vacuum, allowing combustible mixtures into the tail tower or beyond.
Loss of the N2 purge utility
When the N2 purge source is lost it is advisable to immediately shutdown the synthesis unit. Therefore, an emergency source of N2 liquid should be considered. In addition, a special DCS procedure can also be implemented to replace Cl2 with combustion air. This locks down the synthesis unit down in a safe manner until the O2 condition at the free burner coincides with the H2/O2 equivalence point.
Almost all synthesis unit system suppliers offer designs with rupture disks or weighed vents to mitigate explosion hazards. These countermeasures may not be enough in venting high over pressures associated with O2/H2/Cl2 explosive mixtures.
It is imperative that the safety and supervisory systems not allow the synthesis unit to enter a dangerous operating region.
CO Stack emissions control.
CO emissions from synthesis units are frequently overlooked. CO2 is almost always present in the Cl2 feed streams. The amount depends on the cell characteristics and the brine treatment steps.
CO2 is converted to CO in the flame due to high adiabatic temperatures approaching 2200-2500⁰C. As combustion products cool down, the reaction CO + H2O==> CO2 + H2 approaches equilibrium in the 750-1000⁰ C region. This can cause approximately 30-60% of all CO2 entering the burner to be converted to CO and appear in the stack gas.
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