Scrubbers - cont'd
Several key factors must therefore be taken into consideration when designing and sizing the scrubbing system. These include:
Composition of gas load
The composition of the gas load must be known with respect to:
Solids loading, particle size distribution and chemical composition
Water vapor loading
Toxic gas loading
Inerts loading
Variations in gas load
The basis of the scrubber design should take into consideration
the peak gas loading, the minimum gas loading and the mean gas loading in addition
to corresponding variations in inert gas loading.
Depletion / saturation of scrubbing liquid
Analysis of the reaction stoichiometry between the gas and the scrubbing liquid will give some indication of the minimum scrubbing liquid strength at which the absorption process can occur for a recirculatory system. A methodology must be in place that ensures replenishment of the scrubbing liquid at an appropriate point. Hence monitoring of depletion of scrubber liquid and modeling of breakthrough concentrations is critical. Furthermore, the process may specify a maximum concentration of absorbed gas in the scrubbing liquid at which the scrubber liquid should be replenished.
Disposal of scrubbing liquid
If the regeneration of the scrubbing liquid for re-use is
not practical or economical, the disposal of the scrubbing liquid becomes an
important operational issue. Factors to consider include:
- the quantity generated during an emergency could be large
- the liquid could contain hazardous or controlled materials even in spent condition
- the liquid may pose operational, readiness or logistical difficulties because
of the infrequent or sporadic occurrence
- possible scrubbing liquid decomposition, e.g. create additional concern if
the decomposition generates additional heat loads or generate high volume of
gases
- lack of proper attention if the plant is shutdown from the incident or as
a result of concurrent safety concerns
Provision of Back-up systems
In the vent of scrubber failure, it is sometimes possible to isolate plant and process to prevent toxic gas emission by implementation of appropriate interlocks and control systems. However, if temporary isolation of plant and process is unfeasible a back up system should be provided.
Control systems
The control system for the scrubber operation should be interlocked with the plant and processes that the scrubber services such that in the event of scrubber failure process operations can be isolated and / or suspended. The control system should feature scrubber diagnostics that verify and indicate that the scrubber is healthy and working.
Monitoring and instrumentation
Typical instrumentation on a toxic gas scrubbing system should include:
Stack gas analyzer
Scrubbing liquor flow indicator
Scrubbing liquor tank level indication
Flow indication or DP instrumentation across scrubbing fan
Process interlocks for event of scrubber failure