Process engineering

Engineering insights focused on the relationship between industrial processes and air pollution control systems.

Topics include emission generation mechanisms, process variability, pollutant speciation, and the impact of upstream conditions on system design and performance.

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    Scrubber Recirculation Pump Selection

    Why the scrubber recirculation pump is critical

    Choosing the right scrubber recirculation pump is one of the most critical decisions in wet scrubber design.

    • the actual liquid-to-gas ratio (L/G)
    • the stability of the system
    • the effectiveness of mass transfer
    • the long-term behavior under fouling, salts, and process variability

    Most scrubbers do not fail because of incorrect flow rate.
    They fail because the recirculation system becomes unstable over time.
    And the pump is the core of that system.

    Types of scrubber recirculation pumps

    In industrial wet scrubbers, three main types of wet scrubber recirculation pumps are commonly used:

    • Vertical centrifugal pumps in Polypropylene (PP)
    • Horizontal centrifugal pumps in Stainless Steel (AISI 316)
    • Magnetic drive (seal-less) pumps

    These technologies are often treated as interchangeable.
    In reality, they behave very differently under real operating conditions.

    Vertical polypropylene pumps for wet scrubbers

    vertical scrubber recirculation pump operating with solids and foam in industrial wet scrubber tank

    Configuration

    • Vertical shaft, immersed or semi-immersed
    • Wetted parts in polypropylene
    • Seal arrangement depending on pump design

    Advantages

    • Excellent resistance to aggressive chemicals (acids, alkalis, oxidants)
    • No external leakage to the environment, as the pump operates in immersed configuration
    • High tolerance to:
    • Solids
    • sludge
    • salt crystallization

    Limitations

    • Lower hydraulic efficiency
    • Limited head
    • Reduced structural rigidity compared to metal pumps

    Engineering perspective

    Vertical PP pumps are often the most reliable scrubber recirculation pumps in real industrial environments.

    They are not optimized for efficiency, but they are optimized for survival:

    • they tolerate contamination
    • they continue operating with unstable chemistry
    • they do not fail when solids or salts accumulate

    In systems with variable loads and dirty liquids, they are frequently the only pumps still operating after extended periods.

    Stainless steel (AISI 316) recirculation pumps

    ndustrial wet scrubber recirculation pump system with horizontal stainless steel pump and chemical resistant piping

    Configuration

    • Horizontal installation
    • Mechanical seal
    • Stainless steel construction

    Advantages

    • High hydraulic efficiency
    • Higher head capability
    • Strong mechanical structure

    Limitations

    • Mechanical seal is a critical failure point
    • Sensitive to:
    • crystallization
    • suspended solids
    • intermittent operation
    • Risk of leakage over time
    • Limited chemical resistance depending on process conditions

    Engineering perspective

    Stainless steel pumps perform well as scrubber recirculation pumps only under controlled conditions:

    • clean liquid
    • stable chemistry
    • predictable operation

    In real scrubber applications:

    • seals degrade
    • deposits form
    • leakage becomes a maintenance issue

    They are efficient, but not inherently robust.

    Magnetic drive pumps in scrubber applications

    Configuration

    • Seal-less design using magnetic coupling
    • No direct mechanical connection between motor and impeller

    Advantages

    • Zero external leakage
    • Suitable for hazardous or toxic fluids
    • Reduced maintenance on seals

    Limitations

    • Not tolerant to solids or particles
    • Sensitive to crystallization
    • Risk of overheating or magnetic decoupling
    • Higher cost

    Engineering perspective

    Magnetic drive pumps are often selected as scrubber recirculation pumps for safety reasons.
    However, they require:

    • clean fluids
    • stable operating conditions
    • controlled chemistry

    These conditions are rarely guaranteed in industrial scrubbers.

    When solids, salts, or variability are present, magnetic drive pumps tend to become unstable or fail.

    Which scrubber recirculation pump is best?

    The best scrubber recirculation pump is not the most efficient one.

    It is the one that:

    • tolerates solids
    • handles salt accumulation
    • remains stable under variable conditions

    In most real scrubber systems, this leads to a clear conclusion:

    Vertical polypropylene pumps provide the highest operational robustness.

    How to choose the right scrubber recirculation pump

    Choose a vertical PP pump if:

    • solids are present
    • salt buildup is expected
    • operating conditions are variable

    Choose a stainless steel pump if:

    • the liquid is “clean”
    • higher head is required
    • maintenance can be managed

    Choose a magnetic drive pump if:

    • zero leakage is mandatory
    • the fluid is “clean”
    • operating conditions are stable

    Common mistakes in scrubber pump selection

    A frequent error in selecting a scrubber recirculation pump is focusing on:

    • efficiency
    • brand
    • initial cost

    Instead of:

    • process variability
    • fouling potential
    • long-term operation

    Most articles about scrubber recirculation pumps focus on materials and performance curves.

    Very few address what actually matters:

    how the pump behaves when the system is no longer ideal.

    Ignoring this leads to:

    • unstable L/G ratios
    • reduced removal efficiency
    • increased maintenance and downtime

    Conclusion

    A scrubber recirculation pump should not be evaluated based on nominal performance.

    It should be evaluated based on:

    • behavior over time
    • tolerance to real process conditions
    • ability to operate without constant intervention

    The real question is not:

    Which pump is the most efficient?

    but:

    Which pump keeps working when the process stops being ideal?

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    Hydrogen Sulfide Removal in Air: Why Caustic Scrubbers Become Unstable

    Hydrogen sulfide removal is not a steady-state problem

    Hydrogen sulfide removal is one of the most common challenges in industrial air treatment.
    In many systems, caustic scrubbers are used as the primary solution to remove H2S.
    On paper, the chemistry is simple.
    In real operation, these systems often become unstable.

    • H2S is absorbed into the liquid phase
    • it reacts with sodium hydroxide (NaOH)
    • it forms soluble sulfide compounds

    On paper, the process is stable.

    In real operation, it is not.

    The chemistry is predictable. The system is not

    The reaction between hydrogen sulfide and caustic soda is well known:

    • H2S + NaOH → NaHS + H2O
    • NaHS + NaOH → Na2S + H2O

    This is not where the problem lies.

    The issue is that industrial air systems do not operate under controlled, steady conditions.

    In real plants:

    • H2S concentration fluctuates
    • emissions occur in peaks, not averages
    • humidity changes continuously
    • aerosols and particulates interfere with mass transfer

    Why hydrogen sulfide removal becomes unstable

    A caustic scrubber is typically designed around average inlet conditions.

    But real systems are defined by variability.

    When peak loads occur:

    • the reaction accelerates locally
    • sulfide concentration in the liquid increases rapidly
    • the recirculation loop changes composition

    Over time:

    • salinity increases
    • mass transfer efficiency drops
    • system response becomes non-linear

    The scrubber still works.

    But it stops working predictably.

    pH control does not guarantee performance

    In many hydrogen sulfide removal systems, pH is used as the primary control parameter.

    The assumption is straightforward:

    high pH ensures effective H2S removal.

    In practice:

    • pH does not reflect sulfide accumulation
    • it does not represent real absorption capacity
    • it does not capture mass transfer limitations

    Operators may observe a stable pH

    while the system performance is deteriorating.

    As a result:

    • caustic consumption increases
    • removal efficiency fluctuates
    • corrective actions become reactive instead of controlled

    pH is an indicator.
    It is not a control strategy.

    Field reality: transient H2S release in industrial processes

    In many industrial processes, hydrogen sulfide is not released at a constant rate.

    A clear example is found in tannery operations during the pickling phase.

    In these conditions:

    • acidic environments react with residual sulfides
    • H2S is released rapidly
    • emissions occur in short, high-intensity peaks

    The key issue is not the average concentration.

    It is the transient load.

    A caustic scrubber exposed directly to these peaks:

    • reacts, but not in a controlled way
    • accumulates reaction products quickly
    • loses operational stability

    Hydrogen sulfide removal becomes a continuous correction process.

    Hydrogen sulfide removal requires load control

    The most critical mistake in H2S treatment design is assuming that the final stage can handle everything.

    In reality, hydrogen sulfide removal requires load management upstream.
    This is where pre-treatment becomes essential.

    The role of pre-treatment in hydrogen sulfide removal

    Iron-based media systems (CIF) are often used as a pre-treatment stage.

    clean catalytic iron media used in CIF systems for hydrogen sulfide removal before sulfur formation
    severe sulfur fouling on pall rings in H2S removal system reducing airflow and efficiency

    They are not designed to remove all hydrogen sulfide.

    Their function is to:

    • reduce peak concentrations
    • stabilize inlet conditions
    • protect downstream scrubbers

    When pre-treatment is correctly applied:

    • transient peaks are dampened
    • load becomes more uniform
    • the caustic scrubber operates within a manageable range

    This changes the system from reactive to controlled.

    Designing hydrogen sulfide removal for real conditions

    Effective hydrogen sulfide removal systems are not designed around average values.

    They are designed around:

    • variability
    • peak loads
    • real process behavior

    A caustic scrubber without pre-treatment is forced to absorb all fluctuations.

    And no single stage can do that reliably.

    A necessary distinction in hydrogen sulfide removal

    Hydrogen sulfide removal in industrial air systems must be treated as a specific engineering problem.

    Air treatment systems are characterized by:

    • low but highly variable concentrations
    • intermittent emissions
    • complex mixtures of contaminants

    Design approaches taken from other processes
    do not apply directly.

    Each system must be designed based on its actual operating conditions.

    Hydrogen sulfide removal does not fail because the chemistry is wrong.
    It fails because the system is designed for average conditions
    instead of real variability.

    Evaluate whether your hydrogen sulfide removal system is designed as a machine or as a process

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    Why Your Ammonia Scrubber Is Not Working

    On paper, ammonia removal with a wet scrubber looks simple.
    Add acid. Control pH. Remove NH₃.

    In reality, many industrial scrubbers fail to remove ammonia consistently — even when all “parameters” seem correct.

    If your ammonia scrubber is not working, the problem is rarely a single variable.
    It is almost always a process issue, not a component issue.

    The Real Problem

    Ammonia absorption is not just a chemical reaction.
    It is a mass transfer process limited by:

    You don’t have “a pH”.

    Most scrubbers fail because they are designed as equipment, not as process systems.

    Why It Happens

    1. pH control is not enough

    Lowering pH shifts ammonia (NH₃) into ammonium (NH₄⁺), which is soluble.

    But:

    • local pH ≠ bulk pH
    • poor mixing creates zones where ammonia is not absorbed

    You don’t have “a single pH value”.
    You have a distribution of pH conditions inside the tower.

    2. L/G Ratio Is Too Low

    Ammonia absorption requires sufficient liquid flow.

    If the liquid-to-gas ratio (L/G) is too low:

    • contact area is reduced
    • absorption capacity is limited

    Typical failure:

    Scrubber sized on airflow only → insufficient liquid phase

    3. Poor Liquid Distribution

    Even with correct flow rate:

    • uneven spray
    • asymmetric piping
    • clogged nozzles

    create preferential paths.

    poor liquid distribution in wet scrubber nozzle spray pattern
    • Gas bypasses the liquid
    • Efficiency collapses

    This is one of the most underestimated causes of failure.

    4. Buffering Systems in the Liquid

    This is where most engineers get it wrong.

    In real systems:

    • carbonates
    • bicarbonates
    • ammonium salts

    create buffer systems

    Result:

    • pH does not drop as expected
    • acid consumption increases
    • ammonia removal becomes unstable

    The problem is not dosing.

    The problem is chemistry equilibrium.

    5. Insufficient Contact Time

    Short towers, high velocity, or undersized packing:

    • reduce residence time
    • limit absorption

    Ammonia removal is not instantaneous

    It requires time + interface

    6. Gas Conditions Are Ignored

    Temperature and saturation matter.

    • hot air reduces solubility
    • low humidity limits absorption
    • low humidity limits absorption

    A scrubber designed for “nominal conditions” will fail in real operation.

    What Most Engineers Get Wrong

    • Designing based only on airflow
    • Assuming pH = performance
    • Ignoring liquid distribution
    • Treating ammonia as a “simple pollutant”
    • Oversimplifying chemistry

    What Actually Works

    An ammonia scrubber works when:

    • L/G ratio is correctly sized
    • liquid distribution is uniform
    • pH is controlled considering buffering systems
    • contact time is sufficient
    • process variability is included in design

    This is not about adding more acid.
    It is about designing the system as a whole.

    When the System Cannot Work

    There are cases where:

    • tower is undersized
    • gas load is too high
    • variability exceeds design limits

    In these situations:

    • no adjustment will fix the problem
    • only a redesign or revamping will

    Can an Existing Scrubber Be Fixed?

    Sometimes, yes.

    Typical interventions:

    • increase liquid recirculation
    • redesign distribution system
    • introduce a second stage
    • modify chemical control strategy

    But:

    • not all systems are recoverable
    • and not all problems are “operational”

    Final Insight

    Ammonia removal is not difficult.

    But stable ammonia removal in real industrial conditions is.

    Is your ammonia scrubber underperforming?

    Contact us for a technical evaluation
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    Scrubber foam problem: root causes

    Scrubber foam problem: context

    In many industrial wet scrubbing systems, foam formation is often dismissed as a secondary or cosmetic issue. In reality, persistent foam is almost always the visible symptom of a deeper process inconsistency.
    This insight originates from real operational evidence: a recirculation tank completely filled with stable foam, generated during normal plant operation.

    What foam really indicates

    Foam inside a scrubber is not random. It is the result of a combination of physical and chemical conditions:

    • Presence of surfactants or organic compounds
    • High gas velocities and turbulence
    • Unstable liquid chemistry (pH, oxidation state, salinity)
    • Accumulation of reaction by-products

    When these factors align, foam transitions from transient to persistent.
    At that point, it stops being a nuisance and becomes a process variable.

    Why Foam Is a Problem

    The consequences are rarely limited to aesthetics:

    • Loss of effective liquid–gas contact
    • Carryover of droplets and contaminants
    • Unstable pressure drop
    • Reduced mass transfer efficiency
    • Risk of downstream fouling
    • Cavitation of recirculation pump

    In extreme cases, the scrubber continues to operate, but no longer performs its intended function.

    The Typical Reaction (and Why It Fails)

    The most common response is the addition of antifoam agents.
    This approach is appealing because it is immediate and simple.
    However, it rarely addresses the root cause.
    Antifoam acts on the symptom, not on the mechanism that generates foam. As a result:

    • Consumption increases over time
    • Performance becomes inconsistent
    • Operating costs rise without solving the instability

    Engineering Perspective: Treat the Cause, Not the Symptom

    Foam must be interpreted within the broader process context.
    The correct approach requires asking fundamental questions:

    • What compounds are actually present in the gas phase?
    • Is the chemical regime (acid/base/oxidative) coherent with those compounds?
    • Is the liquid recirculation accumulating by-products?
    • Is the hydraulic design promoting local imbalances?

    Only by reconnecting the scrubber to the upstream process can the issue be solved.

    Design Implications

    Persistent foam often reveals deeper design limitations:

    • Systems designed for steady-state conditions operating under variable loads
    • Lack of pollutant speciation during the design phase
    • Oversimplified assumptions about liquid chemistry
    • Inadequate purge and make-up strategies

    In other words, the problem is not the foam itself.
    The problem is that the system was never designed to handle the real process conditions.

    Key Takeway

    A scrubber does not fail when foam appears.

    It fails when foam is treated as an isolated issue.

    Understanding and controlling the origin of the emission is the only way to restore performance.

    Final note

    Visible phenomena in industrial systems are rarely superficial.
    They are signals.
    Ignoring them — or masking them with quick fixes — only delays the real engineering work.