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What Is Dehumidification? | Process Engineering Glossary
What Is Dehumidification?
In piping engineering and process engineering, dehumidification is the removal of water vapour from an air or gas stream to reduce its moisture content to a target level. The target is expressed as a dew point temperature, a relative humidity, or a specific humidity. The dew point is the temperature at which the gas would become saturated and moisture would begin to condense. Reducing the dew point protects downstream equipment, prevents condensation on cold surfaces, and maintains the environmental conditions required by sensitive processes and products.
Water vapour in process air and gas streams causes corrosion, promotes microbial growth, interferes with process chemistry, degrades electrical insulation, and causes condensation on cold pipe surfaces and instrumentation. Dehumidification addresses all of these effects by removing the moisture before it causes harm.
Applications of Dehumidification
Natural Gas Processing
Natural gas produced from wells contains water vapour at saturation. This moisture must be removed to prevent hydrate formation in the transmission pipeline and to meet the pipeline gas specification. Glycol absorption is the standard method for gas dehydration in field applications, but molecular sieve desiccant dehumidification achieves the very low dew points required for cryogenic gas processing.
Compressed Air Systems
Industrial compressed air systems use refrigerant dryers for general service applications and desiccant dryers for instrument air and critical process air applications. Refrigerant dryers reduce the pressure dew point to approximately two to ten degrees Celsius, sufficient for most general pneumatic tool and actuator applications. Desiccant dryers achieve pressure dew points of minus 40 degrees Celsius or lower for instrument air and speciality gas applications.
Pharmaceutical Manufacturing
Pharmaceutical manufacturing environments require controlled humidity to prevent product degradation, prevent tablet coating defects, and protect hygroscopic active ingredients. Desiccant dehumidification systems maintain the manufacturing area relative humidity below 30 to 40 percent regardless of the ambient outdoor conditions. This controlled environment is particularly important in tropical or humid climates where outdoor humidity is high year-round.
Corrosion Protection During Plant Preservation
During plant shutdowns, storage, or long-term preservation periods, process equipment and piping are vulnerable to internal corrosion from residual moisture. Dehumidification of the internal atmosphere to a relative humidity below 40 percent prevents condensation on internal metal surfaces and stops corrosion during the preservation period. Temporary dehumidifiers connected to the vessel or piping system maintain this dry internal environment for months or years if required.
Benefits of Dehumidification
Prevention of Condensation and Corrosion
Maintaining the gas or air stream below its dew point throughout the downstream system prevents condensation on cold surfaces. Condensation is the primary cause of corrosion, biological growth, and ice formation in process piping and instrumentation. Dehumidification at the source of the gas stream protects all downstream equipment and eliminates the need for individual corrosion protection measures at each vulnerable location.
Process Quality Protection
Many manufacturing processes are sensitive to humidity. Hygroscopic materials absorb moisture from humid air and change their physical properties, causing caking, sticking, or degradation of product quality. Pharmaceutical tablets absorb moisture and soften. Electronic components corrode. Food products clump and spoil. Maintaining low humidity in the manufacturing environment through dehumidification protects product quality and reduces waste.
Equipment Reliability
Dry instrument air, dry seal gas, and dry process gas all extend the service life of pneumatic equipment, rotating equipment seals, and process instrumentation. Moisture-related failures in control valves, compressor seals, and instrument transmitters are among the most common maintenance problems in process plants. Effective dehumidification of all gas streams that contact these components eliminates these failure modes and reduces maintenance costs.
Limitations to Consider
Energy Consumption
Cooling dehumidification consumes refrigeration energy to cool the gas below its dew point and then additional heating energy to reheat the dried gas to the delivery temperature. Desiccant dehumidification consumes heating energy for regeneration and, in some configurations, cooling energy to cool the desiccant before the next adsorption cycle. In both cases, the energy consumption is significant and must be accounted for in the plant energy balance and operating cost analysis.
Desiccant Degradation
Solid desiccant materials degrade over time through thermal cycling, contamination by hydrocarbon vapours, and mechanical attrition. Hydrothermal degradation occurs when liquid water or high-pressure steam contacts the desiccant during regeneration at conditions outside the design basis. Contaminated or degraded desiccant loses capacity and eventually fails to achieve the required outlet dew point. The desiccant must be replaced at the interval specified by the vendor based on the operating conditions and the contamination history of the feed gas.
Condensate Handling
Cooling dehumidification produces liquid condensate that must be collected, disposed of, or reused. If the inlet gas contains contaminants such as hydrocarbons or dissolved salts, the condensate carries these contaminants in concentrated form. Contaminated condensate requires treatment before disposal and cannot be returned to the cooling water system or discharged to drain without analysis and approval. The condensate handling system is an essential but sometimes overlooked part of the dehumidification system design.
Dehumidification FAQ
What is dehumidification in process engineering? Dehumidification is the removal of water vapour from an air or gas stream to reduce its moisture content to a required dew point. It prevents condensation, corrosion, ice formation, and product contamination in process plants, manufacturing environments, and utility systems. The two main industrial methods are cooling dehumidification, which chills the gas below its dew point to condense moisture, and desiccant dehumidification, which adsorbs moisture onto a solid desiccant material. Desiccant systems achieve lower dew points and suit cryogenic and pharmaceutical applications where cooling systems cannot reach the required moisture level.
What is the difference between cooling and desiccant dehumidification? Cooling dehumidification reduces the gas temperature below its dew point, causing water vapour to condense and drain as liquid. It suits moderate dew point applications down to approximately zero degrees Celsius. Desiccant dehumidification passes the gas over a solid adsorbent material that captures water vapour by physical adsorption without condensation. It achieves dew points of minus 40 degrees Celsius or lower and operates effectively at low ambient temperatures. The desiccant regenerates by heating with a hot dry gas to drive off the adsorbed moisture and restore capacity for the next cycle.
Why must instrument air be dehumidified to a low dew point? Instrument air must be dry to prevent corrosion inside pneumatic actuators and positioners, to prevent freezing in instrument air distribution piping during cold weather, and to maintain reliable operation of all pneumatically operated control valves and instruments. A pressure dew point of minus 40 degrees Celsius at system operating pressure is the standard requirement for instrument air in most process plants. This dew point ensures the air remains dry even when the pressure drops at instrument connections and even in the coldest ambient conditions at the plant site.
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