Equipment

Molecular Sieve Dehydration

Molecular sieve dehydration removes water vapour where very low water-dew-point specifications are required, particularly upstream of cryogenic gas processing.

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Molecular sieve dehydration adsorber towers in swing service with interconnecting switching manifold

Project reference

LPG Production Facility at Umutu Delta State, Nigeria 2009

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ScopeMolecular sieve dehydration unit within a 100 tonne a day LPG production facility.

Application

When Molecular Sieve Is the Right Route

Molecular sieve units are selected where glycol cannot reach the required specification. They are the standard choice ahead of cryogenic processing, where even trace water would freeze in the cold section.

Very low water dew points, as required for cryogenic plants
Large variations in flow rate across the operating envelope
Higher inlet gas temperatures than glycol can accommodate

Process

Adsorption and Regeneration

Wet gas enters the top of an adsorption tower containing the molecular sieve desiccant and leaves the bottom of the vessel dry. While one tower is adsorbing, the other is regenerating.

For regeneration, gas is heated to around 550 °F (290 °C) and sent to the tower being regenerated to strip water from the sieve. As the saturated gas leaves the top of that vessel it is cooled in a condenser, and the condensed water is separated. Once the water has been driven off, the bed is cooled with unheated gas before the tower returns to adsorption service. The two towers switch between adsorbing and regenerating on a fixed cycle.

Several variations on this flow scheme are possible depending on client preference and the overall process schematic.

System configuration

Major Equipment Within the System

System configuration varies with process duty, operating conditions and project requirements.

Inlet Filter Separator

Removes liquid contaminants from the wet gas so that they do not reach the desiccant bed.
Adsorption Towers

Hold the molecular sieve desiccant that dries the gas; at least two, so that one adsorbs while the other regenerates.
Regeneration Gas Heater

Heats the regeneration gas to about 290 °C (550 °F) to strip the water from the sieve.
Regeneration Gas Cooler

Cools the wet regeneration gas leaving the tower so that its water condenses.
Regeneration Gas Separator

Separates the condensed water from the cooled regeneration gas.

Also in the scope of supply

Piping and control: interconnecting piping, automatic switching valves and instrumentation

Units are designed skid-mounted. Final equipment arrangement and delivery format depend on process duty, transport constraints and site requirements.

Specifications

Key Figures

Parameter	Value
Regeneration gas temperature	Approximately 550 °F (290 °C)
Tower configuration	Minimum two, switching on a fixed cycle
Typical duty	Very low dew point, upstream of cryogenic processing

Design pressure, capacity and cycle time to be confirmed by SPEC engineering against your process conditions.

Advantages

Why Molecular Sieve

Very low dew points

Reaches water specifications glycol cannot, which is what cryogenic processing demands.

Tolerant of flow variation

Handles large swings in throughput without losing outlet specification.

Higher inlet temperatures

Accepts warmer inlet gas than a glycol contactor would tolerate.

Detail

Adsorber Towers and Regeneration Loop

Tower diameter, bed depth and cycle time are set from the inlet gas analysis and the dew point the downstream process needs. The regeneration loop (heater, cooler and separator) is sized alongside the towers so the sieve is fully reactivated within the cycle.