Introduction
Hydrocarbon Dew Point Control Units (DPCUs) are designed to inhibit the formation of solid hydrates in gas streams and by default they perform a level of dehydration. This is achieved through the injection of a liquid agent directly into the gas stream. The liquid agent is selected based on low viscosity and low freezing temperature rather than dehydration ability.
There are 2 common types of liquid agents used for Dew Point Depression:
- Mono-Ethylene Glycol (MEG)
- Methanol (MeOH)
The type of liquid agent used and the package design depends on several factors, and the end-users specific requirements and objectives for the gas stream being processed.
Each package is typically designed in close consultation with the client to ensure the best overall design is achieved.
Design Basis
The design of a DPCU follows a basic structure. However, there are a number of approaches to achieve the end means – Dew Point Depression.
Each system is typically designed and built as a complete turn-key package with particular emphasis given to the following issues:
- Discharge gas hydrocarbon dew point
- Discharge gas moisture content
- Minimum glycol losses
- Minimum power consumption
- Optimum plant efficiency & design integrity
- Compliance with HSE requirements
- Environmentally conscientious design
The resulting design will be influenced by all of these factors, and the emphasis & importance given to each particular issue.
MEG DPCU (155TJ/day) package for Minerva project, Vic. Australia.
Process Description
In a typical MEG Injection Dew Point Control Unit (DPCU), the gas is first passed through an Inlet Separator (1) where any free liquids are removed.
Just prior to entering the gas/gas exchanger pressurised Lean MEG is injected into the gas stream (2). Additional MEG is injected and the MEG-gas mixture is then further cooled through either Expansion (Joule-Thompson Valve/Orifice) or Refrigeration (3).
The further cooling condenses any residual water and/or hydrocarbons. A Low Temperature Separator (LTS) (4) is located downstream to remove any free liquids – now including the condensed water, condensed hydrocarbons and injected MEG. The cooled gas with a depressed dew point is circulated back into the gas/gas exchanger for pre-cooling of the inlet gas (2).
There are two possible process trains available to the free liquids removed in the Inlet Separator and LTS. If significant condensate is present it may be necessary to install a Condensate Stabilisation Unit (5). Otherwise, the liquids – typically containing MEG, water and condensate – will be processed in a MEG Regeneration Package (6) to remove sufficient amounts of water and condensate to produce a Lean MEG product. This Lean MEG is then recycled back into the system and injected back into the DPCU.
A ‘typical’ Hydrocarbon Dew Point Control Unit – a number of process components are added / modified / removed to suit the requirements of each individual application.