In the production of natural gas, dehydration is almost always required in order to remove water from gas to prevent problems such as the formation of hydrates and corrosion. As environmental controls become a greater concern for the offshore industry, Prosernat/OPC Drizo has developed a dehydration unit - the Drizo gas/glycol process - which uses a recoverable liquid solvent alongside well-proven glycol dehydration units. This process strips lean glycol to produce almost no emissions and water dew points to below -80° C with the additional benefit of converting aromatics in the gas into a saleable liquid stream.
While the traditional gas stripping process normally attains a water dew point close to -60° C it has a major disadvantage when the treated gas contains aromatics since a significant portion of these are absorbed by the glycol. The same partial pressure effect of the stripping gas process results in atmospheric aromatic emissions. As the vaporized aromatics cannot condense during distillation, they are vented with other non-condensables into the atmosphere. The impact of these emissions can be reduced by burning the stripping gas in either the LP flare or as fuel for the reboiler (if a fire-tube is used), but this only results in forming another unwanted environmental pollutant, carbon dioxide.
Prosernat recognized that environmental concerns had to be balanced with cost imperatives such as investment levels and production costs and as a result developed the Drizo solvent stripping process, originally pioneered by Dow Chemicals in the 1970s, by utilizing a liquid solvent in a closed loop rather than dry gas. This solvent is vaporized for stripping, then recondensed in a still. The range of solvents has also been extended to a variety of hydrocarbons with boiling points of 60-180° C.
Process
When the dehydrated gas in the contactor contains aromatics (Benzene, Toluene, Ethyl-benzene and Xylene - BTEX), a proportion of those BTEX elements absorbed by the glycol are vaporized in the reboiler, recovered in the overhead separator, and then act as stripping solvent. With richer gases, more aromatic solvent is recovered in the separator than is needed for stripping the glycol, resulting in excess liquid, which is recovered as a saleable product.
In addition to low dew point levels, this distillation process results in a high efficiency stripping of the gas to produce lean glycol concentrations up to 99.99%. Depending on the gas dew point requirement and consequent glycol purity, three different methods are used to dry the recycled stripping solvent:
- Drizo STD, where free water is removed from the solvent by coalescence, providing glycol concentration of up 99.995% weight and a corresponding water dew point to -60° C.
- Drizo Plus which refluxes the solvent flow to an azeotropic vaporizer producing lean glycol at concentrations to 99.997% weight and a resulting water dew point to -80° C.
- Drizo HP where solvent is dried by solid desiccant. This method has proved that -110° C dew point can be achieved with lean glycol at 99.98% weight.
The Drizo process has been operating successfully at locations around the world for a number of years, providing lean glycol purity and consequent low water dew points.
Prosernat is licensing Phillips in Australia for the Bayu-Undan project which, with an initial expenditure of approximately $US1.4 billion on the liquids stripping/lean gas recycle phase, makes it the largest investment proposed to date in the Timor Sea. The project is utilizing a permanently moored floating storage and offlfoading vessel facility with storage capacity for 820,000 bbl of condensate, 300,000 bbl of propane, and 300,000 bbl of butane. Prosernat is providing a Drizo license which includes process guarantee and detailed critical component design.
In addition to the Bayu-Undan project, Prosernat is licensing the Agip/BG/Texaco/ Lukoil consortium for the Karachaganak project in Kazakhstan, BP's Nam Con Son offshore Vietnam and K5PK in the Netherlands operated by Total/Fina/Elf. For the latter project, Prosernat is also supplying the full fabricated modules.
An other application of the Drizo process is in the revamping of existing process units to produce higher glycol concentration or to recover the BTEX elements. This application is currently being carried out on the Phillips Norge Ekofisk Field.
