Hydrocarbon Equipment - Dew Point Control

Stabilization of natural gas liquids (NGLs) or field condensate is a process utilizing controlled flashing and in some cases, a distillation of the liquid to allow it to be stored in atmospheric vessels. The distillation of the liquid can also used to remove objectionable non-hydrocarbon components, most notably CO2, from the sales liquid.

The DPC NGL stabilization process uses tried-and-true conventional technology, employing the same modular fabrication techniques DPC uses for its standard gas conditioning equipment, allowing for high levels of quality control. All of the equipment and piping is designed, fabricated and tested per ASME and TEMA codes and standards. The system is all electric, minimizing permitting issues and reducing the hazards associated with direct fired equipment in an oil field production environment.

DPC was founded in early 2003 with the idea that it would lease JT and refrigeration units to the local producers needing to meet the 45°F HDP pipeline specifications that where prevalent in the East Texas region. The conditioning units employed a simple two phase separator to remove the cold separator liquids from the lighter volatile gaseous hydrocarbons (sales gas). In the typical application, the cold liquids flow to the producer's low pressure crude separator (60 to 100 psig) at which point additional light volatile hydrocarbons are flash off and recovered by low pressure compression. The remaining NGL's mix with the field produced crude or condensate for sale. For applications with cold separators operating at temperature greater than 30°F, this is still the most common method of handling the NGLs and generates the highest market value for the product. Many of DPC's later installations happened to be installed on systems requiring lower HDP specifications. In conditioning units operating in colder operations with cold separator temperatures below 30°F, the amount of light volatile hydrocarbons held by the cold separator liquids increases dramatically. This requires the recovered NGLs to be stored in pressurized tankage. The cold liquids are typically flashed directly from the cold separator into a pressurized storage tank. The storage tank has a back pressure regulator controlling the pressure of the vapor space to maintain a pressure in the 100 psig to 175 psig range which allows the most volatile hydrocarbons to weather off from the liquids. The flash gas vented from the pressurized storage vapor space is either consumed as fuel or recompressed as a recycle stream back to the front of the conditioning unit. The pressurized NGLs are marketed as a mixed NGL product with 2% to 6% methane content.

There is a need by some of clients for further liquid stabilization of the produced hydrocarbon liquids beyond the simple single flash stabilization technique. The pressurized NGL storage is becoming increasingly subject to Federal regulation under OSHA's PSM (Process Safety Management) program and the RMP (Risk Management Plan) requirements. The associated pressurized truck loading is also considered an operation with exposure to both personnel safety and air emissions. In some instances, it has become increasingly difficult for our clients to find local sales outlets for these pressurized liquids, and in some locations to schedule pressurized transport trucks in a timely manner.

The equipment used in the DPC stabilizer process simply provides a way to "cook" the volatile hydrocarbons liquid mix, at moderate pressure (50 to 200 psig) and temperature levels (200°F to 350°F). This is done in a distillation tower, controlled in such a way as to drive off the light gaseous hydrocarbons and other gaseous contaminants to be used as a fuel stream or recycled through the conditioning equipment, eventually to be sold as part of the pipeline quality natural gas. The resulting "stabilized" liquid thereby has a much reduced volatility.

In many cases the liquid can be stabilized to a point that it can be stored and transported in non-pressurized vessels, both enhancing safety in handling, and improving the liquid's marketability. This liquid can be marketed as a crude or "dead" condensate. In other instances, our clients may want to limit the amount of vapor driven off from the liquid, and sell a "moderately" pressurized product to an available market. Meeting a MAPCO NGL pipeline specification is a typical application. This choice only requires an adjustment in operating temperature and pressure of the stabilization unit.

DPC has designed a standard unit to stabilize a nominal 500 BPD of NGL production with an easily operated system in a very energy efficient manner. The heat source is an electric resistance process type immersion heater, procured along with its control system, from an "old-line" process heater manufacturer. The electric heat allows for a simple installation, a stable and safe operation and avoids producing any air emissions associated with direct fired equipment.

The maximum allowable operating design pressure (500 psig) and temperature ratings (650°F) of the system components are far in excess of the anticipated operating conditions. In other words the equipment is over-designed and under-stressed in operation. This is deliberately done for enhanced safety and durability.

The stabilizer is a top feed quench tower design. Bubble cap trays are used to give the tower a stable operation over a large range of flow conditions. The stabilizer tower has both top and middle tray liquid feed points. Multiple heat exchangers are used to recover the heat from the stabilized hot bottom product and return the energy to both the middle and the top feed trays for maximum energy conservation.

The stabilization system is built on two process skids. The tower is mounted on a stand-up skid which then bolts to the heat exchanger and control skid for a quick and simple installation. The skids are designed to be set on stabilized ground, concrete foundations are not typically required.

Dew Point Control, LLC
P.O. Box 18887
Sugar Land, TX 77496-8887
Phone: (281) 265-0101
Fax: (281) 265-0107