Tank Gas Recovery System

This paper will focus on the implementation of Vapor Recovery Units (VRU\s) for processing the associated gases produced in the storage tanks in the oil fields. The first Vapor Recovery system installed in Libya was executed by Akakus Oil Operations which was focused on flare reduction of Tank vapor/ gas by recovering the condensate and diverting the remaining gas to the main plant compression system. The Vapor Recovery Units design, and fabrication was awarded to Hy-bon Engineering, Inc. of Midland, Texas. A company with 55 years experience in this field. This oil was stabilized in three steps: Production Separator at +/- 4 bara = +/- 58 psia Degasser at +/-1.25 bara = +/- 18 psia Surge Tank at 27 mbarg = +/- 0.4 psig Stock Tank at 6 mbarg = +/- 0.1 psig

The gas from the storage and surge tanks was released to flare. The flared gas contains significant amounts of hydrocarbon condensate in the gas phase. To recover the condensate, Akakus installed three Vapor Recovery Units at NC-115. The VRUs maximize condensate recovery by compressing and cooling The vapors. The recovered condensate is then re-injected into the crude oil. The remaining gas will be used as fuel gas for on-site electric power generation. The VRUs enable Akakus to generate revenue as well as reduce greenhouse gas emissions. The crude oil from the surge tanks has a low solution gas oil ratio of 17 scf/bbl. The low gas-oil-ratio typically precludes a VRU from economic viability; however, the VRUs objective is to recover the hydrocarbon condensate. The VRUs installed at NC-115 exhibit the full function. The surge tanks operate at 0.4 psig. From the surge tanks the vapors enter an inlet manifold prior to the VRUs. The VRUs will compress the vapors to 65 psig and 220 F. To recover the condensate the vapors ar e subsequently cooled to 130 F. Throughout the year this will result in significant condensate recovery. The recovered condensate from the discharge scrubber is discharged via level control to the condensate flash drum of the fuel gas compression system. In the condensate flash drum any free water is separated off and the condensate is stabilized to the suction pressure of the shipping pumps. The hydrocarbon phase of the condensate is spiked into the crude oil.

This paper reports field results after VRU units have been started up and stabilized, resulting in good business opportunities for other Akakus locations (as well as others throughout Libya), due to the confirmation on production rates of the condensate. The suppression of environmental issues and economical attractiveness of recovering substantial volumes of condensate in the NC 115 should be considered an opportunity to be evaluated in other fields operated by all Libyan oil producing companies.

The most cost effective technical solution for compressing the tank gas due to the following:

1- Independent integrated system for condensate recovery. 2- Access gases can be boosted to the secondary compression system (this

allow 100% utilization of tank gases).

Dealing with extremely low pressure gases (0.2 to 0.3 psig) presents a unique set of challenges in case multiple units are required to treat the tanks gas production.

VRUs are skid mounted packages with the following components: Suction scrubber. Compressor (screw type). Gas/oil separator. Heat exchanger. Discharge scrubber. Automated bypass and liquid transfer system. Programmable PLC.

Each VRU package contains an oil flooded screw compressor, a discharge fin fan cooler, scrubber, lube oil vessels and condensate pumps. Liquids obtained as a result of compression and cooling are routed to the process (low-pressure condensates) and the export line (high pressure condensates). Non-condensable compressed vapors are routed back to the flare. A total of five Vapor Recovery Units were purchased from Hy-bon Engineering Company. The Hy-Bon units employ a LeROI oil flooded screw compressor that compress low pressure tank vapors to 60 psig. Each compressor contains four poppet valves that allow the control system to vary compressor throughput according to suction pressure conditions

The Vapor Recovery units were installed on two different locations NC115 and NC186 as indicated bellow as part of the Gas Utilization Project Phase I, this phase has been successfully commissioned and is in operation since beginning of year 2009.

Location

Processed Tank Gas MMSCFD

VRU Recovered

Condensate

Additional recovered condensate

NC-115

4

1200 blls/day@

60 psig 1750 blls /day @ 150 psig

NC-186

2

275 blls/day@ 60

psig 1121 blls/day @ 250 psig

Location

No of VRU

Pay back Period Co2 emission reduction

NC-115 3 3 months 866 tons/day

NC-186 2 8 months 406 tons/day

Benefits:

1) Gas: The gas captured by the VRUs is directed to the suction of booster compressors that go to the turbines that generate electricity.

2) Condensate: Along with the gas being utilized the condensate is sent to the

flash drum at the front of the system to be blended back into the crude oil.

3) Pollution: Less pollution to the atmosphere is always a benefit to any application. It is truly a waste to just burn these resources and pollute the planet in which we live.

4) Goal: The goal is to eliminate these flares completely. They would only be used

in an upset situation. At that time all gas and condensate would be used for the purpose of making Akakus Oil more money and help clean up the environment at the same time.

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SC

FD

1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011

Year

Produced Gas Utilized Gas Flared Gas

5. ECONOMICS Recovered Vapors

Heating Value - 3080 Btu/ft3

Wobbe Number - 2289 Btu/ft3 Volume - 6.0 MMSCFD Condensate Production

Volume - 1475 bbl/d Valuation Savings per Month (gas at a conservative worldwide rate of $3 mscfd) - $1,600,000 Savings per year - $18,630,000 While not as easily estimable (due to spiking the condensate into the crude oil) it can be determined that the produced volume of condensate adds another $2,655,000 per month (based on a conservative $60/bbl rate) or $31,860,000 per year to the Akakus bottom line.