BANAGAS operates LPG plant facilities to recover propane, butane and naphtha from oil wells associated gas, Arab gas and refinery gas. Liquefied propane and butane are transferred to refrigerated storage for ship loading at the Sitra Wharf. Naphtha is sent to the Bahrain Petroleum Refinery (Bapco) for storage and subsequent export.
The tail or residue gas is used as fuel for Banagas furnaces and gas turbines, and the rest is supplied to Aluminum Bahrain (Alba) Power Stations and the Bapco Refinery. This residue gas, 245 million standard cubic feet per day (mmscfd), represents 25 per cent of Bahrain’s daily fuel gas consumption.
The original plant was commissioned in 1980 with capacity of 110 mmscfd of associated gas.
At present Banagas operates two gas-processing trains with a total feed gas through put of 280 mmscfd.
The low pressure associated gas is collected from 16 well manifolds in the Bahrain Oil Field. The gas is separated in gas oil separators (GOSPS) at a controlled pressure of about 35 psig. This collected gas will later be sent to compressor stations 1 to 7. Additionally, low pressure non dehydrated refinery gas is also supplied from Bapco refinery to CS-7, whereby it is compressed along with the associated gas and delivered to the central gas plant for LPG recovery.
At each compressor station, gas from several GOSPS is gathered into an inlet header and compressed by a two-stage centrifugal compressor driven by gas turbines to about 420 pound-force per square inch gauge (psig). The gas is then cooled by air fans to 110 degree F, and dehydrated.
During the process some of the gas is condensed and sent to Banagas’s Central Gas Plant (CGP). The compressed gas however contains some water vapour and acidic gases like carbon dioxide and hydrogen sulphide.
The water vapour may condense in the pipe line due to natural cooling and the free water, together with hydrogen sulphide and carbon dioxide present in the associated gas stream, will form corrosive acid. To avoid this, the compressed gas is dried in a glycol dehydration unit before being pumped through gas pipelines to the CGP.
CENTRAL GAS PLANT
The original plant was commissioned in 1980 with a capacity of 110 mmscfd of associated gas.
After a series of plant expansions, Banagas now operates two process trains with a total gas throughput of 280 mmscfd, of which 25mmscfd is high pressure Arab Gas. The two process trains are identical and use refrigerated lean oil-heavy naphtha to recover LPG from the feed gas. Each train consists of condensate, absorption, fractionation, treating and intermediate storage sections.
Condense from the compressor stations including existing ones from gas absorption system enters the condense system where fluctuation in condense feed flow is absorbed and any solids and water are separated. The condense draw off from the condense system is fed to the Depropaniser for fractionation. The gas from the condense system is combined with the feed gas for the absorption system.
CONDENSATE SYSTEM
Stabiliser: Condensate from the compressor stations and from the gas separation system is distilled in a stabiliser column to remove light gas components such as methane and ethane. The stabilised condensate is fed to the Depropaniser, while the light gases join the inlet associated gas.
Separation section: Associated gas, at 420 pound-force per square inch absolute(psia) and a dew point of 40 degree F, enter the separation section where liquids are removed. Any solid residue is separated and removed from the process.
GAS ABSORPTION SYSTEM
Dehydration: The gas leaving the separation section is dehydrated to a dew point of -45 degree F to prevent the formation of hydrates in the subzero absorption section. Triethylene glycol is used to absorb any water from the gas.
Absorber: In the absorber column the refrigerated feed gas and lean oil are intermingled at a temperature of -35 degree F to allow the lean oil to absorb the LPG products of propane and heavier components from the gas and become a rich oil. The residue gas leaves the absorber to be used as fuel gas by Banagas as well as neighboring industries.
FRACTIONATION SECTION
The purpose of the fractionation system is to separate the stabilised condense and the rich oil still overhead liquid into three marketable products, propane, butane and naphtha.
De-ethaniser: The purpose of the de-ethaniser is to remove ethane and lighter components from the rich oil in order to maintain a maximum of 2 per cent ethane in propane. The overhead vapour is recycled to the absorber section to recover any escaped propane and heavier components. The de-ethaniser bottoms are fed to the rich oil still column.
Hot oil system: In the rich oil still, lean oil is separated by distillation from the absorbed LPG products. These LPG products go overhead and are condensed by the rich oil still reflux condenser and then routed to the fractionation columns. The liquid from the bottom of these stills is circulated through the hot oil system as a source of heat for all the plant reboilers and part of it is used as absorbent lean oil. The hot oil temperature is maintained by a gas fired heater.
Depropaniser and debutaniser system: The purpose of the columns is to separate the stabilised condensate and the absorbed LPG products into three marketable products, propane, butane and naphtha.
Depropaniser: The condensed LPG Product and the stabilised condensate are fed to the depropaniser column. A distillation process takes place where the light components of propane are separated and condensed in a reflux condenser prior to sending to the treating section.
Debutaniser: The lighter components of butane vapour are separated and condensed in a reflux condenser prior to sending to the treating section.
TREATING SECTION
Propane and butane contain contaminants such as hydrogen sulphide, carbonyl sulphide and mercaptans.
For the removal of carbonyl sulphide and mercaptans both propane and butane are treated in a fluidised bed of solid potassium hydroxide (KOH). Prior to this, propane is treated by diethanol amine solution to remove hydrogen sulphide.
INTERMEDIATE STORAGE
Treated propane and butane products are routed to horizontal tanks (bullets). After the product is circulated and analysed for purity and water content it is routed to spherical tanks. If any product fails to meet the required specification it is returned to the gas plant to be reprocessed. Only “On-spec-products” are pumped from their respective spheres to the refrigerated storage at Sitra.
A very small quantity of propane product is sold in the local market, after being loaded in pressurised trucks. Naphtha product is stored in floating roof tanks. “On-spec” naphtha is pumped directly to the Bapco Refinery for storage and subsequent export.
STORAGE AND SHIPPING
Refrigerated storage: At Sitra, liquid propane from the Gas Plant is received into the surge drum at 356 psig and 120 degree F. Some of the propane is used as refrigerant, first in the butane chillers and then in the butane sub-coolers. During this process butane is cooled to +25 degree F and the liquid propane is flashed into vapour. The balance of the propane is flashed to its boiling point of -45 degree F into the propane storage tank.
The propane vapours from the storage tank, butane chillers and sub-coolers are compressed by reciprocating compressors. The compressed vapours are condensed and used as refrigerated propane to cool down both propane and butane products to their atmospheric boiling point of -45 degree F and +23 degree F respectively. Each product is stored at its boiling point in separate refrigerated tanks of 200,000 barrels capacity. A 100,000 barrel dual purpose storage tank is also available.
Product shipping: The LPG shipping facilities consist of two loading pumps each with a rated capacity of 5,000 barrels per hour, a 16-inch loading line and a 10-inch vapour return line, each of which is fitted with fully articulated arms.
Just before a ship’s arrival the loading line is cooled down with the product to be loaded. The vapour generated is returned to its respective tank.
Banagas produces approximately 3,000 barrels of propane, 2,700 barrels of butane and 4,500 barrels of naphtha per day and these products are exported to various parts of the world where they are used as industrial and domestic fuel. Some also finds its way into petrochemical feedstock.
