Cooling water systems, treatment & losses recovery in Ras lanuf ethylene plant, Libya

Cooling water systems, treatment & losses recovery in Ras lanuf ethylene plant, Libya

Abstract:

Based on more than ten years of petrochemical complex operation, an overview of cooling water systems used at Ras lanuf petrochemical complex is presented here with emphasizing chemical treatment programs and corrosion, scaling and bio-fouling control. Heat exchangers and net work pipe material selection, cooling water losses and it's recovery were high lighted.

Introduction:

Ethylene plant consumes a large amount of energy. Where as much of this energy is reused in the process. It is however necessary to reject considerable quantity of heat to cooling water.

For Naphtha feed stock ethylene plant at Ras Lanuf oil and gas processing company (Rasco), Libya, two separate cooling water supplies are used for heat removal in the plant, sea water used directly in the turbine steam condensers making use of the maximum allowable temperature rise, a closed loop fresh water system used in relatively large number of smaller heat exchanger within the process plant, the fresh cooling water re-cooled by sea water in titanium plate heat exchangers.

In this paper, for Ras Lanuf ethylene plant, the cooling water systems are described, based on more than ten years plant operation experience, the chemical treatment program for the fresh cooling water system, losses reduction and recovery & heat exchangers materials have been highlighted.

Ethylene production:

Ras Lanuf ethylene plant has been built to produce 330,000 MTA ethylene by cracking naphtha feed stock.

Overall material balance is given in table (1) together with a typical naphtha analysis.

Table (1)

Feed stock properties Overall material balance

At 823 C coil outlet temperature

Outline of ethylene production process:

An ethylene plant can be considered as six processing steps. These steps for naphtha feed stocks plant are shown in figure (1).

1- Pyrolysis cracking and indirect quenching:

In the first step vaporized feedstock is cracked in the presence of steam in a coil in the radiant section of a fired furnace. The coil outlet temperature is controlled to give the desired ethylene and associated product yields.

The cracked gases are rapidly quenched in exchanger to limit the cracking reaction. High-pressure steam is generated in the exchanger and then superheated in the convection section of the furnaces for use in the plant.

2- Direct quenching:

The furnaces effluent gases are oil quenched. Primary fractionation is used to remove the heavy hydrocarbons, which forms the fuel oil components of the furnaces effluent gases. Fuel oil is taken as side stream from the quench oil tower. With a demand for

low-grade heats, to reboil a column, the gases leaving the oil quench tower are further quenched with water. Gasoline product is taken as side stream from the quench water system. The process heat extracted from the water and the quench oil are used to heat the reboiler, process streams and finally rejected at lowest temperature level.

3- Compression and dehydration:

The furnaces cracked gases are cooled in the quench systems to about 40 C before feeding a4stage centrifugal compressor, which rises the cracked gas from near atmospheric pressure to between34to37Bar. Heat of compression is removed by cooling water after each stage. Sulphur & CO are removed by using caustic wash system installed between third and forth stages. After compression the cracked gas is dried before entering to the low temperature fractionation system.

4- Cold fractionation:

The compressed and dehydrated gases consisting mainly of C hydrocarbons and lighter are passed to the cold fractionation sections of the process where they are progressively cooled by successive levels of refrigeration.

The liquids produced at each stage being passed by separate streams to the demethanizer tower.

The demethanizer tower is reboiled by condensing propylene vapor from the refrigeration compressor. The cooling to the condenser is provided from the ethylene refrigeration system. The bottoms from the demethanizer are fractionated in a deethanizer, depropanizer and debutanizer sequence.

The deethanizer tower is reboiled with steam, and the overheads condensed with propylene refrigerant. The overhead ethylene, ethane and acetylene stream is catalytically hydrogenated to remove acetylene, and further fractionated in the C splitter tower to separate ethylene from ethane. This tower is reboiled with propylene vapor from the refrigeration compressor and the overheads are condensed against propylene refrigerant. Ethane from the base of the column is vaporized against a refrigerant stream before being cracked or passed to fuel gas. Bottoms from the deethanizer together with liquid from condensate stripper are passed to the depropanizer from which a C hydrocarbon stream is produced overhead. Methyl acetylene & propadiene in this stream are catalytically hydrogenated before passing to a fractionation system, which produces polymer grade propylene. Propane from the base of the column is vaporized against a refrigerant stream before being cracked or passed to fuel gas. The depropanizer base flow is debutanized and C product produced. Bottom from the debutanizer together with liquid condensed in the initial quenching constitute the pyrolysis gasoline.

5- Gasoline treatment unit:

The raw gasoline feed stock is pumped from the unit feed surge drum, hydrogen and make-up gas charged through the first step hydrogen compressor and injected to the gasoline stream, the mixing stream are charged to the top of the first step hydrogenation reactor after preheated and brought up to the desired temperature, the hydrogenation reaction occur in the fixed bed reactor, the effluent is flashed in the hot separator, one part of the liquid phase is used after cooling as quench for the reactor, in order to limit the temperature rise. The other part is sent directly to the depentanizer tower, the hot vapors from the hot separator are condensed and cooled then flashed in the cold separator. From depentanizer tower, C cut is produced as a side stream and pumped to the battery limit after cooling, the light ends produced as overhead off-gas and sent back to the cracked gas compressor, depentanizer bottom sent as feed to the deoctanizer, from the deoctanizer reflux drum C-C aromatic cut produced and sent to the second step hydrogenation, the deoctanizer bottom products C+ are then sent to the rerun tower where the heavy ends are eliminated, in order to remove the gums and adjust the final boiling point of C-199C cut produced, the wash oil is withdrawn from the rerun tower as vapor side stream, condensed in the water cooler and then sent to the wash oil tank where it is used internally for the cracked gas compressors suctions as fouling inhibitor, from the rerun tower bottom residue pumped back to the oil quench tower. C-C aromatic cut produced from the deoctanizer reflux drum is mixed with the make-up and recycle gas and hydrogen from the second step hydrogen compressor, the mixed stream is preheated and vaporized against the second step reactor effluent, then brought to the required temperature in the furnace, the hydrogenation reaction occurs in the second step hydrogenation fixed bed reactor, the effluent of the reactor is condensed and cooled, then flashed in the separator, the vapor phase is recycled to the hydrogen compressors system, the liquid net product is sent to the stabilizer tower where light ends and HS eliminated through the reflux drum vent to the cracked gas compressor, the saturated C-C aromatic cut produced from the stabilizer bottom.

6- Refrigeration:

Liquefaction of the cracked gas before fractionation and condensation of reflux in the low temperature fractionation section is accomplished by a refrigeration system using ethylene and propylene as refrigerants in a cascade sequence. The system is designed with five temperature levels of refrigeration. Selected to give the optimum balance between minimum heat exchanger surface area and compressor horsepower requirements. The refrigeration system operates independently from the rest of the process.

The propylene refrigerant is used at three temperature levels -36C, -15C and +8C. The propylene refrigerant vapor is compressed in a three stage centrifugal compressor without interstage cooling. Since vapors from the intermediate flash drums provide sufficient cooling effect. Heat removed from the propylene compression system by a water cooled desuperheater and condenser. Condensed propylene is collected in a surge drum from which it flows back into cascade system.

The ethylene refrigerant is employed at two temperature levels, normally -100C and

-68C. The refrigerant vapor is compressed in a two stage centrifugal compressor without interstage cooling, heat removed by cooling water and propylene refrigerant system.

Cooling water systems:

Seawater is used direct in the turbine condensers making use of the maximum allowable temperature rise of 15C. In relatively large number of smaller heat exchangers within the process plant a closed loop fresh cooling water systems are used, recooled by sea water when approach temperature difference of 5C between the fresh cooling water and the sea water will be cooled in titanium plate exchangers. The fresh cooling water coolers arranged for a split-level temperature range to optimize on heat exchange surface and approach temperature differences.

Seawater cooling system:

Ras Lanuf, Libya ethylene plant as other ethylene plants in the world is part of more comprehensive petrochemical complex with crude oil refinery and polymer plants. For transport reason, etc., it is located close to the sea, so, seawater is available in unlimited quantities and is the coldest, cheapest natural water at the site.

Fresh cooling water systems:

Closed loops fresh cooling water systems are used for cooling in large number of heat exchangers within the process plant, for economic and efficient operation, fresh cooling water systems are designed to fully use the allowable/available temperature rise. A three level split system where some services operate over lower temperature range, some others over a higher range and the remainder the whole range can be used.

The use of split range cooling system means economic in exchangers at the lower range and overall water pumping savings. The adoption of a split level system means the process cooler most sensitive to approach temperature differences and for which maximum cooling is required are designed for the lower half of the temperature rise. Other process users, which can accept a higher cooling water temperature such as some quench coolers can utilize water, which has already passed through other process coolers.

Seawater cooling system, material selection & treatment:

Material selection:

The use of seawater requires careful attention to choice of materials. Due to the normal constituents of sea water, sea water intakes pipes constructed to be very resistant to sea water attack, material selected is glass fiber reinforced plastic (GRP) and main pipe work material is carbon steel pipe with internal cement lined.

Turbines condensers use titanium (T ASTM B-338 GR) tube materials, to compact any HS present in the seawater. The use of titanium is prohibitively expensive; for relatively large components such as condenser water boxes (shell side) which are carbon steel with a variety of protective treatment of which epoxy type surface preparations. Turbines condensers are designed as twin units to allow on-load cleaning/maintenance of either half without serious reduction in output. Water velocities are roughly double during this operation which is considerably in excess of recommended long term operation but is satisfactory for short time needed when on-load cleaning and maintenance.

Chemical treatment:

When the Ras Lanuf complex is in full operation, vast quantities of seawater are required to indirectly cool the process streams. In being pumped from the sea through the heat exchanger systems, this sea water becomes warmed and as such becomes an ideal growth medium for microscopic plant and animal cells which will not necessarily be removed by the three banks of sea water filters at the sea water intake inlet. If these organisms are allowed to grow, unchecked they will form layers of materials on heat exchangers surfaces, inside of pipes, etc. which will offer resistance to heat transfer and flow, so, lowering the overall energy efficiency of the complex.

To overcome this problem, it is necessary to inject to the seawater something that will remove this organic contamination by preventing its initial growth. Chlorine is widely used for this function and it is a form of chlorine that is used for this purpose at Ras Lanuf complex.

Passing a direct electric current through a seawater (solution of sodium chloride in water) cause hydrogen gas to be evolved at the cathode (positive terminal) and chlorine gas at the anode (negative terminal). The chlorine gas immediately is absorbed into seawater and exists as a chemical compound known as sodium hypochlorite. The plant to produce sodium hypochlorite has been installed at Ras Lanuf, located adjacent to the seawater intake. The sodium hypochlorite so produced injected to the sea water circulation to prevent the growth of organisms in the system, sodium hypochlorite is also introduced to fire fighting water system and into sanitary sewage to render it sterile before it is mixed with the plant effluent in the biological oxidation system and discharge to the sea.

Fresh cooling water systems, description & treatment:

At Ras Lanuf, Libya petrochemical complex, there are three fresh cooling water system in operation. These are TCW-1 (Treated cooling water system 1), TCW-2 (Treated cooling water system 2) and MCW (Machinery cooling water system in the Refinery).

1- TCW-1: services the Refinery, Ethylene and Polyethylene plant.

2- TCW-2: services the Utilities and cools such items as lube oil coolers (on boiler feed water, demineralized and desal pumps, turbines, sea water cooler and forced fan lube oil cooler).

3- MCW: services Refinery machinery and interchanges with TCW-1

TCW-1 and TCW-2 are desalinated water systems inhibited with low-level zinc chromate. MCW is again desalinated water inhibited with Jof corr 1470

Cooling water systems description:

1- TCW-1:

The treated cooling water system No. 1 supplies cooling water to the ethylene plant, refinery, polyethylene plant and all other process equipment outside of the utilities area. The system consists of9pumps arranged in parallel. All pumps are motor driven. The pumps transfer the cooled cooling water to the equipment being serviced. The water is returned through the plate heat exchangers (were it is cooled by sea water) and back to the pump suction. Any six pumps must be running at any time with one pump on automatic standby, one pump on manual standby and one pump provided as maintenance spare. Each pump is sized for a capacity of 8,000 M/hr at a developed head across the pump of4.4kg/Cm. The system has34plate heat exchangers; each heat exchanger is rated at 1547 M/hr. During normal operation, the inlet water is at approximately 44 C and is cooled to 32 C (Based on maximum sea water temperature of 27 C). The plate exchangers are arranged in rows of six with the supply and return piping separate into each exchanger. One row of six exchangers will be normally valved out and serve as maintenance spare if another row of exchangers ever need to be taken out of service. The system pressure at the plot limits for normal supply is approximately5.68kg/Cmg and for normal return is approximately 2.43 kg/Cmg. One treated cooling water system head tank is provided, located close to the treated cooling water pump, the tank is378M nominal capacity, un-insulated carbon steel tank. The capacity of the head tank is large enough to accommodate treated cooling water thermal expansion within the system. This thermal expansion capability is provided in the top part of the tank10Meter above grade. The 10 Meter elevations ensure that the head tank is the highest point in the system and provides adequate net positive suction head to the pump.

Treated cooling water oil removal system provides water cleanup capability in the event of oil contamination of the treated cooling water. The system consists of plate type oil separator and induced air flotation type oil removal unit. Two 50% capacity trains are provided with all associated chemical feed and auxiliary equipment.

During normal operations, treated cooling water is pumped from the tank to the users in the Ethylene plant, Refinery, Polyethylene plant, etc. and is returned to the pump/plate exchangers area where it is passed through the plate exchangers and then to the pump suctions. Make-up to the treated cooling water head tank is provided as required by the desalinated water system.

2- TCW-2:

The treated cooling water system No. 2 supplies cooling water excessively to the utilities area. The system consists of four pumps arranged in parallel. Two pumps transfer the cooling water to the equipment being serviced through the plate heat exchangers and back to the pump suctions. The system is split into east and west sides configurations. Any two pumps must be running at any one time, one pump for the east side, and one pump for the west side. The remaining two pumps will be on automatic standby. Each pump is sized for a capacity of1225M/hr. The system has four plate heat exchangers. Each heat exchanger is rated at1516M/hr.

During normal operation, the inlet water is at approximately44C and is cooled to32C (based on a maximum sea water temperature 0f 27 C). The plate exchangers are arranged in a raw with the supply and return piping separate into each heat exchanger. The system pressure at the plot limits to and from the utilities area for normal supply is approximately3.67Kg/Cmg and for normal return is approximately2.0Kg/Cmg.
One treated cooling water system head tank is provided. The tank is located close to the treated cooling water pumps. The tank is11.3M nominal capacity, un-insulated carbon steel tank. The capacity of the head tank is large enough to accommodate treated cooling water thermal expansion within the system. The tank has a baffle installed in the bottom of the tank so that a piping failure in either east or west side piping would not cause a drawn of the entire tank. The tank is located 10 Meter above grade. The 10 Meter elevations ensure that the head tank is the highest point in the system and provides adequate net positive suction head for the pumps.

During normal operations, treated cooling water is pumped from the pumps to the users in the utilities area and is returned to the pump/plate exchangers area where it is passed through the plate exchangers and then to the pumps suctions. Make-up to the treated cooling water head tank is provided as required by the desalinated water system.

3- MCW:

The machinery cooling water system is a facility internal to the refinery and independent from utility area, except for supply of desalinated water make-up. The system is a recirculating close circuit with TCW-1 cooler, to provide cooling water required by machinery equipments (pumps & compressors). Water is pumped from a surge tank to which is returned after passing through the users then a heat exchanger where the circulating water is cooled with cooling water (TCW-1) from utility plant. Desalination water make-up is provided to balance water losses. The system consists of two pumps, both motor driven pumps, normally one on-load and the second is auto standby. Each pump is sized for a capacity of100M/hr, with suction and discharge pressure of2and7Kg/Cmg respectively. One surge tank with nominal capacity of 75 M.

During normal operation, the return water from the process equipment is at approximately50C and cooled by TCW-1 to40C.

Machinery cooling water system treatment:

It is treated with anodic corrosion inhibitor, Joef-corr 1470 (Nitrate basis inhibitor), due to the dangerous of anodic inhibitors from pitting corrosion point of view, it's dosing rate is high compared with zinc chromate injection to fresh cooling water systems, normally600PPM, minimum not lower than500PPM.

Fresh cooling water systems treatment:

Successful cooling water treatment program must control corrosion, scale, deposit formation and microbiological fouling. All of these problems are interrelated, and no one problem can be isolated from the others. For example: scaling occurs more rapidly in a corroding system.

Fresh cooling water systems are susceptible to:

1- Scale deposition.

2- Corrosion.

3- Biological fouling.

Additives are required to control all three. However, due to the quality of the water and the make-up, we need not concern ourselves with1and3above.

Corrosion:

In cooling water systems, corrosion usually refers to damage of steel and other metals in the system by reactions of the metal with dissolved salts, organic compounds and gases (oxygen) in the cooling water. Corrosion can be slow and insidious, or catastrophic in nature. Major problems can result from uncontrolled corrosion. Among these problems are damage to heat transfer equipment, plugging of narrow cooling water flow passes with corrosion products, losses of heat transfer by fouling with corrosion products and deactivation of cooling water treatment chemicals by reactions with soluble or precipitated iron in the system.

The principle cause of corrosion in cooling water system is dissolved oxygen in water, other principle factors which affect the corrosion rate are: PH, temperature and dissolved salts. From the available evidence and in the absence of corrosion inhibitor, it would seem reasonable to predict corrosion rate in the region of150MPY or more for our cooling water circuits. The factors, which tend to give this very large corrosion rate, are as follows:

- Dissolved salts: the absence of calcium and presence of low (<5PPM) chromate levels will make pitting more likely. Chloride in TCW also aids corrosion.

- Temperature: the warmer parts of the cooling water circuits will be at greatest risk.

The only safe guard we have against very high corrosion rates and sever pitting is to maintain the correct level of corrosion inhibitor in the cooling water and to initiate a program of corrosion monitoring for the cooling water circuits.

Corrosion inhibitors:

Corrosion inhibitors for cooling water systems are usually chromate-, silicate- or nitrate-based, although poly phosphate, phosphonates, filming amines and vanadium based corrosion inhibitors are also available.

Inhibitors work by interfering with either the anodic or the cathodic corrosion reaction, and are, respectively, called anodic or cathodic inhibitors. A third type, called film forming inhibitors, that are formed by adsorbed films over the entire surface.

Cathodic inhibitors are effective because they reduce the available cathodic area on the normal surface. Since the corrosion reaction is under cathodic control, this effectively reduces the corrosion rate; similarly anodic inhibitors reduce the available anodic areas. However, in this case, the results are quite different, because anodic inhibitors do not reduce the corrosion rate, they simply reduce the area over which the corrosion is spread. If the entire anodic surface is covered, corrosion protection is usually excellent, but if even small part of anodic surface is exposed, the entire corrosion current will be concentrated on that exposed area. This often leads to aggravate pitting attack, and for this reason, anodic inhibitors are sometimes called "dangerous" inhibitors.

Chromates are the best known, and the most effective, of anodic inhibitors. Chromate is a strong oxidizing agent, and it encourages rapid formation of a protective gamma iron oxide film. The danger of pitting attack with chromate is minimized by combining chromate with an effective cathodic inhibitor, usually zinc. The combination also permits chromate to be used at much lower dosages in cooling water. Zinc is a cathodic corrosion inhibitor; zinc ions react with hydroxyl ions at the cathodes to form an adsorbed film of zinc hydroxide. This film effectively polarizes the cathodes so that oxygen cannot reach the metal surface, and the corrosion current is reduced. Zinc is not usually very effective when used by itself. For this reason, zinc is usually formulated with other inhibitors, such as chromate; this combination program can provide excellent corrosion protection.

In our fresh cooling water systems TCW-1 and TCW-2, zinc chromate is used as corrosion inhibitor.

Make-up and injection of zinc chromate: Zinc chromate dosed when the levels of either zinc or chromium fall below3or5PPM respectively, as determined by laboratory analysis. The zinc chromate make-up tank, only half bag (25 Kg/Bag) of zinc chromate is added to one tank of water (size 1800 Liter). This tank full, when injected into TCW-1 will raise the CrO level by about1PPM, or for TCW-2 by about2PPM.

Fresh cooling water systems, water losses recovery: The treated cooling water systems have been closely monitored, in order to control the corrosion inhibitor injection to the cooling water and to reduce the amount of water losses. It was estimated that 75 M/day of treated cooling water is being lost from TCW-1 system and 200 M/day of treated cooling water is being lost from TCW-2 system.

It was found that 30% of this water losses can be recovered by recycling back to the system (instead of draining) the cooling water used on some pumps which use cooling water for mechanical seals and rotating parts cooling, these pumps selected after analysis ensures that no risk of contamination when recycling this water to the system. Knowing that, by reducing the water losses, the corrosion inhibitor consumption also reduced.

Conclusion: This paper has tried to highlight the cooling water systems used in a petrochemical complex where feed stock is heated to high temperature, some of the products cooled to very low temperatures and others delivered at medium temperature levels. In achieving these temperature levels, low-grade heat is rejected to cooling water systems.

By successful cooling water systems treatment program, the scale deposition, corrosion rate and biological fouling are controlled/reduced, the energy utilization improved.

References:

1- Process industries corrosion -Theory and practice, an official NACE publication, A.J. Freedman, cooling water treatment.

2- Ras Lanuf oil and gas petrochemical complex, operating manuals.

3- Ras Lanuf oil and gas processing co. (Rasco), process engineering division files.

------------------------

About Author:Ahmed Eltayef is anOperation Engineer (Technical Coordinator) at Qatar Petroleum (QP) Qatar. He has more than 20years of experience in petrochemical complex operation andcooling water systems process. He has wide experience in Oil and Gas companies operations includingplant projects and studies, planning and scheduling, management reporting, manpower planning, strategic planning, corporate plans / business plans, systems and procedures, QMS, contracts, budgets,coordination with existing / new customers etc. His other articles can be seen here