Olefines Manufactures Ethylene From Ethane Biology Essay

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Qenos was formed in 1999, bringing the people and plants are formerly Kemcor and Orica polyethylene. The sole share holder for Qenos Pty Ltd is China National Chemical Corporation (ChemChina) (Qenos 2008). Qenos covers around 37 hectares and located approximately 17 km south from Sydney central business district and there is shipping terminal at the Botany Bay as shown in Picture 1 (Qenos 2007). Now, Qenos is the largest manufacturer of plastic in Australia and employs around 1000 employers on two sites in Botany, New South Wales and Altona, Victoria (Safety, Health, and Environment Update Brochure 2008).

Picture 1: Qenos Sites at Botany Bay

Qenos produces olefins and full range of polyethylene products such as high density polyethylene (HDPE), low density polyethylene (LDPE) and linear low density polyethylene LLDPE (Qenos 2008). Besides that, Qenos also supply any types of specialty polymers and thus make it a vital link in Australian manufacturing chain (Qenos 2008). At Botany, Qenos operates 4 plants:-

1) Olefines-manufactures ethylene from ethane

2) Alkathene-uses ethylene to make low density polyethylene

3) Alkatuff-uses ethylene to make high density and linear low density polymer

4) Site utilities-operates three boilers and supplies steam, cooling water, towns water, firewater, compress air, electricity and drainage services to all BIP plants (Safety, Health, and Environment Update Brochure 2008)

Furthermore, Qenos has excellent record in safety, health and environmental management because there are no serious incidents being recorded for several years (Safety, Health, and Environment Update Brochure 2008).

For this report, only one area will be covered which is ethylene production in Olefine plant.

2.0: Introduction

The Olefines plant was built in 1983 and originally operated on a naphtha and liquid petroleum gas (LPG) feedstock producing both ethylene and propylene. In 1996 the operation was converted and began to process ethane feedstock and the plant now produces approximately 250,000 tonnes of ethylene per year for the two downstream polyethylene plants-Alkatuff and Alkathene and other domestic and export customers(Qenos 2007).

3.0: Ethylene Properties

Relative Vapour Density

0.980

Flash Point

-135.0°C

Percent Volatile by volume

100

Boiling point

-104°C

Explosive Limits

2.7 mol%-36 mol%

Automatic Ignition Temperature

543°C in air at atmospheric pressure

Table 1: Properties of Ethylene (Orica 1999)

In addition to above table, ethylene is a colourless gas with sweet odour, slightly soluble in water and soluble in a lot of organic solvent. Besides that, it burns readily in presence of oxygen (Orica 1999). The chemical formula for ethylene is C2H4 and chemical Structure of ethylene is

(Wikipedia 2008)

4.0: Production of Ethylene

Production of ethylene can be described in several sequences. In Olefine plant, there are six steps before pure ethylene is extracted. Qenos also produces ethylene using liquid gas oil in totally different plant to the ethane cracking plant. But, the process of producing ethylene from liquid oil is very similar to ethane cracking process as the below explanation is all about ethane cracking process.

Cracking and steam generation (steam cracking)

Quenching process

Compression

Treating

Chilling chain

Distillation (Qenos Corporate Affairs Department 2001)

4.1: Cracking and Steam Generation

Steam cracking is a process where the hydrocarbon (ethane) in presence of steam and heat, changes to other hydrocarbon (ethene). Ethane (principal raw material) gases are piped from Bass Strait via Long Island Point. As shown in figure 1, the feed (ethane) is fed into five gas-fired furnaces. Steam is immediately injected when the feed is fed. In order to minimise the carbon deposits (coke) and to increase the petrochemical rates, the steam is added at controlled rates. This is important as coke deposits prevent the ethane gases from heating to the right temperature and therefore it will lower the effectiveness of the cracking reaction (Qenos Corporate Affairs Department 2001).

Figure 1: Cracking Step

The optimum temperature is about 850 °C. This optimum temperature will cause the ethane to split into smaller molecule which is ethene.

C2H6 (g)  C2H4 (g) + H2 (g)

Then, in transfer line exchanger (TLE), the effluent is cooled. The heat gain from the cooling process is used to raise 10000 kPa steam (Orica Limited). During the reaction, only 60 % of ethane is consume and reacted in the furnaces. The composition of the effluent gas is approximately 50% of ethylene, 35% of ethane and the remainder are hydrogen, methane, acetylene, propane, propylene and some others hydrocarbons. The remaining ethane is fed back to the furnaces for further reaction to become ethene (Qenos Corporate Affairs Department 2001). The cracking process occurs in the ethane cracker in the picture 2.

Picture 2: Ethane Cracker at Qenos Olefins Manufacturing Plant

4.2: Quenching Process

Figure 2: Quenching Step

Then, the effluents (the gases from the furnaces) are immediately cooled (quench) by water. The temperature of the effluents drops from 840 °C to 700 °C to ensure no more cracking reaction occurs and no formation of coke.

Picture 3: Quench Tower

Based on the figure 2, the furnace effluents are combined and cooled to 30 °C and accomplished by direct contact with water. This quench water is recovered and re-used. This process is done in quench tower (Qenos Corporate Affairs Department 2001). The heat recover from the water is used to re-boil three distillation columns (this process will be explained later) (Orica Limited 1999). All the quenching process is done in the quenching tower in picture 3.

During these two processes, ethylene is produced and cooled. However, it is not pure ethylene as it is mixed with other hydrocarbon compounds. Thus, it needs to be separated and then can be sold to customers as 99% wt (weight percent) pure ethylene tower (Qenos Corporate Affairs Department 2001).

4.3: Compression Process

Compression process is very important as it convert the effluent gases to liquid which will be used later. A gas can be liquefy by increasing the pressure of the gas and then cool it down until a liquid is formed. This process is done in gas compressor (refer to figure 3).

Figure 3: Gas Compressor

In the gas compressor, the cracked gas is compressed using a centrifugal compressor. The gas compression in this section of the plant occurs in four stages. When the gas is compressed, the temperature of the gas increases. Thus, heat exchangers are used to cool the gas between each stage. The cooling process is crucial to prevent the gas from becoming too hot. The gas is compressed to a pressure around 3500 kPa (Qenos Corporate Affairs Department 2001).

4.4: Treating Process

The cracked gas steam contains impurities for example carbon dioxide, hydrogen sulphide and acetylene. These types of impurities must be removed before ethylene can be sold to customers because the impurities reduce the ethylene quality. The treatment for the impurities occurs in the third and fourth stages in the gas compressor.

Figure 4: Treatment to remove impurities

In figure 4, hydrogen sulphide and carbon dioxide will be removed first in the caustic's tower. The caustic's tower's purpose is to take out the unwanted chemicals from the ethylene. Dilute sodium hydroxide is used to remove both gases. The reaction occurs when the gas stream is contacted with sodium hydroxide solution in the tower. The reactions are:-

2NaOH (aq) + H2S (g)  Na2S (aq) + 2H2O (aq)

2NaOH (aq) + CO2 (g)  Na2CO3 (aq) + H2O (aq)

When the reactions take place, the impurities will be removed. Then, the waste sodium hydroxide stream is removed from the caustic tower and treat on site in the Spent Caustic Carbonization Unit* which uses waste flue gas from a boiler to convert the stream into a benign baking soda solution that will be dispose later on (Qenos Corporate Affairs Department 2001). (*technology to treat waste sodium hydroxide without any oxidiser and acids)

After hydrogen sulphide and carbon dioxide have been treated, acetylene then will be removed in vessel called the acetylene converter. The conversion process occurs in the large oval-shaped vessel filled with nickel-iron catalyst. Acetylene will react with hydrogen when the gas stream passes the catalyst.

C2H2 (g) + H2 (g)  C2H4 (g)

The catalyst is carefully selected to only and selectively promote the hydrogenation of acetylene. Otherwise, some undesirables' reaction will occur. For instance, ethylene will be converted to ethane. This is unwanted reaction because the plant will lose some of the ethylene.

C2H4 (g) + H2 (g)  C2H6 (g)

When the treatment for impurities is completed, the gas will be cooled to around -100 °C (dried). Consequently, the remaining water would form ice compounds that will be blocked by blocking pipes. This process is archived by passing the gas stream through an apparatus called the molecular sieve desiccant. This apparatus can absorb water. As a result, the gas cooled (Qenos Corporate Affairs Department 2001).

4.5: The Chilling Train

Figure 5: Chilling train process

Chilling train contains a series of heat exchangers. According to figure 5, on the side of the heat exchanger is the gas that needs to be cooled. The refrigerant, liquid ethylene or propylene acts as cooling agent and located on the other side of heat exchanger. During the process, no gas comes to direct contact with the other. When the gas enters the chilling train, it cools and then condenses or liquefies. The liquefy or condenses stream will go to the next process which is distillation (Qenos Corporate Affairs Department 2001).

4.6: Fractionation or Distillation

Figure 6: Fractionation Process

Figure 7: The workings of the de-ethyleniser column

This is the last process in ethylene production before the ethylene is stored in storage tank. Basically, there are three distillation columns as shown in figure 6. Figure 7 shows how these three column works.

The first column is the de-methaniser. This column will separate out the hydrogen and methane from the remaining components. The hydrogen and methane are used as fuel gas later on. At the bottom of the de-methaniser, the remaining heavy gas will exits such as ethylene and ethene. Both of these gases are then fed into the second distillation column to be separated from each other.

The second column is called de-ethyleniser. At this column, ethylene will be split up from other heavier components of the de-methaniser bottom. The separation process is conducted at a pressure of 1950 kPa and the purity of ethylene produces is greater than 99.85%w (Qenos Corporate Affairs Department 2001). The ethylene product is sent to Alkatuff and Alkathene plant. The ethylene is also sold right away to customers and export to other places (Qenos Intro 2008).

At the last column, the ethaniser, ethane is separated from propylene and other heavier gas in de-ethaniser bottom. Ethane is sent back to the cracking furnace for cracking process and the bottom stream is piped into the gas oil cracker plant for further separation (Qenos Corporate Affairs Department 2001).

5.0: Usage of Ethylene

Figure 8: The usage of ethylene after production

Figure 8 illustrate the usage of ethylene. The ethylene is then piped to Alkatuff and Alkathene plants for further chemical process. At Alkatuff plant, ethylene is convert to HDPE and LLDPE while ate the Alkathene plant, ethylene is converted to LDPE. The other ethylene will be sold and export. But, before that, it is stored in large sphere at Botany (Qenos Intro 2008).

6.0: Handling of Ethylene

6.1: Storage and Transport

For transport purpose, ethylene is categorised as flammable liquid. Thus, Qenos must refer to state regulation to store and transport the requirements. The requirement is the products must not be loaded with flammable liquid (class 3) in large quantity, flammable solids (class 4) and oxidising agents (class 5) (Orica Limited 1999).

6.2: Personal Protection

Impervious gloves, full face shield or chemical goggles, rubber boots and overalls must be worn by the worker. Use with enough ventilation. If inhalation risk occur wear air supplied mask. As the liquid can cause cold burn, so contact with ethylene must be avoided (Orica Limited 1999).

6.1: Disposal

Vapour should be discharged to a burning flare. Otherwise, dissipate gas in remote area using water or steam jets or spray to ensure that concentration is below explosive limit (Orica Limited 1999).

7.0: Health Effects

Practically, short term exposure by inhalation is considered to be non-harmful (Orica Limited 1999).

7.1: Skin and eyes

Freezing injury to skin and eye tissue when there are contact with liquid ethylene (Orica Limited 1999).

7.2: Inhalation

Ethylene vapour is an asphyxiant, depriving respired air of oxygen. Inhalation of vapour at moderate to high concentrations can result in headaches, dizziness and possible nausea. Ingestion, inhalation and prolonged or repeated skin contact must be avoided (Orica Limited 1999).

8.0: Appendix

Figure 9: Complete Diagram for Ethylene production

Above figure demonstrate the overall process of ethylene production from the cracking process until the last step which is fractionation. The description of each process has been explained on the above section.

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