Hydrocarbon Steam Cracking for Petrochemicals | Chemical Technology - Chemical Engineering PDF Download

15.1 Introduction 

In industrial processes, hydrocarbons are contacted with H₂O, depending upon the desired effect. When hydrocarbon vapors at very high pressures are contacted with water, water which has a very high latent heat of vaporization quenches the hydrocarbon vapors and transforms into steam. In such an operation, chemical transformations would not be dominant and energy lost from the hydrocarbons would be gained by water to generate steam. The quenching process refers to direct contact heat transfer operations and therefore has maximum energy transfer effeiciency. This is due to the fact that no heat transfer medium is used that would accompany heat losses. The steam cracking of hydrocarbons is an anti-quenching operation, and will involve the participation of water molecule in reactions in addition to teh cracking of the bnydriocarbond on their own. Since steam and the hydrocarbons react in the vapour phase the reaction products can be formed very fast. Therefore cracking of the hydrocarbons on their own as well as by steam in principle is very effective.  

When steam cracking is carried out, in addition to the energy supplied by the direct contact of steam with the hydrocarbons, steam also takes part in the reaction to produce wider choices of hydrocarbon distribution along with the generation of H₂ and CO.

• Hydrocarbons such as Naphtha and LPG have lighter compounds.
• When they are subjected to steam pyrolysis, then good number of petrochemicals can be produced.
• These include primarily ethyelene and acetylene along with other compounds such as propylene, butadiene, aromatics (benzene, toluene and xylene) and heavy oil residues.
• The reaction is of paramount importance to India as India petrochemical market is dominated by this single process.

15.2 Reaction

C_xH_y + H₂O + O₂→ C₂H₄ + C₂H₆ + C₂H₂ + H₂ + CO + CO₂ + CH₄ + C₃H₆ + C₃H₈ + C₄H₁₀ + C₄H₈ + C₆H₆ + C+ Heavy oils 

• The reaction is pretty complex as we produce about 10 to 12 compounds in one go
• The flowsheet will be reaction-separation-recycle system only in its topology. But the separation system will be pretty complex.
• Almost all basic principles of separation appears to be accommodated from a preliminary look.
• Important separation tasks: Elimination of CO and CO₂, Purification of all products such as ethylene, acetylene etc.-
• The process can be easily understood if we follow the basic fundamental principles of process technology
• Typical feed stocks are Naphtha & LPG
• Reaction temperature is about 700 – 800 ^oC (Vapor phase reaction).

 15.3 Process technology (Figure 15.1)

Figure 15.1 Flow sheet of Hydrocarbon Steam Cracking for Petrochemicals

• Naphtha/LPG saturates is mixed with superheated steam and fed to a furnace fuel gas + fuel oil as fuels to generate heat.  The superheated steam is generated from the furnace itself using heat recovery boiler concept.
• The C₂-C₄ saturates are fed to a separate furnace fed with fuel gas + fuel oil as fuels to generate heat.
• In the furnace, apart from the steam cracking, steam is also generated. This is by using waste heat recovery concept where the combustion gases in the furnace.
• After pyrolysis reaction, the products from the furnace are sent to another heat recovery steam boiler to cool the product streams (from about 700 – 800 ^oC) and generate steam from water.
• After this operation, the product vapours enter a scrubber that is fed with gas oil as absorbent. The gas oil removes solids and heavy hydrocarbons.
• Separate set of waste heat recovery boiler and scrubbers are used for the LPG furnace and Naphtha steam cracking furnaces
• After scrubbing, both product gases from the scrubbers are mixed and fed to a compressor.  The compressor increases the system pressure to 35 atms.
• The compressed vapour is fed to a phase separation that separates the feed into two stream namely the vapour phase stream and liquid phase stream. The vapour phase stream consists of H₂, CO, CO₂ C₁-C₃+ components in excess.  The liquid phase stream consists of C₃ and C₄ compounds in excess.
• Subsequently, the vapour phase and liquid phase streams are subjected to separate processing.

Gas stream processing:

• CO₂ in the vapour phase stream is removed using NaOH scrubber.  Subsequently gas is dried to consist of only H2, CO, C₁-C₃ components only.  This stream is then sent to a demethanizer which separates tail gas (CO + H₂ + CH₄) from a mixture of C₁-C₃ components.  The C₂-C₃+ components enter a dethanizerwhich separates C₂ from C₃ components.
• Here C₂ components refer to all kinds of C₂s namely ethylene, acetylene etc. Similarly, C₃ the excess of propylene, and propane.
• The C2 components then enter a C2 splitter which separates ethane from ethylene and acetylene.
• The ethylene and acetylene gas mixture is fed to absorption unit which is fed with an extracting solvent (such as N-methylpyrrolidinone) to extract Acetylene from a mixture of acetylene and ethylene.
• The extractant then goes to a stripper that generates acetylene by stripping. The regenerated solvent is fed back to the absorber.
• The ethylene stream is fed to a topping and tailing still to obtain high purity ethylene and a mixture of ethylene and acetylene as the top and bottom products. The mixture of ethylene and acetylene is sent back to the C2 splitter unit as its composition matches to that of the C2 splitter feed.

Liquid stream processing

• The liqiuid stream consists of C₃,C₄, aromatics and other heavy oil components is fed to a NaOH scrubber to remove CO₂
• Eventually it is fed to a pre-fractionator. The pre-fractionator separates lighter components from the heavy components.  The lighter components are mixed with the vapour phase stream and sent to the NaOH vapour phase scrubber unit.
• The pre-fractionator bottom product is mixed with the deethanizer bottom product.
• Eventually the liquid mixture enters a debutanizer that separates C₃, C₄ components from aromatics and fuel oil mixture.  The bottom product eventually enters a distillation tower that separates aromatics and fuel oil as top and bottom products respectively.
• The top product then enters a depropanizer that separates C₃s from C₄ components.  
• The C₄ components then enter an extractive distillation unit that separates butane + butylenes from butadiene.  The extractive distillation unit consists of a distillation column coupled to a solvent stripper.  The solvent stripper produces butadiene and pure solvent which is sent to the distillation column.
• The C₃ components enter a C₃ splitter that separates propylene from propane + butane mixture. Thesaturates mixture is recycled to the saturates cracking furnace as a feed stream.

15.4 Technical questions 

1. Why two separate furnaces are used for C₂-C₄ saturates and Naphtha feed stocks?

Ans: The purpose of steam cracking is to maximize ethylene and acetylene production. For this purpose if we mix C₂-C₄ saturates and naphtha and feed them to the same furnace, then we cannot maximize ethylene and acetylene production. The napntha steam cracker has its own operating conditions for maximizing ethylene and acetylene and so is the case for C₂-C₄ saturates.

2. Why the product gases from naphtha and C₂-C₄ saturates steam cracker processed separately before mixing them and sending them to the compressor?

Ans: Both crackers produce products with diverse compositions.  Both cannot be fed to a single scrubber and remove the heavy hydrocarbons and oil components.  While the scrubber associated to naphtha steam cracking needs to be remove significantly the oil and heavy hydrocarbons, this is not the case for steam cracker product vapour processing.

An alternate way of designing a single scrubber is to design a complex scrubber that has multiple feed entry points correspond to both product gases entering from various units.  This refers to process intensification and would be encouraging.

3. Why specifically the gases are compressed to 35 atm?

Ans: The distribution of light and heavy components in vapour and liquid streams is critically dependent on the pressure.  Therefore, the pressure of the system plays a critical role in the distribution of these key components.

4. Why is it  not possible to sharply split C₃ components in the phase separator?

Ans: This is the basic problem of the phase equilibrium factors associated to the intermediate components.  Usually, phase equilibrium factors are highest for lighter components and lowest for the heavier components. But intermediate components such as C₃s have phase equilibrium factors in between. Therefore, C3s get distributed between both vapour and liquid equally.  This will be the case even with higher pressure and going for higher pressure is not economical as the pressurizing costs will be significantly.

5. Why a tailing and topping still is required for ethylene production?

Ans: The distillation column for separating ethylene from ethylene from C₂ components needs to carry out a difficult separation. This is also due to the fact that the boiling points of C₂ components is very close. Therefore, there needs to be two columns (indicating good number of trays).

6. Explain how extractive distillation enables the separation of butadiene?

Ans: Dimethyl formamide (solvent) is fed to the distillation column fed with butadiene, butane and butylenes.  The solvent interacts differently with the components and therefore adjusts the relative volatility of the mixture which was close to 1 previously.  Thereby, the solvent forms a high boiling mixture at the bottom with butadiene and thereby enables the difficult separation of butadiene from the C₄ compounds.  Thereby, the solvent + butadiene is fed to a stripper which removes butadiene from the DMF.  One important issue here is that the solvent does not form an azeotrope with the butadiene and is therefore, easy to separate.

7. When acetylene is not required, what process modifications will exist to the technology?

Ans: When acetylene is not required, then the top product from C₂ splitter (which is a mixture of acetylene and ethylene) is fed to a packed bed column and H₂ to convert the acetylene to ethylene. Eventually, one does not require the absorber-stripper technology for acetylene purification.