cracking · hydrocarbons technology :: ssp 1 cracking dissociation of high molecular weight...

Hydrocarbons Technology :: SSP

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Cracking Dissociation of high molecular weight hydrocarbons into smaller fragments.

Thermal Cracking When, High molecular weight hydrocarbons are heated at > 4000C, they split into two, almost at middle & producing one saturated and other unsaturated H.C. i.e. C12 → C6 + C6 C6 → C4 + C2 or C3 + C3

Unsaturated crack again C4H8 → C2H6 + H2 C2H4 + CH4 + C → under severe conditions CH4 + C3H4 (diolefin/alkyne)

One is Unsaturated


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General formula CnH(2n+2) → C(n/2)H(n+2) + C(n/2)Hn Dehydrogenation:

- Upto some extent, 1. Hydrogenation of low mol. wt. & unsaturates 2. Ring opening

Temperature of Thermal Cracking : 450-500 oC Feed : Heavy gas oil to light vaccum gas oil

Mechanism

Free Radical Mechanism Radicals are atoms or group of atoms with bare unpaired electrons. CH4 → CH.

3 + H. H2 → H. +H. R.CH2.CH3 → R.CH.

2 + CH.

3 Hydrogen charged ions or Hydrogen compound with charged ions known as the carbonium Ions. H2 → H. + H.

O2 → O. + O. C5H12 → C5H.

11 +H.

Free Radicals

Radical

production


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C14H30 + H. → C7H16 + C7H.

15 C5H12 + H. → C5H.

11 + H2

C5H12 +C2H.5 → C5H.

11 + C2H6

C3H.7 + C5H12 → C4H10 + C4H.

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C3H.7 + H. → C3H8

C5H.11 + C3H.

7 → C8H18 H. + H. → H2

- Endothermic Nature (Temp.)

High temperature is desirable. - Pressure:

Low P is desirable but at low ‘P’ coke formation is more (also gas ↑) therefore, 15 atm P is used P↑ light fractions ↓

- Time: Time of cracking ↑ for a given T&P, light fractions ↑ also gas C↑ Optimum time is to be set.

- Recycle Ratio: 2 to 3

CATALYTIC CRACKING

Most widely used process in refinery

Fluidized bed catalytic cracking (FCC) and

Moving bed catalytic cracking are in use .

Purpose: heavy oils into gasoline and light olefins.

Feed: Atmospheric gas oil (AGO), vaccum gas oil

(VGO), coker gas oil, vaccum residue (20%).

Catalysts:

Zeolites (heavy acidic) X&Y type

ZSM-5

Pt/zeolites (Pt for CO-CO2 in regeneration)

Termination

Propagation


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Typical process conditions:

Feed temperature: 150-3700C

Reactor T: 500-5500C

Regenerator T: 650-7300C

Cat./oil ratio: 4-10

Reactor pressure: 1-3 atm

Typical product yields: (wt %)

Gasoline: 45-56%

Light crude oil: 13-20%

Butanes+ butenes : 9-12%

X: 70-85%

Propane: 4-6%

Coke: 5-6%

Other gases: 3-5%

Catalytic cracking processes:

1. Fixed bed process: obsolete due to problem in regeneration of catalyst and heat control.

2. Moving bed process: (Thermofor) 10-20% of cat. Cracking is done using this process.


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Thermofor moving bed catalytic cracker


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Houdri moving bed catalytic cracker

Advantages:

Low maintenance cost

High cost activity

Flexibility in charged stocks and conversions.


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Fluidized bed (fluid) catalytic cracking (FCC) or FCC Riser-Reactor or Flexi FCC

FCC

Typical capacity:

Bombay & H.P. Refinery: 7500 B/day.

Cat. Circulation : 8 T/hr.

Cat. Hold up : 15T cracker

19T Cat. in Regenerator

Cat. Particle size : 40-80 m


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S.A. =500m2/g fresh and 200m2g spent.

R.T. 465-5100 C

P=1.5 atm

Catalyst: Zeolite is crystalline Alumino-silicate material.

Al and Si atoms form tetrahedral, which are linked by shared oxidation atoms.

More than 150 zeolites are available Natural and synthetic.

As Alumina increases, Acidity increases.

Zeolites have high adsorption capacity,

High S.A. 300 to 500 m2g.

High acidity.

Favourable for catalytic cracking:

Pore size: 60-80A0. Too low pore size causes the problem in ‘C’ removal,

where as large pore size decreases cracking.

Brosted acid sites are responsible for the cracking.

During reaction course and heating

Bronsted acid sites → Lewis site → B.A. sites + H2O (steam) Activation at 7000C

Bronsted Site Lewis Site


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X & Y Zeolites are used for FCC NaP AlpSi102.pO384.gH2O

P = 96 to 74 for X zeolite

74 to 48 for y zeolite

g= 250 to 270 as Al ↓

Reactor variables for FCC:

Temperature: RT : 450- 520 0C

Regenerator temp: up to 7000C

‘No Net Heat Addition’

Regenerator

C + O2 → CO2 + CO , ΔH= negative

Heated catalyst Particles from regenerator are used to exchange heat with preheated feed to achieve reactor temperature and heat of cracking reactions.

H.T. → olefins (C3,C4)

M.T. → gasoline

L.T. → Improper cracking

Short contact time → high reactor temperature can be maintained.

Pressure: 1.5 atm or so, As such no effects

L.P ↑ ‘C’ formation

H.P. ↓ light H.C.

Riser contact time: 2 to 6 sec

Short contact time and high temperature give High O.N. compounds.

Catalyst to oil ratio: 2 to 7


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Ratio has to be varied over the period of time due to ↓ in catalytic activity with usage of it even if regenerating the catalysts. For fresh catalysts keep the ratio 2 to 3.

Slip velocity:

𝑠𝑙𝑖𝑝 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦 =𝑣𝑎𝑝𝑜𝑢𝑟 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦

𝑐𝑎𝑡𝑎𝑙𝑦𝑠𝑡 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦

=𝑉𝑎𝑝𝑜𝑢𝑟 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦

𝑣𝑎𝑝𝑜𝑢𝑟 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦 − 𝑇𝑒𝑟𝑚𝑖𝑛𝑎𝑙 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦 𝑜𝑓 𝑝𝑎𝑟𝑡𝑖𝑐𝑙𝑒

As slip velocity ↑ contact between vapour and particles is poor → Improper cracking.

Heat balance:

Breakdown of FCC Heat Requirements

Heat consuming event % of Total

Heat up and vaporize fresh feed 40-50%

Heat for recycled oil 0-10% Heat of reaction(ΔH= +ve) 15-30%

Heat: steam 2-8% Heat : losses 2-5%

Heat: air to regeneration temp. 15-25% Heat: C to from Reactor Temp. to Regeneration Temp.

1-2%


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Hydrocracking H2 + Catalyst + heat → Hydrocracking.

Hydrotreating deals with removal of impurities using mild operating conditions low H2 P.P. and diff. Catalysts then hydrocracking.

Hydrocracking deals with the in addition to removal of impurities, cracking of highly heavy feed stock to get the products like Diesel, Gasoline, Naphta, & Fuel oil.

Hydrocracking:

Purpose: heavy H.C. to lighter H.C.

Feed: VGO,HGO, LGO, visbreaker gas oil, FCC residue.

Products: Naphtha (gasoline), Diesel, kerosene, olefins.

Licensors: Axens (IFP) Exxon Mobil

Shell global solution UOP

Catalyst: Ni Mo or Ni W on Zeolite

Ni Mo or Ni W on amorphous silica alumina.

Pd on zeolite

Catalytic Reaction Conditions:

Reactor temp.: 315-4250C

Pressure: 60-70 atm.

Typical Reactions:

Additional reactions: HDS, HDN ,HDO, HDM. Impurity Removal.

Aromatic saturation

Isomerization n-R.H → i-R-H

Hydrocracking of parrafins R-R’ + H2 → RH + R’H

Dealkylation of Aromatic Rings.


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Coke formation:

Due to H2 presence, coke formation is slow or very less. The high pressure of system also help in reducing coke formation.

Without regeneration catalyst can be used for 1 yr.

Reactors:

Trickle Bed Reactor

Ebullated Bed Reactor

Trickle Bed Reactor: {H.C.(liq.) + H2 (gas) } reactants are pass down over a fixed bed of catalysts.

Once -Through Trickle Bed Hydrocracker

Hot water injected to remove NH3 & H2S

Series of fractionators are used to separate L.N, H.N, middle distillates. Unconverted oil.

Feed B.P. range: 340-5500C , API: 22, Sp. g.r. : 0.922, S=2.5%,N:950rpm.

Product quality: Research O.N. 79-80 (Heavy Naphtha)

Jet fuel: Flash point: 380C , Smoke point: 34mm, Aromatics: 7%.

Diesel: Cetane No: 55, flash point: 520C.


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Ebullating Bed Hydrocracker


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Ebullated Bed Reactor

It can treat Residual oils (ADU, VDU etc)

Hydrocracking is an exothermic Reaction

Ebullated bed provides turbulent mixing of all three phases→ better H.T., uniform temp.

Conversion upto 90%

Better quality of products


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Catalytic Reforming Improvement of low O.N. cut of Heavy Naphtha to high O.N. gasoline.

Done by re-orienting or re-forming the low octane compounds.

Also, produces BTX (feed stock for petrochem) & H2 (can be used for Hydrotreating).

Catalysts: MoO3/Al2O3 , Co/MoO3, Pt/Al2O3. Cr2O3/Al2O3 , Pt-Re-Sn or γ-Al2O3 ----most recent & highly effective.

T: 475-6000C

P: 10-25 atm

Space velocity: 1-7 s.

ΔH= +ve

Type of Reactors: Fixed bed , fluidised bed

Licensors: UOP, Exxonmobil, BP

Reactions: (Endothermic)

Dehydrogenation of Nahtenes (Aromatic production)

Dehydrogenation of paraffin (with H2 generation)

Isomerization


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n-Hexane → 2 methyl pentane (32) (66) n-Pentane → 1 methyl butane (63) (90) Also, some reaction like, (extent is very less)

Hydrogeneation of unsaturates C4H8 + H2 → C4H10

Paraffin cracking C8H18 → 2 C4H8 + H2

Hydrodesulfurization Catalyst selectivity plays an Important role to inhibit undesirable reactions.

Process Flows: There are three major process flows for catalytic reforming:

A. Semi-regenerative B. Cyclic C. Continuous catalyst regeneration (CCR)

Semi-regenerative reformer: It is a fixed-bed unit in which catalyst cycles last from 6 to 12 months. A

catalyst cycle ends when the unit is unable to meet its process objectives – typically octane and overall C5-plus yields. At the end of a cycle, the entire unit is brought down and catalyst is regenerated. In a cyclic reformer, catalyst cycles are shorter – 20 to 40 hours –but they are staggered so that only one reactor goes down at a time. In a CCR unit, the catalyst is slowly but constantly moving from the reactor to the regenerator and back again.

In a semi-regenerative unit, desulfurized naphtha is mixed with hydrogen, heated to >480°C and passed through a series of fixed-bed reactors. The major chemical reactions – dehydrogenation and dehydrocyclization – are


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endothermic (heat absorbing), and the reactors themselves are essentially adiabatic. This means that heat can’t enter or leave except by the cooling or heating of reaction fluids. Consequently, the temperature drops as reactants flow through a reactor. Between reactors, fired heaters bring the process fluids back to desired reactor inlet temperatures.

Some catalytic reformers operate at low pressure (791 kPa), while others operate at 3549 kPa. Low operating pressure improves yields of aromatics and hydrogen, but it accelerates catalyst deactivation by increasing the rate at which coke forms on the catalyst. In a CCR reformer, the catalyst always is being regenerated, so increased coking is less problematic. Therefore, CCR units can operate at very low pressures. In most reformers, the feed is spiked with an organic chloride, which converts to hydrogen chloride (HCl) in the reactors. The HCl increases catalyst acidity and helps to minimize catalyst coking. The effluent from the last reactor is cooled and sent to a separator, from which hydrogen-rich gas is removed and recycled to the reactors. The liquid product flows to a stabilizer column, where entrained gases are removed, before going to the gasoline blender or aromatics plant.

Semi-regenerative catalytic reforming


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Continuous catalytic reforming (CCR) or UOP platforming process:

The hydrotreated feed mixes with recycle hydrogen and goes to series of adiabatic, radial-flow reactors arranged in a vertical stack. Catalyst flows down the stack, while the reaction fluids flow radially across the annular catalyst beds. The predominant reforming reactions are endothermic, so heaters are used between reactors to reheat the charge to reaction temperature. Flue gas from the fired heaters is typically used to generate steam.

The effluent from the last reactor is cooled and sent to a separator. Part of the vapor is compressed and recycled to the reactors. The rest is compressed, combined with separator liquids, and sent to the product recovery section. Liquids from the recovery section go to a stabilizer, where light saturates are removed from the C6-plus aromatic products.

Partly deactivated catalyst is continually withdrawn from the bottom of the reactor stack and sent to the regenerator. As the catalyst flows down through the regenerator, the coke is burned away. Regenerated catalyst is lifted by hydrogen to the top of the reactor stack. Because the reactor and regenerator sections are separate, each operates at its own optimum conditions. The regeneration section can be temporarily shut down for maintenance without affecting the operation of the reactor and product recovery sections.

CCR catalytic reforming


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Alkylation Alkylation Objectives Alkylation processes combine light olefins (primarily propylene and butylene) with isobutane in the presence of a highly acidic catalyst, either sulfuric acid or hydrofluoric acid. The product (alkylate) contains a mixture of high-octane, branched-chain paraffinic hydrocarbons. The illustrative reaction between isobutane and trans-2-butene is shown below. Alkylate is a highly desirable gasoline blend stock because, in addition to its high octane, it has a low vapor pressure. The octane of the product depends on the operating condition and the kinds of olefins used.

Alkylation of trans-2-butene

Process Flow: Sulfuric Acid Alkylation

In the sulfuric acid alkylation units, the feeds – propylene, butylenes, amylene, and fresh isobutane enter the reactor and contact sulfuric acid with a concentration of 85 to 95%. The reactor is divided into zones. Olefins are fed through distributors to each zone, and sulfuric acid and isobutanes flow over baffles from one zone to the next.

The reactor effluent goes to a settler, in which hydrocarbons separate from the acid. The acid is returned to the reactor. The hydrocarbons are washed with caustic and sent to fractionation. The fractionation section comprises a depropanizer, a deisobutanizer, and a debutanizer. Alkylate from the deisobutanizer can go directly to motor-fuel blending, or it can be reprocessed to produce aviation-grade gasoline. Isobutane is recycled.

Process Flow: HF Alkylation


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Olefins and isobutane are dried and fed to a reactor, where the alkylation reaction takes place over the HF catalyst. The reactor effluent flows to a settler, where the acid phase separates from the hydrocarbon phase. The acid is drawn off and recycled. The hydrocarbon phase goes to a deisobutanizer (DIB). The overhead stream, containing propane, isobutane, and residual HF, goes to a depropanizer (DeC3). The DeC3 overhead goes to an HF stripper. It is then treated with caustic and sent to storage. Isobutane from the DIB main fractionator is recycled. The bottom stream from the debutanizer goes to product blending.

HF Alkylation

Coking Coking processes come in two basic forms – delayed coking, which is a semi-batch process, and fluid-bed coking, which is continuous.

Delayed Coking: In a delayed coker, vacuum residue feed is heated to about 487 to 520°C

and sent to a large coke drum. Cracking begins immediately, generating coke


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and cracked, vaporized products. Coke stays behind in the drum while the vapors rise to the top and flow to the product fractionator. Liquid products include coker naphtha, light coker gas oil (LCGO), and heavy coker gas oil (HCGO). All of these require further processing due to direct blending into finished products. The coker naphtha and LCGO are hydrotreated. The HCGO can go either to an FCC unit or a hydrocracker

Representative thermal-cracking reaction. The reaction shown here is the sum of a condensation reaction, which generates hydrogen, and dealkylation, which consumes hydrogen.

Meanwhile, hot residue keeps flowing into the drum until it is filled with

solid coke. To remove the coke, the top and bottom heads of the drum are removed. A rotating cutting tool uses high-pressure jets of water to drill a hole through the center of the coke from top to bottom. In addition to cutting the hole, the water also cools the coke, forming steam as it does so. The cutter is then raised, step by step, cutting the coke into lumps, which fall out the bottom of the drum. Typically, coke drums operate on 18- to 24-hour cycles, which include preheating the drum, filling it with hot oil, allowing coke and liquid products to form, cooling the drum, and decoking.

Coke can account for up to 30 wt% of the product. It can be shipped by rail, truck, or conveyor belt to a calciner, which converts green coke fresh from the drum into various grades of petroleum coke. Green coke can also be used for fuel. Types of Coke: Sponge Coke. Sponge coke is named for its sponge-like appearance. It is produced from feeds that have low-to-moderate asphaltene concentrations. If sponge coke meets certain specifications, it can be used to make carbon anodes for the aluminum industry. Otherwise, it is used for fuel. “Green” sponge coke must be calcined before it can be used for anodes. Fuel coke may not require calcination.


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Needle Coke. Needle coke, named for its needle-like structure, is made from feeds that contain nil asphaltenes, such as hydrotreated FCC decant oils. Needle coke is a high-value product used to make graphite electrodes for electric-arc furnaces in the steel industry. Shot Coke. Shot coke is an undesirable product because it is inconsistent and in some cases dangerous. It is produced when the concentration of feedstock asphaltenes and/or coke-drum temperatures are too high. Excessive feedstock oxygen content can also induce its formation. Shot coke begins to form as the oil flows into the coke drum. As light ends flash away, small globules of heavy tar are left behind. These globs of tar coke rapidly grow due to the heat produced by asphaltene polymerization, producing discrete mini-balls 0.1 to 0.2 inches (2 to 5 mm) in diameter. In the center of the drum, the mini-balls can stick together to form clusters as large as 10 inches (25 cm). On occasion, a cluster breaks apart when the coke drum is opened, spraying a volley of hot mini-balls in every direction. Adding aromatic feeds, such as FCC decant oil, can eliminate shot coke formation. Other methods of eliminating shot coke – decreasing temperature, increasing drum pressure, increasing the amount of product recycle – decrease liquid yields, which is not desired.

Fluid Coking: Fluid coking, also called continuous coking, is a moving-bed process for

which the operating temperature is higher than the temperatures used for delayed coking. In continuous coking, hot recycled coke particles are combined with liquid feed in a radial mixer (reactor) at about 446 kPa. Vapors are taken from the reactor, quenched to stop any further reaction, and fractionated. The coke goes to a surge drum, then to a classifier, where the larger particles are removed as product. The smaller coke particles are recycled to a preheater, where they mix with fresh feed. Coking occurs both in the reactor and in the surge drum. Installation costs for fluid coking are somewhat higher than for delayed coking, but feeds can be heavier and heat losses are lower.


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Alternative Fuels

Alternative to Gasoline Fuels:

1). LPG:

Mixture of propane and butane liquefied and stored at 15 atm.

Propane: 25, Butane: 75% (i &n-butane)

Used for domestic heating but can also be used for automobiles.

LPG= Dual cars.

2). CNG:

Compressed Natural gas.

Methane=90% others C2-C4 compounds

O.N. of CNG=130

Tail pipe emission reduces greatly.

Very less ‘C’ formation

NG is almost insoluble in engine oil, consequently lubricating oil retains its properties longer.

Vol. required is higher.

Large amount of N.G. is found world wide.

3). Syn gas

Synthetic fuels like fuel from coal, oil shale, tar sands etc.


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“Sasol” world leader in production of liq. Fuel from coal by coal gasification.

Also, world leader in gas to liq technology.

Two step process :

1st step= R/M to H2, CO, CH4

2nd step= gas to liquid Fuels in the presence of Iron catalysts.(Fisher Tropsch synthesis).

4). Alcohols:

Good burning quality but low calorific values.

All above alternatives are used in the “SI” engine.

Alternative Diesel Fuels:

ADF should preserve global environment, give solution to long term supply for auto-mobiles.

1). Vegetable oils:

90-98%: Triglycerides with small amount of mono & Di-glycerides, free fatty acids etc.

Veg. Oils from castor, Zethropa, corn, cotton seed, linseed, peanut, soyabean, palm, sunflower.

High viscosity compared to diesel fuel due to large molecular structure and mass (30-40 centi-stoke)

Mol. Wt. 3 times the diesel.

Vol. heating value 39-40 mJ/Kg, Diesel 45 mJ/Kg

Problems in engine:

High viscosity leads problem of injection (poor automization), insufficient mixing, high flash point→ low volatility, more carbon deposits, high gum formation.

Therefore, to use veg. Oil into conventional diesel engine some modifications of veg. Oil is to be done.

‘Trans-esterification’ is done to ↓ .


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Tri glyceride + 3 CH3OH → Glycerol + Fatty acid methyl esters

(known as BIODIESEL)

Characteristics are close to Diesel

C.N. =45 to 64

Viscosity↓ by a factor of 8 to 10 compared to Triglyceride.

Adv:

Renewable

Net emission too low or almost zero

2). Petro crops:

Several plants with their latex contain petroleum like H.C. which can be extracted using organic solvents known as ‘Bio crude’.

These plants known as the ‘Petro crops’ plants found in uncultivated areas cactus plants.

3). Coal:

Coal gasification

Coal→ CO + H2 + CH4 → liquid fuel.

4). Agricultural wastes:

Bagasse to Diesel.

Any waste to fuels (solid, liquid, gas phase). 3rd Generation Fuels

Some other Alternatives being developed:

Shale Gas and Shale Oil: Mass production started

Ice Gas (Japan, 2014)

Fuel cell based Automobiles

H2 + ½ O ↔ H2O

Anode, cathode ≈ electricity → automobile parts.

Production of ultra pure H2


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- methane, ethanol, glycerol (bio-diesel by-product).

Sugar to H2 = fuel cell.

Plastic waste to liq. fuel.

Methane Decomposition to H2 and Nano-Carbon.

H2 from water electro-splitting.

cracking · hydrocarbons technology :: ssp 1 cracking dissociation of high molecular weight...

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