LIQUEFACTION - Universitas Diponegoroasepmusa.blog.undip.ac.id/files/Coal-liquefaction.pdf · Complete breakdown of coal with steam and oxygen Sulfur is removed from the syngas Syngas

LIQUEFACTIONAsep Muhamad Samsudin

DEPARTEMENT OF CHEMICAL ENGINEERINGFACULTY OF ENGINEERING DIPONEGORO UNIVERSITY

SEMARANG

Introduction

Liquefaction is the conversion of coals into liquidproducts.The three methods by which liquids can be derived from

coals are :1. pyrolysis,2. indirect liquefaction, and3. direct liquefaction.

Introduction

In pyrolysis processes, the liquids are a by-product ofcoke production.The term liquefaction refers to the conversion of the coalto a product that is primarily a liquid.In indirect liquefaction, the coal is gasified into a

mixture of carbon monoxide and hydrogen (i.e., syngas).The syngas is then processed into liquid products usingFischer–Tropsch synthesis.

Introduction

In direct liquefaction, also referred to as coalhydrogenation, coal is mixed with a hydrogen-donorsolvent and reacted with hydrogen or syngas underelevated pressures and temperatures to produce a liquidfuel.Indirect liquefaction is used quite extensively through out

the world, while direct liquefaction has not be enable tocompete with other liquid or gaseous fuels.

Introduction

6

Adds hydrogen to break down the coal

Dissolves in a solvent followed by hydrocracking

Operates at 450 C and 170 bars Light products are distilled Medium and heavy distillates

obtained from vacuum distillation Liquid yields of 70% of the dry

weight of coal feed Further upgrade is needed for

use as transportation fuels

Complete breakdown of coal with steam and oxygen

Sulfur is removed from the syngas

Syngas reacted over catalyst at300 C and 20 bars

Produces a lighter suite of products; high quality gasoline and petrochemicals

Oxygenated chemicals

DIRECTLIQUEFACTION

INDIRECTLIQUEFACTION

Direct Vs Indirect Liquefaction

Fuel H/C ratio

Brief History of Coal Liquefaction

Coal was hydrogenated in the laboratory by Berthelot as early as1869. The reaction was carried out with hydriodic acid at 520◦F for 24hours, and a 67% yield of oil containing aromatics and naphtheneswas obtained.

In the early 1900s, German scientists and engineers invented anddeveloped two processes that enabled them to produce syntheticpetroleum from their coal supplies and to establish the world’s firsttechnologically successful synthetic liquid fuel industry.

In 1911, Friedrich Bergius obtained oil by hydrogenating coalwithout a catalyst under hydrogen pressure at 570 to 660◦F.

At the end of 1925, I.G. Farben, a chemical company, hydrogenatedcoal using a molybdenum oxide catalyst. The presence of thecatalyst allowed the hydrogenation of coal in the presence of excesshydrogen at low pressure and at temperatures of 750 to 840◦F.

Brief History of Coal Liquefaction

In 1920, Franz Fischer and Hans Tropsch at the Kaiser–WilhelmInstitute for Coal Research in Mülheim, invented a second processfor the synthesis of liquid fuel from coal.

By the mid-1930s, I.G. Farben and Ruhrchemie had started toindustrialize synthetic liquid fuel production, resulting in theconstruction of 12 coal hydrogenation and 9 Fischer–Tropsch plantsby the time World War II ended.

South Africa, fearing boycott as a result of their racial policies,decided to proceed with a synthetic fuels plant in 1951,

Pyrolysis

High-temperature carbonization is the oldest route for the productionof liquids from coal, wherein hydrocarbon liquid is predominantly abyproduct of coke-making.

The low yields (<~5%) of liquid product and relatively high upgradingcosts mean that traditional high-temperature carbonization is not anoption for the production of liquid fuels on a commercial basis.

Mild pyrolysis is also a carbonization technology but with less severeoperating conditions. Mild pyrolysis consists of heating the coal to atemperature in the range ~450-650ºC, driving off volatile matter fromthe original coal and generating other volatile organic compoundsformed by thermal decomposition during the treatment.

Liquid yields are higher than for high-temperature carbonization, but are still no more than 15-20% at most with the main product is chart.

Pyrolysis

A higher yield of liquids can be obtained by rapid pyrolysis. Theseprocesses operate at temperatures up to 1200ºC, but the residencetime of the coal is significantly reduced, to a few seconds at most.

Rapid pyrolysis is aimed at producing chemical feedstocks ratherthan liquid fuels and process economics are likely to be highlyunfavorable for production of liquid fuels. There also appear to beunresolved engineering difficulties.

The latest technology in USA is the Liquids from Coal (LFC) process.It has been in commercial-scale operation since 1992.

The LFC process is a mild pyrolysis method for upgrading coal andwas developed by SGI International. There are two saleableproducts: a low-sulphur, high heating-value solid known as ‘process-derived fuel’ (PDF) and a hydrocarbon liquid known as ‘coal-derivedliquid’ (CDL). PDF yields are considerably higher than CDL yields.

Pyrolysis

The coal is crushed and screened and then heated by a hotgas stream on a rotary grate dryer.

The dried coal is then fed into the main rotary grate pyrolyserwhere it is heated to ~540ºC by a hot recycled gas stream.

On discharge from the pyrolyser the solids are cooled andpass to a deactivation step, consisting of treatment in avibrating fluidized bed with a gas stream of controlled oxygencontent.

The gas stream leaving the pyrolyser is cooled in a quenchtower, condensing CDL but leaving water in the gas phase.

Most of the residual gas is recycled to the pyrolyser, somebeing burned in the pyrolyser combustor to provide thenecessary heat.

Pyrolysis

The remaining gas is burned in the dryer combustor andpasses into the dryer gas recycle loop.The purge from this loop is wet-scrubbed to remove

particulates and sulphur oxides. Purge liquor from thescrubbers is discharged to ponds for evaporation.After being stabilized by mild oxidation, the PDF, a low-

sulphur reactive fuel suitable for pulverized coal-fired boilers, is shipped by rail to power plant. CDL is shipped by rail to a fuel oil distributor.

Pyrolysis

Indirect Coal Liquefaction

Indirect coal liquefaction differs fundamentally from directcoal liquefaction in that the coal is first converted to asynthesis gas (a mixture of H2 and CO) which is thenconverted over a catalyst to the final product.The synthesis gas is produced in a gasifier, where the

coal is partially combusted at high temperature andmoderate pressure with a mixture of oxygen and steam.In addition to H2 and CO, the raw synthesis gas contains

other constituents (such as CO2, H2S, NH3, N2, and CH4),as well as particulates.


Before being fed to the synthesis reactor, the synthesis gasmust first be cooled and then passed through particulateremoval equipment.

Following this, depending on the catalyst being used, it may benecessary to adjust the H2/CO ratio.

Modern high-efficiency gasifiers typically produce a ratiobetween 0.45 and 0.7, which is lower than stoichiometric forthe synthesis reaction.

Some catalysts, particularly iron catalysts, possess water gasshift conversion activity and permit operation with a low H2/COratio.

Other catalysts possess little shift activity, however, andrequire a ratio adjustment before the synthesis reactor.


After shift conversion, acid gases (CO2 and H2S) arescrubbed from the synthesis gas.

A guard chamber is sometimes used to remove the last tracesof H2S.

The cleaned gas is sent to the synthesis reactor, where it isconverted at moderate temperature and pressure, typically498 to 613 K (435 to 645°F) and 1.52 to 6.08 MPa (220 to880 psia).

Products from the process depend on operating conditionsand the catalyst employed, as well as reactor design.

Typical products include hydrocarbons (mainly straightchain paraffins from methane through n-C50 and higher),oxygenates (methanol, higher alcohols, ethers), and otherchemicals (olefins).


Fischer-Tropsch Synthesis

The best-known technology for producing hydrocarbonsfrom synthesis gas is the Fischer-Tropsch synthesis.This technology was first demonstrated in Germany in

1902 by Sabatier and Senderens when theyhydrogenated carbon monoxide (CO) to methane,using a nickel catalyst.In 1926 Fischer and Tropsch were awarded a patent for

the discovery of a catalytic technique to convertsynthesis gas to liquid hydrocarbons similar topetroleum.


The basic reactions in the Fischer-Tropsch synthesis are:


Other reactions may also occur during the Fischer-Tropschsynthesis, depending on the catalyst employed and theconditions used:



The Synthol reactor developed by SASOL is typical of high-temperature operation.

Using an iron-based catalyst, this process produces a verygood gasoline product having high olefinicity and a lowboiling range.

The olefin fraction can readily be oligomerized to producediesel fuel.

Low-temperature operation, typically in fixed-bed reactors,produces a much more paraffinic and straight-chainproduct.

Selectivity can be tailored to give the desired chain growthparameter.

The primary diesel fraction, as well as the diesel-range productfrom hydrocracking of the wax, is an excellent diesel fuel.


SASOL

Oxygenated Chemicals

A whole host of oxygenated products, i.e., fuels, fueladditives, and chemicals, can be produced from synthesis gas.

These include such products as methanol, isobutanol,dimethyl ether, dimethyl carbonate, and many otherhydrocarbons and oxyhydrocarbons.

Typical oxygenate-producing reactions are:

Reaction (27-37) can occur in parallel with the methanolreactions, thereby overcoming the equilibrium limitation onmethanol formation.

Oxygenated Chemicals

The production of methyl acetate from synthesis gas iscurrently being practiced commercially.

Following methanol synthesis, as shown by Reaction (27-35), the reactions are:

Acrylates and methacrylates, which are critical to theproduction of polyesters, plastics, latexes, and syntheticlubricants, can also be produced from these oxygenatedintermediates.

DIRECT COAL LIQUEFACTION

Introduction

Direct liquefaction transforms coal into liquidhydrocarbons by directly adding hydrogen to the coal.All direct-liquefaction processes consist of three basic

steps:1. Coal slurrying in a vehicle solvent,2. Coal dissolution under high pressure and temperature, and3. Transfer of hydrogen to the dissolved coal.

However, the specific reaction pathways and associatedkinetics are not known in detail.

Introduction

Bergius/I.G. Farben Process

The Bergius process was put in to commercial practiceby I.G. Farben in Leuna, Germany, in 1927, andadditional plants were erected in the 1930s.The process operated in two stages.

1. Liquid-phase hydrogenation first transformed the coal intomiddle oils, with boiling points between 300 and 615◦F,

2. vapor-phase hydrogenation then converted these oils togasoline, diesel fuel, and other relatively light hydrocarbons.

Bergius/I.G. Farben Process

The Bergius process converts 1 short ton of coal to 40-45gallons of gasoline, 50 gallons of diesel fuel, and 35 gallons offuel oil.

The gasoline fraction contains 75 to 80% paraffins and olefinsand 20 to 25% aromatic compounds.

Liquid-phase hydrogenation of low-rank coals was usuallyaccomplished at 890 to 905◦F under pressures of 250 to 300atm using an iron-oxide hydrogenation catalyst.

For liquid-phase hydrogenation of bituminous coals,temperatures were similar, but pressures were in the range of350 to 700 atm.

Solvent Refining Processes

The solvent-refined coal (SRC) process is considered theleast complex of the various process schemes.Hydrogenation of the coal in the SRC process occurs

at elevated temperatures and pressures in thepresence of hydrogen.

SRC-I Process

In the SRC-I process, ground and dried coal is fed to thereact or as a slurry.The transport liquid, or process solvent, is a distillate

fraction recovered from the coal hydrogenation productand serves as the hydrogen donor.The coal slurry and hydrogen are introduced into a

dissolver at 840◦F and 1500 psig.The process produces a low-ash (∼0.1%), low-sulfur

(∼0.3%), solid fuel with a heating value of ∼16,000Btu/lb.The SRC-I fuel was envisioned as a boiler fuel to

replace natural gas and fuel oil.

SRC-I Process

SRC-II Process

The SRC-I process was modified to eliminate anumber of processing steps and to produce an all-distillate, low-sulfur fuel oil from coal rather than asolid fuel.In the SRC-II process, pulverized and dried coal is mixed

with recycled slurry solvent from the process.The slurry mixture is pumped to reaction pressure

(∼140atm), preheated to about 700 to 750◦F, and fed intothe dissolver, which is operated at 820 to 870◦F.The dissolver effluent is separated into vapor-and liquid-

phase fractions.

SRC-II Process

The overhead vapor stream undergoes several stagesof cooling and separations, and the condensed liquid isdistilled to produce naphtha and a middle distillate oil,which a reconverted to gasoline and diesel fuel,respectively.The gaseous products are purified to remove hydrogen

sulfide and carbon dioxide, and the hydrogen-rich gas isthen recycled to there actor with make-up hydrogen.The liquid-phase product acts as the solvent for the

SRC-II process.

SRC-II Process

Costeam Process

The costeam process, investigated at the PittsburghEnergy Research Center (DOE) and the University ofNorth Dakota/Grand Forks Energy Research Centerwasintended to produce low-sulfur liquid products fromlignites and subbituminous coals.This process uses crude syngas containing about 50

to 60% carbon monoxide and 30 to 50% hydrogen, ratherthan pure hydrogen.The high moisture content of the low-rank coals

provides the water that is converted to steam.

Costeam Process

Ground coal is slurried with recycled product oil from theprocess.

The slurried coal is pumped to unit pressure (4000 psig),mixed with syngas, heated to 750 to 840◦F, and maintained atconditions for periods of 1 to 2 hours to liquefy the coal.

Severe operating conditions are required for the dissolution ofthe lignite.

These operating conditions were a drawback for the process,as long residence times and high pressures are costly andpresent major engineering problems.

This process, although a potential candidate for liquefying low-rank coals, was only tested in small-scale equipment.

Catalytic Processes

The most important process of this group is the H-Coalprocess, developed by Hydrocarbon Research, Inc. (HRI)as an outgrowth of previous work on the hydrogenation ofpetroleum fractions.The development of this process was sponsored by the

ERDA and a large group of oil companies.

H-Coal Process

In the H-coal process, pulverized coal is slurried with recycledoil and, along with hydrogen, fed to an ebullated-bed reactor,a feature that distinguishes this process from others.

Reactor conditions are normally in the range of 825 to 875◦Fand 2500 to 3500 psig.

The reactor contains a bed of catalytic particles, cobaltmolybdate on alumina oxide.

The products from the reactor include hydrocarbon gases, lightand heavy distillate oils, and bottoms slurry.

Variations of the processing scheme can produce fuel oil,naphtha, synthetic crude, ammonia, and fuel gas.

Further processing can produce gasoline and jet fuel.

H-Coal Process

H-Coal Process

H-Coal Process

In 1995, when it became an employee-owned company,HRI changed its name to Hydrocarbon Technologies, Inc.(HTI).HTI has modified its process, now known as HTI’s direct

coal liquefaction (DCL) process and shown schematicallyin Figure 5-36, and it has gone from a single- to double-stage reactor system.

H-Coal Process

In the HTI DCL process, pulverized coal is dissolved inrecycled, coal-derived, heavy-process liquid at about2500 psig and 800◦F while hydrogen is added.Most of the coal structure is broken down in the first-

stage reactor.Liquefaction is completed in the second-stage

reactor, at a slightly higher temperature and lowerpressure.A proprietary GelCat catalyst is dispersed in the slurry

for both stages.The process produces diesel fuel and gasoline.

H-Coal Process

Donor Solvent Processes

The main representative of this process is the Exxondonor solvent (EDS) process.In a donor solvent process, the solubilization of the

coal is done by the hydrogen donor liquid, andhydrogen molecules come from the donor liquid andnot gaseous hydrogen.The donor liquid is then recycled and hydrotreated to add

hydrogen back into the donor liquid.

Exxon Donor Solvent Process

In the EDS process, finely ground coal is mixed with thedonor solvent, and the coal is liquefied in a noncatalytictubular plug-flow reactor in the presence of molecularhydrogen and a hydrogen-rich donor solvent.The liquefaction reactor operates at 800 to 880◦F and

1700 to 2300 psig.The products from the liquefaction reactor are separated

by distillation into light hydrocarbon gases, rangingfrom methane to propane and methylpropane, a naphthafraction, a heavy distillate, and a bottoms fraction.The naphtha and heavy distillate fractions are treated by

conventional petroleum-refining technology.


About 85% of the naphtha is recovered as gasoline, andabout 50% of the heavy distillate is recovered as amixture of benzene, toluene, and xylenes.Further processing of the heavy distillate produces

fractions comparable to jet fuel and heating fuel.A portion of the heavy distillate is hydrotreated and

recycled to slurry the coal.In addition, the bottoms are either coked in a fluid coking

plant or recycled to the liquefaction reactor.Recycling to the liquefaction unit results in a dramatic

increase in the conversion of the coal to liquid products.



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LIQUEFACTION - Universitas Diponegoroasepmusa.blog.undip.ac.id/files/Coal-liquefaction.pdf · Complete breakdown of coal with steam and oxygen Sulfur is removed from the syngas Syngas