The extraction of coal with supercritical hydrocarbon mixtures

Fluid Phase Equilibria, Elsevier Scientific

10 (1983)

Publishing

327-336

Company,

327

Amsterdam

-

THE EXTRACTION OF COAL WITH SUPCRCRITICAL

M.J.

FINN,

National

C.J.

Coal

BOWER and R.D. Board,

Gloucestershire

GL52

Printed in ‘rhe Netherlands

HYL)xClCARBO:JMIXTURES

HUGHES

Coal

Research

4RZ

(England)

Establishwart,

Stoke

Orchard,

Cheltenham,

ABSTRACT The National Coal Board has developed a process for converting coal into distillable liquids based on the extraction by supercritical fluids of fragments of coal molecules produced by thermolysis. The resultant coal extract has desirable properties for subsequent hydroprocessing. This paper outlines process requirements and describes how these were realised in a 10 kg h-1 pilot plant. Results illustrating the effects of main process variables are given, using examples taken from experimental work employing a simulated processderived solvent as extractant. Yields and analyses are presented for products obtained under varied processing conditions. INTRODUCT ION It the

is

apparent

supply

of

happens

both

coal,

fuels

The in

chemical

reserves

Two processes

have

coal

that

extract

principal

been

developed

is

coal

is

de-ashed

in

first

taken

a high-boiling

liquid

solvent

.

The

employs

a much

second

temperatures Extract results

ion

above (SGE) .

the

critical

This

illustrating

the

paper

and

the

into

a stage

at

which

projected into

society,

past

most

chemical

0378-3812/83/$03.00

compounds

used

to for

is

values.

constituting

will

decade,

distillable

and

obvious been liquids,

termed

of

It the the

a 25

and yield

Liquid

thus

the

latter

the

at

has

liquids.

The One process

Extraction

pressures

and

Supercritical technique

coal

have

0 1983 Elsevier Science Publishers 13.V.

high

Gas

and presents

process.

extremely

the

resultant

coal.

Solvent

termed

experimental

Each

that light

dissolving

solvent, is

tonne/day

PROCESS REQUIREMENTS The

that

it

source,

the

continuation.

solution

lower-boiling

describes

operation

for

is

this

Assuming

by

an alternative

coal

hydroprocessing

(LSE)

from

surpluses,

before

occur.

petroleum,

of

the

method

uses

will required

therefore,

to is

the

that

products.

plant

before

is

has

of

and

price

still

them

those

conversion

petroleum

pilot

difference

the

in

are

replace

Board

at

short-term

exhausted,

rise

times

crude

development feature

Coal

aimed for

common

notwithstanding

become

to

many

National

research

a replacement

equipment

later,

feedstocks

and necessary

proven

candidate.

or will

and progressive

and

economic with

engaged as

sooner

petroleum

a considerable

automotive be

that

crude

molecular

328 weights.

They

are

together

fused

these

consist

structures

oxygen,

are

nitrogen

large, coal

molecules

this

is

the

large

tend

to

weaker

process

by

under

using

to

use

of

a gas

is

that

confers

the

should

be

of

significance,

to

extract

is

not

employ

the

have

with

for

solvent

fluid

is

that

soluble

components, by heating,

are

by

reactive broken

the

form,

and

formation

order

radicals

achieved

above

its

critical

in

the

density

is

into

solution.

tension

of

the

solvent

that

the

critical

to

must

be

the

SGE

temperature

taken

surface

and penetration

of

In

the

thus

originally

is

be

very

produced

This

a gas, to

benefit

of

in

to

be

sufficiently The

effects

are

into

the

coal

with

higher

a regeneration be

in

the

liquid

but

use.

latter

at

It

can

be

be

be

even

possible pressure

stable

at

the

inclusion

solvent

as

advantageous

it

would

to extract

in be

losses

also

oxygen

atmospheric

the

scale

though

is

stabilising

implies

is

would

It

must

thus

this

even

aliphatic,

the

phase

On an industrial

process-derived,

than

containing

extract.

solvent

properties,

yields,

stage.

The

repeated

donor

of

density

type

better

although

solvent

Chemical

Compounds

processing

iLs

hydrogen

being

c’sal.

temperature

a high

pressure.

general the

selection.

permit

allowing

effective,

flirther

solvent to

is

excess.ive

of

Solubility

for

to

use

nature

in

choice

temperature,

solvents

shown

and obtaining

the

the

been

water.

compounds

of

a liquid

fragments

are

irreversible.

residut,.

and

non-destructively.

bonds

is

s,olvent

solvent

the

conditions

radicals

the

process

in the

the

occur,

from

a criterion

process

the

low-boiling

extraction

contaminants

operating

frorr

additional

aromatic

and nitrogen

formed

many of recombination

molecular

without

expected

undesirable

but

into

strains

w!lich

containing

molecules

coal

down

the

rings

complexity,

rings

the

dissolving

radicals

material

influencing to

maintained

be

and carbon

broken

free

carbon of

hindered.

close

be

be

rendering of

problems

factor

to

of

Under

although

large

not

The main

to

removed

even

no wetting

particle

would

so

enable

absent,

yield

degrees

Because

capable

bonds,

and

aromatic

heteroatoms.

the

bonds,

of

various

rings

Furthermore,

a relatively

pressure,

high

up:

heteroatom

solution

of

thermolysis.

break

an acceptable into

as found

rapidly.

carbon-carbon

clusters

alicyclic

therefore

by

molecules

produce taken

have

achieved

recombine

the

by

been

of

structures

linked

has

and

stronger

unit

and sulphur

no solvent

The

are

principally

into

the

necessary

are

small.

LABORATORY EXPERIMENTS Initial stirred solvent, passed the before

work

on developing

Lhe ideas

in

into

a coal

charge

autoclaves, the

autoclave

through

disadvantages extraction),

when

which was

electrically

critical

that

heating

relatively

large

had

slow

and

been

(allowing

volunes

laboratory

employed

WRS placed.

heated

conditions was

the

of

filling

of

solvent

a stream

achieved.

This

had

with was

technique

had

to occur

recombination solvent

conventional

After

to

be used,

and it

329 was

difficult

to

relate

the

results

continuous

process.

These

miniature,

unstirred

autoclave

porous

plates.

heating

were

Results

from

this

proportions

of

operating

by

work

the

and

batch-wise

respective

effects

SOLVENT

solvent

was

whole

of

in

experiments. to

the

to

coal

However, it

ratio

and

and

rapid

properties

rates

the

not

in

relation

to

for

process

possible

contact

operations

to

separate

the

time.

COAL

4iJv

RESIDUE SEPARATORS

HIGH PRESSURE WMP

RECYCLE PUMP

STILL f

LET- DOWN L

FI G. 1. DIAGRAMMATIC PILOT In

order to

batches

to

balance of

overcome

build

Figure

extract

of In

the

limitations

plant, operate

large

enough

processing

operation,

to

of

which

would

conditions

capable 1.

a pilot

data,

hydroprocessing unit,

FLOWSHEET

OF 10kgti’

PILOT

PLANT

PLANT

decided mass

EXTRACT

WATER

STORAGE

in to

be

small

enable

carried

1.0 kg

h-1

of and

coal,

on

of

mode,

experiments out

batch

capable

a continuous

be

temperatures

using

would

and into

a consistent .is shown

pressures

are

of

sandbath.

sufficient

since

was

between

substantial

quantities

coal,

a

retained

a fluidised

extracting

s;olvent

extract

using

was

included, in

a scaled-up

by

charge

assembly

desirable

to

overcome

g coal

feasibility

produced

solvent

experiments

partly

a 50

the

the

relative of

for

determined

hydroprocessing

still

which

coil

showed

batch

were

immersing

coal,

conditions,

preliminary were

in

A preheating achieved

from

limitations

apparatus

generating would the

was

produce effect

of

feedstock.

diagraun~atically stabilised

it

reliable

by

The in circulat-

330 ing

before

only,

solvent

solvent.

This

Pump 9 and

preheated

sandbath.

coil,

slurry

of

the

Since the

the

whirh

of

extract/solvent is

collected,

by

the

and

mixture

partial

produced

Solvent

by

Extract to

is

which

normally

off

are

cooled

from

feed

a heated

in

a larger resulting

the of

still

as

kettle

of

cyclone-

solids

the

fluid.

in

a single

of

where

the to

they and

which

is end

the

the

solvent, reuse.

each

in

virtually

on cooling pressurised

run.

The

almost

indistinguishable

course

it

has

followed

separator.

for

solidifies

still

vapour

condenser,

collected of

extract the

by water

leave

a by

The

heat.

and returned

returned

a liquid,

as

accompanied

latent

a total

as

is

is

accompanied

settling,

solid,,

although

boiling-point, precipitated

a still

out

to

sampled

the

is

to

vapour,

Residue at

pressure

Expansion

system,

emptied

coal,

and

The

depressurised

extract

scrubbed

gravity

the

particulate

the

the

passes

and

solid.

and

a free-flowing

appearance

the

brittle

in

a primary

viscosity

is

converted

the

being

measured

from

black

is

into

by

low

droplets.

heat

through

to

completes

atmospheric

Solvent

separated

the

and

decomposition,

passage

run

a remeltable

vessels

are

its

solvent

sandbath.

which

by

its

liquid

droplets

are

by

passes

solution

vapour,

solvent.

immersed

the

coal

a reciprocating

valve.

above

passed

coal

Gases

and water

unchanged

is

cf

by

a separator.

the

sensible

escaping

reflux

easier

to

coal

crushed

by

a coil

a second

filter

made

far

in

residual

solids,

a tenuous

SOOC as

any

gases

by

is

to

coalesces

some

and

of

pressure

in

pressure-control

temperature

rapidly

a cooling

all

a slurry

extracted

a secondary

being

expands

are

temperature

to

to

critical

temperature

radicals at

thence

system

changed the

selected

coal

virtually the

solvent

mist

the

separation

across

is

solution

and

removed

stage

to

super-critical

removal,

feed above

maintained

separator

Having

the

pressurised

Thermolysed

contactor

type

is

residue in

a high

is size

degree

and

of

porosity.

RESULTS The

solvent

aromatic

from

mixture

solvent

the had

critical

was

products the

pressure

It

rather

higher,

since

either

the

more

than

critical one

the

3.1 is

Some

as

was

reported

by

that Rzasa

mixtures

by

in

& Katz,,

results the

4400C

the

actual

normally

anomalous

present.

the

present

in

by

critical

obtained

in

pseudo-

and

pressure (ref

pressures experiments

investigations

The

and the

Prausnitz

Comings

an

distillation

(713oK),

modified

of

isomeric

process.

pseudo-critical

quoted have

mixture

two

a particular

hydrogenation

of

calculated

a made-up each

of

extract

temperature

M Pa,

was

compound,

representative

subsequent

temperature phase

here

aromatic:

probable

work

hydrocarbon

component.

near

work

chosen

of

of

(ref.1).

binary

the

pseudo-critical

method

that

in

a fully-hydrogenated

This

forms. cut

employed

and

also

.2),

was showed

higher at

Gunn

than

5 M Pa

suggested

that

331 of

Yield

using

to

ments

of

coal

coal

possible

some

(for

a non-reactive

solvent

as

extract

has

ation

of

pressure

60

its

of too

process

rate

with

prevent

become

and

The

increases to

is

solvent,

ratio.

recombination

which

a given

coal

controlled of

will

in

conditions

occur,

matched

the

and residence

temperature.

extract

causing to

a careful to

the

must

insoluble

weight

requires

teaperature,

to

molecular

size

solvent

be

removed Even

precipitation

choice

in of

type

of

solvent

used.

a

s

30 1,

FIG.2

,

I

1

I

5 EXTRACTION

10

15

20

EFFECT

PRESSURE

OF

(M F’al

PRESSURE

ON EXTRACT

LLO

L20

EXTRACTION

FIG. 3

EFFECT

YIELD

A

A

Lao

160 TEMPERATURE

OF TEMPERATURE

(“Cl

ON EXTRACT

as in

quickly solution,

material

quantity,

W

0,

frag-

Optimiz-

solution.

I .. is

and

molecular

residue.

remain

time),

pressure

aof extractable

and

temperature,

extract

by

liberation

recombination

high

particle

type,

YIELD

Extract yield, shown as a percentage approximately

of dry, ash-free coal, increases in an

linear fashion with pressure

temperature

of 420%,

significant

increase.

where doubling

(Fig.2), except at the lowest

the pressure from 10 to 20 ?I Pa produced no

In contrast, the results for the efrect of temperature

suggest (Fig.3), but by no means establish, yield in the region 44Oo-460°C.

the presence of a broad maximum for

These results can be explained by the conjecall available material has been extracted

ture that, at the lowest temperature,

even at 10 M Pa, whilst at the higher temperatures extract recombination

and precipitation

inefficient

extraction or

occurs as the solvent density falls.

It was difficult for us to determine accurate values for solvent density at condition,

but calculations

temperature,

and generalised

values quoted above.

have been made based on reduced pressure, reduced compressibility

should have some significance, calculated

8y replotting

density, Figure 4 is obtained.

is, within experimental of the subcritical

factor, using the pseudo-critical

Whilst the absolute values are unreliable, relative values the results for yield against

This shows that the yield of extract

limits, linear with solvent density, with the exception

results at 420°C which lie below the line

/

300

5M3

LOO

CAlflUTED

SOWENT

FIG. &. EFFECT

DENSITY

600 (kg m -31

OF DENSITY

ON EXTRACT

The effect of solvent to coal ratio is dcmonstratfd 10 M Pa and 20 M Pa, 460%.

At the lower pressure,

YIELD

in Figure 5 for tests at

the yield is linear with

solvent to coal ratio in the range covered, indicating

that reduced solvent

density can be at least partly compensated by increased dilution. yield at 4:l

However,

the

and 10 M Pa is still below that obtained at 2:l and 20 M Pa, and it

is likely Lhat process economics would

favour the higher pressure and lower

333 ratio. 2:l to

It LO 3:l

4:l

will at

be

extract

of

that

whilst

20 M Pa produces

and 5:l

extraction

seen

is

a larger

negligible.

soluble

It

material

concentration

in

increasing

the

can

yield, be

deduced

was virtually solvent

nf

the

solvent

to

coal

the

benefit

in

further

thaL

complete

about

ratio

from

increasing

a~

this

pressure

at

3:1,

representing

the

15%.

60 ./--A-

8 cl

10 M Pa

/

* I-

I

I

I

I

1

2

3

1

5

MLVENTl

FIG

5 EFFECT The

molecular

isopiestically, information

OF

COAL

SOLVENTlCOAL

weight

of

the

and corrected on the

(w/w)

extracts for

mechanisms

ON EXTRACT

produced traces

or

yield

(number

average, present)

(Figure

6).

OF

CALCULATED

SOLVENT

determined provides

At

rach

pressure

1

500 SOLVENT

YIELD

:solvent

I

LOO

EFFECT

thtl

determining

LOO1

FIG. 6.

RATIO

600 DENSITY

DENSITY WEIGHT

1 kg

m-3

1

ON EXTRACT

MOLECULAR

an

334 LIW

molecular

weight

falls progressively

solvent density is thereby reduced.

as the temperature

Similarly,

is increased and the

increasing the pressure and The

thereby the density in general produces an increased molecular weight. implication material

is that as the temperature

increases the amount of extractable

produced also increases, whilsl the size of the molecular fragments

decreases.

However, the ability of the solvent to hold material

is reduced as its density

in solution

falls, leading to a tendency for the heavier species

to precipitate.

This is the likely explanation

with temperature

at each pressure suggested by LIW results o1 sigure

for the maximum value of yield

Changes in extract composition with temperature Table 1. hydrogen

3.

and pressure are shown in

When compared with the coal, all extracts show higher carbon and contents, and a higher hydrogen

to carbon atomic ratio.

sulphur are reduced below the coal values, but nitrogen presumably

due to its occurrence

Results at constant pressure composition with temperature;

Oxygen and

tends to increase,

in stable ring-structures. (10 M Pa) show a progressive

change in extract

the carbon content increases and the hydrogen,

oxygen and sulphur contents fall as tenlperature increases,

At the same time

extract molecular weight falls, even though the extract yield does not change significantly.

Increasing pressure at constant temperature

(44O@Cj produces

an increase in yield from 41.6% to 57.82, accompanied by compositional

changes.

Carbon content is constant, but the increasing molecular weight is accompanied by an increase in oxygen and sulphur and a reduction in hydrogen.

TABLE 1 Product yields, extract and coal analyses Extraction conditions

Yield Extract analyses of Extract Ash C HO Pressure Temp Solvent % w daf content density* coal M Pa kg m-3 % w daf oC %W 5

10

20

440

470

41.6

0.21

82.5 6.6

N

S

Molecular weight atomic (number ratio average) H/C

8.4 1.29 1.19 0.94

520

420

557

46.1

0.07

82.3 6.1

8.9 1.31 1.24 0.90

630

440

538

41.5

0.09

82.5 6.4

8.6 1.37 1.17 0.93

580

460

511

47.1

0.11

83.3 6.3

8.0 1.37 1.11 0.90

540

480

483

47.6

0.13

84.4 6.1

7.1 1.45 0.98 0.86

500

440

599

57.8

0.09

82.51 6.1

8.7 1.21 1.39 0.89

600

12.0

80.0 5.1 11.6 1.33 1.94 0.74

Typical coal analysis * calculated

-

335

TABLE 2 Composition of gaseous products (nitrogen and oxygen free basis) PressurP:

10 M Pa

Constituent

% w of gas

Extraction temperature oC

Gas yield % w daf coal

CO

CO2

H2S

H2

CH4

Other hydrocarbons

420

3.2

6.1

50.8

15.0

0.3

13.7

14.1

440

4. 8

4.7

42.5

10.7

0.4

15.5

26.2

460

5.7

5.7

34.5

14.7

1.0

19.2

24.9

480

7.0

4.8

30.3

9.6

1.1

21.3

32.9

Yield of gas increases progressively changes, Table 2.

with

At the lowest temperature

temperature

the proportion

PROCESS

of carbon dioxide falls to one-third,

of hydrocarbon

over conventional

can be adjusted

which utilises

enhanced

in the extract,

The process

bituminous different

conditions,

advantage

the proportions

such that, in the context

of the SGE

coal conversion processes. of extract and

of a self sufficient

plant,

only part of the coal the hydrogen leading to lower hydrogen

to carbon ratio is

requirements

in downstream

This does not reduce the utility of the residue which has an ash-

free calorific

processed

a potential

the residue as a fuel, the most economic ratio can be obtained.

Secondly, by extracting

processing.

illustrate

liquid solvent extraction

First, by choice of operating residue

and aver half

constituents.

IMPLICATIONS

The results of this program process

and its composition

As the gas yield increases with

dioxide, with about a quarter hydrocarbons.

the gas consists

temperature,

0ve.r half the gas is carbon

value similar is adaptable

to that of the coal. to a wide variety

of feedstocks.

Coals successfully

in the 10 kg h-1 unit include British coking as well as non-caking coals, Australian types of solvent.

perhydrous With

coal and North American

these attributes,

lignite, using

it is apparent

conversion

of coal may prove to be a field in which supercritical

technology

has a future role.

that the fluid

336

ACKNOWLEDGEMENT The Authors

thank the National Coal Board for permission

to publish this

paper, and the European Coal and Steel Community for financial support. views expressed

are those of the authors and not necessarily

The

those of the Board.

REFERENCES 1. R.C. Reid and T.K. Sherwood, The Properties of Gases anh Liquids, McGraw Hill Book Co.. 2nd Edition, p.316. 2. E.W. Comings, High Pressure Technology, McGraw Hill Book Co., 1956, p.300.