Chemicals Produced in a Commercial Fischer-Tropsch Process

Chapter 2 Chemicals Produced in a Commercial Downloaded by LOUISIANA STATE UNIV on September 1, 2014 | http://pubs.acs.org Publication Date: December 16, 1987 | doi: 10.1021/bk-1987-0328.ch002

Fischer-Tropsch Process Mark E. Dry

Sasol Technology (PTY) Limited, P.O. Box 1, Sasolburg 9570, Republic of South Africa

In this article a general review of the Fischer-Tropsch process is given, but since Sasol operates the only proven commercial plants the emphasis will fall on the Sasol operations. The first Sasol plant (Sasol One) came on stream in 1955 and is still in production. The profitability of this operation initially was low because the price of crude oil remained depressed for many years due to the discovery and exploitation of the huge oil deposits in the Middle East. After 1973, however, the price of crude oil rose rapidly and consequently the profitability of the Fischer-Tropsch process in South Africa improved dramatically. This lead to the construction of two much bigger plants (Sasol Two and Sasol Three) which came on stream in 1980 and 1982 respectively. Sasol is now a public company and its shares are listed on the Johannesburg Stock Exchange. Despite the fluctuations in the price of crude oil, the commercial success of the process is reflected by the four-fold increase in the share prices since 1979. The reason for the viability of the process in South Africa is the combination of three factors:

a) b)

there are no petroleum deposits in the country the industrial activity (and therefore the market) is predominantly concentrated in the Witwatersrand area which is about 400 miles from the sea

c)

the huge deposits of coal which are in the same area are cheap to mine because the thick seams allow a high degree of

mechanization.

The present plants are geared predominantly at the production of gasoline and diesel fuel but a significant fraction of chemicals are also produced in both the coal gasification and in the FischerTropsch (FT) synthesis process. The latter process can be manipulated to increase the production of chemicals, should this be desired. For fuller detail of the production of fuels, the reader is

referred to other reviews (}_)(2). 0097-6156/87/0328-001 8$06.00/0

© 1987 American Chemical Society In Industrial Chemicals via C1 Processes; Fahey, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

2. DRY

The Fischer-Tropsch Process

19

The Fischer-Tropsch Process Primary and Secondary Reactions Examples of some of the primary reactions are:

Olefins

: 2n H2 + nCO ~» CnH2n + n H20

Alcohols : 2n H0 + nCO Z

Downloaded by LOUISIANA STATE UNIV on September 1, 2014 | http://pubs.acs.org Publication Date: December 16, 1987 | doi: 10.1021/bk-1987-0328.ch002

Acids

(1)

-» C H0 -OH + (n-1) Ho0 n

2n+l

(2)

l

: 2n H2 + n+1 CO -* Cn H2n+]C00H + (n-1) H20

(3)

The primary products are linear. The formation of branched products probably result from secondary isomerization reactions. Ketones are also apparently secondary products. If the catalyst used is active for the water gas shift reaction:

nH20 + nCO — » nC02 + nhL

(4)

then C02 can be formed in a secondary reaction. The extent to which this happens depends on the H2/C0 ratio inside the reactor and on

the temperature. In principle therefore mixtures of CO« and hL or of

HLO + CO can be used as feed gases provided the waxer gas shift reaction is active. For instance, for the production of olefins

3n H2 + nC02 -» Cn H2n + 2nH20

(5)

"Equation 5" is simply the sum of "Equation 1" and the reverse of "Equation 4". Overall Process Scheme

A generalized scheme of the Sasol process is illustrated in Figure 1. The basic raw materials are coal, water and air. The plant is a complex operation consisting of many interlinked processes. This complexity is, however, not an important factor in the economics

when

it

is

borne

in mind that the main

cost

is the

production of synthesis gas, which accounts for over 50 % of the

total.

Synthesis Gas Production

All the Sasol plants use Lurgi gasifiers to convert coal to raw synthesis gas. The units are fed counter currently (coal in at the top and the gasifying agents, oxygen and steam, in at the bottom). These gasifiers are well-suited to the low grade high-ash coal used by Sasol. The composition (volume percent) of the raw gas is

approximately, 60 (hL + CO); 9 Cl-L; 29 C02; 1 Inerts (A + Nj;

0.5 H2S and lesser amounts of components such as C2Hß and tars etc.

The raw gas is purified in the Lurgi Rectisol process (cold methanol scrubbing) where various by-products (eg tar naphthas) and

impurities (eg C02 and H2S) are removed.

In Industrial Chemicals via C1 Processes; Fahey, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

20

INDUSTRIAL CHEMICALS VIA C , PROCESSES

coal 1 — «— *\ "r \*

1^

STEAM


1

*"

I

*"

GASIFICATION

•*

I

.

NH3 -^

1 02 I

PLANT

1 I ^ —

AIK

1

Tl

ARGON

I

TAR ACIDS **

OXYGEN

,

N2

? PITCH

RAW GAS

? CREOSOTE

PURIFICATION

C02*H2S

^

? TAR NAPHTHA PURE GAS

j:

jr F - T SYNTHESIS

ALCOHOLS

KETONES

'

1

t F-T WATERt „

1

f ACIDS

REFINING

f

Ï

WATER

,

+¦ ^rc^T"

GASES

________ 1

NAPHTHA

?

T

OIL

WAX

CRYOGENIC

PURIFICATION

SEPARATION CH4

REFORMING

I

ÎT

STEAM

,

SEPARATION

WORK -UP

______

'

02

?

Fig. 1.

^0

wc2 W

ETHYLENE PLANT

\i

FRACTIONATION

OLIGOMERISATION

\

IT

T

T

"

Generalized Sasol process scheme


^0ILS

2. DRY

The Fischer-Tropsch Process

21

Other coal gasifiers, eg Shell -Koppers and Texaco, have been commercially tested elsewhere. These operate at higher temperatures than the Lurgi reactors and consequently produce a gas with a lower

ChL content and a lower H2/C0 ratio. They use less steam but more oxygen.

A

discussion of the

pros

and

cons of the various

coal

gasifiers is beyond the scope of this review. Unless there is a viable market for methane (eg as a fuel gas) the methane (both that from the coal gasifiers as well as that formed in the downstream FT process) is reformed using nickel

catalyst at high temperatures to produce more CO and H2 which is


then recycled to the FT reactors (see Figure 1). The ChL reforming

reaction is typically:

1.0 CH4 + 0.24 H20 + 0.62 02~>0.52 CO + 0.48 C02 + 2.24 H2

(6)

Fischer-Tropsch Synthesis Reactors

In its present commercial operations Sasol uses two types of reactors. In the fixed bed "low" temperature Arge reactors the gas enters at the top (see Figure 2). The catalyst is packed into the narrow tubes. The FT reaction heat is absorbed by the water surrounding the tubes and steam is generated. The desired reactor temperature is maintained by controlling the steam pressure above the water jacket. The catalyst formulation and the reactor process conditions are set for the maximum production of high quality paraffinic waxes. Only the Sasol One plant utilizes these reactors. In the "high" temperature Synthol reactor (see Figure 3), the gas enters at the bottom where it picks up the powdered catalyst

(flowing down the stand-pipe) and sweeps it up the right-hand side

of the reactor where the FT reaction occurs. Two banks of heat

exchangers (see Figure 3) remove some of the reaction heat. As for all well fluidized catalyst beds the whole reactor is near isothermal. At the top of the reactor the gas and catalyst disengage (in the wide settling hopper) and the gas leaves the reactor via cyclones while the catalyst flows back down the stand-pipe. The Synthol reactor is used for the production of low molecular weight olefins and light oils. At the big new plants (Sasol Two and Three) only the Synthol reactors

are

used.

Per

unit

cross-section

of

the

reactors

the

Synthol reactor has a much higher gas throughput than the Arge reactor.

There are two other kinds of reactors which should be suitable

for the FT reaction. In the slurry phase reactor finely divided

catalyst is suspended in a heavy oil and the synthesis gas bubbles

through the bed. The fixed fluidized bed (FFB) reactor is in principle the same as the slurry bed except that no oil medium is used. Like the Synthol reactor it operates at a "high" temperature. Sasol is currently investigating the feasibility of both these two alternative types of reactors. More detail of the commercial and developmental reactors is

published elsewhere {])_ (2^.


22

INDUSTRIAL CHEMICALS VIA Ci PROCESSES


J C

fl [

* "^""^GAS INLET

-]

)

y

Chemicals Produced in a Commercial Fischer-Tropsch Process

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