Industrial Chemicals via C1 Processes - American Chemical Society

Chapter 1

Synthesis Gas Feedstock for Chemicals W. Keim

Downloaded by 79.110.17.68 on November 21, 2016 | http://pubs.acs.org Publication Date: December 16, 1987 | doi: 10.1021/bk-1987-0328.ch001

Institut für Technische Chemie und Petrolchemie der Rheinisch-Westfälischen

Technischen Hochschule Aachen, Worringer Weg 1, D-5100 Aachen, Federal Republic of Germany This paper deals with the application of synthesis gas as feedstock for chemicals. Distinction is made between a direct and

indirect hydrogenation path of CO. The direct route leading to paraffins, olefins, oxygenates and N-containing derivatives is briefly discussed. Economics

will favour reactions which retain at

least one of the oxygen atoms of the original CO reactant and potential appli-

cations exist for alcohols. The indirect

path considers methanol and methyl formate as intermediates. Starting from methanol, base and fine chemicals can be

synthesized by carbonylation, reductive carbonylation and oxidative carbonylation. Special emphasis is given to the homologation of methanol to acetaldehyde. Finally various chemical reactions are discussed

describing methyl formate as a potential building block. The efforts to develop processes for fuels and chemicals from resources other than crude oil have seen a prodigious growth since 1973. The use of coal or natural gas derived synthesis gas has especially been considered to be a most promising route for the future production of fuel and chemicals. Numerous patents and papers have appeared and the reader is referred to the following

books ( _l-6. ) .

With the recent global decline in crude oil prices, the attractiveness of coal derived syngas as an alternate feedstock has ebbed bringing many efforts and considerations in this field back to the research stage. 0097-6156/87/0328-0001$06.00/0

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

2

INDUSTRIAL CHEMICALS VIA C , PROCESSES

Many arguments in favour of continuation of research and developments in syngas chemistry can be made on the following grounds: a) The long range availability of plentiful and low cost coal and natural gas will neces-


sitate their utilization as raw materials. Since the

development of a process from the bench scale to a commercial operation takes about 10-20 years, the need for doing research and development work today is obvious, b) The availability of alternative feedstocks and processes will ease monopoly positions of crude oil suppliers and will stabilize raw material prices, c) The shortage of foreign currency or strategic considerations will create regions where it is now sensible to produce fuels or chemicals starting from raw materials other than crude oil. d) Special situations can be envisioned where syngas derived chemicals can compete favourably as in the production of acetic acid, esters, and anhydrides .

Synthesis gas offers many routes to industrial chemicals. They can be classified in a direct and an indirect path as shown in Figure 1. The direct conversion deals with the straight hydrogénation of carbon monoxide to paraffins, olefins and heteroatom (oxygen, nitrogen) containing products. The indirect conversion invokes intermediates such as metha-

nol, methyl formate and formaldehyde. The latter ones in a consecutive reaction can yield a variety of desired chemicals. For instance, acetic acid can be synthesized

directly from CO/H2 , but for reasons of selectivity the

carbonylation of methanol is by far the best commercial

process .

For the synthesis of chemicals from CO/H2 three considerations will influence the economics and process

feasibility, namely the ratio of C0:H2, the loss of oxygen as by-product water or CO2 , and the interrelation

of chemicals/fuels. The two first points are exemplified in Table I.

Depending on the nature of the raw material (coal, natural gas, naphtha), both the reaction conditions of gasification, and the employment of reforming (shift conversion) will determine the composition of the syngas. For instance, synthesis gas produced from coal nor-

mally has a H2:C0 ratio of unity and therefore the ratio

has to be adjusted to 2.0 prior to the use of the gas for methanol synthesis. Accordingly, syngas derived from

CH4 bears a cost advantage for the needed ratio of 2.0.

To remove oxygen atoms, water or CO2 is formed as by-product, which in Table I is expressed in reactant loss as water. The synthesis of n-octane, representative for gasoline in the Fischer Tropsch synthesis, yields 57 % H2O indicating a poor raw material utilization. A similar picture is given for the ethene synthesis (Mobil route) from methanol. Therefore, both the ratio of C0:H2 and the formation of water affect the economics of CO/H2

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

1. KEIM

Feedstock for Chemicals

Table I.

3

CO/H2 usage ratio

reactant

CO:H2

as H20

loss (%)


Direct conversion

CO + 2 H2 2 CO + 2 H2 2 CO + 2 H2

? methanol 1:2 *~ acetic acid 1:1 ? methyl formate 1:1

2 CO + 4 H

?

ethanol

3 CO + 6 H2

? propanol

2 CO + 3 H

?

4 CO + 8 H2 2 CO + 4 H2 8 CO + 34 H2

^ isobutanol ? ethylene ? n-octane

1:2

28

1:2

38

ethylene glycol 2:3

1:2 1:2 1:2.1

50 56 57

Indirect conversion

CH-OH + CO

^

CH-COOCH^ + CO

? acetic anhydride -

2 CH3OH

*~ ethylene

Natural

acetic acid

-

56

Gas . Coal , Biomass

CO/ H2

_______ f Direct

I

Conversion

Indirect

Conversion

via

Paraffins and olefins ( Fischer - Tropsch ) Oxygenates

Methanol

chemistry

Methylformate

chemistry

Formaldehyde

chemistry

( alcohols , esters , acids )

N- Containing derivatives

Figure 1.

Direct and indirect conversion of synthesis

gas

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

4

INDUSTRIAL CHEMICALS VIA C, PROCESSES

technology significantly. The chemical industry has always been tied very closely to the usage and development of energy whether it was coal in the past or crude oil today. Also in the future the fate of the chemical

industry will be connected to the development of the energy resources. If methanol, ethanol or Fischer Tropsch products should ever enter into the energy sector, those products will be available as feedstocks for the chemi-

cal industry in practically any amount at rather low cost .


Within the frame work of this article special attention will be given to: 1. Direct conversion of synthesis gas

2. Indirect conversion of synthesis gas. Direct conversion of synthesis gas The direct conversion of synthesis gas can yield paraffins, olefins, oxygenates and nitrogen containing compounds. Best known here is the Fischer Tropsch synthe-

sis yielding mixtures of mainly linear alkanes and/or

alkenes. There have been many attempts to tailor the

product slate to C2/C3 olefins only. However, all data indicate, that the Schulz-Flory selectivity dominates.

Mechanistically, the Fischer-Tropsch synthesis can be described as a reductive oligomer izat ion of carbon monoxide following a geometric progression (SchulzFlory distribution) in which the chain growth probabili-

ty, a, can possess various values (1_) . Prior to entering a costly research and development program, simple

calculation can help to evaluate whether the mathematically determined product distribution can be balanced economically on the market. Tailoring the product distribution (selectivity) remains a challenging goal. Some research groups attack this problem by catalyst modifications (

Industrial Chemicals via C1 Processes - American Chemical Society

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