GB2155482A - Process for the production of protein-containing material - Google Patents

Abstract

A process for the production of protein-containing material comprises cultivating micro-organisms under aerobic conditions in a culture medium based on methanol produced by synthesis, from a synthesis gas which contains an excess of hydrogen, carbon dioxide from exhaust gas formed during growth of the micro-organisms being returned into the synthesis gas or into the synthesis gas production stage, and the cultivated biomass being separated to yield single-cell protein.

Description

SPECIFICATION A process for the production of protein-containing material The present invention relates to the production of protein-containing material by the cultivation of micro-organisms under aerobic conditions in a culture medium based on methanol, produced by synthesis from a synthesis gas which contains an excess of hydrogen, the cultivated biomass being separated to yield as single-cell protein (sometimes hereinafter referred to as SCP).

There are a number of processes for the production of single-cell protein. The chief differences between these processes resides in the use of various sources of carbon, various microorganisms and various medium-gassing systems. The sources of carbon are, for example, sulphite waste liquors, sugar-containing solutions particularly molasses, hydrocarbonaceous substances particularly paraffins, and alcohols particularly ethanol and methanol. The microorganisms used are primarily bacteria, yeasts, fungi and algae. The culture medium can be gassed with air, oxygen or with oxygen-enriched air.

It is already known to obtain single-cell protein directly from natural gas by means of bacteria.

This has the disadvantage that the source of carbon is not well utilized by the bacteria. Hence, this process has not been generally accepted, since the main cost factor in the industrial production of single-cell protein is usually the consumption of the source of carbon. In this respect, the starting material is of great importance.

Processes based on alcohols, particularly methanol, have been of special interest in recent times, since alcohols are industrial products and not agricultural products and hence are available at all times of the year. Also they are satisfactorily miscible with water and are available both in large quantities and in a pure form.

Hence it is known to cultivate bacteria in a culture medium, based on methanol, which is gassed with air.

The present invention provides a process of the kind described above but which is distinguished by particularly good utilization of the source of carbon. Such an advantage is achieved by returning carbon dioxide from exhaust gas formed during growth of the microorganisms into the synthesis gas used in production of the methanol or into the synthesis gas production stage.

According to the present invention there is provided a process for the production of proteincontaining material which comprises cultivating micro-organisms under aerobic conditions in a culture medium based on methanol produced by synthesis, from a synthesis gas which contains an excess of hydrogen, carbon dioxide from exhaust gas formed during growth of the microorganisms being returned into the synthesis gas or into the synthesis gas production stage, and the cultivated biomass being separated to yield single-cell protein.

The source of carbon is hydrocarbons, such as natural gas, which are converted, preferably by steam reformation, to a synthesis gas suitable for methanol synthesis and which chiefly comprises carbon monoxide and hydrogen. This synthesis gas, without the feedback of carbon dioxide, has an excess of hydrogen which, together with the inert consistuents (nitrogen, methane) is discharged from the methanol system by way of purging gas. By virtue of the feeback of carbon dioxide in accordance with the present invention, this excess of hydrogen, together with the carbon monoxide, can be additionally converted to methanol in the synthesis.

Optimum methanol synthesis is only possible when the following condition is fulfilled:

the amounts being expressed im moles.

It follows from this formula that portions of the carbon dioxide from the growth of the microorganisms may be usefully fed into the stream of synthesis gas between the production of the synthesis gas and the methanol synthesis, instead of, or in addition to, the adding of carbon dioxide to the raw gas for the production of synthesis gas. In order to grow the microorganisms, methanol produced in this way is fed to a vessel in which the micro-organisms are cultured, in the presence of oxygen, in an aqueous culture medium containing nutritive salts.

Exhaust gases containing carbon dioxide are produced during growth of the micro-organisms. In accordance with the present invention, the carbon dioxide of this exhaust gas is fed back to, for example, a steam reformation stage. Therein, the carbon dioxide reduces the formation of carbon dioxide from the hydrocarbons and water, and so provides methanol formation.

The consumption of hydrocarbons can be reduced by feeding back the carbon dioxide into the steam reformation stage. Hence, in the process in accordance with the invention, the yield resulting from the hydrocarbon used for the production of methanol is, on the whole, increased.

The energy balance of the overall system is improved at the same time, so that less heating medium is required. By way of example, the quantity of energy saved can be used to advantage to dry the separated biomass. In addition, the prime costs for a plant for the synthesis of methanol from hydrocarbons with the feedback of carbon dioxide are less than those for a corresponding plant without the feedback of carbon dioxide.

In accordance with one feature of the present invention, it is particularly advantageous to use yeasts as the micro-organisms, that is to say, the carbon dioxide of the exhaust gas formed during the growth of yeasts is fed back into the synthesis gas or into the synthesis gas production stage.

Yeasts have fundamental advantages compared with, for example, bacteria. Plants for the growth of yeasts can be operated unsterile. Owing to the low risk of contamination, the operational readiness of the plants is substantially increased, the demands on the skill and training of the operating personnel are reduced, and the economic operation of the plants is improved owing to the lower costs for preparing the raw materials and nutrients. Furthermore, there is less risk of damage to health than in the case of bacteria, since yeasts have long been used for human nutrition. When used in animal fodder, yeast protein is frequently superior to bacteria protein, as is reported in the literature (for example G. Cardini, SCP-products from methanol-grown yeasts, Dechema-Monographie, Volume 83, 1979, pages 219 to 225).Finally, yeasts are substantially simpler to separate from the culture medium than bacteria.

The chief disadvantage of yeasts resides in the fact that they oxidise a higher proportion of the carbon source to carbon dioxide than bacteria. Their generation of carbon dioxide is greater than that of bacteria, and their utilization of the carbon sources to produce protein is smaller than that of bacteria. Therefore their consumption is greater and their use more expensive. Bacteria have the relatively higher content of proteins and a relatively low yield of carbon dioxide. This means that bacteria exhibit the best utilization of carbon sources. Since, as already mentioned, the source of carbon constitutes the chief proportion of the costs of the end product, bacteria have been used as the micro-organisms in processes used hitherto, owing to their better utilization of the source of carbon.The disadvantages of bacteria, particularly the necessity for sterile operating conditions, were consciously accepted.

By virtue of the process in accordance with the present invention, it is now possible to improve the utilization of the source of carbon when using yeasts, such that the difference, existing in the previous processes, between bacteria and yeasts in the utilization of the source of carbon is virtually balanced out. At the same time, the advantages offered by the yeasts compared with bacteria are retained.

Example According to details given in the literature, the utilization of the carbon source by bacteria is 0.45 to 0.55 kg of single-cell protein per kg of methanol. This value is 0.4 to 0.45 in the case of yeast, that is to say, it is approximately 10% lower.

Table I shows that the consumption of hydrocarbons, (natural gas in the present Example), is reduced by approximately 8% by feeding back carbon dioxide into the steam reformation stage.

The thermal balance is at the same time improved, so that excess steam is produced and can be used in the drying of the yeast. A further 4% of natural gas is thereby saved for heating purposes. Hence, in this manner, the difference between bacteria and yeasts in the utilization of the carbon source is approximately balanced out.

Table I Production of 100000 X 103 kilograms within one year of dry yeast Methanol requirement: approximately 30 t/h with CO2 without CO2 feedback feedback Natural gas requirement (process and heating 996.9 kmol/h 1088.2 kmol/h CO2 feedback 218.4 kmol/h Excess steam 16.3X103kg/h kg/h Natural gas requirement for the production of the differential quantity of steam - 44.5 kmol/h Total natural gas requirement 996.9 kmol/h 11 32.7 kmol/h Difference 135.8 kmol/h Percentage (relative to 1132.7 kmol/h 12.0% The process in accordance with the invention can also be used, in principle, in the growth of bacteria or fungi.

Basically, it is possible to use carbon dioxide from the exhaust gas formed during the growth of micro-organisms in a culture medium gassed with air. However, for this purpose, the carbon dioxide would first have to be separated from the exhaust gas. When using air for gassing, the exhaust gas predominantly comprises nitrogen with small quantities of other gases, for example, carbon dioxide and oxygen.However, the separation of the highly diluted carbon dioxide from the other constituents of the exhaust gas (see gas balance in accordance with Table II) requires a more expensive process which chiefly comprises the following steps: compression of the exhaust gas, separation of carbon dioxide (carbon dioxide scrubbing, regeneration of the scrubbing agent containing carbon dioxide, and the production of a fraction, rich in carbon dioxide, under low pressure, or PSA pressure-swing-absorption), renewed compression of the carbon dioxide for feedback into the production of synthesis gas and/or methanol. These steps are so expensive that the carbon dioxide obtained is too expensive for it to be fed back in an economical manner.

Hence, in a preferred embodiment of the invention, the exhaust gas used is formed during the growth of micro-organisms in a culture medium gassed with oxygen-enriched air. In this case, the proportion of carbon dioxide in the exhaust gas is greater than that in an exhaust gas produced during air-gassing, so that the separation of carbon dioxide is correspondingly simpler and more economic.

In accordance with a further variant of the process in accordance with the present invention, it is particularly advantageous to gas the culture medium with oxygen when the exhaust gas produced largely comprises carbon dioxide with admixtures of oxygen, water, nitrogen and traces of methanol.

In this case, it is a particularly simple matter to purify the exhaust gas. If the carbon dioxide is purified, the residual gas fraction chiefly comprises oxygen with small quantities of nitrogen.

Advantageously, in accordance with a preferred embodiment, this gas may be used directly for gassing the medium.

Table II.

Typical exhaust gas composition.

with 02 gassing with air gassing mol % kmol/h* mol % kmol/h* C 2 59.9 2.59 3.5 2.59 02 30.6 1.32 13.0 9.62 N2 0.8 0.03 75.8 56.09 CH30H 200ppm 0.000 9 200 ppm 0.015 H20 8.7 0.38 7.7 5.70 * relative to the production of 100 kg/h SCP, dry Temperature: 40iC.

Since the use of pure oxygen also increases the yield of oxygen in the exhaust gas, the purity of the carbon dioxide in the exhaust gas is still further improved compared with gassing with air.

In accordance with a further embodiment of the process in accordance with the invention, the exhaust gas, formed during the growth of micro-organisms in a culture medium gassed with oxygen, can be purified. The components contained in the exhaust gas are generally not detrimental to the production of synthesis gas. However, traces of impurities, such as chlorine, present in the exhaust gas can be removed by, for example, chemosorption. In accordance with a further embodiment of the present invention, the exhaust gas may, if desired, be further purified in, for example, a PSA plant. In this manner, a virtually 100% pure, wet carbon dioxide can be obtained in a relatively simple manner for feedback to the production of synthesis gas and/or the production of methanol. In a further advantageous embodiment of the present invention, the exhaust gas can be purified by a gas scrubber.As already mentioned, the residual gas fraction can be used directly for gassing the medium.

In accordance with a particularly preferred embodiment of the present invention, the carbon dioxide is preheated and is admixed with the hydrocarbons fed to the synthesis gas production stage.

In a further embodiment of the present invention, the carbon dioxide is mixed with the raw material upstream of the synthesis gas production stage and/or with the synthesis gas downstream of the synthesis gas production stage. These measures enable particularly flexible adaptation of existing plants for the production of methanol.

In accordance with a further preferred embodiment of the present invention, steam-reformation of hydrocarbons is preferably used in the synthesis gas production stage. Advantageously, methane or natural gas may be used as the hydrocarbon raw material for the production of the synthesis gas.

One embodiment of the present invention will be described hereinafter with reference to the accompanying drawing which is a flow diagram.

Natural gas and steam are fed to a steam reformer 1 by way of a line 4. The methanol synthesis gas (carbon monoxide and hydrogen) formed in the steam reformer 1 is subsequently synthesized to form methanol in vessel 2. The methanol is fed to a vessel 3 which contains an aqueous culture medium. Oxygen for the gassing of culture medium and nutrient salts, are fed to the vessel 3 by way of lines (not illustrated). Yeasts are grown in the vessel 3. The exhaust gas formed during the growth of the yeasts is drawn out of the vessel 3. Since the culture medium is gassed with pure oxygen, the exhaust gas flowing through line 5 largely comprises carbon dioxide with small admixtures of oxygen, water, nitrogen, and traces of methanol.

Hence, it is unnecessary to separate the carbon dioxide from the exhaust gas. On the contrary, the exhaust gas rich in carbon dioxide only has to be compressed by means of a compressor 8 to which the exhaust gas is fed by way of a line 7. The exhaust gas is then fed into into the steam reformer 1, usually together with the charge.

The processes taking place in the three stages may be represented by the following typical reaction equations: Steam-reformer: CH4 + H20 + (CO2) = CO + 3 H2 + (CO2).

The carbon dioxide fed back serves to reduce the formation of carbon dioxide from natural gas and water.

Methanol synthesis: CO + 2 H2 = CH3 OH SCP-Production: x CH3OH + y O2 + z NH3 + salts = a SCP + b CO2 + c H2O Typical values of a, b, x, y, z (typical composition of SCP in accordance with E. Klug, Annual Report on Fermentation Processes, Academic Press, New York, Volume 3, pages 1 41 to 145) are: Table 111 Bacteria Yeasts x 6.4 kmol 7.5 kmol y 5.4 kmol 7.0 kmol z 1.0 kmol 0.6 kmol a 1 kmol 1 kmol b 2.4 kmol 3.5 kmol If yeasts are the micro-organisms used, the quantity of carbon dioxide contained in the exhaust gas is similar to the quantity which can be usefully fed back to the steam reformer.

Surplus carbon dioxide can be drawn off from the plant by way of line 6. However, if need be, surplus carbon dioxide formed may also be fed to the steam reformer. If more methanol is synthesized in this variant of the process than can be utilized in the vessel 3, methanol can be taken from the plant by way of line 9.

Thus the yield resulting from natural gas used for the production of methanol is increased by using the exhaust gas from a culture which is gassed with oxygen or oxygen-enriched air. The process in accordance with the present invention particularly enables the use of the advantages of the growth of yeasts, and at the same time avoids their disadvantage, namely the poorer utilization of the carbon source.

Claims (14)

1. A process for the production of protein-containing material which comprises cultivating micro-organisms under aerobic conditions in a culture medium based on methanol produced by synthesis, from a synthesis gas which contains an excess of hydrogen, carbon dioxide from exhaust gas formed during growth of the micro-organisms being returned into the synthesis gas or into the synthesis gas production stage, and the cultivated biomass being separated to yield single-cell protein.

2. A process as claimed in claim 1, in which the micro-organisms are yeasts.

3. A process as claimed in claim 1 or 2, in which the exhaust gas used is formed during the growth of micro-organisms in a culture medium gassed with oxygen or oxygen-enriched air.

4. A process as claimed in any of claims 1 to 3, in which the exhaust gas, formed during the growth of micro-organisms in a culture medium gassed with oxygen or oxygen-enriched air, is returned directly into the synthesis gas production stage and/or into the synthesis gas.

5. A process as claimed in any of claims 1 to 4, in which the exhaust gas is purified before being returned into the synthesis gas and/or into the synthesis gas production stage.

6. A process as claimed in claim 5, in which the exhaust gas is purified by chemosorption.

7. A process as claimed in claim 5 or 6, in which the exhaust gas is subjected to a PSA (pressure-swing-adsorption) treatment.

8. A process as claimed in any of claims 5 to 7, in which the exhaust gas is purified by a gas scrubber.

9. A process as claimed in any of claims 5 to 8, in which the residual gas from the purification of carbon dioxide is used for the gassing of the culture medium.

10. A process as claimed in any of the claims 1 to 9, in which the carbon dioxide is preheated and is admixed with the hydrocarbons fed to the synthesis gas production stage.

11. A process as claimed in any of the claims 1 to 10, in which carbon dioxide is mixed with the raw material upstream of the synthesis gas production stage and/or with the synthesis gas downstream of the synthesis gas production stage.

12. A process as claimed in any of the claims 1 to 11, in which steam reformation of hydrocarbons is used in the synthesis gas production stage.

1 3. A process as claimed in any of the claims 1 to 12, in which methane or natural gas is used as the hydrocarbon raw material for the production of synthesis gas.

14. A process for the production of protein-containing material substantially as hereinbefore described with particular reference to the foregoing Example, or as illustrated in the accompanying drawing.

1 5. Protein-containing material whenever produced by a process as herein described and claimed.