GB2149390A

GB2149390A - Anaerobic fermentation process

Info

Publication number: GB2149390A
Application number: GB8325287A
Authority: GB
Inventors: Kevin Joseph Mckeown
Original assignee: BIOMASS INTERNATIONAL
Current assignee: BIOMASS INTERNATIONAL
Priority date: 1983-09-21
Filing date: 1983-09-21
Publication date: 1985-06-12
Also published as: EP0155960A1; GB8325287D0; WO1985001281A1; AU3431384A

Abstract

A process for the anaerobic fermentation of a liquid substrate comprises feeding the substrate to a fermenter comprising a closed vessel (1) having a generally horizontal liquid-pervious layer of packing material (2) extending across the body of the vessel so that the average height of the lower surface of the layer above the floor of the vessel is from 100 to 3000 mm, means (8) for introducing liquid substrate to the vessel below the layer of packingmaterial, means (7) for removing gas from the upper part of the vessel above the layer of packing material, means (10, 11) for removing treated liquid from the vessel at or above the top of the layer of packing material and means (5) for removing sludge from the bottom of the vessel; and fermenting the liquid substrate in the fermenter whilst removing gas and treated liquid from the fermenter.

Description

SPECIFICATION Anaerobic fermentation process This invention is concerned with improvements in and relating to the anaerobic fermentation of liquid, generally aqueous, substrates.

The anaerobic fermentation of liquid substrates, such as aqueous liquid wastes or effluents, is a well-established process. Typically, such a process comprises feeding the liquid substrate, continuously or semi-continuously, to a closed fermentation vessel in which the substrate is subjected to anaerobic fermentation, often at moderately elevated temperatures, whereby a gaseous product, commonly methane, is produced. During the course of the fermentation, solid material, unfermented material and/or fermentation residue, settles to the bottom of the vessel as a sludge and excess fermented liquid substrate is removed from the fermentation vessel. The fermentation may be carried out with the liquid in the vessel in a more or less quiescent condition or undisturbed conditions.

One problem encountered in such a process is that particles of solid matter, possibly bearing the micoorangisms responsible for the fermentation, may be upwardly entrained in the body of liquid in the vessel, especially by the flotation action of gas evolved during the course of fermentation.

It has now been found, in accordance with the present invention, that particles of solid material may be de-entrained from the liquid by means of a liquid-pervious layer of packing material; that is a layer formed of randomly arranged (dumped) or regularly arranged pieces of fluid-fluid contact elements such as are used in fluid-fluid contact apparatus in the chemical engineering industry, for example in distillation columns, absorbtion columns, and humidifiers. Such fluid-fluid contact elements may, for example, take the form of regularly arranged grids or regularly arranged arrays of generally vertical shaped sheets.Preferably, however, the fluid-fluid contact elements are discrete elements such as Raschig rings, Lessing rings, Pall rings, Tellerette rings, saddles (for example Berl or Intalox saddles) or Cascade minirings (i.e. rings having an overall length less than their diameter). When the fluid-fluid contact elements are discrete elements of the above described sort it is preferred that the layer of packing material take the form of a randomly arranged (dumped) layer of the elements supported on a perforate support.

It has been further found that the depth of the body of liquid below the packing material is suitably from 100 to 3000 mm; that is by employing depths of liquid below about 3000 mm the rate of fermentation per unit volume area of the fermentor is reduced whilst by employing depths above about 3000 mm, no useful increase in the rate of fermentation per unit volume is obtained, i.e. increasing the total volume of liquid in the fermentor below the layer of packing material over a certain amount does not lead to a concomitant increase in overall fermentation rate per unit volume.

According to the invention, therefore, there is provided a process for the anaerobic fermentation of a liquid substrate which comprises feeding the substrate to a fermentor comprising a closed vessel having a generally horizontal liquid-pervious layer of packing material extending across the body of the vessel so that the average height of the lower surface of the layer above the floor of the vessel is from 100 to 30000 mm, preferably from 200 to 15000 mm, means for introducing liquid substrate to the vessel below the layer of packing material, means for removing gas from the upper part of the vessel above the layer of packing material, means for removing treated liquid from the vessel at or above the top of the layer of packing material and means for removing sludge from the bottom of the vessel; fermenting the liquid substrate in the fermentor whilst removing gas and treated liquid from the fermentor.

In order that the invention may be well understood reference will now be made to the accompanying diagrammatic drawing which is a vertical cross-section through a fermentor for use in accordance with the invention.

As shown in the drawing, a fermentor for use in accordance with the invention comprises a closed vessel 1, for example of generally circular cross-section, having extending thereacross a generally horizontal liquid-pervious layer of packing material 2 supported on perforate support 3 above the floor, 4, of the vessel. The floor 4 of vessel 1 is dished or conical in shape and is provided at its lower point with a valved discharge conduit 5 for removal of sludge. The top 6 of vessel 1 is of inverted conical shape and is provided, at its upper point, with a valved vent 7 for the discharge of gas from the fermenter. Mounted within vessel 1 is perforated tube 8 for the introduction of liquid into vessel 1 below the bottom surface of the layer of packing material 2 and connected to valved liquid feed conduit 9.In this connection it may be noted that is generally preferred, in accordance with the invention, that the means for feeding liquid into the vessel, i.e. the perforate pipe 8, be provided with perforations or nozzles such that the feed liquid is introduced horizontally or downwardly into the body of liquid contained within the vessel.

Finally, a channel 10 is located in the periphery of vessel 1, above the top of the layer of packing material 2, and is connected to valve liquid discharge pipe 11, the inner lip of the channel serving as a weir.

The packed layer 2 is formed of dumped packing elements and suitable has a thickness or depth of from 50 to 1500 mm, preferably from 75 to 1000 mm In operation, the substrate to be treated is fed, continuously or semicontinuously into vessel 1 via distributor tube 8 and is fermented in vessel 1 by flowing upwardly through vessel 1, particles of solid material entrained in the upward flowing body of material being dis-entrained by the action of packed layer 2. During the course of the fermentation, gas evolved is taken off, generally as a continuous operation, through vent 7 and, at the same time, treated liquid flowing over weir 10 is removed (continuously or semi-continuously depending upon the mode of introduction of feed liquid to the vessel) via offtake pipe 12. Sludge accumulating in the bottom of the vessel is removed via pipe 5, generally on a semi-continuously or intermittent basis.

Fermentation operations in accordance with the invention may be carried out at moderately elevated temperature, e.g. 10 to 70 C, and to facilitate this the fermentor may be provided with internal or external heating means and further may be provided with an outer layer of thermal insulation.

The fermentation process of the invention may be used for the anaerobic fermentation of a wide variety of substrates for example effluents from citric acids production efflucents from paper and pulp production, effluents from molases fermentation and effluents from the edible oil dairy, pharmaceutical, brewing and food industries.

Claims

1. A process for the anaerobic fermentation of a liquid substrate which comprises feeding the substrate to a fermenter comprising a closed vessel having a generally horizontal liquid-pervious layer of packing material extending across the body of the vessel so that the average height of the lower surface of the layer above the floor of the vessel is from 100 to 3000 mm, means for introducing liquid substrate to the vessel below the layer of packing material, means for removing gas from the upper part of the vessel above the layer of the packing material, means for removing treated liquid from the vessel and/or the top of the layer of packing material and means for removing sludge from the bottom of the vessel and fermenting the liquid substrate in the fermenter whilst removing gas and treated liquid from the fermenter.

2. A process as claimed in claim 1 in which the packing layer comprises a layer of dumped discrete fluid-fluid contact elements.

3. A process as claimed in claim 1 or claim 2 in which the average height of the lower surface of the layer of packing material above the floor of the vessel is from 1500 mm.

4. A process as claimed in claim 1 substantially as hereinbefore described with reference to the accompanying drawing.