Chemistry: molecular biology and microbiology – Micro-organism – tissue cell culture or enzyme using process...
Reexamination Certificate
2002-02-28
2002-12-10
Redding, David A. (Department: 1744)
Chemistry: molecular biology and microbiology
Micro-organism, tissue cell culture or enzyme using process...
C435S293100, C435S295200, C435S813000
Reexamination Certificate
active
06492135
ABSTRACT:
FIELD OF THE INVENTION
In its broadest aspect the present invention relates to U-shape and/or nozzle-U-loop fermentors and methods of the operation of the same.
More specifically the invention relates to U-shape and/or nozzle-U-loop fermentors and methods for the operation of the same, which are particularly appropriate for production processes with methanotrophic bacteria and similar processes, whereby different gases and other nutrients are to be supplied to the fermentation liquid in order to obtain an optimally proceeding fermentation process with the highest possible yield of fermentation product in the shortest possible period of time.
PRIOR ART
Stirred Or Back-mix Fermentors
In conventional bioreactors (fermentors) the mixing of gases with the fermentation liquid is effected by means of stirrer blades placed centrally in the fermentor. The stirrer blades generate turbulence in the liquid, which means that gas, usually injected at the bottom of the reactor, will be dissipated in the liquid in the form of small fine gas bubbles. This type of reactor provides a relatively homogenous mixing, i.e. that about the same concentrations of gases and substrates will be found whether measuring at the top or at the bottom of the reactor. This type of reactor is, however, not particularly appropriate for up-scaling, since it is difficult to obtain the same homogeneous mixing and the same mass transportation in large reactors as can be obtained in small laboratory and pilot reactors. Besides, the vigorous mixing implies a significant heating of the fermentation liquid.
Airlift and Loop Fermentors
In order to avoid the mechanical stirring, different types of airlift reactors have been developed. The majority of these reactors are so-called loop reactors having two sections: an upstream part and a downstream part, which are interconnected at both ends. Gases are supplied at the bottom of the reactor in the upstream part in an arrangement, which yields small gas bubbles (e.g. through a vitrified ceramic plate or an array of small nozzles). The bubbles mix with the liquid whereby the total density is reduced and the gas-liquid mixture ascends displaced by new liquid emerging from the downstream part. The gas-liquid mixture moves up through the upstream part of the reactor and releases its gas bubbles at the top, whereupon the liquid descends down through the downstream part. In order to obtain a long residence time for the gas bubbles in the liquid, airlift reactors are conventionally tall slender reactors. This implies that the gas must be supplied at a high pressure for overcoming the hydrostatic pressure at the bottom of the reactor. If the gas is air, this implies the use of compressors. Furthermore, airlift reactors have a relatively poor exploitation of the injected gas. Typically only 20-40% of the gas is utilized. Besides, it is difficult to obtain good and quick release of the gas bubbles from the fermentation liquid at the top of the reactor and separation of the gas phase thus produced (which may be rather foaming) from the liquid phase before the fermentation liquid moves down in the downstream part of the reactor.
U-shape Reactor
The U-shape reactor is constructed with a view to provide:
Non-compressed or nearly non-compressed gas injection
Long residence time and thus high degree of exploitation of the injected gas
Low energy consumption for liquid circulation
Simple design
Good separation of gases and liquid at the top.
In principle the U-shape reactor is also a loop reactor. However, contradictory to conventional loop reactors the liquid circulation is effected by means of one or more in-line pumps. This (or these) pump(s) may be of the propeller pump type, wherein the propeller blades are designed for pumping a mixture of liquid and gas. The gases can be introduced at different locations in the U-shape loop, but typically they will be supplied at the upper end of the downstream part of the loop. By introducing the gases at the upper end of the downstream part of the loop a nearly non-compressed injection is obtained, since the gases only have to overcome a hydrostatic pressure of some few meters. The gases can be introduced by means of particular gas dispensers providing for a distribution across the downstream part of the loop. Fine dispersion of the gases in the liquid is effected by means of static mixing elements placed immediate below the gas injectors (the mixing elements may be of e.g. Sulzer manufacture). The liquid flow in the downstream part of the loop must be sufficiently high so that all the injected gas is carried along down through the static mixers. Here a comminution of the gas is effected so that a large number of small gas bubbles is obtained, which are dispersed uniformly in the liquid. The bubbles are carried along with the liquid flow down through the downstream part of the loop to its lower end and further on through a U-bend to the upstream part of the loop so that the gas bubbles are redispersed (e.g. by means of static mixing elements) several times in the liquid.
The U-bend causes a centrifugal effect and thus some separation of gas bubbles and liquid.
Therefore, the in-line pump is preferably placed adjacent the U-bend, partially because it then assists in producing a redispersing of the gas in the liquid and partially because it is practical to have it placed at the bottom of the fermentor.
In order to obtain a good bubble distribution in the upstream part of the loop more static mixers may be provided therein.
The top of the fermentor is designed so that the upstream part of the loop via a bend is passed horizontally onto the side of a widening of the upper end of the downstream part of the loop. This particular construction feature assists in yielding a good separation of liquid and gas bubbles, as centrifugal forces act in the bend and in the very widening of the upper end of the downstream part of the loop a vigorous circulation of the liquid with corresponding accompanying centrifugal forces arise, which also bring about separation of liquid and gas bubbles. Thereby, one of the great problems associated with airlift reactors—viz. separation of the gas and liquid phases—is solved in an utmost elegant fashion.
Furthermore, the U-shape reactor provides for a long contact time between the gas and liquid phases, as the injected gas is present both in the downstream and in the upstream parts of the loop. This means that an essentially higher utilization of the gas is obtained compared with conventional airlift reactors.
Gas bubbles in liquids have a tendency to fuse together to larger volumes (coalesce). This tendency contributes to making conventional airlift reactors ineffective inasmuch as the bubbles become larger and larger upward through the upstream part, partly due to coalescence and partly due to a reduced hydrostatic pressure. In the U-shape reactor here described, this tendency in the upstream part is counteracted by providing static mixers appropriately spaced apart at distances, which depend on the medium applied. In the downstream part, the increasing hydrostatic pressure counteracts the tendency to increased bubble sizes. To the extent that this effect cannot balance the fusion (coalescence) of the gas bubbles there is provided for a redispersing by means of static mixers.
The amount of gas, which advantageously can be dispersed in the liquid, depends on the hydrostatic pressure. In the case of tall reactors it will therefore be advantageous to have several locations for the introduction of gases in the downstream part. The only requirement to the gas inlets is that at least one static mixing element is placed immediately after each inlet for dispersing the gas in the liquid.
In order to give an impression of the dimensions, which such a U-shape reactor may have it may be mentioned that its total height can be about 40 metres and its width can be about 6.6 metres, the said width is to be understood as the perpendicular distance between the portions of the vertical walls of the downstream and upstream parts being spaced furthest
Coleman Sudol Sapone P.C.
Redding David A.
Sapone William J.
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