Abstrict
The invention is a process for the separation of biotechnologically
produced valuable materials from a fermenter broth by crossflow
microfiltration and/or ultrafiltration, in which at least two modules
arranged in series and fitted with porous membranes are used per
stage, to produce concentrate and a permeate which contains the
valuable material. The pressure of the permeate is controlled so
that the absolute pressure on the permeate side is at different
absolute pressure in each module.
Claims
We claim:
1. A process for the separation of valuable biologically produced
compositions from a fermenter broth which comprises: providing universally
applicable maximal recovery of said compositions under high flux
by passing the fermenter broth through at least two modules which
can be microfiltration or ultrafiltration separation modules arranged
in series wherein the fermenter broth, at a first pressure, contacts
a separation membrane in the first module to produce a concentrate
and a permeate in the first module, the permeate is produced at
a pressure lower than the first pressure, and the fermenter broth
is passed through each succeeding module in the series at a pressure
lower than the pressure in the preceeding module to produce a permeate
at a pressure lower than the pressure in the module and controlling
the pressure of the permeate in each module so that the permeate
absolute pressure in each module is different.
2. A process of claim 1, wherein permeate pressure in each module
is reduced from the input module to the output module in such a
way that a mean transmembranal pressure difference between the concentrate
(6) and the permeate (7) is substantially equal in all the modules
(1).
3. A process of claim 1 wherein the pressure of the permeate is
controlled below ambient pressure in the last module.
4. A process of claim 1 wherein a microfiltration is carried out
at flow velocity higher than about 4 m/s.
5. A process of claim 4 wherein the flow velocity is from about
5 to about 7 m/s.
6. A process of claim 1 wherein the membrane material comprises
at least one material selected from aluminium oxide, silicon carbide
or zirconium dioxide on a support.
7. A process of claim 1 carried out under sterile conditions with
recycling of the concentrate to dilute the fermenter broth.
8. A process of claim 1 comprising a microfiltration followed by
an ultrafiltration wherein the permeate from the ultrafiltration
is used in the microfiltration to dilute the fermenter broth and
also as a diallizing liquid.
9. A process of claim 1, wherein in microfiltration, a salt solution
is used to dilute the fermenter broth and also as a diallization
liquid.
Description BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention is a process for the separation of biotechnologically
produced valuable materials from a fermenter broth by crossflow
microfiltration and/or ultrafiltration, in which at least two modules
arranged in series and fitted with porous membranes are used per
stage.
2. Statement of Related Art
Processes such as these for the recovery of valuable materials
from fermenter solutions are known. In crossflow membrane microfiltration,
the dissolved valuable material is separated from the fermenter
broth as permeate. The valuable material is then concentrated in
another unit, for example by precipitation or by ultrafiltration
or reverse osmosis. Both microfiltration with pore diameters of
0.02 to 10 .mu.m and ultrafiltration with pore diameters of 0.001
to 0.02 .mu.m are carried out in modules arranged in series. A sufficiently
high pressure has to be applied to the fermenter broth entering
the first module to ensure that, due to the pressure losses occurring
in the modules, a sufficiently large transmembranal pressure difference
for separation is provided in the last module.
The disadvantage of the known processes lies in the problem of
concentration polarization which always arises in membrane processes.
In microfiltration, due to the flux of materials onto the membrane
surface, the valuable materials arrive at the pores of the membrane,
along with materials which are unable to pass through the pores
because of their size. A surface layer of complex composition is
thus formed on the membrane surface and obstructs the flow of material,
particularly the valuable material, through the membrane during
the concentration phase. Retention (of the valuable materials) is
thus increased and permeate flux reduced. To maintain the separation
efficiency of the process, the concentration phase has to be terminated
at a relatively low degree of concentration and the membrane surfaces
have to be cleaned.
Applicants' patent No. DE 35 15 650 describes a process which largely
obviates this disadvantage. However, this process is based on the
use of certain membrane materials and precisely defined hydrodynamic
conditions. Accordingly, this process is only suitable for certain
biotechnology products.
Accordingly, an object of the present invention is to provide a
microfiltration process which provides for minimal retention of
the valuable material despite high permeate flux and which can be
universally applied.
BRIEF DESCRIPTION OF THE INVENTION
According to the invention, a process is provided whereby minimal
retention of the valuable material occurs at high permeate flux
wherein a different pressure in relation to the ambient pressure
is maintained on the permeate in each module.
In the process of the invention, a valuable material (biologically
produced) is separated from a fermenter broth by being passed over
a microfiltration or ultrafiltration member. In microfiltration,
the valuable material is recovered in a permeate. In ultrafiltration
the valuable material is recovered in the concentrate. Whenever
the microfiltration or ultrafiltration member are arrange in at
least two modules which modules communicate in series to receive
the fermenter broth, the pressure of the permeate is controlled
so that the permeate pressure in each module is different in relation
to ambient pressure.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows the principle of crossflow membrane filtration of
the separation of valuable materials from culture or fermenter solutions.
FIG. 2 is a simplified process diagram of a crossflow membrane
filtration plant for carrying out a microfiltration process according
to the invention.
DETAILED DESCRIPTION OF THE INVENTION
It has been found that the retention of the valuable material and
the permeate flux are dependent on the mean transmembranal pressure
difference. As used herein mean transmembranal pressure difference
is the arithmetic mean of input and output pressure difference across
a membrane in a module. The average pressure of a module is the
arithmetic mean between the inlet and outlet pressure. As the mean
transmembranal pressure difference increases, permeate flux increases
rapidly at first and then reaches a limit. By contrast, in microfiltration
retention of the valuable material increases with increasing pressure
difference. According to the invention, the pressure on the permeate
in relation to the ambient pressure is controlled to obtain an optimal
transmembranal pressure difference in all successive modules to
which different pressures are applied on the concentrate side. The
optimum pressure difference can thus be established in each module.
Retention reaches its minimal value while permeate flux is high
enough to enable the process to be carried out economically.
In an embodiment of the invention, the pressure on the permeate
side is controlled so that the pressure on the permeate side of
the module is reduced from the input module to the output module
in such a way that a substantially equal mean transmembranal pressure
difference between concentrate and permeate is applied in all the
modules.
In view of the pressure losses in the modules, the entry pressure
into the first module on the concentrate side is considerably higher
than the exit pressure from the last module in a series. In conventional
membrane separation processes, therefore, the transmembranal pressure
difference is reduced from module to module. Accordingly, the transmembranal
pressure difference is so high, at least in the first module, that
the retention of the valuable material is high. By contrast, according
to the present invention, a constant or optimal transmembranal pressure
difference is established in all modules, guaranteeing constant,
low retention.
According to the invention, the pressure can be controlled below
ambient pressure on the permeate side of one or more modules at
the end of a series of modules. Depending on the particular application,
it is preferred to apply a reduced pressure on the permeate side
of the last stage in order to obtain a uniform or optimum transmembranal
pressure difference in all the modules.
In another embodiment of the invention, microfiltration is carried
out at flow velocities higher than 4 (meters/second) and, more particularly,
in the range from 5 to 7 m/s. It has been found that these flow
velocities are particularly favorable.
In another embodiment, inorganic materials, such as aluminium oxide,
silicon carbide or zirconium dioxide on a support, are used as the
filtration member materials. These membranes have the advantage
that they can be sterilized by treatment with steam, no membrane
compaction occurs and the membranes are not subjected to abrasion.
However, corresponding organic membranes may also be used in accordance
with the invention.
Another preferred embodiment of the invention in which microfiltration
is used in conjunction with ultrafiltration, the permeate from the
ultrafiltration is mixed with the fermenter broth feed to dilute
the fermenter broth and as a diallization liquid. A salt solution
may also be used.
The invention is described in more detail in the following with
reference to the accompanying diagrammatic drawings, wherein:
A tubular separation module 1 as used in the process is illustrated
in principle in FIG. 1 for the separation of biotechnology produced
valuable materials from a cell suspension (fermenter broth, optionally
after cell disintegration) by crossflow membrane filtration. The
separation module 1 consists of a support tube 2 in which is arranged
a carrier tube 3 to the inner surface of which a separation active
membrane layer 4 is applied. In the embodiment illustrated by way
of example, the tubular separation module 1 has a length of approximately
1.2 m. If the carrier tubes are stable, there is no need for the
support tube 2.
The culture or fermenter mixture 5 is passed over the separation-active
membrane layer 4 in the arrowed direction at flow velocities of
approximately 5-7 m/s, the valuable materials passing radially through
the membrane layer 4 while cells, cell fragments and solids are
retained by the membrane layer 4.
A liquid concentrate 6 containing the cells, cell fragments and
solids and a liquid permeate 7 containing the valuable material
are formed. The driving force for the separation of concentrate
6 and permeate 7 is a transmembranal pressure difference. This transmembranal
pressure difference is established through a higher pressure on
the concentrate side in relation to the permeate side. In order
to be able to apply a uniform or optimal transmembranal pressure
difference in all of the modules 1, a predetermined pressure is
established on the permeate side, by passing the permeate through
flow restrictors 8 to discharge the permeate 7.
The tubular separation modules 1 are the separation means of the
simplified flow diagram of a crossflow microfiltration plant shown
in FIG. 2. The fermenter solution 5 is introduced through a pipe
9 into a temperature-controlled holding vessel 10 or is taken directly
from the fermenter from which it enters the tubular separation modules
1 through pipes 11, 12 by means of a feed pump 13 in the pipe 11
and a circulation pump 14 in the pipe 12. In this case, three series
arranged separation modules 1 form one stage. In each of the three
tubular separation modules 1, the culture or fermenter solution
1 is separated into a concentrate 6 and a permeate 7, the concentrate
6 passing through pipes into the next module 1 or being for the
most part recycled from the last module through pipes 6 and 15 by
the circulation pump 14 while the permeate 7 flows from each separation
module 1 into an outflow pipe 16. In the case of batch operation,
a relatively small part of the concentrate 6 passes back into the
vessel 10 or fermenter through a branch pipe 17 and the pipe 9.
In continuous operation, the concentrate 6 is taken directly from
the pipe 17. FIG. 2 shows a single-stage construction of the separation
plant. In practice, up to ten separation stages are normally used.
In one typical embodiment of the invention, three modules 1 each
1.2 m in length are arranged in series and operated in such a way
that the exit pressure of the first module on the concentrate side
corresponds to the entry pressure of the second module, etc. The
pressure drop per module is 1.8 bar, i.e. in all 5.4 bar. The fermenter
composition 5 enters the first module 1 at an entry pressure of
6.1 bar in relation to the ambient pressure. The exit pressure in
the last module is thus 0.7 bar gauge. To obtain a uniform mean
transmembranal pressure difference in each stage, a decreasing pressure,
in relation to the ambient pressure, is applied from stage to stage
on the permeate side 7. The pressure on the permeate side measures
3.6 bar in the first module, 1.8 bar in the second module and 0
bar in the third module. A constant mean transmembranal pressure
difference of 1.6 bar is thus obtained in every stage. The permeate
7 pressure is reduced by the flow restrictions 8 before entering
the outflow pipe 16. The process principle may be applied to any
number of successive modules from two modules upwards.
In an other embodiment, the permeate 7 flows through the outflow
pipe 16 into a separation stage, for example a precipitation stage
or an ultrafiltration unit, for concentration of the valuable materials.
If necessary, the ultrafiltration unit may also be operated by the
same counter-pressure technique as applied in crossflow microfiltration,
i.e. with application of a stagedependent pressure on the permeate
side.
The permeate from the ultrafiltration may also be used to dilute
the fermenter broth and as diallization liquid. Good results have
also been obtained through preliminary dilution and diallization
with salt solutions, particularly of salts which stabilize the proteins.
The process is also suitable for the recovery of valuable materials
when they are not secreted into the fermentation medium from the
microorganisms producing them, but instead can only be recovered
after destruction of the cells by mechanical or chemical (enzymatic)
means.
The invention is not limited to the embodiments illustrated by
way of example in the accompanying drawing. The modules may be differently
constructed, for example as plate modules or as multichannel units,
and may be differently subdivided in length, and the like.
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