Liquid purification or separation – Processes – Liquid/liquid solvent or colloidal extraction or diffusing...
Reexamination Certificate
1999-12-09
2001-10-02
Fortuna, Ana (Department: 1723)
Liquid purification or separation
Processes
Liquid/liquid solvent or colloidal extraction or diffusing...
C210S650000, C210S195200, C430S030000, C430S567000, C430S569000
Reexamination Certificate
active
06296770
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to the reproducible separation and purification of photographic emulsions from mixtures containing photographic emulsion combined with impurities such as low molecular weight gelatins, salts, and other addenda added in the previous batch process step. More particularly, the present invention relates to methods utilizing spiral wound ultrafiltration (UF) for achieving the aforementioned while reducing yield loss and additive variability and in addition reducing equipment stress in ultrafiltration processes is addressed.
BACKGROUND OF THE INVENTION
In U.S. Pat. No. 5,256,294, titled “TANGENTIAL FLOW FILTRATION PROCESS AND APPARATUS,” by Robert D. van Reis, et al., Oct. 26, 1993, and U.S. Pat. No. 5,490,937, titled “TANGENTIAL FLOW FILTRATION PROCESS AND APPARATUS,” by Robert D. van Reis, et al., Feb. 13, 1996, a system for concentrating and washing proteins is described that employs cassette-type (similar to plate-and-frame) ultrafiltration membranes of various decreasing sizes for separating protein mixtures while maintaining a flux ranging from 5% to 100% of transition flux. Transmembrane pressure is maintained substantially consistent along the length of the membrane at a level no greater than the transition flux. A backpressure control valve is used on the permeate flow such that a constant pump speed is maintained while the permeate flow can dictate a constant concentration of material to be retained at the wall of the membrane. In this way, a vast reduction of the concentration polarization layer allows for more consistent purification of the proteins. The disadvantages in these methods are that they are not proven for spiral wound confirmation, an additional control valve is required for the permeate and the pump speed and pressure on the modules are not reduced.
In U.S. Pat. No. 5,164,092, titled “2-STAGE ULTRAFILTRATION PROCESS FOR PHOTOGRAPHIC EMULSIONS,” by William D. Munch, Nov. 17, 1992, a two-stage system for concentrating and washing photographic emulsions to high viscosities is described that employs spiral wound ultrafiltration as the first stage. Plate-and-frame (of which cassettes are a variety) ultrafiltration is the second stage. Although the goal of further concentrating emulsions to appropriate curtain-coating levels achieved by the combination of the ultrafiltration module designs, first batch yield losses and yield variability was not addressed. These are important due to the cost of lost product in lost yield which, in this case, occurs only in the first batch after a chemical cleaning. Furthermore, since it only occurs on a first batch after a chemical cleaning, the yield will be lower for that batch than for the one to seven additional batches that follow before the next chemical cleaning. Since up to 50 additional chemicals can be added in the downstream batch process, the variability in the yield can lead to manually having to cut the amounts of those 50 chemicals depending on the yield. If the yield were dependably high, computer generated amounts of chemicals could be added in the downstream batch process at a constant amount for each batch. Without a dependably high yield it is necessary to manually adjust depending on the yield variability. These issues were not addressed since the traditional process control strategy was used. This process employs the much-used process control strategy of a positive displacement pump operating at constant, relatively high speed. The pump speed control is nested with a feed flow backpressure control valve. Together, the pump speed, which is approximately 300 rpm, and feed backpressure control in the process result in a relatively high feed inlet pressure setpoint. In this scheme, transmembrane pressure for the filtration is maintained along the membrane substantially higher than the transmembrane pressure at the transition point of the filtration.
The resulting concentration polarization layer build up prevents concentration to levels required in certain applications. Thus, the additional concentration apparatus, i.e. the plate-and-frame UF module is required to achieve the target concentrations with the addition of considerable process time due to the severely reduced surface area of the plate-and-frame arrangement.
SUMMARY OF THE INVENTION
A novel process control process for UF washing and concentration (of photographic emulsions) results in increased first batch yield and reduced yield variability, and finally reduced stress on the following process equipment and components: feed pumps, UF membranes, and back pressure control valves. Reduced stress on process components is typified by 20% lower pump speed for C
wall
(constant concentration at the UF membrane wall) batches; 4% lower pump speed overall; 19% reduction in use of feed backpressure control valves; 40% reduced tube pressure on C
wall
batches, and 8% reduced tube pressure overall.
The process control strategy does not require additional equipment (plate-and-frame UF) and uses the Blatt Stagnant Film Theory. The controlling algorithm uses a predetermined, gel-based mass transfer coefficient of the feed, the gelatin concentration of the feed, and a C
wall
setpoint wherein;
PFR=k
LN (
C
wall
/C
bulk
)
Where
PFR=Permeate flow rate in L/m
2
hr
k=mass transfer coefficient in L/m
2
hr
C
wall
=Concentration of gel at wall of membrane in wgt/vol % gel
C
bulk
=Concentration of gel in bulk solution in wgt/vol %=V
0
C
0
/V
V
0
=volume at time=0 in L
V=volume at time=t in L
C
0
=concentration of gel at time=0 in wgt/vol %
The C
wall
setpoints are derived from the Blatt Stagnant Film Theory. Through experimentation, it was determined that the mass transfer coefficient (k) is controlled largely by the gelatin in the silver halide emulsions and not by the silver halide grains.
FIG. 1
demonstrates how the permeate flowrate of a traditionally processed batch of emulsion is regressed against the wgt/vol % gelatin during the initial concentration step. The negative of the slope of a line is the mass transfer coefficient (k). The initial C
wall
setpoint can be determined using the following equation:
C
wall
setpoint=
e
(y intercept/k)
In this application, the backpressure control valve is used, when necessary, on the feed flow, while an algorithm based on the Blatt Stagnant Film Theory is used to control the concentration polarization layer retained at the wall of the membrane (spiral wound configuration in this case). This is achieved by adjusting the pump speed to attain the permeate flow rate required. The operating range of inlet and outlet pressures for each tube of membranes is reduced while at the same time the pressure differential is increased. The resulting increased feed flow aids in reducing the concentration polarization layer retained at the wall of the membrane.
The C
wall
(Constant Concentration at the UF Membrane Wall) UF process control operates usually without a back pressure control valve, and at lower pump speeds (such as 258 rpm to 324 rpm with the average being 275 rpm) that vary according to the required permeate flow. The membrane module inlet and outlet pressures, both variable, are lower for most of the process. Thus, in addition to better yield and reduced yield variability, there is reduced stress on the process equipment due to the lower pump speeds and operating pressures.
REFERENCES:
patent: 5164092 (1992-11-01), Munch
patent: 5242597 (1993-09-01), McArdle
patent: 5248418 (1993-09-01), Munch
patent: 5256294 (1993-10-01), van Reis
patent: 5270159 (1993-12-01), Ichikawa et al.
patent: 5490937 (1996-02-01), van Reis
patent: 5693229 (1997-12-01), Hartmann
Gately William R.
Hall Raymond F.
Wilcox Margaret B.
Bocchetti Mark G.
Eastman Kodak Company
Fortuna Ana
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