Fixed cycle time ultrafiltration process

A method is presented for achieving constant purification cycle times over the life of an ultrafiltration membrane in a silver halide purification system. Silver halide emulsion pumped from a feed vessel enters an ultrapurification module wherein a permeate stream is separated from the emulsion. The permeate stream is then divided into a permeate-to-drain stream and a recycle stream. The recycle stream is returned to the feed vessel. By varying the amount of permeate recycled, a constant permeate-to-drain rate can be maintained, and hence the purification cycle time can be maintained constant despite fouling of the membrane.

FIELD OF THE INvENTION 
This invention relates to the purification of silver halide emulsions and 
more particularly to a method of ultrafiltration in which the purification 
cycle time is standardized by diverting a portion of the permeate back 
into the reaction vessel. 
BACKGROUND OF THE INVENTION 
Ultrafiltration is a useful way to concentrate and purify various liquid 
compositions. Among such compositions are silver halide photographic 
emulsions. Ultrafiltration is used to remove alkali metal nitrates and 
other impurities formed in precipitation of the silver halide. 
Silver halide emulsions normally are prepared in a batch process by mixing 
a silver nitrate solution with an alkali metal halide solution in a 
gelatin medium. The composition is then washed to remove soluble salts. 
One type of ultrafiltration process suitable for use with the present 
method is described in Research Disclosures Vol. 102, October 1972, Item 
10208 and Vol. 131, March 1975, Item 13122, which are incorporated herein 
by reference. The soluble impurities, e.g., alkali metal nitrates, 
permeate through the ultrafiltration membrane and the permeate is 
discarded. 
One problem, however, is that the membrane becomes progressively fouled 
during use and the rate of permeation decreases. As a consequence, a batch 
of silver halide emulsion treated with a fresh membrane and one treated 
with a used membrane will be subjected to different processing conditions, 
including reactant concentrations, residence time in the reaction zone, 
fluid level in the reaction vessel, etc. Having been subjected to 
different processing conditions, the silver halide emulsions prepared in 
different batches will not have identical physical and photographic 
characteristics. It is possible to clean the membrane and thereby 
partially restore its original flux rate. However, some debris remains 
after each cleaning and the membrane progressively degrades with repeated 
use until it is too clogged to be of use. 
As a result, successive batches of photographic emulsion will experience 
concentrates that vary widely over the course of one reaction to the next. 
The concentrate varies because the unwanted salts are removed at different 
rates because the semipermeable membrane becomes progressively clogged. 
So, salts are initially rapidly removed thereby initially quickly lowering 
the salt concentration, but over time, the rate of change of salt 
concentration is much lower. However, if a membrane is changed in the 
middle of a reaction, the rate of concentrate will increase again. Such 
changes make it difficult to time the reactions and the varying changes in 
concentrate result is inconsistent quality of photographic emulsions. 
SUMMARY OF THE INVENTION 
It has been discovered that more consistent reactions can be obtained by 
recycling some of the normally discarded permeate back into the reaction 
vessel. In accordance with the present invention, an improved apparatus 
and method are provided by means of which different batches of silver 
halide emulsion can be prepared and purified under substantially identical 
conditions. The products, therefore, are highly uniform. The invention 
separates components from a liquid emulsion by ultrafiltration in such a 
way as to provide constant purification cycle times throughout the life of 
a membrane. The invention flows a stream of the liquid composition from a 
first vessel to a second vessel containing an ultrafiltration membrane. 
One side of the membrane is maintained at a higher pressure than the 
other, with the stream flowing in contact with the higher pressure side. 
The difference in pressure between the sides induces salt-laden fluid 
(permeate) to flow from the higher pressure side to the lower pressure 
side. A stream of concentrated emulsion (concentrate) is withdrawn from 
the higher pressure side and is recycled to the first vessel. A permeate 
stream is withdrawn from the lower pressure side and is divided into one 
stream which is recycled to the first vessel and another stream which is 
withdrawn at a constant flow rate from further contact with the liquid 
composition. Hence, instead of entirely discarding the unwanted permeate, 
a portion is recycled into the liquid emulsion from which it was filtered. 
The amount recycled into the first vessel is dictated by the current 
condition of the membrane. As the membrane progressively fouls, 
progressively less permeate is recycled to the first vessel. The net 
effect is that there is a constant, fixed, and controlled flow of permeate 
withdrawn from the first vessel, regardless of the condition of the 
membrane. It is believed novel and unobvious to recycle a portion of this 
unwanted permeate to reduce variability among emulsion batches.

DETAILED DESCRIPTION 
FIG. 1 shows a preferred apparatus for carrying out one embodiment of the 
method of the present invention. Solutions of silver nitrate (AgNO.sub.3) 
and potassium bromide (KBr) are combined in the presence of gelatin and 
react to form of an aqueous emulsion 3 of silver halide crystals. The 
reaction occurs in a kettle or reservoir 2 that is stirred by agitator 4. 
In the course of the reaction, crystals of silver halide are precipitated 
and ions of potassium and nitrate remain as dissolved salt contaminants in 
the emulsion 3. The emulsion 3 is coupled via line 5 and pump 8 to 
ultrafiltration module 6. The resulting emulsion 3 is purified of the 
dissolved salts by ultrafiltration module 6. The ultrafiltration module 6, 
as shown in FIG. 8, has a high pressure chamber or side 7 and a low 
pressure chamber or side 9. The differential pressure across a 
semipermeable membrane 22 causes flow from the high pressure, concentrate 
side 7 of the module 6 to the lower pressure, filtered, solvent side 9. 
Semipermeable membrane 22 differentiates the flow of chemicals, retaining 
molecules having less than a predetermined molecular weight. This 
principle is illustrated schematically in FIG. 8 by showing materials 11 
larger than the predetermined molecular weight and materials 13 smaller 
than the predetermined molecular weight. 
A concentrate stream 30 is then withdrawn from the high pressure chamber 7 
and recycled to kettle or reservoir 2 via concentrate line 10. Permeate 31 
on the low pressure side 9 of semipermeable membrane 11 is withdrawn from 
the low pressure side 9 and discharged from ultrafiltration module 6 via 
permeate output line 32. 
As shown more fully in FIGS. 1 and 2, permeate output line 32 is in fluid 
communication with a fluid control valve 12. A longitudinally moveable rod 
34 is coupled to a valve seat 16. Such a valve may be a spool valve with 
seat 16 formed from a raised land or seat 16 on rod or spool 34. The 
function of valve 12 is to divide the permeate flow 31 between a first 
outlet 36 in fluid communication with a drain and a second outlet 38 in 
fluid communication with reservoir or kettle 2. Rod or spool 34 is 
moveable to one of an infinite number of intermediate positions to divide 
the permeate 31 between the drain and reservoir 2. In the preferred 
embodiment, rod 34 is positioned to provide a constant rate of flow of 
permeate 31 to the drain via valve outlet 36. 
The valve 12 is controlled by a microcontroller 40 that operates an 
actuator 42, such as a solenoid, for moving the spool or rod 34. 
Microcontroller 40 receives control signals from flow meter 20. Meter 20 
has a flow control sense line 45 coupled to the drain outlet 36 for 
monitoring the flow rate of permeate to drain. Microcontroller 40 is 
programmable to move spool 34 via actuator 42 in order to keep the 
permeate-to-drain flow a constant. 
At the start of a filtration process, the flow rate of permeate directed to 
drain is a minor portion of the total permeate stream. This minor portion 
initially is on the order of 20%. As the flow of liquid concentrate 
continues over the course of a purification process, the rate of formation 
of the permeate stream, i.e., the flux rate, will decrease as the membrane 
becomes fouled. This deterioration of flux rate over the course of a 
purification batch is shown graphically in FIG. 3. Because permeate is 
directed to drain at a constant rate, the permeate-to-drain rate will 
become a larger proportion of the total permeate flow rate as flux rate 
declines. 
The proportion of permeate returned to kettle 2 will depend upon the 
desired constant flow rate of permeate to drain and on the age of the 
semipermeable membrane 22. The flow rate to drain must be less than the 
minimum flux rate of the membrane 22 at any time during the life of the 
membrane. Thus, with a new permeable membrane and dilute, low viscosity 
emulsion 3, the amount of permeate returned to kettle 2 is at its highest. 
As the membrane ages and as the viscosity or concentration rises, the 
proportion of permeate recycled will decrease. The membrane 22 is 
regenerated by chemical cleaning procedures before the recycled permeate 
rate reaches zero. 
Cleaning does not restore the membrane to its original condition, however. 
Throughout its useful life, the membrane gradually becomes permanently 
fouled and cleaning will not remove all matter clogging it. As a result, 
the flux rate vs. time plot for a series of batch purifications followed 
by membrane cleaning resembles the path of a bouncing ball. See FIG. 5. 
Uniform flux rates for a given feed flow and/or pressure, therefore, are 
not possible over the life of a membrane, making uniform purification 
cycle times for a given membrane impossible without varying feed rate 
and/or filter pressure. 
The volume of liquid composition in the kettle 2 preferably is allowed to 
decrease as permeate is directed to drain. In another embodiment of the 
present method, the volume in kettle 2 is maintained at a constant level 
by adding a washing liquid, preferably water, at the same rate as the 
constant rate at which permeate is sent to drain. The purpose of the 
washing liquid is to remove soluble impurities such as alkali metal 
halides without changing the concentration of the batch. The effect of 
this washing phase on flux rate is shown in FIG. 4. The combination of 
concentrating and washing phases may be necessary to ensure that adequate 
washing is achieved within aim concentration limits. 
As the membrane becomes fouled, the total permeate rate will eventually 
decrease until it equals the rate at which permeate is diverted to the 
drain. A signal can be initiated when the valve reaches a position at 
which, e.g., 90% of the permeate is diverted to the drain. This would 
signal a need either to clean or to replace the membrane. 
EXAMPLE 1 
At the start of the process, three way valve 12 is positioned to divert all 
permeate to kettle 2. When both the designated feed flow rate and system 
pressure have been obtained, three way valve 12 is repositioned via 
feedback control from flow meter 20 in the drain line to allow the desired 
flow of permeate to drain. FIG. 4 graphically shows the change in permeate 
flux as a function of the various process cycles of concentration and 
washing. 
The process has three phases. They are: 1) an initial concentration (CONCO) 
phase whereby approximately 50 to 75% of the water contained in the 
concentrate 7 received from the reaction process is removed; 2) a washing 
phase (WASH) where the concentration of salts and other addenda are 
removed to a specified, lower concentration; accomplished by the addition 
of process water, typically, distilled (DI), demineralized (Dmin), or 
reverse osmosis (RO) at a rate equivalent to the permeation rate so that 
the level in the kettle 2 is maintained constant within a specified kettle 
volume (known as constant volume diafiltration). The degree of washing is 
controlled by one of several means--turnovers, conductivity, or specific 
ion concentration. After the concentrate 7 has reached the desired level 
of purity, the third phase is entered--a concentration step (CONCF) where 
additional water is removed to further increase the concentration of the 
macrosolute to a predetermined value that is particular for that product. 
Because three way valve 12 is automatically repositioned in response to 
permeate to drain flow changes, a decline in permeate flux due to 
increasing product concentration or viscosity will not decrease the 
quantity of permeate to drain. Typically, when the wash starts, the 
relative change in the total flux is significantly less than the flux 
change due to concentration. Upon completion of the wash cycle, additional 
concentration may be necessary, with the accompanying further decline in 
permeate flux. In the absence of membrane fouling, additional batches of 
the same product could be assumed to produce the same permeate flux rates 
at each point in the process, be it either concentration or wash. 
FIG. 5 shows the effects of membrane fouling as consecutive batches of 
emulsions are processed through the ultrafiltration unit 6 according to 
the prior art during initial concentration (CONCO), wash (WASH) and a 
final concentration (CONCF). Although each batch is followed by a system 
cleaning procedure, each succeeding batch produces slightly lower maximum 
flux rates due to the permanent fouling occurring at the membrane surface. 
The result of this fouling is an increased process time to reach each of 
the designated cycle end points and an increased overall process time. 
Turning to FIG. 6, there is shown the results of five successive three 
phase batch processes made according to the subject invention. During each 
phase of each process, the permeate rate of discharge to drain is held 
constant. By selecting a fixed permeate rate of discharge to drain for the 
entire process, the increased batch processing times are eliminated and 
individual cycle times and overall process times become fixed. Table 1 
displays simulated data that might be expected in a series of 
ultrafiltration processes followed by membrane cleaning. 
TABLE 1 
______________________________________ 
Fixed Recycle Process 
Permeate-to- 
Permeate Cycle 
Flux Rate 
Drain Rate Rate Time 
Start 
End Start End Start 
End Min. 
______________________________________ 
BATCH #1 50 30 10 10 40 20 90 
BATCH #2 45 28 10 10 35 18 90 
BATCH #3 40 25 10 10 30 15 90 
BATCH #4 37 21 10 10 27 11 90 
BATCH #5 33 16 10 10 23 6 90 
______________________________________ 
EXAMPLE 2 
In another embodiment, the premeate discharge rate is varied, preferably in 
a stepwise manner prior to the final phase of each process. Turning to 
FIG. 7, there is shown the results of such variations. The rate is varied 
by pre-programming the microcontroller 40 to step down the 
permeate-to-drain rate as the filtration process enters the final 
concentration stage. While permeate-to-drain rate varies within a given 
process in this example, the overall goal of a constant process time is 
achieved while maximizing the efficiency of the permeable membrane 22. 
Although described in terms of the purification of silver halide, the 
method of the present invention can be used in other fields utilizing 
ultrafiltration processes, including the purification and concentration of 
gelatin, and the purification of waste effluent, water, food products, and 
pharmaceuticals. 
The invention has been described with particular reference to preferred 
embodiments thereof, but it will be understood that variations and 
modifications can be effected within the spirit and scope of the 
invention. For example, any suitable means can be used to maintain a 
constant permeate discharge rate. By maintaining a constant discharge rate 
of permeate and recycling the rest of the permeate back to the reaction 
vessel 2, the concentration of undesired components is gradually and 
controllably reduced over time. The overall operation of reaction is thus 
more consistent from batch-to-batch and its end point is more predictable. 
In effect, the time for completion of a reaction can be held constant.