Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
2000-06-08
2002-07-09
Cain, Edward J. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C521S040000
Reexamination Certificate
active
06417251
ABSTRACT:
This invention relates to ultrafiltration processes for the recovery of polymeric latices from whitewater. In particular, the invention relates to an ultrafiltration process which utilizes a flat membrane run in the turbulent regime, or a vibrating membrane.
Polymer latices, also termed polymer emulsions, are widely used in industrial applications, including binders for paints, printing inks, non-woven fabrics and the like. These latices may be prepared in continuous or batch processes by polymerizing monomers, usually ethylenically unsaturated compounds, in the presence of water, surfactants and other adjuvants that affect the manufacturing process or the properties of the latices.
Economics may dictate that the same kettles, piping and other equipment be used to produce different latices, therefore the equipment must be cleaned between batches. Even where a single latex is produced on a continuous basis, the equipment must still be cleaned periodically.
Cleaning usually comprises washing the equipment with water. This creates large volumes of dilute aqueous latex known as “whitewater”. Whitewater thus created may have a solids concentration of about 5% by weight or less, although it may be higher. This solids concentration represents emulsion-sized particles of the original polymer latex. In addition to these particles of the original polymer latex, whitewater may also contain alcohols or other organic liquids, surfactants and the like. As produced, the solids concentration of the whitewater emulsion is far below the typical 40% or greater found in the original polymer latex, but it represents enough suspended organic matter to cause a serious waste disposal problem.
Typical whitewater may contain emulsion-sized particles of polymers such as styrenics, acrylics such as polymers of esters of acrylic or methacrylic acid, acrylonitrile, vinyl polymers such as poly(vinyl chloride) and vinyl acetate, and complex copolymers of two or more such materials, with crosslinkers, graftlinkers and the like, such as butadiene, divinylbenzene, ethylene glycol dimethacrylate, allyl methacrylate and the like.
In typical manufacturing operations, whitewater generated by various batches of different polymer types throughout a plant are combined, the entire mixture is treated as a single waste stream, and then the mixture is disposed of, generally by incineration. This represents a financial loss for the manufacturer, both by a decrease in yield with some latex product going to waste, and then having to dispose of the whitewater waste stream generally by incineration.
In order to address the cost issues, U.S. Pat. No. 5,171,767 described an ultrafiltration process and apparatus for polymeric latices, whereby the polymer obtained from the whitewater could be recycled into good product. The patent was directed towards the use of hollow fiber membranes in polymeric latex whitewater ultrafiltration operations. The patent further taught that the polymer latex will be unstable under turbulent flow, and therefore laminar flow (a Reynolds number of 3000 or lower) is required in the process.
Ultrafiltration systems may utilize many different configurations, including hollow fiber membranes, tubes, sheets, spiral, or flat membranes. Ultrafiltration membranes have “semi-permeable” walls. As used herein, by semi-permeable is meant that low molecular weight materials such as water, surfactant, and salt pass through, but high molecular weight materials such as polymer do not pass through.
Ultrafiltration flat membranes are an assembly of sheets of membrane material stacked and bound to form a “cassette”. Whitewater enters an inlet manifold on the cassette and passes across the parallel membrane surfaces. As the material passes across the membrane surfaces, non-polymer containing material permeates the membrane and passes behind the membrane surfaces, then exits the cassette through a separate “clean” water manifold.
The ultrafiltration process generates a concentrated whitewater stream which contains polymer, and a non-polymer containing stream, known as the permeate. Generally, the concentration of polymer in the concentrated whitewater stream does not exceed 40% by weight. The permeate may contain surfactants, salts, and small organic compounds.
There are several problems associated with the use of hollow fiber membranes in polymeric latex whitewater ultrafiltration operations:
1) the hollow fibers have been prone to breakage;
2) the hollow fibers are more expensive than a flat membrane;
3) the hollow fibers build a non-permeable layer on the membrane which is known as “fouling”; and
4) the hollow fibers do not have as long of a life as a flat membrane.
Flat membranes are known to be utilized in polymeric latex whitewater ultrafiltration operations. It is well documented that flat membranes should be run in the laminar flow regime to avoid problems with the stability of the polymer latex. However, the flat membranes tend to foul under these operating conditions.
Therefore, there is a need for a process for the recovery of polymeric latices from whitewater, which provides longer membrane life. It would also be useful if the process could provide a higher concentration of polymer recovered from the whitewater.
The present inventor has now discovered that with the ultrafiltration systems described herein, it is possible to provide a process for the recovery of polymeric latices from whitewater, which provides longer membrane life through reduced fouling of the membrane. The inventor has also provided a process which can provide a concentration of polymer recovered from the whitewater as high as 60% by weight.
In one aspect of the present invention, there is provided a process for recovering a polymer latex product from a whitewater emulsion which includes a) contacting the whitewater emulsion with a stationary ultrafiltration flat membrane; b) removing water and other low molecular weight material from the whitewater emulsion; and c) recirculating the whitewater emulsion across the ultrafiltration flat membrane repeatedly to generate a concentrated whitewater emulsion having a polymer concentration greater than the initial polymer concentration of the whitewater emulsion; wherein the whitewater emulsion flows through the ultrafiltration flat membrane in turbulent flow.
In a second aspect of the present invention, there is provided a process for recovering a polymer product from a whitewater emulsion which includes contacting the whitewater emulsion with a vibrating ultrafiltration membrane to remove water and other low molecular weight materials from the whitewater emulsion to generate a concentrated whitewater emulsion having a polymer concentration greater than the initial polymer concentration of the whitewater emulsion.
The process of the invention may be useful for whitewater containing any polymer. Suitable polymers include, but are not limited to styrenics; acrylics such as polymers of esters of acrylic or methacrylic acid; acrylonitrile; vinyl polymers such as poly(vinyl chloride) and vinyl acetate; and complex copolymers of two or more such materials, with crosslinkers, graftlinkers and the like, such as butadiene, divinylbenzene, ethylene glycol dimethacrylate, allyl methacrylate and the like. Acrylic latex polymers are preferred. The concentration of polymer in the whitewater is not critical, but typically is 10 percent by weight or less, more typically 5 percent by weight or less.
In one embodiment of the invention, the whitewater emulsion is contacted with an ultrafiltration flat membrane. The structure of ultrafiltration flat membranes is described above. The channel height is the height between flat membranes. For the flat membranes useful in this invention, the channel height may be from 25 to 75 mil, preferably from 30 to 50 mil. The transmembrane pressure is the pressure across the membrane wall. Transmembrane pressures typically range from 70 to 1400 kiloPascals (kPa), more typically from 70 to 300 kPa for the stationary membranes and 300 to 1400 kPa for the vibrating membranes. The operati
Cain Edward J.
Lee Katarzyna W.
Rohm and Haas Company
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