The following invention relates to an anionic polymer having a skewed molecular weight distribution useful as a scale inhibitor and anti-precipitant. The anionic polymer may be made by a process which varies the amount of chain transfer agent during continuous polymerization, or the anionic polymer may be made by a physical blending.
The interest in scale inhibitors has been a continuous one. Many liquid flow applications require scale free operation, or, as a minimum, reduced scale buildup, to perform in a maintenance free and consequently economical manner. This is because these applications usually require heat transfer systems. For example, it is of particular importance to inhibit or minimize scale on the surfaces of boilers, heat exchangers, turbines, steam generators, pumps and steam and other fluid condensers. As a general statement, it can be said that any machine or other piece of equipment using water as at least one component and/or having a metal thermal transfer surface performs more efficiently when scale is kept to a minimum.
When certain alkaline earth metals are present in the liquid, there seems to be a great propensity for scale buildup. The alkaline earth metals preponderantly present and, therefore, of particular concern are calcium and magnesium.
There is, therefore, an almost continuous search in the art for compositions which will reduce or arrest the scale buildup of alkaline earth metal salts onto heat transfer and other surfaces.
As a general statement, there is an on-going need for scale inhibitors in industrial operations. Industrial processing with such equipment as air conditioning, refrigeration, heat exchange and evaporation, requires that the alkaline earth metal compounds be inhibited from depositing out of the water.
When certain industrial operations are being performed, for example, cooling operations, the alkaline earth metal compounds may be present in mineral matter such as slime or roiled sediment. These compounds would tend to cement themselves together as a strongly adherent scale which would interfere with the water flow.
In the prior art, anionic polymers have been used as scale inhibitors. See, for example, U.S. Pat. Nos. 3,663,448 and 3,463,730. These patents are incorporated herein by reference. Generally, scale inhibitors in the prior art have been used in the molecular weight range of about 500 to about 12,000.
There appears to be confusion in the art as to the definition of scale inhibitors and anti-precipitants. The following terms seem to have been used interchangeably either singly or in combination: scale inhibitor which would also include the term anti-scalant, anti-precipitant, anti-nucleation agent, and dispersing agent.
In this invention, the term scale inhibitor means a composition which inhibits the deposition of adherent scale deposits on the surfaces or parts of metal heat exchange surfaces. The term anti-precipitant means a composition which hinders the precipitation of a solid or the formation of turbidity in bulk solutions.
In certain applications there is a need for an anti-precipitant but not necessarily for a scale inhibitor. An example of this is in oil field brines where precipitated particles would tend to clog the porous rock. A scale inhibitor would not be necessary in this application because there are no heat transfer surfaces.
In other applications, the use of a scale inhibitor and anti-precipitant would complement each other. An example of this application is in recirculating cooling water systems where the heat transfer surfaces must be clean, and where the restricted flow areas must be clear of precipitated particles.
In yet another application, there is a need for a scale inhibitor without the need for an anti-precipitant. An example of this is in a boiler, where a scale inhibitor is necessary to reduce or eliminate the scale buildup in a boiler tube.
The confusion in the art has caused anti-precipitants to be used as scale inhibitors at a great loss of economic efficiency. However, as indicated above, anti-precipitants have been made for other applications. In fact, as indicated below, they require a different MW range.