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IP.com Disclosure Number: IPCOM000250064D
Publication Date: 2017-May-25
Document File: 2 page(s) / 32K

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Improved Distribution of a Polymerization Inhibitor in an Ester-Water Phase Separation Unit Operation Containing an Aqueous Layer of High Salt Content via Direct Injection

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Improved Distribution of a Polymerization Inhibitor in an Ester-Water Phase Separation Unit Operation Containing an Aqueous Layer of High Salt Content via Direct Injection

Nitroxides, such as 4-hydroxy-2,2,6,6-tetramethylpiperdine-1-oxyl, have been well studied for the inhibition of free-radical polymerization in (meth)acrylate monomer production[1],[2].   For example, 4-hydroxy TEMPO (4-HT) (1) and derivatives of TEMPO (2) are commonly used as in-process inhibitors for the esterification[3] of acrylic acid in the production of (meth)acrylate esters such as methyl (MA), ethyl (EA),  butyl (BA), and 2-ethyl hexyl acrylate (2-EHA), to mitigate free-radical polymerization. 

(Meth)acrylate manufacturing processes often have biphasic streams that require inhibition of both phases to mitigate polymer formation.  One advantage of 4-hydroxy TEMPO (1) over other common inhibitors such as phenothiazine (PTZ) is its solubility in both organic and aqueous phases.  In addition, it is effective under both aerobic and anaerobic conditions. 

Many (meth)acrylate processes incorporate a neutralization stage to recover un-reacted (meth)acrylic acid and therefore improve the overall process yield.  Process polymerization inhibitors are routinely added to the organic phase.  Those inhibitors can then partition into the corresponding aqueous phase to provide polymerization inhibition in both phases.

For example, analysis of a biphasic system composed primarily of alkyl acrylate and basic water[*] showed the partitioning behavior summarized in Table 1.

Table 1. Partitioning behavior of 4-HT in a biphasic system

Process Stream


Separator Organic layer

100 ppm

  Separator Aqueous layer

13 ppm

As these data show, 4-HT was added to a biphasic system of BA and basic water, the 4-HT inhibitor was found to partition preferentially to the organic phase, driven in part by the salt content in the aqueous phase.  In Table 1, the salt content of the aqueous phase was in the range of 20 – 30 % by weight. 

To improve the performance of the inhibitor in the aqueous phase, it is sometimes desirable to increase its concentration above the levels shown in Table 1.  We have discovered that the concentration of inhibitor in an aqueous layer with high salt content can be significantly increased by injecting inhibitor directly into this layer instead of solely relying on the partitioning behavior of the inhibitor when added to a biphasic ester-water system.

In a large-scale trial, 4-HT (1) was injected directly into the aqueous phase of a phase separation unit operation in addition to the 4-HT being added to the organic phase feeding the separator.  Samples were collected to determine the concentration of inhibitor in the organic and aqueous phases.  The analyses found that 4-HT (1) remained relatively constant in the organic layer while the concentration in the aqueous layer remained higher than the basel...