Tackling fouling in vinyl monomer manufacture
Conventional and new approaches to the control of fouling in vinyl monomer processes are discussed in this article, which describes the fouling characteristics of three vinyl monomer processes: acrylic acid, vinyl acetate and vinyl chloride
Deborah A Foster, Allen R Syrinek, J Paul Street & Frederic Hisbergues
Ondeo Nalco Energy Services
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Fouling is a significant problem in vinyl monomer plants. It is common practice to add antioxidants or antipolymerants to curb fouling, but deposition continues to be a critical issue. Deposits can form in almost every vessel in a facility, including reactors, preheaters, distillation columns, reboilers, condensers, vacuum systems, heavy ends equipment, recovery boilers and even waste tanks.
Fouling is an expensive problem. It leads to poor heat transfer in reboilers and condensers, which in turn causes distillation towers to run less efficiently, reducing separations. In many cases, fouling progresses at a rapid rate and necessitates shutting down the plant to clean affected equipment. Production revenues are lost during plant shutdowns, while equipment cleaning and repair costs mount.
Fouling also introduces concerns for employee safety and environmental contamination. Vinyl monomer processes involve extremely hazardous substances, such as carcinogens and corrosives, flammables and even explosives. When facilities shut down, vessels are opened to the atmosphere and residues from the process must be removed. Employees must enter the vessels to perform necessary cleaning and repairs. The maintenance work necessitated by process fouling may put employees and the environment at risk.
In order to make improvements, it is important to understand the causes of fouling in vinyl monomer facilities, particularly with some of the fouling mechanisms in vinyl monomer processes that include acrylic acid, vinyl acetate and vinyl chloride. More accurate information on how deposits form in these processes has led to innovative chemical treatment options. Advanced anti-foulants are available that are more effective in controlling deposition than is possible with conventional antioxidants or antipolymerants.
A vinyl monomer contains a reactive carbon-carbon double bond, called a vinyl group. In acrylic acid, the vinyl group is attached to a carboxyl group. In vinyl acetate, it is attached to an acetate group. In the case of vinyl chloride, the vinyl moiety is attached to a chlorine atom (Figure 1).
The term monomer refers to molecules that can combine to form polymers. Polymerisation of vinyl monomers occurs across the double bonds. The ease with which vinyl monomers react to form polymers and copolymers makes them valuable in industry. They are building blocks in the manufacture of a variety of commercial polymers, plastics, and fibres.
The very property that makes vinyl monomers so valuable commercially is also the one that creates fouling problems during their manufacture. It is difficult to purify them without the monomers reacting. Processes and special equipment are designed to minimise temperatures and other reaction initiators. Chemical additives are also widely used to limit fouling.
The three vinyl monomer processes in the following discussion are examples demonstrating the broad range of fouling problems to be found in the reactor and purification sections, and the additives available to control such fouling. Acrylic acid, vinyl acetate and vinyl chloride were chosen because of their substantially different synthetic processes and unique fouling mechanisms, although they are all vinyl monomers.
It is also important to note that these processes are described in general terms in the sections that follow. Every vinyl monomer plant design is confidential and customised. There are a number of process licensors, many of the plants combine portions of several different licenses, and most processes have been altered over time to meet the needs of the specific manufacturer.
The primary route to acrylic acid (AA) is through the catalytic oxidation of propylene with steam and air. AA is produced as an aqueous solution from which it is extracted with an organic solvent in the absorbing column. In the purification section, crude AA is isolated by first removing water and solvent in the solvent recovery area. Acetic acid is then removed in the light-ends column. Purified acrylic acid is distilled overhead in the final product column (Figure 2).
Undesired reaction of acrylic acid monomer takes place throughout the purification section. Fouling severity increases as the temperature increases and the monomer concentrates in the latter part of the distillation train. Fouling occurs in liquid-wetted portions of the distillation section. Deposits build in pre-heat exchangers, on tower trays and in reboilers and associated equipment. The foulant can take the form of a dry powder or fluffy popcorn or gelatinous tar. Colours of the foulant range from white to pink to red-orange to purple to black.
Fouling also takes place in vapour spaces in the purification section. Foulant grows in stagnant regions underneath tower trays, in the overhead, in vent lines, and in condensers.
The effect of all this deposition is to impede heat transfer and clog process equipment, reducing distillation efficiency, and ultimately reducing production of AA.
Different parts of the acrylic acid process produce different kinds of foulant. Purification section foulant is a mixture of poly(acrylic acid) and polyester. The mechanism for its formation is radical co-polymerisation of acrylic acid with oligomeric acrylate esters (formed by addition reaction).
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