Fouling, reactors

Maintaining high annual reactor production requires good reactor continuity with smooth product transitions and low amounts of off-grade material. One aspect of this objective is reducing and/or eliminating reactor fouling. 

The prevention of fouling of the reactor within the slurry loop is an important part of the operating procedure. Some process parameters that may change to indicate the possibility of internal fouling are  

The kill system is automatically used if the computer system, which monitors the key polymerization parameters outlined above, detects serious reactor continuity problems, thus eliminating direct operator action to initiate the kill system. Removing the manual reactor kill decision to the computer model monitoring the polymerization process, eliminates the difficult decision required by the reactor operator to shut down a commercial reactor. However, one downside of the automatic injection of the reactor kill system by the computer system is that it may lead to an occasional unnecessary shut down. However, the benefits of such an automatic kill system have resulted in the almost complete elimination of extreme fouling, which requires the reactor to be opened up and cleaned out by the plant maintenance personnel. 

Reaction (A) takes place immediately after the introduction of the carbon monoxide to the polymerization reactor, thus stopping the insertion of ethylene into the growing polymer chain and therefore stopping the heat of polymerization. Reaction (B) will take place very slowly, which results in the incorporation of a carbonyl group into the growing polymer. This slow reaction is responsible for the reversibility of the kill system. Once the added carbon monoxide has been incorporated into the polymer, then the active center will again react with ethylene and the polymerization process will restart. 

---

Emulsion Process. The emulsion polymerization process utilizes water as a continuous phase with the reactants suspended as microscopic particles. This low viscosity system allows facile mixing and heat transfer for control purposes. An emulsifier is generally employed to stabilize the water insoluble monomers and other reactants, and to prevent reactor fouling. With SAN the system is composed of water, monomers, chain-transfer agents for molecular weight control, emulsifiers, and initiators. Both batch and semibatch processes are employed. Copolymerization is normally carried out at 60 to 100°C to conversions of - 97%. Lower temperature polymerization can be achieved with redox-initiator systems (51). 

By-products from competing Diels-Alder reactions Reactor fouling - heat-transfer and product-loss issues Distillation required - energy intensive Poor reactor volume utilization Hazardous catalyst... 

Jones and co-workers976 have recently reported the use of catalyst 49 (Figure 5.15) with perfluorinated alkanesulfonic acid sites anchored to SBA-15 as a methylaluminoxane-free supported cocatalyst for ethylene polymerization. When catalyst 49 and trimethylaluminum were used in combination with Cp 2ZrMe2 as the metallocene precatalyst, productivities as high as 1000 kg polyethylene molZr-1 h 1 were obtained without experiencing reactor fouling. 

The coagulum formed during polymerization may take many forms and is commonly referred to by many names, often colloquial, e.g., reactor fouling, filterable solids, button, sediment, silt, grit, seeds, sand, waste, scrap, or worse. In this discussion, the term "coagulum" will be used to denote any polymer recovered in a form other than stable latex. 

The aromatic and hydroaromatic components of coal-derived feedstocks are potential sources of carbon laydown, and steam would be needed to reduce reactor fouling. 

At this stage, we should mention another important factor that can affect drastically the heat-transfer capability of a polymerization reactor fouling by polymer... 

From an operational point of view, the choice of an appropriate polymerization reactor depends on six requirements temperature control mixing product accumulation and reactor foul-up follow-up separation processes the desired form of the product and safety. Heats of polymerization are typically high, so that maintaining the reactor at a desired temperature level is not always a simple task. Temperature can become spatially nonuniform and globally out of control (causing inconsistency of the reaction medium). Nonuniformity in temperature can lead to localized zones of poor mixing or even dead zones. In a polymerization reactor, temperature, mixing, viscosity,... 

Many polymerization reactions start out in a low-viscosity medium. As polymerization proceeds, the viscosity of the reaction medium increases, and in many cases the product agglomerates and deposits on the reactor wall and on the stirrer, causing product accumulation and subsequent reactor foul-up. The steady-state operation of a polymerization reactor thus requires constant cleaning of the reactor and stirrer walls and the product outlets under varying viscosity conditions. 

The reactor foul-up due to the depositions of viscous and sticky polymer products on the walls of the reactor and stirrer, as well as at the reactor outlet, has led to many special designs of mechanically agitated reactors. These designs are largely published in the patent literature. Some of these novel... 

The main reason that the branching which takes place in suspension polymerization does not lead to reactor fouling is primarily due to the fact that each droplet of monomer dispersed in the continuous water phase is like a tiny isolated reactor. The water phase is continuously in contact with the reactor... 

The development of commercially viable plastic pyrolysis processes has up to now been hindered by the need to engineer around various process problems such as reactor fouling by carbon deposits, poor heat transfer of molten plastics, the requirement for integrated fractionation of products, separation of water and suspended carbon from the liquid fuels and integrated desulphurization. 

It is also possible, and in some cases more desirable, to use a continuous tubolar reactor as the first seedittg reactor (Nomura. 1980). Oscillations should not he a problem in such a reactor, but reactor fouling might he a more important consideration. 

Solution—polymerization of monomers dissolved in solvents in which the polymer products are also soluble Lower viscosity than bulk and better heat transfer and mixing direct application of solution less reactor fouling Smaller reactor capacity than bulk high separation cost often inflammable and toxic solvents lower molecular weights SBR, polyvinyl acetate, polystyrene, acrylics... 

Less reactor fouling for corrosive type of catalyst because the catalyst used is normally held within some structures and hence will not be in direct contact with the reactor wall. 

The "gas-phase" (fluidized-bed) process is much like the slurry process in that polymer particles are formed at similar temperatures, but a liquid hydrocarbon diluent is not used. A bed of catalyst/polymer is stirred either by mechanical means, or, more often, by fluidization, while ethylene, N2, and other hydrocarbons that act as a coolant are circulated [718-722], Although the "gas phase" process offers many advantages, including the lack of a diluent that can cause polymer swelling, its weak point is poor heat removal from the polymer particles, because of the low heat capacity of a gas. Thus, reactor fouling still occurs,... 

Normal Swollen Fused mass Commercial Log FIGURE 228 Progression of polymer swelling and reactor fouling. 

The slurry polymerization process with prepolymer has been scaled up from bench-scale (100 grams) to pilot-plant operation (0.5-ton batches) without major difficulties. An important aspect of this process is the absence of reactor fouling. 

The metallocene is added to the supported MAO as a solution in toluene or aliphatic hydrocarbon. Subsequently subjecting the mixture to microwaves has been claimed to fix the metal component on the support and reduce reactor fouling (adhesion of polymer to reactor surfaces). The metallocene can also be dry-blended with the support, avoiding solubilization of the finished catalyst. ... 

Investigators at BASF have noted that (n-BuCp)2-ZrCT contacted with AlMes-treated wet silica and used to polymerize ethylene in an isobutane slurry results in severe reactor fouling. Addition of a small amount of n-butyllithium to the autoclave eliminates this fouling and allows for smooth continuous reactor operations. " ... 

---