Exotherm Control

Solution Polymerization. Plant scale polymerizations ia water are conducted either adiabaticaHy or isotherm ally. Molecular weight control, exotherm control, and reduction of residual monomer are factors which limit the types of initiators employed. Commercially available high molecular weight solution polyacrylamides are usually manufactured and sold at about 5% soHds so that the viscosities permit the final product to be pumped easily. 

Resoles. Like the novolak processes, a typical resole process consists of reaction, dehydration, and finishing. Phenol and formaldehyde solution are added all at once to the reactor at a molar ratio of formaldehyde to phenol of 1.2—3.0 1. Catalyst is added and the pH is checked and adjusted if necessary. The catalyst concentration can range from 1—5% for NaOH, 3—6% for Ba(OH)2, and 6—12% for hexa. A reaction temperature of 80—95°C is used with vacuum-reflux control. The high concentration of water and lower enthalpy compared to novolaks allows better exotherm control. In the reaction phase, the temperature is held at 80—90°C and vacuum-refluxing lasts from 1—3 h as determined in the development phase. SoHd resins and certain hquid resins are dehydrated as quickly as possible to prevent overreacting or gelation. The end point is found by manual determination of a specific hot-plate gel time, which decreases as the polymerization advances. Automation includes on-line viscosity measurement, gc, and gpc. 

The peracid—exotherm control agent mixtures can be granulated using a variety of techniques common in the industry, including agglomeration. As with peracid precursors, the surface area to volume ratio can impact the stabiUty of the peracid. Particles are thus made as large as possible to maintain stabihty (141). 

Low inventory Stationary condition Stationary condition Agitation provides safety tool Streams may be diluted to slow reaction Agitation provides safety tool Controllable addition rate Agitation provides safety tool Large exotherm controllable... 

Exo-receptors, divergent, 16 774 Exothermal chemical runaways, controlling, 22 720 Exotherm control, 10 457... 

This process though simple has some drawbacks associated with heat transfer. As polymer formation proceeds the medium becomes viscous and heat transfer becomes difficult. If polymerisation is exothermic, control becomes more difficult and sometimes it may lead to explosions. Because of the possibility of localised overheating leading to degradation and discolouration of polymer, this process is seldom used with large batches. 

Temperature is commonly the most dominant variable in reactor systems. Since many chemical reactors are exothermic, controlling the dominant variable in these systems amounts to removing the exothermic reaction heat through temperature control. We gave many examples of how that is done. In cases where temperature is not the most dominant variable, compositions typically dominate. In this case unit control is not localized to the reactor since composition control is affected by other parts of the plant. 

Figure 1.5 Schematic of BASF s early tower process for the continuous polymerization of styrene. This configuration was designed by C. Wulff and E. Dorrer in the early 1930s. Polymerization was thermally initiated and the exotherm controlled by heat transfer tubes (courtesy of BASF, Ludwigshafen)...

Cast allylics. The ally lie ester resins possess excellent clarity, hardness, and color stability and thus are used to cast them into optical parts. These castings can be either homo or co-polymers. The free radical addition polymerization of the allylic ester presents some casting difficulties such as exotherm control, monomer shrinkage during curing and the interaction between the exotherm, the free radical source, and the environmental heat required to decompose the peroxide and initiate the reaction. 

Easy to install Very high energy consumption for large components Extra cooling strategy needed Issues with exotherm control... 

Solution Polymorization. Commercial production of polyacrylamides by solution polymerization is conducted in aqueous solution, either adiabatically or isothermally. Process development is directed at molecular weight control, exotherm control, producing low levels of residual monomer, and control of the polymer sohds to ensure that the final product is fluid and pumpable. A generic example of a solution polymerization follows. 

Use of safer chemicals including solvents satisfies one of the criteria of green chemisfry principles. From green chemistry perspective, solvent-free chemical fransformafions are indisputably more desirable than reactions in any kind of solvents. However, feasibility of solvenf-free reactions is often complicated by problems with reaction (especially exothermic) control, mass transfer limifafion, viscosity and melting point of reacfanfs, etc. Water is the solvent of first choice provided by nature. This is the most commonly used green solvent and heterocyclic S5mthesis in aqueous medium is discussed in details in Chapters 7 and 8 of this book. Unfortunately, very often water is not suitable for organic reactions and choice of another more efficient solvent becomes critical. 

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