Keep cool when designing batch reactors

The stirred batch reactor is a workhorse at many plants. In designing such a unit, focus on effective temperature control to achieve optimum performance.

By John Edwards

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The stirred batch reactor is a workhorse at many fine and specialty chemical plants, frequently serving multiple purposes. It can handle not just reaction but solvent extraction, crystallization and distillation. The successful installation of such a reactor depends to a great degree upon the proper design of its temperature-control system. 

The most critical factor is the design operating temperature range. This, coupled with a site’s practices and the initial fill cost, drives the selection of a heat transfer fluid (HTF).

An HTF must not be used at temperatures above the manufacturer’s recommended maximum. It is considered good practice to select a fluid with temperature capabilities at least 20°C (36°F) higher than the required process maximum to safeguard against fluid breakdown. Table 1 summarizes the temperature capability of some common HTFs.

Note that the food industry prefers propylene glycol, due to its low oral toxicity, to ethylene glycol. Glycol water-based systems require inhibitors to keep dissolved oxygen from forming organic acids, which can cause corrosion and fouling.

Wherever possible, avoid pressurized systems by selecting a fluid with acceptable vapor pressure at the maximum operating temperature. This will simplify system design and operation. Then, evaluate the suitability of other crucial physical properties over the operating temperature range. The specific heat of water-based and organic HTFs can vary significantly. As Figure 1 shows, water has a higher heat-removal capability.

 

Fig. 1 Liquid specific heats
Liquid specific heats


The liquid viscosity throughout the operating temperature range is a key parameter. At low temperatures, viscosity effects can become limiting, resulting in low jacket/coil-side heat-transfer coefficients and high pressure drops (Figure 2). Selection of an HTF with reasonable viscosity characteristics and an acceptable freeze point will allow operations down to -90°C (-130°F) [1].

 

Fig. 2 Liquid viscosities
Liquid viscosities

Fluid tradeoffs
Organic HTFs offer a number of advantages:
• liquid state throughout the operating temperature range, which simplifies the control system, equipment configuration and operation;
• stable fluid properties over a wide temperature range;
• less corrosion and erosion of heat-transfer surfaces than water;
• controllable temperature differences, which minimize thermal shock effects; and
• flexibility to handle a variety of services.
However, they also pose disadvantages:
• lower thermal efficiency than water-based systems;
• higher initial equipment and installation costs;
• significantly greater cost for initial fluid charge;
• propensity to leak;
• special commissioning, operational and maintenance procedures;
• longer downtime on equipment failure; and
• flammability, toxicity, odor and good manufacturing practice (GMP) concerns.

These HTFs will aggressively search for any leak paths and this must be considered when selecting equipment and specifying piping. Use sealless pumps for fluid circulation. To achieve the flow required to prevent bearing damage, install a restriction orifice in a spillback line. The piping design should specify ANSI 300 flanges, as a minimum, to allow for high bolting torques. The gaskets should consist of an asbestos-free filler reinforced with a stainless steel spiral.

These systems have to be thoroughly dried out by heating during commissioning to prevent operational problems and equipment damage. This can take days on large installations and needs to be done slowly to avoid equipment damage due to steam hammering. Manufacturers do not recommend water for pressure testing, preferring a suitable dry alternative. However, this is not usually feasible during the construction phase.

Water breakthrough, due to contamination or equipment failure, can result in considerable downtime to identify and rectify the problem. At low temperatures, water breakthrough will result in freezing, leading to loss of circulation and possible equipment damage.

Reactor parameters
The heat-transfer-area/reactor-volume ratio increases as the reactor size decreases (Figure 3). This needs to be considered carefully during scaleup and underscores the importance of correctly matching reactor size to batch size. Partially filled reactors not only reduce the heat-transfer area but can cause mixing problems and exothermic reaction instability.

 

Fig. 3 Reactor volume versus heat-transfer gas
Reactor volume versus heat-transfer area


The thermal conductivity of materials of construction significantly impacts reactor wall temperatures and, thus, can limit cycle times. Extreme temperature differences can result in product quality problems on certain processes. (The density and specific heat differences among materials aren’t a critical factor in heat transfer.)

As vessel size increases, so too does the cross-sectional area for fluid flow, which is determined by the annulus width for jackets and the pipe diameter for coils. Unbaffled jackets result in laminar flow, which gives poor thermal performance. Baffling in the jacket annulus, dimple jackets, half coils and inlet agitating nozzles can provide higher velocity. Mechanical design, construction and cost constraints can limit options [2, 3].

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