Plug Flow Reactors

In the production of both low & high viscosity organic and inorganic chemicals, a simple jacketed stirred tank with supporting auxiliary equipment is the traditional means of completing chemical reactions. In many cases and for various reasons, a batch tank reaction system may be best for a particular chemical synthesis and operating plant environment and should continue to be used.

It is sometimes desired to operate continuously in order to eliminate batch-to-batch variations, produce a consistent product at all times and to have a compact dedicated continuous reactor to free up use of existing batch tank equipment. There are many equipment types and arrangements used to achieve continuous operations such as with the use of empty pipe reactors, cascades of stirred tanks, extruders, scraped surface reactors, etc.

The typical process goals of a continuous reactor system are that the reaction product be homogeneous with regard to degree of reaction achieved, molecular weight, molecular weight distribution, viscosity, temperature and other operating conditions and chemical/physical properties. For this to be achieved, all the material within the reactor must be well mixed and have the same residence time (plug flow).

In fast chemical reaction systems such as when a + b > c with no side reactions possible, simple empty pipe reactors are often used. When fluid viscosities are water like, empty pipe reactors operating in deep turbulent flow most often work well. With increasing viscosity resulting in laminar flow operating conditions in the pipe reactor, static mixing elements will require to be installed in the pipe to assure good mixing of the reacting materials.

In slow chemical reaction systems, long residence times are required to complete the reaction. These chemical reaction systems may be low or high viscosity throughout their entire reaction life cycle or viscosity may increase/decrease at various stages of the reaction. In addition, secondary reactants may require to be added after initial reaction and heat may be required to be added or removed.

Because slow chemical reaction systems normally require long residence times, tubular reactors will operate in deep laminar flow regardless of fluid viscosity. The tubular reactors are typically skid mounted in a sloped horizontal back-and-forth configuration (Figure #1) or in a vertical up-flow followed by a vertical down-flow configuration. The pipe/tubing is normally filled with static mixing elements such as shown in Figure #2, #3 and #4 where the choice of static mixing element insert is highly dependent on the chemistry of the reaction, diameter of unit, operating Reynolds number and Bodenstein number, fluid viscosity, heating/cooling requirements, etc.

Plug Flow & Radial Mixing

A tubular reactor is a fundamentally simple continuous reactor where there are no moving parts other than pumps that deliver the reactants. In a typical slow chemical reaction system where long residence times are required, the fluids passing through a tubular reactor are in the laminar flow regime even when the materials are low viscosity water-like fluids.

An empty tube is unfortunately not suitable as a chemical tubular reactor in slow chemical reaction systems when the material processed is in the laminar flow regime. The addition of static mixing elements inside the tubular reactor creates the ideal conditions of radial mixing and near plug flow conditions necessary to perform chemical reactions in the laminar flow regime.

Figure #5: Laminar flow in an empty tube (top) and through a High Performance GX™ static mixer (bottom).

Radial Mixing:

In an empty tube, viscous material in laminar flow will exhibit no radial mixing (Figure #5 top) where material will exit the pipe in virtually the same position as where it was introduced into the pipe. Addition of GX™ mixing elements in the same tube will create a high degree of radial mixing (Figure #5 bottom).