Tubular reactors may be empty for homogenous reactions or packed with catalyst particles for heterogeneous reactions.  Packed reactors require upper and lower supports to hold particles in place. Uppermost packing is often of inert material to serve as a pre-heat section. Pre-heating can also be done with an internal spiral channel to keep incoming reagents close to the heated wall during entry.

It is often desirable to size a tubular reactor to be large enough to fit 8 to 10 catalyst particles across the diameter and be at least 40-50 particle diameters long. The length to diameter ratio can be varied to study the effect of catalyst loading by equipping the reactor with “spools” to change this ratio.

Temperature is typically controlled by thermocouples located on the outer wall of an externally heated tubular reactor. A moveable internal thermocouple is often employed to observe the temperature changes occurring as the reaction proceeds through the reactor.

Tubular reactor systems are highly customizable and can be made to various lengths and diameters and engineered for various pressures and temperatures not specified above.
 
We provide a split-tube furnace for heating these vessels. Insulation is provided at each end so that the end caps are not heated to the same temperature as the core of the reactor. The heater length is normally divided into two or three separate heating zones, although it can be split into as many zones as required.

We can furnish either a fixed internal thermocouple in each zone or a single movable thermocouple that can be used to measure the temperature at points along the catalyst bed. External thermocouples are typically provided for control of each zone of the heater. 

Gas Feed Systems

Various gas feeds can be set up and operated from a Gas Distribution Panel. In order to deliver a constant flow of gas to a reactor, it is necessary to provide gas at a constant pressure to an electronic Mass Flow Controller. This instrument will compare the actual flow rate delivered to the set point chosen by the user, and automatically adjust an integral control valve to assure a constant flow. Care must be taken to size these controllers for the specific gas, the flow rate, and the pressure of operation. A mass flow controller needs a power supply and read-out device, as well as a means of introducing the desired set point.

When ordering mass flow controllers, you will need to specify:

1. Type of gas to be metered (e.g. N2, H2, CH4)
2. Maximum operating pressure of the gas (100 or 300 bar)
3. Maximum flow rate range in standard cc’s per minute (sccm)
4. Pressure for calibration of the instrument

Mass flow controllers are available for use to 1500 psi and to 4500 psi. Considerable savings can be obtained if the mass flow controller is to be used only to 1500 psi.

The schematic below depicts the installation of a mass flow controller for the introduction of gas to a continuous-flow reaction system. Such installations can be enhanced with the addition of a by-pass valve for rapid filling.

High pressure piston pumps are most often used to inject liquids into a pressurized reactor operating in a continuous-flow mode. For low flow rates, HPLC pumps, many of which are rated for 5000 psig, are excellent choices. Typical flow rates for pumps of this type range up to 10 or 40 mL per minute. Pumps are available to accommodate manual control from their digital faceplate or computer-controlled from a 4871 Process Controller.

Chemical feed pumps are our recommendation for continuous feeding of liquids when the desired flow rate is greater than 2 liter per hour. Here we will try to help you find a suitable pump. We will need to know the type of liquid; the minimum, typical, and maximum desired feed rate; the maximum operating pressure; and any special operating considerations such as explosion proof operation or corrosion possibilities.

Back Pressure Regulators

In addition to supplying gases to a reaction through electronic mass flow controllers, the reactor is kept at a constant pressure by installing a Back Pressure Regulator (BPR) downstream of the reactor. This style of regulator will release products only when the reactor pressure exceeds a preset value.

When a BPR is used in conjunction with mass flow controllers, the user is assured that a constant flow of gas is passing through a reactor, which is being held at a constant pressure. This provides for the highest degree of control and reproducibility in a continuous-flow reactor system.

Cooling Condensers

It is often desired to cool the products of the reaction prior to handling them. For this purpose, tube-and-shell heat exchangers are available to act as the cooling condensers. An adaptation of our standard condensers provides an excellent design. Descriptions and available sizes are found in the 4500 Catalog, Stirred Reactors and Pressure Vessels.

Gas/Liquid Separators

Tubular reactors operating in continuous-flow mode with both gas and liquid products will also require a Gas/Liquid Separator for smooth operation. The separator is placed downstream of the reactor, often separated from the reactor by a cooling condenser. In the separator vessel, liquids are condensed and collected in the bottom of the vessel. Gases and non-condensed vapors are allowed to leave the top of the vessel and pass on to the back pressure regulator. It is important to operate the BPR with a single fluid phase to prevent oscillation of the reactor pressure.

The gas/liquid separator can be sized large enough to act as a liquid product receiver that can be manually drained periodically. Many of the non-stirred pressure vessels listed in our 4500 catalog are ideally suited for use as gas/liquid separators. Vessels of 300, 600, 1000, or 2000 mL are commonly chosen.

Control and Data Acquisition Systems

A variety of solutions exist to meet the needs of system operators. System accessories such as heaters, mass flow controllers, and pumps can be obtained with individual control packages to create a manual, distributed control system. Each of the various controllers that are equipped with an output signal can be incorporated into a PC-based data acquisition system.

As the number of channels to be controlled and logged increases, economics will often dictate that the distributed system of individual controllers should be replaced with the computer-based Model 4871 Process Controller. This controller is described in detail in our catalog and on our web site, www.parrinst.com.

Below are a series of pictures and schematic representations of some typical systems, along with a symbols chart to facilitate understanding.