The common method for monitoring product viscosity and acid number is for an operator to draw samples from the reactor at regular intervals. These samples are then sent to the laboratory for analysis. After receiving an analytical result that matches the value specified in the operating procedures, the operator proceeds to the next processing step. The analytical results are usually available 15-30 minutes after sampling. Because the reaction continues during this time interval, the analytical results are not representative of the actual product inside the reactor. Because this method is inconsistent, the produced resins vary significantly between batches.

To improve consistency between batches, many operators have attempted to get more accurate viscosity data during the production of alkyd and polyester resins. Operators have measured the torque of the reactor agitator, but this only generates trend data (i.e., "the viscosity is going up"). Others have installed viscometer sensors inside the reactor. However, the data from these sensors is not reliable because of temperature measurement problems, vapor bubbles in the product mix, solid raw material particles precipitating on the sensor, etc.

To avoid all these problems, RHE America, the North American affiliate of RHE H?ndel Engineering GmbH, developed a system in which the viscosity and acid number are continuously measured in a bypass around the reactor. The Continuous Measurement System includes a product circulation pump, tempering device, viscometer, NIR spectrophotometer with transmission probe, heating/cooling circuit with pump, interconnecting pipe, and instrumentation, all incorporated on a single skid (see process flow diagram).

A constant stream of product is pumped from the lower part of the reactor through a jacketed pipe, through the tempering device and into the viscometer. The viscosity is measured at a constant, preset temperature maintained by circulating thermal oil through the jacketed pipe and the tempering device. A rotary type viscometer is used because it is largely insensitive to solid particles, encrustations and solidifications. After flowing through the viscometer and the spectrophotomer, the product returns to the reactor. A signal from the viscometer is available for use by the client's control system.

The temperature of the sample stream is controlled by a thermal oil system, which pumps thermal oil through the jacketed pipe and the tempering device. The sample stream temperature is independent of the reactor temperature. This secondary thermal oil system is connected to the client's primary thermal oil system and accurately controls the product temperature within ?0.5?C of the preset temperature. For cooling of the product stream, the thermal oil in the secondary circuit is pumped through a distribution valve to the thermal oil cooler.

The measured values of the viscosity and the acid number, as well as the corresponding measuring temperature are displayed either digitally on a control panel or on a monitor if the measurement system is integrated into a computerized process control system.

The online viscosity and acid number measurement system allows the automatic resin production to preset target values. Because all the measuring signals are available continuously, the resin production process is terminated at the optimum point in time.

An additional and important advantage is that the resin manufacturer can supply his customer with a complete computer printout as documentation of that particular product batch.

RHE H?ndel Engineering GmbH, a German engineering firm, specializes in the design and supply of plants for the manufacture of synthetic resins, polymer emulsions and related products. They recently developed a new and unique method for the continuous, online measurement of many different product parameters including viscosity and acid number.

For more information on resins, contact RHE America, 2525 Cabot Drive, Suite 205, Lisle, IL 60532; phone 630/5770400; fax 630/577.0401; or e-mail cantrall@amcec.com.

Case History

In 2001, RHE supplied a viscosity and acid number measurement system, which was connected to the existing reactor. The manipulation and control of the system was integrated into the existing process control system. After only a few days of instruction and operator training by RHE, SYNTEKO could operate the system without any problems.

Since the installation of the system, the alkyd resins are manufactured to a very narrow specification range. In particular, viscosity fluctuations between different batches of the same resin formulation have been drastically reduced. The variation between resin batches is now so small the customer no longer needs to adjust the paint formulation for variations in the viscosity and acid number of the supplied alkyd resins.