Improving productivity.

Q We have an old alkyd and polyester resin manufacturing plant. We want to increase productivity. Can you tell us some cost-effective ways to do it?

A Productivity measured as the annual production of a given plant is a key factor in assessing manufacturing costs. It is not surprising that many alkyd and polyester resin manufacturers, under competitive pressures, want to improve the productivity of their existing equipment at minimum cost.

The first area to look at is Batch Time. In modern plants, typical batch times are 16-19 hours for alkyds, 14-16 hours for saturated polyesters (with phthalic anhydride feed) and 18-25 hours for saturated polyesters (with isophthalic acid feed), respectively. Of course, batch times vary somewhat from one plant to another, due to different recipes and types of operations. Generally speaking, batch times are 20-50% longer in old plants. This leaves a lot of room for management to spend a few dollars and justify a return on investment.

Shorter batch times mean higher production rates with the same equipment. Consequently, one must look for the obvious bottlenecks in the process such as:

(1) The raw material loading and/or product unloading steps may take too long. This can be easily fixed by retrofitting larger equipment with proper automation.

(2) The heating and cooling transfer rates may be too slow. This is a more difficult task to tackle, since they are mainly limited by the design of the reactor and the agitator. Assuming that replacing the reactor is not a solution due to its prohibitive cost, we are left with the following alternatives:

(a) retrofitting a new agitator that can provide gentle but more efficient horizontal and vertical mixing.

(b) modifying the existing heating/cooling system to provide better control of the heating/cooling rates, thus minimizing potential fouling or degradation of the product and accelerating the energy transfer.

(c) the distillation and decantation systems may be improperly designed or undersized. Revamping the existing equipment, adding more controls, or operating the distillation column under vacuum may be the answer.

The next area to look at is Process Controls. It is important to examine the state of the existing instrumentation. Modern resin manufacturing plants use Programmable Logic Control (PLC) based control systems with Human Machine Interface (HMI) as a minimum, and with recipe management software for the more sophisticated ones. When touring an old plant, one of the most striking points is the lack of reliable instrumentation. Adding computers and basic instruments, such as mass flow meters, load cells, temperature transmitters etc., is a relatively inexpensive solution to the problem.

Monitoring product viscosity and acid/hydroxy numbers is paramount to quality control in alkyd and polyester resin manufacturing processes. Most plants use the old-fashioned method which consists of grabbing a sample from the reactor and testing it in a lab. During this procedure, valuable time (typically 30 minutes) is lost, the chemical reaction continues, and the reaction end point always remains a guess.

The net result is an approximate chemical composition of the final product. To overcome this problem, some plants have installed reactor-mounted viscometers, but without much success. The state-of-the-art solution is to retrofit continuous inline viscosity and acid/hydroxy number measurement systems. Such systems are proven, reliable, and give instantaneous measurements of these key parameters.

The operator knows the state of the reaction and the product characteristics at anytime during the batch process. The signals sent by the instruments can be integrated into the existing process control system. The benefits are numerous: (1) consistent, on-spec product; (2) shorter batch time; (3) less labor involvement, and (4) safer procedure.

Each plant is different, and only a complete technical audit of the existing equipment and a thorough review of the operating procedures by experts will determine which of these cost effective solutions are the best to improve productivity.

--Bernard Seguy, RHE America ([e-mail]).