T. Anthony ("Tony") Royal, a vice president with Jenike & Johanson, was named as a co-inventor on a patent for a unique pressure vessel and lock hopper system used to decaffeinate green coffee beans. Tony's work on this project started several years ago, when Maxwell House, a division of Kraft General Foods, asked us to assist in the design of the pressure vessel.

During the process, caffeine-free liquid carbon dioxide enters near the bottom of the vessel and flows in an upward direction, through a contact bed of stationary beans. The liquid becomes caffeine-laden and exits near the top of the vessel. Using lock hoppers, small batches of fresh beans are periodically introduced into the top of the 5,000 psi processing vessel, with an equal amount of decaffeinated beans withdrawn from the bottom.

It was critical that the new equipment would operate as intended, because its expected high cost would make future modifications economically unfeasible.

To process the green coffee beans economically, a relatively large capacity vessel (almost 60 ft. tall with an inside diameter of 6.5 ft.) was required. The vessel had to be operated at pressures of up to 5,000 psi to maintain the carbon dioxide in its supercritical (liquid) form.

For the process to be effective, the beans had to proceed through the vessel in a uniform mass flow pattern. The resulting first-in first-out flow pattern would achieve uniform residence times for each batch of material in the vessel. A minimum residence time was critical for proper decaffeination of the beans.

The size of the outlet was another concern. While a relatively large outlet would provide high discharge rates, the resulting large, high pressure valve would be expensive to purchase and maintain. Therefore, the valve had to be as small as possible, but still provide the maximum required discharge rates.

In addition to outlet size, the direction of fluid flow during discharge had to be considered. Beans had to discharge from the large pressure vessel to a smaller one. As the beans filled the smaller vessel, the fluid occupying that vessel would be displaced. Without another exit, that fluid would have to leave the smaller vessel though the same opening as the beans were entering. Such a countercurrent fluid flow would severely limit the achievable discharge rate of the beans from the large vessel.

Tony and his associates at Jenike & Johanson developed a design that met the above requirements. The design provided uniform mass flow and the required discharge rates. Cost considerations and technologies for fabricating vessels operating at high pressure were important constraints on the design.

Mass flow was achieved by making the vessel's hopper walls smooth and steep enough for the beans to flow along them. We ran laboratory tests to determine the wall angles required for mass flow, but, due to the difficulty of testing at high pressures, we substituted water for the liquid carbon dioxide. We determined that the test results would be conservative and provide a reasonable factor of safety for the design.

Since our tests indicated that relatively steep walls would be required to ensure mass flow with a simple conical hopper design, we designed an insert to be placed inside the conical hopper. This allowed the walls to be less steep and therefore minimize the overall height of the vessel—which reduced its capital cost.

The insert consisted of two cones (attached end-to-end) instead of a typical single cone design. This provided a more uniform discharge of material than was possible with single cone inserts. To ensure that such an insert would be efficient, we conducted small scale modeling at our facilities in San Luis Obispo, California.

We recognized that, while mass flow would ensure uniform travel of the beans through the vessel, the outlet conditions would determine the maximum rate of material discharge. We determined the proper outlet dimensions by using our proprietary two-phase flow computer program. By simulating potential operating conditions, we established an optimum design. In order to minimize the size of the outlet valve and its associated costs, we concluded that fluid flow had to be in the same direction as that of the discharged beans, and at a specified rate. By providing another exit for the fluid displaced from the lower lock hopper, this fluid could be injected into the larger vessel through a distributor located just above its outlet, thereby providing a flow co-current with that of the beans.

The laboratory testing, modeling work, and innovative thinking by Tony Royal and his associates at Jenike & Johanson provided a sound basis for the design of the pressure vessel. The equipment was installed and is operating as expected. Initial costs were minimized, and no modifications were necessary. The design was unique and a patent was awarded, naming Tony as a co-inventor.