Molecular sieves use a desiccant process to remove water vapor from hydrocarbon streams. The process involves flowing hydrocarbon fluid over an adsorption bed to remove moisture. As a bed reaches saturation, switching valves direct the hydrocarbon gas to another bed while the first bed is regenerated with hot, reverse flowing gas.
Switching valves are key
From the article, here’s how important the switching valves are to this process. The performance of a molecular sieve unit is critically linked to the performance of the switching valves. The drying process itself requires tight sealing between the adsorption and regeneration streams to maintain efficiency. Mechanical reliability is also paramount since these units can run for years between turnarounds, while switching two to three times a day.
The valves must seal reliably, despite the presence of abrasive dust and constant thermal cycling between ambient feed and 500°F regeneration gas. Increasingly strict fugitive emission requirements make the design problem even more difficult because the valve must both seal and avoid packing leaks, even as it is constantly operated.
Historically, the industry has used rising Segment Ball Valve for molecular sieve service. This design employs a mechanical arrangement that first moves the ball off the seat, then turns the ball away from the seat as it opens. The valve closes by first twisting toward the seat, then jamming against it to achieve tight bidirectional shutoff. The RSBV design has two design issues. The mechanical stress on the stem is excessive, eventually warping the stem and creating seat leaks. More detrimental are the higher fugitive emissions associated with a rising stem design, which tend to be 100 times higher than with a rotary valve.
A modern solution
As operating units continue to extend times between turnarounds, industry has sought better performing valve alternatives. One design, the double eccentric C-ball valve, has emerged as the best choice. This design offers the same mechanically assured shutoff as an RSBV, with zero leakage bidirectional shutoff and low-wear eccentric motion, but has further advantages, as I explain: The double eccentric C-Knife Gate Valve has a much simpler and more rugged design than an RSBV. It also has no stem wear, and it utilizes true rotary motion to greatly minimize emissions. With the same face-to-face and flow coefficients as an RSBV, a double eccentric C-ball valve is a drop-in replacement.
The new valve design can operate in molecular sieve service for eight years or more without maintenance, and during that time it continues to meet fugitive emission requirements, without the need for packing adjustment.