Effective Scrubber Design Requires Holistic Approach
The construction of an efficient scrubber requires the careful evaluation of several factors, from the inlet design to the demisting technology, to ensure a consistent flow of gas and liquids.
In a presentation held by the SPE Separations Technology Technical Section, Jimmie Riesenberg, senior process adviser and separations lead at Chevron, discussed the finer points of a scrubber design.
Gas scrubbers remove liquid droplets from gas streams to protect downstream equipment from damage and failure. They are typically used upstream of gas treating equipment or downstream of equipment where liquids have condensed from the gas. Before a design takes place, the scrubber’s expectations must be known. The liquid load should be taken into account, as should other factors such as startup, shutdown, and carryover from an upstream separator. Riesenberg said the typical specification of less than 0.1 gal/MMscf for a liquid carryover is extreme, and high inlet liquid loads make the expectation unrealistic and unachievable. Carryover performance cannot be evaluated without an analysis of the size of the droplets that need to be separated.
“The typical spec is that you have 99.5% removal of all particles 10 µm or larger,” Riesenberg said. “Is that a good spec? Maybe. The question is what is the actual droplet size distribution that you’re expecting in that scrubber?”
Most of the data available for gas scrubbing technology comes from air/water testing. However, the testing does not translate well to hydrocarbon performance, Riesenberg said. Air and water have a high gas/liquid density ratio, which is key to gravity separation, but it is not the same as testing hydrocarbons because water has an extremely low viscosity and a high surface tension. He placed the same caveat on other commonly used methods such as nitrogen/water tests. The only way to accurately predict hydrocarbon performance is by hydrocarbon testing in a controlled setting, he said.
Hydrocarbon gas must be tested with a hydrocarbon liquid at relevant pressures and properties. This liquid may be a model fluid, crude oil, or condensate.
“If you don’t have the hydrocarbon testing, how do you correlate those results to reality? Just because it works in the field does not mean you know how well it’s separating. Unless you’re going to put in the equipment to actually determine the liquid carryover and the drop size distribution in the gas leaving the separator—and it is very rare that it is done—you don’t know how well it’s separating,” Riesenberg said.
The overall goal of a scrubber is to create plug flow, or flow where the velocity of fluid is constant across any cross-section of the pipe, up to the vessel.
Riesenberg listed four key components of the design as follows:
- Inlet piping size and routing
- Inlet nozzle and distributor
- Vessel size and internals
- Gas outlet and liquid outlet
Generally, the internal diameter of the inlet piping should match that of the inlet nozzle for a distance from at least 10 to 20 pipe diameters upstream of the scrubber. Riesenberg said computational fluid dynamic modeling helps to determine the exact distance for a particular system. It is also important for a piping design to prevent a swirling flow of gas or liquids entering the scrubber.
Within the demister, the mesh pad should be positioned so that the difference in backpressure between the head and outlet nozzle is negligible. If it is impossible, an alternative is to install a perforated plate on top of the demister to even the flow. Liquid collection will decrease the flow area of the separation device and it should be accounted for in velocity, momentum, and k-factor calculations, Riesenberg said....
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