Emerging Frontiers in Hydrodynamic Cavitation

6 Degassing and Gas Stripping

6.1 Two Step Mechanism: Nucleation and Disengagement

Degassing requires two distinct operations. First, the liquid must become supersaturated with respect to the local pressure so that dissolved gas forms bubbles or expands existing nuclei. Second, the bubbles must coalesce, rise, or be separated before they redissolve during pressure recovery. A cavitation device can perform the first operation very effectively but may fail as a complete degasser if the outlet immediately repressurizes the bubble laden liquid without a venting vessel [14,17,34].

This form of Henry's law uses a concentration based Henry constant Hi. Other conventions use the reciprocal constant; the convention and temperature must be stated.

For stripping, bulk concentration $C_{\text{i}}$ exceeds the equilibrium concentration $C_{\text{i*}}$ associated with the gas phase partial pressure.

For an ideal first order contactor, ηstrip is fractional removal and τ is gas liquid contact time. Actual systems may be limited by bubble disengagement and back mixing.

Figure 6.1. Cavitation assisted gas stripping: nucleation followed by disengagement. [70].

Figure 6.2. Packaged vortex treatment skid used in industrial degassing and cooling water trials. [14].

6.2 Demonstrated Uses and Limitations

The ISPT project compared three end uses. Low viscosity dairy liquids deaerated faster after vortex treatment than by spontaneous holding tank deaeration, although a small venting tank was still required. In an evaporative condenser, the treatment successfully supported chemical free scale and corrosion control. In contrast, processing reverse osmosis permeate did not reduce dissolved gases and was judged to add no value in that application [14]. These contrasting results are instructive: a low pressure zone is not sufficient if the dissolved gas loading, gas speciation, pressure recovery, temperature, or downstream separator does not create a net removal path.

Potential stripping targets include carbon dioxide, oxygen, hydrogen sulfide, methane, ammonia under favorable pH, and volatile organic compounds. Each requires a gas specific mass balance and off gas treatment. Removing oxygen may reduce oxidation and corrosion in some closed processes, but aeration may be required elsewhere. Carbon dioxide removal raises pH and can intentionally precipitate carbonate hardness but may also cause uncontrolled scaling downstream. Hydrogen sulfide and VOC stripping require enclosure, ventilation, explosion assessment, and air pollution control. The choice between cavitation, vacuum degassing, membrane contactors, steam stripping, inert gas stripping, or thermal deaeration should be based on the required residual concentration and the value of integrating the hydraulic device with another process duty.

Table 6.1. Cavitation assisted degassing applications

Liquid and target gas	Potential benefit	Required downstream equipment	Evidence or risk
Low viscosity food liquids; entrained air	Reduced holding time and air related process defects	Small vent or disengagement tank	Positive industrial experiment [14]
Cooling water; CO2	pH shift and bulk carbonate precipitation	Side stream solids separator and basin venting	Field use, but chemistry must be controlled [7–14]
RO permeate; dissolved air/CO2	Potential reduction in chemical demand	Vent, vacuum or sweep gas if pressure recovery is insufficient	Uploaded field experiment found no removal [14]
Closed process liquid; O2	Reduced oxidation or corrosion	Gas tight separator and oxygen control	Application specific; avoid microbial anaerobiosis
Leachate or wastewater; H2S/VOCs	Odor and toxicity reduction	Enclosed off gas collection and treatment	Uncontrolled stripping transfers pollution to air

6.3 Engineered Cavitation Degassers and Bubble Coalescers

Patented cavitation degassers illustrate why nucleation and separation should be designed as distinct hydraulic functions. The Soviet apparatus SU 1258460 A1 places a cavitation element in a working chamber, draws gas through a vacuum outlet, and uses a diffuser and conical recovery chamber to enlarge the gas liquid interface and discharge liberated gas. The stated design objective was higher throughput with lower specific energy [60].

Figure 6.3. Hydrodynamic liquid degassing apparatus with a cavitation element, vacuum gas withdrawal, and diffuser [60].

A later United States patent by Kozyuk uses a cavitating restriction to nucleate dissolved gases, followed by a sequence of annular chambers, turns, and velocity reductions that promote bubble coalescence before the liquid exits. Small bubbles are deliberately converted into fewer, larger bubbles because larger bubbles separate more readily and are less likely to be carried into downstream equipment [61].

Figure 6.4. Cavitation and staged coalescence geometry for liquid degassing. [61].

Table 6.2. Functional comparison of two cavitation degasser concepts

Design function	SU 1258460 A1	U.S. Patent 10,159,915 B2
Nucleation	Cavitation element in the working chamber	One or more local restrictions create cavities and release dissolved gases.
Gas growth	Gas enriched cavities expand through the diffuser and low pressure zone	Repeated changes in velocity and direction provide time for dissolved gas to enter cavities.
Coalescence	Conical recovery chamber and developed interface	Successive annular chambers reduce velocity and merge small bubbles.
Separation	Vacuum gas outlet and downstream discharge	Coalesced bubbles leave the liquid through a downstream separator or vented vessel.
Primary design risk	Insufficient venting or gas entrainment at high throughput	Re dissolution if pressure recovers before bubbles are physically separated.

A complete degassing skid therefore requires a pressure controlled cavitation stage, a low velocity coalescence volume, a vent or vacuum connection, mist elimination where volatile liquids are processed, and controls that prevent air ingress or unsafe gas accumulation. Gas composition, flash point, toxicity, oxygen sensitivity, foam, and condensable vapor loading determine the required vent treatment. Performance should be reported as residual dissolved gas at a stated temperature and pressure, together with electrical energy per treated volume and the fraction of gas actually captured by the vent system.