Compressors are the workhorses that support industry – they power pneumatic tools, they push out oil from the deep inside the earth, and they transport natural gas. The US Department of Energy estimates that compressors account for 10% of the US total annual energy consumption – 85,000 Gigawatt-hours per year. Industry spends $10 billion annually powering them, and that electricity demand emits 128 metric tons of CO2. Over the lifetime of a compressor, its energy consumption costs account for over 75% of the overall cost of ownership – energy costs ultimately dwarf initial capital cost and maintenance expenses. Compressor energy savings is the priority for industry and the environment.
The Problem & Solution
Typical compressors waste 85% of the electricity they consume. The compression process generates lots of heat and high temperatures, and that heat is rejected (wasted) to protect the compressor and the gas.
Our novel Foam Compressor can save up to 60% on the energy consumed. The technology is foam – a stabilized mixture of the compression gas & a cooling liquid that keeps the temperatures low thereby reducing the energy consumed and wasted. Our Foam Compressor consumes less energy, has fewer compression stages, and requires a fraction of the cooling equipment (i.e. no intercoolers).
|Industrial Gases||Process Gases||Oil & Gas|
It is well known in the compressor industry that isothermal compression of gases yields the highest discharge capacity per unit input power (kg/s per kW). Historically, the means of achieving near-isothermal conditions has been met by the use of multi-stage compressors with inter-stage cooling. Still, the reciprocating piston compressor in-cylinder process is near-adiabatic resulting in high stage discharge temperatures thus limiting the pressure ratio per stage. The result is a compromise between the desired reduction in energy and the cost and complexity of increased number of stages.
We have developed a near-isothermal compression process that reduces both the number of stages required for a given pressure ratio while maximizing overall compression efficiency. This process is illustrated in Figure 1. Low pressure gas is mixed with a liquid to form a uniformly dispersed foam matrix. The foam is drawn into the suction side of the low pressure stage and compressed near-isothermally by heat transfer from the gas to the liquid.
The uniform foam is discharged from the compressor and enters a gas-liquid separator. The warm, dry gas is transported to the end process while the warm, recovered liquid is returned to near-suction-side pressure through an energy recovery machine and then cooled prior to being reintroduced to the suction side gas stream.
Near-Isothermal Compression Benefits
The advantages of near-isothermal compression over multi-stage adiabatic compression are shown in Table 1. Near-isothermal compression saves on power, energy, electrical demand charges & consumption rates, all while doing this in fewer stages with no intercoolers, and at significantly lower gas temperatures. Additionally, the aftercooler for the near-isothermal compressor is smaller & cheaper because it cools low pressure liquid as compared to cooling high pressure, high temperature gas as is the adiabatic compressor’s case.
Foam technology requires the direct contact between gas and liquid phases to achieve near-isothermal compression. As a result, the gas and the compressor must be tolerant of multiphase flows – gas, foam liquid and liquid vapor – but only during the compression process. Bulk liquid & liquid droplets are removed by the foam separation system leaving a clean gas. Foam technology also requires positive displacement compression and works well with reciprocating piston compressors due to their high compression ratios.
Given equal discharge capacity, total process pressure ratio, and isothermal process liquid/gas mass ratio, different gases will yield different energy savings. Figure 2 shows the first order estimated energy savings for a near-isothermal compressor versus a four-stage adiabatic compressor with intercooling for four common gases: natural gas (CNG), carbon dioxide (CO2), air, and argon (Ar). Nitrogen and oxygen savings are similar to air. In general, the near-isothermal process favors gases with higher molecular weight (low specific gas constant), higher ratio of specific heats (k), and higher process discharge pressures. Furthermore, the benefits of improved compression efficiency will favor applications with high utilization factors or where utility demand charges ($/kW) and/or energy costs ($/kWh) are high.