(32c) Energy Efficient Vacuum Pressure Swing Adsorption System

A high energy efficiency vacuum pressure swing adsorption system design with direct drive centrifugal compressor has been developed and will be presented.

Vacuum Pressure Swing Adsorption (VPSA) technology is commonly used to supply oxygen to a wide range of industries.  Glass, waste water treatment, pulp and paper, mining and cement, metal fabrication, pharmaceutical, and fermentation are a few examples. Many of these applications use oxygen rather than air which includes a substantial portion of nitrogen for environmental sustainability and productivity improvements. The competitive cost of oxygen supply is an important consideration factor when choosing an oxygen supply system.

Electric energy cost is the largest operating cost for VPSA system since the raw material, air, is free.   Typical VPSA oxygen plants are designed to be operated with minimum operator assistance. With increasing electricity costs, it becomes more important to develop technology to reduce VPSA energy consumption, and therefore reducing the total oxygen cost to the customers.

Conventional VPSA technology uses displacement rotary lobe types of blowers operating at fixed speeds to compress air to the desired process pressure. These machines generally have low efficiency with no capability of turn down. Up to 40% of the energy supplied to compressor is wasted through various inefficiencies. In addition to high energy requirements and poor utilization, these blowers are generally very noisy. In order to meet 85 DBA requirements, state of the art silencers and a building is typically required to mask the excessive noise to protect the operators hearing.

It is desirable to develop a new VPSA compressor technology which has higher energy efficiency and is quieter.

Comparing to displacement rotary blowers, centrifugal compressors have a demonstrated much higher energy utilization (efficiency).  The primary difference of utilizing a centrifugal compressor in a VPSA process is that the vapor pressure swing process requires routine rapid pressure changes.   In order to maintain centrifugal compressor peak efficiency, the compressor drive system is required to match the head pressure requirement of both the pressurizing and evacuating step of the adsorbent beds by continuously varying the speed.

A direct drive, high speed permanent magnetic motor with magnetic bearings was developed to provide the centrifugal compressor with the variable speed required by VPSA process.  In contrary to the conventional induction motor and gearbox system, the direct drive centrifugal compressor is capable of accelerating and decelerating rapidly to meet the short cycle time requirement for VPSA system, and also eliminates the losses from gearbox, conventional bearings, and lube oil system.

Comparing to the conventional VPSA there are many benefits attributed to the direct drive centrifugal VPSA system:

1.  Electric power usage is significantly lower, up to 20% power reduction is expected. 
2. System layout is more compact, since the motor and compressor are smaller and there is no gearbox and lube oil system. 
3. The system is significantly quieter therefore the building is not required.
4. Seal water and associated piping are no longer needed.