A number of processes used in the industry operate at pressures below atmospheric (i.e. under vacuum). The main reason for vacuum processes is the ability to lower operating temperatures, thereby saving fuel and avoiding decomposition of hydrocarbon streams. Pressures range from coarse vacuum, just under atmospheric, to ultra-high vacuum, down to < 10-7 Torr, as defined below.
In refining, the most widespread use is in vacuum distillation of atmospheric residues. A notable use in upstream operations is enhanced hydrocarbon recovery for aquifer remediation. There are three primary types of vacuum systems: vapour ejector, liquid ring vacuum pump, and hybrid systems using a combination of these
The most common system for creating a vacuum is the gas ejector, which uses a high pressure motive gas to entrain a lower pressure stream. Historically, ejectors have been used for a range of vacuum applications from small systems on ancillary plants to very large systems, often with several ejectors in series in refinery vacuum distillation units. Ejectors are commonly used in upstream applications for flare gas recovery, restart of dead wells, and for boosting production. (Refer to the template for Ejectors for a discussion of ejector
Liquid ring vacuum pump
An alternative technology is the liquid ring vacuum pump (LRVP), which uses a liquid ring formed in the annular space between the pump casing and the off-centre impeller to compress the gas as shown below.
Figure 1: General LRVP schematic (Reference 2)
One of the advantages of LRVPs is their ability to discharge at higher than atmospheric pressure. LRVPs can be used on their own or in combination with ejectors, the latter usually being preferred to create a mild vacuum while the LRVP further reduces pressure to the desired level.
Ejectors use gas (e.g. steam) while LRVPs are electro-powered which can be a consideration for the technology selection.
Hybrid systems use a combination of ejector and LRVP to achieve the desired vacuum conditions. Commercial systems, such as the Hijet system, may use parallel ejector systems to achieve higher efficiency.
Range of application:
From small systems with coarse vacuum to very large with ultra-high vacuum
> 30% depending on the design of the system
Guideline capital costs:
(Cost Basis 2007; Reference 6): Steam ejector systems: ~$20,000 to ~$120,000, depending on the size and type of system; Vacuum pumps: ~$15,000 to ~$400,000, depending on the size and type of pump.
Guideline operational costs:
(Cost Basis 2007; Reference 6) ~$750,000 in utilities for a three-stage ejector system handling a gas flow rate of 1150 kg/h
Typical scope of work description:
For new applications, it is important to analyse the vacuum duty required based on the suction pressure and pumping capacity needed. The availability of utilities such as steam and electricity are also a determining factor regarding the choice of vacuum technology to be employed. The operating costs for a vacuum system are typically more significant than the initial capital cost, so system optimization or use of variable speed drive LRVPs may be important considerations for reducing energy consumption.
Size of installation, type of gas, depth of vacuum, availability of utilities (steam or electricity), cooling water capacity, sour water treatment capacity Consistent steam quality is important for proper ejector performance
Non-condensable gas loading Ejector erosion potential, or product build-up
Steam / electricity price
Variable speed drives on vacuum pumps to allow for production variability may result in energy savings
The following are technologies that provide similar benefits and may be considered as alternatives to vacuum systems:
VOC recovery systems
Issues and risks are few and known. The technology has been used for many years for refinery applications.
Reduce energy consumption of existing vacuum systems
Install / improve vacuum systems to reduce fuel consumption
The existing steam ejector system at the SINOPEC Tahe Refinery in China needed regular maintenance due to corrosion, and was difficult to operate due to fluctuations in steam quality and seasonal variations in cooling water temperature. The refinery also wanted to lower energy consumption, reduce GHG emissions, and eliminate sour water production. A specialized system, utilizing two parallel liquid jet ejector systems, was commissioned in December 2005. Instead of steam, the liquid ejector system uses the LVGO fraction as the motive stream. The system produces a 15 mmHg vacuum at the top of the column and also includes a separator, centrifugal pump and cooler. The minimum direct yearly savings were estimated at US $226,632 per year (cost basis 2010), as summarized below.