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Jul-2015

Energy savings from electric 
capacity control

Updated stepless capacity control of reciprocating compressors saved energy and improved process stability at a petrochemicals site.

KLAUS STACHEL
Hoerbiger
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Article Summary
The largest refining, petrochemical and agrochemical complex in the Czech Republic lies just outside the town of Litvínov, 80 km north west of Prague and close to the German border. The plant is owned by Unipetrol, the leading oil and petrochemical company in the Czech Republic and a member of the Polish PKN Orlen group. The Litvínov site is also home to a complex producing polymers and other chemical products, including ammonia for use as a fertilizer. Ammonia manufacture requires nitrogen, which in this case comes from a dedicated plant on the site operated by industrial gases company Air Products.

Recently, reciprocating compressor technology company Hoerbiger had the opportunity to help Air Products and Unipetrol save money and improve process stability by installing its new eHydroCOM capacity control system on a nitrogen compressor (see Figure 1). Both Air Products and Unipetrol say that the new control system saves an estimated annual €330 000 in electricity costs and allows the ammonia plant to operate more smoothly.

Looking for power savings

The Air Products plant at Litvínov liquefies and distills air, splitting it into oxygen and nitrogen. It is one of Air Products’ larger facilities in Europe.

The refinery uses the oxygen to produce hydrogen from heavy oil residues. Some of the hydrogen is then used in the refinery’s hydroprocessing facilities to make clean fuels. The rest, with nitrogen added, is converted into ammonia, an important fertilizer for crops.

In the Air Products plant, nitrogen leaving the air separation unit at 60 bar is compressed to 324 bar by a Peter Brotherhood (Dresser-Rand) reciprocating compressor driven by an electric motor. The nitrogen is piped to Unipetrol’s ammonia plant, where hydrogen is added, and the resulting mixture is sent to an ammonia reactor.

Electricity accounts for 70% of the plant’s variable operating costs, so saving on power is a key driver. With a power rating of 2300 kW, the nitrogen compressor costs Unipetrol close to €2 million each year in electricity alone. That made it an obvious target when Air Products and Unipetrol were looking for energy savings.

In this case, savings were possible because the original method of varying the compressor’s throughput to suit the needs of the ammonia process was far from optimal. To run at 75% of rated capacity, clearance pockets were opened on the cylinder heads. For 50% capacity, the suction valves were held open to reduce the amount of nitrogen to be compressed.

At either 75% or 50% capacity, this ‘stepwise’ arrangement worked well. At any other throughput, however, nitrogen had to be throttled from the discharge back to the suction side of the compressor via a recycle valve, wasting energy consumed in compressing it. As well as using unnecessary electricity, stepwise control had undesirable effects on the downstream process.

Advantages of stepless control
At just the time when Air Products and Unipetrol were seeking to cut their operating costs at Litvínov, Hoerbiger was looking for customers willing to host pilot installations of its new eHydroCOM capacity control system. In 2013, Hoerbiger brought the new system to the attention of the two companies. Straight away, they spotted an opportunity to save money.

Five years ago, Unipetrol installed Hoerbiger’s established HydroCOM stepless capacity control system on five hydrogen compressors, and was pleased with the resulting energy savings and reliability. The prospect of an all-electric version of the control system, equally capable yet with lower project costs, seemed a good opportunity.

For its part, Hoerbiger believed the Air Products compressor would be a suitable candidate for eHydroCOM because of its size and variable flow rate.

Hoerbiger’s original system provides stepless capacity control for reciprocating compressors. Around 1000 HydroCOM systems are in operation worldwide. The working principle is known as reverse flow control. At part load, the system delays the closure of the suction valves at the start of the compression stroke. As a result, some of the gas in the cylinder is pushed back out through the suction valves before the actual compression starts. This means that only the amount of gas required by the process is compressed, so the compressor uses less energy.

HydroCOM uses hydraulic actuators to move the ‘unloaders’ that hold the suction valves open at the appropriate points in the compression cycle. A central hydraulic unit supplies oil to the actuators. The timing of the actuators is controlled electronically from a unit located in the plant’s switch room, which communicates with the main distributed control system.

Old-style capacity control by recycling discharge gas back to the compressor inlet, in contrast, is wasteful because it requires the compressor to run at full power regardless of throughput. Stepwise capacity control systems such as clearance pockets fare a little better, but they waste energy unless the process demand exactly matches one of the control steps – typically 25%, 50%, 75%, and 100%.

All-electric stepless capacity control
The new eHydroCOM capacity control system does everything that the original system can do, but with lower project and maintenance costs. Because it uses electric actuators for the suction valves, instead of the hydraulic actuators used by HydroCOM, it is simpler to install and easier to service. As a result, it is well suited to small compressors of a few hundred kW – though, as the Air Products project demonstrated, it is equally suitable for MW-class compressors.

The system architecture gives high-level control dynamics, so the compressor’s output can be changed within just three revolutions of the crankshaft. Avoiding trips caused by incorrect suction, interstage or discharge pressures is key to the smooth operation of any reciprocating compressor. With control performance no longer limited by a slow recycle valve, problems with upstream or downstream equipment are much less likely to result in a compressor trip.

eHydroCOM can be installed or retrofitted quickly, because of its ‘plug and play’ connections, and requires no cooling water or hydraulics. Maintenance is simple, with comprehensive self-test features, and no adjustments required after valve servicing. 
The system has been extensively field tested at customer sites including RAG Rohöl-Aufsuchungs Aktiengesellschaft in Haidach, Austria, and Air Products’ Litvínov plant.
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