Enhance processing with capacity control of reciprocating compressors

Improvements to refinery, petrochemical and gas processing are available from the full-range stepless capacity control of reciprocating compressors

Klaus Stachel and Markus Wenisch
Hoerbiger Compression Technology

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Article Summary

Industrial compression of gases such as hydrogen or hydrocarbons in refineries is growing due to increased demand for high purity fuels, bottom-of-the-barrel conversion technologies for heavy oil feedstock and increasing volumes of shale-based paraffinic crudes. In combination with low or even varying molecular weight hydrocarbons, reciprocating compressors are often the best 
and most economic solution 
for compression. Reciprocating compressors (‘recips’) are flexible, energy efficient and suitable for high-pressure applications.

In the past, reciprocating compressors were often considered unreliable, with difficulties in achieving precise control of their output capacity. Electric power was wasted because the compressor capacity was typically controlled by recycle valves, often referred to as bypass or spillback valves, in combination with step control. Step control is realised by means of compressor suction valve unloading systems or fixed volume clearance pockets. Due to the large control steps provided by such systems, compressors were not operated efficiently in terms of the electrical energy consumption of the main driver. Furthermore, the generally slow reaction speed of recycle valves does not allow for the most effective optimisation of the process.

With today’s increasing capacity and power ratings of new recips, advanced control systems can significantly reduce energy consumption by avoiding excessive recycling of process gas. On top of this, reciprocating compressors are frequently integrated into complex processes, with side streams or multi-stream compression, which requires precise control and operational flexibility.

Today’s refinery challenges

The refining industry is undergoing continuous technical changes in order to comply with stringent environmental regulations for clean fuel products against a backdrop of a wider variety of crude feedstocks ranging from heavy asphaltenic crudes to high API gravity shale crudes.

Processing of highly contaminated crude feedstock, with high sulphur content, represents one of the biggest challenges for existing plants. On the other hand, the industry has to face a clear market trend towards the quality improvement of light fuels for road, air and marine transportation, while demand for heavy fuel products for industry and power generation is declining. The conversion of such ‘sour’ crude into light and mid-
distillate products through hydration processes is a vital part of the production chain of a refinery, but it is also afflicted with high capital and operating costs and significant investments in the erection of new facilities or conversions of existing plants. Hence, flexible and sustainable production is of utmost importance.

As substantial parts of many 
refining processes, such as hydrocracking, hydrotreating, isomerisation and reforming plants, multi-stage reciprocating compressors are used to compress hydrogen at high pressure (up to 220 bar) for essential parts of the process.

A change in feedstock quality requires flexible plant operation involving advanced capacity control concepts for any reciprocating compressor. Therefore, many end users, compressor manufacturers (OEM), engineering companies (EPC) and process licensors are already taking advantage of such advanced control systems, either as an integrated system for new equipment or as compressor upgrade solutions for existing machines.

Conventional control concepts for reciprocating compressors
In previous years, many existing refineries were upgraded to produce clean fuels or process feedstock with higher sulphur content. Before the modification, the sole capacity of one compressor, running at full load, was sufficient to meet hydrocracker or hydrotreater hydrogen demand. A second compressor was kept in stand-by mode only to ensure equipment redundancy. With today’s growing demand for 
hydrogen, the sole capacity of a single compressor is no longer sufficient, and therefore many stand-by compressors have now become main units in order to satisfy the additional hydrogen demand. For new greenfield projects, the plant layout has also changed.  For new plants, a stand-by unit is often not foreseen, as a result of the desire to limit capital cost and also taking into account the higher reliability of modern recips.

Typically, the output capacity of two compressors running in parallel is higher than the maximum anticipated process requirement at full production output. Therefore, gas flow into the process itself has to be reduced by a control system to match precisely the prevailing process demand. Conventional control systems do not allow for effective process control. Some of the systems enable only incremental adjustment of the capacity (for example, 0%, 50%, 100%), while others drastically decrease the efficiency of the plant by the recycling of compressed gas back to the suction side of the machine. Various conventional methods used to control compressor capacity include recycle valve control, step control and clearance valve control.

Recycle valve control
The most common way of controlling the flow of a reciprocating compressor is the recycle or spillback control. Here, the compressor itself runs at full load or at defined load steps. Stepwise capacity variation is achieved by additional control devices, such as pneumatic compressor suction valve unloaders or fixed volume cylinder clearance pockets. In order to regulate the gas flow according to process demands, part of the compressed gas is re-expanded and recycled to the suction side, resulting in significant energy losses. Figure 1 shows a typical load scenario of a hydrogen make-up recycle application. The production requires a hydrogen (H2) flow, which corresponds to 80% of the compressor’s rated capacity. Consequently, the compressor delivers 100%, while 20% of the excess process gas is recycled to the suction side.

Another important aspect of recycle control is the risk that the process gas can become contaminated with, for example, catalyst debris, which is entrained in the compressor,  causing performance degradation, higher wear rates and reduced lifetime of performance determining components such as compressor valves, rider bands and main packings.

Step control
Step control, also known as ‘on/off’ control, is another widespread method to adjust the output of a reciprocating compressor. Capacity variation is achieved by permanently unloading the compressor suction valves of one or more cylinder ends. The possible variation of load steps is defined by the number of cylinders per compression stage. Looking, for instance, at a two-cylinder double-acting compressor with two stages, 0%, 50% and 100% capacity steps can be realised. Suction valve unloading is achieved by means of pneumatic actuators that deactivate the compression in the specific compression chamber.

It is an inherent disadvantage of step control that this method is only efficient as long as the required process gas flow is equal to the adjusted load step on the compressor. Otherwise, excessive gas has to be recycled through a spillback control valve, resulting in energy losses due to the re-expansion of already compressed gas.



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