Enhance processing with capacity control of reciprocating compressors.pdf

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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ndustrial 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.

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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 middistillate 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 equip-

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capacity of a single compressor is no longer sufficient, and therefore many stand-by The spillback valve adjusts compressors have now become the required recycle main units in order to satisfy the gas-flow by recycling either to the make-up feed additional hydrogen demand. or to the refinery network. For new greenfield projects, the plant layout has also changed. For new plants, a stand-by unit Recycle / net gas is often not foreseen, as a result compressor Depending on the of the desire to limit capital cost feedstock quantity and and also taking into account the quality, more or less 20% recycling of H2 higher reliability of modern recycle gas is to the suction side. generated and needs to recips. Since recycling rate is be re-compressed. proportional to consumed Typically, the output capacity power, energy is wasted by using the bypass valve for of two compressors running in continuous load control. parallel is higher than the maxiRecycle valve mum anticipated process 20% requirement at full production recycle Process / Recycle gas output. Therefore, gas flow into Reaction separation the process itself has to be 80% load required reduced by a control system to Gas feed match precisely the prevailing e.g. H2 100% capacity process demand. Conventional Treated product, Feed e.g. low sulphur control systems do not allow for diesel, gasoline, etc. compressor effective process control. Some (Controlled with pneumatic unloaders) Required H2 load, e.g. 80% of the systems enable only increof rated compressor Compressor supplies mental adjustment of the capacity. 100% H2, the remaining capacity (for example, 0%, 50%, Load depending on feed 20% of the gas has to be quantity and quality (sulphur 100%), while others drastically recycled through the content or other gas impurities). bypass. decrease the efficiency of the plant by the recycling of Feed compressed gas back to the suction side of the machine. Feed pump Various conventional methods Figure 1 Schematic layout of a compressor running on recycle-/spillback used to control compressor control capacity include recycle valve control, step control and clearment or as compressor upgrade solutions for ance valve control. existing machines. Bypass/spillback valve

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

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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 perfor- Figure 2 Compressor cylinder equipped with a fixed clearance volume mance determining components pocket (in front of the cylinder) such as compressor valves, rider lead to operational problems with upstream or bands and main packings. downstream equipment. Furthermore, with lubricated compressors, continuous cylinder Step control Step control, also known as ‘on/off’ control, is unloading leads to an accumulation of lubricaanother widespread method to adjust the output tion oil inside the unloaded compression of a reciprocating compressor. Capacity variation chamber, which is then discharged together with is achieved by permanently unloading the the process gas as soon as the specific cylinder compressor suction valves of one or more cylin- end is loaded again. The resulting oil slug can der ends. The possible variation of load steps is damage the compressor valves and consequently defined by the number of cylinders per compres- lead to an unplanned compressor stop for sion stage. Looking, for instance, at a maintenance. Due to the low degree of automation of many two-cylinder double-acting compressor with two stages, 0%, 50% and 100% capacity steps can be existing step control systems, changes of realised. Suction valve unloading is achieved by compressor load often have to be initiated means of pneumatic actuators that deactivate manually by the operators. In practical terms, the compression in the specific compression these devices are rarely used and the compressor runs at full load. In many cases, experienced chamber. It is an inherent disadvantage of step control plant personnel are required to be available for that this method is only efficient as long as the compressor starts, stops and switch-over procerequired process gas flow is equal to the adjusted dures in order to maintain stable process load step on the compressor. Otherwise, exces- conditions at all times. sive gas has to be recycled through a spillback control valve, resulting in energy losses due to Clearance volume control the re-expansion of already compressed gas. If the compressor is equipped with fixed or variWhenever the compressor is switched to a able clearance pockets, additional clearance higher or lower load, sudden flow changes are volume can be added to the compression chamgenerated. These flow changes cause pressure ber. The compressor output reduces if the fluctuations in the process, since the recycle clearance volume of the cylinder increases. control valves are not fast enough to compensate Figure 2 shows a fixed volume clearance pocket this disturbance effectively. Pressure fluctuations installed on the cylinder cover. The fixed clearhave a significant impact on the process and can ance pocket enables a 75% load step in addition


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Therefore, this technology is not suitable for refining and petrochemical applications. In most cases, the operators do not utilise variable clearance pockets and, consequently, the capacity is again controlled by the recycle valve.

Advanced control system

Figure 3 Electro-hydraulic reverse flow control system

to the 0%, 50% and 100% load steps per cylinder, which can already be achieved by suction valve unloaders. However, recycle valve control is still required to precisely match the process requirement. Stepless capacity variation can be achieved by means of variable clearance pocket control, which enables a capacity turndown down to around 60% per cylinder. However, variable volume clearance pockets for make-up and recycle gas compressors can only be adjusted when the compressor is stopped and de-pressurised. P

P

Dr

A

A solution to overcome the previously discussed obstacles is to make use of a stepless, fullrange control system based on the ‘reverse flow’ principle. Here, compressor output is regulated steplessly by means of hydraulically actuated and electronically controlled actuators (see Figure 3), which are directly installed on the suction valve covers of the compressor (stepless valve unloaders). The reverse flow principle achieves capacity variation by actively controlling the start of the compression stroke. The working principle is outlined in Figure 4. At full load, compression starts at point C when the piston is in bottom dead centre (BDC). At part load, the compression starts at Cr. Since the volume in the cylinder gets reduced from C to Cr, a portion of the gas

Dr

A D

D Energy savings compared to recycle valve control Cr

Cr B

C

C

B V

TDC

BDC

V

TDC

BDC

Figure 4 Working principle of the stepless reverse flow control: left: 100% capacity; right: part load

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Indicated power, %

Annual savings, ×1000€

flows back to the suction cham3000 ber. Hence, only the gas flow 70 €/MWh required by the process is 100 €/MWh 2500 compressed. 120 €/MWh Since the indicated power of 2000 the compressor is proportional to the gas flow rate being 1500 compressed, the power consumption of the electric 1000 driver decreases at part load, resulting in significant energy 500 savings. Figure 5 shows the potential of power cost savings 0 100 90 80 70 60 50 40 30 20 10 0 for a mid-size hydrogen Average compressor load, % compressor with 3.0 MW indicated power (power required for compression). The graph Figure 5 Possible annual power cost savings for a 3.0 MW compressor based shows annual savings in power on 70 100 and €120/MWh energy costs as function of required compressor costs as a function of annual capacity to match process demand average compressor output and various costs for electric power 100 (70, 100 and €120 per MWh). Step control 90 This reveals huge savings Reverse flow control Energy savings 80 potential, thus allowing for a Ideal control system high return on investment and 70 very short payback periods for 60 the stepless control system. 50 Furthermore, active control of 40 the suction valve closure veloc30 ity reduces the impact forces of the valve sealing element, 20 hence increasing suction valve 10 durability and maintenance 0 intervals, and reducing mainte0 10 20 30 40 50 60 70 80 90 100 nance costs. Since the run time Capacity, % between compressor stops is often limited due to the lifetime of the compressor valves, Figure 6 Comparison of the compressor power consumption between step service intervals can thus be control and reverse flow control as a function of the compressor capacity increased, and production periods can be extended. Provided such a system is considered for new By using advanced capacity control technology, compressor projects, it can considerably reduce the recycle valve remains fully closed during the capital costs for the compressor where enernormal compressor operation with process gas. gy-efficient operation is required (see Figure 6). Compared to conventional systems, stepless By using an electro-hydraulic control system reverse flow control features the highest control as an integrated part of the compressor, the dynamics and contributes significantly to number of cylinders per stage can be reduced. improved process flexibility. In case of process Hence, the capital costs and footprint of the disturbances, such as those caused by problems compressor are minimised. Furthermore, fewer with up- or downstream equipment, the system cylinders mean reduced maintenance costs, immediately manages to control pressure fluctua- shorter shutdown periods for overhauls, and tions and most likely avoids a plant shutdown. fewer spare parts to be kept in stock. The higher


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Fast response time of control system allows for precise control of hydrogen and feed (H2-to-feed ratio) for optimised reactor operation and extended catalyst cycle time. Feedstock

Possibility of heater power savings and emissions reduction. Fired heater

Recycle gas

Reactor

Increased product quality and more process stability due to advanced compressor control.

HP separator Heat exchanger

Product − low sulphur diesel

Bypass/spillback valve


Compressor A make-up

Compressor A recycle

Reverse flow control




Compressor B make-up

Compressor B recycle



Possibility of additional compressor power saving due to stepless control of recycle stage and accurate adjustment of recycle gas stream

Pump

Hydrogen

Possibility of compressor power savings due to stepless control of make-up stages. Multistage operation is perfectly balanced due to automatic interstage pressure control.

Figure 7 Schematic HDS plant layout with integrated stepless flow control on make-up/recycle compressor

efficiency reduces electric power costs, and the high degree of automation allows for precise plant control and fine-tuning of process parameters, such as gas pressure, hydrogen/feed ratio and gas flow rate. The following example explains the impact on the compressor arrangement based on the chosen capacity control system. A reciprocating compressor shall compress process gas in two compression stages into a process that typically runs at 70-90% load of the rated capacity. When

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considering pneumatic suction valve unloaders and spillback valve control, a fourcylinder compressor with two cylinders per compression stage would be required to enable at least 25% load increments to run, relatively efficiently, at loads between 70% and 75%. However, between 100% and 76% load, the dispensable process gas still needs to be recycled through the spillback valve. In this worst case, up to 24% of the rated compressor power is wasted. Additionally, installed fixed clearance


0-100%

MIN selector

50-100%

0-50%

MIN selector

0-100%

0-100%

Split range

PIC 001

8PT 001

H2 feed

Split range

PIC 002

Split range

8PIC 003

8PT 002

Make-up 1st stage New signals / DCS logic Existing signals / DCS logic

FIC 001

8FT 001

Recycle stage

Make-up 2nd stage

HP separator

8PT 003

Amine scrubber

Figure 8 Control concept integration based on existing spillback control

pockets would improve the situation slightly, but the control logic would become more complicated and access to the compressor for maintenance impeded. However, two cylinders per stage are still required as compared with stepless control. The same application with a stepless full-range flow control system would require only one cylinder per compression stage (two cylinders in total for the compressor) since the load steps for the complete required turndown range are negligibly small. New compressors are normally designed with 10-15% surplus capacity to provide a margin for performance losses and degradation. Therefore, new compressors are running at part load even at full production output.

Advanced control system cases: refining applications Hydrogen desulphurisation unit

Two make-up/recycle compressors are integrated into a hydrogen desulphurisation unit. Each compressor has four cylinders and a motor rating of 5.0 MW. The plant scheme is provided in Figure 7. The make-up section of the compressor includes two stages, with one cylinder per stage, and discharges hydrogen at a pressure of


around 80 bar. The control requirement is to automatically maintain pressure in the high-pressure separator. The flow of the recycle gas, coming from the amine absorber, is recompressed by the recycle stage of the compressor and mixed with the fresh hydrogen stream coming from the make-up section. Prior to the plant upgrade, one compressor was operated between 90% and 95% and the second machine was kept in stand-by mode. In the course of the plant modification, to enable better treatment of highly contaminated feedstock, hydrogen demand increased to about 120% of the rated capacity of one compressor. Thus, both compressors were required to run in parallel — ideally, each at 60% part load. Therefore, the proposed plant layout included a stepless capacity control system, enabling efficient part load operation on both machines, without the use of the recycle control valve. The existing compressors were upgraded with electro-hydraulic unloaders and their associated auxiliaries, such as the control signal interface unit and hydraulic unit. Neither civil works nor modifications to process gas piping were necessary. The existing control logic and operator interface in the distributed control system (DCS), which were originally based on recycle

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fresh hydrogen coming from a side stream. The third stage forwards the hydrogen mix as make-up gas for the hydrotreater. Due to the integrated machinery concept and the requirement to control each stage individually, the licensor and the engineering company jointly decided to implement an electro-hydraulic capacity control system. But the resulting energy savings are also worth mentioning: the compressors were operated at 60% load on average for the Figure 9 Multi-service compressor with stepless reverse flow control on test bench first few years, until the final expansion phase of the plant valve control, were slightly modified by adding was finished. During this time, the refiner saved split range function blocks for each stage (see approximately €3.5 million/y in power costs, Figure 8). The programming efforts for the allowing a return on investment within two distributed control system were low and the weeks. Stepless full-range capacity control entire installation was carried out within a shut- systems are an enabler for cost-effective intedown period of two weeks. grated machinery arrangements.

Multi-service hydrogen compressor

Conclusion

Reverse flow control is capable of controlling each compression stage individually. Therefore, multi-stage compressors or applications with side streams between compression stages are perfectly controlled and pressure balanced. Furthermore, reverse flow control fully supports integrated machinery concepts and plant layouts where one machine handles multiple processes (multi-service compressors). The next case study shows the application of reverse flow control to three 14MW hydrogen compressors, which were part of a large refinery project in Asia (see Figure 9). Two compressors run in parallel and the third one serves as a stand-by unit. Each compressor consists of three stages and compresses hydrogen into three different process sections of the refinery. The first stage supplies fresh hydrogen into a desulphurisation plant for light gasoline products. The hydrogen flow is mixed with the feed and has to be accurately balanced, depending on feed quantity and quality. The second stage receives the hydrogen recycle gas from the amine scrubber of the same unit and recompresses it into a treat gas unit together with

With today’s growing compressed hydrogen demand for treating lower quality crude oil, plant flexibility has become more and more important. In order to control refining processes more precisely, stepless reverse flow control on reciprocating compressors allows for fine-tuning of process parameters, such as gas flow or pressure. Compared to conventional traditional control systems, the stepless reverse flow principle enables energy savings because recycle or spillback operation is avoided, since just the required gas amount is compressed and forwarded to the plant. Furthermore, pressure peaks due to sudden load changes caused by cylinder unloading can be avoided and the consequential risk of plant shutdown is reduced to a minimum. The high degree of automation allows for individual control of compression stages, thus enabling balanced compression of multistage machines, as well as the implementation of complex control concepts (multi-service compressors). With more than 1000 advanced control systems installed, the system has proved to be a reliable and sustainable solution for new and

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existing compressor applications in the refining, chemical, natural gas and shale gas industry. This article is based on a presentation by Alberto Vargas, Hoerbiger Corporation of America, Inc., at the 5th World Refining Technology & Shale Processing Summit 2013 (5th WRTS) on 4-5 December 2013 in Houston. Klaus Stachel is Product Manager, Monitoring and Controls, with Hoerbiger Compression Technology in Vienna, Austria. Email: [email protected]


Markus Wenisch is a Senior Product Expert with Hoerbiger Compression Technology in Vienna, Austria. Email: [email protected]

LINKS More articles from: Hoerbiger Compression Technology More articles from the following category: Rotating Equipment

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Enhance processing with capacity control of reciprocating compressors.pdf

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