White Paper
Ten Ways to Reduce Energy in a Process Facility
September, 2011
Seven Ways to Reduce Energy in a
Process Facility
White Paper
Bart Carpenter
Senior Consultant
Merrick & Company
2450 S. Peoria Street • Aurora, CO 80014-5475
Tel: 303-751-0741 • Fax: 303-751-2581
www.merrick.com
White Paper
Seven Ways to Reduce Energy in a Process Facility
September 2011
Seven Ways to Reduce Energy in a Process Facility
Energy conservation has been practiced for many years but there are surprisingly good
opportunities still available within the process industries. Many opportunities can be realized
without capital investment; those that do have the benefit of being “low risk” because the
technology is well proven and savings are easy to quantify. In the future, rigorous energy
management will be increasingly important as countries and companies work to reduce their
carbon footprint. There are literally hundreds of ways to save energy. Listed below are
seven common ways to save energy in a process facility and improve plant profitability.
1. Tune Fired Heaters Frequently
Fired heaters often represent the single largest energy use in a process facility and they
should be tuned frequently to insure excess air is maintained at target levels. This can be
achieved by implementing a furnace monitoring program to include the following activities:
o
Determine excess air targets for each fired heater in the facility
Most fired furnace data sheets will indicate the excess air (or excess oxygen) rate
the furnace was originally designed to meet. However, the original targets can
sometimes be unrealistic for a variety of reasons. New targets should be established
by a furnace expert who can monitor draft, stack combustibles, flame patterns, etc. to
establish realistic operating targets. This expert can also provide refresher training
to the people responsible for tuning the heaters.
o
Quantify the incentive to meet excess air target for each fired heater
The economic incentive to tune fired heaters generally follow the “80/20 Rule”, i.e.,
80% of the savings can be achieved by focusing on 20% of the heaters. A good
plant engineer can set up real-time charts to monitor furnace excess air levels and
the added operating cost of not meeting targets. This information helps the
operators to prioritize their time on the heaters that have the largest economic
incentive. Large heaters with high stack temperatures generally provide the best
opportunity for improvement.
o
Make meeting furnace excess air targets a priority
Tuning furnaces is nothing new, unfortunately, it is often treated as a mid-to-low
priority task. Meeting excess air targets needs to be a priority, especially for those
heaters that provide the largest savings. If furnace excess oxygen levels are not a
priority for management, they won’t be a priority for the operators. Treat off-spec
heaters, especially large heaters without stack heat recovery, with the same urgency
as an off spec product.
Recommended Reading: “Optimize Fired Heater Operations to Save Money”,
Hydrocarbon Processing, June 1997
2. Produce On-Spec Products
Two easy ways to waste energy in a process facility are to produce off-spec product and
product at a better quality than necessary. Both need to be eliminated.
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©Copyright 2011 Merrick & Company
White Paper
Seven Ways to Reduce Energy in a Process Facility
September 2011
Off-spec product is often recycled through the facility, consuming as much energy as it did
the first time through and consuming valuable plant capacity. A careful review of the yield
statement and the operator log will help pin-point the largest opportunities. A thorough
review of the process, including statistical analysis, can help identify and correct the root
cause.
More often than not, a facility wastes more energy making better quality than what is
necessary to avoid producing off-spec product. Every specification should be reviewed
periodically to insure they meet market requirements or internal plant needs.
o
Most process facilities produce a variety of intermediates, i.e., streams that must be
further processed before they become finished products. Specifications for these
streams must also be reviewed to insure they have a good basis as opposed to “this
is the way we always do it”. Answer the question “What quality is actually required
and why?”
o
Occasionally there is confusion or misunderstanding concerning product
specifications. If in doubt, check with your customer. For example, one large midwest refiner was producing 208 psia propane when the vapor pressure specification
called for 208 psig. This small discrepancy was worth over $500k/year once
corrected.
o
Sometimes variation in the process requires operations to consistently produce a
better quality product than what is needed. In these cases, one must reduce the
variation such that the operating target can be shifted closer to the actual
specification. Statistical processes and tools, such as Six Sigma and Minitab, can
help identify the sources of variation so they can be eliminated.
Minimum Specification
Reduce variation and
shift the mean
Reduced
Product
Giveaway
Savings
Average Product
Giveaway
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©Copyright 2011 Merrick & Company
White Paper
Seven Ways to Reduce Energy in a Process Facility
September 2011
Recommended Reading: “Six Sigma: What It Is and How to Use It”, Harvard
Management Update, June 1999
3. Optimize Feed Preheat Trains
There are numerous opportunities within a process facility to recover heat from hot streams
to cold streams and minimize the heat rejected to air or cooling water. Sometimes this
requires integrating heat sources to heat sinks in separate process units. Simple examples
include:
o
Preheating distillation tower feed with hot distillation tower bottoms product
o
Preheating reactor feed with hot reactor effluent, especially in units where the two
streams are about the same flow rate and tight temperature approaches can be
achieved
o
Reboiling a distillation tower using pumparound heat supply from a multi-draw
fractionator
o
Reboiling a distillation tower using the overhead stream from another distillation
tower
o
Preheating feed or generating steam using hot flue gas from a fired heater
Good engineering design practice balances the cost of incremental exchanger area against
the energy savings. Note that in many applications, as heat recovery is increased the
downstream air or water cooler is reduced in size and the load on the cooling tower is
reduced.
More complicated systems, such as a crude unit preheat train, can best be optimized using
modern pinch analysis tools such as Aspen® Energy Analyzer. These tools help optimize
complicated exchanger networks by matching hot and cold process streams with a network
of exchangers so that demands for externally supplied utilities are minimized.
Exchanger fouling is also an area that is receiving considerably more attention, especially as
turnaround cycles have increased to 3-4 years and the opportunities to clean exchangers
during turnarounds is reduced. Exchangers should be designed to minimize fouling by
choosing the proper exchanger type and designing for minimum velocities. Additionally,
certain exchangers that exhibit high fouling rates should include the appropriate piping so
they can be cleaned between turnarounds as unit monitoring dictates for energy efficiency
or unit throughput.
Recommended Reading: “Optimization Application: Pinch Technology Analysis”,
http://highered.mcgraw-hill.com/sites/dl/free/0072392665/53827/ch09excerpt.pdf
4. Minimize Reflux Requirements
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©Copyright 2011 Merrick & Company
White Paper
Seven Ways to Reduce Energy in a Process Facility
September 2011
Distillation continues to be one of the work-horses for most process facilities to purify various
intermediate and product streams on the basis of boiling points. It is fairly common to see
distillation columns operating with more reflux than what is required to produce on-spec
products, resulting in higher than needed reboiler and condenser duties. Strategies to
reduce reflux include:
o
Replace existing trays with more efficient trays or packing and/or consider adding
additional trays
o
Upgrade the control system to avoid making a better quality product than required
o
Insure feed tray location is optimal via process simulation
o
Operate at lowest possible pressure as limited by tower flooding and/or overhead
condensing constraints
Reboiler duty can also be reduced by preheating feed to the column using a feed/bottoms
exchanger although the savings is generally not a 1:1 ratio. The optimal feed preheat
temperature should be determined via simulation to insure excessive feed preheat is not
actually increasing reboiler duty.
Recommended Reading: “Distillation Feed Preheat – Is It Energy Efficient?”
Hydrocarbon Processing, October 1993
5. Eliminate Steam Vents
A typical steam system consists of several different steam pressure levels as illustrated in
the sketch below. High pressure steam (600 psig is typical) is produced in fired boilers to
meet high temperature process needs and/or turbine drivers for pumps and compressors.
600 psig steam is also “letdown” into the mid-pressure header to maintain pressure on that
system, which is supplemented with additional steam produced in waste heat boilers. In a
similar fashion, mid-pressure steam is letdown to the low pressure header to maintain
pressure on the low pressure system.
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©Copyright 2011 Merrick & Company
White Paper
Seven Ways to Reduce Energy in a Process Facility
September 2011
A well designed and operated plant steam system will minimize steam vents, however, this
is rarely achieved due to the complexity and limited understanding of the plant-wide steam
system. Ideally, the boiler feedwater deaerator vent(s) should be the only visible steam vent
that operates on a continuous basis. Even though steam must be vented to insure oxygen
removal, the vent should be designed to only use what is required for good deaerator
operation. The deaerator vent should not serve as the plant low pressure steam vent.
Other vents that can and should be eliminated include:
o
Unbalanced System Vents – Ideally, the total steam condensing load is matched
against the steam produced by the primary and waste heat boilers. In order to
maintain a balance, the load must be matched thermally and on a pressure-level
basis.
o
Steam Turbine Vents – In general, topping turbines take steam at one level and
exhaust into a lower pressure header and should be utilized to minimize steam rates
across the letdown stations. Ideally, letdown stations should be limited to 5 to 15%
of the low pressure header demand for stable pressure control. Turbines that
exhaust to atmosphere should only be used in emergency services. On a similar
note, condensing turbines should also be avoided as they are also inherently
inefficient versus electric motors.
o
Hidden Vents - Steam vents are sometimes “hidden” from view, making their
identification and elimination more difficult to diagnose. Real life examples include
condensing steam in air or water coolers or discharging a steam vent into a cooling
tower.
A plant-wide steam audit is usually required to fully understand the overall balance,
including all flows into and out of boilers, steam and condensate headers and boiler
feedwater treating systems. A review of the steam system controls is also highly
recommended as oftentimes the control system is creating at least a portion of the
imbalance and the venting that results.
Recommended Reading: “Steam-System Design: How it Evolves”, Chemical
Engineering, October14, 1985
6. Reduce Pressure Drop Across Control Valves
It’s generally a good practice to survey control valve pressure drops in hydraulic systems
using centrifugal pumps and compressors and especially where the pump/compressor
horsepower requirements are large. Ideally, the centrifugal driver is sized to provide a
reasonable pressure drop across the control valve, typically 10-15% of the total hydraulic
losses or 10 psi, whichever is greater. If the driver is mis-matched with the hydraulic
requirements, the system will balance itself by taking additional pressure drop across the
control valve, consuming more horsepower than required for process control.
Evaluating centrifugal pump systems is fairly straight forward providing one considers the
operating flexibility required between minimum and maximum flow rates. If the pump is
putting up too much head, consider trimming the impeller or replacing it with a smaller pump.
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©Copyright 2011 Merrick & Company
White Paper
Seven Ways to Reduce Energy in a Process Facility
September 2011
One can also add a variable speed drive to the existing pump and eliminate control valve
losses altogether.
Fixed speed centrifugal compressors, on the other hand, are a little more difficult to evaluate
and “trimming the impeller” is more complicated. One must also understand the entire
system, including the process operating pressure and pressure drop, to evaluate the system
properly. Since the head requirement depends on the ratio of the discharge pressure to the
inlet pressure (absolute basis), small changes in inlet pressure have a much larger impact
on horsepower than corresponding change in discharge pressure. Efforts to increase
suction pressure can lead to very significant horsepower savings.
Sometimes even compressors with variable speed drives are putting up too much head as
limited by minimum speed constraints. In these instances, consider rewheeling the machine
to better match compressor differential pressure to system requirements.
Finally, spillbacks used for surge control are another easy way to waste horsepower and
should normally be closed. Check that they are fully closed and that they are not leaking.
Bottom line - it’s always a good idea to look at every large compressor while performing your
energy audit.
Recommended Reading: “Understanding Centrifugal Compressor Performance in a
Connected Process System”, Petroleum Technology Quarterly, Spring 2002
7. Design for Energy Efficiency the First Time
It is always easier to justify energy efficient technologies in the original design versus after
the process equipment is engineered and constructed. For example, a good process
engineer will balance the capital cost of additional distillation trays against the heat input
requirements to affect a given separation. Obviously, adding additional trays is much easier
to do during the original design versus adding additional tower height and trays with a
retrofit.
An Energy Checklist is a valuable tool to help insure good design practices are not
inadvertently overlooked and that energy efficient technologies are utilized to achieve a
reliable and efficient design. For example, the following questions should be asked when
designing new pumps:
o
o
o
o
o
o
Have I selected an efficient pump and motor?
Is the pump well matched to the hydraulic requirements?
If minimum flow protection is required, is it designed to be zero flow at sufficient flow
rates?
Can a smaller impeller be installed in the future?
Would a variable speed drive make sense to eliminate control valve losses?
Is the inlet and outlet piping sized correctly?
Recommended Reading: “Energy Efficiency Improvement and Cost Saving
Opportunities for Petroleum Refineries, An ENERGY STAR® Guide for Energy and
Plant Managers”, February 2005
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©Copyright 2011 Merrick & Company
White Paper
Seven Ways to Reduce Energy in a Process Facility
September 2011
Summary
An in-plant review is an excellent way to get a thorough review of your process facility to
uncover ways to save energy such as the seven ways described above. During this
evaluation, particular emphasis is placed on sometimes overlooked opportunities that occur
between process units for energy improvements. Once opportunities are identified, they can
easily be screened and prioritized based upon value, ease of implementation and cost. Best
of all, many opportunities can be implemented with little or no capital.
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©Copyright 2011 Merrick & Company