Unit Operations

Improve Fuel Efficiency with Economizers

Boilers, fired furnaces, and kilns are extremely inefficient and are often the major consuming of energy in many different processes. The ability to recover wasted heat is critical to achieving optimal energy efficiency, capacity and profitability.

The majority of heat and energy losses in a furnace can be split into the following categories:

  • Waste gas
  • Shell losses
  • Heat in the product - i.e. steam

Waste gas energy losses can then be further divided:

  • Quantity of waste gas - excess air
  • Temperature - enthalpy
  • Composition - CO content, humidity, incomplete combustion

The amount of heat lost through the waste gas is a significant portion of every unit operation involving combustion, however it can be harnessed. This will improve the furnace’s thermal efficiency and reduce total plant fuel consumption. There are two main ways this can be done

  1. Reduce the quantity of waste gas through excess air control
  2. Reduce the temperature of the waste gas by installing an economizer

Simply put an economizer is a heat exchanger which preheats a boiler’s water feed by using the hot waste gas.

For furnaces other than boilers, for examples calciners or kilns, the same concept can be used, however another liquid stream may be required to reduce total plant energy requirements.

The economic justification for upgrading a boiler by installing an economizer can be made by simply looking at the potential in temperature reduction on heat loss. The potential enthalpy that can be recovered by reducing the stack temperature by 10 degrees can often be significant, and the temperature drop is usually much greater than this.  From this the design of an economizer in terms of heat exchange area, and liquid flows can be made. From this it is virtually the same as designing a heat exchanger, although the conditions particularly for high sulfur fuels can be extreme.

General rule of thumb’s suggest that more than 5% fuel consumption reductions can be achieved by installing an economizer, with additional 5% improvement possible with a condensing economizer - which condense the stack gases to additionally recover the latent heat of vaporization.

The re-use of heat is the primary method for improving fuel efficiency, and an economizer is the re-using heat in the most basic sense. Looking further afield synergy between unit operations can be taken into consideration when looking for even greater energy gains.

More Reading:

Consider installing a condensing economizer - US Department of Energy

Reduce Energy Losses by Monitoring Steam Trap Failure

Steam traps are used throughout process plant’s steam systems to remove condensate while losing the minimum amount of steam to the atmosphere. In large plants there can be over 1000 steam traps.

A significant amount of energy can be lost if a steam trap fails open - releasing steam to the atmosphere on a continual basis. For plants which do not have steam trap monitoring systems or preventative maintenance, these steam traps can leak for months or even years.

Steam is extremely expensive to produce, so any losses can result in huge costs very quickly. When considering the capital cost for boilers, the cost of makeup water, fuel, maintenance, and operation it can come to as much as $20 per tonne.

A single steam trap can lose as much as 15 kg/hour after failure, which depending on the temperature and pressure is a significant energy loss over time. This can be calculated by looking at the enthalpy of the stream and the number of traps that have failed.

Based on a trap lifespan of 4 years, it can be assumed that 25% of traps will fail every year, and knowing the cost of producing steam the cost of not repairing the failed traps can be calculated. This is an exponential cost over time as more and more traps fail.

Using the following assumptions, the difference in costs between not having a steam trap monitoring system and repairing all steam traps on a 6 month basis can be seen below:

  • $10 per tonne of steam
  • 15kg/hour steam loss for a failed steam trap
  • 25% yearly failure
  • All steam traps repaired after 6 months
  • $500 to replace a steam trap

In this example the $60,000 repair bill every 6 months can seem a lot just to save a little steam, and as can be seen it takes over 12 months for the savings to start to be seen. Savings of over $800,000 can be seen over a 3 year period, and saving nearly 150,000 tonnes of steam.

Introducing an extensive steam trap survey can be expensive and time-consuming, but it can pay for itself by reducing total plant losses. As steam traps fail their continual loss of steam becomes common place - often to the point where operators do not realize they should not be expelling so much steam. This can lead to a lack of urgency in repairing traps and stopping the energy loss.

Emerson were able to reduce energy losses on a major food manufacturing facility by installing wireless flow meters on steam traps. They found in a survey that 22% of the steam traps had failed for an unknown amount of time. With the installation of flow meters these steam traps could then be monitored continually, and if a leak was discovered it could be repaired or replaced - saving months of energy losses and tonnes of steam.

The use of instrumentation also introduces the additional capital cost as well as the cost of maintaining and calibrating more equipment, which may not be the preferred option of sites with limited resources. The cheaper option may be to simply set up a regular survey of all failed steam traps so that maintenance can be planned.

With ever increasing energy costs reducing steam losses is an easy way to improve process plant’s profitability by improving total fuel efficiency.

More Reading:

Understanding Steam Traps - Chemical Engineering Process

Improve Combustion Efficiency with Excess Air Control

Boilers, fired furnaces, and calciners are among some of the most common pieces of equipment in any processing plant, and often consume the majority of the site’s fuel. The increase in their fuel efficiency will often result in significant economic benefits and the simplest way to do this is through excess air control.

Combustion is used in an extremely wide range of practices - boiling water into steam as a heat transfer media, calcining product such as limestone or alumina, transforming chemical energy into mechanical energy or electricity. Combustion requires a certain amount of oxygen for the reaction to fully occur, which is provided through air. Unfortunately air only contains approximately 21% oxygen, with the majority of the remainder being nitrogen. Due to the exothermic nature of the combustion reaction the nitrogen is also heated, which is then vented to the atmosphere through a stack. This is a direct loss of energy as heat to the atmosphere.

Most furnaces are run with too much excess air to ensure complete combustion, as well as to reduce the risk of an explosion if there is a potential for secondary ignition - i.e. the presence of an electrostatic precipitator to clean the waste gas on fired calciners. Insufficient air will also result in incomplete combustion, leading to high levels of Carbon monoxide, soot, ash, and unburnt fuel in the waste gas emissions.

Depending on the accuracy and confidence in the air flow meters, an air to fuel ratio control can be used to stabilize the excess air regardless of the furnace load. From this an online O2 or CO monitor can be used as a trim to ensure the optimal amount of air is being used at all times. The optimal excess O2 can be determined by measuring the CO concentration in the flue gas and reducing the oxygen content until the CO concentration begins to increase.

This control system requires strict control over both the fuel flow rate and the air flow rate through blower output/speed.

With the ever decreasing cost of instrumentation control systems such as this can be installed on smaller and smaller pieces of equipment and still prove to be economically viable.

The potential energy savings can be easily estimated:

  • Calculate the stoichiometric oxygen requirement for a set fuel flow
  • Determine an appropriate level of excess oxygen - usually 2-3%
  • Convert the oxygen requirements into total mass flow of air
  • Calculate the total enthalpy of the stack gases
  • Compare the difference between current stack energy losses and with excess oxygen control
  • Determine the cost of energy through fuel cost and calorific value

Introducing an excess air control system will also result in the following additional benefits:

  • Increased combustion efficiency
  • Reduced fuel costs
  • Reduced NOx emissions

Once excess air control is in place further fuel efficiency upgrades can be made, such as economizers to reduce stack temperature, or extra insulation to reduce shell losses.

Troubleshooting Guide Compilation for Centrifugal Pumps

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Centrifugal pumps are one of the most common forms of unit operations in the world. They are the heart of any processing plant and their reliability and performance are intrinsically linked to total plant performance. The ability to identify pump performance degradation, impeller wear, cavitation, as well as perform adequate preventative maintenance is critical to getting optimal results.

Luckily, because they are the most popular piece of equipment for transporting liquids there is also a huge amount of literature and guides available, and one of the best pieces of advice is not to re-invent the wheel. To go through every possible pumping problem and troubleshooting tips is extremely time-consuming and tedious. This guide is a compilation of some of the better quality pump troubleshooting guides available online, and is an informative resource for engineers seeking answers.

The Centrifugal Pump - Grundfos

Slurry Pumping Manual: A Technical Application Guide for Users of Centrifugal Slurry Pumps and Slurry Pumping Systems - Warman International

Warman Slurry Pumping Handbook - Warman International

Pump Principles Manual - Chesterton

Slurry Handbook: Guidelines for Slurry Pumping - Flygt

Centrifugal Pumps: Basic Concepts of Operation, Maintenance, and Troubleshooting

Pumps: Selection and Troubleshooting - Birla Institute of Technology and Science

Pump Vibration Troubleshooting - Mechanical Solutions

 

Offline Resources:

Centrifugal Pumps - J. Gulich

Troubleshooting Centrifugal Pumps and their Systems - R. Palgrave

Pump Selection and Troubleshooting Field Guide - R. Beverly

Practical Centrifugal Pumps: Design, Operation and Maintenance - P. Girdhar

Troubleshooting Process Operations - N. Lieberman

 

Still having problems - ask the manufacturer.

If you have other guides or pieces of information that you believe is important please share it.

Maximize Plant Throughput by Optimizing Wash Frequency

Fouling and scale buildup is the bane of any processing plant. It restricts capacity, reduces heat transfer, increases cleaning requirements, fuel consumption, and total costs.

In some high fouling industries pumps and pipelines can require cleaning and washing on extremely high frequencies, and the ability to identify when these become a ‘bottleneck’ can increase total plant throughput.

The type of equipment which can foul and require cleaning is vast, and can include anything and everything:

But how can you identify when a piece of equipment’s throughput or performance is degrading?

Every piece of equipment is different, but the key identifier is the same - historic key performance indicator degradation. The Key Performance Indicator (KPI) should be identified for every critical piece of equipment, but can be looked at for individual pipelines as well.

For example, take a heat exchanger with the purpose of recovering as much heat as possible (that is, it is not controlling to a set point):

In terms of fouling and wash requirements, the KPI would be total heat transfer (MW). If the heat transfer is continually reducing over time then the economic breakeven point can be used.

Another example is a simple pump. Looking at the pumping efficiency, or another common identifier is pumping throughput at maximum speed. If 6 months ago a pump was able to achieve 300 kL/hr at 100% speed, but is now only able to achieve 250 kL/hr, then the possibility of a wash should be considered (as well as pumping maintenance). This could be a result of solids buildup on either the pump suction or discharge or both. Looking at line pressures is often enough to identify the location of any restrictions.

Another tip to identify solid buildups within pipes where the fluid is hot is a thermal imaging camera - the buildup will act as an insulation and will result in a cold spot, further proof of washing requirements.

All different pieces of equipment have different KPIs which can be used to identify performance degradation. This degradation can then be tracked and measured to estimate the rate of scale build up, or fouling rate, leading to an improvement in planning and wash frequency.

“Success is simple. Do what’s right, the right way, at the right time”

Arnold Glasow

Washing frequency is often one of the simplest ways to maximize plant performance, throughput, and efficiency. The rule of thumb is to complete the wash before the restriction becomes a restriction.

Heat Exchangers - Optimizing Operations through Cleaning Frequency

Heat exchangers are one of the most common piece of equipment and are present in almost all processing plants. Their design is very well understood, however their operation often has opportunities for improvement.

Deutsch: Industrielle Hochdruckreinigung von W...

Photo credit: Wikipedia

Depending on the circumstances and fluid conditions heat exchanger condition may degrade rapidly due to tube leaks, or fouling buildup. This results in poor heat transfer, which can impact downstream processes or lessen heat recovery leading to increased costs.

Q = UA * LMTD

Once installed the only way to improve heat exchanger performance is through the temperature differential or by increasing the heat transfer coefficient, usually through optimizing cleaning schedules. Without replacing the heat exchanger or expensive modifications the heat exchange area is difficult to increase.

There is a big difference between calculating the rate of degradation in performance and scheduling and organizing heat exchanger cleaning. Graphing the change in UA over time through several cycles vs the required outlet temperatures the cleaning schedule can be identified more accurately. A minimum required performance can be targeted based on required temperature pickup or desired heat recovery, allowing an appropriate level of cleaning to be schedules ahead of time.

This data can also be used to be converted into dollars, which is always more justifiable. The reduction in performance and temperature vs the heat recovery and cost of energy will result in a breakeven point around which cleaning may be based. As can be seen in the graph below, an increased frequency of cleaning may or may not be economically justifiable. Naturally this will depend on the circumstances.

Before optimizing a heat exchanger the question must be asked of the purpose of the exercise - what are your trying to achieve?

  • Is the heat exchanger fouling more quickly than expected and requires additional cleaning just to maintain required outlet temperatures?
  • Is there potential to recover more heat to reduce overall fuel consumption?
  • Is the heat exchanger currently cleaned far too frequently, and the required performance can be obtained by reducing the schedule frequency?
  • Is the cleaning not as effective as historically, leading to further concerns about equipment condition?

There are large opportunities in reducing costs and improving fuel efficiency through optimizing heat exchanger performance, but only if the correct equipment indicator is targeted.

Filtration Fundamentals - Optimizing and Improving Filter Performance

Filters are generally used to separate the liquid and solids from within a slurry. Their impact on downstream processes are critical and their performance can have a large impact on their entire sites operability. Filters are very well understood, and the ability to optimize them for improved performance is well documented. Although every filter is slightly different they all follow the same principles. The process of optimization should always be done in a systematic manner.

Source: CDEGlobal

Filtration may be defined as the separation of solids from liquids by passing a suspension through a permeable medium which retains the particles.

L. Svarovsky, Solid-Liquid Separation

1. Viscosity

The viscosity of the slurry/filtrate is a key consideration as it will have a significant impact on the total filtration rate. The lower the viscosity the easier the liquid flows, and the less time it takes to be removed from the solid cake. The viscosity of any solution can be controlled in a variety of ways, such as changing the temperature or the concentration. Specialty chemicals, also known as drainage aids, are also available which can change the properties of the solution to improve filterability.

When considering your ability to alter the viscosity to improve the filtration rate a number of questions must be asked:

  • Will the increased energy consumption required to increase the temperature be economically justified by the increase in filter performance?
  • Will the change in filtrate properties have an impact on downstream processes?
  • What will the impact of diluting the solution be?

2. Specific Cake Resistance

The actual resistance of the cake is critical and one of the most important factors. It can be measured relatively easily through lab experimentation, so any changes can be clearly observed when experimenting or in site trials. The resistance caused by the cake can also viewed as the voidage between individual particles.

  • Particle size
  • Particle shape
  • Cake thickness
  • Compressibility

There are also several methods which can be used to reduce the specific cake resistance which leads to improved filterability:

  • Increase the particle size to increase the voidage
  • Use of filter aids - additional larger particles added to the slurry which are used to increase the voidage
  • Reducing the cake thickness - can be a result of reducing the online time or filter speed

3. Medium Resistance

The medium is the physical barrier which separates the solid cake from the liquid filtrate and is one of the easier filter attributes the change. The weave size is chosen to satisfy a combination of filtration rate and filtrate solids. There are lots of circumstances which can change the resistance of the filter medium, including:

  • Cloth blinding
  • Tears and rips

Filter cloths blind when the particles block up the pores, resulting in decreased filter performance. The impact of blinding can be reduced by washing or re-clothing, which can be optimized by identifying the relationship between operating time, particle size, and the blinding rate.

Tears and rips in the medium also have a significant impact because they allow the solids to bypass the filter through the path of least resistance. Changing to a more durable cloth can often be economically justifiable in filter performance increases.

4. Driving Force - Pressure/Vacuum

The appliable pressure on a filter is limited by the maximum operating pressure of the mechanical components or the maximum pressure available from the pumping systems. Additional or larger pumps may not be possible if the filter cannot handle it. Increasing the number of vacuum pumps is often a costly process when considering the change in filtration rate.

Filters are a great unit to work with and operate because they are one of the few systems where their performance can often be visually observed as well as through lab samples and online monitoring.