1. Energy efficiency in process industries

Environmental concerns are more and more pressing as global warming manifestations are causing various damages across the world. Many companies have accepted to play their part and included environmental protection, often through energy savings, in their company policies. If for some business, the main energy consumers are well identified, companies working in the field of bulk solids handling sometimes struggle to find obvious ways to improve their environmental footprint, evaluate the impact of changes or build viable investment proposals aiming at energy savings.

This article aims at reviewing the different energy savings potential that companies operating or designing powder handling processes can unlock. It focuses on common unit operations present in more or less all bulk solids processes (pneumatic conveying, mixing, pulse jet filters,…). Other sources of savings can be found but may be more specific to the industries where the process is operated and are not detailed in the article (spray drying, air handling in food or pharmaceuticals...).

Examples of savings calculation are given in the article. They aim at helping the reader in assessing the order of magnitude of the gain that can be expected, and guide him in building a case towards factory management to implement the changes required.

2. How to reduce energy consumption of bulk solids processes ?

2.1 Pneumatic conveying : Dilute Phase

Pneumatic conveying stands as a cornerstone in bulk solids processing, offering versatile material transport solutions. Dilute phase conveying, characterized by suspended solids in the transport air, represents a prevalent method due to its simplicity and adaptability. However, optimizing dilute phase conveying systems is paramount to curbing energy consumption. Modifications to consider : Enhanced Operational Practices: Collaborate with pneumatic conveying experts to fine-tune conveying parameters, optimizing blower speed and solids loading ratio for energy efficiency. Automation Integration: Implement timers and frequency drives to regulate blower operation, reducing energy wastage during idle periods.

In bulk solids processing industries, pneumatic conveying is one of the preferred solutions to transport materials in between unit operations. It can accommodate all sorts of layouts and offers a good containment of the materials. Dilute phase pneumatic transport, for which the solids conveyed is in suspension in the transport air is one of the most common widespread processes as it is quite simple to design and operate.

The air mover is generally a Roots blower that can be positioned either at the beginning of the line (pressure dilute phase) or at the end of the line (vacuum dilute phase).

Experience shows that the operation of the conveying line is not always well mastered by factory operators. What is at 1st important to the operators is that the line does not block and achieve a defined throughput, they are therefore tempted to run the blower at high speed which appears to them as being safer (which is in reality not quite true). Those blowers have motors of several kW.

The engineer looking for energy savings can then apply a quick sense check by calculating the air velocity in the conveying pipe and the solids loading ratio. Indeed, dilute phase conveying line have typical speed of 20 to 30 m/s at the end of the line and solids loading ratio around 5 to 10. If the values calculated for the line are significantly different with a higher conveying speed and a lower solids loading ratio, then there may be possibility to reduce the blower speed and save energy. A trial can be organized (always perform a risk analysis) with the help of a pneumatic conveying expert to find the optimum conveying parameters.

Example

A blower is used to convey a product at 4.5 t/h in a factory while the factory is complaining about high breakage of the product.

Current conditions

• Blower at 100%
• Performance datasheet of the blower indicate under 520 mbar g discharge pressure a flow of 1066 Nm3/h and a power on the shaft of 20.6 kW.
• The air velocity at product pickup is then 29.8 m/s at the discharge conditions of the blower
• The solids air ratio is 4.5/1.3 = 3.46

In most of the cases, the required pick-up velocity is around 16 to 20 m/s (it can be determined by experience or estimated thanks to correlations calculating the saltation velocity). The solids load ratio for a dilute phase conveying can be in between 5 and 10. The Engineer advises a trial to run the blower slower targeting 20 m/s at the beginning of the line.

New conditions

• Blower at 66% discharging around 700 Nm3/h of air
• The trial shows a pressure drop of 400 mbar g. Performance data of the blower are used to calculate the power on the shaft for this speed and pressure : 11 kW
• Solids load ratio is 4.5/0.858=5.2
• The pressure is stable and the Engineer decides to continue running those conditions

Savings can be calculated the following way

• The line runs 8000 h/y
• Total savings (20.6-11)*8000 = 76800 kWh
• Price of kWh in the country = 10 cents
• Savings = 7680 USD/year

It is interesting to note that optimizing the air velocity has also as advantages to reduce the material attrition during transport and reduce the pipe wear, thus bringing advantages in terms of quality and maintenance.

1 more interesting to note about dilute phase conveying is that in many cases the line is running… without conveying product ! To simplify the automation and avoid risks related to frequent start and stop of the blower, factory operators may be tempted to just let the blower run in between 2 transfers of product. This is an energy waste that can be tackled easily by implemented a timer to automatically stop the system after a certain time and, for blowers on frequency drive, decrease the blower speed during waiting time. This can bring quickly interesting savings for the factory at no cost.

2.2 Dense phase pressure conveying

Optimization Strategies: Conveying Regime Evaluation: Assess air velocity and solids loading ratio to ensure operation within the dense phase mode, minimizing compressed air consumption. Parameter Adjustment: Fine-tune timers and valve settings to achieve efficient material transport while minimizing energy expenditure. Flush Operation Elimination: Avoid unnecessary line flush operations post-transfer to further reduce energy consumption.

Dense phase pressure conveying can also be an important source of energy wastes if the process is not well mastered or maintained. Solids are conveyed thanks to compressed air that is pressurizing blow tanks and the conveying line. As compressed air is actually expensive to produce, any over consumption will quickly cost substantial amounts of money.

Checking the conveying parameters and especially calculating the mass air flowrate that is actually used to transport product is a good reflex. Pressure dense phase conveying should be designed to convey at low velocity, typically 3-8 m/s, and high solids loads ratio, typically > 30. If the Engineer arrives to the conclusions that air velocity is significantly higher and the solids load ratio significantly lower than these values, there is likely room for improvement. Getting in the right conveying regime will reduce the air consumption and generate savings.

Example

A factory is transporting a raw material in dense phase. When the Engineer is studying the process trends he realizes that the pressure is quite low, it prompts him to calculate the solids load ratio. The line conveys 4.5 t/h of product and uses around 350 Nm3/h.

Current conditions

• Solids air ratio = 4500 / (350*1.2) = 10.7

10.7 is a low solids load ratio, the conveying is probably NOT dense phase. The Engineer consults the pilot plant tests report and gets the info that this product can convey at a ratio of 30.

New conditions

• The air conveying flow rate can be calculated for a ratio of 30 : 4500/(30*1.2) = 125 Nm3/h
• A test is organized. The Engineer adjust the timers for opening the different air valves in the system and obtain a stable pressure. The conveying pressure is higher, as there is more material in the line, but the conveying is done in dense phase.

Savings

• The line is consuming 225 Nm3/h less of compressed air
• Cost of 1 Nm3 of compressed air in the factory = 1 cent

The line runs around 4000 h a year (batch conveying)

• Savings = 225*4000*0.01 = 9000 USD

Dense phase conveying systems should be designed to avoid strong line flush at the end of a transfer (even if this may be required due to some product specificities). Eliminating such operation from the conveying sequence can also lead to savings.

2.3 Filters

Operational Efficiency: Pulsing Interval Optimization: Determine optimal pulsing intervals based on dust load and filter size to minimize compressed air consumption. Pressure Drop Monitoring: Implement pressure drop monitoring to refine pulsing intervals and ensure effective filter cleaning without unnecessary energy expenditure. Automation Utilization: Employ automation systems to regulate pulsing intervals and synchronize filter cleaning with operational requirements for enhanced energy efficiency.

Many filters are equipped with a compressed air pulse jet system to unclog them. It is particularly the case of filters located at receivers of conveying lines, which are usually quite big (several m2). The pulse jet system is very often governed by a timer whose interval is very frequent. Visiting a factory, you can easily hear a filter whose pulse jet system is not properly set and therefore wastes compressed air.

For such filters, pulsing every 30s to 1min is usually a good starting point which can be refined by monitoring the pressure drop through the filter and adjusting in consequence. There is also no need to run the back-flush of the filter for long after the conveying has ended. Potential savings are usually more significant than expected by factory owners when considering all the filters present in a process.

Example

A small size filter of 6 m2 has a pulse jet system equipped with a 10 liters air tank compressed at 4 bar g for cleaning the filter. The filter is cleaned every 15 s but the dust load is low, prompting the engineer to try modifying the pulsing to 1 min.

Current conditions

• Air consumption / h = 3600/15*4*10 = 9600 l/h = 9.6 Nm3/h

New conditions

• Air consumption / h = 3600/60*4*10 = 2400 l/h = 2.4 Nm3/h

Savings

• Operation of the line = 8000 h/y
• Savings = (9.6-2.4)*8000 = 57600 Nm3/y
• With a cost of 1 Nm3 of 1 cent, the savings is 576 USD/y
• As there are many such filters in a plant, savings can amount from hundreds to thousands dollars a year. The savings will be more or less important depending on the size of the filter, as large filters can have compressed air manifold of up to 30 l.

2.4 Mixing (batch)

Mixers are usually fitted with several kW motors. Optimizing the process here is not always yielding very large savings but help to set up a mindset that, applied to the whole powder handling process line, will contribute positively to cost savings and environmental care.

The action here is to optimize the mixing time so that the mixer’s motor is turned on only when necessary. Industrial observations indeed show that batch mixers are very often operated away from their optimum point. Carrying out a proper mixing validation will help to define the minimum mixing time necessary to reach the manufacturer homogeneity target. Saving 1 to 5 minutes / batch can count when a mixer is operated very often. Calculating potential savings is explained below, one must note however that optimizing the cycle time of mixer will have as primary consequence to increase the production capacity, reducing the cost of production, avoiding the investment in new equipment and thus generating strong savings.

Example

A factory is operating a double shaft paddle mixer equipped with a 15 kW motor

Current situation

• Total mixing time = 4 min

The Engineer reckons that double shaft paddle mixers are designed for short mixing time in the order of 1-2 min. As there may be a potential optimization, a new validation of the homogeneity is carried out. It shows that the mixture reaches the homogeneity target after 2 minutes mixing.

New situation

• Total Mixing time = 2 min

Savings

• 10 batches / h, 5000 h/year (considering downtime for changeover...etc...)
• 10*5000 = 50000 batches / y
• Savings = 50000 * 2 / 60 * 15 = 25000 kWh
• Cost assumed 10 cent / kWh, savings = 2500 USD / y

2.5 Design

Energy-Saving Design Principles: Layout Simplification: Opt for straightforward layout configurations to minimize pressure drop and energy consumption. Equipment Optimization: Ensure equipment sizing is appropriate to avoid unnecessary energy expenditure. Automation Integration: Design automation systems to optimize equipment operation and minimize energy waste during idle periods. Gravity Flow Maximization: Prioritize gravity-based material flow to reduce reliance on mechanical or pneumatic conveyors, conserving energy.

As many Energy savings can be done by optimizing an existing installations, savings will even yield better environmental and economical results if they are embedded in the design. The following advices should be followed to design a competitive bulk handling installation :

• Simple layout of pneumatic conveying line : additional length and bends cause additional pressure drop and therefore energy spendings
• Determine experimentally the minimum conveying velocity of the material to be transported. As shown, a velocity higher than required will lead to strong energy overspending
• Control the margins considered by the manufacturer : blower are sometimes over sized
• An aftercooler is often positionned after a blower in pressure dilute phase conveying : this is actually not always required, only install it if proven that the system or the product cannot stand the temperature increase due to the blower compression.
• For dense phase, carry out experimental trials to design the line able to convey as high a ratio as possible
• Design the automation system to avoid that a line runs without product. Have a timer to switch off the line if no request is done for a certain time.
• Prefer to have flow of powders by gravity which allow to reduce the usage of conveying line, mechanical or pneumatical
• Make sure treated air is not lost but recycled

2.6 Maintenance

Sustaining Efficiency: Regular Inspections: Conduct periodic inspections of compressed air systems to detect and rectify inefficiencies, ensuring optimal operation. Leak Detection: Utilize leak detection mechanisms to identify and repair compressed air leaks promptly, preventing energy wastage. Operator Training: Provide comprehensive training to operators on maintaining and optimizing compressed air systems, fostering a culture of energy consciousness and efficiency.

Making sure the process is energy efficient by design is a 1st and necessary step, however on the long term, savings will materialize through a strict maintenance of the process equipment. It is particularly true for the consumption of compressed air as pressure regulator can easily be set improperly or solenoid fail with time, leading to an increased consumption of compressed air.

All compressed air settings should be mapped and controlled on a regular basis, at least a week, by the operators. Those regular inspections are also the occasion to identify leaks of compressed air that happen from time to time at fittings and which can be very costly if not immediately adressed.

3. Conclusions

It is possible to find many sources of energy savings in a bulk handling process, the article focusing only on some of them. Savings are not always individually very high but a systematic approach will allow to sum up individual sources of savings which can altogether significantly improve the competitiveness of a process. A proper instrumentation and maintenance of the installation will make sure that it performs close to its energy optimal and will help to develop environmental consciousness within the company.

In summary :

Table 1 : Tips for energy savings in powder handling process industries

Area	Design	Savings of	Operation
Dilute phase pneumatic conveying	Electricity	# Do not overdesign too largely the installation # Work on the pipe layout to make it as simple as possible, this will reduce pressure drop and energy consumption # Use proven design calculation tools to optimize design	# Foresee timers to stop blower if no transport happens for some time # Adjust blower speed to optimize conveying velocity. Conveying with too high velocity will consume a lot of electricity and may damage the material # Follow up pressure to make sure there is no blockage that could lead to higher compression rates and energy spendings
Dense phase pneumatic conveying	Compressed air (Electricity)	# Perform pilot plant tests to optimize the solids load ratio, less air per kg of conveyed product will reduce the load on compressors and generate savings	# Avoid flush at the end of conveying when not necessary, this consumes a lot of compressed air # Check pressure regulator to avoid over consumption of compressed air. Recalculate from time to time the solids load ratio to verify the line performs properly
Mixing	Electricity	# Optimize design so that the asset is used as much as possible	# Optimize mixing time to avoid mixer's several kW motor run too long for no homogeneity improvement
Air conditioning	Electricity, steam, chilled water	# Reduce building air leakage # Recircule clean process air whenever it is possible, it will reduce the need for treatment of external air. The AHU must be designed for this purpose	# Follow-up building air temperature and relative humidity, make sure it stays within specifications
General maintenance	Compressed air (Electricity)		# Create inspection checklist of all pressure regulators in the process lines, especially air flush of bearing seals, filters pulse jet system or fluidizing systems in hoppers