Chemical tank cleaning: Small things make a big difference

Technical article by Ivan Zytynski, marketing director of BETE Ltd:

The cleaning of tanks and vessels is often an overlooked source of inefficiency in the chemical manufacturing process. Also, as an ‘unglamorous’ and non-core process the true cost of cleaning, is often neglected. However, with some thought significant efficiency gains can be achieved.

These efficiencies come from a combination of the four key elements of the cleaning process; time, mechanical action, heat and chemical action. The focus of this article is on how quick, yet substantial savings can be achieved by improving the mechanical action element of the cleaning mix.

Any cleaning application has four components that contribute towards effective cleaning.

1. Time. The longer the cleaning if performed the greater the cleaning.

2. Chemicals. This is the dissolving effect of chemical cleaning fluids including water.

3. Mechanical action. This is the physical action of the cleaning spray to dislodge residue.

4. Heat. Generally the hotter the cleaning fluid the better the cleaning action.

Increasing any of these 4 components will improve overall cleaning but there will be a cost associated with each. The cost of each of these elements will differ depending on the application and there may well be other constraints in place. For example, in food processing or other ‘sensitive’ applications there will be limit on the types of chemical that can be applied.

The differential cost of each element is the key to efficient cleaning. Optimising the mix of elements is the process of increasing one element of the mix that has a lower cost (e.g. mechanical action) so that another element that has a higher cost (e.g. heat) can be reduced. The net cleaning power will remain the same but the cost associated with the cleaning process will be reduced.

Absolute efficiency gains
Whilst overall efficiency can be gained by reconfiguring the contributions from each element it is clearly beneficial to strive for efficiencies in each element.

If, for example, a cheaper method of heating can be found then this element in its own right becomes more efficient, and thus the whole process is more cost effective.

An absolute gain in one element, however, might be better utilised by reducing the contribution from another more costly element. For example, if a more efficient heating method was found then either heat could be maintained at the current level for a lower cost OR heat could be increased for the same cost.

If heat is increased then perhaps time could be reduced whilst keeping overall cleaning power at the same level. If the opportunity cost saved by reducing cleaning cycle time is greater than the savings made by improved heating efficiencies then this configuration is optimum. In other words a gain in efficiency in one element is not always best deployed in that element.

Maximising gains

In order to leverage hard won efficiency gains it’s sensible to consider how they are best deployed and how they might be used to re-configure the cleaning elements mix.

The true cost of water is often under appreciated in terms of: Raw utility bill cost per m3 of water: Cost of filtering and sanitising if recycling wash off; Cost of caustics or other cleaning fluids; On top of this if we reduce the water needed to clean we can lower pump running costs, increase its lifetime for and potentially use a smaller pump (reduced capital expenditure)

As the cost of energy and water are both increasing, and likely to continue to increase, reductions in water usage have significant financial benefits to any organisation. Further more the green / environmental benefits are seen as a moral imperative by many organisations. Future green legislation is only likely to increase the need for more efficient water usage.

It may be a cliché but it’s still true. The time spent cleaning between production runs, whilst necessary, still represents downtime. The opportunity cost associated with this downtime will vary greatly depending on the application, but in almost all cases a reduction on cleaning cycle time will have a direct financial benefit.

Generally there will be limitations on each of these elements. Perhaps more importantly both elements have very rapidly diminishing returns after a certain point.

For example, the cost associated with raising cleaning temperatures from 60º - 70º is unlikely to be worth it for most applications. It is likely that chemical and heat action will be the elements that are reduced in any efficiency drive. The typical scenario is that improved mechanical action will result in the reduction of chemical usage or lowering the temperature of cleaning.

When performing these savings calculations it is important to remember the true cost of heat and chemicals. Heat is relatively simple to calculate as it is essentially an energy cost but consideration should be given to maintenance spares and overall capital expenditure versus working life of the heating system. The cost of using chemicals should obviously include the raw cost of the chemical but also the cost of disposal of spent chemicals and recycling plant costs (if used).

Often the simplest absolute efficiency gains can be found by improving the mechanical action element of the mix. These gains can then be deployed to reduce other elements of the mix, if this is appropriate.

For any impact cleaning process water serves two purposes. Firstly it acts to dissolve residue, this is part of the chemical element of cleaning mentioned above. More importantly, however, water is the mechanism by which the mechanical action element is delivered. The efficiency of a water spray for delivering mechanical energy for cleaning will be greatly affected by the nature of the spray and thus the nozzle used.

Mechanical action is essentially the process of transferring energy from a pump to the surface to be cleaned via water. As with all energy transfer systems efficiency is less than 100%. Much energy is wasted but by reducing this waste through improved nozzle selection we can significantly improve the efficiency of the tank washing system. If this is achieved then we can reduce the amount of energy/water used and achieve the same level of mechanical action.

Effective nozzle selection will obviously not directly affect pipe friction losses but it will affect losses of energy through fluid atomisation and turbulent flow.

Fluid atomisation

The process of breaking apart a fluid into droplets to form a spray pattern uses energy and once used, is then not available for cleaning the surface in question. The benefits of an atomised spray are that it can be formed into a full cone or flat fan pattern delivering the spray to a larger area, but this means that the overall energy transfer will be less due to the energy used in the process of atomisation.

Turbulence
Further energy will be lost in atomised sprays due to turbulent flow. In flat fan or full cone spray patterns the droplets are moving in a less uniform direction than in a solid stream of water. Whilst the whole fluid has a definite direction the individual droplets will have a random, turbulent, element to their motion. This effectively wastes energy meaning the overall transfer of energy in full cone and flat fan patterns is much lower than in solid stream nozzles.

So the most efficient spray system would be solid stream jet followed by a flat fan spray pattern and finally a full cone pattern (omni-directional spray nozzles can be considered as 360o full cone nozzles). Indeed the efficiencies gained can be considerable. For example, it is not untypical to require 10 times less water per square meter being cleaned.

The drawback of solid stream cleaning systems is that they require a defined wash cycle time to cover the whole of the tank. This could increase clean cycle times thus adding to the cost associated with the time element of the cleaning process.

Designs of tank washing nozzle - What are the options?

Spray Balls
These nozzles are spheres with multiple holes producing an omni-directional spray. Whilst the individual jets may appear to be solid stream spray patterns, in actual fact, when considered together, they are better approximated to cone pattern. In terms of overall energy transfer efficiency spray balls are very inefficient. They do, however, have the advantage of being very cheap and can deliver a low impact rinse more or less instantly to the whole container.

Typical uses would include: rinsing of highly soluble powers or non viscous fluids, cleaning in explosive environments
For anything other than very light cleaning applications in small containers though it is likely that efficiencies can be gained by changing from spray balls.

Spiral Wide angle nozzles
These nozzles produce a “cone” of spray up to 270 degrees wide. This makes them suitable for cleaning tanks by inserting them towards the top of the vessel. The spray is cone and thus inefficient at energy transfer but nevertheless offers significantly greater impact than spray balls per volume of water used. Spirals should be considered for low impact cleaning of small tanks.

Typical uses would include: rinsing and washing fluid containing vessels, rinsing of vessels containing soluble powders, cleaning in explosive environments

Multi-headed nozzles

Various nozzle manifold tank cleaners exist on the market. Several full cone nozzles will be positioned on a single head giving an omni-directional spray. Because multiple nozzles are being used to produce more direct sprays the impact per volume of water is increased.

However, the individual sprays are still full cones and thus inherently inefficient. Such systems are suitable for small-medium sized tanks that require light to moderate cleaning. For very corrosive environments the relatively simple design of these nozzles allow for manufacture in a variety of resistant metals or plastics such as PTFE.

Typical uses would include: rinsing and washing fluid containing vessels, rinsing of vessels containing soluble powders, cleaning in explosive environments

Rotary Flat Fan Nozzles
These nozzles will have several flat fan sprays that rotate under the pressure of the fluid. The rotating flans will sweep the whole of the tank. As with static nozzles complete coverage happens only after a few moments but the nozzle spray will need to be working for while before any significant residue is removed. As a flat fan pattern is being used there is a moderately efficient energy transfer resulting in a medium impact cleaning spray.

These nozzles are often the most efficient choice for small to medium sized tanks that have moderately stubborn residues to remove. The complexity of these tank cleaners means the materials of construction are more limited when compared to static nozzle but for highly corrosive environments there are some rotary flat fans that can be manufactured entirely in materials such as PTFE.

Cleaning mid sized storage or reaction vessels, cleaning of caked on powders and viscous fluids, cleaning in explosive environments (some models)

Rotary solid stream

These nozzles have two, four or eight solid stream jets which rotate sweeping the inside of the tank. A gearing mechanism changes the angle of the rotation so over time complete cleaning of the tank is achieved. These nozzles are by far the most energy efficient tank washers due to the solid stream jets being deployed. They are generally more expensive than other types of tank washer but for medium to large tanks or tanks with stubborn residues the additional capital expenditure is often paid for rapidly through efficiency gains.

Typical uses include: cleaning large processing or storage tanks, cleaning highly viscous liquids or caked on solids.

Other constraints

Within the chemical processing industry other constraints may affect nozzle selection. If, for example, the environment being cleaned is highly volatile then rotary nozzles may represent an explosion risk. Some ATEX approved models exist but choice may be limited, particularly when it comes to the larger solid stream rotary cleaners.

A further constraint may be the available materials of construction. Static nozzles can be made from a wide variety of materials, in particular highly chemically inert plastics, but the larger machines are not generally available in plastics and so for some environments will not be suitable. Another point to consider against metal nozzles is if they are being used to clean glass lined tanks.

Plastic nozzles are far more forgiving on expensive fragile vessels should they come loose. A large metal nozzles rotary jet cleaner, weighing 10Kg, may represent an unacceptable risk to the tank despite being the most efficient.

Conclusions

Quick savings can often be found by looking at the mechanical action element of the cleaning mix. The efficiency of the mechanical action delivery mechanism can often be improved by correct nozzle selection. This improvement in efficiency will mean the same contribution to overall cleaning and can be achieved using less water and these water savings translate directly to a reduction in operating costs.

These gains can be further multiplied by considering how the 4 elements of the cleaning mix can be reconfigured. It may, for example, be more beneficial to reduce the heating and chemical elements of the mix rather than use all the efficiency gains to reduce water consumption. The optimum configuration of the mix will vary from application to application but regardless of improvements in the mechanical action component of the mix, quick gains can be achieved with a payback time measured in weeks - rather than months.

Four steps to improve efficiency

Step 1 - Consider the true cost of each of the 4 elements of the cleaning mix.

Step 2 - Consider the current contribution to cleaning from each element.

Step 3 - Look at which elements have absolute efficiency gains to be made i.e. the same cleaning contribution for less cost.

Step 4 - Assess how the new cleaning mix will be optimised. ENDS.