Optimizing Tank Cleaning Operations

By Christine Pagcatipunan | August 27, 2007
Tank cleaning is not a process conducive to shortcuts—especially in ethanol production. The contamination and bacteria growth that can result from improperly cleaned yeast and fermentation tanks can put a significant dent in production and profitability. Yet, tanks need to be cleaned in as little time as possible because empty tanks hinder production and profitability.

This article provides information on the various types of tank cleaning machines—how they work, maintenance requirements, performance validation options and the relative cost/value of operation. Operational guidelines also help ensure optimal cleaning in the shortest amount of time at the lowest possible cost.

Tank Cleaning Technology
Most ethanol plants are faced with cleaning anywhere from one to dozens of fermentation and yeast tanks. Most plant personnel have a good understanding of cleaning requirements and experience with tank cleaning equipment. However, as technology advances and new products become available, there may be room for improvement.

Conducting a quick evaluation of the process and current equipment won't take much time. This small time investment may be the first step to more efficient and cost-effective cleaning.

Let's start with the basics. Is the level of cleaning appropriate? Most producers require high-impact cleaning to thoroughly clean tanks. This usually consists of three steps. First, tanks are cleaned with water to remove corn mash and yeast residue. Then, they are washed with hot water and/or a heated caustic solution to kill micro-organisms and prevent bacterial growth. Finally, tanks are thoroughly rinsed to remove any remaining residue and/or the caustic solution.

If tanks are currently being cleaned to an acceptable standard and in a reasonable amount of time, the correct level of cleaning is likely being used.

Another basic item is determining whether the tank cleaning equipment is designed specifically for the tanks needing cleaning. Tank cleaning equipment manufacturers provide information on the distance the spray will reach to effectively clean the internal surface of the tank. This is usually expressed in terms of tank diameter. So, if the diameter of a tank is 40 feet, be sure the equipment has a spray distance of at least 40 feet. Consider tank length as well. If tanks are 40 feet in diameter and 80 feet long, one tank cleaning machine that cleans up to 40 feet in diameter won't get the job done. Figure 1 provides more information on spray distance and how it is defined for cleaning, rinsing and wetting.

Because tanks used in ethanol production are typically large—40 feet to 60 feet in diameter—conventional tank wash nozzles and spray balls are not suitable. Typically, tank wash nozzles/spray balls effectively clean tanks with diameters ranging from 2 feet to 25 feet and don't provide adequate impact for cleaning yeast and fermentation tanks.

The final basic tank-cleaning item is determining if the level of spray impact is adequate. The level of impact needed to thoroughly clean tanks depends on the residue, cleaning chemicals and water temperature. If contamination is a problem or cleaning cycle times are too long, an increase in impact may help. The most effective way to do this is to increase flow rate instead of increasing pressure. As shown in Figure 2, doubling the flow rate can increase impact by as much as 100 percent while doubling pressure only provides 40 percent more impact.

Solid stream sprays provide the greatest level of impact, which is why most tank cleaning machines are equipped with solid stream spray nozzles.

The next step is to evaluate equipment options.

Tank Cleaning Machines: Fluid- and Motor-Driven
Fluid-driven tank cleaning machines use fluid to spin a turbine. The turbine powers a gear set. The nozzle assembly rotates as the hub revolves around its central axis. The higher the liquid pressure and flow, the faster the rotational speed will be.

Motor-driven tank cleaning machines use an external electric or air motor to drive the nozzle assembly. The nozzles revolve around the central axis of the nozzle assembly. Tank washers powered by an air motor offer independent control of the liquid pressure for cleaning and the rotational speed (cycle time) to optimize the cleaning process.

The cleaning performance of fluid- and motor-driven machines can be comparable. However, it may be possible to shorten cleaning time by using motor-driven units. When an electric motor is used, cycle times are constant despite operating pressure and flow rate. When equipped with an air motor, air pressure can be increased to make the nozzle hub rotate faster.

Fluid-driven machines can usually achieve cycle times comparable to motorized machines by increasing pressure. However, some manufacturers don't endorse this approach because operating at higher operating pressures means increased wear on internal parts and shorter service life for the machine.

From an operational standpoint, there are significant differences between the types of machines, and these differences should be considered.

Costly Clogging
Most plants use recycled water for tank cleaning. Debris in the water can cause clogging. Fluid filtration is recommended prior to use in tank cleaning machines.

In fluid-driven machines, clogging is a definite source of concern. When debris clogs internal flow passages or gets caught in the gears, the machines simply stop working because they can no longer rotate. Once clogged, fluid-driven machines must be serviced immediately in order for cleaning to resume.

Motor-driven machines are better equipped to operate with less-than-pristine water. Debris may accumulate in the nozzles but the machine will continue to operate since it is powered by an external motor rather than fluid. There may be some degradation of the spray pattern but cleaning is not interrupted.

Maintenance Time and Expense
The maintenance requirements for fluid-driven machines vary by brand. In general:

› Many units require service at manufacturer-specified intervals based on hours of use. The turbines can spin at very high rpm, requiring replacement of wear parts on an ongoing basis.
› Most units have many internal pieces, and maintenance requires disassembly, replacement of worn parts and reassembly. Some manufacturers require that special tool kits be purchased to maintain and repair the units.
› Factory refurbishing is also recommended by some manufacturers based on hours of operation.

With motor-driven machines, the motors are positioned outside the tank. This helps ensure long wear life and minimizes the motor's exposure to harmful solvents. In general:

› Motor-driven machines require minimal maintenance as rotational speed can stay constant over a wide range of operating pressures and flows.
› Motor-driven machines have fewer parts, so if service is required it can be quickly completed by the end-user. No special tools are required other than hex keys, also known as Allen wrenches.

Nozzles on fluid- and motor-driven units will wear over time. Nozzle orifices should be inspected for wear and flow rate should be monitored. An increase in flow rate can indicate nozzles are spraying over capacity as a result of wear.

After the nozzles have been in use for a pre-determined period of time, it's recommended to contact tank cleaning machine manufacturers to conduct wear testing using specialized equipment. The testing will document wear rate, and the manufacturer can provide recommendations for replacement. Some manufacturers will provide nozzle wear testing at no charge.

Purchase Price versus Operation Cost
Motorized tank cleaning machines typically cost more than fluid-driven machines. However, over time, motorized units are less costly to operate and maintain for the reasons noted above. The additional expense incurred at purchase is quickly offset—sometimes in the first few months of operation. How quickly the cost is offset will depend on water quality, the chemicals being used, hours of use and cycle times. Also, consider the cost of lost production time to resolve clogging issues and the labor expense for maintenance in addition to parts and refurbishment costs.

A Final Consideration: Performance Validation
No matter which type of tank cleaning machine is used, there is always the issue of monitoring. Are the machines working? Testing for bacteria after cleaning is a widely used method. Some producers supplement that method by having personnel routinely listen for sound and/or detect vibration in order to confirm tank cleaning machines are operating.

Some tank cleaning machine manufacturers offer an optional monitoring system. These systems include a sensor, monitoring device and software to validate tank cleaning machine operation. The need for human monitoring is eliminated. Some systems can alert personnel to performance changes at the moment they occur via visual and audible alarms for immediate resolution. Another benefit of these monitoring systems is the data can be stored, providing documentation for adherence with quality control and compliance requirements.

Some monitoring systems use a pressure sensor; others use an acoustic sensor. There are important performance differences that should be considered. Acoustic sensors are mounted to the tank exterior. They monitor rotation speed and frequency, spray loss, nozzle clogging and pressure variations. Pressure sensors must be precisely placed inside the tank and require tank modification. They are unable to detect rotation frequency, variations in pressure and clogged nozzles.

Monitoring systems can be easily added to most tank cleaning operations although there may be some compatibility issues between the type of tank cleaning machine and the monitoring system. Be sure to consult with the machine manufacturer to avoid integration problems.

Christine Pagcatipunan is an applications engineer at Spraying Systems Co. Reach her at [email protected] or (630) 665-5000.

The claims and statements made in this article belong exclusively to the author(s) and do not necessarily reflect the views of Ethanol Producer Magazine or its advertisers. All questions pertaining to this article should be directed to the author(s).