Cleaning ampoules the ultrasonic way
In an injectable plant, sterile liquids are filled in vials, ampoules and pre-filled syringes. Each one of the above has certain advantages and disadvantages. Since the opening of an ampoule is very small, internal cleaning becomes very difficult, as it cannot be cleaned manually. To properly clean an ampoule, ultrasonic waves are used. Therefore, it becomes very important to know the basics of ultrasonic cleaning to validate ultrasonic ampoule washing machines. This article is trying to throw light on ultrasonic validation for ampoule washing machine. An ampoule washing machine cleans the ampoules with a jet of water and by applying ultrasonic waves. Ultrasonic waves are used to loosen particles that stick to the ampoules.
Ultrasonic cleaning offers several advantages over conventional methods. Ultrasonic waves generate and evenly distribute cavitation implosions in a liquid medium. The released energies reach and penetrate deep into crevices, blind holes, and areas that are inaccessible by other cleaning methods. The removal of contaminants is consistent and uniform, regardless of the complexity and geometry of the part being cleaned. Cleaning is usually completed in one to three minutes.
An ultrasonic cleaning system has three components—ultrasonic generator, transducer and the vessel, where it is attached. The ultrasonic generator generally works on electrical power, which is usually 240 volt 50 Hertz, and converts it into a higher voltage and faster cycle to activate the transducer, usually 2,000 volts at 40,000 Hertz, for a cleaning system.
The function of a transducer in an ultrasonic cleaning system is to convert electrical pulses from the generator into a sound wave or pressure wave. This pressure wave travels through liquid and forms cavitation, which works as a scrubbing force in an ultrasonic cleaning system. The transducer is bounded to sides or bottom.
Constructional aspects of transducer
Figure 5. Expansion & contraction of piezoelectric transducer
Ultrasonic transducers are made up of a number of materials, the most common one being a piezoelectric material used in ceramic, called lead zirconate titanate. During the manufacture of ceramic, it is subjected to high potential difference, which causes polarisation. When this is put in service and has an electrical potential applied to it from the ultrasonic generator, it swells and changes dimension. When the electrical potential is removed, it reverts to its normal dimensions.
Figure 6. Piezoelectric transducer (compare to magnetostictive iezoelectric transducer is very small & compact)
By cyclic application of voltage, the transducer expands and emits a pressure wave. The frequency of the pressure wave is decided by the frequency of the transducer and output frequency of the generator.
Magnetostrictive transducers are generally less efficient than their piezoelectric counterparts. This is primarily due to the fact that the former requires a dual energy conversion from electrical to magnetic, and then from magnetic to mechanical. Some efficiency is lost in each conversion. A magnetic hysteresis effect also detracts from the efficiency of the magnetostrictive transducer. On the other hand, piezoelectric transducers expand and contract when potential difference is applied. There is no conversion from one form of energy to another.
Fixing of the transducer
Transducers are bounded either to the surface or immersed in the liquid. In both the cases, if it is a piezoelectric transducer, they are attached by epoxy, and if it is a magnetostrictive transducer, which is heavier, they are attached by brazing to the wall of the equipment.
Fig 7. Magnetostictive transducer
Magnetostrictive transducers utilise the principle of magnestriction in which certain materials expand and contract when placed in an alternating magnetic field. Alternating electrical energy from the ultrasonic generator is first converted into an alternating magnetic field through the use of a coil of wire. The field is then used to induce mechanical vibrations at the ultrasonic frequency in resonant strips of nickel or other magnetostrictive materials, which are attached to the surface to be vibrated.
Ultrasonic generators create ultrasonic waves in three forms—full wave, half wave, continuous wave. Ultrasonic ampoule washing machine suppliers should provide a frequency and a watt meter to verify the output frequency and output wattage of the generator, which will ensure proper cleaning, if all parameters are maintained. To protect the generator from electrical fluctuation the generator output may be adjusted to +10 percent.
Ampoules are cleaned by ultrasonic waves by the creation of cavitations. A high intensity pressure wave is transmitted to a liquid to tear the liquid apart in rarefaction high cycle and drop the pressure within the liquid below its point of vaporisation. When this has been achieved, millions of vacuum bubbles called cavitation events are developed. Every half cycle will create such cavitation bubbles. The cycle will repeat 40,000 times per second. You can imagine the shear force with which they release and break the bonds of the particles in glass ampoules. Cavitation depends on ultrasonic power, density of the liquid, vapour pressure and temperature. The imploding cavitation bubble conducts majority of cleaning. This is directly proportional to the applied frequency—as the frequency increases, the cavitation events also increase.
Fig 8. Ultrasonic waves
The larger the imploding bubble, the greater is the implosion force. As we lower the frequency, the number of cavitation events also lowers.
Properties of liquid effecting ultrasonic cleaning.
Viscosity: If viscosity is low it promotes cavitation.
Density: Density should be high to create intense cavitation events, although high-density liquid requires additional energy to initiate cavitation.
Vapour pressure: Vapour pressure of medium volume is more suitable for ultrasonic cleaning.
Surface tension: Like vapour pressure surface tension should be moderate for good cavitation.
Distilled water is not recommended for ultrasonic cleaning applications because they lack nuclei in the water from which the vacuum bubble is formed. Hence, purified water is used for cleaning ampoules.
Fig 9. Cavitation
Liquid temperature affects the cavitation quantity, density and cleaning action. Cooler the liquid, more difficult it is to go below the point of vaporisation to begin cavitation development process, the applied energy will not be sufficient. If we increase the liquid temperature, it is easier to go below the vaporisation point so we can initiate cavitation with less energy. The number of cavitation events increase as temperature increases. Normally, the temperature of water in ampoule washing machine is between 60-65° C.
For cavitation to become effective, dissolved gases trapped in the liquid must be removed. If not removed, these bubbles act like cushions, lessening the effectiveness of cavitation. To avoid the introduction of gasses in an ultrasonic bath of ampoule washing machine, the following precautions should be taken-
The recirculation water through pump should not reach above the liquid
Ampoules should not be introduced in the bath in jerks.
The movement of the product should be done smoothly inside the water so that gases are not introduced
Testing performance of ultrasonic cleaner in ampoule washing machine
Aluminum foil ablation test: Using the foil ablation test, the activity of the ultrasonic cleaner is verified by the erosion pattern, which is created on the aluminium foil exposed in the bath for a short period.
Aluminium foil of the type sold as a wrap for cooking.
Adhesive tape (e.g. autoclave indicator tape or masking tape).
A watch or clock with a second hand.
Make small pieces of aluminium foil, measuring about 10 cm x 20 cm each. Fold each piece over a rod that is suspended over the foil in a tank. A clothes hanger works well. Your cleaner should be filled with cleaning solution, degassed, and brought up to normal operating temperature. Suspend stripes in the centre of the tank and the other two a couple of inches from each end of the tank. The tank is filled to the fill line, and the ultrasonic turned on for about ten minutes. All three pieces of aluminium foil should be perforated and wrinkled to the same extent.
Result and interpretation
When the foil strips are inspected, the areas that show maximum erosion should be at similar positions on all foils and each should be eroded to a similar extent. On re-testing the extent of erosion, the pattern should remain consistent. If the zones of erosion are markedly different on the foils, it indicates poor uniformity of cleaning. A significant change between tests indicates a deterioration or failure in the transducers. If there is no erosion, this indicates complete failure.
Chemical indicator for checking cavitation
Ultrasonic generator Ultrasonic transducer
Chemical indicators available in the market can be used to verify the operation of ultrasonic cleaning. This chemical comes in a vial containing fluid beads in it. When it is introduced in the ultrasonic operation, it changes its colour, by which we can find out whether proper cavitation has taken place or not.
The vial with the cavitation indicator contains glass beads and chemicals, which are initially green in colour. When exposed to adequate ultrasonic cleaning cycle, theircolour changes to yellow. This can be used to find the appropriate time for the ampoules to remain in the bath before being lifted for cleaning.
In the case of an ultrasonic ampoule washing machine, empty ampoules, which are loaded into the machine for cleaning, should be placed in line with the bubble propagation i.e. the ultrasonic generation surface should be perpendicular to ampoules. The cavitation can happen inside the ampoule surface. This is very important for cleaning the inner surface of the ampoules. Ultrasonic cleaning works best on hard materials like metals, glasses, ceramics etc. Normally, cleaning is satisfactory between 20-40 kHz. The power density of ultrasonic cleaner is about 10 watt/sq inch of driving area. Here the driving area is ampoule internal surface and external surface. An attempt to increase the wattage to a larger extent will result in an erosion of metals. It is generally assumed that the outer surface area is easier to clean as compared to the inside surface area of ampoule. Whatever is the surface area kept inside the ampoule will be increased by 100 percent for calculating wattage/sq inch, thereby, ensuring the cleanliness of the ampoule.
For e.g, following is a calculation to find out what the wattage required for an ultrasonic generator, where the ultrasonic machine has a sump of 0.5 metre x 0.5 metre, in which 10 ml ampoules are loaded. The driving surface area is the outside the surface area of the ampoule and the inside surface area of the ampoule. Once the total power of ultrasonic generator, where 10 watts/sq inch as a power density, is known, calculate/estimate how many 10 ml vial should be loaded at a time.
0.5 metre = 19.6 inches = 20 inches
If the l= 20 inches and w=20 inches,
Then the total surface area exposed to ultrasonic pressure waves is 400 sq inch. That means that the ultrasonic generator for an ampoule washing machine should be around 4000 watts.
The internal surface area of A is 1.52 sq/inch The internal surface area of B is 1.661 sq/inch The total internal surface area of ampoule is 3.18 sq/inch.
The total surface area available is 400 sq inches.
One ampoule of 10 ml is 3.18 sq inch i.e. ideal loading is 125 ampoules at a time and this ampoule should remain in the bath for one to three minutes (this time has to be arriving by doing it practically).
In actual condition the ultrasonic generator should be 30 percent higher than the calculated value i.e. for the example stated above, the ultrasonic generator will be of the capacity 4000 watt+4000 X 0.3 = 5.2K watt.
Choice of cleaning frequency is determined by the surface condition of the ampoule. The cavitational shock intensity is higher at 25 KHz than at 40 KHz, however, lower frequencies are found more damaging, and hence, 40 KHz is preferable as it is a quieter operation. Also at 25 KHz, implosions created are less in number as compared to those created at 40 KHz. At also 25 KHz the bubbles created are much larger in size as compared to these created at 40 KHz. The number of bubbles entering inside the ampoule will be more at 40 KHz than in 25 KHz. Therefore, in all respects, 40 KHz is preferred.
(The author is Chief Engineer at VHB Medisciences. He can be contacted at firstname.lastname@example.org)