Patent Application
20090158821
The invention relates generally to devices, methods and systems for measuring one or more characteristics of a suspension.
Slurry concentration is one of many important parameters in chromatography column packing process. The current concentration measurement by resin settling and manual reading is time consuming, lacks accuracy (approximately +1-5%), is error-prone and also wastes large amounts of valuable resin materials.
In the column packing process, resin beads such as agarose beads are mixed with solvent liquid buffer to create a slurry suspension. Stirring and agitation help to ensure that the beads are fully mixed with the buffer and homogenously distributed in the slurry. Then the slurry is pumped into the column. When the entire column is filled with slurry, the beads gradually settle on the bottom and accumulate upwards to form a solid bed. The bed height, when the beads are completely settled, is recorded to calculate Volume Gravity Settled (Vgs) based on a known column diameter. Then the column is compressed using a movable plate to push the buffer out of the column through filters, while leaving beads packed in the column. After the beads are compressed, the bed height is recorded again to calculate Column Volume (Vc). The ratio of Vgs and Vc is defined as the compression factor, which is generally viewed as the most important parameter in the column packing process. The compression factor is indicative of column qualities, such as, resolution, capacity and throughput.
In current column packing methods, media is packed to a certain bed height (Vc), which is determined in advance. Slurry volume is measured using a flow meter. The compression factor (CF) is determined by the slurry volume concentration (solid beads volume concentration in slurry). If the slurry concentration (%) is faulty when packing these columns, the compression factor of the media will not be optimal.
Slurry volume % measurement is currently determined using a sedimentation method, which is manual, slow and prone to errors. Generally, the user mixes the slurry to a homogenous state and takes a sample. This sample must be settled overnight or longer in a graduated cylinder. The Vgs level is read after the slurry has settled, and slurry concentration is calculated as:
Slurry %=Vgs in the sample/total slurry volume.
At best, current methods provide slurry % measurements with approximately +/−2% error, however, the error rate is generally higher. Slurry % measurements can also be carried out in mass % and then converted to volume %. Although mass measurements can be accurate, bead volume changes significantly in different buffers. This leads to large errors when converting mass % to volume %. For the purpose of description only and is not intended to be limiting, the term slurry concentration, as used herein in the description of the embodiments, is based on volume %.
The limitations of current methods demonstrate that there is a need for a slurry concentration sensor. The ideal sensor should be fast, robust and reliable for determining slurry concentration to avoid re-packing columns in production processes. An in-line (real time) slurry concentration sensor would also enable automatic column packing, which would greatly simplify industrial workflow, reduce human errors and improve large-scale production repeatability and cost effectiveness.
Many ultrasonic measurement instruments have been developed over the past two decades for slurry concentration measurements for different industrial applications. Some of them require off-line measurements, taking slurry samples out of the original container and measuring in a specially designed container. Some other systems use in-line measurements but do not provide sufficient accuracy.
Besides ultrasonic methods, optical methods have also been explored for slurry concentration measurement. However, most of these optical systems are only capable of measuring slurries with low concentration (usually <10%) and that are relatively transparent. For high concentration and opaque slurry samples, optical methods are insufficient.
The ultrasonic devices, methods and systems of the invention are more accurate, faster and more efficient than previous methods and may be readily adapted for automation and portability and for use in a variety of industrial and biomedical processes. For example, these devices, methods and systems improve column-packing quality and reduce the amount of resin materials needed.
One or more of the embodiments of the devices, methods and systems comprises an immersible device with a two-step reflector system that, in some of the embodiments, is adapted to calibrate either or both velocity and attenuation based on buffer alone and/or on homogeneous slurry measurements. One or more of the embodiments of the methods and systems may also use dual devices and data analysis processors that are adapted to incorporate a dual device system. These devices, methods and systems may be adapted for in-line or off-line use and may be used to measure a variety of suspension parameters including, but not limited to, concentration, particle density and a rate of settlement. These devices, methods and systems may be adapted for in-line or off-line use, and may be adapted for a flow-through system and/or a system in which the ultrasound device is built in to the suspension processing system.
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
a and 3b are schematic views of an embodiment of an immersible device with at least two reflective surfaces.
a and 7b are graphs showing examples of the potential difference in velocity between liquid carriers.
One example embodiment of the system comprises two ultrasonic probes, two reflector blocks, a housing to fix the probe and reflector in relative positions, and a portable data analysis instrument. One of the probe/reflector pairs (otherwise referred to herein as an immersible device) is immersed in the slurry directly, and the other probe/reflector pair comprises a filter, which only allows only the liquid carrier, such as a buffer, to flow between the probe surface and the reflective surface and blocks resin beads from entering when immersed in the slurry. By measuring ultrasonic velocities, attenuations, and reflection/transmission coefficients in slurry, the concentration of the particles in the slurry may be determined with an accuracy that is +/−1%. Buffer liquid variations may be removed from the data analysis algorithms when using a dual immersible device system. The immersible device may also comprise two or more staggered reflective surfaces, which reduce distance variation between the probe and the reflector, to improve the accuracy of the ultrasound measurements.
Data analysis algorithms used in the systems and methods may be adapted to calculate calibrated ultrasound parameters based on buffer only and homogeneous slurry measurements. This process for calibrating the parameters greatly reduces the influence that buffer variations have on measurement accuracy. The dual probe design helps to acquire both the buffer only and slurry ultrasound parameters in one measurement without requiring time consuming settling steps. The data analysis steps may also incorporate data interpolation and correlation to accurately calculate TOF.
Although ultrasound parameters related to slurry concentration generally comprise velocity, attenuation, reflection coefficient and resonant frequency, the latter is not conducive to an in-line measuring system. Velocity is quite sensitive to slurry concentration change (<1%). Velocity may be divided into phase velocity and group velocity. Phase velocity is the speed of phase change along the wave-propagating path while group velocity is the wave profile moving speed, also called energy speed. If a propagation media is non-dispersive, then phase velocity and group velocity are the same. If the media is dispersive, then phase velocity and group velocity are different at different frequencies. Media dispersion is related to slurry bead size distribution. Most slurry concentration measurements are taken at a single frequency (for example, 1 Mhz) and then the group velocities are measured. For descriptive purposes only and without any intended limitation on the scope of the invention, velocity, when used to describe the example embodiments, refers to group velocity.
With known wave propagation distance, velocities may be calculated based on time difference measurements. There are three widely used time measurement methods: zero crossing, peak amplitude, and cross correlation. Zero crossing locates the time when the wave first crosses zero, either from positive to negative or vice versa. Zero crossing may be efficiently implemented by waveform interpolation and root finding algorithms. Two zero crossing points will provide the time difference from which velocity may be calculated. Peak amplitude methods measure at least two peaks relative to time and calculate the time difference, from which velocity may be measured. Cross correlation methods shift one of at least two waveforms and then compare the similarities between the two waveforms. When the correlation reaches maximum, this is the time difference between the two waveforms. Zero crossing is used in one or more of the embodiments in part because of its high accuracy and robustness in the presence of waveform distortions.
Acoustic field radiated from an ultrasound probe may be divided into near field and far field. In the near field, wave amplitude changes dramatically while phase is relatively accurate (<0.005% error). In the far field, amplitude changes gradually with monotonic decay and phase error increases because of wave diffraction. The optimal location for time or velocity measurements is in the near field and the optimal location for attenuation measurements is in the far field.
Attenuation may be measured based on the rate of waveform decay, which is usually measured in dB/m or Neper/m (1 Np/m=8.686 dB/m). Different concentrated slurries have different wave attenuations. The measured attenuation represents the overall attenuation, which includes the attenuation associated with the probe (and a buffer rod if it is attached to the probe), the probe and slurry interface, the slurry, the far field diffraction and the plate reflection, if a reflector is used. To optimize the methods and systems that comprise ultrasound attenuation measurements, multiple reflections are preferably recorded rather than just one reflection. To do so, distance between the probe surface and reflector should be tightly controlled.
Reflection coefficient is the ratio of amplitudes of the incoming wave and the reflected wave at the interface between two materials with different acoustic impedances. Acoustic impedance is defined as the multiplication of density and ultrasound velocity in the material where wave propagates through. When slurry concentration changes, both slurry density and ultrasound velocity changes accordingly.
Resonant frequency methods measure vibration frequency change due to liquid mass change with a known volume in a vibrating tube. Then density may be converted into a concentration at a known temperature. Because it is an offline measurement technique, it is generally not suitable for in-line slurry concentration measurement. Velocity is used in one or more of the embodiments because of its high sensitivity and accuracy.
Velocity measurements may use pulsed waves or continuous waves. Pulsed wave based method may use a pulse-echo method wherein a single ultrasonic transducer acts as a transmitter as well as a receiver; and/or a through-transmission method wherein two ultrasonic transducers are used in which one is the transmitter and the other is the receiver. Continuous wave based methods may use interference or generation of stationary waves due to multiple reflections from a sample, where the sample is place between two transducers or is placed between transducer and a reflector. The pulse-echo method is combined with zero crossing in one or more of the embodiments to achieve the high velocity accuracy.
One embodiment of the immersible device of the invention is generally shown and described in
Reflector 18 has a polished flat surface on one end and a cone 22 on the other end. The flat surface is used to reflect ultrasound waves and the cone shape helps to reduce reflections from the other end. Both the probe and reflector are fixed in position by housing 12. The device may be immersed directly into a suspension. Ultrasound waves are radiated from the probe surface, propagate through the suspension, and are reflected back to the probe by the reflector surface.
An embodiment of the system of the invention is generally shown and referred to in
Another embodiment of the system of the invention is generally shown and referred to in
Another embodiment of the system of the invention is generally shown and referred to in
System 180 may further comprise an ultrasound wave generator/receiver device 193 to transmit and receive the ultrasound waves to and from probe 180; and a signal processing device 195, in communication with the ultrasound wave generator to receive and process the ultrasound waves from the ultrasound wave generator/receiver device, an oscilloscope and a processor for processing and analyzing the ultrasound signals. Device 186 may communicate with device 192 through a cable or wirelessly.
To achieve highly accurate measurements using a single-surface reflector such as the embodiments shown in
To reduce possible distance measurement errors, a two-surface reflector may be incorporated into the immersible device. An embodiment of such a device with at least two reflective surfaces is generally shown and described in
Depending on whether the device, methods and systems of the invention are use in an off-line application or are incorporated into an in-line application at a point in the system in which the suspension may need to be maintained in a more homogenized state, stirring bars may be incorporated into the system to maintain appropriate distribution of the particles in suspension to obtain accurate measurements. An example of such stirring bars is mechanical stirring bar such as Caframo Model RZR1 mechanical stirrer, which has a variable speed control and a stirring head that can be clamped in a fixed vertical position.
Without stirring, particles in the slurry start to settle downward. Ultrasound parameters can be measured at multiple times during the particle settlement process. For example, the ultrasound velocity and/or attenuation may be measured every 30 seconds multiple times (e.g. 20 times) as the particles settle. The ultrasound parameter change versus time during particle settlement process (rate of settlement) may be used to determine other valuable information, such as, but not limited to, particle size, particle contamination status, particle aging status, and particle density.
The liquid carrier, such as a buffer, also may introduce variations into a system, as illustrated by the graph in
V(resin %)=V_slurry−V_buffer.
a shows the velocity of QFF vs. slurry concentration for two sets of samples. More specifically,
Buffer variation may be reduce using an off-line calibration method, such as the following:
Although this sample preparation method will ensure that the same buffer % for all the slurry samples is used, in-line applications typically require an in-line calibration method. Therefore, to reduce buffer variation in an in-line system, one or more of the embodiments of the methods and systems may incorporate dual or multiple immersible devices. Two ultrasound devices or probes are used in combination: one to measure slurry velocity and the other to measure buffer only velocity with a filter around the probe to block bead entrance and only allow buffer solution to go through the filter. Dependent on the filter pore size, time varies for buffer to enter and fully occupy the ultrasound path. As a non-limiting example, several seconds may be sufficient time for a Q Sepharose big bead slurry sample using a 12 μm filter. Any air bubbles in the ultrasound path are preferably removed by a variety of methods, such as, but not limited to, slight agitation of the device or liquid in the flow space.
Temperature also may play a significant part in determining one or more of the ultrasound parameters of the suspension. For example, temperature variations may significantly affect velocity measurements as shown in
A 3D regression plot of velocity vs. concentration and temperature is shown in
Velocity (m/s)=1624.753672+0.307557*concentration−0.581831*temperature
The regression equations are different for different slurries depending on bead and buffer combinations. To accurately compensate for the temperature variation, temperature in slurry should to be measured precisely, preferably within +/−0.05° C. accuracy. Although it may be desired to control the temperature of the chamber to keep slurry temperature constant during ultrasound measurements, this configuration may not be suited to an industrial manufacturing environment. For applications, where it is not suitable or desired to control the temperature of the suspension, temperature recording and compensation may be used to reduce temperature variation in suspension measurements. From these temperature measurements, a temperature compensation curve is generated that can be applied to the velocity measurements. Temperature compensation curves may be generated using measurements from multiple temperature points.
The immersible devices and the methods and systems may be adapted for use in portable devices such as, for example, field devices. For example,
While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.
