Micro Flow Sensor for Microreactor

TANAKA Yoshiaki¹ TERAO Minako¹ AKUTSU Tomomi¹ ISOZAKI Katsumi¹

Recently, a state-of-the-art technology called a microreactor has been drawing a lot of attention in the small-lot production processes for high value added functional materials, such as pharmaceuticals and fine chemical materials. We have been focusing on this technology, and have developed a new micro flow sensor, which is one of the most important components of microreactor systems. Conventional flowmeters made of stainless-steel narrow tubes for micro flow rate applications are not applicable to micro reactor systems where corrosive liquids are used. The key feature of the developed micro flow sensor is its high resistance to corrosive liquids thanks to the use of glass for all wetted surfaces of the sensor, and arranging the sensing elements in non-wetted areas. This paper introduces the developed micro flow sensor and reports its evaluation results.

1. Advanced Technology Research Center, Corporate R&D Headquarters

INTRODUCTION

Recently, a state-of-the-art technology called a microreactor has been attracting much attention in the chemical process industry. With this technology, it is possible to manufacture chemical materials which cannot be produced by the conventional batch process in a chemical plant. This technology is expected to lead to innovations in the small-lot production process for high value added functional materials, such as pharmaceuticals and fine chemicals.

We have been focusing on this technology and conducting R&D on microreactor systems.¹ We have developed a prototype of a micro flow sensor, which is one of the most important components of microreactor systems, utilizing Yokogawa's expertise with flow measurement technology and micro electro mechanical system (MEMS) technology.²

Figure 1 shows a prototype of the micro flow sensor. The figure on the right is the sensing device of the prototype configured on a glass chip, and that on the left is its case. In this paper, we describe the developed micro flow sensor and report its evaluation results.

FEATURES AND SPECIFICATIONS OF THE MICRO FLOW SENSOR

Figure 1 Overview of the micro flow sensor under development

The first feature of the micro flow sensor is its high resistance to corrosive liquid thanks to all wetted surfaces of the sensor being made of glass, and by placing the sensing elements in non-wetted areas. Many types of corrosive liquid are used in microreactor systems with which functional materials, such as pharmaceuticals and fine chemicals, are manufactured. One of the well-known micro flow sensors is the mass flowmeter, which is commonly used in semiconductor manufacturing equipment. However, this type of flowmeter cannot be used in processes with corrosive liquid, because it is usually made of stainless-steel narrow tubes.

The second feature is its small dead volume owing to no obstacles or moving parts. The conventional mass flowmeters have a large dead volume because they contain obstacles such as bypass tubes. Thus, if the fluid to be measured contains air bubbles, the bubbles stay in the dead volume, causing large measurement errors. Therefore, for accurate measurement, purging is necessary to get rid of the air bubbles, and the mass flowmeter may need to be removed from the system, which hinders maintenance. By contrast, the proposed micro flow sensor has only a straight tube, and so air bubbles pass through easily, which facilitates maintenance.

To achieve these features with the micro flow sensor, both the flow channel and sensing elements are fabricated on a glass chip using the MEMS process. Table 1 shows the target specifications of the micro flow sensor based on the needs of microreactor customers.

Table 1 Target specifications of the flow sensor

Items	Specifications
Flow rate range	0.01 - 10 mL/min
Measurement accuracy	± 5% of Reading
Temperature range	15 - 35 °C

MEASUREMENT PRINCIPLE

Figure 2 Measurement principle

There are several measurement principles for flow rate measurement, and flowmeters of Yokogawa current products are based on various measurement principles for large-scale petrochemical plants. Typical products include differential pressure transmitters using differential pressure through an orifice, vortex flowmeters using the Karman vortices which appear downstream of an obstacle, and electromagnetic flowmeters using Faraday's law. However, these products do not meet the target specifications listed in Table 1. Therefore, we chose a thermal measurement method for the new micro flow sensor as it is most suitable for micro flow rate measurement and can be configured on a glass chip as mentioned above.

The measurement principle of the micro flow sensor is as follows. As shown in Figure 2 (a), three sensing resistors whose resistance varies with temperature are located on the outside surface of the flow channel across the flow direction. The center resistor is used as a heater, and the resistors on both sides are temperature sensors.

The temperature of the heater is controlled at a certain level higher than that of the flowing fluid to generate a temperature distribution. This temperature distribution generated by the heater depends on the flow velocity. For example, as flow velocity becomes faster, heat transfer from upstream to downstream increases and the temperature downstream rises. Consequently, the difference between the temperatures measured by the two sensors can determine the flow velocity, which leads to the flow rate in the channel. The flow sensor described so far is based on the thermotransfer method. These are commonly used for measuring gas flow rate. However, since the thermal diffusivity of a liquid is generally smaller than that of gas by a factor of a hundred, it has been considered difficult to configure a liquid flow sensor with a wide range of flow rate using the thermotransfer method. The small thermal diffusivity causes saturation of the difference between the downstream and upstream temperatures at a lower flow velocity than in the case of gas. Hence, to compensate for the small contribution of the diffusing heat generated by the heater to the measurement, the difference of temperatures is normalized by dividing the difference by the sum of temperatures measured by two sensors. This normalization compensates for the diffusing heat that does not contribute to the measurement, and thus the upper limit of measurable flow rate can be raised successfully.

 

Figure 3 Changes of temperature distribution with flow rate

Figure 3 illustrates the results of the temperature distribution obtained by a numerical simulation. Fully developed laminar flow in the flow channel is assumed because the Reynolds number of the fluid within the analyzed flow rate range does not exceed 200. Therefore, no turbulence models are required for the numerical simulation.

The figure shows that the temperature distribution profile is distorted downstream as the flow rate increases. The average temperature near the heater falls as the flow rate increases because of the small thermal diffusivity of liquid. This can be compensated by the normalization mentioned above.

In order to extend the measurable flow rate range further, we put another temperature sensor downstream as shown in Figure 2 (b). The heater is driven by a pulse wave pattern, and a temperature "mark" is put in the measured fluid. Then the flow rate is determined by the time difference as the temperature "mark" flows between the two sensors downstream, which is called Time of Flight (TOF) method. These two measurement methods are integrated on the same chip, hence covering a wide range of flow rate.

STRUCTURE AND FABRICATION PROCESS OF THE MICRO FLOW SENSOR

Figure 4 Structure of the micro flow sensor

Based on the results of the numerical simulation, we designed and fabricated a sensor chip as shown in Figure 4, where the flow channel and sensing elements are both created on the glass chip. In the future, it will be possible to integrate microreactors and various control devices such as micro valves with the sensor chip by putting the sensing elements on the glass chip.

As mentioned previously, two temperature sensors are placed downstream of the heater on one chip to combine two measurement methods, the thermotransfer method and the TOF method.

The micro flow sensor is fabricated as follows.

1. A flow channel and holes for inlet and outlet are sandblasted on the glass substrate.
2. Another glass substrate 0.1 mm thick is thermal-compression bonded onto the substrate to cover the flow channel.
3. Thin film resistors are deposited on the surface of the cover glass using the MEMS process. These resistors are resistance temperature detectors (RTD) made of platinum film. Platinum film is sputter-deposited and the patterns of RTD are fabricated by the lift-off technique (a patterning method for metal thin film where wet etching is not applicable).

Figure 5 Evaluation result for flow rate range	Figure 6 Evaluation result for zero drift

This simple fabrication process without complicated processes reduces the cost.

EVALUATION RESULTS OF WATER FLOW EXPERIMENT

We evaluated a prototype model of the micro flow sensor with a flow rate test line. The measured object is water flow rate, and the reference is the total flow mass measured with a precision balance at the end of the test line. The evaluation results for the measured flow rate range, zero drift, and temperature characteristics are shown in Figure 5, 6 and 7, respectively. These results satisfy the target specifications in Table 1, and prove that the developed micro flow sensor is applicable to microreactor systems.

CONCLUSION

Figure 7 Evaluation result for temperature characteristics

This paper proposes a micro flow sensor for microreactor systems. The main feature of the sensor is its high resistance to corrosive chemicals.

We demonstrated the micro flow sensor at Inchem TOKYO 2007, one of the largest exhibitions in the chemical industry. In order to develop a product that fully meets customers' needs, we introduced an early-stage prototype model to customers.

As a result, we received two requests: coverage of higher flow rate range and high pressure resistance. To meet the former request, we developed the micro flow sensor that uses the thermotransfer method and TOF method complementarily as described in this paper. We are currently developing a high pressure resistance type.

While our large flowmeters for large-scale petrochemical plants have a high reputation among customers, they do not meet customers' needs in the micro flow rate industry. In addition to application to microreactor systems, we are now developing a flow sensor for various micro flow rate applications where the avoidance of metallic ion depositions is necessary, by taking advantage of the feature of high resistance to corrosion.

REFERENCE

1. Katsumi Isozaki, Makoto Imamura, et al., "Micro-Measurement and Manipulation Technologies," Yokogawa Technical Report English Edition, No. 41, 2006, pp. 7-18
2. M. Terao, T. Akutsu, Y. Tanaka, "Non-wetted Thermal Micro Flow Sensor," Proceedings of SICE Annual Conference 2007, 2007