IT201800006980A1 - SYSTEM FOR IMPROVING THE MEASUREMENT OF THE DEW POINT OF A FLUID OF A PROCESS FLUID - Google Patents

Description

Patent application for an invention entitled "SYSTEM TO IMPROVE THE MEASUREMENT OF THE DEW POINT OF A PROCESS FLUID"

DESCRIPTION

Field of application

The present invention relates to a method and relative system for measuring the dew point of a process fluid and in particular of the process fluid used in the dehumidification of plastic materials.

In particular, the invention which is the subject of the present patent application finds application in the sector of plants and devices for the dehumidification of plastic material, for the subsequent melting and molding in a transforming machine.

In particular, the invention which is the subject of the present patent application relates to a system for measuring the dew point, known as "dew point", of the fluid used by way of example but not exclusively for the dehumidification of the plastic material.

The proposed system is therefore generally applied to those processes in which a fluid with low values of dew point DP is used, by way of example but not exclusive, DP <-30 ° C, and in particular, to dehumidification systems of plastic material .

State of the art

As is known, the plastic material, in the form of granules or flakes, is transformed into finished or semi-finished objects by heating, casting, molding or extrusion. As is known, plastics, given their hygroscopicity, contain water molecules; during the melting processes, the water molecules can compromise the polymeric structure of the plastic materials themselves, causing surface or structural defects in the finished or semi-finished products, compromising the quality of the final product.

It follows that, in order to avoid the formation of bubbles and cavities in the plastic material and any alterations in the chemical structure of the same, the control of humidity in the mass of granules and therefore the dehumidification process, is necessary and fundamental in the process of transformation of plastic materials.

To extract the moisture from the plastic material, the treatment of the plastic material with different drying fluids is currently envisaged.

In the dehumidification systems currently used, we then proceed with the introduction of a certain amount of plastic material to be dehumidified in a hopper in which the material is subjected to the action of the drying fluid, called process fluid, which removes its moisture.

It is known that the process fluid is conveyed by process fluid generators, typically air or air / nitrogen mixtures, in jargon known as the “dryer”. Once the fluid has been introduced into the container where the plastic is stored, it passes through the plastic mass, removing its moisture.

The achievement of the optimal degree of dehumidification for a given plastic material that will subsequently be subjected to melting is conditioned by numerous factors including the residence time of the material in the hopper, the flow rate of the process fluid and the treatment temperature.

To change the degree of residual humidity that the granular material has at the end of the dehumidification treatment, one intervenes both on the residence time and on the characteristics of the process fluid, changing the specific flow rate, the temperature and the dew point temperature ( or dewpoint).

Remember that the dew point or dewpoint is that thermodynamic state in which, at a certain temperature and a certain pressure, a fluid-vapor mixture becomes saturated with water vapor.

Therefore with dew point temperature or dew point temperature, hereinafter referred to as DP, we mean the temperature at which, at constant pressure, the saturation of water vapor of the fluid, in particular of the process fluid, occurs.

It follows that the DP value of the fluid used for the dehumidification of the plastic material is a parameter of fundamental importance in the management of the process.

By way of example, the description of a typical dehumidification process in PET transformation processes is given below.

Notoriously, in the transformation processes of plastic materials, a fluid (air / nitrogen mixtures etc.) is used for dehumidification with the DP indicatively included: -80 ° C <DP <-30 ° C

Table 1 below shows indicative values of the parameters of a known process by way of example.

Table 1

Inside the dryer, there are special towers containing adsorbent material capable of fixing the moisture present in the fluid.

Generally, the fluid leaving the dryer is sent along the delivery pipe to a heater and after passing through the plastic material contained in special hoppers, it returns to the dryer along the return pipe, to transfer the moisture extracted from the material to the towers. .

The process fluid is then pushed again by the pump along the circuit to ensure that the material reaches the temperature / humidity conditions required by the transformation process.

For the reasons already listed, the optimal condition is that the process fluid leaving the towers must have a specific DP generally <-30 ° C.

In the consolidated technique, the measurement of the DP is carried out with special probes, called dewpoint probes.

The measurement is usually carried out on a sample flow Qm tapped from the main flow with a measurement circuit that can be open in the environment or closed on the return to the dryer. It is known that dewpoint probes measure the DP of the Qm flow at a pressure Pm similar to that measurable in the process flow Q at the bleed point (P≈l bar). Under these pressure conditions (P ≈ 1 bar), the dewpoint probes must measure very low% of H2O in the fluid (approximately 1 ÷ 40 ppm), making the measurement uncertain and not always reliable.

Dew Point probes are known to be precision instruments that generally use sensors (metal oxides / capacitive polymers / nano structures etc.) capable of varying their own voltage / current response even with small% H2O variations perceived by the sensor itself.

Table 2 shows the Pressure, Flow and Temperature parameters in which the dewpoint probes usually operate.

Table 2

Table 3 shows the correlation between the quantity of 2O and DP at a pressure of 1 bar

Table 3

From Table 3 it is possible to note the quantity of H2O at DP = -80 ° C is <1/200 of that present at DP -40 ° C.

In general, the dewpoint probes show some problems, listed below, which affect the accuracy of the measurement.

Sensitivity to temperature

Dewpoint probes are generally calibrated at a flow temperature Tf: 20 ° C <Tf <50 ° C.

It follows that with Tf> 50 ° C the measurement accuracy is generally reduced.

Sensitivity to variations in flow on the probe

As described, the dewpoint probes are generally fed by a flow of sample fluid Qm tapped from the process flow.

As a result, flow rates Qm »5 l / min generally lead to rapid aging of the probe.

Furthermore, values of flow rates Qm <1 l / min generally tend to alter the measurement.

In the consolidated technique, the flow rate Qm is obtained by exploiting the ΔΡ existing between the withdrawal point downstream of the towers and that of reintegration at the suction of the dryer pumps.

This ΔΡ is not constant but varies according to various parameters such as: process flow rate (Q), level of material in the hopper, clogging of filters and coolers etc.

Therefore the flow rate Qm is generally not constant, but can vary from Qm ≈ 0 to Qm »5 l / min.

Uncertainty of measurement with Dew Point <-50 ° C / -60 ° C

As reported above, for example, with Dew Point = -70 ° C approximately 2.6 ppm of H2O present in the process fluid must be measured, often in less than ideal temperature conditions. It follows that in common practice it is not prudent to manage the dehumidification process with only the Dew Point probe.

Tendency over time to measure values of D P <of the real

The dewpoint probes are generally subject to natural aging, linked to oxidation phenomena, material deposits, saturation, etc., which reduce the sensitivity of the probe to H2O by making it detect% of HO slightly lower than the real value.

It follows that the dewpoint probe tends over time to overestimate the dehumidification capacity of the dryer while, on the contrary, this capacity actually tends to decrease due to the saturation of the adsorbent towers.

This can easily have negative repercussions on the production process.

These factors therefore generate uncertainties in the measurement at low DP (1 ÷ 40 ppm H2O indicatively) and therefore difficulties in the management of the dehumidification processes of plastic materials, with consequent possible quality problems and disputes of the finished product.

As previously commented, the process fluid DP is a fundamental parameter in the dehumidification process, but it is also a parameter that is not easy to measure as it is often located at the lower limits of the working range of the dewpoint probes currently available.

The purpose of the invention is therefore to make the measurement of the DP in the process fluid safer and more reliable, especially for low concentrations of H2O (approximately 1 ÷ 40 ppm H2O).

In particular, the object of the invention which is the subject of the present patent application is to allow a reliable measurement of the DP while maintaining the value of the measurement flow (Qm) at the optimum value.

In particular, the object of the invention which is the subject of the present patent application is to provide a system capable of guaranteeing a flow (Qm) as constant and adequate as possible for the dewpoint probe (9) using an automatic adjustment system of the measurement flow. (Qm).

The proposed system acts on the measurement range (Qm), which is one of the fundamental parameters to be adjusted during the measurement of the Dew Point.

Since, as already described, the operating conditions of the plants are generally variable and different from those considered in the design phase, the actual measurement flow in working conditions (Qm) differs significantly from the design one and is frequently found in the outside the range recommended by the manufacturers of the Dew Point probes.

Thanks to the proposed system for measuring the Dew Point, it is therefore possible to reduce the uncertainties inherent in the current consolidated measurement technique, improving the quality and reliability of the dehumidification process of plastic materials.

Brief description of the drawings

The technical characteristics of the invention, according to the aforementioned purposes, can be found in the content of the claims and the advantages of the same will become more evident in the detailed description that follows, made with reference to the attached drawings which represent one or more embodiments, exemplary and not. limiting, in which:

- Figure 1 shows a diagram of a generic dehumidification system;

- Figure 2 shows a system diagram of a first embodiment of the invention;

- Figure 3 shows a system diagram of a functional variant to the first embodiment of the invention;

- Figure 4 shows a system diagram of a functional variant to the first embodiment of the invention;

- Figure 5 shows a system diagram of a functional variant to the first embodiment of the invention;

- Figure 6 shows a system diagram of a functional variant to the first embodiment of the invention;

Detailed description

In the description and in the claims reference will be made to a process flow for carrying out the dehumidification of the plastic material; it is understood that the expression "process fluid" is not limited to the use of air but also includes the use of other treatment fluids suitable for the purpose and that this process fluid could also be used for different purposes from the dehumidification of the plastic material.

According to a general embodiment of the invention, the dehumidification plant represented in Figure 1, comprises at least one generator (1) of dry fluid, said generator called dryer, inside which there are special towers (5) containing adsorbent material able to retain the humidity present in the process fluid;

- at least one filter (2);

- at least one cooler (3);

- at least one reintegration point (20);

- at least one sampling point (21);

It is specified that the withdrawal point (21) and the make-up point (20) can also be located in different points of the dehumidification system, based on specific application needs and that the make-up point (20) could be missing if a measurement circuit open in the environment (11A).

The standard dehumidification system typically used in the usual technique also includes

- at least one hopper (12) in which the plastic material (13) is stored;

- at least one heater (7)

- at least one delivery pipe (6)

- at least one return pipe (22)

- at least one pump or fan (4).

Generally the fluid leaving the dryer (1) is sent along the delivery pipe (6) to a heater (7).

From the heater (7) the fluid is sent inside the hopper (12) through a generic diffuser in order to pass through the plastic material (13) contained.

Reached the top of the hopper (12), the process fluid loaded with moisture absorbed by the plastic material (13), crosses the return circuit (22) with a certain flow rate (Q) and is then sent back to the dryer (1).

The fluid is then pushed again by the pump (4) into the towers (5), along the delivery circuit (6), heater (7), hopper (12) and return circuit (22) to ensure that the material (13 ) reaches the temperature / humidity conditions required by the transforming machine (14).

According to an essential aspect of the invention, the dehumidification unit comprises at least one device (9) for measuring the dew point temperature DP of the flow out of the dryer, said at least one device typically consisting of a probe called dewpoint probe,

The at least one dewpoint probe is generally fed by a measuring circuit (8-1 1 / 8-11 A) in which the sample fluid Qm tapped from the process circuit (6) flows to a sampling point (21) and returned to the process flow at a make-up point (20),

As previously commented, in order to achieve optimal conditions, the process fluid leaving the towers (5) must have a specific DP, generally <-30 ° C).

The aim of the invention, which is the subject of the present patent application, is therefore to provide a regulation and measurement process which, for processes with a high flow rate value (Q), involves exclusively the sampling circuit part. In this case, in fact, the energy expenditure connected with the measurement process is very low. If, on the other hand, one operated on low-entity process flows (Q), it would be possible to operate directly on flow (Q) in the condition: Q = Qm. In a first embodiment (Figure 2) the withdrawal circuit (8) provides at least one dissipator (26), at least one modulating valve (29) which separates the pressure delivery circuit (P0), from the return circuit (P2), at least one temperature sensor (27), at least one high pressure sensor (28), at least one dewpoint probe (9), at least one low pressure sensor (34).

In a preferential form (Figure 3) it is possible to provide at least one flow meter (36) to adjust the flow rate Qm to a settable value.

The valve (29) can in fact be managed on the basis of the pressure values measured by the sensors (28-34) or on the basis of the readings of an anemometer (36).

In a further preferential embodiment (Figure 3-4-5-6) it is possible to foresee the presence of a pump / fan (25) possibly managed by an inverter, to guarantee an adequate flow to the DewPoint probe (9) in all operating conditions.

In a further preferential embodiment (Figure 4) it is possible to realize a bypass circuit (31) managed by suitable valves (30), able to allow the use of the measurement system even with a partially bypassed fan / pump (25) or disabled.

In a further preferred embodiment (Figure 5) it is possible to provide at least one gas / liquid exchanger (33-33 ') to replace the dissipator (26-26') in order to reach lower and more stable measurement temperatures.

As reported in the embodiment shown in (Figure 6), the presence of at least one evaporator (37) is considered for cooling the flow entering the probe.

In (Figure 6) an alternative embodiment is reported in which at least one thermoelectric device (35) is used for cooling the fluid entering the probe (9).

This flow rate regulation system is able to manage the measurement flow (Qm) from Qm = 0 up to a suitably defined value Qm = Qmax, both with the fan (25) active, and with the fan (25) bypassed or deactivated.

It follows that this active system for managing the Qm flow on the dewpoint probe (9), is able to solve the problem of the variations in the Qm flow rate found in the typical dehumidification systems of plastic materials, in which the adjustment of the lamination valve ( 10) is fixed.

In common practice, the Qm flow directed to the D.P. probe is cooled. (9) with heatsinks (26 Figures 2-3-4).

This cooling method, although effective, does not allow to drop below the temperatures of the operating environment which, in some cases, reach 60 ° C.

Since cooling water is almost always available in dehumidification systems, it is possible to provide for the cooling of the sample flow (Qm) also by means of a gas / liquid exchanger (33) (Figure 5-6) or, in the absence of cooling liquid, by means of an evaporator (37) (Figure 6) or a thermoelectric device (35) (Figure 6).

Claims (11)

CLAIMS 1. DP dewpoint measuring system, applicable preferably but not exclusively plastic material dehumidification plants having at least one fluid generator (1), at least one dehumidification hopper (12), at least one delivery circuit (6), at least one return circuit (22) and at least one measuring circuit (8-11 / 8-1 1A) equipped with at least one dewpoint probe (9), said system characterized in that the at least one measuring circuit (8-1 1 / 8-11 A) has a modulating valve (29) which regulates a sample flow (Qm). 2. Measuring system as per claim 1 characterized in that the measuring circuit (8-1 1 / 8-11 A) includes at least one dissipator (26). 3. Measuring system according to the preceding claims characterized in that it has at least one temperature sensor (27). 4. Measuring system according to the preceding claims characterized in that it has at least one high pressure sensor (28). 5. Measuring system as per the preceding claims, characterized in that it has at least one low pressure sensor (34) 6. Measuring system as per the preceding claims characterized in that it has at least one flow meter (36). 7. Measuring system as per the preceding claims, characterized in that it has at least one pump or fan (25). 8. Measurement system as per the preceding claims, characterized in that it provides a bypass circuit (31) managed by suitable valves (30). 9. Measuring system as per the preceding claims, characterized in that it provides at least one gas / liquid exchanger (33). 10. Measurement system as per the preceding claims, characterized in that it provides at least one gas / gas exchanger (37). 11. Measurement system as per the preceding claims, characterized in that it provides at least one thermoelectric device (35).