Method of Controlling a Chemical Process Heated by Microwave Radiation

This invention relates generally to the use of microwave radiation in chemical processes, and more particularly, to a method of controlling microwave assisted chemical reactions or solvent extraction processes.

Digestions of materials in digesting liquids, such as strong acids, have been conducted in digesting systems wherein the digesting liquid was heated by microwave radiation. See, for example, European patent 0,416,759, European published specification 0,455,513 and U.S. Pat. No. 5,215,715. Extractions with solvents, wherein the solvent is sufficiently polar to absorb microwave radiation and convert it to heat, have also been employed as reported in Volume 371 of the Journal of Chromatography, at pages 299-306. A mixture of polar and non-polar solvents can be utilized, with the polar solvent absorbing microwave radiation and causing heating of the mixture.

Although digesting acids are corrosive, they are not usually explosive; however, many organic solvents are explosive when their vapors are mixed in a certain ratio with air. Thus, a safety problem can be encountered when conducting microwave heated extractions unless potential explosions are recognized and procedures are followed to prevent any such explosion. In U.S. Pat. No. 5,620,659 issued Apr. 15, 1997 in the name of Revesz, a system for preventing explosive incidents is disclosed. The system operates by detecting the presence of very small quantities of solvent vapors in exhaust air from a microwave chamber. When the solvent vapor concentration increases to explosive proportion, the source of microwave radiation, which otherwise could cause ignition of an explosive mixture of solvent vapor and air in the microwave chamber, is shut down. Alternatively the entire electrical system of the digesting apparatus is shut down in response to such detection. To be safe, such shutdowns occur when the content of solvent vapor in air is less than the lowest explosive limit. Thus, according to the teachings of Revesz, early detection of any leak in the system or of any rupture of a safety diaphragm on the extracting vessel is essential. The purpose of the rupture diaphragm is to prevent explosive failure of the extracting vessel due to excess pressure that might be developed within while the extracting solvent is heated.

European patent specification 0,455,513, the disclosure of which is hereby incorporated by reference, discloses such a rupture diaphragm in a digesting system. Further, U.S. Pat. No. 5,620,659 teaches the presence of a collecting container for collecting any vented vapor and liquid from a digesting container that has blown its rupture diaphragm. Though the system taught by Revesz is useful in preventing explosions, such a system is not useful in controlling an extraction process. From the teachings of Revesz, it is clear that once failure of an extracting vessel has occurred, monitoring for explosive chemicals is necessary. Commonly, when using microwave radiation for extraction, a plurality of vessels is placed within a microwave oven for simultaneous extraction. When a rupture diaphragm on a single vessel ruptures, the escaping solvent vapours may result in shutting down the system. Unfortunately, such a result affects all vessels and samples within the device. As such, it is not a desirable outcome. Further, when an extraction process is progressing, the rupturing of a rupture diaphragm results in an open topped vessel where a closed top vessel was desired. This open top often renders a resulting extraction unsuitable for analysis. Therefore, though a catastrophic failure such as an explosion is averted, it is only at the cost of the reactions in progress.

In U.S. Pat. No. 5,382,414 issued on Jan. 17, 1995 in the name of Lautenschlager and herein incorporated by reference, a resealable venting vessel cover is disclosed. Such a vessel cover obviates a need for a rupture disc by releasing gas from a vessel when pressure within the vessel is excessive and resealing the vessel when the pressure within the vessel is again below the threshold pressure. The released gas may accumulate in a collection chamber when present or in the microwave chamber itself when no collection chamber is used. The gas is vented through a venting system. The use of the resealable venting vessel cover, allows for continued extraction, digestion or hydrolysis of the sample contained within the vessel with only nominal loss of vessel contents.

In U.S. Pat. No. 5,447,077 issued Sep. 5, 1995 to Lautenschlager, which is hereby incorporated by reference, a device for microwave radiation heated evaporation is disclosed. Such a device employs vessels with outlets therein through which evaporated chemicals are discharged and an inlet through which air or other gases are fed into the vessels. It would be advantageous to provide a system for controlling a process heated by microwave radiation such as extraction, digestion or hydrolysis in order to improve the results of the process. It would also be advantageous to control a process heated by microwave radiation in order to avert explosive situations or potentially explosive situations without stopping the process.

In accordance with the invention there is provided a method of controlling a microwave heated chemical process. The method comprises the steps of: sensing a chemical substance within gases in one of the microwave chamber and the microwave exhaust system to provide sensed chemical information; analyzing the sensed chemical information to determine a change; in response to a change in the sensed chemical information, performing one of reducing microwave radiation energy by an amount less than the current microwave radiation energy level and increasing microwave radiation energy.

In accordance with another embodiment the invention there is provided a method of controlling a microwave heated chemical process. The method comprises the steps of: applying microwave radiation at a microwave radiation energy level to a sample contained within a vessel; sensing a chemical substance escaping from the vessel to provide sensed chemical information; analyzing the sensed chemical information for a change; in response to a change in the sensed chemical information altering an external stimulus to the system while maintaining application of some microwave radiation to the sample.

In accordance with another embodiment the invention there is provided a method of controlling a microwave heated chemical process. The method comprises the steps of: sensing a chemical substance within gases in one of the microwave chamber and the microwave exhaust system to provide sensed chemical information; analyzing the sensed chemical information to determine a change; and in response to a change in the sensed chemical information, without terminating the process, altering an aspect of the process.

According to another aspect of the invention, there is provided a microwave system for controlling a microwave heated chemical process comprising:

a sensor for sensing a chemical substance within gases in one of the microwave chamber and the microwave exhaust system to provide sensed chemical information;

a processor for analyzing the sensed chemical information to determine a change in the chemical process that could result in the change and for providing a first signal in dependence upon the change; and,

a controller responsive to the first signal for altering an aspect of the process without terminating the process.

Exemplary embodiments of the invention will now be described in conjunction with the following drawings, in which:

FIG. 1 is a drawing of a microwave oven having an open door and sample vessels within as known in the prior art;

FIG. 2 is a drawing of a microwave oven having an open door and sample vessels within according to the invention;

FIG. 3 is a drawing of a microwave chamber containing a vessel having a resealable venting vessel cover;

FIG. 4 is a drawing of a cracked vessel with a sealed vessel cover within a microwave chamber;

FIG. 5 is a simplified diagram of a sensor comprising a plurality of integrated circuits for sensing each of a plurality of different chemicals;

FIG. 6 is a simplified block diagram of an analysis circuit;

FIG. 7 is a simplified flow diagram of a method according to the invention.

FIG. 8 is a simplified flow diagram of another method according to the invention wherein a second other process is controlled based on chemical concentrations detected; and

FIG. 9 is a simplified diagram of a microwave oven having two separate exhaust systems.

Referring to FIG. 1 a prior art microwave chamber is shown containing a plurality of vessels and a collection chamber. The microwave extraction apparatus or system 11 comprises a chamber 13 which includes a ceiling 15, three side walls 17, a door 19 and a floor 21, with the ceiling having mounted therein a light or lamp 23. A source of microwave radiation, such as a magnetron (not illustrated) directs microwave radiation into the chamber. In the chamber is mounted a turntable 25 on which are positioned a plurality of extraction containers 27.

In FIG. 1, a view of extraction vessels on the turntable in a microwave chamber is shown. The extraction vessels are connected to a collection container wherein any discharges from the extraction vessels, including solvent vapor and entrained liquid, are collected. The collection container head space is connected to solvent detecting and magnetron controlling means external the microwave chamber. This connection allows solvent vapor within the collection container head space to pass from the collection chamber to the solvent detecting means. Extraction containers 27, all of which include rupture diaphragms, each connect to tubes 31, which carry discharged vapor and any entrained material from containers whose rupture diaphragms have burst due to the containers having reached their pre-set maximum design pressure to collection container 33. The headspace in container 33 is connected by another tube 35 to an exhaust duct that contains a vapor detector. In operation such a device monitors vapor concentration to shut down the magnetron in case of sufficient solvent vapor drawn from the collection container through the tube 35 into the exhaust, possibly because of a vapor surge.

It is well known to have chronologically controlled microwave devices. A process involving several process stages each requiring a predetermined microwave radiation energy level and each requiring a predetermined length of time is entered as a plurality of chronological stages. For each stage a time and energy level/temperature are entered.

When the process is begun, each stage, except a first stage, is executed following a previous stage. Thus, a plurality of stages is preprogrammed and the process stages are performed one after another. From a scientific perspective, chronological control is optimal since it enhances repeatability and consistency of results. Unfortunately, when using such a programmed process, it is difficult to effect changes to the process in dependence upon information gathered during processing. According to the invention, a method of utilizing chemical data gathered during process execution to modify process parameters and thereby, increase microwave apparatus flexibility is disclosed.

Referring to FIG. 2, a microwave oven is shown. The microwave oven 201 comprises chamber 202, fan 204, exhaust in the form of an outlet tube 206, and a microwave radiation generator in the form of a magnetron 208. Within the outlet tube 206 is disposed a sensor 210 for sensing chemical content of exhaust gases. The fan 204 draws the exhaust gases from the microwave chamber 202. An output signal from the sensor 210 comprises chemical information in the form of chemical concentration information for analysis to determine progress of a current microwave heated chemical process. This determination is then used to adjust aspects of system control.

Referring to FIG. 2b, a microwave oven is shown. The microwave oven 201 comprises chamber 202, fan 204, exhaust in the form of an outlet tube 206, and a microwave radiation generator in the form of a magnetron 208. Within the chamber 202 is disposed a sensor 210 for sensing chemical content of exhaust gases. The fan 204 draws the exhaust gases from the microwave chamber 202. An output signal from the sensor 210 comprises chemical information in the form of chemical concentrations for analysis to determine progress of a current microwave heated chemical process. This determination is then used to adjust aspects of system control. Of course, many other configurations are employable with a method according to the invention.

Referring to FIG. 3, a microwave chamber 202 containing a vessel 327 having a resealable venting vessel cover is shown. It has been found that vessel covers as described in U.S. Pat. No. 5,368,820 hereby incorporated herein by reference, often seat during the first few minutes of an extraction process. Sometimes, when the microwave heating begins, vessel covers are not completely seated on a mating vessel surface. With application of heat and resulting pressure, these vessel covers each move a small amount, thereby adjusting their placement to a correctly seated placement. During this stage, minute quantities of reaction gas are known to escape from such vessels. This quantity of gas, when it escapes, is detected by the sensor 210 and a signal from the sensor dependent upon detected chemical concentrations is analyzed.

In a typical application of the present invention, a microwave heating apparatus is provided with a ventilation system drawing air through the microwave chamber and out an exhaust. The sensor 210 measures gas concentrations within this moving stream of air. Therefore, in such an embodiment, variations in gas concentrations and gas concentrations over known periods of time are monitored. When a concentration of predetermined chemicals detected by the sensor 210 within the gas is reduced over that previously detected, the process is adjusted as appropriate for seated vessel covers. For example, microwave heat energy is modified, ventilation rate is adjusted, stirring is initiated, stirring is terminated, stirring rate is adjusted, or so forth. When the process for seated and unseated vessel covers is identical, no adjustment is performed.

Alternatively, a microwave energy level is increased in order to begin a process in earnest only after vessel cover seating has occurred. Alternatively, a microwave radiation energy level is decreased once seating has occurred to allow a process to proceed more slowly. Further alternatively, the process is modified when chemical concentrations indicative of an improperly seated vessel cover are detected. Thus, depending on a chemical process and in accordance with known chemical principles, the microwave radiation energy level and/or another aspect of a chemical process is adjusted in dependence upon chemical sensor feedback. When using a vessel cover constructed of a material that softens with application of heat, it has been found that seating is improved significantly once the material is softened--heated. For such covers when an unseated vessel cover is detected, microwave radiation energy is reduced to allow heat from the previously applied power to warm the vessel cover and thereby better seat the cover. Once the cover is seated, the process proceeds as programmed. Of course, for different vessel covers and different chemical processes, different corrective actions may be applied.

Referring again to FIG. 3, it is known that some gases diffuse through vessel walls during a microwave heated chemical extraction, digestion or hydrolysis process. This phenomenon is significant under predetermined conditions of, for example, process temperature, pressure or time. A sensor 210 detects concentrations of chemicals within gases passing by the sensor. Since some chemical species are more likely to diffuse than others, a time resolved signal from the chemical sensor 210 indicates whether the detected chemicals within the gas likely result from diffusion or from another source such as an unseated vessel cover. When diffusion is determined to be a cause of sensed chemical concentrations within the outlet tube 206, microwave radiation energy levels are adjusted to maintain diffusion in a desired range. Alternatively, microwave radiation energy levels are adjusted to return detected diffusion to a predetermined range. The range is determined based on the chemical process performed. Of course, similar process control is achieved when the chemical sensor 210 is within the microwave chamber 202 as shown in FIG. 2b. For clarity, since some chemical species are more likely to diffuse than others, a presence of higher than normal concentrations of those chemicals indicates that diffusion is likely occurring. By determining, in advance, information relating to diffusion rates of different chemicals, diffusion signatures are achieved and it is sometimes possible to distinguish between chemical concentrations resulting from diffusion and chemical concentrations within a same quantity of gas that result from other causes.

Referring to FIG. 4, a cracked vessel 327a with a sealed vessel cover is shown in a microwave chamber. The crack 427 is in the form of a minute fissure in the side wall of the vessel. When the vessel cracks, an amount of gas escapes through the crack. This gas is drawn through the outlet tube 206 past the sensor 210 where it is detected. An amount of gas detected indicates a size of an opening in a vessel. When the opening is small, it appears similar to venting of a vessel cover during venting; however, unless the fissure is resealable, the venting of gases through the fissure continues over a lengthy period of time. For example, when a vessel undergoes catastrophic failure, the gases within the vessel are drawn through the outlet tube and the sensor detects chemical concentrations within the gases. It may be that sufficient explosive chemicals exist to necessitate a system shut down as taught by Revesz. Alternatively, fanning is increased substantially in order to diffuse the gases more quickly. Starting fans 204b near the outlet port of the outlet tube 206 increases fanning. The additional fans disperse the gas quickly and effectively. Alternatively, the fan 204 is turned at a faster rate to increase airflow through the microwave chamber and thereby vent the gases more quickly. Alternatively, inert gases are caused to flow into the microwave chamber in order to displace oxygen from the chamber and thereby suppress the potential for ignition or explosion of explosive gases. Of course, other means of venting the microwave chamber more quickly are also available.

When a resealable venting vessel cover exhausts gas, the gas is drawn past the sensor 210. The gas is detected and the chemical process is adapted in response to the sensed chemical concentrations. For example, venting of the microwave chamber is increased while microwave radiation energy is reduced to prevent further venting of sample vessels. Of course other adjustments are possible and these are determined in dependence upon chemical processes occurring in each vessel. By analyzing the time resolved change in chemical concentrations within gases detected, it is possible to distinguish between venting of a vessel, chemical diffusion through the vessel wall, or incomplete vessel cover seating. This allows for detection and/or monitoring of three separate phenomena simultaneously and for controlling an ongoing process somewhat independently for each of the three phenomena detected. In order to determine chemical concentration signatures for each of said phenomena, experimentation with the phenomena is performed for a predetermined chemical process and results are used for analysis of chemical concentrations detected when performing same or similar chemical processes. The sensor comprises one of a single sensor for detecting a single chemical substance, a plurality of sensors for detecting a plurality of chemical substances, or a single sensor for detecting a plurality of chemical substances.

Referring to FIG. 5, a sensor is shown. The sensor is in the form of a plurality of integrated circuits each for detecting a different chemical and/or a different chemical concentration. Sensor 501 is an Nox sensor for detecting NOx within the exhaust fumes. Sensors for detecting Nox are known in the art of chemical sensor design and are available--for example, sensor type NAP-11AN, from MLS GmbH, Mikrowellen-Labor-Systeme, Leutkirch, Germany. Sensor 503 is a sensor for a group of organic molecules that are potentially explosive. Other sensors detect other chemicals such as hydrofluoric acid, etc. Each sensor provides a signal based on a composition of a gas in which the sensor is placed. The sensor shown in FIG. 5 is assembled in a fashion such that air drawn through the outlet tube 206 passes around each integrated circuit. Signals from each integrated circuit are provided to an analysis circuit.

Referring to FIG. 5a, another embodiment of a sensor is shown wherein two integrated circuits are mounted on opposing sides of a probe. Of course, when a sensor is disposed on a wall of the outlet tube 206 or of the microwave chamber 202, a probe is not necessary.

Referring to FIG. 6, a block diagram of an analysis circuit is shown. The analysis circuit comprises a processor in the form of a digital signal processor (DSP) for receiving signals from the sensors 210 and for determining a concentration of some chemical substances proximate to the sensors 210. The concentration of predetermined gases and other information are used to determine process adjustments to maintain safety and where possible viability of a current chemical process. Only in extreme circumstances is the entire system shut down.

Referring to FIG. 7, a flow diagram of a method according to the invention is shown. Chemical concentrations are detected using a sensor within the microwave chamber 202 or within the microwave exhaust 206. The detected chemical information is provided to a processor in the form of a DSP. The information is analyzed along with past detected chemical information, past control responses, and stored data in the form of anticipated outcomes of past control responses. Based on the analysis, a control response is determined and the chemical process is controlled based on the determined control response. Specific control responses are outlined above and below. Of course, a combination of control responses is employed when determined beneficial. In an embodiment employing a combination of control responses, one of a neural network and an expert system is used to determine optimal control responses for detected chemical concentrations. The design and implementation of such a control system is well known in the art of computer science and more particularly in those areas of computer science focused on design and implementation of the above mentioned systems.

In an embodiment, stirring speed is modified according to the determined control response. When stirring is used to assist a process, the stirring is commenced once the process has progressed sufficiently to ensure that vessel covers are well seated. Alternatively, stirring speeds are increased or decreased in accordance with detected chemical concentrations. Depending on a chemical process within a sample vessel, gases formed within the vessel and therefore emitted by diffusion, venting, unseated caps, etc. are reduced or increased through application of increased stirring. Sometimes, gas production is unaffected by stirring. When gas emissions are affected by stirring, controlling stirring allows for an increase or reduction in gas within sample vessels is during a process. For example when stirring increases gas formation, it is not desirable to stir until vessel covers are well seated. Therefore, a stirring mechanism is disabled or very slight stirring occurs until vessel cover seating is determined to be acceptable.

In an embodiment, chemical concentrations are analyzed to determine chemical profiles for assessing causes of predetermined portions of chemical concentrations detected within the gas. For example, when a first chemical diffuses more than a second, a profile of the diffusion ratio allows a predetermined algorithm to process chemical concentrations and determine what portion results from venting of vessel covers and what portion results from diffusion. These results form a chemical profile in the form of chemical information, concentration information, and sources of each concentration or of portions of the concentrations. Of course, chemical profiles may incorporate other data for determining control responses.

According to another embodiment, microwave radiation energy is modified according to the determined control response. For example, when a profile of detected chemicals is similar to a chemical profile indicative of an unseated vessel cover, the power provided to the magnetron is reduced and, therefore, the microwave radiation energy level is reduced. Subsequently, when the profile of detected chemicals is similar to a chemical profile indicative of well seated vessel covers, the power level is returned to a normal level.

According to another embodiment, process time or times for different aspects of a process are modified in order to control said process. The times are, for example, extended when insufficient diffusion occurs or are foreshortened when excessive vessel venting occurs. Preferably, process time modification and microwave radiation energy modification are used together to effect controlled processes achieving substantially similar results to those desired by an operator. In a common digestion process, a sample within a vessel is heated to a predetermined temperature and maintained about that temperature for a predetermined period of time. Using a method according to the invention, the heating of the sample is effected in a more controlled manner until it reaches the predetermined temperature. Effects of diffusion and unseated vessel covers are minimized when that is desirable. The process control functions do not effect the repeatability or effectiveness of the overall process. Though the term chemical concentration is used throughout this disclosure, a more suitable phrase is time resolved concentration or the change in concentration as a function of time.

It is apparent to those of skill in the art that time resolved concentration is determined by comparing a detected concentration to a previously detected concentration. For example, an ill fitted vessel cover results in a rising concentration with slope dependent on the applied power and reactions going on inside the vessel; the ill fitted vessel cover is usually only of concern early on in a process. Diffusion through vessel walls results in a low and slowly rising concentration. Venting results in a detected chemical concentration spike that is narrow in time; the concentration rises rapidly when venting occurs and falls off rapidly since the vessel cover reseals. A cracked, broken or exploded vessel likely results in a large detected concentration spike that settles down to a new, higher baseline that decreases over time as the process progresses and exhausting of fumes within the microwave chamber continues. Molecules of the form NOx are easily detected. This is desirable because of the corrosiveness of the nitric acid used in aggressive sample digestions and the potential for damage to equipment and injury to personnel. Some chemicals are monitored because they are carcinogenic; some chemicals are extremely toxic even to the point of being lethal such as hydrofluoric acid and its fumes. Typical digestion acids are hydrochloric acid, nitric acid, hydrofluoric acid, phosphoric acid, hydrogen peroxide, perchloric acid. Toxicity, hazardous exposure, corrosiveness to equipment, and potentially lethal exposure are all important issues in controlling a process and, according to the invention, are monitored and prevented when possible; however, for control of a process, it is important to continue the process in the form of digestion, extraction, and/or hydrolysis to its conclusion when feasible.

In an embodiment shown in FIG. 8, instead of affecting process control, the sensor signal is used to affect another process--for example to enhance process safety. This is achieved by providing safety processes for initiation upon detection of predetermined chemical concentrations. For example, should a vessel catastrophically fail--explode--the contents are dispersed within the microwave chamber. When the contents are hazardous in the form of toxic chemicals or explosive chemicals, upon detection of a high concentration of the chemicals, neutralizing chemicals or inert gases are provided at an inlet to the microwave chamber in order to dilute and/or neutralize the hazardous chemicals. Since such neutralizing process or agent is sometimes very costly, it is advantageous to employ same only when a hazard occurs. Another safety process according to the invention involves a second other exhaust for providing chemical neutralization as shown in FIG. 9. When detected hazardous chemical concentrations exceed predetermined thresholds, the first exhaust 206 is substantially sealed and the gas including the hazardous chemicals is directed through the second other exhaust 207 to a system 207b for neutralization or appropriate storage. This effectively, improves safety in processing of hazardous chemicals.

Alternatively, instead of a single sensor within a microwave apparatus, a plurality of sensors are used to detect gas concentration information.

The detected concentrations are analyzed to produce gas flow related information for adjusting a process, venting, safety processes, stirring, etc. according to the invention. The use of a plurality of chemical sensors allows for more accurate estimation of chemical sources and causes of chemical emissions. Also, the use of a plurality of sensors within the microwave chamber and within the exhaust enhances control options and fault tolerance.

In a further embodiment, the method of controlling a process disclosed herein is used during microwave radiation heated evaporation. The evaporation is performed, for example, in a device such as that disclosed in U.S. Pat. No. 5,447,077. A chemical sensor disposed so as to detect chemical concentrations of evaporated gases is used to monitor an ongoing evaporation process. For example, chemicals, which evaporate above but very near the desired evaporation process temperature, are monitored and when an increase in concentration is detected, the process temperature is reduced to prevent loss of those chemicals. Of course, monitoring of an evaporation process is also useful for implementing safety processes, preventing catastrophic failures, recording data regarding process progression and execution, and for controlling the evaporation process in numerous other ways. As is evident to those of skill in the art, monitoring of an evaporation process is possible when a sensor is disposed within a flow of evaporated gases. For example, a sensor disposed at an outlet to a sample vessel or at an evaporation exhaust to an evaporator apparatus detects chemical concentrations within exhausted gases. Of course, chemical sensors disposed outside of the evaporation apparatus allow for detection of leaks in the evaporation apparatus, diffusion, catastrophic failure, etc.

Numerous other embodiments are envisaged without departing from the spirit or scope of the invention.