CN1206829A - Electro-chemical sensor for detecting chloro in phosgene - Google Patents

Abstract

An online system for detecting the presence of low levels of chlorine in a stream of phosgene is described. Detection is based on the electrochemical reduction of chlorine at a platinum electrode at potentials at which other components of the phosgene stream and electrolyte cause no interference. A continuous flow of electrolyte over the surface is maintained by gravity feed or by positive pumping mechanism. The electrolyte flow rate is about 1 to about 10ml per hour. Organic electrolytes are preferred although aqueous electrolytes can be used. The sensor of this invention can detect very low levels of chlorine. The electrode can be regenerated in place. The detector system comprises five main components including a gas delivery subsystem, the electrochemical sensor, a temperature control unit, a signal measurement subsystem, and a scrubber subsystem.

Description

Be used for surveying the electrochemical sensor of phosgene chlorine

The present invention relates to a kind of electrochemical sensor that is used for surveying and measuring the phosgene chlorinity, more particularly, relate to the sensor that is used for monitoring phosgenation air-flow trace amounts of chlorine.

Thereby by with bis-phenol for example bisphenol-A direct sunshine gasification phosgene or phosgene are used to produce carbonate polymer.In phosgene in addition the existence of a spot of chlorine can cause in the polycarbonate production process and polycarbonate resin in produce serious quality problems.Forming coloured reaction product in resin is exactly an example.

Because the toxicity and the corrosivity of these two kinds of materials, be complicated to the online detection of chlorine in the phosgene.Also can utilize the UV absorption that is not all the basis with spectrum between the chlorine photoreactive gas, but its susceptibility and reliability and unsatisfactory.The electrochemistry in industry sensor that is used for chlorine monitoring that is applicable to sudden leak detection is because limited electrolyte source and in the damage of phosgene stream electrode surface and can not continued operation.

The invention provides a kind of machine system of company that is used for surveying phosgene stream chlorine lower content.Detection is based on the electrochemical reduction of platinum electrode place chlorine under voltage, and other composition in the phosgene stream does not cause tangible mutual interference mutually with electrolyte under this voltage.In order to ensure the long-time stability of electrode, keep electrolytical continuous flow from the teeth outwards by gravity supply or pressure pumping installations.Electrolyte flow speed approximately is 1-10ml/ hour.The variation of flowing velocity is to the not obviously influence of susceptibility of sensor electrode.Although can use the water quality electrolyte, preferably organic bath.Sensor of the present invention may detect the chlorine of utmost point low content, for example from being less than 1ppm to a few percent.Electrode can restore in position.

Detector system is made up of five critical pieces, and they comprise gas delivery subsystem, electrochemical sensor, temperature control component, signal measurement subsystem and optional scrubber subsystem.

Fig. 1 is the synoptic diagram of whole detector system.

Fig. 2 is the cut-open view of electrochemical sensor unit and associated components.

System comprises three gas lines, i.e. chlorine pipeline in phosgene pipeline, nitrogen pipeline and the nitrogen. Every gas line is furnished with flow governor and metering device. We test with two kinds of such devices. Suitable flow governor comprise the mass flow controller and with the flowmeter of needle valve (although by mass flow controller delivery air specific discharge meter more accurately and lasting since the particle erosion in the air-flow or decompose due to hole plug may need often it to be keeped in repair). In fact flowmeter does not require maintenance, and can satisfy the accurately requirement of flowing for most on-line measurement.

Gas delivery system is installed in the inside of hermetic enclosure, and in the situation of phosgene pipeline leakage phosgene, thereby this shell provides the isolated protection of air operator, thereby and at low temperatures this shell provide thermal isolation to avoid the phosgene condensation. Control all air-flows and measure by flowmeter by the suitable needle valve that is installed in the box top. Nitrogen can be used for the chlorine standard sample that forms to demarcate, and also is used for the flushing sensing system. Stainless steel is connected all pipelines and connects with teflon tube. Check valve can be used for avoiding the cross pollution between the gas line.

Electrochemical sensor system comprises that one has the electrochemical sensor of at least two electrodes, a voltage stabilizer and an electrolyte conveying device.The most important parts of sensor are platinum (Pt) sensing electrodes, and it is also referred to as working electrode.Chlorine is electrochemically reduced to cl anion (Cl at the working electrode place ^-), and the reduction current that produces by working electrode goes on record on surveying instrument.Cl concn is high more in the air-flow, and the electric current that measures is also high more.Other electrode is an auxiliary electrode, is also referred to as the reading electrode.Use one section silver-colored line.When chlorine was reduced at the working electrode place, silver-colored auxiliary electrode was oxidized to silver chloride (AgCl) simultaneously in containing the electrolyte of cl anion.Thereby pure sensor response is a chlorine forms silver chloride with silver-colored oxidation, and does not produce the accessory substance of solubility.Third electrode is a reference electrode, and its major function is to make voltage stabilizer provide constant voltage to working electrode.Reference electrode is silver/silver chloride electrode, and it can contain Cl by one section silver-colored line is inserted into simply ^-Thereby forming thin silver chloride layer in the anionic electrolyte on silver surface makes.Also auxiliary electrode and reference electrode can be merged into an electrode.Yet more advantageously use electrode separately.

In order to ensure the stable surface property of platinum sensor electrode, on the platinum surface, use continuous electrolyte flow.Use reproducible electrolyte obviously to improve stability, reproducibility and the operation lifetime of sensor.Because electrolyte flows on sensor electrode continuously, electrode surface in the electrode bilayer and electrolyte are upgraded termly.Obtain this continuous electrolyte flow by the electrolyte induction system of forming by pump and teflon check valve.Pump not only is used to control electrolyte flow, thereby also is used to provide the electrolyte malleation to avoid phosgene might pass through the porosint seepage.The purpose of teflon check valve is to avoid unexpected phosgene seepage when electrolyte induction system generation seepage (for example compression pump tracheal rupture).Can successfully use two kinds of electrolyte of water quality and non-water quality.The major advantage of aqueous solution is that its cost is low; Shortcoming is the phosgene hydrolyzable and forms the hydrogen chloride that produces corrosive atmosphere.

The chlorine sensing system is completely cut off in the shell or box of heat control.This box has two purposes.Thereby it maintains sensor and prevents under the stationary temperature that phosgene is in low temperature (＜15 ℃) condensation down.It has prevented that also phosgene escapes under the situation of sensor seepage.Can provide heat to this shell by any device easily.Thermal source can be operated by temperature controller.For fear of because the box due to the malfunction of thermal coupling device is overheated, second temperature controller of connecting with main temperature controller can be installed.

Available following several method survey sensor electric current: (1) uses voltage instrumentation amount open circuit voltage, (2) measure closed current with ammeter, (3) electric current is fallen by resistor (＜100 ohm) measuring voltage of load, (4) use the current-voltage converter measuring voltage based on the operation amplifying circuit.Last a kind of measuring method is preferred, because it the problem that exterior I R descends can not occur.Current-voltage converter is a kind of integrated circuit of voltage stabilizer, and it is easy to analog-and digital-record.For the signal that satisfies manufacturer in 4-20 milliampere scope transmits standard, can in system, add the shapable excluder of the voltage signal in the current-voltage converter being changed back the current signal of 4-20 milliampere.

Under breadboard test condition, with caustic solution washing phosgene and chlorine.For fear of the phosgene seepage, use two step scrubber system.First step scrubber liquor is 1: 2: 3 45% concentrating KOH aqueous solution, water and methyl alcohol and form by volume ratio; The second step scrubber liquor by volume ratio be 1: 2 45% concentrate the KOH aqueous solution and methyl alcohol is formed.Phenol red PH indicator as scrubber liquor.The red explanation scrubber liquor that disappears uses up.Salt (the K that importantly prevent at the solution interface place, particularly in first kind of scrubber liquor, separates out ₂CO ₃) outlet of obstruction phosgene.Utilize its most advanced and sophisticated and phosgene outlet (1/4 " OD teflon tube) to link to each other and V-type connector that the bigger end of its diameter (about 1 " ID) is immersed under surperficial about 1.5 inches of the scrubber liquor is helpful to this.Under the operating environment of factory, phosgene can be led to scrubber column.

Following several composition is arranged: solvent, electrolytic salt, special reagent and PH buffering agent in electrolyte solution.The silver ion that electrolyte solution has to contain with silver-colored auxiliary electrode place forms insoluble product; And can not be by electrochemical reduction at platinum working electrode place; And not with the composition of chlorine generation chemical reaction.Cl anion is preferred.

The electrolyte solvent system that two kinds of main types are arranged, i.e. hydrosolvent system and organic solvent system.When water when the solvent, the salt that is used to form electrolyte solution is inorganic salts or the very big organic salt of polarity typically, because their solubleness in water is bigger.Such salt-aqueous solution is called as electrolyte aqueous solution.Metal chloride, can be about 1 mM-saturated as the working concentration of lithium chloride, sodium chloride and potassium chloride.Approximately the concentration of 0.1-1 mole can make solution conductivity and avoid salt to cross the balance that reaches good between the fractional precipitation.When organic compound was used as solvent, used salt was those organic salts that have enough solubleness in organic solvent.This salt-organic solution is called organic electrolyte solution.Appropriate organic solvent comprises acetonitrile (AN), carbonic allyl ester (PC), dimethyl sulfoxide (DMSO) (DMSO) and dimethyl formamide (DMF).Also can be with other polar organic matter as electrolyte solvent, as long as they have suitably high specific inductive capacity for example approximately greater than 10 and don't can disturb the detection of chlorine.Suitable organic salt comprises TBAC tetrabutylammonium chloride (TABCl), hexafluorophosphoric acid tetrabutylammonium (TBAHFP), the negative ion alkylammonium salt that tetrafluoro boric acid tetrabutylammonium (TBATFB) is relevant with other, comprises the potpourri of these salt.The potpourri of TBAC tetrabutylammonium chloride and hexafluorophosphoric acid tetrabutylammonium provides as the good performance of electrolytic salt.TBAC tetrabutylammonium chloride is the required chloride-ion source of sensor operation.Yet its solubleness is very limited.In order to increase the electric conductivity of electrolyte solution, can in electrolyte composition, add other organic salt.Except cost was higher slightly, organic electrolyte solution was preferred.A restriction of electrolyte aqueous solution is that phosgene and water reaction form hydrochloric acid and carbon dioxide.For identical cl concn, organic bath produces the electrode current higher than Water-Electrolyte.It may be that the unevenness that distributes in aqueous medium of chlorine is bigger that organic bath produces a reason than high sensitive.This will obviously reduce the chlorine quantity of carrying out electrochemical reduction.

Although continuous electrolyte flow has prolonged the operation lifetime of electrochemical sensor significantly, possibly can't normal running after long-time the use for sensor.The silver auxiliary electrode may be assembled the thicker silver chloride of one deck on electrode surface, therefore limited the output of electric current.The platinum sensing electrode also may stand owing to there is the effect that poisons of gathering due to the sorbing material on the platinum surface.Electrolyte flow can be reduced to sintered glass and occur stopping up.Upgrade on the spot by electrode and can overcome these primary two problems.In the conventional sensors operation, the chlorine reduction takes place, simultaneously oxidized the and formation chlorination silverskin of silver electrode at the platinum electrode place.At reproducting periods, electrochemical process is reversed.Oxidation takes place at the platinum electrode place, and reduces at the silver electrode place.Carry out this process by apply higher positive voltage to platinum electrode.When chlorion being oxidized to elemental chlorine (Cl at the platinum electrode place ₂) time, the chlorination silverskin on the silver-colored auxiliary electrode is reduced to elemental silver.Great majority can oxidizedly be removed at the lip-deep chemisorption thing of platinum (poisonous substance).The silver auxiliary electrode will carry out reduction process, thereby make the chlorination silverskin be oxidized to elements A g.But this reduction process is arrived in the change color actual observation by silver surface.

Content in conjunction with the accompanying drawings, people will more be expressly understood the present invention.

Fig. 1 represents whole detection system with the form of sketch, and it comprises phosgene intake line 2, gas-carrier pipeline 4 and chlorine pipeline 6, and pipeline 6 also is used for carrier gas and demarcates purpose.Every gas piping is furnished with two-port valve door 8 and pressure gauge 10.Phosgene pipeline 2 also is furnished with check valve (being illustrated in 12 places) and aciculiform valve 14.Gas-carrier pipeline has three-way valve 16 and check valve 17 and aciculiform valve 18.Flowmeter 18 and 20 is positioned on phosgene pipeline and the gas-carrier pipeline.Chlorine and gas-carrier pipeline are furnished with aciculiform valve 24 and check valve 26.Article three, gas piping all enters in the flowmeter 28.Output from flowmeter 28 is sent in the electrochemical sensor unit 30 that will more specifically describe hereinafter.

By pump 34 electrolyte of sensor unit 30 is delivered to check valve 35 by reservoir 32.Also can carry electrolyte by gravity flowage.Electrolyte flows through sensor unit 30 and arrives in the waste cell melt gatherers 36, and gatherer 36 is furnished with a vent pipe and one and opens and closes valve 38, and valve 40 leads to the gas vent of scrubber (not shown) on inspection.

Sensor unit 30 contains three electrodes that will further describe in Fig. 2.Electrode and voltage stabilizer 50 electrically connect, and voltage stabilizer electrically connects with electric current excluder 52 and datalogger 54.

Hot shell 40 contains and comprises flowmeter 10,20 and 28 and the part of the system of sensor unit 30.Hot shell is furnished with a temperature controller 42, one or more well heater 44 and one or more circulation fan 46.

Chlorine monitoring sensor of the present invention has very excellent linear response to cl concn, and it is shown in following table 1.

Table 1. hangs down chlorine transducer calibration table

Cl concn (ppm)	Sensor current (microampere)
		????5.2	????38.13
????4.0	????27.81
		????2.1	????12.03
????0.7	????4.19
		????0.0	????-0.06

The high chlorine transducer calibration of table 2. table

Cl concn (ppm)	Sensor current (microampere)
		????0.0	????0.0
????20.0	????75.5
		????28.3	????125.0
????43.5	????200.0
		????46.6	????181.3
????58.5	????240.6
		????88.2	????428.1
????117.4	????540.6
		????153.0	????793.8
????180.2	????1068.8
		????215.2	????1162.5
????224.5	????1331.3
		????293.6	????1475.0

Sensor of the present invention has bigger susceptibility, and is as shown in table 1, but it also has the dynamic response scope of broad, as shown in table 2.Electrode current is the true electric current of exporting from working electrode that is produced by the chlorine reduction.Data clearly illustrate that between the cl concn of sensor signal and 0-300ppm to have excellent linear response.Collect these data under 150 cubic centimetres/minute the flow being approximately.Further test findings discloses: in addition still better in higher phosgene flow lower linear relation.Test condition comprises platinum working electrode, silver/silver chloride reference electrode and auxiliary silver electrode.Electrolyte is 0.6 mole a lithium chloride, and its flow is approximately 5 milliliters/hour.The flow of phosgene is 150 cubic centimetres/minute, and temperature is remained under 40 ℃, and the voltage that applies is 0 volt.Chlorine reduces preferred operating voltage and is approximately-0.1 to+0 volt.Under the higher voltage, platinum electrode surface is oxidized and reduced the efficient that chlorine is surveyed.

The sensor 30 of Fig. 2 presentation graphs 1.Electrochemical sensor system comprises an electrochemical sensor that has at least two (being depicted as 3) electrodes 30, voltage stabilizer or an ammeter and an electrolyte conveying device (not shown).The most important parts of sensor are platinum sensing electrodes 11, are also referred to as working electrode.Is cl anion (Cl) at working electrode place chlorine by electrochemical reduction, and by the reduction current of surveying corresponding to the internal measurement device in the voltage stabilizer of Fig. 1 registering instrument 54 by working electrode produced.Also can use external device (ED).Cl concn is high more in the air-flow, and the electric current that detects is big more.Auxiliary electrode 13 (being also referred to as counter electrode) is one section silver-colored line.When at working electrode place reduction chlorine, in containing the electrolyte of cl anion, silver-colored auxiliary electrode is oxidized to silver chloride (AgCl) simultaneously.Silver forms silver chloride thereby pure sensor response is oxychlorination, and does not produce the solubility accessory substance.Third electrode is a reference electrode 15, and its function is to make voltage stabilizer 50 provide constant electric current to working electrode.Reference electrode is silver/silver chloride electrode, can contain Cl by one section silver-colored line is inserted into simply ^-Thereby form monochlor(in)ate silver thin layer at the silver surface place in the anionic electrolyte and prepare reference electrode.Working electrode 11 is one section and is wrapped in and has the suitable porosity and make platinum line on the sintered glass pipe 17 that electrolyte evenly flows through.Shown pipe 17 has is convenient to pore-free bottom 18 mobile and that dropping liquid forms.Any short-cut method by comprising O-ring or cement is seal sensor cap 20 on the spot at neck 22 places of sensor body.Electrolyte enters in the sensor through conduit 24, and phosgene and any other gas enter in the sensor through conduit 26.Gas and electrolyte are discharged outside the sensor through conduit 28.

Claims (7)

1. be used for surveying and measuring the device of phosgene small amounts of chlorine, it comprises gas delivery system and electrochemical sensor, temperature control system, is used for measuring the Signal Measurement System of the electric current that is produced by electrochemical sensor and the optional scrubber system that is used for removing from the sensor waste liquid dechlorination photoreactive gas.

2. according to the device of claim 1, wherein gas delivery system comprises inert gas conveying device and chlorine standard model potpourri conveying device, photoreactive gas conveying device, and each said device has flow control apparatus, measuring apparatus and check valve separately.

3. according to the device of claim 1, wherein electrochemical sensor comprise be used for the chlorine electrochemical reduction for muriatic working electrode, silver-colored auxiliary electrode, chloride electrolyte, the chloride electrolyte induction system, two that are used for electrolyte is transported to sensor continuously and electrolyte electrodes in contact, pipe guide from sensor to working electrode that the voltage stabilizer of electric current is provided and is used for electrolyte is removed from.

4. according to the device of claim 1, wherein electrochemical sensor comprise be used for the chlorine electrochemical reduction for muriatic working electrode, silver-colored auxiliary electrode, reference electrode, contain muriatic electrolyte, chloride, the electrolyte induction system, two that are used for electrolyte is transported to sensor continuously with the electrolyte electrodes in contact, guarantee the pipe guide that voltage stabilizer provides the reference electrode of constant voltage and is used for electrolyte is removed from sensor to working electrode.

5. according to the electrochemical sensor of claim 4, it comprises:

One sensor body, it has upper gas input media, bottom gas and electrolyte output unit, comprise glass tube and the electrode system that has reference electrode and auxiliary electrode in pipe, the glass tube with porous region and bottom imperforate section that link to each other with the electrolyte induction system, be wrapped in the glass tube working electrode that contacts of the electrolyte with flowing through porous region on the porous region outside surface at least.

6. according to the electrochemical sensor of claim 5, wherein electrode system comprises the sintered glass pipe that has polytetrafluorethylecoatings coatings on its outer surface.

7. according to the device of claim 1, wherein electrochemical sensor is arranged in and has heating arrangement and be used to keep steady temperature and prevent the temperature control case of the heat distribution device of phosgene condensation.