EP0025100A1 - Method for the condensation of carbonisation gas produced by waste pyrolysis - Google Patents

Method for the condensation of carbonisation gas produced by waste pyrolysis Download PDF

Info

Publication number
EP0025100A1
EP0025100A1 EP80104112A EP80104112A EP0025100A1 EP 0025100 A1 EP0025100 A1 EP 0025100A1 EP 80104112 A EP80104112 A EP 80104112A EP 80104112 A EP80104112 A EP 80104112A EP 0025100 A1 EP0025100 A1 EP 0025100A1
Authority
EP
European Patent Office
Prior art keywords
cooling
carbonization
condensate
condensation
gases
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP80104112A
Other languages
German (de)
French (fr)
Inventor
Erwin Dipl.-Chem. Thomanetz
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
HERKO PYROLYSE GmbH AND CO RECYCLING KG
Original Assignee
HERKO PYROLYSE GmbH AND CO RECYCLING KG
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by HERKO PYROLYSE GmbH AND CO RECYCLING KG filed Critical HERKO PYROLYSE GmbH AND CO RECYCLING KG
Publication of EP0025100A1 publication Critical patent/EP0025100A1/en
Withdrawn legal-status Critical Current

Links

Images

Classifications

    • C—CHEMISTRY; METALLURGY
    • C10—PETROLEUM, GAS OR COKE INDUSTRIES; TECHNICAL GASES CONTAINING CARBON MONOXIDE; FUELS; LUBRICANTS; PEAT
    • C10K—PURIFYING OR MODIFYING THE CHEMICAL COMPOSITION OF COMBUSTIBLE GASES CONTAINING CARBON MONOXIDE
    • C10K1/00—Purifying combustible gases containing carbon monoxide
    • C10K1/04—Purifying combustible gases containing carbon monoxide by cooling to condense non-gaseous materials

Definitions

  • the invention relates to a process for the condensation of carbonization gases from waste pyrolysis.
  • lumpy waste is thermally decomposed in a heated carbonization reactor with the exclusion of air, forming a carbonization gas rich in hydrocarbons, which must be condensed for its reuse as an oil.
  • the object of the invention is to provide a method which allows efficient cooling of the carbonization gases produced in continuous operation.
  • the hot smoldering gases obtained in the pyrolysis are passed through a spray veil of condensed smoldering gases, in that the smoldering gas condensate formed is further cooled and is used in part to produce the spray veil.
  • the cooling effect is here by the stän reached evaporation of the spray curtain.
  • the direct evaporative cooling offers an optimal heat exchange and avoids disturbances, such as the clogging of heat exchange surfaces and the like. With the subsequent cooling of the carbonization gas condensate, the heat originating from the hot carbonization gases is removed from the condensate and its use as a coolant is made possible.
  • the carbonization condensate is filtered in a heat exchanger before it cools down. Furthermore, the residual gas remaining in the condensation of the carbonization gas is preferably cooled in a second cooling stage for the separation of residual condensate, it being possible for the cooling in the second cooling stage to take place by low-temperature cooling. For aerosol separation, it is preferred to subsequently filter the residual gas.
  • hot smoldering gas at a temperature of around 900 K is passed from above into a cooling tower 1.
  • a cooling tower 1 In the jacket area there are, arranged in a ring, inward spray nozzles 2.
  • the spray nozzles are fed with cooled condensate, which is conveyed by a pump 3 and which is in the liquid state, via feed lines from outside the cooling tower and the condensate thus to a liquid veil or spray mist atomized inside the cooling tower 1.
  • the hot smoldering gases flow through the liquid curtain in the cooling tower, the cooling effect resulting from the constant evaporation of the liquid curtain.
  • the cooled carbonization gas rains out and collects as a liquid in the sump 4 at the bottom of the cooling tower 1.
  • This type of direct evaporation offers because of the large Surface that the liquid veil offers to the hot carbonization gas, an optimal heat exchange. There is no clogging or contamination of heat exchange surfaces and no risk of clogging.
  • the bottom condensate is passed through a filter 6 and from there into a heat exchanger 7 operated, for example, with a closed cooling water circuit with an air cooler, where it is cooled further and the heat originating from the hot carbonization gases is removed.
  • the (for example, 320 K) cooled condensate is pumped back to the part by the pump 3 via the spray nozzles 2 in the cooling tower 1, and excess condensate ge g e-appropriate, removed after cooling to ambient temperature for further use such as in the chemical processing industry.
  • the residual gas which predominantly consists of permanent gases during cooling in cooling tower 1 is also further cooled in a second cooling stage for the separation of residual condensate.
  • This second cooling stage can be, for example, an electrically operated low-temperature cooling stage of approximately 250 K.
  • This cooling stage is optionally followed by an electrostatic precipitator for aerosol separation.

Landscapes

  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Combustion & Propulsion (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • General Chemical & Material Sciences (AREA)
  • Oil, Petroleum & Natural Gas (AREA)
  • Organic Chemistry (AREA)
  • Treating Waste Gases (AREA)
  • Production Of Liquid Hydrocarbon Mixture For Refining Petroleum (AREA)
  • Vaporization, Distillation, Condensation, Sublimation, And Cold Traps (AREA)
  • Coke Industry (AREA)

Abstract

The hot carbonisation gases produced in the pyrolysis are passed through a spray curtain of condensed carbonisation gases, and the carbonisation gas condensate produced in this process is cooled further and partly used to produce the spray curtain.

Description

Die Erfindung betrifft ein Verfahren zur Kondensation von Schwelgasen aus der Abfallpyrolyse.The invention relates to a process for the condensation of carbonization gases from waste pyrolysis.

Bei der Abfallpyrolyse wird stückiger Abfall in einem beheizten Schwelreaktor unter Luftabschluß thermisch zersetzt, wobei ein an Kohlenwasserstoffen reiches Schwelgas gebildet wird, das zu seiner Wiederverwendung als öl kondensiert werden muß.In waste pyrolysis, lumpy waste is thermally decomposed in a heated carbonization reactor with the exclusion of air, forming a carbonization gas rich in hydrocarbons, which must be condensed for its reuse as an oil.

Aufgabe der Erfindung ist es, ein Verfahren zu schaffen, das eine effiziente Kühlung der anfallenden Schwelgase im Dauerbetrieb gestattet.The object of the invention is to provide a method which allows efficient cooling of the carbonization gases produced in continuous operation.

Diese Aufgabe wird erfindungsgemäß dadurch gelöst, daß die bei der Pyrolyse anfallenden heißen Schwelgase durch einen Sprühschleier kondensierter Schwelgase geleitet werden, daß das dabei entstehende Schwelgaskondensat weiter abgekühlt und zum Teil zur Erzeugung des Sprühschleiers verwendet wird. Der Kühleffekt wird hierbei durch die ständige Verdampfung des Sprühschleiers erreicht. Die direkte Verdampfungskühlung bietet einen optimalen Wärmeaustausch und vermeidet Störungen, wie das Zusetzen von Wärmeaustauschflächen und dergleichen. Mit der nachfolgenden Abkühlung des Schwelgaskondensats wird die den heißen Schwelgasen entstammende Wärme aus dem Kondensat abgeführt und seine Verwendung als Kühlmittel ermöglicht.This object is achieved according to the invention in that the hot smoldering gases obtained in the pyrolysis are passed through a spray veil of condensed smoldering gases, in that the smoldering gas condensate formed is further cooled and is used in part to produce the spray veil. The cooling effect is here by the stän reached evaporation of the spray curtain. The direct evaporative cooling offers an optimal heat exchange and avoids disturbances, such as the clogging of heat exchange surfaces and the like. With the subsequent cooling of the carbonization gas condensate, the heat originating from the hot carbonization gases is removed from the condensate and its use as a coolant is made possible.

Gemäß einer bevorzugten Ausgestaltung der Erfindung wird das Schwelgaskondensat vor seiner Abkühlung in einem Wärmetauscher gefiltert. Ferner wird bevorzugt das bei der Kondensation des Schwelgases verbleibende Restgas in einer zweiten Kühlstufe zur Abscheidung restlichen Kondensats gekühlt, wobei die Kühlung in der zweiten Kühlstufe durch Tieftemperaturkühlung erfolgen kann. Zur Aerosolabscheidung wird bevorzugt, das Restgas nachfolgend zu filtern.According to a preferred embodiment of the invention, the carbonization condensate is filtered in a heat exchanger before it cools down. Furthermore, the residual gas remaining in the condensation of the carbonization gas is preferably cooled in a second cooling stage for the separation of residual condensate, it being possible for the cooling in the second cooling stage to take place by low-temperature cooling. For aerosol separation, it is preferred to subsequently filter the residual gas.

Eine bevorzugte Ausführungsform des erfindungsgemässen Verfahrens wird im folgenden anhand der einzigen Figur der Zeichnung beschrieben.A preferred embodiment of the method according to the invention is described below with reference to the single figure of the drawing.

Bei der Pyrolyse von stückigem Abfall in einem beheizten Reaktor anfallendes heißes Schwelgas einer Temperatur von rund 900 K wird von oben in einen Kühlturm 1 geleitet. Im Mantelbereich desselben befinden sich, ringförmig angeordnet, nach innen gerichtete Sprühdüsen 2. Die Sprühdüsen werden mit durch eine Pumpe 3 angefördertem gekühltem Kondensat, das in flüssigem Zustand vorliegt, über Zuleitungen von außerhalb des Kühlturms gespeist und das Kondensat so zu einem Flüssigkeitsschleier bzw. Sprühnebel im Inneren des Kühlturms 1 zerstäubt. Der im Kühlturm stehende Flüssigkeitsschleier wird dabei von den heißen Schwelgasen durchströmt, wobei sich der Kühleffekt aus der ständigen Verdampfung des-Flüssigkeitsschleiers ergibt. Das abgekühlte Schwelgas regnet aus und sammelt sich als Flüssigkeit im Sumpf 4 am Boden des Kühlturms 1. Diese Art der direkten Verdampfung bietet wegen der großen Oberfläche, die der Flüssigkeitsschleier dem heißen Schwelgas bietet, einen optimalen Wärmeaustausch. Es gibt kein Zusetzen oder Verschmutzen von Wärmeaustauschflächen und keine Verstopfungsgefahr. Das Sumpfkondensat wird durch ein Filter 6 und von dort in einen etwa mit einem geschlossenen Kühlwasserkreislauf mit Luftkühler betriebenen Wärmeaustauscher 7 geleitet, wo es weiter abgekühlt und die den heißen Schwelgasen entstammende Wärme abgeführt wird.During the pyrolysis of lumpy waste in a heated reactor, hot smoldering gas at a temperature of around 900 K is passed from above into a cooling tower 1. In the jacket area there are, arranged in a ring, inward spray nozzles 2. The spray nozzles are fed with cooled condensate, which is conveyed by a pump 3 and which is in the liquid state, via feed lines from outside the cooling tower and the condensate thus to a liquid veil or spray mist atomized inside the cooling tower 1. The hot smoldering gases flow through the liquid curtain in the cooling tower, the cooling effect resulting from the constant evaporation of the liquid curtain. The cooled carbonization gas rains out and collects as a liquid in the sump 4 at the bottom of the cooling tower 1. This type of direct evaporation offers because of the large Surface that the liquid veil offers to the hot carbonization gas, an optimal heat exchange. There is no clogging or contamination of heat exchange surfaces and no risk of clogging. The bottom condensate is passed through a filter 6 and from there into a heat exchanger 7 operated, for example, with a closed cooling water circuit with an air cooler, where it is cooled further and the heat originating from the hot carbonization gases is removed.

Das (z.B. auf 320 K) abgekühlte Kondensat wird zum Teil durch die Pumpe 3 über die Sprühdüsen 2 in den Kühlturm 1 zurückgepumpt, und überschüssiges Kondensat gege-benenfalls nach Abkühlung auf Umgebungstemperatur zur Weiterverwendung etwa in der chemischen Grundstoffindustrie abgeführt.The (for example, 320 K) cooled condensate is pumped back to the part by the pump 3 via the spray nozzles 2 in the cooling tower 1, and excess condensate ge g e-appropriate, removed after cooling to ambient temperature for further use such as in the chemical processing industry.

Das bei der Kühlung im Kühlturm 1 vorwiegend aus Permanentgasen bestehende verbleibende Restgas wird in einer zweiten Kühlstufe zur Abscheidung von restlichem Kondensat ebenfalls weiter abgekühlt. Bei dieser zweiten Kühlstufe kann es sich beispielsweise um eine elektrisch betriebene Tieftemperaturkühlstufe von ca. 250 K handeln. Dieser Kühlstufe ist gegebenenfalls zur Aerosolabscheidung ein Elektrofilter nachgeschaltet.The residual gas which predominantly consists of permanent gases during cooling in cooling tower 1 is also further cooled in a second cooling stage for the separation of residual condensate. This second cooling stage can be, for example, an electrically operated low-temperature cooling stage of approximately 250 K. This cooling stage is optionally followed by an electrostatic precipitator for aerosol separation.

Claims (6)

1. Verfahren zur Kondensation von Schwelgasen aus der Abfallpyrolyse, dadurch gekennzeichnet, daß die bei der Pyrolyse anfallenden heißen Schwelgase durch einen Sprühschleier kondensierter Schwelgase geleitet werden, daß das dabei entstehende Schwelgaskondensat weiter abgekühlt und zum Teil zur Erzeugung des Sprühschleiers verwendet wird.1. A process for the condensation of carbonization gases from waste pyrolysis, characterized in that the hot carbonization gases obtained in the pyrolysis are passed through a spray curtain of condensed carbonization gases, that the carbonization condensate formed is further cooled and is used in part to generate the spray curtain. 2. Verfahren nach Anspruch 1, dadurch gekennzeichnet, daß das Schwelgaskondensat vor der weiteren Abkühlung gefiltert wird.2. The method according to claim 1, characterized in that the carbonization gas condensate is filtered before further cooling. 3. Verfahren nach Anspruch 2, dadurch gekennzeichnet, daß die weitere Abkühlung in einem Wärmeaustauscher erfolgt.3. The method according to claim 2, characterized in that the further cooling takes place in a heat exchanger. 4. Verfahren nach einem der vorstehenden Ansprüche, dadurch gekennzeichnet , daß bei der Kondensation des Schwelgases verbleibendes Restgas in einer zweiten Kühlstufe zur Abscheidung restlichen Kondensats gekühlt wird.4. The method according to any one of the preceding claims, characterized in that residual gas remaining in the condensation of the carbonization gas is cooled in a second cooling stage for the separation of residual condensate. 5. Verfahren nach Anspruch 4, dadurch gekennzeichnet, daß die Kühlung in der zweiten Kühlstufe durch Tieftemperaturkühlung erfolgt.5. The method according to claim 4, characterized in that the cooling in the second cooling stage is carried out by low-temperature cooling. 6. Verfahren nach Anspruch 4 oder 5, dadurch gekennzeichnet, daß das Restgas nachfolgend zur Aerosolabscheidung elektrisch gefiltert wird.6. The method according to claim 4 or 5, characterized in that the residual gas is subsequently filtered electrically for aerosol separation.
EP80104112A 1979-07-16 1980-07-15 Method for the condensation of carbonisation gas produced by waste pyrolysis Withdrawn EP0025100A1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
DE2928676 1979-07-16
DE19792928676 DE2928676B1 (en) 1979-07-16 1979-07-16 Process for the condensation of carbonization gases from waste pyrolysis

Publications (1)

Publication Number Publication Date
EP0025100A1 true EP0025100A1 (en) 1981-03-18

Family

ID=6075839

Family Applications (1)

Application Number Title Priority Date Filing Date
EP80104112A Withdrawn EP0025100A1 (en) 1979-07-16 1980-07-15 Method for the condensation of carbonisation gas produced by waste pyrolysis

Country Status (3)

Country Link
EP (1) EP0025100A1 (en)
JP (1) JPS5653701A (en)
DE (1) DE2928676B1 (en)

Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5707592A (en) * 1991-07-18 1998-01-13 Someus; Edward Method and apparatus for treatment of waste materials including nuclear contaminated materials
EP3031881A1 (en) 2014-12-08 2016-06-15 Innord sp. z o.o. S.K.A. Method of pyrolytic processing of polymer waste from the recycling of food packaging and a system for carrying out such method
CN113891754A (en) * 2019-06-10 2022-01-04 耐思特公司 Method for processing waste plastic pyrolysis gas

Families Citing this family (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE3545202A1 (en) * 1985-12-20 1987-06-25 Bbc Brown Boveri & Cie Process and apparatus for cleaning and cooling pyrolysis gas
DE3721475C1 (en) * 1987-06-30 1989-03-23 Asea Brown Boveri Plant for pyrolysis of waste material
DE3721450C1 (en) * 1987-06-30 1988-12-08 Asea Brown Boveri Process for cooling hot pyrolysis gas
DE19529536B4 (en) * 1995-08-11 2005-10-20 Schroeder Sascha Process for the treatment and conditioning of fuel gas
CN105407717B (en) 2013-08-08 2018-04-03 赛诺菲-安万特德国有限公司 Recovery component in cryopreservation application scenario

Citations (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE1471633A1 (en) * 1963-12-03 1968-12-12 Westfalia Dinnendahl Groeppel Process for wet dedusting of gases or vapors
DE1494802A1 (en) * 1966-01-27 1969-08-07 Metallgesellschaft Ag Process for the separation of methane and ethane from carbonic acid-rich compressed gases
DE2323654A1 (en) * 1972-05-12 1973-11-22 Harald F Funk Solid waste disposal - by high temp gasification and sepn of useful products from gas
DE2542055A1 (en) * 1975-09-20 1977-03-31 Metallgesellschaft Ag PROCESS FOR TREATMENT OF A CRUDE GAS FROM THE PRESSURE GASIFICATION OF COAL
DE2701800A1 (en) * 1977-01-01 1978-07-20 Heinz Hoelter Purification of pyrolysis gas - by oil scrubbing and contact with sorbent above dew point
DD133572A1 (en) * 1977-11-07 1979-01-10 Faramund Fabian PROCESS FOR COOLING AND CLEANING GASES
DE2756556A1 (en) * 1977-12-19 1979-06-28 Heinz Hoelter Textile waste gases decontamination - in scrubber column and chemisorption bed
DE2853989B1 (en) * 1978-12-14 1979-11-22 Metallgesellschaft Ag Process for the treatment of water-containing condensate from the cooling of the raw gas of the pressure gasification
DD141033A1 (en) * 1979-02-28 1980-04-09 Gerhard Dalluege METHOD FOR DIVING AND COOLING OF RAW GASES

Patent Citations (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE1471633A1 (en) * 1963-12-03 1968-12-12 Westfalia Dinnendahl Groeppel Process for wet dedusting of gases or vapors
DE1494802A1 (en) * 1966-01-27 1969-08-07 Metallgesellschaft Ag Process for the separation of methane and ethane from carbonic acid-rich compressed gases
DE2323654A1 (en) * 1972-05-12 1973-11-22 Harald F Funk Solid waste disposal - by high temp gasification and sepn of useful products from gas
DE2542055A1 (en) * 1975-09-20 1977-03-31 Metallgesellschaft Ag PROCESS FOR TREATMENT OF A CRUDE GAS FROM THE PRESSURE GASIFICATION OF COAL
DE2701800A1 (en) * 1977-01-01 1978-07-20 Heinz Hoelter Purification of pyrolysis gas - by oil scrubbing and contact with sorbent above dew point
DD133572A1 (en) * 1977-11-07 1979-01-10 Faramund Fabian PROCESS FOR COOLING AND CLEANING GASES
DE2756556A1 (en) * 1977-12-19 1979-06-28 Heinz Hoelter Textile waste gases decontamination - in scrubber column and chemisorption bed
DE2853989B1 (en) * 1978-12-14 1979-11-22 Metallgesellschaft Ag Process for the treatment of water-containing condensate from the cooling of the raw gas of the pressure gasification
DD141033A1 (en) * 1979-02-28 1980-04-09 Gerhard Dalluege METHOD FOR DIVING AND COOLING OF RAW GASES

Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5707592A (en) * 1991-07-18 1998-01-13 Someus; Edward Method and apparatus for treatment of waste materials including nuclear contaminated materials
EP3031881A1 (en) 2014-12-08 2016-06-15 Innord sp. z o.o. S.K.A. Method of pyrolytic processing of polymer waste from the recycling of food packaging and a system for carrying out such method
CN113891754A (en) * 2019-06-10 2022-01-04 耐思特公司 Method for processing waste plastic pyrolysis gas
US11471817B2 (en) 2019-06-10 2022-10-18 Neste Oyj Method for processing plastic waste pyrolysis gas
US11969689B2 (en) 2019-06-10 2024-04-30 Neste Oyj Method for processing plastic waste pyrolysis gas

Also Published As

Publication number Publication date
JPS5653701A (en) 1981-05-13
DE2928676B1 (en) 1980-06-04

Similar Documents

Publication Publication Date Title
DE3442400C2 (en)
EP0519225A1 (en) Process and apparatus for cleaning flue gases from furnace installations
DD250166A5 (en) METHOD FOR OBTAINING ENERGY FROM ALKALINE FUELS OF LOW HEATING VALUE
EP0167702A1 (en) Process for the subsequent treatment of a gas obtained by pyrolyzing trash
EP0025100A1 (en) Method for the condensation of carbonisation gas produced by waste pyrolysis
DE3423798C2 (en)
DE4300921C1 (en) Solvent and diluent recovery for re-use - esp. in polymer prodn., processing or moulding, e.g. spinning
EP0297424B1 (en) Process for cooling hot pyrolysis gas
DE2632092A1 (en) METHOD FOR COOLING GASES CONTAINING SOLIDS, TAR AND NAPHTHALINE
DE3602586A1 (en) METHOD FOR REFURBISHING ALTOEL
DE4402793C1 (en) Method for separating through screen glass tubes
WO1986005507A1 (en) Process and apparatus for the cooling of coke
EP0356554A1 (en) Process for gasifying coal and treating the gas produced
DE2416558A1 (en) METHOD AND DEVICE FOR COALING COAL
DE4040334A1 (en) Process and assembly to recover solvent - and similar materials from effluent air
DE3307737A1 (en) METHOD AND DEVICE FOR DESTROYING LIQUID OR PASTE-SHAPED WASTE
DE600460C (en) Process for the recovery of pitch and tar oil fractions
DE4231306C2 (en) Process for cleaning machine-contaminated machine parts and device for cleaning metallic machine parts contaminated by polymers
DE3538745A1 (en) METHOD AND SYSTEM FOR WATER-FREE SOLID EXTRACTION WITH ORGANIC SOLVENTS
DE4012146A1 (en) Precooling of coke-oven gas - with electrostatic filtration and naphthalene scrubbing
DE2508189C3 (en) Process for extracting smoldering tar and system for carrying out the process
EP0712920A2 (en) Apparatus for thermally decomposing plastics
DE510940C (en) Process for the direct production of road tar in a coke oven plant with by-product recovery plant
WO2003016437A2 (en) Method for scavenging precoolers of a coking plant
DE3823660A1 (en) Process for removing pollutants from a gas resulting from the low-temperature carbonisation of waste materials

Legal Events

Date Code Title Description
PUAI Public reference made under article 153(3) epc to a published international application that has entered the european phase

Free format text: ORIGINAL CODE: 0009012

AK Designated contracting states

Designated state(s): AT BE CH DE FR GB IT LU NL SE

STAA Information on the status of an ep patent application or granted ep patent

Free format text: STATUS: THE APPLICATION IS DEEMED TO BE WITHDRAWN

18D Application deemed to be withdrawn

Effective date: 19830203

RIN1 Information on inventor provided before grant (corrected)

Inventor name: THOMANETZ, ERWIN, DIPL.-CHEM.