Potamon edule supporte l’eau saumâtre et même l’eau douce ; il remonte les cours d’eau méditerranéens et creuse des terriers dans les berges. Mais le Crabe d’eau douce fouisseur le plus inquiétant est incontestablement le Crabe chinois (Eriocheir sinensis), originaire d’Extrême-Orient, introduit accidentellement en Allemagne au début du siècle et qui envahit depuis les rivières européennes sans qu’on puisse s’y opposer ; il commet des dégâts importants en minant les rives et les digues ; on le reconnaît aux manchons de poils qui ornent ses pinces.
Quelques Crabes tropicaux ont des mœurs franchement terrestres. Gecarcinus, des Antilles et d’Afrique, ne regagne la mer que pour se reproduire ; ses cavités branchiales fonctionnent comme des poumons ; il creuse des terriers, d’où il ne sort que la nuit.
Parmi les Anomoures à faciès de Crabes, on peut citer les Porcellanes, qui ne dépassent pas 2 cm de long et qui abondent sous les pierres à marée basse ; on peut les distinguer des vrais Crabes par la présence de pattes-nageoires (uropodes) à l’extrémité de l’abdomen et par le doigt mobile de leur pince, qui est placé vers l’intérieur. Dans les régions chaudes, le Crabe des Cocotiers (Birgus latro) reste près de l’eau lorsqu’il est jeune et abrite son abdomen dans une coquille, comme un Pagure ; puis il devient terrestre ; son abdomen se replie sous le céphalothorax, et ses chambres respiratoires servent de poumons ; il vit dans des terriers, mais grimpe également aux Cocotiers, dont il fait tomber les fruits pour les manger à terre.
M. D.
➙ Crustacés / Décapodes.
A. Bauchau, la Vie des crabes (Lechevalier, 1966).
cracking ou craquage
Procédé industriel consistant à modifier la composition d’un produit pétrolier sous les effets conjugués de la température, de la pression et, le plus souvent, d’un catalyseur.
La réaction de base est la dissociation, ou « craquage », de molécules longues d’hydrocarbures (essence lourde, gasoil, distillats lourds, résidus) pour donner des molécules courtes (gaz, essence légère, carburants).
Types de cracking
Cracking thermique (sans catalyseur)
On a prétendu que le cracking avait été découvert accidentellement par la négligence d’un ouvrier oubliant d’arrêter à temps une distillation de pétrole brut. Quoi qu’il en soit, des installations rudimentaires faisaient déjà appel, aux États-Unis, avant 1914, à des températures de 400 °C et à des pressions de 5 bars pour augmenter les rendements en essence. Après la réalisation du premier four tubulaire en 1926, le procédé connut un grand essor, toutes les raffineries étant dotées d’une installation pour traiter les gas-oils et les distillats paraffineux ou pour « réformer » les essences lourdes à 550 °C et 60 bars. À l’heure actuelle, le cracking thermique est encore utilisé couramment sous deux formes : — le visbreaking (réduction de viscosité), qui craque modérément les fuel-oils, ou résidus de la distillation atmosphérique du brut, à 450 °C et 20 bars ; — le coking (cokage), qui est un craquage poussé jusqu’au résidu solide ultime du pétrole, le coke.
Cracking catalytique (avec catalyseur)
Découvert vers 1935 par un Français, Eugène Houdry (1892-1962), le cracking catalytique, dont le principal avantage est une pression de marche de 1 bar et une température de 450 °C seulement, se développa très rapidement aux États-Unis pendant la Seconde Guerre mondiale pour la fabrication de carburants militaires et de toluène. Il s’est actuellement substitué au cracking thermique dans la plupart des raffineries pour la conversion du gas-oil en essence, avec des rendements atteignant par recyclage 80 p. 100. Le catalyseur est composé d’oxydes métalliques, généralement silicate et alumine, amorphes ou zéolitiques et se présente sous forme granuleuse ou pulvérulente. Pour maintenir son activité, il est nécessaire de le « régénérer », c’est-à-dire de brûler la couche de coke qui s’y dépose au cours de la réaction.
Crackings spéciaux
Certains procédés de raffinage spéciaux dérivés du cracking, tels le reforming catalytique, le steam-cracking et l’hydrocracking, ont pris une grande importance pour la fabrication de bases pétrolières et pétrochimiques.
Théorie du cracking
Le cracking modifie la nature des hydrocarbures qui le subissent et réalise des synthèses de nouveaux corps en regroupant différemment les atomes de carbone et d’hydrogène. Les longues chaînes saturées sont ouvertes pour donner des oléfines (hydrocarbures non saturés à double liaison C = C) et des aromatiques (hydrocarbures à noyau benzénique) : les produits légers ainsi obtenus sont riches en hydrogène, et il y a même une formation non négligeable d’hydrogène, tandis que les produits lourds en sont pauvres, allant jusqu’au coke de pétrole, carbone pur évidemment dépourvu d’hydrogène. Les oléfines gazeuses (éthylène, propylène, butylène), qui sont rares dans le pétrole brut, constituent à leur tour d’excellentes matières premières des synthèses pétrochimiques. Au cours du cracking, des réactions de polymérisation et d’alkylation — cette dernière combinant l’isobutane et le butylène en isooctane — produisent une coupe d’essence qui, séparée par fractionnement, donne un carburant antidétonant à haut indice d’octane.
Rentabilité
Tout cracking est un procédé de conversion de produits excédentaires de moindre valeur en produits plus chers demandés par le marché. Aux États-Unis, les besoins en carburant automobile atteignent 60 p. 100 du pétrole brut, contre 25 p. 100 seulement en Europe, car les véhicules lourds y sont équipés de moteurs à essence et non de diesels, et le chauffage domestique s’y fait essentiellement au gaz naturel et non au mazout. Il n’est donc pas étonnant que la capacité de cracking y représente 60 p. 100 de celle de distillation du pétrole brut, dont les deux tiers sous forme de cracking catalytique, au lieu de 10 p. 100 en Europe occidentale. Ces chiffres ne comprennent pas les reformings et les autres procédés spéciaux. Le coût des unités de cracking varie suivant leur taille et le procédé mis en œuvre. Pour un même tonnage traité, l’investissement pour un cracking catalytique est cinq fois plus élevé que pour une unité de distillation du brut et peut dépasser 20 dollars par tonne de capacité annuelle. Les installations de raffinage aux États-Unis coûtent donc trois fois plus cher qu’en Europe. Les frais d’exploitation (main-d’œuvre, énergie, combustible, catalyseur, produits chimiques, entretien) d’un cracking catalytique sont de l’ordre de 5 dollars par tonne traitée.
A. H. S.
➙ Hydrogénation / Reforming / Steam-cracking.
K. Vinnacker et L. Kuchler, Traité de chimie appliquée, t. V : Chimie organique (Eyrolles, 1966).
>
We Care About Your Privacy
We and our 916 partners store and access personal data, like browsing data or unique identifiers, on your device. Selecting "I Accept" enables tracking technologies to support the purposes shown under "we and our partners process data to provide," whereas selecting "Reject All" or withdrawing your consent will disable them. If trackers are disabled, some content and ads you see may not be as relevant to you. You can resurface this menu to change your choices or withdraw consent at any time by clicking the ["privacy preferences"] link on the bottom of the webpage [or the floating icon on the bottom-left of the webpage, if applicable]. Your choices will have effect within our Website. For more details, refer to our Privacy Policy.
We and our partners process data to provide:
Use precise geolocation data. Actively scan device characteristics for identification. Store and/or access information on a device. Personalised advertising and content, advertising and content measurement, audience research and services development.
About Your Privacy
Your Privacy
Targeting Cookies
Functional Cookies
Performance Cookies
Strictly Necessary Cookies
Store and/or access information on a device 741 partners can use this purpose
Personalised advertising and content, advertising and content measurement, audience research and services development 882 partners can use this purpose
Use precise geolocation data 282 partners can use this special feature
Actively scan device characteristics for identification 150 partners can use this special feature
Ensure security, prevent and detect fraud, and fix errors 568 partners can use this special purpose
Deliver and present advertising and content 568 partners can use this special purpose
Match and combine data from other data sources 406 partners can use this feature
Link different devices 351 partners can use this feature
Identify devices based on information transmitted automatically 539 partners can use this feature
Save and communicate privacy choices 413 partners can use this special purpose
Your Privacy
We process your data to deliver content or advertisements and measure the delivery of such content or advertisements to extract insights about our website. We share this information with our partners on the basis of consent. You may exercise your right to consent, based on a specific purpose below or at a partner level in the link under each purpose. Some vendors may process your data based on their legitimate interests, which does not require your consent. You cannot object to tracking technologies placed to ensure security, prevent fraud, fix errors, or deliver and present advertising and content, and precise geolocation data and active scanning of device characteristics for identification may be used to support this purpose. This exception does not apply to targeted advertising. These choices will be signaled to our vendors participating in the Transparency and Consent Framework.
More information
Targeting Cookies
These cookies may be set through our site by our advertising partners. They may be used by those companies to build a profile of your interests and show you relevant adverts on other sites. They do not store directly personal information, but are based on uniquely identifying your browser and internet device. If you do not allow these cookies, you will experience less targeted advertising.
Functional Cookies
These cookies enable the website to provide enhanced functionality and personalisation. They may be set by us or by third party providers whose services we have added to our pages. If you do not allow these cookies then some or all of these services may not function properly.
Performance Cookies
These cookies allow us to count visits and traffic sources so we can measure and improve the performance of our site. They help us to know which pages are the most and least popular and see how visitors move around the site. All information these cookies collect is aggregated and therefore anonymous. If you do not allow these cookies we will not know when you have visited our site, and will not be able to monitor its performance.
Strictly Necessary Cookies
Always Active
These cookies are necessary for the website to function and cannot be switched off in our systems. They are usually only set in response to actions made by you which amount to a request for services, such as setting your privacy preferences, logging in or filling in forms. You can set your browser to block or alert you about these cookies, but some parts of the site will not then work. These cookies do not store any personally identifiable information.
Store and/or access information on a device 741 partners can use this purpose
Cookies, device or similar online identifiers (e.g. login-based identifiers, randomly assigned identifiers, network based identifiers) together with other information (e.g. browser type and information, language, screen size, supported technologies etc.) can be stored or read on your device to recognise it each time it connects to an app or to a website, for one or several of the purposes presented here.
Personalised advertising and content, advertising and content measurement, audience research and services development 882 partners can use this purpose
Use limited data to select advertising 705 partners can use this purpose
Advertising presented to you on this service can be based on limited data, such as the website or app you are using, your non-precise location, your device type or which content you are (or have been) interacting with (for example, to limit the number of times an ad is presented to you).
Create profiles for personalised advertising 561 partners can use this purpose
Information about your activity on this service (such as forms you submit, content you look at) can be stored and combined with other information about you (for example, information from your previous activity on this service and other websites or apps) or similar users. This is then used to build or improve a profile about you (that might include possible interests and personal aspects). Your profile can be used (also later) to present advertising that appears more relevant based on your possible interests by this and other entities.
Use profiles to select personalised advertising 562 partners can use this purpose
Advertising presented to you on this service can be based on your advertising profiles, which can reflect your activity on this service or other websites or apps (like the forms you submit, content you look at), possible interests and personal aspects.
Create profiles to personalise content 249 partners can use this purpose
Information about your activity on this service (for instance, forms you submit, non-advertising content you look at) can be stored and combined with other information about you (such as your previous activity on this service or other websites or apps) or similar users. This is then used to build or improve a profile about you (which might for example include possible interests and personal aspects). Your profile can be used (also later) to present content that appears more relevant based on your possible interests, such as by adapting the order in which content is shown to you, so that it is even easier for you to find content that matches your interests.
Use profiles to select personalised content 224 partners can use this purpose
Content presented to you on this service can be based on your content personalisation profiles, which can reflect your activity on this or other services (for instance, the forms you submit, content you look at), possible interests and personal aspects. This can for example be used to adapt the order in which content is shown to you, so that it is even easier for you to find (non-advertising) content that matches your interests.
Measure advertising performance 806 partners can use this purpose
Information regarding which advertising is presented to you and how you interact with it can be used to determine how well an advert has worked for you or other users and whether the goals of the advertising were reached. For instance, whether you saw an ad, whether you clicked on it, whether it led you to buy a product or visit a website, etc. This is very helpful to understand the relevance of advertising campaigns.
Measure content performance 390 partners can use this purpose
Information regarding which content is presented to you and how you interact with it can be used to determine whether the (non-advertising) content e.g. reached its intended audience and matched your interests. For instance, whether you read an article, watch a video, listen to a podcast or look at a product description, how long you spent on this service and the web pages you visit etc. This is very helpful to understand the relevance of (non-advertising) content that is shown to you.
Understand audiences through statistics or combinations of data from different sources 511 partners can use this purpose
Reports can be generated based on the combination of data sets (like user profiles, statistics, market research, analytics data) regarding your interactions and those of other users with advertising or (non-advertising) content to identify common characteristics (for instance, to determine which target audiences are more receptive to an ad campaign or to certain contents).
Develop and improve services 602 partners can use this purpose
Information about your activity on this service, such as your interaction with ads or content, can be very helpful to improve products and services and to build new products and services based on user interactions, the type of audience, etc. This specific purpose does not include the development or improvement of user profiles and identifiers.
Use limited data to select content 162 partners can use this purpose
Content presented to you on this service can be based on limited data, such as the website or app you are using, your non-precise location, your device type, or which content you are (or have been) interacting with (for example, to limit the number of times a video or an article is presented to you).
Use precise geolocation data 282 partners can use this special feature
With your acceptance, your precise location (within a radius of less than 500 metres) may be used in support of the purposes explained in this notice.
Actively scan device characteristics for identification 150 partners can use this special feature
With your acceptance, certain characteristics specific to your device might be requested and used to distinguish it from other devices (such as the installed fonts or plugins, the resolution of your screen) in support of the purposes explained in this notice.
Ensure security, prevent and detect fraud, and fix errors 568 partners can use this special purpose
Always Active
Your data can be used to monitor for and prevent unusual and possibly fraudulent activity (for example, regarding advertising, ad clicks by bots), and ensure systems and processes work properly and securely. It can also be used to correct any problems you, the publisher or the advertiser may encounter in the delivery of content and ads and in your interaction with them.
Deliver and present advertising and content 568 partners can use this special purpose
Always Active
Certain information (like an IP address or device capabilities) is used to ensure the technical compatibility of the content or advertising, and to facilitate the transmission of the content or ad to your device.
Match and combine data from other data sources 406 partners can use this feature
Always Active
Information about your activity on this service may be matched and combined with other information relating to you and originating from various sources (for instance your activity on a separate online service, your use of a loyalty card in-store, or your answers to a survey), in support of the purposes explained in this notice.
Link different devices 351 partners can use this feature
Always Active
In support of the purposes explained in this notice, your device might be considered as likely linked to other devices that belong to you or your household (for instance because you are logged in to the same service on both your phone and your computer, or because you may use the same Internet connection on both devices).
Identify devices based on information transmitted automatically 539 partners can use this feature
Always Active
Your device might be distinguished from other devices based on information it automatically sends when accessing the Internet (for instance, the IP address of your Internet connection or the type of browser you are using) in support of the purposes exposed in this notice.
Save and communicate privacy choices 413 partners can use this special purpose
Always Active
The choices you make regarding the purposes and entities listed in this notice are saved and made available to those entities in the form of digital signals (such as a string of characters). This is necessary in order to enable both this service and those entities to respect such choices.
