Principles of Free-Radical Polymerization Kinetics

In order for polymerization to take place, we have seen that the monomer has to have double functionality. In step growth polymerization this was provided by the co-reactive functional groups of the monomers. In free radical polymerization the opening of a double bond provides this functionality. The organic molecule reacts with an active species as follows:

Therefore, primary free radicals must be introduced to start the reaction. This is done by the use of an initiator, a thermo-labile compound, typically peroxides or azo-compounds which decompose in the following way:

The primary radicals react with the monomer producing monomer radicals:

Where f is the initiator efficiency or the fraction of radicals that are captured by monomer molecules. From this point, the site of the reactive center changes by the propagation (addition or transfer) reactions:

The rate of addition is not affected by the size of the organic macro-radical, therefore the rate of propagation can be expressed as:

In free radical polymerization, long chains of polymer are formed for a low monomer conversion, since the time life of free radicals is very small.

Chain transfer reactions can occur by transfer of an atom to the macro-radical. The macro-radical is terminated and the new active species can a) add more monomer b) terminate without further addition (inhibition). In any case, the result of chain transfer reactions is a lower molecular weight of the polymer. Schematically, the reaction is shown below:

Transfer reactions may take place from macro-radical to monomer, initiator, solvent or to an agent intentionally introduced to control MW. Only transfer to monomer cannot be controlled and determines the maximum degree of polymerization. The transfer constant is the ratio between the kinetic rates of the transfer reaction with respect to the propagation, where i depends of the nature of the transfer reaction:

The successive addition reactions terminate when two radicals encounter each other. This termination can take place by combination or disproportionation of the two radicals:

It is noteworthy that the termination coefficient is highly dependent on diffusion of the macro-radicals, and when these become entangled with the polymer chains, k_t values decrease significantly, leading to auto-acceleration of polymerization for high monomer concentrations due to lower termination and higher propagation rates (see below).

In order to determine the molecular weight in free radical polymerization, we must calculate the ratio between the depletion of monomer with respect to the production of polymer. For relatively high molecular weight products, the initiation rate is negligible compared to the propagation. If we take the inverse of this ratio:

We obtain the Mayo equation. If we assume the transfer reaction negligible when compared to the termination reactions, and a steady state is reached, the initiation rate is equal to the termination rate. The concentration of radicals can be therefore calculated and introduced in the Mayo equation to determine the degree of polymerization:

We observe that the propagation rate is proportional to k_p and inversely proportional to k_t^1/2 in steady state conditions.

Carlos Arnaiz del Pozo

This summary was elaborated from the outcomes of the course ‘’Polymer Reaction Engineering’’ in the TU/e for the PPD Designers program and from the book: Elements of Polymer Science and Engineering. A.Rudin.

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Estación de depuración de aguas residuales. Proceso de Aireación prolongada.

Una Estación Depuradora de Aguas Residuales es una planta de tratamiento de aguas en la cual, mediante unos procesos físico-químicos y biológicos se adecúa un caudal de agua contaminada para poder verterla a un cauce de agua natural y que ésta siga su curso natural.

Las E.D.A.R. de aireación prolongada se encuentra dentro del proceso de lodos activos para el tratamiento de aguas.

Se suele aplicar a pequeñas poblaciones (<10000 habitantes) y suele emplear equipos prefabricados.

Las ventajas respecto de la tecnología convencional de fangos activos son:

• Bajos requisitos de superficie
• Buenos rendimientos de eliminación de sólidos en suspensión y materia orgánica
• Los lodos salen de la cuba biológica ya estabilizados

Las desventajas respecto de la tecnología convencional de fangos activos son:

• Elevado  consumo energético
• Control del proceso más complejo que en las Tecnologías Convencionales
• En las instalaciones que emplean aireadores de superficie se forman aerosoles que pueden vehicular agentes patógenos
  Bajos rendimientos de eliminación de nutrientes y de patógenos

Los procesos de los que consta esta tecnología son:

LÍNEA DE AGUAS.

·         Pretratamiento: : El objetivo del pretratamiento es separar de los influentes que llegan a la E.D.A.R. y mediante procesos físicos y mecánicos la mayor parte de los elementos que causarían problemas en nuestra E.D.A.R. en el pretratamiento se separan arenas, piedras, grasas, latas, botellas, trapos…

El pretratamiento contará con los siguientes equipos:

-         Aliviadero de caudal: Es un canal exterior para aliviar el exceso de caudal que llegue a la E.D.A.R. en días de muchas precipitaciones.

-         Pozo de gruesos: El agua residual a tratar, una vez aliviado el exceso, pasa al pozo de gruesos en el que se separan todos los sólidos y objetos que por su tamaño podrían dañar las bombas de elevación. Se retiran materiales muy pesados (piedras, arena gruesa…) y muy voluminosos (madera, trapos…)

-         Pozo/Arqueta de bombeo: El caudal de aguas residuales a tratar pasa al pozo de bombeo, para su elevación al pretratamiento, y así continuar el resto del tratamiento por gravedad.

-         Desbaste: Es un tratamiento que consiste en la eliminación de las sustancias excesivamente grueso cuya presencia en el efluente perturbaría el tratamiento total y e eficiente funcionamiento de las máquinas, equipos e instalaciones de la E.D.A.R.

-         Desarenado – Desengrasado: El objetivo de los desarenadores es separar arenas, grava, cenizas y cualquier otra sustancia pesada que tenga velocidad de sedimentación o peso específico superiores a los de los sólidos orgánicos putrescibles del agua residual.

Los desarenadores deberán proteger los equipos mecánicos móviles de la abrasión y desgaste anormales. En nuestro caso el desarenador utilizado es aireado con el objetivo de eliminar olores, favorecer la separación con independencia del caudal de entrada mediante el aire. Aceites y grasas se eliminarán también en este equipo. La cantidad de aceites y grasas será reducida al tratarse de aguas provenientes, básicamente, de domicilios. La cantidad de aire inyectado a través de los difusores determina la velocidad de giro o de rotación del agua, de manera que cuanto mayor sea la cantidad de aire inyectado, mayor será la velocidad de giro, pudiéndose establecer por lo tanto la velocidad adecuada actuando sobre la cantidad de aire inyectado.

Tratamiento primario: El objetivo primordial de este tratamiento es la reducción de sólidos en suspensión. En este tratamiento se reduce, además, una cierta cantidad de DBO₅.

Decantador primario o tanque de tormentas: Este equipo tratará el agua que no pueda llegar al tratamiento secundario. En él sedimentarán los sólidos en suspensión.

·         Tratamiento secundario En el tratamiento secundario se eliminan los componentes disueltos o en suspensión.  Este tratamiento favorece el crecimiento de microorganismos (bacterias y protozoos)

-         Reactor biológico: El agua será tratada en dos reactores biológicos de aireación prolongada.  El agua pretratada entrará en la zona anóxica, que dispondrá de agitadores y que se incorporará también la recirculación interna. El reactor tendrá una circulación tipo flujo pistón, mediante canales de oxidación con zonas óxica y anóxica para conseguir la nitrificación y desnitrificación del efluente, además de la eliminación de la materia orgánica carbonosa.

-         Decantador secundario: La unidad de sedimentación secundaria es parte fundamental del proceso biológico para conseguir los objetivos de calidad relativos a los sólidos en suspensión y DBO asociados a los sólidos suspendidos volátiles del efluente, es decir, biomasa.

-         Caudal de recirculación: El objetivo de la recirculación externa es concentrar la biomasa en el reactor.

-         Caudal  de purga: El caudal de purga va a la línea de fangos. Es necesario para mantener la biomasa constante en el reactor.

LÍNEA DE FANGOS.

Los fangos generados durante la depuración necesitan de un tratamiento. En el caso de la aireación prolongada se tratan en dos equipos diferentes.

-         Espesador: Su finalidad es la homogeneización de la masa así como la generación de canales preferenciales que faciliten la evacuación del agua intersticial y de los gases incluidos en los fangos que se generan por fenómenos de fermentación.

-         Deshidratador: El objetivo de este tratamiento es aumentar el porcentaje de materia seca al máximo para disminuir el volumen al mínimo para favorecer el posterior tratamiento de compostaje.

Bibliografía:

 Apuntes de la asignatura Depuración de Efluentes Líquidos. profesor Virgilio Llorente. Escuela Técnica Superior de Ingenieros Industriales

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Engine Lubricant oils. Current challenges.

A lubricant is defined as a substance introduced between two surfaces in relative motion with the aim of reducing the friction between them.

The basic functions of a lubricant are friction reduction and wear prevention. Other desirable features are contribution to the cooling, being anticorrosive, having  a cleaning action, preventing the formation of deposits and facilitating the dispersion of pollutants.

Composition

The lubricant oils are formulated from a range of basestocks and additives. The basestock provides the fluid layer that separates the moving surfaces and eliminates heat and particles that cause the wear, minimizing friction. Some properties of lubricating oils are improved by additives, whose functions are described later.

Basestocks. Classification

The basestocks are the major components of lubricating oils for engines. They can be classified according to their main components, its origin and its production process, among other classifications.

Depending on the major components, one can speak of paraffinic base oils, naphthenic, and others, such as white oils, electrical oils and process oils.

The paraffinic base oils are made from crude oil with a high content of alkanes. They are characterized by good viscosity - temperature relation, high viscosity index, good low temperature properties and stability.

The naphthenic base oils are characterized by containing no paraffins and high solvent power. The viscosity - temperature relationship is worse than in the previous case and its viscosity is low - medium.

The base oils can be classified according to their origin, within two groups: mineral and synthetic base oils. Mineral base oils are those obtained directly from the refinery, while the synthetic are obtained after other processes. The oils can be classified according to their production process:

• Mineral oils: produced by distillation and refining using:

• Conventional technologies (e.g. acid refining and  solvent extraction).
• Modern technologies such as  hydrotreating and hydrocracking.

• Synthetic oils: produced by chemical reaction:

• Synthetic hydrocarbons: Polyalphaolefins, polyisobutenes dialkylbenzenes.
• Other synthetic fluids: dicarboxylic esters, polyesters, neopentyl polyalkylene glycols, phosphate esters, silicone oils, polyphenol ether, perfluoroalkyl ether clorofluoroalquil ether, PAMA / PAO - cooligomers.

Mineral oils can be considered worse in reference to properties like temperature stability, oxidation stability, viscosity - temperature behavior, low temperature fluidity, high temperature volatility, application temperature range, radiation stability and resistance to ignition.

However, mineral oil may be superior with regards to properties such as behavior towards hydrolysis, corrosion, toxicology, compatibility with other materials, miscibility with mineral oils, compatibility with seal materials, additive solubility, availability (in general or of certain viscosity grades) and price.

Additives for engine oils

The additives are substances capable of improving the properties of the base oil. They are added from 5 to 20 % by weight. They can be classified by their function. These include the anti-wear additives, antioxidants, friction modifiers, demulsifiers, antifoaming agents, detergents and dispersants. One additive usually have several functions.

• Antiwear additives: The aim of the antiwear additives is to prevent wear of the parts that are in contact in the engine. The best known are the ZDDP (zinc alkyl dithiophosphate) but following the trend in the reduction of metal content in the engine lubricating oils anti-wear additives based primarily on phosphorus and sulfur are used more and more.

• Antioxidant Additives: Antioxidants additives are used to avoid the decomposition of oils by oxidation reactions. The mechanisms to prevent degradation of the lubricant by oxidation are the removal of free radicals and the decomposition of hydroperoxides. Some examples of compounds that are usually used for this purpose are overbased phenates and salicylates. The antiwear additives ZDDPs and P - S also have antioxidant properties.

• Friction modifiers: The aim of the friction modifiers is to reduce the friction and increase the fuel efficiency. Some of the compounds that are commonly used for this purpose include carboxylic acids, phosphoric, phosphonic acids and derivatives. Depending on the metal content, we often speak of friction modifiers with metals (mainly molybdenum and tungsten) and organic friction modifiers, which do not contain metals.

• Detergent and dispersant additives: The detergent and dispersant additives are used to prevent carbon deposits on hot engine parts and encourage the dispersal of pollutants to prevent the formation of soot and sludge deposits. Compounds commonly used as detergent additives are sulfonic acids, carboxylic acids and salicylic. With regards to dispersant additives, succinimides and Mannich dispersants are generally used.

• Antifoaming additives: Anti-foaming additives are used to avoid foaming, which may jeopardize the effective engine lubrication. The effectiveness of the foams in the lubricant in wear prevention is low Chemical compounds commonly used in the treatment of non-aqueous foams are polydimethylsiloxanes. They are available in a wide range of molecular weights. Furthermore, silicone-based fluids are characterized for having limited solubility in the oil and less surface tension than oils from hydrocarbons. Fluorosilicones show lower surface tension, but its use is limited by its high price. Other commonly used antifoam compounds are aliphatic alkoxylated acids, polyethers such as polyethylene glycol, branched polyvinyl ethers and poly alcoxyamines.

New challenges of lubricating oils for engines

Currently, manufacturers of lubricating oils for engines are facing the following challenges:

• Reduction of metal content: Due to environmental concerns, there is a trend in the reduction of metal content. Particularly this reduction is considered in the sulfur, phosphorus and total metal contents.

·   Fuel economy: fuel saving is one of the main research issues, because of the economic and environmental consequences that it entails.

Due to this challenges, the research on this area is full of opportunities to improve the properties of lubricant oil for engines.

Author: Silvia Fernández Castejón. Chemical Engineer. Complutense University of Madrid.

Bibliography:

Automotive Lubricants Reference Book. R.F. Haycock and J.E. Hillier. Editorial SAE. 2nd

Edition.

Chemistry and Technology of Lubricants. R.M. Mortier, M.F. Fox and S.T. Orszulik.

Editorial Springer. 3rd Edition.

Trends in lubricant oils for engines. British Petrol.

http://www.bp.com/genericarticle.do?categoryId=9013610&contentId=7021437

Trends in lubricant oils for engines. Shell.

http://www.shell.com/home/content/innovation/smarter_mobility/energy_efficienct_fuels_lubri

cants/advanced_lubricants/

Trends in lubricant oils for engines.. Shell. (2).

http://www.shell.com/home/content/lubes/media_centre/news_media_releases/2011/shell_conc

ept_lubricant.html

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Jatropha oil, an alternative raw material for the production of biodiesel. Analysis of the pretreatment process.

As an alternative fuel, biodiesel must compete economically with conventional fuels. One way to reduce costs is to use less expensive feedstocks such as non-edible oils, animal fats, waste cooking oil and by-products of vegetable oil refining. Non-edible vegetable oils, mostly from seeds of trees and shrubs, can be a good alternative. Within them it is Jatropha oil.

Jatropha oil is toxic. Therefore, it has the advantage of not being edible and thus not competing with other uses like other oils that may be intended for human or animal consumption. However, it has the disadvantage of having a high content of free fatty acids (FFA). This requires a special pretreatment process to be used as a feedstock in the biodiesel production process.

The biodiesel production process consists of the following steps:

Graphic 1. Steps in the production process in the chemical industry. Biodiesel production process.

Starting with the oil, it is necessary to perform several pretreatment steps in order to obtain a raw material which fulfills the specifications for been used in the production of biodiesel. Graphic 2 shows the general steps of the pretreatment process of a crude oil in the biodiesel production process.

Graphic 1. Pretreatment process of an oil as raw material for the biodiesel production process.

Making an analysis of pretreatment steps in the particular case of Jatropha oil, we conclude the following:

• Degumming: Vegetable oils often contain gums such as phospholipids. These substances deactivate the transesterification catalyst, due to its content in phosphorus. The purpose of degumming is removing these gums.

In this case, the effect of the phospholipids in the esterification reaction has been investigated by adding them to the oil after the degumming or the refining step. Phospholipids were shown to show little influence on the final conversion of free fatty acids in the esterification. It can be concluded that in this particular case this step is not necessary.

• Neutralization. Due to the high FFA content (up to 40% by weight) in Jatropha oil, its elimination would not be advisable, since raw material is lost. The best option would be one of the proposals for raw materials with high content in AGL. The literature consulted showed that the most common method for reducing the FFA content is acid catalysis or esterification.

• Deodorization and bleaching: Because the goal of the process is to produce biodiesel, these operations will not be performed, since it is only recommended in the case of  oils for human consumption.

• Washing and drying: The two stages are necessary in all oils pretreatment, being of great importance drying due to the influence of the water on the kinetics of the transesterification reaction.

It is concluded that the pretreatment process for Jatropha oil case consists of the following phases:

Graphic 3. Steps of the pretreatment process for the Jatropha oil in the biodiesel production process

The esterification reaction

An AGL reversibly reacts with an alcohol in the presence of an acid catalyst to form ester and water. FFA by this reaction may be transformed into esters. The reaction product contains less amount of FFA and after separation of the water, can be used as an economical raw material for the transesterification.

Graphic 4. Esterification reaction using sulphuric acid as a catalyst.

Secondary reactions

FFA reactive with the basic catalysts of the transesterification  reaction (NaOH or KOH) to form soap (saponification reaction).

 Graphic 5. Saponification reaction of FFA in the presence of the transesterification catalyst.

Furthermore, the presence of moisture results in the hydrolysis of the triglycerides present in the oil, that should give AGL esters as a result and dilute the acid catalyst so that the reaction rate decreases. Being a reversible reaction, the presence of water, a reaction product, influences the conversion and the reaction rate.

Free fatty acids

The FFA content in Jatropha oil may exceed 40 % wt, above the values ​​for a desirable feedstock for transesterification, being recommended less than 0.5 % wt.

Alcohols

At this stage the most widely used alcohols are methanol and ethanol, being the first more widely used.

Catalysts

The esterification reaction does not occur in the absence of catalyst. The catalyst is acid and the catalysis may be:

 Homogeneous catalysis: with catalysts such as sulfuric, hydrochloric, sulphonic and phosphorus acids,  being the first the most used.

 Heterogeneous Catalysis: various solid acid catalysts have been studied such as D72 resin HM molecular sieve, SAPO-11, HZSM-5, H.beta and metatitanic acid (SO 4 2 / TiO 2), showing the latter a higher activity in the reaction .

Conditions of the esterification reaction

Doing a research, the usual conditions of the esterification reaction are shown in the following table:

 Table 1. Usual Conditions in the esterification reaction.

	Units	Range
METANOL PURITY	% wt.	98 - 99,8
CATALYST PURITY (H2SO4)	% wt.	98 - 99
PRESSURE	atm	1
TEMPERATURE	ºC	30-60
TIME	min	60-120
CATALYST CONCENTRATION	% wt.	1-10
METHANOL / OIL RATIO	v/v	0,16-12

Author: Silvia Fernández Castejón. Chemical Engineer. Complutense University of Madrid.

Bibliography:

- H.J. Berchmans, S. Hirata. Biodiesel production from crude Jatropha curcas L. seed oil

with a high content of free fatty acids. Bioresource Technology 99 (2008) 1716 – 1721

- M. Berrios y col. A kinetic study of the esterification of free fatty acids (FFA) in

sunflower oil. Fuel 86 (2009) 2383 - 2388

- W.M.J. Achen y col. Review. Jatropha bio-diesel production and use. Biomass and

bioenergy 32 (2008) 1063 – 1084

- E.Santacesaria y col. Comparison of different reactor configurations for the reduction

of free acidity in raw materials for biodiesel production.Ind.Eng.Chem.Res.2007, 46,

8355-8362.

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