se io fossi piccolo
come il grande oceano,
camminerei sulla punta dei piedi delle onde
nell’alta marea
sino a sfiorar la luna
dove trovare un’amata
uguale a me;
angusto sarebbe il cielo
per potermi contenere

se io fossi povero
come un miliardario,
che cos’è il denaro per l’anima?
è un ladro insaziabile,
si annida in essa
all’orda di tutti i miei più sfrenati desideri,
non basterebbe l’oro
di tutte le Californie

se io potessi balbettare
come Dante, o Petrarca
accendere l’anima per una sola
ordinarle coi versi di bruciare
le parole del mio amore sarebbero
un arco di trionfo
pompose ed inutili
vi passerebbero le amanti
di tutti i secoli
dei secoli
e così sia

se io fossi silenzioso
come il tuono
gemerei, abbracciando in un tremito
il decrepito eremo terrestre
urlerò con la mia voce immensa
le comete torceranno le ali fiammeggianti
e giù si getteranno, a capofitto
per la malinconia
coi raggi degli occhi rosicchierei le notti

se io fossi buio
come il sole
ma perché mai dovrei io
abbeverare
con il mio splendore
il ventre dimagrato
della terra
morirò
porterò via con me
il mio amore immenso
in quali notti
quali malattie
da quali Golia fui generato
così grande
così inutile.

Majakovskij

La conoscenza delle costanti della Legge di Henry di grassi e lipidi è importante per discutere della respirazione. L’effetto dello scambio di gas tra sangue e aria all’interno degli alveoli nei polmoni, significa che la composizione dell’aria nei polmoni varia durante il ciclo respiratorio. Il gas alveolare è infatti una soluzione dell’aria appena inalata e dell’aria che sarà espirata. La concentrazione dell’ossigeno presente nel sangue equivale ad una pressione parziale di circa 40 Torr (5,3 kPa), mentre la pressione parziale dell’ossigeno atmosferico è di circa 104 Torr (13,9 kPa). Il sangue arterioso resta nei capillari che passano attraverso la superficie degli alveoli per circa 0,75 s, ma il gradiente della pressione parziale è tale che esso si satura di ossigeno in circa 0,25 s. Se i polmoni incamerano dei fluidi (come in una polmonite), le membrane respiratorie si addensano, la diffusione è enormemente rallentata e i tessuti risentono della carenza d’ossigeno. Il diossido di carbonio segue il percorso contrario all’interno dei tessuti respiratori, ma, in questo caso, il gradiente della pressione parziale è inferiore (circa 5 Torr nel sangue e 40 Torr nell’aria all’equilibrio). Tuttavia, dal momento che il diossido di carbonio p molro più solubile nel fluido alveolare di quanto non lo sia l’ossigeno, ad ogni respiro vengono scambiate quantità uguali di ossigeno e diossido di carbonio ad ogni respiro.

Una camera di ossigeno iperbarico, nella quale l’ossigeno ha un’elevata pressione parziale, viene usata per il trattamento di alcune malattie. L’avvelenamento da diossido di carbonio, per l’appunto, può essere curato con questo sistema, come anche l conseguenze di uno shock. Malattie causate da batteri anaerobici, come la gas-cancrena o il tetano, possono anch’esse esser trattate con il medesimo metodo, in quanto tali batteri non sono in grado di proliferare in ambienti ad elevate concentrazioni d’ossigeno.

Nello scuba diving (immersione subacquea, dove SCUBA sta per “Self-contained underwater breathing apparatus”), l’aria viene fornita ad una pressione maggiore, così che la pressione all’interno della cassa toracica del sommozzatore eguagli la pressione esercitata dall’acqua circostante (l’incremento di pressione esercitato dall’acqua è di circa 1 atm ogni 10 m di profondità). Una conseguenza dell’inalare aria ad alte pressioni è data dal fatto che l’azoto è mlto più solubile nei tessuti grassi che in acqua, perciò esso tende a dissolversi nel sistema centrale nervoso, ne midollo osseo e nelle riserve di grassi. Il risultato è la narcosi da azoto, con sintomi quali l’intossicazione. Se il sommozatore risale troppo rapidamente in superficie, l’azoto fuoriesce dalla soluzione lipidica sotto forma di bolle, che causano la dolorosa e a volte fatale condizione conosciuta come “The bends”. Molti casi di annegamento sembrano essere conseguenza di emboli arteriosi (ostruzioni nelle arterie causate da bolle di gas) e perdita di conoscenza non appena le bolle d’aria raggiungono il cervello.



Da Atkins’ Physical Chemistry di P. Atkins e J. De Paula – Traduzione di Michele Formica

Introduction of the physico-chemical properties of the supercritical fluids

A pure supercritical fluid (SCF) is any compound at a temperature and pressure above the critical values (above critical point). Above the critical temperature of a compound the pure, gaseous component cannot be liquefied regardless of the pressure applied. The critical pressure is the vapor pressure of the gas at the critical temperature. In the supercritical environment only one phase exists. The fluid, as it is termed, is neither a gas nor a liquid and is best described as intermediate to the two extremes. This phase retains solvent power approximating liquids as well as the transport properties common to gases.

A comparison of typical values for density, viscosity and diffusivity of gases, liquids, and SCFs is presented in Table 1.

Table 1. Comparision of physical and transport properties of gases, liquids, and SCFs.

The critical point (C) is marked at the end of the gas-liquid equilibrium curve, and the shaded area indicates the supercritical fluid region. It can be shown that by using a combination of isobaric changes in temperature with isothermal changes in pressure, it is possible to convert a pure component from a liquid to a gas (and vice versa) via the supercritical region without incurring a phase transition.

The behavior of a fluid in the supercritical state can be described as that of a very mobile liquid. The solubility behavior approaches that of the liquid phase while penetration into a solid matrix is facilitated by the gas-like transport properties. As a consequence, the rates of extraction and phase separation can be significantly faster than for conventional extraction processes. Furthermore, the extraction conditions can be controlled to effect a selected separation. Supercritical fluid extraction is known to be dependent on the density of the fluid that in turn can be manipulated through control of the system pressure and temperature. The dissolving power of a SCF increases with isothermal increase in density or an isopycnic (i.e. constant density) increase in temperature. In practical terms this means a SCF can be used to extract a solute from a feed matrix as in conventional liquid extraction. However, unlike conventional extraction, once the conditions are returned to ambient the quantity of residual solvent in the extracted material is negligible.

The basic principle of SCF extraction is that the solubility of a given compound (solute) in a solvent varies with both temperature and pressure. At ambient conditions (25°C and 1 bar) the solubility of a solute in a gas is usually related directly to the vapor pressure of the solute and is generally negligible. In a SCF, however, solute solubilities of up to 10 orders of magnitude greater than those predicted by ideal gas law behavior have been reported.

The dissolution of solutes in supercritical fluids results from a combination of vapor pressure and solute-solvent interaction effects. The impact of this is that the solubility of a solid solute in a supercritical fluid is not a simple function of pressure.

Although the solubility of volatile solids in SCFs is higher than in an ideal gas, it is often desirable to increase the solubility further in order to reduce the solvent requirement for processing. The solubility of components in SCFs can be enhanced by the addition of a substance referred to as an entrainer, or cosolvent. The volatility of this additional component is usually intermediate to that of the SCF and the solute. The addition of a cosolvent provides a further dimension to the range of solvent properties in a given system by influencing the chemical nature of the fluid.

Cosolvents also provide a mechanism by which the extraction selectivity can be manipulated. The commercial potential of a particular application of SCF technology can be significantly improved through the use of cosolvents. A factor that must be taken into consideration when using cosolvents, however, is that even the presence of small amounts of an additional component to a primary SCF can change the critical properties of the resulting mixture considerably.

Application of supercritical fluid extraction

Supercritical extraction is not widely used yet, but as new technologies are coming there are more and more viewpoints that could justify it, as high purity, residual solvent content, environment protection.

The basic principle of SFE is that when the feed material is contacted with a supercritical fluid than the volatile substances will partition into the supercritical phase. After the dissolution of soluble material the supercritical fluid containing the dissolved substances is removed from the feed material. The extracted component is then completely separated from the SCF by means of a temperature and/or pressure change. The SCF is then may be recompressed to the extraction conditions and recycled.

Some of the advantages and disadvantages of SCFs compared to conventional liquid solvents for separations:

Advantages

* Dissolving power of the SCF is controlled by pressure and/or temperature
* SCF is easily recoverable from the extract due to its volatility
* Non-toxic solvents leave no harmful residue
* High boiling components are extracted at relatively low temperatures
* Separations not possible by more traditional processes can sometimes be effected
* Thermally labile compounds can be extracted with minimal damage as low temperatures can be employed by the extraction

Disadvantages

* Elevated pressure required
* Compression of solvent requires elaborate recycling measures to reduce energy costs
* High capital investment for equipment

Solvents of supercritical fluid extraction

The choice of the SFE solvent is similar to the regular extraction. Principle considerations are the followings.

* Good solving property
* Inert to the product
* Easy separation from the product
* Cheap
* Low PC because of economic reasons

Carbon dioxide is the most commonly used SCF, due primarily to its low critical parameters (31.1°C, 73.8 bar), low cost and non-toxicity. However, several other SCFs have been used in both commercial and development processes. The critical properties of some commonly used SCFs are listed in Table 2.

Table 2. Critical Conditions for Various Supercritical Solvents

Organic solvents are usually explosive so a SFE unit working with them should be explosion proof and this fact makes the investment more expensive. The organic solvents are mainly used in petrolchemistry.

CFC-s are very good solvents in SFE due to their high density, but the industrial use of chloro-fluoro hydrocarbons are restricted because of their effect on the ozonosphere.

CO2 is the most widely used fluid in SFE.

Beside CO2, water is the other increasingly applied solvent. One of the unique properties of water is that, above its critical point (374°C, 218 atm), it becomes an excellent solvent for organic compounds and a very poor solvent for inorganic salts. This property gives the chance for using the same solvent to extract the inorganic and the organic component respectively.

Industrial applications

The special properties of supercritical fluids bring certain advantages to chemical separation processes. Several applications have been fully developed and commercialized.

Food and flavouring

SFE is applied in food and flavouring industry as the residual solvent could be easily removed from the product no matter whether it is the extract or the extracted matrix. The biggest application is the decaffeinication of tea and coffee. Other important areas are the extraction of essential oils and aroma materials from spices. Brewery industry uses SFE for the extraction of hop. The method is used in extracting some edible oils and producing cholesterine-free egg powder.

Petrolchemistry

The destillation residue of the crude oil is handeled with SFE as a custom large-scale procedure (ROSE Residum Oil Supercritical Extraction). The method is applied in regeneration procedures of used oils and lubricants.

Pharmaceutical industy

Producing of active ingradients from herbal plants for avoiding thermo or chemical degradation. Elimination of residual solvents from the products.

Other plant extractions

Production of denicotined tobacco.

Enviromental protection

Elimination of residual solvents from wastes. Purification of contaminated soil.

Mi aiuteranno?

Saretta

Just run.

Semplicemente metti qualcosa di te sotto forma di onde sonore.

Frequenze impalpabili, ma concrete.

Phoenix From Fire di Michele Formica

Stop.

Una sigaretta in solitaria.

Musica. Penetra. Fluida e pungente.

Squilla il telefono, ma non risponderò. Per non rovinare questo momento.

La perfezione, spesso, risiede in ciò che non si è faticato raggiungere.

Stop.