washing is a mild alkali process for emission control by the removal of acid gases (such as carbon dioxide and hydrogen sulfide) from gas streams and uses the principle that the rate of absorption of carbon dioxide by potassium carbonate increases with temperature. It has been demonstrated that the process works best near the temperature of reversibility of the reactions:
In the Benfield process, acid gases are scrubbed from the feed in an absorber column using potassium carbonate solution with Benfield additives to improve performance and avoid corrosion.
Water washing, in terms of the outcome, is analogous to washing with potassium carbonate (Kohl and Riesenfeld, 1985), and it is also possible to carry out the desorption step by pressure reduction. The absorption is purely physical, and there is also a relatively high absorption of hydrocarbon derivatives, which are liberated at the same time as the acid gases.
See also: Gas Cleaning, Gas Processing, Gas Treating, Hot Potassium Carbonate Process, Scrubbing.
Alicyclic Hydrocarbons
Alicyclic hydrocarbons contain cyclic structures in all or part of the chemical skeleton. The saturated alicyclic hydrocarbons have the general formula CnH2n. When the molecular formula of a saturated hydrocarbon corresponds to the general formula CnH2n-2, then the compound contains two rings; if the formula corresponds to CnH2n-4, it contains three rings, etc. Their boiling points and densities are higher than alkanes having the same number of carbon atoms. In crude oil, the most frequently found rings are those having five or six carbon atoms. In these rings, each hydrogen atom can substituted by a paraffinic alkyl chain that is either a straight chain or branched. Monocyclic naphthenes with one ring are major constituents of the light fraction. Monocyclic naphthene of carbon numbers C20, to C30, with long side chains can be isolated from paraffin waxes.
Cycloalkanes (naphthenes) catalytically crack by both ring and chain rapture and yield olefins and paraffin. In the case of hydrocracking process, polycyclic hydrocarbons aromatic are partially hydrogenated. Naphthene rings in polycyclic compounds are readily removed by ring–opening followed by cracking. Single–ring naphthenes and paraffin are more resistant to cracking. It was also found that single-ring naphthenes appear to react more slowly at high conversion levels. In the catalytic reforming process, the fundamental reaction occurring is (i) dehydrogenation of six members rings naphthenes by platinum catalyst, and (ii) isomerization of alkyl cyclopentane derivatives to cyclohexane derivatives.
The stability of the cycloalkane derivatives increases up to the six member ring, then decreases from seven to eleven member ring, and from the twelve member onwards attains the stability of two six member rings (Table A-12).
Table A-12 Properties of selected alicyclic hydrocarbon derivatives.
| Alicyclic hydrocarbon (naphthene) | Melting point, °C | Boiling point, °C | Density, @20 °C |
|---|---|---|---|
| Cyclopropane | -127 | -33 | |
| Cyclobutane | -80 | 13 | |
| Cyclopentane | -94 | 49 | 0.746 |
| Cyclohexane | 6.5 | 81 | 0.778 |
| Cycloheptane | -12 | 118 | 0.810 |
| Cyclooctane | 14 | 149 | 0.830 |
| Methylcyclopentane | -142 | 72 | 0.749 |
| Cis-1,2- dimethylcyclopentane | -62 | 99 | 0.772 |
| Trans-1,2- dimethylcyclopentane | -120 | 92 | 0.750 |
| Methylcyclohexane | -126 | 100 | 0.769 |
Aliphatic Hydrocarbons
Aliphatic hydrocarbon derivatives (also known as paraffins or paraffin derivatives) are straight chain or branched saturated organic compounds with composition CnH2n+2. Paraffin derivatives are present in large amounts in nature and low molecular weight paraffin derivatives are found in natural gas. Methane is the lowest member of the paraffin series of hydrocarbons. With the increase in size of molecule, several hydrocarbons may exist. Such hydrocarbons have different properties and are known as isomers. On the other hand, olefin hydrocarbon derivatives have carbon atoms joined by two bonds.
Hydrocarbon derivatives are classified into chemical families according to their structure. All structures are members of the homologous series of even hydrogen numbers. The chemical formula is CnH2n +2 or lower in hydrogen content CnH2n. The carbon-carbon molecule chains have different chemical bonding arrangements such as (i) saturated hydrocarbon derivatives that are linked by carbon-carbon single bonds and are given the suffix ane and (ii) unsaturated hydrocarbon derivatives that are linked by multiple bonds double bond C=C (suffix ene) or triple C≡C(suffix yne). However, the same molecule can contain several multiple bonds – if there are two low sets of the double bond, the suffix is diene.
Normal alkanes (straight-chain paraffins) consist of a chain of carbon atoms. Each carbon atom is linked to four atoms, which can be either carbon or hydrogen, their general formula CnH2n+2 . The carbon skeleton can be structured as straight chains as are the normal paraffin, CH3(CH2)nCH3. The boiling points increase with the number of carbon atoms. With the low carbon numbers, the addition of a carbon increases the boiling point to approximately 25°C. Further additions give smaller increase. At the same time, the density increases with the molecular weight 0.626 kg/L for pentane, and 0.791 kg/L for pentacosane; on the other hand, the density is always much lower than 1. The normal alkanes from C1 to C4 are colorless gases; C5 to C17 colorless liquids; and from C18 onwards, colorless solids. Other physical properties, such as melting point, density, and viscosity, also increase in the same way as boiling point (Table A-13). There is a relationship between physical properties and chemical composition. The variation in boiling point of compounds is due to different intermolecular forces such as hydrogen bonding. The alkanes
