Suitable desorbent

The present study suggests the potential application of a method for reducing surfactant losses in reservoirs, thus, ipso facto increasing their effectiveness. This method consists in incorporating a suitable desorbent in the water used to drive the surfactant slug injected into the formation to be treated. 

Poro-xylene is an industrially important petrochemical. It is the precursor chemical for polyester and polyethylene terephthalate. It usually is found in mixtures containing all three isomers of xylene (ortho-, meta-, para-) as well as ethylbenzene. The isomers are very difficult to separate from each other by conventional distillation because the boiling points are very close. Certain zeoHtes or mol sieves can be used to preferentially adsorb one isomer from a mixture. Suitable desorbents exist which have boiling points much higher or lower than the xylene and displace the adsorbed species. The boihng point difference then allows easy recovery of the xylene isomer from the desorbent by distillation. Because of the basic electronic structure of the benzene ring, adsorptive separations can be used to separate the isomers of famihes of substituted aromatics as weU as substituted naphthalenes. 

Desorbent features are illustrated in Table 6.8 using Cg-aromatic adsorbates with BaX and KY adsorbents. The results in Table 6.8 further emphasize the desorbent characteristic requirement mentioned above. For instance, phenyldecane is a suitable desorbent for PX separation using BaX adsorbent. However, phenyldecane is too weak to desorb PX from KY adsorbents. In contrast, diphenylmethane offers good separation of PX with KY adsorbent but not with BaX. With BaX adsorbent, PX is separated from other Cg-aromatics using 1,4-diisopropylbenzene but not with other isomers of diisopropylbenzene, such as 1,3-diisopropylbenzene. 

Displacement of the adsorbate with another substance that is in turn displaced in process is practiced, for instance, in liquid phase recovery of paraxylene from other Cg aromatics. In the Soibex process, suitable desorbents are toluene and paradiethylbenzene. This process is described later. 

Molecular sieve adsorbents have been widely studied and employed to separate saturated, mono- and di-saturated fatty acids or to concentrate them in a mixture. Logan and Underwood (32) patented a method to use zeolite as the adsorbent to separate esters of fatty acids according to their degree of unsaturation. Use of nonionic, hydrophobic cross-linked polymers has also been successful in selective separation of fatty acids (33). The fatty acids are first allowed to adsorb onto the solid bed of a particular adsorbent and then the adsorbed acids may be desorbed by employing a suitable desorbent. 

To develop a cyclic separation process, our next step v as to find a suitable desorbent. A suitable desorbent must allov selective adsorption and yet must desorb at a reasonable rate. Three aromatic desorbents v ere evaluated benzene, toluene, and o-xylene. 

The process is based on the fact that alkali metal-modified or alkaline earth modified zeolites of type X, A, L or ZMS-5 and also titanium dioxide, adsorb p-cresol more strongly than m-cresol. Thus m-/p-cresol mixtures can be separated in an adsorption column and can be dissolved again with a suitable desorbing liquid such as an aliphatic alcohol and ketone. The separating efficiency depends both on adsorption and desorption. 

The McCabe-Thiele approach has been developed to describe the Sorbex process (76). Two feed components, A and B, with a suitable adsorbent and a desorbent, C, are separated ia an isothermal continuous countercurrent operation. If A is the more strongly adsorbed component and the system is linear and noninteracting, the flows ia each section of the process must satisfy the foUowiag constraints for complete separation of A from B ... 

UOP s Parex Process can be used to purify -xylene by adsorption (38). Toray has a similar process. These processes take advantage of the fact that %xylene is adsorbed more easily than the other Cg aromatics by a suitable molecular sieve. The -xylene is desorbed by either a lighter or heavier hydrocarbon which is subsequently removed by distillation. -Xylene is recovered in about 97% yield (see Adsorption). 

Products from the reactions are collected on Tenax cartridges, and the analytes desorbed by heating, or on polyurethane form plugs from which the analytes can be recovered by elution with a suitable organic solvent. 

Solid-phase microextraction (SPME) consists of dipping a fiber into an aqueous sample to adsorb the analytes followed by thermal desorption into the carrier stream for GC, or, if the analytes are thermally labile, they can be desorbed into the mobile phase for LC. Examples of commercially available fibers include 100-qm PDMS, 65-qm Carbowax-divinylbenzene (CW-DVB), 75-qm Carboxen-polydimethylsiloxane (CX-PDMS), and 85-qm polyacrylate, the last being more suitable for the determination of triazines. The LCDs can be as low as 0.1 qgL Since the quantity of analyte adsorbed on the fiber is based on equilibrium rather than extraction, procedural recovery cannot be assessed on the basis of percentage extraction. The robustness and sensitivity of the technique were demonstrated in an inter-laboratory validation study for several parent triazines and DEA and DIA. A 65-qm CW-DVB fiber was employed for analyte adsorption followed by desorption into the injection port (split/splitless) of a gas chromatograph. The sample was adjusted to neutral pH, and sodium chloride was added to obtain a concentration of 0.3 g During continuous... 

The concept of SPME was first introduced by Belardi and Pawliszyn in 1989. A fiber (usually fused silica) which has been coated on the outside with a suitable polymer sorbent (e.g., polydimethylsiloxane) is dipped into the headspace above the sample or directly into the liquid sample. The pesticides are partitioned from the sample into the sorbent and an equilibrium between the gas or liquid and the sorbent is established. The analytes are thermally desorbed in a GC injector or liquid desorbed in a liquid chromatography (LC) injector. The autosampler has to be specially modified for SPME but otherwise the technique is simple to use, rapid, inexpensive and solvent free. Optimization of the procedure will involve the correct choice of phase, extraction time, ionic strength of the extraction step, temperature and the time and temperature of the desorption step. According to the chemical characteristics of the pesticides determined, the extraction efficiency is often influenced by the sample matrix and pH. 

Such a desorbent may be, for example, a hydrophilic nonionic surfactant, which is among the least expensive on the market and is suitable in calcic environment... 

At negative potentials EG can compete successfully with glycolate for adsorption sites, unless the EG concentration is too low, and this accounts for the lack of glycolate at the lowest EG concentration. The simultaneous appearance of glycolate and carbonate was taken by the authors as strong evidence for the existence of a common intermediate, identified as A in the scheme. It seems that A may desorb oxidatively or the second carbon may become bound to the surface if a suitable neighbouring adsorption site is available. Once this last process has taken place, carbon carbon bond scission occurs with complete oxidation to CO3". 

Mangani et al. [13] have described a method for determining extract chlorinated insecticides in soil. In this procedure a short column is packed with the soil sample. The insecticides in the soil are desorbed by a suitable solvent mixture chosen for its polarity characteristics. 

The process runs on a cycle. First the C4 stream is fed to a vessel packed with the molecular sieve. The butene-1 molecules start to fill up the sieves pores. After a while, when the pores are about saturated, the feed is cut off. Another liquid, the desorbent, is flushed back through the vessel, and the butene-1 is washed out of the sieves. The desorbent is selected so that after it picks up the butene-1 from the sieve, it can easily be separated from the butene-1 by fractionation. The key, of course, is to use a desorbent with a boiling temperature a good distance away from butene-Ts. Any run-of-the-mill hydrocarbons that fit in this criterion are suitable. 

To desorb the adsorbate from the zeolitic adsorbent, a desorbent is added. A desorbent is a suitable liquid that is capable of displacing or desorbing the adsorbate from the selective pores of the adsorbent. The process of recovering or desorbing the adsorbate from the adsorbent is known as the desorption step. 

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