Solvents such as acetone, methylethyl ketone and tetrahydrofuran are commonly used as reaction media and for extracting products in the pharmaceutical, speciality chemicals and fragrance industries. For various reasons, there is an increasing interest in recovering solvents, as well as in related tasks such as monitoring oxygen concentrations so as to minimise the use of purge gases.
Perhaps the strongest commercial argument for recovering and reusing solvents is the direct cost savings. In some processes with intensive solvent use, the cost of the solvent can be a significant proportion of the overall product cost.
Another compelling reason for recovering solvents is the increasing environmental legislation against emissions; such emissions may be as a result of a process design where solvent recovery was not incorporated at the outset, or where venting has occurred as a result of plant problems.
With heavy pressures on chemical and pharmaceutical companies to get new products to market as quickly as possible and with high standards of quality, it is hardly surprising that solvent recovery and reuse is not always a priority during the plant design phase. However, plant refurbishments and upgrades often include the installation of a solvent recovery system.
The most popular method of recovering solvents is filtration and distillation. Carbon bed adsorbers are used for filtration and steam is then used to desorb and recapture the solvents.
The disadvantage of this is that water is introduced into the recovered solvents and this must be removed before they can be reused. A batch distillation process is therefore employed to purify the solvent to an acceptable level for reuse -- or possibly for sale to other solvent users.
Clearly there is no advantage in purifying the solvent beyond the required level as this would represent wasted resources, so analysers -- such as the Servomex2500 infrared analyser -- are used to give an accurate, reliable, continuous online measurement of the amount of water in the solvent.
Compared with laboratory testing of samples, online analysis gives faster results, is more accurate (samples can become contaminated while being transported to the laboratory), and allows trends to be visualised and monitored in real time. On balance, online analysis is likely to reduce the number of batches that are rejected, saving both time and money.
Operating at wavelengths in the near-infrared region (1.4 or 1.9microns) the Servomex2500 can be used to measure water at per centage levels in many solvents. It is also possible in some cases, for example chlorinated solvents, to use the mid-infrared band (2.7microns) to measure levels down to
Stainless steel, Hastelloy C, Monel or other exotic materials can be used for the sample cell to resist corrosion, and the infrared source and detector are isolated from the sample. As a result, the system gives long, trouble-free operation, and built-in temperature compensation ensures accuracy even in the event of variation in the sample conditioning process.
Aside from solvent recovery processes, there are also risks associated with solvent spillage, when solvents may enter the drainage system and contaminate wastewater effluent. Although an effluent treatment plant may be able to treat low levels of contamination, a sudden unexpected high concentration of solvent would be problematic. Solvent-using plants therefore need to monitor the wastewater and, if there is a sudden increase in the solvent content of the effluent, the wastewater should be diverted to a holding tank for subsequent dilution and treatment.
One of the easiest ways to monitor wastewater for solvent contamination is by using an infrared analyser to measure the vapour phase concentrations of specified solvents at vpm (volume parts-per-million) levels in the headspace above the flowing effluent in the plant's sub-surface draining system.
In the event of a sudden increase in solvent content, there is an immediate increase in the vapour phase concentration that is rapidly measured and an alarm raised so that corrective action can be taken promptly. This technique is suitable for use with both solvents that are immiscible with water and those that are soluble, since an equilibrium vapour phase concentration can still be detected in the confined headspace.
Instruments such as the Servomex2500 infrared process gas analysers can be configured to suit any solvent. In addition, groups of solvents such as hydrocarbons can be summed together using the common absorption bands.
With increasing commercial and regulatory pressures on process industries, the recovery, reconditioning and reuse of solvents is an important aspect of running plants efficiently. Furthermore, plant managers cannot be complacent about the risks associated with solvent contamination of wastewater effluent.
Tim Wilkes is Market Manager at Servomex. For more information about solvent recovery, and to discuss particular applications for analysis instruments and systems, el +44 1892 652181 or e-mail firstname.lastname@example.org.