|Solvent Recovery: Theory|
There are various procedures and systems to recover or dispose solvents used in the process technology. Three of the most used are explained on the following pages: The last one is the most advantaged method, of which the solvent is directly condensated in the process and thus reusable again in the procedure without any further treatment. It is called direct-condensation.
1. Adsorption Process
Adsorption on activated carbon has been utilized for vapor phase solvent recovery for several decades and has proven to be a relatively simple, reliable and economical method of recovering solvents for reuse and a means of preventing air pollution. It is one of the few air pollution control technologies that can provide an economic payback.
Adsorption is a physical phenomenon in which the solvent molecule adheres to the surface of the activated carbon by virtue of weal forces (akin to surface tension) called Van der Waal's forces. The surface of the activated carbon is highly developed and is typically in the range of 1100 m2/gm. Most industrial solvents can be recovered using activated carbon.
While several methodologies are utilized for regenerating activated carbon, the predominant method in use is steam regeneration. In steam regeneration, dry, low-pressure steam is injected into the carbon bed, usually in a counter current fashion to the solvent laden airflow. The steam, due to its high heat content, desorbs the adsorbed solvent from the activated carbon and carries it out of the adsorber vessel to a condenser/cooler where the mixture is condensed and sub cooled. This general method has been utilized very successfully over the years and is especially effective for solvents which are insoluble in water, in which case, the condensed/sub cooled mixture is decanted to separate the recovered solvent and water. The solvent in most cases is returned to the solvent storage tank for reuse while the water is usually discharged without any further treatment. When the recovered solvents are water soluble, additional processes such as steam stripping, distillation and pervaporation are used to separate the solvent and water.
2. Solvent disposal method: Thermal Afterburner
The following is a brief summary of the various thermal treatment systems available to the process engineer in treating volatile, hazardous organic wastes (VOC) in an air stream. The purpose is to discuss the subject of oxidation.
Oxidation, either catalytic or thermal, is a well-developed technology for VOC control. Although oxidation is not the only treatment available for controlling VOC's, it is advantageous in the fact that the pollutants are destroyed, as opposed to being captured. Other approaches to the control of VOC's are condensation, adsorption, absorption and bio filtration. Oxidation units can destroy nearly 100 percent of the VOC and toxic emissions. In fact, depending on the type of system, destruction efficiencies of over 99.99% can be attained.
The decades-old technology operates on a simple premise: Sufficiently heating a VOC consisting of carbon and hydrogen, in the presence of oxygen, will reduce the VOC to harmless carbon dioxide and water. A general reaction could be depicted as follows:
CzHy + (z + y/4)O2 à (z)CO2 + (y / 2)H2O
In this general reaction, the heat of formation of the products, typically, is less than that of the reactants, thus yielding a heat release. To initiate this reaction, a certain amount of energy is required. If the amount of energy released in the formation of the products is greater than the amount required to initiate the reaction, the reaction will sustain itself without further input of energy. When a catalyst is used, the reaction remains the same except the energy required to activate the reaction is lowered. Typical catalysts used are either precious metal-based or metal oxide-based. The two primary approaches to heat recovery are recuperative and regenerative. The recuperative exchanger transfers heat through a surface (a heat exchanger tube, for instance) from one fluid to another as long as a temperature gradient exists between the two fluids.
In a regenerative system, a hot fluid transfers heat to the surface over a period of time, after which the colder fluid is then passed over the same surface, absorbing heat from the surface.
Depending on the type of oxidation system and recovery design, as much as 95 percent of the thermal energy can be recovered to be reused in the oxidizer, in other industrial processes, or to provide building heat, etc.
3. Solvent circulation method: Condensation
The process situation is ideal with the use of 'Condensation Technology' in which the solvent is used in a 'circular system'. Dry coating material is being added and the finished product is leaving the system also in dry condition. Inside the floating dryer, the solvent is leaving the system in vaporous condition and is being liquefied in the condenser, with the help of heat removal and can be used without further treatment again in the production process.
This solvent recovery method through direct condensation is a quite easy procedure; one of the biggest benefits is the possibility of reusing the solvent without any further treatment in the production process. Another benefit is the small energy demand for the drying process inside the dryer and for the condensation process. The direct-condensation has following advantages:
To achieve acceptable operating costs, it is essential to run the process with relatively high solvent content in the process air. Would be only operated with air, the solvent/air mixture would be explosive. By mixing with N2, there has to be achieved an oxygen content of less than 6%. Combustion or explosions can occur with oxygen contents from 11%. (Or 50 to maximum 80 grams of solvent per Nm3). By using N2 as an inert gas, it will be necessary to use a complete sealed circuit, including a complete sealed dryer with virtually gas-proof in- and outlet. It is very important to take continuous measurements of the Oxygen (O2) content to guarantee save operation. A fully automated N2 Delivery system is necessary to secure all possible process situations.
(C) 2002 by FioTec (Thailand) Co., Ltd. / PTM Engineering Switzerland