Solvent Recycle

Solvent Recycle in Chemical and Medicine Industry

Solvent is one kind of liquid which can dissolve gas, solid and liquid into even mixture. Solvent is also called menstruum which divided into organic solvent and inorganic solvent. Organic solvent usually has strong dissolve capacity which includes aliphatic hydrocarbon, aromatic hydrocarbon, alcohol, polyols, its derivatives, ketone, acid, ester, amide and so on. Inorganic solvent includes water, liquid nitrogen, liquid carbon dioxide, wet acid, fused salt and so on, which among them water is the most important inorganic solvent. 

The rapid development of solvent by having the characteristics of strong dissolving capacity, high electivity, low toxicity or non-poisonous, stable performance, little corrosion, good working performance, low volatility, less operation cost, easy recycle and other characteristics is widely used in the field of cosmetics, detergent, ink, paint, bonding agent, organic synthesis, new material, medicine, pesticide and so on, as well as extensively applied in the process of dissolve, reaction, distillation concentration extraction ,evaporative crystallization, drying and other processes. 

During the process of distillation concentration, evaporative crystallization, drying and other processes which related to solvent removal, there is large amount of solvent evaporate and it is usually operated in a vacuum status, thus vacuum process system design makes a significant influence to efficiency and energy consumption of solvent recycle.

One.  Vacuum Pump Type Selection

The pressure for vacuum system involving solvent removal in chemical engineering, medicine, new material and other filed is generally ranging from hundreds to ten thousand kilopascals, which including steam jet injector vacuum pump, liquid ring vacuum pump, drying vacuum pump and so on

1. Steam Jet Injector Vacuum System

Steam Jet Injector is a vacuum equipment which uses steam as the operation motivation. Working steam accelerate to be supersonic jet in Laval nozzle, and ejection fluid will be attributed to mixture room because of the shear action effect together with working steam room, then ejection fluid form single and even mixed fluid gradually, finally mixed fluid will eliminate jet injector after passing through diffuser who slows down the speed to certain backpressure.Steam jet injector is able to deal with vacuum series which have large vacuum amount and some particulate matter. Muti-stage steam injector can reach higher vacuum degree. However, the operation energy consumption is high in steam jet injector, during the jet process, the steam will mix with vacuum gas, as a result, it will decrease the purity of the solvent in vacuum gas which need to be recovered, if we choose mix condenser after using the steam jet injector, the solvent purity will decrease again and increased the following difficulty of purifying solvent separation and energy consumption.

2. Liquid Ring Vacuum System

Put some proper amount of water in the pump as the working liquid. There is multiple-bladed rotor installed in the acentric liquid ring vacuum pump, the liquid pitched all around by impeller, the liquid formed a similar thick closed ring which determined by the pump shape, the below surface of liquid ring is tangent to the impeller hub, the above inner surface of the liquid ring exactly come into touch with the rotor top.At that time, it forms a crescent space between the impeller hub and liquid ring, and this space will be divided into several locules by impeller which with equal numbers as blades. When we start from the blade bottom 0 degree, the locule volume becomes bigger before blade rotating to 180 degrees, meanwhile locule volume is connected with the intake port, and the air will be sucked in; the locules will be disconnected with the intake port when the air suck in process finished; when the blades continue rotating, the locules become smaller and the air will be compressed, when locules are connected with the exhaust port, the air will be released from pump.It is called water ring vacuum pump while the working liquid is water. Liquid ring vacuum pump is mainly used in the technology which have black vacuum and big amount of sucking rate, can also be used in roots or steam jet pump. Except for the low energy efficient for liquid ring pump, while the working liquid directly touch with the vacuum air, the solvent in the air will dissolve in the working liquid, and working liquid will be eliminated as waste liquid, which result in waste water amount increase , solvent recycle efficiency lower, cooling energy consumption increased in the process of vacuum system emission condensation, vacuum system solvent recycle purity decreased, solvent separation purity difficulty and energy consumption increased.

3.Drying vacuum pump

Drying vacuum pump mainly refers to non-oil drying mechanical vacuum pump, which can exhaust air under atmospheric pressure and release it into the atmosphere, and there is no oil in pump chamber or other working medium, pump limit pressure is equally the same level to oil sealed vacuum pump or close to mechanical vacuum pump. Drying vacuum pump includes of claw , roots, screw and other type of vacuum pump. There is no oil in the pump chamber or other working medium in dry vacuum pump and it can prevent organic solvent from polluting technology medium.

Besides, there is no working medium in drying vacuum system who can work efficiently and with lower energy consumption at the same gas amount and vacuum degree. The root, claw and screw combination can achieve big amount of air extraction and high vacuum degree. Meanwhile, the vacuum gas will not be polluted because of no other medium involved in, and the solvent recycle purity is high. Some recycled solvent can be used again and again, even if need separation purity, the separation and energy consumption difficulty will be lower. For instance, alcoholic liquid concentration and recycle in medicine, new material THF, isopropanol, ethylene oxide, epoxypropane and many other types of solvent recycle have been succeeded.

Two.  Condensation Recycle System Design

The design of vacuum exhaust gas condensation recycle system makes significant influence to solvent recycle efficiency and energy consumption, suitable condensation system design not only improved the recycle efficiency, but also decreased pump gas volume configuration electronical rate and reduced cool energy consumption. 

1.The Front and Interstage Pump Condenser

The front and Interstage Pump Condenser design not only can lower the gas temperature which goes into the vacuum pump, but also can condense as much as solvent which can be condensed in the vacuum, thus to lower the gas volume which goes into the pump and lower the vacuum system gas volume configuration. As for the system which combined with several vacuum pump in series, the interstage condenser is also very necessarily needed, cause as the vacuum gas pressure increased, the series dew point temperature further increased, by using condensation function, the vacuum pump gas volume can be reduced when it comes to next pump. 

2.Condensation after Pump

The gas pressure after pump is higher than the atmosphere pressure, and the series dew point temperature further increased, condensation after pump can further condense recycle of the solvent which difficult to condense in negative pressure, thereby to improve the solvent recycle efficiency and lower the difficulties of exhaust gas treatment and energy consumption.

3.Condensation System Design

Condensation system designed based on the gas pressure, temperature, solvent soiling point, melting point, saturated vapor pressure and equilibrium partial pressure, maximizing the solvent recycle benefit as the constraint goal, balance the condensation temperature and vacuum gas volume and recycle efficient relationship.

There. Recycle Liquid Separation and Purification

If the recycle solvent cannot be recycling, further separation and purification will be needed, the common ways are stated as following;

1. Liquid and liquid separation; It layered the liquid between liquid based on the solvent mutual solubility, mainly refers to immiscible component series like water, hydrocarbon separation and others. The liquid and liquid separation is simple and suitable layered device can meet the separation request.

2. Rectifying; As for many solvent series, rectifying can be adapted based on the difference of component relative volatility.

3. Differential Pressure Rectification; As for the components which have the same boiling point and relative low volatility, to get high purification solvent, differential pressure rectification separation can be adapted based on boiling point difference in different pressure, such as Water-THF, Water-isopropanol and other separation.

4 . Extraction and Rectification; Regarding the component with lower boiling point in the series, extraction and rectification way can be used to separate, which reduced the separation consumption like DMF/DMAC -water component separation. If directly use rectification, all the water needs to be gasified and energy consumption will be very high, but if extraction firstly to lower the water amount in the component, the rectification energy consumption will be saved about above 70% .

All in all, exhausting gas of solvent recycle in industry field, a suitable vacuum pump is necessarily needed, such as drying vacuum pump, such as suitable vacuum system design like reasonable condensation system equipment and solvent purification separation later on. APUMP has been focused on vacuum solvent system design and operation for many years, which accumulated abundant experiences, meanwhile, we have successfully carried out many equipment based on the modernized researching approach, and brought significant economic benefits for enterprises and remarkable social benefit.