Processing, June 2020

Chemical Processing C o n t r o l l i n g c h e m i c a l vac u u m p r o c e s s e s w i t h direct sealing diaphragm valves Advantages include accuracy chemical compatibility and easy automation By Jeff Jennings Equilibar he use of vacuum technology to create Edisons electric light bulb in 1879 is widely known Since then researchers have discovered thousands of other ways T to take advantage of running a process in vacuum While these may not be as famous as Edisons invention modern vacuum processes have had an impact on the world that is di cult to overstate e manufacturing of smartphones solar panels and silicon chips for example would not be possible without vacuum technology Over the past decade the use of vacuum technology for chemical processes has also increased dramatically Vacuum plays an integral role in lifesaving pharmaceutical developments in laboratory research in food and beverage processes and in environmental e orts such as polymer devolatilization Some of these new chemical vacuum processes involve harsh chemistries which present unique and sometimes quite di cult challenges Due to the hazards involved with corrosive chemicals process components must be chemically compatible as well as extraordinarily accurate and precise For any given vacuum application additional process conditions such as high temperatures or automation requirements can add further complications As a result traditional methods of controlling vacuum are not always suitable and alternatives must be considered One relatively new approach for providing vacuum control of chemical processes is the use of direct sealing diaphragm valves A classic example vacuum distillation To illustrate possible reasons for using a direct sealing diaphragm valve to control vacuum in chemical processes consider the relatively familiar process of vacuum distillation Almost everyone understands the method used to make whiskey and other distilled beverages because water and alcohol have different boiling temperatures alcohol boils o earlier than water allowing the alcohol to be collected But what happens if a chemist or engineer needs to separate two delicate compounds that are heat sensitive is situation happens frequently in the manufacturing of chemicals or pharmaceuticals with long chain molecules or polymers Often the best solution is to lower the boiling temperature by reducing the pressure over the liquids In this situation vacuum distillation is used to lower the pressure above a liquid to less than its vapor pressure allowing the most volatile liquids to be selectively boiled o and distilled is is particularly useful if the temperatures required for a uid to boil at atmospheric pressures would be hot enough to damage sensitive molecules through polymerization or by thermal cracking Precisely controlling the vacuum pressure is critical because the mixture being distilled may contain several liquids with close boiling points Accuracy in controlling the vacuum allows much higher selectivity in the distillation Real world vacuum applications and challenges Matthias Bogar manages the German division of Pressure Control Solutions a uid control specialty company that works extensively with research scientists and industries throughout Northern and Western Europe He has years of engineering experience designing vacuum distillation systems According to Bogar one of the most common applications for vacuum distillation is recycling or re ning solvents Vacuum distillation units are also used for quality control of crude oil and crude oil fractions as well as extraction of essential oils In addition for pharmaceutical manufacturing a special thin film and short path evaporators are used for vacuum distillation under very low vacuum Bogar said ese are often used for the production of hormones vitamins or other proteins that are easily destroyed by high temperatures Bogar says that three challenges to consider when designing vacuum distillation systems are Flow Coe cient Cv range precision and chemical resistance Cv range The flow restriction required by a process can be described by its Cv which is a function of ow rate di erential pressure and gas speci c gravity During the start up of a vacuum distillation system a large Cv is required to remove the atmosphere from the system When the system is at its target vacuum on the other hand only minimal gas passes through the vacuum regulator most of the product stays in the system either being condensed at the column head distillate or in the evaporator as residue Only non condensable gases are being pulled through the regulator and into the pump At that point the low Cv capability of the regulator is important In addition batch distillation systems can involve multiple steps for example in some systems distillate needs to be discharged usually by separating a distillate receiver that can be ventilated to discharge product All in all there are many scenarios during a distillation where the required Cv of a vacuum control valve can change rapidly Bogar said Precision When separating two products with similar boiling points even a couple mbar of uctuation can easily contaminate the distillate An excellent example is the re ning of rose oil for cosmetics Bogar said In one instance one small fraction in the oil was suspected to be carcinogenic To be able to 32 Processing JUNE 2020 Stainless steel direct sealing diaphragm valve with an electronic vacuum pilot regulator Images courtesy of Equlibar