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Operating Cost Benefits of Oil-Free Vacuum – Part 2

By Vacuubrand |

In part 1 of this white paper, we reviewed the general advantages of using dry, chemical-resistant diaphragm pumps for process scale-up applications.  In part 2, we will delve into the process of configuring the mini-plant in order to realize the benefits of oil-free pumps.

Mini-Plant Optimization


The processes conducted in DSM’s mini-plants are normally multi-step operations including rectifications, distillations, evaporations, synthesis steps, or reactions and recrystallizations. Solvent mixtures must be separated and purified, and energy balances must be determined. These plants consist largely of glass components, and around 10 process set-ups are used in a single plant. Equipment such as distillation columns with diameters up to 70mm and a height of 14m are used for these purposes. Similarly, rotary evaporators with flask sizes of up to 50 L and reactors in the range of 0.5 L to 25 L are used.

The glass components that comprise the majority of these set-ups have been found to leak at the joints. These leaks are undesirable as they increase the pumping demand (i.e., flow requirements) so that unnecessarily large and less efficient pumps are required. The analysis performed by DSM showed that in many cases the required vacuum levels permitted a switch from rotary vane pumps to a chemical resistant diaphragm pump provided that the company could reduce flow demands on the pumps by minimizing leakage in the pilot plant equipment.

Historically, glass joints have been sealed using PTFE inserts.  PTFE does offer chemical resistance but, as a seal, PTFE is often a poor material choice as it is does not conform well
to the variations in the sealing surface. DSM’s search for a better seal material led them to the perfluoroelastomer (FFKM) to replace their PTFE gaskets. With the FFKM seals installed, the measured pressure rise of the set-up was 0.5 mbar/hr., which corresponds to a leak rate of 0.01 mbar-L/s. During subsequent leak tests, the set-up was found to have an ultimate vacuum of 1x10-2 mbar using a rotary vane pump. This is well beyond the 1 mbar – 500 mbar range identified as critical by DSM staff. Thus, with the change to FFKM seals, the switch to chemical-resistant diaphragm pumps became a practical option in most instances.

The Benefits of VACUUBRAND's Diaphragm Pumps


By switching from rotary vane pumps to VACUUBRAND’s diaphragm pumps, DSM has realized a number of benefits, including more precise control, chemically inert wetted materials, and reductions in maintenance requirements.

The Benefits of VARIO® Control


Previously, process pressure was controlled by solenoid valves. These valves were used to meter in small amounts of air or nitrogen, or used to open and close the intake line to the pump in order to maintain the desired operating pressure. These valves were actuated to control pressure while the pump was run continuously at full speed. The so-called “two-point control” provided by the solenoid valves managed the drift of the actual process pressure around the desired process pressure. In addition to the hysteresis this approach introduces, it also reduces the efficiency of the process and reduces solvent recovery efficiency.

VACUUBRAND’s variable speed pumps featuring VARIO® control allow for a substantially higher degree of precision than that afforded by simple “two-point control” with solenoid valves. By continuously varying pump speed, VARIO control ensures the pump maintains the programmed vacuum level without the hysteresis associated with the “two-point control” approach while simplifying the control system at the same time.

Furthermore, because the VARIO control is achieved with a VACUUBRAND CVC 3000 vacuum controller that is integral to the pump, process data is readily available to the data acquisition system. The precise VARIO control, combined with the ability to integrate with a data acquisition system, greatly helps to realize a pilot plant’s primary goal of process control and optimization. Finally, by adjusting motor speed to match vacuum demands, VARIO control also yields significant power savings benefits. Savings of up to 90% can be achieved with VARIO pump control when compared with a pump that operates continuously at a single speed.

The Benefits of Superior Chemical Resistance


Use of oil-sealed rotary vane pumps meant that harsh chemicals and solvent vapors had to be cooled before they could reach the pump so that these substances would condense and not contaminate the pump oil. The use of cold traps to remove these chemicals by condensing them out of the vapor stream protected the oil-sealed pumps from the damage caused by the incompatibility of the pump oil with the solvents. In spite of the best efforts to remove as much chemical vapor as possible through cold traps, some vapor inevitably reached the pump, necessitating oil changes 1 or 2 times per month under normal circumstances. Extreme circumstances required weekly oil changes or occasional pump rebuilds. These service interruptions required that a stock of standby pumps be maintained in order to avoid system downtime. Once the oil has been removed from the rotary vane pump, the contaminated oil must also be handled according to the appropriate hazardous waste regulations.

In contrast, VACUUBRAND’s chemical-resistant diaphragm pumps feature fluoropolymer wetted materials, making them a much better match to aggressive chemicals used in this
pilot plant while simultaneously requiring less maintenance than rotary vane pumps. In the absence of oil, the residual chemical vapors not caught by the cold trap pass harmlessly
through the pump and are either collected by a post-pump condenser or exhausted.

Summary


DSM’s decision to work with VACUUBRAND was driven by a desire to reduce DSM’s mini-plant operating costs and maintenance requirements. By improving the seals on its mini-plant set-ups, DSM was able to switch from rotary vane pumps to chemical-resistant diaphragm vacuum pumps in many of these set ups. DSM was able to make this switch by working with VACUUBRAND to select and install VACUUBRAND’s VARIO® pumping units. These pumps provide excellent resistance to aggressive chemicals, and the oil-free operation and variable pumping speed reduce operating costs, decrease maintenance requirements, and provide markedly improved process control.

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