Understanding Process Vacuum for Process Improvement

Through this article we wish to give practical tips to boost process capacity, increase product quality and reduce cycle time in batch or continuous systems, performed under vacuum. Attempt is made to broadly discuss the basic process fundamentals, conventional vacuum pumps used in the industry and case studies having achieved process improvements. Distillation systems are widely used in chemical and pharmaceutical industries where the operation goes by several names such as Vacuum Distillation, Solvent Recovery, Vacuum Drying, Tray Drying, Flash drying, Thin-film distillation, High vacuum distillation, Molecular distillation etc. A typical distillation process is discussed below: 

      

 

Typical Vacuum Distillation System consists of an evaporator, Vapor-liquid separator, condenser and a vacuum pumping system. In the evaporator, the product is heated to elevated temperatures to generate vapors, which are passed through Vapor-Liquid separator  (VSL); the condenser and finally the non-condensable are ejected out through the vacuum pumping system. Condensers are maintained at relatively lower temperatures so that the evaporated vapors can condense, causing drop in pressure. It is this differential pressure, which pushes the vapors from the evaporator to the condenser. The vacuum pumping system maintains the process vacuum by continuously pumping out leakages and other non-condensable loads, generated during the process. Most of the chemical processes are carried under vacuum for
maintaining low process temperature and inert working conditions.

“How can process capacity be increased?” The answer to this can easily be worked out by analyzing the following: -
• By how much do we need / can increase the output?
• What is system design capacity?
• Are current process yields optimized?
• Additional constraints needed to be impose (i.e., capital, time or regulatory)?
• Where is the bottleneck?

The answers to these basic questions can dramatically improve the process efficiency. In most of the processes the basic answer to the above would be to  “Improve the Vacuum pumping system capabilities”. We discuss under, basic terminology used frequently in reference to the vacuum process.
 

What is Vacuum?
Vacuum is simply a pressure below atmosphere. To create vacuum in a system, a pump is required to remove mass (gas/vapor) from the system. The more mass is removed; lower is the pressure that exists inside the system. Various vacuum levels are defined depending upon the ultimate absolute pressure, in Torr (mmHg),

    

The performance of a vacuum process largely depends on the right vacuum pumping system. Many companies have been able to  improve process efficiency and reduce process time considerably by making modifications in the vacuum pumping systems only. We shall discuss a few case studies highlighting the fact that proper selection of vacuum system can enhance the process performance, reduce process time, achieve higher product purity and reduce  power consumption. It is, however, important to understand the terms  Capacity, Throughput and Ultimate/ Blank-off vacuum of a vacuum pumping system/pump as they play very vital role in the overall performance of the vacuum system. Too small a pumping system would result in inefficient or no process whereas too large a pumping system would result in high capital and operating
cost. 

Capacity is generally referred to as the volumetric displacement of the pump at either free air Delivery (FAD) conditions or at the inlet conditions. It is also referred as the pumping speed of pump. It is generally calculated based on the geometrical displacement of the pump with conventional units as Lts/min, m3/hr.
Throughput is the product of capacity and Pressure (abs) with conventional units Torr–Lts/min. Therefore, for a pump of constant displacement, the throughput would drop with the drop in the inlet pressure.

Ultimate vacuum/Blank-Off vacuum: - The limiting pressure approached in the vacuum system after sufficient pumping time to establish that further reductions in pressure will be negligible. It is the final pressure achieved by a pump under blank-off condition when the throughput is practically zero. At this stage the pump does not pump any air/vapor and no further drop in pressure is possible. For most of the chemical processes vacuum-pumping system is designed to take care of process load and maintain the process to the desired levels of pressures. Process loads mainly consist of:

• Plant air leakage load.
• Process non-condensable such as dissolved gases.
• Process condensable load - vapors which escape the condenser

The sum of the individual loads must be effectively pumped out to maintain the process vacuum. For example a load of 10Kg of Air Leakage at 100 Torr (660mmhg) vacuum, 20ºC needs a pump of pumping capacity 63 m3/hr and for the same load at 10 Torr the Pumping speed required would be 630 m3/hr and at 1 Torr would need a pumping speed of 6300 m3/hr.

Mechanical Vacuum Boosters, manufactured by  EVEREST, are being extensively used in chemical process industry to boost the performance of the vacuum pumps, especially in low-pressure range, where conventional vacuum pumps have poor volumetric efficiency. Everest Boosters are capable of moving large quantity  of gas at low pressures, with far smaller power consumption than for any other equipment now available.

The internals of a Booster are totally free of any sealant fluid, and therefore the pumping is dry. Due to the vapor Compression by the booster, the pressure at the discharge of the booster is relatively high, resulting in higher volumetric of the backing pump. Everest Twin Lobe Boosters are used in series with a variety  of backing pumps to achieve higher speeds and lower ultimate pressures. 

The Table below gives a rough estimate  of how the boosters enhance the working vacuums of the process when installed in  combination with various types of vacuum pumps. Various types of backing pump can be used, depending upon the system requirement and ultimate vacuum needs.  However, the final vacuum is governed by the suitable selection of the backing pump and booster combination. The table below gives a  broad range of vacuum achieved with various backing pumps combinations.

 Water Ring Pump :- is the most widely used vacuum pump in the chemical process industry and therefore, is broadly discussed for understanding and improvement.

 

 In a cylindrical housing, partially filled with sealing liquid, a multi-blade impeller on a shaft is positioned eccentrically. Port plates with inlet and discharge openings are positioned on either side of the impeller.  A liquid ring is created by the centrifugal force generated by the rotating impeller. The centrifugal force  holds the liquid ring against the inner wall of the pumping chamber. Since the impeller is located eccentric to the pumping chamber, the depth of entry of the blades into the  liquid ring decreases and increases as the impeller rotates. This creates increasing impeller cell volume on the inlet port side, creating a vacuum. On the discharge port side, the impeller cell volume decreases, as the blades move further into the liquid ring, increasing the pressures, until discharge takes place through the discharge port. A continuous flow of fresh sealing liquid is supplied to the pump via the sealing liquid inlet.  

 

 The figure above shows typical pumping speeds of Water ring pump and Ejector, which are very popular in the chemical process  industry. These pumps have water as the sealing/motive fluid and therefore, cannot work beyond the saturated vapor pressure of water, to corresponding temperature. As evident from the curve above, the effective pumping speed drops drastically at low-pressure range and the overall process becomes inefficient, un-economical and slow. Liquid  Ring Pumps are used throughout process
industry.  Unfortunately they suffer from few limitations, such as

The final vacuum achievable is largely dependent on the vapor pressure of the pump fluid  corresponding to the working temperatures. For water sealed pumps, the lowest practical operating pressure for two-stage design would be 60 Torr (700mm Hg) for exit
water temperature at 30-32 ºC. 

• Their energy consumption per unit of gas pumped is higher since most of it is lost in handling pump fluid
• It requires large quantities of sealing fluid
• It adds load on the ETP system

Economical solution to overcome the above limitations is installation of mechanical vacuum Booster to boost the vacuum pumping system performance. Case studies discussed establish that drastic process improvements have been achieved on installation of Mechanical Booster only.  Mechanical vacuum boosters cover a vast capacity & pressure range making them an ideal choice for process engineers for practically all process applications.