Vapor Compression for WFI generation
Water for Injection (WFI) is an essential constituent of sterile manufacturing facilities. It is required for several applications and purposes, such as in parenteral liquid preparation, special solution manufacturing and CIP systems. These applications demand high-volume production of WFI.There are two key processes proven for the production of WFI: distillation and filtration. The distillation process purifies water by the change in phase and entrainment separation. There are several methods used in the production of WFI by distillation, such as single-effect distillation, multiple-effect distillation and vapor-compression distillation. The filtration process uses membrane filtration, such as reverse osmosis (RO) and ultrafiltration. The feedwater is processed through RO in combination with electrodeionization (EDI) and an ultrafiltration membrane to produce WFI.
Various regulatory authorities across the world have different requirements for the production of WFI. The United States Pharmacopeia allows WFI to be produced by distillation or via an equal or superior process. The European regulation permits only distillation, whereas the Japanese Pharmacopoeia permits filtration in addition to the. distillation processes.
While considering the distillation process, multiple-effect distillation and vapor compression distillation are two methods available for producing WFI. The accelerated growth of the pharmaceutical industry today is causing a demand for larger-volume production. Considering different requirements, those in the industry have recently started exploring vapor compression technology for its suitability to their operating conditions.
Hence, understanding vapor compression technology is of the utmost importance.
Vapor compression (VC) is a process used to produce WFI on the principle of distillation, where the water is distilled with phase change and entrainment separation. In the VC process, the vapor produced by evaporating the water is compressed to increase the saturation pressure and temperature. This compressed vapor is then condensed to produce WFI by transferring its latent heat to infeed water in a heat exchanger. This process is similar to the mechanical refrigeration cycle.
VC technology offers various benefits in operation. Although the initial cost of investments is comparatively higher, the operational expenses are reduced substantially. Various factors, like recycling of latent heat, no requirement of cooling water for condensation and reduced energy consumption, contribute to the reduction of the operation cost. Moreover, the pretreatment requirement of infeed water is less stringent for producing WFI through VC. This translates to eliminating the requirement of a purified water generation system, its distribution and its periodic validation.
A typical VC configuration includes an evaporator, a compressor, a heat exchanger (distillate cooler, blowdown cooler and feed heater), a decarbonator, pumps, valves, motors and controls.
The evaporator consists of an evaporator bottom head, a calandria and a dome. The bottom head is provided with a heating coil for initial heating of infeed water. The calandria is composed of a shell and bank of tubes for the exchange of heat from the compressed vapor to the infeed water. The dome consists of an arrangement for the removal of entrainment and the collection of vapor for compression.
As the name suggests, the function of the compressor is to compress the vapor generated in the evaporator. This increases its pressure (and temperature), which creates the temperature gradient necessary for the heat-transfer process and allows for the recovery of latent heat. There are different kinds of compressors available in the market, such as industrial fans, positive displacement compressors, belt-driven compressors andor direct drive compressors.
The distillate cooler is a shell and tube type heat exchanger, with a double tube sheet, construction in SS316L. The function of the distillate cooler is to cool down the outgoing distillate and heat the incoming feedwater, thereby optimizing energy consumption.
The blowdown cooleris a shell and tube type heat exchanger, with a single tube sheet, construction in SS316L. The function of the blowdown cooler is to cool down the outgoing blowdown and heat the incoming feedwater, thereby optimizing energy consumption.
The feed heater is a four-pass shell and tube type heat exchanger, with a double tube sheet, construction in SS316L. The function of the feed heater is to heat the incoming feedwater before it enters the evaporator.
The decarbonator consists of a shell, nozzle and steam sparger, made of SS316L. The water entering the decarbonator is dispersed in a specific pattern, which increases the contact area. A counterflow of steam is introduced to remove dissolved, non-condensable gases, such as CO2 and oxygen. In addition, it adds heat to the feedwater entering the evaporator.
The distillate pump is a sanitary centrifugal pump made of SS316. Its basic function is to remove the distilled water from the evaporator.
Features of Vapor Compression
In a typical VC unit used for pharmaceutical distillation, softened and dechlorinated water is boiled inside the bank of tubes (in the evaporator). The generated vapor then passes through a mist separator to remove entrainment within the vapor. The pure vapor enters the compressor at a controlled saturation pressure. The compressor then compresses this vapor, resulting in a higher saturated pressure steam. This high-pressure compressed steam is discharged onto the shell side of the evaporator. The high-pressure steam, by virtue of its property, possesses a higher temperature. This higher-temperature steam transfers its heat to the boiling water inside the tubes, resulting in condensation to produce WFI by giving up its latent heat energy. Additional vapor is generated, and the process continues.
1. Can produce distillate at ambient temperatures and hot temperatures on demand
2. Typically has less stringent feedwater pretreatment requirements
3. Provides a simpler pretreatment system
4. Makes variable production possible
5. Has a larger capacity, providing more significant utility savings
Advantages of the MECO Vapor Compression System
Advanced Compressor Technology: TheGII Centurbo™ Compressor
The compressor is very compact and is mounted directly onto the evaporator, eliminating problematic ducting. The impeller is made of super alloy, making it highly resistant to oxidation and corrosion. Unlike other drive systems, the MECO direct drive system eliminates intermediate coupling to connect the fan and motor, which requires alignment, additional bearings and a dual circulating oil system with external cooling fans and motors. It uses a once through oil mist system instead of a large-volume oil recirculation system, eliminating oil leaks and oil and filter changes. Maintenance is limited to refilling the oil reservoir every six months. This drastically reduces maintenance time and cost.
Reduced Energy Consumption
For the given feedwater temperature, discharge temperature and flow rates, the steam consumption is drastically reduced compared to conventional systems. The power consumption varies by cubic root of the compressor’s rotational speed. In other words, the efficiency of the VC process increases as the production rate for a given evaporator decreases. A reduction of 25 percent in capacity yields a 50 percent reduction in power consumption, and a reduction of 50 percent yields an 80 percent reduction in power consumption.
It takes only two hours to change the MECO GII direct drive compressor. Being integrally mounted, it eliminates excessive ductwork and gasketing.
Vertical Tube Natural Circulation Evaporator
MECO evaporators are designed with a vertical tube configuration. This design promotes natural circulation, whereas the horizontal tube design uses forced circulation. Forced circulation requires spray nozzles and a circulating pump (additional energy) to force distribution over heating surfaces.
Vertical tubes ensure all evaporation surfaces have the same high wetting rate, whereas horizontal tubes could have instances of isolated dry surfaces, promoting corrosion.
The vertical evaporator constitutes straight annealed tubes in the Evaporator. No residual stress of surface finish problems commonly found in the U-tube of horizontal design are present.
Individual tubes can be replaced rather than replacing the entire bundle as done in the horizontal design.
Even with minimal pretreatment, MECO still routinely runs with a conductivity of about 0.25 microsiemens (µS). This is five times lower than the industry requirements and four times lower than other distillers. This is achieved by stripping the feedwater of gases, such as CO2 and ammonia, which contribute to conductivity, and oxygen, which contributes to corrosion, before they enter the system.
Fast Start-up and Hot Standby: Heating coil
The vertical tube VC has the benefit of utilizing a heating coil within the evaporator combined with an advanced control system for rapid heat-up and hot standby operation. MECO plants typically come online in 45 minutes from cold conditions and are instantaneously and reliably started from hot standby conditions.
The compressor operates at a noise level of only 72-85 dBA. This eliminates the need for an acoustic wall or personal hearing protection, thereby also eliminating the requirement of special OSHA compliance reporting.
MECO’s vertical design reduces footprint. It requires only an overhead space, which normally is not associated with any land cost, whereas the horizontal design requires substantial floor space. It does require additional maintenance space, however, should there be a need to remove the evaporator tubes for maintenance.