MED vs VCD: SELECTING A DISTILLATION TECHNOLOGY FOR WFI

Assessment of WFI equipment system costs

Multiple Effect & Vapor Compression. Criteria for selecting a distillation technology

Multiple effect distillation (MED) and vapor compression distillation (VCD) both produce compendial WFI, but their cost and performance depend strongly on site conditions. There is no universal answer: the choice follows from the demand profile, the available utilities, the feed water quality and the operating strategy of the plant. This page lists the variables that drive the decision and the way a total cost comparison is structured. The process is further enhanced by considering the key features of both the technologies to help gauge which system may be perfectly compatible with site operation concepts and operational tasks, and then fine-tune the selection process by comparing the desired features with what is available from the manufacturer.

An overview of design features should be taken into account when considering the following issues:

1. WFI Equipment system costs. Consider the required volumes of clean utilities needed in the facility.

Will primary water use be WFI or Purified Water (PW)? If there is only a small need for PW, it may be more effective to couple a VCD unit with minimal pretreatment and use the WFI for both applications. Is there a small pure steam requirement? The capability of a multi-effect still to be tapped for pure steam could be more advantageous than the additional investment of a dedicated pure steam generator.

2. WFI Equipment evaluation. What utilities are already available?

If steam is not readily available, electric-driven models are available with both technologies, but only smaller volumes (about 285 L/h (75 US gph) typically for Multi Effect design. If it is necessary to minimize the use of cooling water could also drive the choice in favour of a Vapor Compression Distiller. A PW system already on site could feed a Multiple Effect still, thus taking advantage of the lower acquisition cost as compared to Vapor Compression.

3. WFI Equipment evaluation. What maintenance resources are available?

The Multiple Effect Distiller requires little maintenance due to the minimal moving parts, but as part of a good preventive maintenance schedule it may require annual descaling. The Vapor Compression still will possibly require a more frequent preventive maintenance schedule due to the moving parts, but the newer compressor advances may help minimize the time required.

4. WFI Equipment placement. Are there any space constraints?

Multiple Effect stills in general take up less floor space than a vapor compression still. However, gravity feed to a WFI holding tank may require additional headroom than is available. Of the two Multi Effect technologies, falling film type systems tend to be more compact, but taller; while the natural circulation require more floor space due to the external evaporators. In some models, a raised condenser may alleviate full height demands.

WFI equipment cost evaluation: a Vapor Compression system with customized water pretreatment on the same frame.

Practical cost evaluation in WFI distillers

A simplified and incomplete equipment evaluation for WFI production is based on capital costs. A more helpful and comprehensive approach would include a custom water pretreatment system for a complete evaluation of WFI production plant, with some considerations applied.

Summary of a lifecycle evaluation cost:

• Capital cost of the still, of the water pretreatment it requires, of utility connections, installation and qualification, and of the space it occupies.

• Energy cost: plant steam and/or electricity, according to the technology and the number of effects (in MED the specific steam consumption falls as the number of columns increases, at the price of a higher capital cost).

• Water cost: cooling water and feed water, including the pretreatment consumables.

• Maintenance and availability: preventive maintenance, descaling, spare parts, expected downtime.

• Operation: start-up time, operator effort, automation level.

Quality and validation considerations

• Both technologies rely on the liquid-to-vapor phase change as a barrier, supported by an efficient separation of entrained droplets (labyrinth separators in the columns of a MED, demisters or baffles in a VCD).

• Both systems are validated with endotoxin challenge tests.

• Feed water chemistry, operating temperature and materials of construction influence scaling and corrosion phenomena, including rouging.

• The temperature of the distillate must be compatible with the storage and distribution strategy (hot loop or cold storage with cooling).

Operational considerations

• Redundancy: for continuous operation, plants often install several units in parallel (N+1) instead of a single large still.

• Start-up and shutdown: the start-up times in the table show that both reach production quickly; the frequency of stops and restarts is part of the evaluation.

• Sanitization: the choice interacts with the temperature maintained in the storage and distribution loop.

Data needed for a sizing evaluation

· Peak and average demand and operating hours

· Required WFI temperature and delivery pressure

· Pure Steam demand

· Available utilities and their unit cost

· Feed water analysis

· Redundancy requirements.

Site variables and their influence

*Variable*	*Tends to favor MED*	*Tends to favor VCD*
Plant steam	Available at suitable pressure and cost	Scarce, costly or unavailable
Electricity	Expensive relative to steam	Available at moderate cost
Cooling water	Available and inexpensive	Scarce or costly, or to be minimized
Feed water quality	Pretreatment already in place (e.g. double-stage RO)	Limited pretreatment; higher tolerance on feed water
Demand profile	Steady, continuous demand	Variable demand, with output modulation from 0 to maximum
WFI temperature	Hot storage (85–99 °C)	Hot or cold distillate; intermediate temperatures
WFI delivery pressure	Atmospheric outlet acceptable	Outlet at 1–1.5 bar without additional pump
Pure steam demand	Small pure steam demand covered by the first column	Pure steam produced by dedicated generator
Space	Limited floor area (columns are compact in plan)	Limited height
Maintenance	Few moving parts, periodic descaling	In-house skills for rotating equipment (compressor)

The table indicates tendencies, not rules: more than one variable usually applies at the same time, and the balance shifts with the local price of energy and water.

MED vs VCD design characteristics

MULTIPLE EFFECT DESIGN

This still technology is easily recognized by its multiple columns (i.e. effects) which re-use heat energy through the additional effects. First column is heated by an external source such as plant steam (electric heating is available from some suppliers in smaller models). Plant steam is replaced in remaining effects by the pure steam produced from the initial evaporation process. This pure steam is utilized as a both a heating source for additional columns as well as a preheating source for incoming feedwater. In turn, the feedwater helps condense the pure steam so that the remaining pure steam produced during the process requires minimal cooling water in the final condenser.

VAPOR COMPRESSION DESIGN

Vapor Compression stills have been a dominant method of WFI production for LVPs (Large Volume Parenterals) over the years due to the cost efficiencies provided with the higher capacity WFI production. However, recent improvements with Vapor Compression technology as well as rethinking of WFI usage make it a compelling choice for select smaller-scale operations, allowing them to break under about 2,270 L/h (600 US gph) per hour barrier. The units operate at lower temperatures and larger tubing, allowing them to bypass the more exacting pretreatment requirements of a Multiple Effect still. Thus, the Vapor Compression unit can possibly be the more cost-effective choice when evaluated as a full system. Likewise, without a need for cooling water, utility consumption is further enhanced.

ISPE REFERENCE

ISPE. Baseline Guide, Vol. 4: Water and Steam Systems (3rd ed., 2019).
ISPE. Good Practice Guide: Membrane-Based Water for Injection Systems.

Multiple Effect vs Vapor compression technology

*Parameters*	*MULTIPLE EFFECT DISTILLER*	*VAPOR COMPRESSION DISTILLER*
*Parameters*	*MULTIPLE EFFECT DISTILLER*	*VAPOR COMPRESSION DISTILLER*
OUTPUT FLEXIBILITY	Reduced output modulation	Capacity ranging from 0 to max. cap. of the still
TEMPERATURE FLEXIBILITY	WFI output 85÷99°C	WFI output from infeed water T + 10°C till 99°C
HEATING MEDIA FLEXIBILITY	Industrial steam or electricity	Industrial Steam and/or electricity
COOLING WATER	High consumption depending on quantity of columns	No cooling water required
FEED WATER	SiO₂ < 1 ppm, amines free resins (in case of DI), double stage RO preferred	SiO₂ <30 ppm, Single stage RO or even softened water acceptable
FEED WATER INPUT	Must be higher than primary steam pressure	< 1 bar
WFI OUTPUT	Atmospheric pressure	1 / 1.5 bars
WFI QUALITY	0.2÷0.5 µS/cm with FW <1.2microS	0.15÷0.4 µS/cm with FW <100microS
PREVALIDATION (endotoxin challenge)	Yes	Yes
HEAVY METALS	Free	Free + elimination of chlorine solvents
MOVING PARTS	Feed pump	Compressor, Recirculation pump
PURE STEAM FROM 1ST COL.	Possible	Possible
STRESS CORROSION	Higher tendency under comparable conditions*	Lower tendency under comparable conditions*
CLEANABILITY	More tough than VCD	More easy than MED
START UP	SCADA (15 min for steam heating)	SCADA (15÷40 min, for steam heating)

*Based on Bram-Cor field experience.

COMPARISON TABLE © Bram-Cor 2026 – Typical values for Bram-Cor SMPT and STMC ranges; actual values depend on model and configuration.