How Big Do I Need To Build A Pilot Plant: The Vexing Question of Scale

What size do I need to build my pilot plant? (By size or scale, I am generally refereeing to throughput not physical size.) A smaller unit is usually cheaper to design and build, less expensive to operate, and faster to start producing data. So, the most common answer is as small as possible but how small is small? There are many other considerations that may set a bottom limit on how small you can make the pilot plant and have it meet your goals.

The scale may be determined by the purpose. If, for example, one is developing a new polymer intended for the tire or automotive industry then you may need to produce large quantities of product for customer evaluation or testing. If 1,000 lbs is required for each potential vendor to evaluate and one thinks that they need to send out 12 sets for evaluation in a year then you need a pilot plant that can produce at least 12,000 lb/year. With maintenance and downtime factored in, that may mean a unit that can produce 18,000-24,000 lbs/yr.

The scale may be determined by a key piece of process equipment.  A specialty reactor, a key piece of separations equipment, a high temperature pump or mixer, etc. may have a minimum size in which it can be fabricated or purchased. Many vendors are uncomfortable producing a novel, smaller design and will only provide their smallest commercial unit. Others only feel comfortable scaling down a small amount. Still others will have problems with the construction that limit how small they can go. Whatever the reason the end result is a size larger than you would prefer. This may then set the feed rate and hence, the size of the unit.

The scale may be determined by a contractual agreement. You may need to produce 1 ton/day of a given product to retain or attract investors or qualify for or retain a government subsidy. The logic behind the amount may be impeccable or simply someone’s opinion but it usually is fixed and often not negotiable.

The scale may be determined by equipment limitations. This often sets the basis for the entire unit and has many potential facets. It might be the lowest feed or product rate you believe is achievable with the technology available (or with which you are comfortable). For example, your experience may indicate that the smallest liquid feed pump rate that can be maintained reliably is X or the lowest gas feed rate is Y. It may be how accurately you can measure a small value such as gas consumption or temperature rise. For example, let’s assume that you believe you can measure hydrogen gas flow to 10 SCCM. Let’s assume you are looking for a 10% decrease in consumption. This would require a hydrogen consumption of 10 sccm/10% or 100 SCCM. If hydrogen consumption is usually around 10% of the total feed (excess hydrogen is required) then the flow rate of hydrogen must be at least 100 sccm/10% or 1,000 SCCM.  Similar calculations for other situations can set produce a scale for the proposed unit.

Analytical limitations may set the scale of the pilot plant. If a key component can only be measured above a certain level, then the unit must be designed to produce that product above that level, often several times over for the desired accuracy. If the component is only a small percent of the product stream, this can result in a larger unit.

Scale up considerations often set the scale of the pilot plant. After careful analysis, it may be felt that a process can only be safely and reliably scaled up a given factor. (10X is the most commonly quoted generic figure but the actual value depends on the process, the history, the chemistry and the unit operations). This may be a larger value with a known and well understood process, or a relatively smaller value with an unknown process or a technology like mixing or solids handling that does not scale well. It is probably the most quoted reason for a particular scale.

Occasionally mass balance requirements sets the scale of a unit. Occasionally if the requirements of the research demand a high accuracy in the mass balance (e.g. to understand the economic consequences of a very expensive component or very expensive clean up stream) , this may lead to a – usually small – increase in the size of the pilot plant.

So you need to figure out what is the driving force for your pilot plant to determine the scale. While listed as individual causes above, the reality is that several of these considerations may come into play in deciding he final scale of the unit, although usually one is dominant.

The above assumes the most common situation, one where you are trying to keep the pilot plant as small as possible to save cost and time. Occasionally, however, the opposite is true. You would like to make the pilot plant larger - in effect a demonstration unit or at least something similar - but can’t. The reasons you can’t make it as large as you would like are also varied.

You may not be able to afford the costs of that large a unit. Whether it is the construction or the operating costs or the combination is immaterial and can vary.

You may not have room for the unit. This may be space for the base unit or, more commonly, space for the unit and all the required peripheral equipment and facilities (tankage, storage, etc.)

You may not have enough of a specific utility for the size you would prefer. Limitations in power, cooling water, steam, wastewater treatment, etc., can all be determining and the cost to provide more simply be unsupportable.

You may not be allowed to build the desired scale due to legal reasons. It may be too tall, or too close to a property line and violate a local building code. Your facility may not be zoned for the type of construction required. Recognized national standards or local codes may prohibit that large a facility in the only spaces you have available. Your building may not have the required fire rating and/or may not be properly constructed for the amount of flammable or toxic materials involved.

You may not have enough ventilation for the size hood or laboratory required. You may either not have enough money to increase existing systems or provide new systems or be limited and unable to increase their size nor add others due to limitations in chases, power, utilities, or numerous other reasons.

You may not be able to handle the hazardous waste generated. This can be a straightforward as being too expensive to landfill large quantities of waste or off spec materials or as complex as the need to design, permit, and install extensive air pollution control equipment, waste water separation and treatment equipment, incinerators, flares or similar systems.

Less frequently, there may be organizational reasons preventing you from the size you would otherwise prefer. This may be an insurance carrier reluctant to assume the larger risk, management’s discomfort with the potential hazards of a larger unit, or a local municipalities discomfort at the potential effects outside the facility in the event of an accident.

You may not have enough staff to operate a larger facility. This is acerbated if continuous operation is envisioned for longer periods (weeks to months). While it may be possible to handle this for a time by use of part time or contract personnel this is rarely an effective longer-term solution and is always very expensive. Even if you can afford the extra operating personnel for the duration envisioned, you may not have the infrastructure to properly support them. Lack of trained supervision, inadequate locker rooms, cafeterias, security, and a host of other areas can wreak havoc with an otherwise well-staffed operation. (This is, by the way, one of the most often overlooked problems with larger units.)

You may not be able to get enough catalyst or feedstock for a larger unit. Once commercialized, there may be many ways to deal with this problem as you can enter into long term commitments, but often there are relatively few on a larger pilot plant scale. You may be able to make the missing component yourself or arrange for a toll manufacturer to produce what you require, but again it will be expensive.

So how big to make the pilot plant is not always the easiest question to answer. A careful evaluation of all these issues is necessary to try and develop the right answer as part of defining the pilot plant program.