Mass and Energy Balance

A mass and energy balance is the guiding document for any chemical effort.  It accounts for all the various inputs and outputs of every unit.  Values can easily be assigned to track losses and tie the financial performance of the system to the process control data.  The mass and energy balance is a tool by which to guide the engineers and operators so that the most financial productivity can be extracted from the system.  All lab scale, pilot scale and commercial scale plant units, systems and equipment testing, evaluation, and operation should be based on constituent level mass and energy balances.

These balances shall be compiled from the mass based (not volume disguised as mass) measurements of composition from the analytical lab combined with the mass based flow measurements from the on line systems or by collection and weighing of the batch run materials.  Temperature measurements can be used to determine and resolve the energy balances for all systems.  In cases where the direct mass flow measurement is impractical equivalent volumetric flow measurements combined with a density measurement will be used to determine the calculated mass flow.

The composition measurements shall focus on the needed constituents of concern.  The analytical test methods used in the compilation of the mass balances shall be validated using accepted statistical methods.  The error in the measurement shall be displayed as part of the reported values in any mass balance.  For example:  the total flow shall be reported as 1.0 +/- 0.1 lb/hr.  Another example at the constituent level would be the total Ca concentration in that flow shall be reported as 250 +/- 10 mg/kg.  The mass flow of the Ca in the stream would be calculated using the combination of the analytical composition measurement and the total mass flow measurement.  The correct methods should be used to propagate the error through the calculation.

All commercial runs, test plans and experimental orders should include mass balance sampling plans and energy balance sample plans.  Key learnings reports should issue the mass balance and energy balance according to the corresponding sample plan as the basis for the work.

Process Development

Process development is a complex chemical engineering work flow that requires rigor, discipline and organization.  The product of a development effort is to the technical information needed to commercialize a product.  The Process Development effort should run first prior to and then continue in parallel to the Front End Loading work flow to deliver a Basis of Design (BOD) document that includes reports, spreadsheets, and drawings that cover the many aspects of the system development effort justifying the various anticipated performance parameters for each unit operation conversion.  Many times the Process Development effort is documented in a Process Work Up document.  This document becomes a subset of the information that makes up the BOD.  The more refined and simple that information generated is presented the greater degree of continued success.   Simplification and communication enables data based decision making, well understood mass and energy balances, and clarity in the results from the validated test methods.

Entrepreneurial Engineering

Engineering in an entrepreneurial environment is very different than traditional engineering.  Traditional engineers are conservative.  The majority of engineers spend their career focused upon presenting a solution to a problem with defined parameters.  Work flow processes are systematic.  Work is checked and double checked and then sent out for the next step in the work flow process.  There is creativity in this process, but usually at the expense of many hours of presentations, reviews, and surviving the glass is half empty crowd.

Entrepreneurs are fluid.  They move with the opportunity presented in the moment.  They pivot on the information that emerges and they have to be able to scrap what might have been hours, days or months of painstaking work to evolve with the fast moving changing situation.  The paradox is remaining creative while focusing to deliver the goods without a constant churn of chaos.  Entrepreneurial engineers create clarity from chaos and they need to deliver on blistering fast pace.

A traditional engineering effort is the result of a prescribed method and work flow process.  Engineering products take many forms.  Traditionally the effort follows a progression through development, design, construction, and commissioning.  Each of these having phases and tasks that combine to complete the job.

Entrepreneurial Engineering is all about attitude.  Innovation is messy.  The normal way of doing things must be challenged.  Entrepreneurs must find possibilities.  Most times without having all the information.  They must pivot quickly on new information, and respond to changing needs.  An engineer in an entrepreneurial situation must be able to release the clutch on his or her work modifying it and letting it evolve with the emerging business opportunity. 

Both models are right, depending upon the situation.  Innovation fuels entrepreneurial efforts.  Traditional rigor insures safety.  Bothe approaches must coexist.  In start-ups, entrepreneurial,  and traditional efforts a root cause of stress on both sides is the mis-match of the type of people who are placed in these roles.  A traditional engineering project of building a bridge or a power plant requires methodical and traditional methods.  The driving to market of a new game changing process or product requires entrepreneurial judgment and attitude.

Process Engineering

Process engineering takes the process development effort and carries it through to the commercial scale.  The needs and parameters defined in the development effort are blended with the commercial resources available to embody the functional system.  The process engineer begins by insuring that all development effort completed on the bench and in the pilot follows rigorous scale up principles.  The process engineering effort applies the development learnings to tailor the system unit operations for the performance required to hit the quality and cost parameters of the enterprise.  Safety is the utmost concern of any chemical production system.  The process engineers ensure that the proper analysis and mitigation efforts are employed to insure the safest possible installation and operating system.

For more on the details of a process engineering effort:

Chemical Engineering for Investors

Any enterprise has a set of financial investors who are usually limited in their detailed understanding of the mass and energy balance and the performance of the equipment which drives yield and thus the “cash balance” of the system.  One of the more overlooked, but critically important aspect of any large chemical plant is the ability to tie the technical parameters to the financial parameters in a common language that has the understanding of the business colleagues, but also respects the complexity of the technical complexity from the technical colleagues.  Ususally, it’s the process engineering team that takes on this responsibility and works with the project engineering colleagues to present the information in the usual CapEx and OpEx formats.  Preprocess is hired many times to help coach the financial investors in their understanding of the implications of technical direction on the investment. The earlier in the effort that common language is found to balance these discussions, the higher the probability that the enterprise will meet everyone’s expectations as the product is brought to market.

For more on the details of the chemical engineering aspects important for investors:
Chemical Engineering Basics for Investors

Procurement and Contracts

Many projects require an iterative development for certain complex pieces of equipment that make up the major unit operations for the system.  At some point an engineered specification must be developed to enough detail that the various vendors bidding the job can understand the request for proposal, but also have enough flexibility to understand that the effort is iterative depending upon certain technology applications that each may be able to bring to the table to enable a different cost structure or better performance of the overall system.  In these efforts, it is advantageous to have early scope sessions with prospective vendors so that information is clear and the best possible solution can be developed.  PreProcess is brought in to may early chemical start-ups to act as the owner’s chief engineer in these development efforts.  The ability to guide the discussions and evaluate he development data presented on the various options for equipment is critical to an early stage project.

For more on equipment procurement and contract development:
Equipment Procurement and Contract Development

Agitators, Tanks, and Reactors

Agitators and agitated tanks are one of the basic functional unit operations of chemical processes.  They make up the building blocks of all storage, reactor and separation systems.  Here are the common considerations in their application.

The standard model is a square batch where the height of the liquid matches the diameter of the tank, of T=Z.  Usually included are a center top entry agitator, baffles, and a bottom geometry that depends upon the function of the tank.  If you are trying to suspend a solid or emulsify immicible liquids, a dish bottom tank is used to ensure proper mixing of the phases.  If the resulting emulsion is transparent this is usually a visual indication that it is stable, if it is translucent it is not stable.  If you are trying to separate the liquid phases, settle solids, or precipitate solids that form as a result of a liquid phase reaction, a cone bottom tank is used.    Flat bottoms tanks are for clear liquid storage. 

Usually square batch tanks are 12 foot diameter by 12 foot liquid fill height in the tank.  Many plants use the term “innage” to describe the liquid volume in the tank.  This is the correlation between the height of the liquid from the bottom of the tank and its associated volume.  The opposite approach many other plants use is the “outage” this is equivalent to the freeboard which is the difference in height from the overflow of the tank to the top of the liquid in the tank.  In some applications freeboard is critical for disengagement of foams, gases, and other tank separation products. 

A 12 ft by 12 ft tank is easily scaled down using a 12 inch by 12 inch laboratory scale equivalent.   The other critical parameters in scale down include holding the geometric ratios between impeller diameter, impeller height off the bottom, and baffle width constant.  Other scaling factors include agitator shaft rotational speed, number of baffles, pitch of the impellor blade, and number of impellors.  These geometric ratios for the tank, coupled with fluid characterization ensures success in scale up. 

From a practical matter the largest shop fabricated tanks are usually 30,000 to 35,000 gallons.  This is equivalent to a 14 foot diameter by 33 foot long tank.  This is because of the limitation to shop fabricate and ship to site.  Shipping is usually limited by the 15 foot minimum interstate highway bridge clearance.  The 12 foot by 12 foot basis has evolved over time to include nozzle extensions, lugs, lifting eyes, jackets, and external pipe supports to maintain the 14 foot transportation envelope.  

Tanks can be classified for different purposes.  For instance, reactors and precipitators should be square batch.  Simple mix tanks or blending tanks can be taller than the square batch.  These are the cases where you might use multiple impellors. Multiple impellors are modeled using an upper square batch and a lower square batch model with usually a 20% overlap.  This helps set the dimensions for both the upper and lower mixing zones.  

For slurries a special type of second impellor is a tickler.  A tickler is a smaller diameter impellor that sits about one pipe diameter above the bottom outlet discharge nozzle.  This impellor is usually 20-25% of the diameter of the main impellor.  It is to keep the slurry moving through the discharge without creating a plugged cake. 

Usually in atmospheric tanks you have a half a diameter freeboard above the liquid level to the top of the tank. 

For more on agitators, agitated tanks and reactors:
Agitators and Agitated Tanks


Instruments are the eyes and ears of the process system.  Without good measurements, operations is blind.   Temperature, pressure, level and flow are common process instrumentation that have many measurement points in the field.  pH is the most common on line analytical measurement.  Sample stations are usually implemented to take composition samples to an off line lab for analysis.  In recent years, many different approaches have been applied to bring lab measurements to the process system in the units.

For more on instruments: