Process design

In chemical engineeringprocess design is the choice and sequencing of units for desired physical and/or chemical transformation of materials. Process design is central to chemical engineering, and it can be considered to be the summit of that field, bringing together all of the field’s components.

Process design can be the design of new facilities or it can be the modification or expansion of existing facilities. The design starts at a conceptual level and ultimately ends in the form of fabrication and construction plans.

Process design is distinct from equipment design, which is closer in spirit to the design of unit operations. Processes often include many unit operations.

Documentation

Process design documents serve to define the design and they ensure that the design components fit together. They are useful in communicating ideas and plans to other engineers involved with the design, to external regulatory agencies, to equipment vendors and to construction contractors.

In order of increasing detail, process design documents include:

  • Block flow diagrams (BFD): Very simple diagrams composed of rectangles and lines indicating major material or energy flows.
  • Process flow diagrams (PFD): Typically more complex diagrams of major unit operations as well as flow lines. They usually include a material balance, and sometimes an energy balance, showing typical or design flowrates, stream compositions, and stream and equipment pressures and temperatures.
  • Piping and instrumentation diagrams (P&ID): Diagrams showing each and every pipeline with piping class (carbon steel or stainless steel) and pipe size (diameter). They also show valving along with instrument locations and process control schemes.
  • Specifications: Written design requirements of all major equipment items.

Process designers typically write operating manuals on how to start-up, operate and shut-down the process. They often also develop accident plans and projections of process operation on the environment.

Documents are maintained after construction of the process facility for the operating personnel to refer to. The documents also are useful when modifications to the facility are planned.

A primary method of developing the process documents is process flowsheeting.

Design considerations

There are several considerations that need to be made when designing any chemical process unit. Design conceptualization and considerations can begin once product purities, yields, and throughput rates are all defined.

Objectives that a design may strive to include:

Constraints include:

  • Capital cost: the amount of budget or investment to construct end to end process.
  • Available space: the area of the land to build the plant.
  • Safety concerns: consideration towards risk analysis on industrial accidents or hazardous chemicals.
  • Environmental impact and projected effluents and emissions
  • Waste production/recycling: manage waste produced as side product of the process for not to harm the surroundings.
  • Operating and maintenance costs: represent the variable cost of the operational of the plant.

Other factors that designers may include are:

  • Reliability
  • Redundancy
  • Flexibility
  • Anticipated variability in feed stock and allowable variability in product.

Sources of design information

Designers usually do not start from scratch, especially for complex projects. Often the engineers have pilot plant data available or data from full-scale operating facilities. Other sources of information include proprietary design criteria provided by process licensors, published scientific data, laboratory experiments, and suppliers of feedstocks and utilities.

Design process

Design starts with process synthesis – the choice of technology and combinations of industrial units to achieve goals. More detailed design proceeds as other engineers and stakeholders sign off on each stage: conceptual to detailed design.

Simulation software is often used by design engineers. Simulations can identify weaknesses in designs and allow engineers to choose better alternatives. However, engineers still rely on heuristics, intuition, and experience when designing a process. Human creativity is an element in complex designs.

Yield (chemistry)

In chemistryyield, also referred to as reaction yield, is the amount of product obtained in a chemical reaction.[1] The absolute yield can be given as the weight in grams or in moles (molar yield). The percentage yield (or fractional yield or relative yield), which serves to measure the effectiveness of a synthetic procedure, is calculated by dividing the amount of the obtained desired product by the theoretical yield (the unit of measure for both must be the same):{\displaystyle {\mbox{percent yield}}={\frac {\mbox{actual yield}}{\mbox{theoretical yield}}}\times 100}

{\mbox{percent yield}}={\frac  {{\mbox{actual yield}}}{{\mbox{theoretical yield}}}}\times 100

The theoretical yield is the amount predicted by a stoichiometric calculation based on the number of moles of all reactants present. This calculation assumes that only one reaction occurs and that the limiting reactant reacts completely. However, the actual yield is always smaller (the percent yield is less than 100%), often very much so, for several reasons:

  • Many reactions are incomplete and the reactants are not completely converted to products. If a reverse reaction occurs, the final state contains both reactants and products in a state of chemical equilibrium.
  • Two or more reactions may occur simultaneously, so that some reactant is converted to undesired side products.
  • Losses occur in the separation and purification of the desired product from the reaction mixture.
  • Impurities are present in the starting material which do not react to give desired product

The ideal or theoretical yield of a chemical reaction would be 100%, an ideal that is never reached. According to Vogel’s Textbook of Practical Organic Chemistry, yields close to 100% are called quantitative, yields above 90% are called excellent, yields above 80% are very good, yields above 70% are good, yields above 50% are fair, and yields below 40% are called poor. These names are arbitrary and not universally accepted, and depending on the nature of the reaction in question, these expectations may be unrealistically high. Yields may appear to be 100% or above when products are impure, as the measured weight of the product will include the weight of any impurities.

Purification steps always lower the yield, through losses incurred during the transfer of material between reaction vessels and purification apparatus or imperfect separation of the product from impurities, which may necessitate the discarding of fractions deemed insufficiently pure. The yield of the product measured after purification (typically to >95% spectroscopic purity, or to sufficient purity to pass combustion analysis) is called the isolated yield of the reaction. Yields can also be calculated by measuring the amount of product formed (typically in the crude, unpurified reaction mixture) relative to a known amount of an added internal standard, using techniques like gas / liquid chromatography, or NMR spectroscopy. A yield determined using this approach is known as an internal standard yield. Yields are typically obtained in this manner to accurately determine the quantity of product produced by a reaction, irrespective of potential isolation problems. Additionally, they can be useful when isolation of the product is challenging or tedious, or when the rapid determination of an approximate yield is desired. Unless otherwise indicated, yields reported in the synthetic organic and inorganic chemistry literature refer to isolated yields, which better reflect the amount of pure product one is likely to obtain under the reported conditions, upon repeating the experimental procedure.

Organic chemist Tomas Hudlicky and coworkers have noted the phenomenon of « yield inflation », in which reported yields in the chemistry literature has gradually crept upward in recent decades, a phenomenon attributed to careless measurement of yield on reactions conducted on small scale, wishful thinking and a desire to report higher numbers for publication purposes, or scientific fraud. After performing careful control experiments, they note that each physical manipulation (including extraction/washing, drying over desiccant, filtration, and column chromatography) results in a loss of yield of about 2%. Thus, isolated yields measured after standard aqueous workup and chromatographic purification should seldom exceed 94%.

When more than one reactant participates in a reaction, the yield is usually calculated based on the amount of the limiting reactant, whose amount is lessthan stoichiometrically equivalent (or just equivalent) to the amounts of all other reactants present. Other reagents present in amounts greater than required to react with all the limiting reagent present are considered excess. As a result, the yield should not be automatically taken as a measure for reaction efficiency.

Reliability. Ingénierie de fiabilité

L’ingénierie de fiabilité est un domaine de l’ingénierie, qui traite de l’étude, de l’évaluation et du Product Lifecycle Management de la fiabilité : l’habilité d’un système ou d’un composant à remplir ses fonctions exigées dans des conditions déterminées pour une période de temps déterminé. L’ingénierie de fiabilité est une sous-discipline au sein de l’ingénierie des systèmes. La fiabilité est souvent mesurée en probabilité de défaillance, fréquence de défaillance, ou en termes de disponibilité, une probabilité dérivée de la fiabilité et de la maintenabilité. La maintenabilité et la maintenance sont souvent des parts importantes de l’ingénierie de fiabilité.

Redundancy

In engineeringredundancy is the duplication of critical components or functions of a system with the intention of increasing reliability of the system, usually in the form of a backup or fail-safe, or to improve actual system performance, such as in the case of GNSS receivers, or multi-threaded computer processing.

Pilot plant

pilot plant is a pre-commercial production system that employs new production technology and/or produces small volumes of new technology-based products, mainly for the purpose of learning about the new technology. The knowledge obtained is then used for design of full-scale production systems and commercial products, as well as for identification of further research objectives and support of investment decisions. Other (non-technical) purposes include gaining public support for new technologies and questioning government regulations.[1] Pilot plant is a relative term in the sense that pilot plants are typically smaller than full-scale production plants, but are built in a range of sizes. Also, as pilot plants are intended for learning, they typically are more flexible, possibly at the expense of economy. Some pilot plants are built in laboratories using stock lab equipment, while others require substantial engineering efforts, cost millions of dollars, and are custom-assembled and fabricated from process equipment, instrumentation and piping. They can also be used to train personnel for a full-scale plant. Pilot plants tend to be smaller compared to demonstration plants.

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