Process Engineering

Michael Kleiber

• Publisher: De Gruyter
• Year: 2016
• ISBN:
  9783110383102
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Chemical & Biochemical Engineering
• Index: Technology & Engineering

1Engineering projects

An engineering project is a huge and complex task for usually several hundred people. Coming from the university and just having finished one’s studies, one has usually no clue on what is going on beyond the own desk. In fact, the construction of a chemical plant is often compared to the erection of the pyramids in ancient Egypt. While the weight of a chemical plant is much lower, its complexity is by far greater, and the project can usually be completed in approx. three years instead of twenty. The target for a beginner must be to become a increasingly larger cog in the machine. First, an overview on the particular phases and activities must be obtained.

1.1Process engineering activities

A plant always belongs to somebody. His target is to quickly earn money by producing the substance the plant is designed for. At the beginning, a feasibility study has to be done. Amarket analysis is performed, which hopefully shows that it is worth starting a more detailed project. For a new process, it has to be checked whether it is possible to overcome the technical difficulties. The legal situation with patents and licenses has to be clarified, and possible locations for the plant are compared, whereby it is often necessary to consider different energy prices or transport costs for raw materials and products. A realistic production capacity and an impression of investment (CAPEX) and operation costs (OPEX) must be available before starting a project (Chapter 1.3). For the production capacity, it must be taken into account that no plant is in operation all the time; usually, 8000 h per year are scheduled, giving approx. 90% availability. A corresponding overcapacity must be provided in the design.

It is important to know that an engineering project is not a sequential process, where e.g. first the reactors are planned and finished, then the product purification, and so on. This would actually be impossible, because due to recycle streams in the process a complete engineering design of a single part of the plant could never be achieved. Instead, all parts of the plant are worked out simultaneously, with increasing accuracy and degree of detailing. The advantage is that possible bottlenecks and difficulties are detected as early as possible, the interconnections are identified early, and an appropriate number of project participants can be assigned to work on the various parts of the plant. Certainly, this is not the way we are used to in everyday life, and there is often doubt as to whether it makes sense to perform a design of a piece of equipment while it is clear that the input streams are just preliminary and will change several times during the project. Nevertheless, as mentioned, it is most important to get an overview on the process as soon as possible. And with today’s tools, the design from the previous phase is usually an ideal starting point when the preconditions have been subject of change.

The process engineer should know what the follow-up activities of his calculations are. The first phase in a project is the conceptual design, where the first mass and energy balances are prepared, often based on lab trials and estimations.

The mass and energy balance is a key issue for all the following activities up to the phase of detail engineering. A change in the mass balance has often a major impact on all other participants of the project, so it is desirable to make it as exact as possible, and to update it as soon as it makes sense. There is a certain misunderstanding as to what a mass and energy balance really is. The term “process simulation” is very common, and is also used here, but hardly applies. In fact, what the process does in the steady state for a given set of inlet conditions is calculated, i.e. the streams and the operating conditions of the particular pieces of equipment. Sometimes, the purpose is in fact to find out how the plant or the equipment behaves, at least how it reacts, and what the sensitivities are. However, in most cases, its purpose is to generate the data for the design of the equipment, applying conservative cases concerning process conditions or impurities. The exact process conditions that would enable the process engineer to really “simulate” the plant are usually not known, at least not in the Conceptual Design phase.

Despite these often occurring misunderstandings, “process simulation” is nowadays well acknowledged as a useful tool which requires a well-trained process engineer who has a profound knowledge of the process itself, its thermodynamics (Chapter 2), the various pieces of equipment and their peculiarities, and the simulation experience, in order to achieve convergence in the simulation flowsheet, which often turns out to be complex. Nowadays, some well-established commercial (ASPEN, HYSYS, ChemCAD, PRO/II, ProSim) and inhouse process simulators (Chemasim at BASF, VTPlan at Bayer) are available, performing calculations that would have been considered to be absolutely impossible 30 years ago. The genuine process simulation showing the actual plant behavior with respect to the design of the equipment, the startup-behavior, and the process control is called dynamic simulation. Nowadays, its application becomes more and more popular, and conventional process simulation can be used as a starting point for the dynamic version.

Sometimes, single process steps remain unknown and are represented in the mass balance by simple split blocks. At least, there must be a concept of how to overcome this lack of knowledge and what the effort might be. At the beginning of the basic engineering these points should be completely clarified, and a full mass and energy balance must be available. How this is done is the subject of Chapters 2 and 3. It is desirable that pilot plant activities take place to confirm the mass balance and to make sure of the influence of the recycle streams. The main purpose of such an activity is to see whether all components are regarded and whether none of them accumulates in the process.

The particular pieces of equipment are preliminarily designed according to the current knowledge so that it becomes clear what the critical pieces of equipment are, either because of their size or because of possible delivery limitations. As well, it must be considered whether the plant can be operated at reduced or increased capacity, which might be necessary for a certain period of time. Useful tools are the process flow diagrams (PFD), where the whole process is visualized, including the main control loops (Figure 1.1). A PFD is a document to understand the process, operation data for the important streams and blocks are usually included. The counterpart of the PFD is process description, which describes the PFD inwritten form. It should not be excessively detailed, as its main purpose is to enable the reader to understand the essentials of the process. At the end of the conceptual design phase, equipment and operation costs and hence the feasibility and their basis are better defined, often with respect to a possible location.

Fig.1.1: Example for the detailing in a PFD.

In a so-called HAZID (HAZard IDentification) the main issues concerning the safety of the process are first discussed and listed, often with first recommendations. At a later stage, the so-called HAZOP will take place, where all relevant safety issues are discussed (Chapter 14.1). Finally, lists of utilities, raw products, auxiliary substances (e.g. catalysts) and emissions (exhaust air, waste water, solid and organic wastes) are issued. In the conceptual design phase, the design of the equipment can be done in a preliminary way using rules of thumb. A first optimization of the process should be performed. In process development, optimization is rarely a mathematical problem, where an objective function is defined and somehow minimized. Process simulator programs offer such a function; however, the author’s experience is that in most cases process optimization cannot be translated into an objective function, as many soft factors have to be regarded (e.g. danger of fouling, increasing complexity, material issues, ease of startup etc.). Equipment costs can be estimated by a process engineer as long as only dimension changes are involved; however, it takes a specialist if the type or the material of the equipment changes. The number of team members in the conceptual design phase is comparatively low, as the process engineering tasks are usually complicated but of a limited extent. The complexity of the process development is encountered by an iterative procedure, where many options are tested to achieve a stepwise progress towards an improved process. There is no clear workflow plan; instead, the creativity of the project members is decisive [1]. Nevertheless, it is desirable to compose a comprehensible documentation to save the process knowledge which was gained during the assessment of the various options. As there is no special structure available for this purpose, the documentation is done with a final report.

A successful and systematic way of optimizing a plant is the so-called value engineering procedur...

Table of contents

• Cover
• Also of Interest
• Title Page
• Copyright
• Dedication
• Preface
• Contents
• 1 Engineering projects
• 2 Thermodynamic models in process simulation
• 3 Working on a process
• 4 Heat exchangers
• 5 Distillation and absorption
• 6 Two liquid phases
• 7 Alternative separation processes
• 8 Fluid flow engines
• 9 Vessels and separators
• 10 Chemical reactions
• 11 Mechanical strength and material choice
• 12 Piping and measurement
• 13 Utilities and waste streams
• 14 Process safety
• Glossary
• List of Symbols
• Bibliography
• A Some numbers to remember
• B Pressure drop coefficients
• Index
• Fußnoten