Process Design - Appunti, domande e regole pratiche

PROCESS DESIGN

SUMMARY

TECHNICAL DOCUMENTATION............................................................................................ 2

REPRESENTING a PROCESS......................................................................................... 3

PRELIMINARY INFORMATION.............................................................................................. 9

LEVEL 1: CONTINUOUS vs BATCH PROCESS..................................................................11

LEVEL 2: INPUT OUTPUT STRUCTURE.............................................................................13

LEVEL 3: RECYCLE STRUCTURE...................................................................................... 20

LEVEL 4: SEPARATION SYSTEM........................................................................................29

VAPOR PHASE RECOVERY SYSTEM........................................................................... 30

LIQUID PHASE RECOVERY SYSTEM........................................................................... 31

LEVEL 5: ENERGY INTEGRATION......................................................................................36

PROBLEM TABLE APPROACH...................................................................................... 39

UTILITY PLACEMENT..................................................................................................... 45

POWER SYSTEMS..........................................................................................................46

DISTILLATION PLACEMENT.......................................................................................... 46

REACTOR PLACEMENT.................................................................................................47

EVAPORATORS...............................................................................................................47

ECONOMIC ANALYSIS.........................................................................................................50

CAPITAL INVESTMENT.................................................................................................. 50

COST of MANUFACTURING........................................................................................... 53

REVENUES......................................................................................................................56

PROFITABILITY............................................................................................................... 56

QUESTIONS.......................................................................................................................... 61

1

HEURISTICS SUMMARY TABLE

Category / Level Heuristic

Operating pressures between 1 and 10 bar do not significantly

Operating Conditions increase capital investment.

Temperatures above 400°C require replacing common construction

Operating Conditions materials with more costly ones.

Optimal utility range for standard heating (steam) and cooling (water)

Operating Conditions is 40°C to 260°C.

If plant capacity > 5000 ton/year, continuous processes are usually

Level 1: Process Selection the most convenient choice.

If plant capacity < 500 ton/year, batch processes are typically

Level 1: Process Selection preferred.

Always attempt to recover at least 99% of all valuable chemical

Level 2: Input-Output species.

A byproduct is considered valuable if its net benefit exceeds 200,000

Level 2: Input-Output €/year.

Use a Recycle & Purge (R&P) stream if both reactant and impurity are

Level 2: Gas Phase light (normal boiling point Tb​<−48∘C).

Running a reactor adiabatically is the simplest and cheapest design

Level 3: Reactor Design option.

Adiabatic operation is feasible if the temperature rise is within 10-15%

Level 3: Reactor Design of the inlet temperature.

Maintain 1<L/D<2 for stirred tanks; for tubular reactors, L/D>6 is

Level 3: Dimensions standard.

Approximate pressure drops in the recycle loop as 10-20% of the

Level 3: Compression target delivery pressure.

Level 4: Separation Phase split (flash) separations are the most economical methods.

Level 4: Distillation Distillation is usually cost-effective only if relative volatility αij​>1.1.

Total column height should be Htot​<50 m with an aspect ratio of

Level 4: Column Limits 2<H ​/D<30.

tot

Remove corrosive compounds, reactive monomers, and the largest

Level 4: Sequencing flowrates as early as possible.

Optimal ΔTmin​

is generally 10-20°C for liquid streams and 30-40°C

Level 5: Energy Integration for gas streams.

Level 5: Pinch Rules Above the pinch: CP ​≤CP ​; Below the pinch: CP ​≥CP ​.

Hot Cold Hot Cold

Economics Annualized investment cost is roughly estimated as Fixed Capital / 3.

Costs scale with size according to the 0.6 rule (or 0.7 for the

Economics petrochemical industry).

Compressors are the most expensive units, followed by reactors;

Economics pumps and tanks are often neglected initially. 2

TECHNICAL DOCUMENTATION

PROJECT PLAN

The Project Plan is developed & implemented by the project manager and:

●​ Sets deadlines for completion of activities & intermediate deliverables

●​ Allows scheduling of subordinate or dependent tasks

●​ Allows estimation of the required manpower resources at each stage of the project

●​ Determines an estimate of when cash outlays are expected

It comprehends:

●​ a list of tasks

●​ the duration (or the start and end date) of the task

●​ the predecessor tasks

●​ the resource allocations

●​ gantt chart

In order to create a project plan we could use:

●​ a specialized software for larger projects

●​ cheap software or spreadsheets for small projects

DESIGN BASIS

The design basis is a documentation of design assumptions and boundary conditions. It

includes:

●​ contact information

○​ company name

○​ process unit name

○​ etc

●​ conventions followed, like units of measurement

●​ Product information for primary products

○​ desired grades

○​ safety data sheet

○​ production rate

○​ purity

●​ Information for primary raw materials

○​ feedstock grades

○​ safety data sheet

○​ availability

○​ pricing

○​ feed impurities

●​ Site information

○​ ambient conditions

○​ special conditions (wind, hurricane, earthquake condition for civil engineering

design)

●​ utility information and price 3

DESIGN DOCUMENTATION

The design documentation for a process usually includes a minimum of:

●​ The design basis

●​ A written description of the process

●​ A process flow diagram

●​ At least one mass & energy balance

●​ Product specifications and properties

●​ A list of major plant equipment

●​ Equipment specification sheets

●​ A piping and instrumentation diagram

●​ A cost estimate

●​ HS&E information (e.g., HAZAN, HAZOP, MSDS forms)

●​ additional information as market information, competition studies, alternative design

options etc

REPRESENTING a PROCESS

A process can be represented:

●​ in an easy way with flow diagrams or flowsheets for a graphical representation

●​ with software tools

○​ Draw.io (good choice)

○​ DIA diagram

○​ Microsoft Visio

○​ Professional software like AutoCAD

BLOCK FLOW DIAGRAMS 4

It is composed by several boxes:

●​ each box represents a process task, which can be carried out using multiple

equipment or unit operations, showing the sequence of operations

●​ it presents minimal information on flow materials and material balances (material

flows are represented by arrows, typically from left to right and the lighter streams

(gases) are directed towards the upper part of the diagram

●​ when lines cross, the vertical one is commonly broken

●​ no details on equipment

PROCESS FLOW DIAGRAM

It comprehends:

●​ process topology

●​ stream information (each stream is assigned a number, that links it to a table where

some information are listed)

●​ equipment information (identified through a symbol and an identification code):

○​ first letter identifies the type of equipment

○​ the first number identifies the area in which it is located

○​ the other numbers identifies the actual unit

○​ A/B when there is an identical unit as a backup

○​ equipment tables summarise key information on equipment

●​ main utility streams

●​ key control loops are rarely shown 5

PIPING and INSTRUMENTATION DIAGRAM

It is the most complete diagram (abbreviated P&ID or P&I) since:

●​ comprises all information required for planning plant construction

●​ stream mechanical details like materials, diameters etc

●​ details on each piece of equipment, as well as specification sheets used to purchase

equipment from suppliers

●​ measurement and control instrumentation

●​ utilities

●​ usually does not comprise information on operation conditions

●​ typically for one PFD there are more P&IDs

PLANT LAYOUTS

Usually the plant layout can be of two types:

●​ grade-mounted horizontal in-line arrangement

○​ unit is located in a square or rectangular area

○​ main pipe rack is placed at approximately the center of the area

○​ Advantages: equipment are located at grade so it is easy to construct and

more accessible to operation and maintenance

○​ Disadvantages: required area is large. More length of cabling, utility, feed

and product piping

●​ structure-mounted vertical arrangement

○​ unit is a multilevel of steel or concrete structure

○​ Equipment is placed as per process flow in each level

○​ Advantages: area required is less. This can be arranged as an enclosed house

with a controlled environment

○​ Disadvantages: more difficult to construct. More complex for operation and

maintenance (access is difficult) 6

Equipment placement must be made considering:

●​ required access for maintenance, installation, and removal

●​ required area

During the plant layouts we need also to choose the site in which the units will be

constructed. The most important feature of a site are:

●​ tax regime

●​ availability of raw materials and other resources

●​ cost of labor and fixed costs

●​ transport facilities

●​ availability of skilled workers

●​ political and strategic factors

●​ environmental impact

In general, a good site has:

●​ easy access to plant for workers or emergency responders meaning that there must

be lots of access roads (better if upwind of plant)

●​ building must be located away from the hazardous area and should be upwind of

plant

●​ space between process units (this feature is important if the processes are

hazardous)

●​ storage units are easy accessible from road (without driving though the plant) and

prevailing wind carries vapors away from plant

●​ smooth flow of materials inside the process, reducing pipe length etc

●​ make allowance for future plant expansion 7

PROCESS DESIGN APPROACH

Process design is given by the combination of the analysis (assessment of the effects of

operating conditions and equipment on products) and the synthesis (selection of chemical

route, equipment, connections etc) of a process. It is an unspecified problem where only a

little information is available at the beginning. Process design is affected by market demand

and value of products:

●​ commodity chemicals:

○​ large volumes but low add-value

○​ buyers purchase them according to their chemical composition

○​ can’t be distinguished between companies

○​ are typically manufactured in continuous 24/7 dedicated plants that has very

high cost investment so only the highly efficient processes (energy and

material optimization) can compete in the market

○​ process design (and optimisation) makes the difference

●​ fine chemicals:

○​ small volumes

○​ buyers purchase them according to their chemical composition

○​ may be highly differentiated between companies

●​ specialty chemicals:

○​ very small volumes but very high add-value

○​ buyers purchase them according to their “effects” rather than on their chemical

compositions

○​ are typically produced in multi functional plants so the capital investment

could be lower than continuous processes

○​ process design is important to ensure product quality

Usually the types of flowsheet development problem are:

●​ revamp design where we try to improve the performance of the process

●​ established design where we try to entry a low-risk market or expand an existing

process

●​ novel design where we try to create a new process with radically better cost of

production

The objective of conceptual design is to reach the definition of a PFD for a base-case in a

rapid way, usually by using shortcut methods. Then we must analyze the system in a more

rigorous way.

The flowsheet structure is defined while moving from a simple structure to a more

complex one.

The hierarchical approach may be employed to decompose the flowsheet of an existing

process for:

●​ better process understanding

●​ helps assessing design choices and possibly re-discussing them

●​ allows proposing changes to improve process operation 8

ECONOMICS in CONCEPTUAL DESIGN

If a new process is to be designed, at each design level it is necessary to estimate the

economic performance.

The economic potential (EP) is an approximated profitability index, and is used to

discriminate among design alternatives during conceptual design:

€

⎡ ⎤

= − −

⎣ ⎦

●​ variable costs are raw material, energy, waste treatment costs

●​ for investment, only process equipment annual costs will be accounted for. A rule of

thumb tells that the annualized investment is

= 3

so the economic potential becomes

€ .

⎡ ⎤

= − −

⎣ ⎦

3

If the economic potential at one level, becomes negative then a new configuration must be

searched or the project is abandoned.

Usually during the calculation of the EP we:

●​ neglect the costs of pumps and tanks because they are much less expensive than

other equipment

●​ compressors are the most expensive equipment, followed by reactors

●​ installation equipment is estimated through simplified correlations (Appendix A

of Turton) that accounts for the purchase, the installation, the control system, piping,

insulation etc, like the Bare module cost:

( ) ( )

0 0 ( )

= = +

1 2

0 0

where: 0

○​ is the cost at a certain year (known)

○​ I are cost indices that tells how prices of technical equipment has changed in

i

time

○​ is the installation factor that depends on pressure factor (at how many atm

the equipment works) and material factor

The economic potential will be calculated with additional terms at each design level. 9

PRELIMINARY INFORMATION

The design of a new process requires the availability of some key information:

●​ Main chemical reactions and reaction conditions

●​ Required productivity

●​ Product purity and value (as a function of purity)

●​ Raw material compositions and costs

●​ Reaction rates and catalyst deactivation rates

●​ Constraints

●​ General information on plant site

●​ Physical properties

●​ Health, safety and environmental information of chemicals

●​ Byproduct values (or costs)

●​ Utility costs

●​ Equipment costs

CHEMICAL REACTIONS

Possibly the most important decision in a chemical process is to choose the chemical route

to the desired product. Also secondary reactions may have a critical impact on process

design because side products must be separated, affecting the design of the separation system.

The informations about the reactions are:

●​ Stoichiometry of all (key) reactions

●​ Reaction T and P ranges

●​ Reaction system phase (or phases)

●​ Product distribution w.r.t. conversion (if available, also as a function of temperature,

reactant molar ratio, pressure)

●​ Conversion vs. residence time

●​ State (homogeneous phase; particles; slurry; etc), deactivation rate and regeneration

procedure of catalyst

We must be careful about definitions of conversion, selectivity, and yield since selectivity

and yield have no universally accepted definitions. For example, considering the simple

reaction A → B+C, the selectivity and the yield could be computed as:

●​ =

●​ (suggested)

= × ℎ.

●​ = = = ·

●​ = = = ·

× ℎ.

Selectivity represents the “efficiency” of the reactive system towards the desired product

because in some cases it could be better to have lower productivities with less impurities.

PRODUCTIVITY and PURITY

●​ There is a minimal process productivity to reach profitability 10

●​ The maximal productivity may be limited by availability of utilities, transport of raw

materials and products or land availability

●​ specification of purity for raw materials and products

CONSTRAINTS

A process could have some constraints that limit the operability of some equipment:

●​ flammability or explosive constrains

●​ polymerization or degradation effects (temperature and pressure constraints)

●​ constraints on construction materials

PLANT SITE

The general information about plant site regards:

●​ available fuels

●​ pressure of available steam

●​ inlet temperature for cooling water

●​ available temperature for refrigeration

●​ electric power availability

●​ availability of waste treatment plants

PHYSICAL PROPERTIES

The physical properties of the involved components are really important to achieve a good

estimate of the economic potential since they could affect material/energy balances.

If a property is not known with good accuracy, it is good practice in conceptual design to

assess the sensitivity of the proposed solution to property changes to understand if investing

in more accurate measurement is a good choice.

SPECIAL CONCERN

●​ Usually, atmospheric pressure up to 10 bar does not increase the capital investment

too much so a decision to operate outside this range must be justified.

●​ At elevated temperatures, common construction materials must be replaced by more

costly materials so a decision to operate at temperatures greater than 400°C must be

justified

●​ there are temperature limits for common heating and cooling utilities (steam

provides heat at 250°C while cooling water provides cooling at 25°C) so a decision to

operate at temperatures outside the range of 40 to 260°C must be justified 11

LEVEL 1: CONTINUOUS vs

BATCH PROCESS

BATCH PROCESS

The first thing that must be done when creating a new process is deciding if the process

would be batch or continuous.

A batch process is typically characterised by the production of a limited amount of product

in a time interval:

●​ production is scheduled in production cycles (called batch) that comprises for

example the charge of raw materials, their process and the discharge of products)

●​ similar production cycles define a production campaign

Usually the same plant may be used for different products so some equipment is designed to

be multi-purpose.

Features of the batch processes are:

●​ Capital investment is typically lower than continuous processes (because usually the

same equipment is used for several operations meaning that we need to buy less

equipment)

●​ Energy demand per product unity is much larger

●​ high flexibility of the operations due to:

○​ need of reaching a product specification

○​ adaptation of product quality to the market request

○​ recommended for products that have a strictly quality constraints

(pharmaceutics, food, etc)

○​ seasonal productions

●​ its control is more difficult because in a steady state continuous process the set point

is fixed in time while the batch process is inherently dynamic → need to fix a

trajectory

CONTINUOUS PROCESS

In a continuous process, raw materials and products are continuously fed and removed,

respectively:

●​ works 24h/day

●​ each piece of equipment is designed to perform one single operation in an optimal

way (low flexibility)

●​ lower energy demand than batch processes

●​ higher efficiency with respect to batch processes

It is not uncommon to have hybrid situations. For example:

●​ in a continuous process, some operations may be carried out discontinually like in a

batch process (catalyst regeneration) 12

●​ in a batch process, reaction products could be stored in intermediate tanks and they

are separated continuously.

GUIDELINES

Productivity:

●​ if plant capacity > 5000 ton/year usually the most convenient choice are continuous

processes

●​ if plant capacity < 500 ton/year usually the most convenient choice are batch

processes

Market:

●​ s