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Anteprima

ESTRATTO DOCUMENTO

The concept of flowsheet

UniME 6

Unit operations in chem. processes

pretreatment of the feed (purification, heating, etc.)

▸ chemical transformation (reactor)

▸ continuous or batch-type

◦ mono- or multi-phase

◦ fixed bed, stirred tank, multiphase

separation

▸ mechanical (not usual)

◦ different composition between gas/liq. phase (distillation)

◦ different solubilities (extraction, absorption-stripping,

◦ crystallization, ...)

preferential adsorption (active carbones)

UniME and many more

◦ 7

Flowsheet

The flowsheet is the “road-map’’ of a process, and serves to

▸ identify and focus the scope of the process for all interested and

associated functions of the project. As a project progresses, the

various engineering disciplines read their portions of responsibility

from the flowsheet, although they may not understand the process

or other details relative to some of the other phases of engineering.

The flowsheet also describes the process to management as well as

▸ those concerned with preparing economic studies for process

evaluation.

A good process flowsheet pictorially and graphically identifies the

▸ chemical process steps in proper sequence. It is done in such a

manner and with sufficient detail to present to others a proper

mechanical interpretation of the chemical requirements.

UniME 8

distillation columns

multistep

multi-feed

introductions

UniME 9

gas-.liq separ.

ethylbenzene reactor

vent transalkylation vent

water

benzene furnace

heat out toluen

e

heat exchanger

furnace

ethylene

furnace reactor

compressor alkylation

- input-output, mass flow

sequence of operations

- main equipments (with possible omissions)

UniME - 10

Type of flowsheets

Block Diagram

▸ Usually used to set forth a preliminary or basic processing concept

◦ without details. The blocks do not describe how a given step will be

achieved, but rather what is to be done. These are often used in survey

studies to management, research summaries, process proposals for

“packaged” steps, and to “talk-out’’ a processing idea. For management

presentations the diagrams are pictorial and help illustrate the basic

flow cycle.

UniME 11

Ammonia

Ammonia is an important primary inorganic material. 85% of

▸ the worldwide production is utilized in the manufacture of

synthetic fertilizers.

N2 + 3 H2 2 NH3, H = -91.6 kJ/mol

 

▸ favored by: (i) high pressure, (ii) low temperature,

◦ (iii) active catalyst, (iv) pure gas (low inert gas )

α-Iron

catalyst: (from magnetite), promoted by

◦ (a) K2C03 (increases activity),

(b) Al203, SiO2, (protect against sintering),

(c) CaO (reduces susceptibility to sulfur and

chlorine compounds)

UniME 12

Ammonia and Nitric Acid

ammonia synthesis ammonia (NH3) oxidation

to nitric acid (HNO3)

UniME 13

Nitric acid (HNO3) production by ammonia

(NH3) oxidation

UniME 14 14

Process schemes

It is shown the sequence of unit operations and the

▸ flows between them.

Four types

▸ simplified

◦ only main operations

quantified

◦ also T,P and composition (in, out of main equipments)

with instruments

◦ also the main instruments to control operations

operational

◦ complete

UniME 15

Main symbols (in flowsheets)

1 vertical empty reactor

2 filling (catalyst., resins,

structured filling)

1 2 3 4 3 fixed bed reactor (or column)

4 multitubular reactor

5 continuous mixed reactor

6 gas-lig (or liq/liq) separat.

7 plate distillation column

8 tanks

9 furnace

10 condensator

5 6 7

UniME 8 9 10 heat cyclone valves

pumps (gas, liq.)

exchanger 16

agitated with immersion jacketed

plate column packed column spray column reactor heat exchanger

heat exchangers

pneumatic electric flow manual pneumatic

UniME 17

Simplified flowsheet

UniME absorption-stripping

depuration air from benzene 18

Absorption-stripping

Gas absorption is a unit operation in which soluble components of a

▸ gas mixture are dissolved in a liquid. The inverse operation, called

stripping or desorption, is employed when it is desired to transfer

volatile components from a liquid mixture into a gas. Both

absorption and stripping, in common with distillation, make use of

special equipment for bringing gas and liquid phases into intimate

contact.

Absorption, stripping, and distillation operations are usually carried

▸ out in vertical, cylindrical columns or towers in which devices such

as plates or packing elements are placed. The gas and liquid

normally flow countercurrently, and the devices serve to provide

the contacting and development of interfacial surface through

which mass transfer takes place.

Absorption is favoured at low T and high P, the contrary for

UniME stripping 19

absorption-stripping

Block diagram depuration air from benzene

UniME 20

Quantified flowsheet

UniME 21

Instrumental flowsheet

UniME 22

Operational flowsheet

UniME 23

Reading flowsheets - Reactors 1/...

gas-liquid

high and low conv.

UniME 24

Reading flowsheets - Reactors 2/...

gas-gas, fixed bed gas-liq, slurry (suspended cat.)

UniME 25

Reading flowsheets - Reactors 3/...

liq-liq, moderate exothermicity gas-liq, exothermic reaction

UniME 26

Reading flowsheets - Reactors 4/...

gas-liq, exothermic reaction gas-gas, high exothermic

UniME 27

Mass and heat balances

Base concepts

UniME 28

Quantification of mass/heat transfer 1/..

Chemical processes may be classified as batch, continuous or semi-batch

▸ and as either steady-state or transient. Although the procedure required

for performing mass, or material, balances depends on the type of

process, most of the concepts translate directly to all types.

The general rule for mass balance in a system box (a box drawn around

▸ the complete process or the part of the process of interest) is:

input + generation - output - consumption = accumulation

where,

▸ input is the material entering through the system box. This will include feed and makeup

◦ streams;

generation is the material produced within the system, such as the reaction products in a

◦ reactor;

output is the material that leaves through the system boundaries. These will typically be

◦ the product streams of the process;

consumption is the material consumed within the system, such as the reactants in a

◦ reactor;

accumulation is the amount of material that builds up within the system.

UniME ◦ 29

Quantification of mass/heat transfer 2/..

In a steady-state continuous process, the accumulation should always be

▸ zero, which leads to a more simple mass balance equation:

input + generation = output + consumption

In the case of systems with no reaction, where mass is neither generated

▸ nor consumed, the result is even simpler:

input = output

Example 1: Mass Balance on a Continuous Distillation Process

▸ Suppose that a 1000 kmol h-1 feed stream, consisting of 30.0% by mole n-

◦ pentane and the remainder n-hexane, is to be separated into 95.0% pure

pentane and 95.0% pure hexane streams using a distillation column.

Determine the flow rates of the output streams through the use of mass

◦ balances, assuming steady-state operation.

UniME 30

pentane has a lower boiling point than hexane.

first step is to draw and label a flowsheet

• diagram indicating the process steps and all

the streams

F is the feed stream, D the distillate or tops

• product stream (which will be primarily

pentane, the lighter of the two species), B

the bottoms product stream (primarily

hexane), p refers to pentane and h to

mole fractions hexane

input = output

• we have two unknowns. Therefore, we

• need to generate two independent

total and equations that will allow us to solve for

partial mass these unknowns (degrees of freedom)

balances

UniME 31

Mass Balance on a Process with Reaction

Suppose an initially empty tank is filled with 1000 mol of ethane and the

▸ remainder with air. A spark is used to ignite this mixture and the following

combustion reaction takes place:

2C2H6 + 7O2 4CO2 + 6H2O

Assume that the amount of air provides twice the stoichiometric

▸ requirement of oxygen for this reaction, and that air is composed of 79%

nitrogen and the remainder oxygen. Suppose that the reaction reaches a

90% conversion. What is the composition of the mixture in the tank at the

end of the reaction?

E for ethanol, O for oxygen,

N for nitrogen, C for carbon

dioxide and W for water

UniME input + generation = output + consumption 32

g is used to indicate the amount generated and the subscript c indicates the amount consumed

As there are five species involved, five mass balance equations can be defined.

• We have 19 unknown variables. As we have just defined 5 equations, we have

• 14 degrees of freedom remaining. To solve this problem, therefore, we need to

define at least 14 more equations.

We can write down new equations relating the unknown and known variables by

• making use of the stoichiometric coefficients This set of nine equations

reduces the degrees of

freedom to 5

UniME 33

Further equations can be defined on the basis of the specifications

of the feed and the conversion of the reaction:

The result is that we have

19 equations and 19

unknowns giving zero

degrees of freedom.

Given nl,E = 1000 mol, the initial amount of ethane

UniME 34

1/..

Energy balances

Energy balances can be treated in much the same way as

▸ material balances

Energy may be transferred between a system and its

▸ surroundings in two ways:

As heat, or energy that flows as a result of a temperature difference between a

◦ system and its surroundings. The direction of flow is always from the higher

temperature to the lower. Heat is defined as positive when it is transferred to

the system from its surroundings.

As work, or energy that flows in response to any driving force other than a

◦ temperature difference. For example, if a gas in a cylinder expands and moves a

piston against a restraining force, the gas does work on the piston. Energy is

transferred as work from the gas to its surroundings, including the piston.

Positive work means work done by the system on its surroundings, although

this convention is sometimes not followed and one should be careful to note

the convention used by other people.

UniME 35

the full energy balance equation is

▸ 2/..

As internal energy, U, is typically difficult to measure or estimate, enthalpy is instead used.

Specific enthalpy (enthalpy per unit mass), denoted by H, is defined as

UniME 36

In working with changes of energy, it is often useful to choose a reference 3/..

state, a state in which one of the quantities is assumed to be zero.

UniME (A) 37

Energy Balance on a Distillation Column 1/..

We consider the distillation unit introduced before, updated

▸ with temperature information for each of the streams. The

temperatures have been estimated using a physical property

estimation system. More streams are included. Specifically,

the vapour stream, V, from the top of the column to the

condenser and the liquid reflux stream, L, from the condenser

back into the column.

The relationship between the liquid reflux stream back into

▸ the column and the actual distillate product stream (D) is

given by the reflux ratio (typical value between 1.3 and 1.6;

1.6 in this case):

UniME Neglecting the effect of pressure on enthalpy, estimate the

▸ rate at which heat must be supplied 38


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DETTAGLI
Corso di laurea: Corso di laurea in chimica industriale
SSD:
Università: Messina - Unime
A.A.: 2013-2014

I contenuti di questa pagina costituiscono rielaborazioni personali del Publisher Giuseppe^^ di informazioni apprese con la frequenza delle lezioni di Chimica industriale e studio autonomo di eventuali libri di riferimento in preparazione dell'esame finale o della tesi. Non devono intendersi come materiale ufficiale dell'università Messina - Unime o del prof Perathoner Siglinda.

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