Chimica industriale - struttura dell'industria chimica

Main symbols (in flowsheets)

1 vertical empty reactor

2 filling (catalyst., resins, structured filling)

3 fixed bed reactor (or column)

4 multitubular reactor

5 continuous mixed reactor

6 gas-liquid (or liquid-liquid) separation

7 plate distillation column

8 tanks

9 furnace

10 condensator

11 heat cyclone valve

12 pumps (gas, liquid)

13 exchanger

14 agitated with immersion jacketed

15 plate column

16 packed column

17 spray column

18 reactor heat exchanger

19 heat exchanger

20 pneumatic electric flow manual pneumatic

21 Simplified flowsheet

22 absorption-stripping

23 depuration air from benzene

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.

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 stripping.

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

Quantification of mass/heat transfer

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 ◦ 29Quantification 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% purepentane 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 30pentane has a lower boiling point than hexane.first step is to draw and label a flowsheet• diagram indicating the process steps and allthe 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

waterUniME input + generation = output + consumption 32g 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 5UniME 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 ethaneUniME 341/..

Energy balances

Energy balances can be treated in much the same way as material balances. Energy may be transferred

1. 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.
2. 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.

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

The unit mass (H), denoted by H, is defined as

In working with changes of energy, it is often useful to choose a reference state, a state in which one of the quantities is assumed to be zero.

Energy Balance on a Distillation Column

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):

Neglecting the effect of pressure on enthalpy, estimate the rate at which heat must be supplied

To determine this amount, we will need to determine the change

The enthalpy of the output streams relative to the feed stream and the amount of cooling done in the condenser can be determined using specific values. UniME 393/.. UniME (B) 404/.. The difference between the enthalpies is primarily due to the heats of vaporization, but there is a small contribution from the drop in temperature across the condenser. UniME (C) 41

Using the specific enthalpies in Table (C) to get the actual stream enthalpies allows us 5/.. to then solve for the remaining three unknowns, with the following results. Looking at the system box 1, there are three streams: the feed stream and the two output streams, and one heat input. The energy balance around the column with unknowns Qr and the enthalpies of the feed and bottoms streams can be calculated as before, to finally obtain UniME 42.

Kinetics and thermodynamics Base concepts UniME 43. Kinetics and thermodynamics of reactions DEFINITIONS 1/.. extend of reaction ▸ the change in the number of moles of

any reaction compound◦ (reactant or product) due to the chemical reaction divided by its stoichiometric coefficient.

Conversion.UniME ▸ The conversion of a reactant A (XA) is the fraction of reactant◦ 44 limiting reagent

Kinetics and thermodynamics of reactions

DEFINITIONS 2/..

Yield (YK) molar (or by weight)▸ amount of K produced expressed as a fraction of the◦ maximum amount of K (according to the stoichiometry) that could be produced. This is the amount of reactant A converted to the specific product:

Selectivity:▸ fraction of the converted reactant A (not the initial A) that is◦ converted to the specific product.

Productivity▸ moles (or weight) of product produced per time (h) and per◦ UniME volume (or mass catalyst) of reactor dimensionally equivalent to rate, 45 BUT it is NOT a reaction rate

Reaction rate The reaction rate (r) is the change of the extent of the▸ reaction per unit time per unit volume. for constant density this is equal to dC/dt (C = concentration). In a

Reaction network of R reactions, the total change in the number of moles of a reaction component i is due to many reactions and given by

Classification of reactions

Chemical Reaction Thermodynamics 1/..