CHAPTER 8

POLLUTION PREVENTION AND UNIT OPERATIONS

UNIT OPERATIONS AND POLLUTION PREVENTION

- Chemical Reactors

- Separation Devices

- Separative Reactors

- Storage Tanks and Fugitive Sources

Pollution Prevention for Chemical Reactors

In the designing of chemical reactors for pollution prevention there are important considerations :– Raw materials, products and byproducts– Conversion yield and selectivity for the desired

product– To establish reaction type and make the reactor

choice– Definition of the reactor operation

Selection of Reactors and Material Selection

• The selection of materials has an influence on the environmental impacts caused by reactors in chemical processes.

• For materials used in a reactor it is important to consider :– The choice of feed entering the reactor– The catalyst (if one is needed)– Solvents or diluents

Raw materials and feedstocks used in chemical processes should not pose toxic threats to the environment or to human health.

New process chemistry may need to be adopted if raw materials are eliminated or substituted.

If toxic compounds are being used, they should be replaced with an equivalent, but less toxic, substance.

Selection of Reactors and Material Selection

Raw Materials and Feedstocks

Solvents are very useful and important in the chemical process and due to their high volatility cause :

- Tropospheric (low-level) ozone occuring from smog

- Health concerns for workers

- Health concerns for the general population in the vicinity of the facility

Selection of Reactors and Material Selection

Solvents

Some criteria to consider for solvent selection :

• The solvent’s properties

• The byproducts generated and their properties

• The potential health impacts to the workers and to the general population

• The potential environmental repercussions

Selection of Reactors and Material Selection

Solvent Selection

Catalyst : a substance that is added to a chemical reaction mixture in order to accelerate the rate of reaction.

Type of Catalysts :

homogeneous, which is one phase with the reaction mixture

heterogeneous, which exists in a phase other then the reacting mixture, (typically existing as a solid within a reacting fluid mixture)

Selection of Reactors and Material Selection

Catalysts

- Allow the use of more environmentally benign raw materials

- Directly create more environmentally acceptable products from reactions

- Increases selectivity toward the desired product and away from unwanted byproducts (wastes)

- Converts waste chemicals into raw materials

Selection of Reactors and Material Selection

Benefits of Catalysts

Important considerations regarding catalyst selection include : - End disposal of the catalyst - The possiblity of regenerating it

Because these two issues may have important environmental impacts, it is desirable to regenerate the catalyst as many times as possible and /or use minimal energy or materials to regenerate it instead of disposing of it.

Selection of Reactors and Material Selection

Catalyst Selection

Selection of a Reactor :Reaction Type and Reactor ChoiceFeatures that influence pollution prevention

opportunities and strategies for selecting a chemical reactor :

• Chemical reaction mechanism• The reaction order• Series or parallel reaction pathways, • ReversiblityThese details will determine the optimum reactor

temperature, residence time, and mixing.

As a general rule, a desirable reaction includes :

- A very high conversion of the reactants

- High selectivity toward the desired product

- Low selectivity toward any byproducts.

Selection of a Reactor :Reaction Type and Reactor Choice

Reactant conversion (reaction yield): ratio of the existing concentration of product to inlet reactant:

R P

[P] / [R]o

Reaction selectivity: ratio of existing product concentration to the undesired byproduct concentration

R P + W[P] / [W]

Modified Selectivity is the ratio of exiting product concentration to the sum of product and byproduct (waste) concentrations:

[P] / ([P]+[W]) = [P] / [Reactant consumed]

Conversion and Selectivity Equations

Parallel reactions pathways are very common in the chemical industry. These kind of reactions are in competition of the main reactions (undesirable).

R P

R W

Also series reactions, it’s when a reaction is followed of other one, consecutively (one after other).

R P W

kp

kw

Types of Reactions

Optimizing the reaction in serial reactions:

* kp / kw must be as large as possible * Control of the reaction residence time

kp and kw [=] reaction constants of product and waste respectively

Reversible reactions, are reactions where there is a competition between two reactions: one towards the desired product and the other one is of the decomposition of the product (in the opposite direction).

R P

This kind of reaction inhibits full conversion of reactants to products.

Types of Reactions

- Continuous-flow stirred-tank reactor (CSTR), consists of a batch of reaction stirred at certain parameters settled.

- Plug flow reactor, can be staged and each stage can be operated at different conditions to minimize waste formation

- Fixed-bed catalytic reactor, that is used when hot spots are a problem for highly exotermic reactions (it will likely avoid the unwanted temperature excursions)

Types of Reactors

Can influence the :- Degree of conversion of reactants to products- Product yield- Product selectivity

Temperature is changed above or below the initial temperature:

ΔT = + To or – To

Ratio of [product] / [byproduct] ≈ T (direct ratio)

Therefore pollution can be prevented in parallel (and also series) reactions by increasing reactor temperature generally

Reaction Temperatures

It’s the addition of two or more raw materials into a reactor to assure contact or collisions occur between substances in order for the chemical reaction to occur.

This effect of mixing occurs for both :

- Homogeneous and heterogeneous reaction systems

- Batch or semi-batch reactors

Mixing

The factors that influence/are affected by mixing are:

- The reaction must be mixed instantaneously at a molecular level.

- The rate of reaction can be reduced because of diffusional limitations between segregated elements of the reaction mixture.

- Problems with imperfect mixing are particularly evident for rapidly reacting systems. In these situations, reactants are significantly converted to products and byproducts before mixing is complete.

Mixing

Separation tecnologies are one of the most important unit operations found in chemical processes :

As the mixtures and chemical reactions are not 100% efficient, it is necessary to separate chemical components from one another prior to subsequent processing steps.

Separation unit operations generate waste because:• The separation steps themselves are not 100% efficient• Require addtional energy input • Require waste treatment to deal with off-spec products

Pollution Prevention for Separation Units

Selectin a mass seperation agent (chemical compound like solvent) is an important issue for pollution prevention in order to avoid:

- Exposure to toxic substances for facility workers and consumers who use the final product.

- Excessive energy consuption in the recovery of the solvent or other raw materials

- Associated health impacts of the emitted criteria pollutants (CO, CO2, NOx, and SOx, particulate matter)

Pollution Prevention for Seperation Units

Choice of Mass Separating Agent

Adsorption is when a chemical dissolved in a liquid or a gas phase will preferentially become immobilized on the surface of a solid matrix (adsorbent) packed with a column.

- Separation and recovery of toxic metal ions from aqueous streams is one of the very important application of adsorption

Choice of Mass Separating Agent

Pollution Prevention for Separation Units

While it may be difficult or impossible to eliminate all waste streams, it is certain that wastes can be minimized by:

- Judicious choice of mass separating agent- Correct choice and sequencing of separation

technologies- Careful control of system parameters during

operation.

Chemical process

WASTE

Air

Water

Soil

Operation Design and Operation Heuristics for Separation Technologies

- Making the correct choice of type of separating process (i.e.: adsorption, distillation, dialysis, etc.)

- Considering several pollution prevention heuristics to guide the design of the flowsheet and operation of the units

- Associating streams of the process by simmilar compositions

- Avoidint the addition of chemical compounds to improve separation (i.e. corrososive), unless necessary

Minimizing Waste from Seperation Technologies

- Reduce as much as possible the number of components of simmilar component properties

- When adding separating agents, remove it in the next step of the process of separation using an energy separating agent technology

- The process should avoid separation technologies that operate far from ambient temperature and pressure.If large variations from ambient temperature are required, it is more economical to operate above rather than below.

Minimizing Waste from Seperation Technologies Continued

It is used for over 90% of the separation applications in chemical processing.

Distillation columns contribute to process waste in four ways:

a) By allowing impurities to remain in a product

b) By forming waste within the column itself

c) By inadequate condensing of overhead product

d) By excessive energy use

Types of Separation Technologies : Distillation Columns

The most common way to increase product purity in distillation are:

- To increase the reflux ratio. - If a column is operating close to flooding (increasing reflux

ratio is not an option), then adding a section to the column leads to higher-purity products.

There are several ways to decrease the generation of tars in the reboiler of the column:

- Reducing the column pressure, resulting in lower reboiler temperatures.

- Improving the process control technology = product purity specifications will be met

Types of Seperation Technologies : Distillation Columns

Optimizing Use of Distillation Columns

REACTOR + SEPERATOR = REDUCTION OF BY- PRODUCTS

Examples:Distillation: - Solvent recovery from waste water- Ink and solvent recycle- Batch distillation of used antifreeze- Solvent recovery and reuse in automobile paint

operations

Pollution Prevention for Reactors and Seperators

Extraction: - Extraction of a batch process residue- Hydrocarbon recovery from refinery wastewater

and sludge

Reverse Osmosis: - Closed-loop rinsewater for process electroplating- Recovery of homogeneous metal catalysis

Ultrafiltration: - Polymer recovery from wastewater

Pollution Prevention for Reactors and Seperators (cont)

Adsorption: - Natural gas dehydration- Replacement of azeotropic distillation (benzene,

ciclohexane)

Membranes: - Recovery and recycle of high-value volatile organic

compounds- Recovery of organic compounds fron wastewater

streams- Metal ion recovery from aqueous waste streams

Pollution Prevention for Reactors and Seperators (cont)

The separative reactor has a very high potential for reducing waste generation:

- Hybrid systems that combine chemical reactions and product separation in a single process unit.

- When chemical reaction and separation ocurr in concert, the requirements for downstream processing units are reduced, leading to lower capitals costs.

- Unwanted byproduct generation can be minimized in series reactions by the removal of the desired product

- Separation units that have been integrated with reaction include distillation, membrane separation, and adsoption.

Pollution Prevention Applications for Separative Reactors

Reaction coupled with membrane separation also is used to increase the efficiency of chemical reactions:

- Can be used to selectively remove either products or byproducts from the reaction zone

- Overcoming low conversions in equilibrium-limited reactions and reduce waste generation in series reactions.

- Can also be used to selectively permeate reactants into the reaction zone in order to control excessive byproduct formation (e.g. Permeation of O2 in partial oxidation or oxidative coupling reactions).

Pollution Prevention Applications for Membrane Separative Reactiors

Additional challenges remain before commercial application of membrane separative reactors can be realized. These include:

a) Economical manufacture of thin, defect-free selective membrane layers over large surface areas

b) Leak-free reaction systems with high temperature seals

c) Elimination or reduction of sweep gases which dilute product streams

d) Enhanced membrane and catalyst performance, including resistance to foulding and deactivation

Pollution Prevention Applications for Membrane Separative Reactiors

- Storage Tanks

- Fugitive Sources (valves, pumps, piping conectors, pressure relief valves, sampling conections, compressor seals, and open-ended lines)

Pollution Prevention for

Storage Tanks and Fugitive Sources

Storage tanks are very common unit operations in several industrial sectors:

• Petroleum production and refining

• Petrochemical and chemical manufaturing, storage and transportation

• Other industries that either use or produce organic liquid chemicals.

Pollution Prevention for

Storage Tanks

The main environmental impact of storage tanks:

- Continual occurrence of air emissions of volatile organic compounds (VOC’s) from roof vents

- Periodic removal of oily sludges from tank bottoms.

Tank bottoms are:- Solids or sludges composed of rusts, soil particles,

heavy feedstock constituents, and other dense materials that are likely to settle out of the liquid being stored.

Pollution Prevention for

Storage Tanks

Sludges from tank bottoms may be periodically removed and either treated via land aplication or disposed of as hazardous waste.

They may be prevented from settling to the tank bottom:

• By the action of mixers that keep the solid particles suspended in the liquid.

• The use of emulsifing agents that keep water and solids in solution and out of the tank bottoms.

Minimizing Pollution from Tank Bottom Sludges

Air emissions of VOC’s from storage tanks are mainly from petroleum and chemical processing facilities:

Working losses are the emission that stem from the normal operation of the chemical processing in response to the changes in liquid level within the tank

Standing losses are the emissions caused by the action of ambient changes in temperature and pressure

Minimizing Pollution from Storage Tank Air Emissions

The emissions from tanks are dependent on :- Vapor pressure of the stored liquids- Tank caracteristics (it´s type)- Paint color and condition (of the tank)- Geographic location of the tank

There are 6 major types of storage tanks:- Fixed roof- External floating roof- Internal floating roof- Domed external floating roof- Variable vapor space- Pressure tanks

Minimizing Pollution from Storage Tank Air Emissions

Within individual components, leaks are localized near seals, valve packing and gaskets. These leaks are of two types:

- Low-level leaks that may persist for long periods of time until detected

- Sudden episodic failures resulting in a large release

Leaks can be prevented or repaired, and leakless technologies are available for situations where even small rates of release cannot be tolerated

Reducing Emissions from Fugitive Sources

There are two methods for reducing or preventing emissions and leaks from fugitive sources in the industry :

1) Leak detection and repair (LDAR) of leaking equipment

2) Equipment modification or replacement with emissionless technologies

Methods to Reduce Fugitive Emissions

- Equipment such as pumps and valves are monitored periodically using an organic vapor analyzer (OVA).

- The wand of OVA is directed towards the suspected source of leakage on each piece of equipment ( i.e. at a packing nut on a valve, at a shaft seal on a pump.)

- If the source registers an OVA reading over a threshold value, the quipment is said to be leaking and repair is required.

Methods to Reduce Fugitive Emissions

LDAR Program

The nature of the repairs varies :- It may involve something as simple as tightening a packing

nut on a valve- It may require replacement of a seal on a pump or a gasket

in a connector.

Industrial LDAR programs vary greatly:- Frecuency of monitoring and their effectiveness (monthly,

quarterly, or annual basis using an OVA is the preferred approach)

- Problems in detecting low concentrations of VOCs- Intensity of monitoring (area)- Costs of monitoring: establich the frecuency of monitoring

Methods to Reduce Fugitive Emissions

Reducing fugitive emissions might involve:

- Equipment modification (redesigning a process)

- Considering fewer pieces of equipment and connections

- Replacing leaking equipment with new conventional equipment.

Methods to Reduce Fugitive Emissions