chapter 8 · pollution prevention for chemical reactors in the designing of chemical reactors for...

CHAPTER 8POLLUTION PREVENTION AND

UNIT OPERATIONS

UNIT OPERATIONS AND POLLUTION PREVENTION

- Chemical Reactors

- Separation Devices

- Separative Reactors

- Storage Tanks and Fugitive Sources

Pollution Prevention forChemical Reactors

In the designing of chemical reactors forpollution prevention there are importantconsiderations :– 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 influenceon the environmental impacts caused by reactors in chemical processes.

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

Raw materials and feedstocks used in chemicalprocesses should not pose toxic threats to theenvironment or to human health.

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

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

Selection of Reactors and Material SelectionRaw Materials and Feedstocks

Solvents are very useful and important in thechemical process and due to their highvolatility cause :

- Tropospheric (low-level) ozone occuringfrom smog

- Health concerns for workers

- Health concerns for the general populationin the vicinity of the facility

Selection of Reactors and Material Selection

Solvents

Some criteria to consider for solventselection :

• 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 toaccelerate the rate of reaction.

Type of Catalysts :homogeneous, which is one phase with the

reaction mixtureheterogeneous, which exists in a phase other

then the reacting mixture, (typically existingas a solid within a reacting fluid mixture)

Selection of Reactors and Material Selection

Catalysts

- Allow the use of more environmentallybenign raw materials

- Directly create more environmentallyacceptable products from reactions

- Increases selectivity toward the desiredproduct and away from unwantedbyproducts (wastes)

- Converts waste chemicals into raw materials

Selection of Reactors and Material Selection

Benefits of Catalysts

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

Because these two issues may have importantenvironmental impacts, it is desirable toregenerate the catalyst as many times as possible and /or use minimal energy ormaterials to regenerate it instead ofdisposing 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 reactionincludes :

- A very high conversion of the reactants

- High selectivity toward the desiredproduct

- Low selectivity toward any byproducts.

Selection of a Reactor :Reaction Type and Reactor Choice

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

R P[P] / [R]o

Reaction selectivity: ratio of existing product concentration tothe undesired byproduct concentration

R P + W[P] / [W]

Modified Selectivity is the ratio of exiting productconcentration 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 reactionsare in competition of the main reactions(undesirable).

R PR 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 thedesired product and the other one is of thedecomposition of the product (in the oppositedirection).

R P

This kind of reaction inhibits full conversion of reactantsto products.

Types of Reactions

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

- Plug flow reactor, can be staged and eachstage can be operated at different conditionsto minimize waste formation

- Fixed-bed catalytic reactor, that is used whenhot spots are a problem for highly exotermicreactions (it will likely avoid the unwantedtemperature 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 initialtemperature:

ΔT = + To or – To

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

Therefore pollution can be prevented in parallel (and alsoseries) reactions by increasing reactor temperaturegenerally

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 mixingare:

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

- The rate of reaction can be reduced because ofdiffusional limitations between segregatedelements of the reaction mixture.

- Problems with imperfect mixing are particularlyevident for rapidly reacting systems. In thesesituations, reactants are significantly converted toproducts and byproducts before mixing iscomplete.

Mixing

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

As the mixtures and chemical reactions are not 100% efficient, it is necessary to separate chemical components from oneanother 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 forSeparation Units

Selectin a mass seperation agent (chemicalcompound like solvent) is an important issuefor pollution prevention in order to avoid:

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

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

- Associated health impacts of the emitted criteriapollutants (CO, CO2, NOx, and SOx, particulatematter)

Pollution Prevention for Seperation UnitsChoice of Mass

Separating Agent

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

- Separation and recovery of toxic metal ions fromaqueous streams is one of the very importantapplication of adsorption

Choice of MassSeparating Agent

Pollution Prevention for Separation Units

While it may be difficult or impossible to eliminate allwaste 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.

Chemicalprocess

WASTE

Air

WaterSoil

Operation Design and Operation Heuristicsfor Separation Technologies

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

- Considering several pollution prevention heuristics toguide the design of the flowsheet and operation ofthe units

- Associating streams of the process by simmilarcompositions

- Avoidint the addition of chemical compounds toimprove separation (i.e. corrososive), unlessnecessary

Minimizing Waste fromSeperation Technologies

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

- When adding separating agents, remove it in the nextstep of the process of separation using an energyseparating agent technology

- The process should avoid separation technologiesthat operate far from ambient temperature andpressure.If large variations from ambient temperatureare required, it is more economical to operate aboverather 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 columnleads to higher-purity products.

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

- Reducing the column pressure, resulting in lower reboilertemperatures.

- Improving the process control technology = product purityspecifications 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 forReactors 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 reducingwaste generation:

- Hybrid systems that combine chemical reactions andproduct 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 reactioninclude distillation, membrane separation, and adsoption.

Pollution Prevention Applicationsfor Separative Reactors

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

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

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

- Can also be used to selectively permeate reactants intothe reaction zone in order to control excessivebyproduct formation (e.g. Permeation of O2 in partialoxidation or oxidative coupling reactions).

Pollution Prevention Applications forMembrane Separative Reactiors

Additional challenges remain before commercialapplication of membrane separativereactors can be realized. These include:

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

b) Leak-free reaction systems with high temperatureseals

c) Elimination or reduction of sweep gases whichdilute product streams

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

Pollution Prevention Applications forMembrane Separative Reactiors

- Storage Tanks

- Fugitive Sources (valves, pumps, pipingconectors, pressure relief valves, samplingconections, compressor seals, and open-ended lines)

Pollution Prevention forStorage Tanks and Fugitive Sources

Storage tanks are very common unitoperations 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 forStorage Tanks

The main environmental impact of storage tanks:

- Continual occurrence of air emissions of volatileorganic 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 liquidbeing stored.

Pollution Prevention forStorage Tanks

Sludges from tank bottoms may be periodicallyremoved and either treated via land aplication ordisposed of as hazardous waste.

They may be prevented from settling to the tankbottom:

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

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

Minimizing Pollution fromTank 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 fromthe normal operation of the chemicalprocessing in response to the changes in liquid level within the tank

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

Minimizing Pollution fromStorage 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 localizednear seals, valve packing and gaskets. Theseleaks are of two types:

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

- Sudden episodic failures resulting in a largerelease

Leaks can be prevented or repaired, and leaklesstechnologies are available for situations whereeven small rates of release cannot be tolerated

Reducing Emissions from Fugitive Sources

There are two methods for reducing orpreventing emissions and leaks fromfugitive sources in the industry :

1) Leak detection and repair (LDAR) ofleaking equipment

2) Equipment modification orreplacement with emissionlesstechnologies

Methods to Reduce Fugitive Emissions

- Equipment such as pumps and valvesare monitored periodically using anorganic vapor analyzer (OVA).

- The wand of OVA is directed towards thesuspected source of leakage on eachpiece of equipment ( i.e. at a packing nuton a valve, at a shaft seal on a pump.)

- If the source registers an OVA readingover a threshold value, the quipment issaid 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 preferredapproach)

- 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 andconnections

- Replacing leaking equipment with newconventional equipment.

Methods to Reduce Fugitive Emissions