Distillation Column

If distillation columns contain large inventories of hazardous materials or the materials are particularly liable to leak from pump glands, then the reflux and bottoms pumps should be fitted with emergency isolation valves.

From: Critical Aspects of Safety and Loss Prevention , 1990

Process Design

In Lees' Loss Prevention in the Process Industries (Fourth Edition), 2012

11.8.4 Distillation Columns

Distillation columns present a hazard in that they contain large inventories of flammable boiling liquid, usually under pressure. There are a number of situations which may lead to loss of containment of this liquid.

The conditions of operation of the equipment associated with the distillation column, particularly the reboiler and bottoms pump, are severe, so that failure is more probable.

The reduction of hazard in distillation columns by the limitation of inventory has been discussed above. A distillation column has a large input of heat at the reboiler and a large output at the condenser. If cooling at the condenser is lost, the column may suffer overpressure. It is necessary to protect against this by higher pressure design, relief valves, or HIPS. On the other hand, loss of steam at the reboiler can cause underpressure in the column. On columns operating at or near atmospheric pressure, full vacuum design, vacuum breakers, or inert gas injection is needed for protection. Deposition of flammable materials on packing surfaces has led to many fires on opening of distillation column for maintenance.

Another hazard is overpressure due to heat radiation from fire. Again pressure relief devices are required to provide protection.

The protection of distillation columns is one of the topics treated in detail in codes for pressure relief such as APIRP 521. Likewise, it is one of the principal applications of trip systems.

Another quite different hazard in a distillation column is the ingress of water. The rapid expansion of the water as it flashes to steam can create very damaging overpressures.

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Distillation Columns and Towers

Seán Moran , in Process Plant Layout (Second Edition), 2017

Abstract

Distillation columns are key unit operations in traditional chemical engineering, especially in the oil and gas industry. They are usually tall structures filled with heated flammable fluids, and are consequently inherently hazardous. Many serious accidents have centered on columns and their ancillary operations. Where they are present, the layout of distillation columns should receive early investigation since the layout of a number of other major items of equipment usually depends upon their placement, and they can have a high potential for initiating domino effects from fire, explosion, and/or collapse.

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Distillation

James G. Speight , in The Refinery of the Future (Second Edition), 2020

4.2.3 Columns

Distillation columns (distillation towers) are made up of several components, each of which is used either to transfer heat energy or enhance material transfer. A typical distillation column consists of several major parts:

1.

A vertical shell where separation of the components is carried out.

2.

Column internals such as trays, or plates, or packings that are used to enhance component separation.

3.

A reboiler to provide the necessary vaporization for the distillation process.

4.

A condenser to cool and condense the vapor leaving the top of the column.

5.

A reflux drum to hold the condensed vapor from the top of the column so that liquid (reflux) can be recycled back to the column.

The vertical shell houses the column internals and together with the condenser and reboiler constitutes a distillation column (Fig. 4.5).

Figure 4.5. Individual parts of an atmospheric distillation column.

In a crude oil distillation unit the feedstock liquid mixture is introduced usually near the middle of the column to a tray known as the feed tray. The feed tray divides the column into a top (enriching, rectification) section and a bottom (stripping) section. The feed flows down the column where it is collected at the bottom in the reboiler. Heat is supplied to the reboiler to generate vapor. The source of heat input can be any suitable fluid, although in most chemical plants this is normally steam. In refineries the heating source may be the output streams of other columns. The vapor raised in the reboiler is reintroduced into the unit at the bottom of the column. The liquid removed from the reboiler is known as the bottoms.

The vapor moves up the column, and as it exits the top of the unit, it is cooled by a condenser. The condensed liquid is stored in a holding vessel known as the reflux drum. Some of this liquid is recycled back to the top of the column and this is called the reflux. The condensed liquid that is removed from the system is known as the distillate or top product. Thus there are internal flows of vapor and liquid within the column as well as external flows of feeds and product streams, into and out of the column.

The column is divided into a number of horizontal sections by metal trays or plates, and each is the equivalent of a still. The more trays, the more redistillation, and hence the better is the fractionation or separation of the mixture fed into the tower. A tower for fractionating crude oil may be 13   ft. in diameter and 85   ft. high according to a general formula:

c = 220 d 2 r

where c is the capacity in bbl/day, d is the diameter in feet, and r is the amount of residuum expressed as a fraction of the feedstock (Parkash, 2003; Gary et al., 2007; Speight, 2014a, 2017; Hsu and Robinson, 2017).

A tower stripping unwanted volatile material from gas oil may be only 3 or 4   ft. in diameter and 10   ft. high with less than 20 trays. Towers concerned with the distillation of liquefied gases are only a few feet in diameter but may be up to 200   ft. in height. A tower used in the fractionation of crude oil may have from 16 to 28 trays, but one used in the fractionation (superfractionation) of liquefied gases may have 30–100 trays. The feed to a typical tower enters the vaporizing or flash zone, an area without trays. The majority of the trays are usually located above this area. The feed to a bubble tower, however, may be at any point from top to bottom with trays above and below the entry point, depending on the kind of feedstock and the characteristics desired in the products.

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Common Hazards

Ian Sutton Author , in Plant Design and Operations (Second Edition), 2017

Reboiler Leak

Many distillation columns are heated by steam reboilers. Typically the process materials are on the tube side of the exchanger with the steam being on the shell side, as shown in Fig. 19.3.

Figure 19.3. Representative reboiler arrangement.

If the steam is at higher pressure than the process (which is frequently the case) and if one of the reboiler tubes develops a leak, steam will enter the process and flow up the distillation column. Given that the water has a much lower molecular weight than most chemicals that are being distilled, the volumetric flow of gas up the column can be so large that the trays or packing in the column are lifted up off their supports and seriously damaged.

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Distillation

James G. Speight , in The Refinery of the Future, 2011

4.2.3 Columns

Distillation columns (distillation towers) are made up of several components, each of which is used either to transfer heat energy or enhance material transfer. A typical distillation column consists of several major parts:

•

A vertical shell where separation of the components is carried out.

•

Column internals such as trays, or plates, or packings that are used to enhance component separation.

•

A reboiler to provide the necessary vaporization for the distillation process.

•

A condenser to cool and condense the vapor leaving the top of the column.

•

A reflux drum to hold the condensed vapor from the top of the column so that liquid (reflux) can be recycled back to the column.

The vertical shell houses the column internals, together with the condenser and reboiler constitutes a distillation column (Figure 4.6).

Figure 4.6. Individual Parts of an Atmospheric Distillation Column.

 Source: Speight, J.G. 2007. The Chemistry and Technology of Petroleum 4th Edition. CRC Press, Taylor & Francis Group, Boca Raton, Florida.

In a petroleum distillation unit, the feedstock liquid mixture is usually introduced near the middle of the column, to a tray known as the feed tray. This tray divides the column into a top (enriching, rectification) section and a bottom (stripping) section. The feed flows down the column where it is collected at the bottom in the reboiler. Heat is supplied to the reboiler to generate vapor. The source of heat input can be any suitable fluid. In most chemical plants it is normally steam, and the heating source may be the output streams of other columns. The vapor raised in the reboiler is re-introduced into the unit at the bottom of the column. The liquid removed from the reboiler is known as the bottoms.

The vapor moves up the column; as it exits the top of the unit, it is cooled by a condenser. The condensed liquid is stored in a holding vessel known as the reflux drum. Some of this liquid is recycled back to the top of the column and this is called the reflux. The condensed liquid that is removed from the system is known as the distillate or top product. Thus, there are internal flows of vapor and liquid within the column, as well as external flows of feeds and product streams into and out of the column.

The column is divided into a number of horizontal sections by metal trays or plates, and each is the equivalent of a still. The more trays, the more redistillation, and hence the better is the fractionation or separation of the mixture fed into the tower. A tower for fractionating crude petroleum may be 13 feet in diameter and 85 feet high, the height being determined by a general formula:

c = 220 d 2 r

Where c is the capacity in bbl/day, d is the diameter in feet, and r is the amount of residuum expressed as a fraction of the feedstock (Nelson, 1943).

The feed to a typical tower enters the vaporizing or flash zone, an area without trays. The majority of the trays are usually located above this area. The feed to a bubble tower, however, may be at any point from top to bottom with trays above and below the entry point. This, of course, depends on the type of feedstock and the characteristics desired in the products.

4.2.3.a Tray Columns

The tray column typically combines the open flow channel with weirs, downcomers, and heat exchangers. Free surface flow over the tray is disturbed by gas bubbles coming through the perforated tray; with possible leakage of liquid dropping through the upper tray.

Usually, trays are horizontal, flat, specially prefabricated metal sheets, which are placed at a regular distance in a vertical, cylindrical column. Trays have two main parts: (1) the part where vapor (gas) and liquid are being contacted – the contacting area and (2) the part where vapor and liquid are separated, after having been in contact – the downcomer area.

Classification of trays is based on:

1.

The type of plate used in the contacting area.

2.

The type and number of downcomers making up the downcomer area.

3.

The direction and path of the liquid flowing across the contacting area of the tray.

4.

The vapor (gas) flow direction through the (orifices in) the plate.

5.

The presence of baffles, packing or other additions to the contacting area to improve the separation performance of the tray.

Common plate types, for use in the contacting area are:

1.

Bubble cap trays, in which caps are mounted over risers fixed on the plate (Figure 4.5). The caps come in a wide variety of sizes and shapes, round, square, and rectangular (tunnel).

2.

Sieve trays, which come with different hole shapes (round, square, triangular, rectangular (slots), star), various hole sizes (from about 2 mm to about 25 mm) and several punch patterns (triangular, square, rectangular).

3.

Valve trays that are also available in a variety of valve shapes (round, square, rectangular, triangular), sizes, weights (light and heavy), orifice sizes and either as fixed or floating valves.

Trays usually have one or more downcomers. The type and number used mainly depends on the amount of downcomer area required to handle the liquid flow. Single pass trays are those which have one downcomer delivering the liquid from the tray above. This is a single bubbling area across which the liquid passes to contact the vapor and one downcomer for the liquid to pass to the tray below.

Trays with multiple downcomers, and hence multiple liquid passes, can have a number of layout geometries. The downcomers may extend in parallel from wall to wall, or they may be rotated by 90 or 180 degrees on successive trays. The downcomer layout pattern determines the liquid flow path arrangement and liquid flow direction in the contacting area of the trays.

Giving a preferential direction to the vapor flowing through the orifices in the plate will induce the liquid to flow in the same direction. In this way, liquid flow rate and flow direction, as well as liquid height, can be manipulated. The presence of baffles, screen mesh or demister mats, loose or restrained dumped packing and/or the addition of other devices in the contacting area can be beneficial for improving the contacting performance of the tray, and hence its separation efficiency.

The most important parameter of a tray is its separation performance and four criteria are of importance in the design and operation of a tray column. To ensure that the required separation is achieved:

1.

The level of the tray efficiency, in the normal operating range.

2.

The vapor rate at the "upper limit", i.e., the maximum vapor load.

3.

Tthe vapor rate at the "lower limit", i.e., the minimum vapor load.

4.

The tray pressure drop.

The separation performance of a tray is the basis of the performance of the column as a whole. The primary function of a distillation column is the separation of a feed stream into (at least) one top product stream and one bottom product stream. The quality of the separation performed by a column can be judged from the purity of the top and bottom product streams. The specification of the impurity levels in the top and bottom streams and the degree of recovery of pure products set the criteria for a successful operation of a distillation column.

It is evident that tray efficiency is influenced firstly by the specific component under consideration. This holds especially for multi-component systems in which the efficiency can be different for each component, due to its different diffusivities, diffusion interactions, and different stripping factors. The vapor flow rate is also a factor in the efficiency of the tray. Usually, increasing the flow rate increases the effective mass transfer rate, whilst simultaneously decreasing the contact time. These counteracting effects lead to a roughly constant efficiency value for a tray in its normal operating range. Upon approaching its lower operating limit a tray starts weeping and looses efficiency.

4.2.3.b Packed Columns

A packed column is similar to a trickle-bed reactor, where a liquid film flows down over the packing surface, in contact with the upward flow of gas. A small fragment of packing geometry can be accurately analyzed assuming the periodic boundary conditions. This allows calibration of the porous media model for a big packing segment.

The packing in a distillation column creates a surface for the liquid to spread on. This provides a high surface area for mass transfer between the liquid and the vapor.

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Cosolvent Machines

John B DurkeeII, in Cleaning with Solvents: Methods and Machinery, 2014

3.8.3 Management of Azeotrope Integrity with a Distillation Column

The distillation column associated with a Class IV cosolvent process is not that found within a conventional single-solvent vapor degreasing process.

•

That in the single-solvent vapor degreasing process only operates when needed (batch mode), and produces a different quality of overhead (OH) product throughout the time span of processing a batch of soiled solvent. The quality is initially rich in the volatile solvent components, and finally rich in the least volatile soil components AAAA .

•

That in the Class IV cosolvent process operates continuously, and produces the same quality of overhead (OH) product throughout its use.

These capabilities can be compared visually, first by comparing the two types of distillation columns in Figures 3.39 (batch BBBB ) and 3.40 (continuous); and second by comparing the full installations of distillation column and vapor degreaser in Figures 3.41 (single solvent) and 3.42 (Class IV cosolvent).

Figure 3.39. Batch Distillation Column

Figure 3.40. Continuous Distillation Column

Figure 3.41. Single Solvent Vapor Degreaser

The batch-use distillation column exists to separate single cleaning solvents from soil components. The fluid distilled is a single cleaning solvent.

Water is removed from cleaning systems that use single solvents in two ways: (1) insoluble water is removed in a decanter (Figure 3.14), and (2) soluble water is indirectly but effectively removed by reaction with solvent (halogenated) which reaction product then is consumed by reaction with sacrificial stabilizer chemicals CCCC .

The continuous-use distillation column exists to recover the pure binary azeotrope for further use, and to exclude all other contaminant(s) in the cleaning fluid – including soil components. Identification of non-soil contaminants, as noted in Endnote UUU and Footnote 124, is uncertain. But almost certainly they include water and the products of its interaction with the binary azeotrope or its components (Box 3.9 ); in addition to any excess of one azeotropic component 115 ; and occasionally some unexpected materials.

Box 3.9

Management of Multiple Azeotropic Components

In Figure 3.40 a ternary azeotrope is shown as being slightly less volatile than the binary azeotrope – it is recovered nearer the top section of the distillation column. That assignment was made for visual convenience in this book.

It may be, in actual cleaning operations, that ternary azeotropes or multiple binary azeotropes (both are reported formed in the reference of Endnote TTT) are the most volatile materials, and the binary azeotrope used for cleaning operations is not the overhead (OH) product of the distillation column.

In that case the binary azeotrope being used for cleaning is recovered for reuse as a side draw (product) from the continuous distillation column. Water is the likely tramp impurity producing this conversion of the binary azeotrope to other coordinated solvent assemblies.

In such situations, the material values of the overhead product must be recovered. The image above shows one method of doing this (albeit incompletely) – phase separation, a side draw to recover the intact binary azeotrope for reuse in cleaning operations, and another side draw to remove the water (presumed to be less volatile than the binary azeotrope or its more volatile products of conversion.

Integration of the batch-use distillation column into a single-solvent vapor degreaser and the continuous-use distillation column into a Class IV azeotropic cosolvent vapor degreaser is shown in Figures 3.41 (and Ref. 2, Figure 7.27 116 ) and 3.42, respectively.

Note in Figure 3.41 that: (1) there are three refrigerated zones within the vapor degreaser– the third one at the top being used to minimize entry of water, (2) the distilled solvent cannot be returned to the vapor degreaser as vapor – to save energy – because it is produced in batch mode and provision for storage of vapor is impractical, (3) cleaning quality can't be constant because the soil level in the cleaning sumps varies as soil accumulates prior to a batch purge to the distillation column, and (4) distilled solvent is added to the sump where the solvent is most clean – the rinse sump 117 .

Integration of the continuous distillation column into a Class IV cosolvent vapor degreaser is shown in Figure 3.42.

Figure 3.42. Class IV Cosolvent Degreaser

Note in Figure 3.42 that there are four facilities employed to remove water, because of the above concerns: (1) a top refrigeration coil, (2) a purge station to displace humid air with dry air or nitrogen from the parts basket prior to its entry into the freeboard area of the degreaser, (3) a decanter-type water separator to remove any water condensed in the two lower refrigeration coils, and (4) a second decanter-type water separator to remove any insoluble water prior to the distillation column 118 .

Further note in Figure 3.42 that: (1) it is assumed (as most commonly occurs) that there is no interaction of the binary azeotrope with water or any other tramp impurity, (2) azeotrope vapor can be returned to the vapor rinse zone of the vapor degreaser if desired (to save energy), (3) it is necessary through experience to learn the locations of positions in the continuously operating distillation column from which purge streams must be taken and when and how much 119 , and (4) cleaning operations in the vapor degreaser should be conducted normally.

While necessary, and possibly appearing to be complex, these facilities are commonly and straightforwardly used in the production of all cleaning solvents. They should be familiar to any reputable supplier of vapor degreasers.

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Damage control

Trevor A. Kletz DSc, FEng, FIChemE, FRSC , in Critical Aspects of Safety and Loss Prevention, 1990

Distillation

Many distillation columns contain large inventories of hazardous materials, on the trays or packing and in the base. The hold-up per theoretical plate varies from 20 mm to 100 mm for various trays and packings. Whenever possible, designers should choose a tray or packing with a low hold-up. (See intensification .)

If the bottoms product is liable to degrade, its inventory is often reduced by narrowing the base, so that the column appears to balance on the point of a needle (see Figure 7). This can be done just to reduce the inventory, though it rarely is.

Figure 7. The inventory in a distillation column can be reduced by narrowing the base

Much bigger reductions in inventory, up to a 1000 times, can be achieved by the use of ICI's Higee distillation process (see Figure 8). It is based upon the observation that distillation (or any liquid-vapour contacting process) will take place more efficiently if gravity can be increased, or simulated by centrifugal force. The rotating packed bed is about 1 m thick and about 1 m diameter. The liquid travels outwards and the vapour inwards. The diameter corresponds to the height of a normal column and the thickness to the diameter. Two units are required, one for the stripping section and one for the rectifying section. (See innovation .)

Figure 8. ICI's Higee distillation unit

If distillation columns contain large inventories of hazardous materials or the materials are particularly liable to leak from pump glands, then the reflux and bottoms pumps should be fitted with emergency isolation valves.

If flammable materials are handled in vacuum stills, the vacuum should be broken with nitrogen, not air.

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ADAPTIVE MULTIVARIABLE CONTROL APPLIED TO A BINARY DISTILLATION COLUMN

L. Bárcenas-Uribe , J. Alvarez-Gallegos , in Adaptive Systems in Control and Signal Processing 1983, 1984

INTRODUCTION

Control of distillation columns has been a popular topic for years. Interest in the topic has intensified in the lasts years for reasons that include increased raw material and energy costs, developments in hard ware and the emerging developments in control techniques that uses some kinds of chemical processes as a benchmark plants to evaluate the performance of the new algorithms. This is the case of Adaptive Control Schemes. Distillation columns are - suitable plants to demonstrate the feasibility of adaptive control - techniques. However, applications of adaptive control algorithms to distillation columns are not very common (see as example, LIEUSON and Coworkers (1980), SASTRY and Colleagues (1977), UNBENHAVEN and - SCHMID (1979), VOGEL and EDGARD (1982) and WIEMER and others (1983)).

This paper describes the results of simulation tests made to evaluate the performance of the discrete multivariable adaptive control algorithms - due to GOODWIN, RAMADGE and CAINES - (1980) when it is applied to a nonlinear process. Apparently, the present paper is the first application of the GOODWIN and Colleagues (1980) adaptive multivariable control to a - distillation column and appears that this performance scheme may be better than some previous similar studies in a deterministic ambient.

In this approach both top and bottom compositions control of a simulated - binary distillation column is performed. The nonlinear, time varying parameter model of one pilot distillation column due to ESPAÑA, (1976) is used like a "Process". It has been showed that it acts like the true pilot - plant in a very wide operation conditions.

The two control algorithms used-monovariable or SISO and Multivariable or MIMO- drive both output tracking and regulation. In order to obtain a - feasible application to the process - we have kept the inputs and outputs within the space constrained by the physical limits by doing a slightly modification on the algorithm. On - the other hand, the usual large initial transients are avoided by - doing first an offline process identification and using these identified parameters to initialize the control algorithms. This offline identification is described in BARCENAS, (1983) and BARCENAS and ALVAREZ, (1983). The next section deals with the process description, then it is described the - simulation study done to verify the performance of the algorithm.

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Enhanced distillation types

A. Kayode Coker , in Ludwig's Applied Process Design for Chemical and Petrochemical Plants (Fourth Edition), Volume 2, 2010

A Case Study: Reactive Distillation of Methyl Acetate

A reactive distillation column having 15 stages with a total condenser and reboiler is to be designed for the production of methyl acetate, with mass flow rate of 45 kgmole/h. A reflux ratio of 5 mol to 1 of the product is to be used. The feed enters at a temperature of 75°C and a pressure of 101.3 kPa at the 10 th stage. The overhead product rate coming out of the column is 20 kg mole/h. The pressures at the condenser and reboiler are 90 kPa and 97 kPa respectively. Simulate:

1.

the reactive distillation column, assuming no pressure drop across the reboiler and condenser. Determine the mole fractions of components in both the distillate and the bottoms, and also the compositions, temperature and pressure profiles throughout the column.

2.

using the reaction mechanism, the stoichiometric coefficients and the sparse continuation solver in Honeywell UniSim® simulation software, determine the mole fractions in the distillate and bottoms, when the chemical reaction occurs on stages 5–10 of the column.

The mole fractions of the components in the feed are:

Methanol 0.4
Acetic acid 0.4
Methyl acetate 0.1
Water 0.1

Operating conditions are:

Feed stream conditions

Stream Name Feed
Temperature 75°C (165°F)
Pressure 101.3 kPa (14.7 psia)
Mass flow 45 kgmole/h (100 lbmole/hr)
Composition Mole fraction
Methanol 0.4
Acetic acid 0.4
Methyl acetate 0.1
Water 0.1

Add a Distillation Column with the following connections:

Column name Reactive distillation
Number of stages 15
Feed Feed, stage 10
Condenser Type Total
Overhead liquid Distillate
Bottoms liquid Bottoms
Condenser Energy Cond Q
Reboiler Energy Reb Q
Pressure
Delta P, Condenser and Reboiler 0 kPa (0 psi)
Condenser 90 kPa (13 psia)
Reboiler 97 kPa (14 psia)
Specification
Reflux ratio 5
Distillate rate 20 kg mole/hr (44 lb mole/h)

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Advances in Chemical Heat Pumps and Heat Transformers

P. LE GOFF , ... R. RIVERO , in Heat Pumps, 1990

Cells in Thermal Parallel and Material Series Arrangement: The Quasi-Isothermal non-Isobaric Distillation

Conventional fractional distillation columns are quasi-isobaric and strongly non-isothermal. For instance, distillation of the mixture ammonia-water at 9 bar to obtain almost pure separated products requires a temperature of 190 °C at the reboiler and 20 °C at the condenser in a column with 20 stages. The average temperature loss in each stage is (190-20)/20 = 8.5 °C. It will be assumed the possibility of turning each stage 90° to achieve a thermal parallel arrangement instead of the conventional series arrangement ( Fig. 7), each stage being heated on one side at 28.5 °C and cooled on the other side at 20 °C. The system can no longer be isobaric so it will be called Quasi-Isothermal Non-Isobaric Distillation.

Figure 7. A separator with cells in thermal parallel and material series arrangement: quasi-isothermal non-isobaric distillation.

One possible realization technique for the evaporation-condensation effects is a system made of finned vertical tubes with external evaporation-condensation falling films.

The lower section of the system is heated by warm water at 20-40 °C and the upper section is cooled by a cold fluid, e.g. the wind at 10 °C. The mixture to be separated contains 50 % of ammonia. The rich phase has 72 % and the poor phase 38 %. Working pressures for the different effects are 0.66,0.84, 1.02 and 1.30 bar respectively.

A heat transformer made with this operation principle is shown in Fig. 8. It is designed to produce heat at 140 °C from sources at 30 and 10 °C. The separator is composed by 13 effects, e.g. a group of 13 finned tubes. Feed is a 93 % mixture (in water) at 40 °C. At one end it produces almost pure water and at the other end pure ammonia. Mixing is made under a pressure of 9 bar and it conducts to the point E at 145 °C. A counter-current heat exchanger allows water and ammonia preheating from 30 to 135 °C, by cooling the mixture from 145 to 40 °C. A heat exchanger obtaining useful heat, Qu, at 140 °C is located within the mixer which operates at 145 °C. In practice, this exchanger is formed by a column containing a tubular coil in which the working fluid circulates and on which pure water falls in a film, absorbing the vapor ammonia supplied to the column.

Figure 8. A multiple-effect heat transformer, operating with the pair HN3-H2O, for the production of heat at 140 °C from a heat source at 30 °C and a cold source at 10 °C.

The enthalpy efficiency is Qu/Qsep = 3.4 % and the exergy effectiveness is Quθu/Qsepθsep = 16.2 % with To = 283 K. However, these parameters have little significance in this case. It is better to speak in economic terms: the economic yield is the ratio of sale price for steam at 140 °C to buying price of heat at 30 °C and coldness at 10 °C. These last values depend essentially on the investment amortization in recovery devices of those low value thermal sources. It can be estimated that this yield is extremely high in all cases where such heat sources are available at low prices.

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