What Principle is Used in the Continuous t e Ab Sorption Refrigerator

Theoretical and Actual Cycle Analyses

Meherwan P. Boyce , in Gas Turbine Engineering Handbook (Fourth Edition), 2012

Absorption Cooling Systems

Absorption systems typically employ lithium–bromide (Li–Br) and water, with the Li–Br being the absorber and the water acting as the refrigerant. Such systems can cool the inlet air to 50°F (10°C). Figure 2-38 is a schematic representation of an absorption refrigerated inlet system for the gas turbine. The cooling shown on the psychometric chart is identical to the one for the mechanical system. The heat for the absorption chiller can be provided by gas, steam, or gas turbine exhaust. Absorption systems can be designed to be either single or double effect. A single effect system will have a coefficient of performance (COP) of 0.7–0.9 and a double effect unit will have a COP of 1.15. Part load performance of absorption systems is relatively good and adiabatic thermal efficiency does not drop off at part load like it does with mechanical refrigeration systems. The costs of these systems are much higher than the evaporative cooling system; however, refrigerated inlet cooling systems in hot humid climates are more effective due to the very high humidity.

Figure 2-38. Absorption refrigerated inlet cooling system.

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The Refrigeration Cycle

G.F. Hundy , ... T.C. Welch , in Refrigeration, Air Conditioning and Heat Pumps (Fifth Edition), 2016

2.6 Heat powered cycles

2.6.1 Absorption Cycle

The principle of the absorption cycle is given in Fig. 2.12. The refrigerant flowing through the condenser, expansion valve and evaporator performs in just the same way as in the vapour compression cycle (Fig. 2.3). The difference is that the compressor is replaced by a thermal compressor. The refrigerant leaving the evaporator is absorbed by a liquid absorbant; the strength of the solution increasing as the liquid absorbant passes through the absorber. This solution is then pumped up to condenser pressure and the vapour is driven off in the generator by direct heating. The high-pressure refrigerant, now a gas, can then be condensed in the usual way and passed back through the expansion valve into the evaporator. The weak solution from the generator is passed through another pressure-reducing valve and back to the absorber.

Figure 2.12. Absorption cycle – basic circuit.

Refrigerant compression is replaced by a thermal compressor.

There are two main refrigerants in use, ammonia with water absorbant and water with lithium bromide absorbant. The water/lithium bromide systems are suited to air-conditioning water chiller temperatures, whereas ammonia systems are suited for evaporating temperatures below 0°C. With water as a refrigerant the whole system operates well below atmospheric pressure. Ingress of air due to leakages can cause problems.

The diagram in Fig. 2.12 is illustrative only. The low-pressure vessels (evaporator and absorber) and the high-pressure vessels (condenser and generator) are commonly combined within one shell each. Dual effect systems can be used. Overall thermal efficiency may also be improved by a heat exchanger between the two solution paths and a suction-to-liquid heat exchanger for the refrigerant.

Absorption system can be used to advantage where:

•

a CHP unit has spare heat available.

•

a low-cost supply of waste heat is available.

•

heat from landfill gas or geothermal can be used.

•

electrical load limits apply at the site.

•

low noise and/or vibration are major considerations.

•

solar energy can be harnessed.

Because of the need for additional processes, an absorption systems can be significantly more costly than a vapour compression alternative.

The energy input to an absorption cycle is higher than for a compression cycle, but the main driving energy is in the form of low-grade heat, not electricity. Typical figures are shown in Table 2.2. The ideal COP can be shown to be

Table 2.2. Energy per 100 kW cooling capacity at 3°C evaporation, 42°C condensation

	Absorption	Vapour compression
Load	100.0	100.0
Pump/compressor (electricity)	0.1	30.0
Low-grade heat	165	–
Heat rejected	265.1	130.0

$\text{COP}=\frac{\frac{1}{\text{Ta}}-\frac{1}{Tg}}{\frac{1}{\text{Te}}-\frac{1}{\text{Tc}}}$

If the absorber and condenser temperatures are the same,

$\text{COP}=\frac{\text{Te}(\text{Tg}-\text{Ta})}{\text{Tg}(\text{Ta}-\text{Te})}$

Where Te, Tg, Tc and Ta are the evaporating, generator, condenser and absorber respective temperatures. The ideal absorption COP is 1.4 for the conditions in Example 2.1, with Tg = 100°C (steam heat source) and Ta = 25°C. Compare this with 6.7 for a Carnot COP, and 5.4 for an ideal vapour compression process with R134a. An analysis of the ideal cycle is given by Tozer and James (1997).

COP enhancement can be achieved by using higher heat supply temperature, higher refrigerant evaporating temperature, lower heat rejection temperature. A typical COP for an air-conditioning absorption cycle in would be about 0.7, compared to approximately 3.5 for a vapour compression system. With double and triple effect generators COP can be between 1.2 and 1.7.

2.6.2 Adsorption Cycle

Adsorbents such as active carbons, zeolites or silica gels can adsorb large quantities (c. 30% by weight) of many gases within their micropores. The most widely used combinations are active carbons with ammonia or methanol, and zeolites with water, but the choice of which adsorbent and which refrigerant to use depends on the application. The quantity of refrigerant adsorbed depends on the temperature of the adsorbent and the system pressure. Heat is required to drive out the refrigerant in desorption and heat is generated during adsorption.

A basic adsorption cycle is illustrated in Fig. 2.13. Initially (a) the whole assembly is at low pressure and temperature, the adsorbent contains a large concentration of refrigerant within it and the other vessel contains refrigerant gas. The adsorbent vessel (generator) is then heated, driving out the refrigerant and raising the system pressure. The desorbed refrigerant condenses as a liquid in the second vessel, rejecting heat (b). Then the generator is cooled back towards ambient temperature, re-adsorbing the refrigerant and reducing the pressure (c). The reduced pressure above the liquid in the second vessel causes it to boil, absorbing heat and producing the refrigeration effect (d). The cycle is discontinuous since useful cooling only occurs for one half of the cycle. Two such systems can be operated out of phase to provide continuous cooling and heat rejected by one sorption bed can be used to pre-heat the other, thereby improving efficiency. Adsorption technology is also being developed for hybrid heat pumps, see chapter: Heat Pumps and Integrated Systems. A review of thermally driven heat pumps is given by Kühn (2013).

Figure 2.13. Adsorption cycle – idealised process (a) pressure raised, (b) refrigerant de-sorbed and moved to condenser, (c) pressure lowered, (d) refrigerant boiled in evaporator and adsorbed (University of Warwick).

2.6.3 Desiccant Cooling

Desiccant cooling is an open cycle which uses a desiccant wheel and thermal wheel to achieve both cooling and dehumidification. Solar thermal energy or waste heat can be used to re-activate the desiccant. The thermal wheel is a rotary heat exchanger positioned within the supply and exhaust air streams in order to recover the heat energy, and the desiccant wheel works in a similar way, additionally re-activating the desiccant. The principle of the cycle is shown in Fig. 2.14. There are a number of variations depending on the condition of ambient outdoor air, air change requirements and humidity control requirements. It offers high efficiency but requires large air flows and is limited in operational range, but it can be applied in conjunction with a conventional vapour compression cycle. Referring to Fig. 2.14, the outdoor air moisture content is reduced and its temperature increased as it passes through the desiccant wheel. It is sensibly cooled through the thermal wheel, further cooled to the required supply temperature, with some moisture gain as it passes through an evaporative cooler. The return air from indoors, at, say 25°C is passed through an evaporative cooler so that it enters the thermal wheel at a lower temperature and higher moisture content. As it passes through the thermal wheel, it is sensibly heated and further heated by the heater so that it can re-activate the desiccant before exhausting. The by-pass can be controlled so that unnecessary heat is not applied. The thermal wheel is illustrated in Fig. 25.1. More detail on desiccant cooling systems can be found in CIBSE Guide B3 (2016).

Figure 2.14. Desiccant cooling – principle of operation.

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Water and wastewater treatment: biological processes

B. Sizirici Yildiz , in Metropolitan Sustainability, 2012

Soil absorption systems

The typical absorption system is a series of gravel-filled trenches preceded by a septic tank, in which organic material is consumed by anaerobic bacteria. Gravity causes effluent from the septic tank to flow into trenches, often referred to as leach lines or drain lines. The lines together are often referred to as a tile field. The system design is based on the settling of suspended solids and the biodegradation of organic materials in the septic tank, and then the percolation of wastewater through the soil profile. Soil absorption systems have been adopted by individual homes and small communities ( Gohil, 2000; WEF, 2003).

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28th European Symposium on Computer Aided Process Engineering

Catarina G. Braz , ... R. Alvim , in Computer Aided Chemical Engineering, 2018

3 Results and discussion

The absorption system presented in Figure 1 was simulated using data supplied by the licensor of an industrial unit for formaldehyde production as input values of the model. The temperature and the formaldehyde mass percentage profiles are presented in Figure 3. Although these profiles were not yet compared with the real plant data, the simulation results are in agreement with what would be expected.

Figure 3. Temperature and formaldehyde mass composition profiles of the absorption system.

Table 1 compares the simulation results with the typical values from the industrial unit and presents the absolute relative deviation between them, calculated by equation (15). Note that the T_liq from the bottom of column T-2 is not the temperature of the liquid after the first tray, but the temperature after mixing with the liquid coming from the top packing of column T-1. The simulation results are in general within the typical ranges of the plant operation values, presenting absolute relative deviations below 6.5%.

Table 1. Simulation results compared with the typical operational values.

	Typical operational values	Simulation	ε rel (%)
W% bottom of T-1	50-58	51.9	5.8
T liq bottom of T-2 (°C)	52-56	51.9	3.9
T gas top of T-2 (°C)	22-38	33.2	6.4
% of Recovered HCHO	98-100	100	1

(15) ${\epsilon }_{\mathit{rel}}=\frac{\left|x_{\mathit{operational}}-x_{\mathit{simulation}}\right|}{x_{\mathit{operational}}}\times 100$

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Absorption capture systems

Stephen A. Rackley , in Carbon Capture and Storage (Second Edition), 2017

Enzyme-catalyzed chemical absorption

In aqueous absorption systems, the hydration of dissolved CO ₂ to form bicarbonate ions ( ${\mathrm{HCO}}_3^{-}$ ) according to Reaction (6.15):

(6.15) ${\mathrm{CO}}_2+{\mathrm{H}}_2\mathrm{O}\leftrightarrow {\mathrm{HCO}}_3^{-}+{\mathrm{H}}^{+}$

is commonly either a rate limiting step (e.g., in the case of K₂CO₃) or a secondary absorption mechanism that makes minimal contribution to the overall CO₂ loading of the solvent solution (e.g., for amine systems). The use of the enzyme CA as a catalyst to address these limitations has been widely investigated since it was first proposed in 1961.

Carbonic anhydrases are a broad class of enzymes found in all living organisms—from microbes to man—that catalyze the hydration of CO₂ (see Section 10.1.3). The catalyst has the largest effect if it is dissolved in the solvent solution, typically at <1   g/l, with a 12-fold rate improvement observed in laboratory tests with K₂CO₃ solvent. Since the enzyme is susceptible to process conditions such as high temperatures and pH, it can be protected from these extremes by immobilization in microparticles, applied as a coating to the absorber tower packing, or in membranes.

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Mass transfer applications of nanofluids

Bharat Bhanvase , Divya Barai , in Nanofluids for Heat and Mass Transfer, 2021

11.2.3 Liquid-liquid interphase mass transfer in nanofluids

Similar to absorption systems, extraction also depends upon the interfacial area of the two contacting liquids for which the main parameters are droplet size distribution, mean droplet diameter, and dispersed phase holdup, which can be improved by using several techniques like imposing pulsing motion or agitation and addition of surface-active agents ( Amani et al., 2018). The droplet Sauter mean diameter (d ₃₂) given by Eq. (11.35) is a measure often used to estimate the magnitude of stability of a drop (Raji-Asadabadi et al., 2013).

(11.35) $d_{32}=\frac{{\displaystyle {\sum }_n^{i=1}}{n}_i{d}_i^3}{{\displaystyle {\sum }_n^{i=1}}{n}_i{d}_i^2}$

where n _i is the number of drops having diameter d _i . Also, the dispersed phase holdup (x _d ) is given by Eq. (11.36).

(11.36) $x_d=\frac{V_d}{V_d+V_c}$

Both of these are useful in calculating the interfacial area (a) by Eq. (11.37).

(11.37) $a=\frac{6x_d}{d_{32}}$

Empirical correlations for determining d ₃₂ for toluene/water system incorporating silica nanoparticles in a horizontal mixer settler have been derived as given in Eq. (11.38) (Raji-Asadabadi et al., 2013), where D′ is the agitator impeller diameter, φ _f is the feed holdup, ω is the weight fraction of the silica nanoparticles in the system, and We is the Weber number (Eq. 11.39).

(11.38) $\frac{d_{32}}{D^{\prime }}=0.054\left(1+0.824{\phi }_f\right){\left(1+\omega \right)}^{-2.115}W{e}^{-0.54}$

(11.39) $We=\frac{{\rho }_c{N}^2{D^{\prime }}^3}{\sigma }$

In the above equation, ρ _c is the continuous phase density, N is the impeller agitation speed, and σ is the interfacial tension.

Generally, mass balance on a droplet dispersed in the continuous phase in an extraction process is given by Eq. (11.40) (Amani et al., 2018).

(11.40) $-k_{od}\left(C-C^{\ast }\right)4\pi r_d{}^2=\frac{4}{3}\pi r_d{}^3\frac{dc}{dt}$

where k _od is the overall mass transfer coefficient of the dispersed phase, r _d is the droplet radius, and C and C ^∗ are the solute concentrations in output of dispersed phase and in equilibrium with continuous phase, respectively. Integrating this equation and rearranging give Eq. (11.41).

(11.41) $-k_{od}=-\frac{d}{6t}ln\left(1-\frac{C_0-C}{C_0-C^{\ast }}\right)=-\frac{d}{6t}ln\left(\frac{C-C^{\ast }}{C_0-C^{\ast }}\right)$

Here, C ₀ is the solute concentration in droplet before contact. Thus, k _od can be given by Eq. (11.42) (Amani et al., 2018; Ashrafmansouri and Nasr Esfahany, 2015), assuming a complete mixing of continuous phase, where E is the extraction fraction estimated by Eq. (11.43).

(11.42) $k_{od}=\left(-\frac{d}{6t}\right)ln\left(1-E\right)$

(11.43) $E=\frac{C_0-C}{C_0-C^{\ast }}$

In a packed column, 0.05   vol.% of SiO₂ nanoparticles (10   nm) enhanced the mass transfer coefficient by 42% (Nematbakhsh and Rahbar-Kelishami, 2015). The estimation for the effective diffusivity (D _eff ) using SiO₂ nanofluids in the water-acetic acid-toluene system as a function of Reynolds number (Re), drop diameter (d), nanoparticles size (n _s ), and nanoparticle concentration in dispersed phase (n _c ) is given in Eq. (11.44).

(11.44) $D_{eff}=8.9\times {10}^{-6}{\mathrm{Re}}^{0.45}\times \frac{d-0.003n_c}{{\left(159.5+n_s\right)}^{0.37}}-1.2\times {10}^{-8}$

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COOLING OF BUILDINGS

R.K. Swartman , in Solar Energy Conversion, 1979

4.1.1 Absorption systems

In an absorption system, thermal energy produces a cooling effect. In general, an evaporating refrigerant is absorbed by an absorbent on the low-pressure side, the absorbed refrigerant is generated by direct thermal energy input on the high pressure side, generated refrigerant is liquified in the condenser, and liquid refrigerant evaporates in the evaporator. In a continuous cycle, the generator and condenser are contained in one reservoir with the evaporator and absorber in another reservoir as in Fig. 4. Refrigeration is accomplished as the liquid refrigerant evaporates, and heat is rejected as the refrigerant liquifies in the condenser. The performance of the system depends on the temperatures in the generator, absorber, condenser and evaporator, and the capacity depends on the generator and cooling water temperatures.

Fig. 4. Absorption cooling system

The behaviour of absorption refrigerators depends on the thermodynamic characteristics of the refrigerant/absorbent mixture. There is a threshold value of generator temperature which must be exceeded if the machine is to function. If the evaporator temperature is specified for a particular application, this determines the pressure in the evaporator and absorber. When the temperature at which heat is rejected is specified, then this temperature and the absorber pressure determine the concentration of the refrigerant in the absorber. Specifying the heat-rejection temperature also determines the pressure in the condenser and generator. For a given pressure, there is a unique relationship between temperature and refrigerant concentration in the generator. The system cannot function, however, unless the concentration of refrigerant is lower in the generator than in the absorber. This determines the lower limit for the generator temperature above which operation is possible.

The cooling effect is approximately equal to the enthalpy of evaporation at the evaporator temperature and the heat supplied is approximately equal to the enthalpy of evaporation at the generator temperature. The ratio of the cooling effect to the heat supplied is the coefficient of performance (COP). For an ammonia/water system just above the threshold temperature, the COP is about 0.7, falling to about 0.6 when the generator temperature is at 130°C.

The water/lithium bromide system operates at around 10°C, so is suitable for air conditioning. Most machines using LiBr/H₂0 have a water-cooled absorber and condenser, which in turn requires a cooling tower. The pressure differences between the high and low pressure sides are low enough that these systems can use a vapour-lift pump and gravity return from absorber to generator, instead of a mechanical pump. The COP of LiBr/H₂0 systems is usually in the range of 0.6 to 0.8. If water is used to cool the absorber and condenser, the generator temperature is in the range of 75 to 95°C. Variations in the generator temperature with changes in solar flux vary the capacity of the cooler. The operating temperatures required of the solar collector because of the high generator temperatures make flat plate collectors marginal in this application. For this reason, focusing solar collectors are being considered for many solar cooling projects.

The ammonia-water system is similar to the water/lithium bromide system except that a rectifying section must be added to the top of the generator to remove water vapour from the ammonia vapour going to the condenser. The pressures and pressure differences of the NH₃/H₂0 system are much higher, so mechanical pumps are required to return solutions from the absorber to the generator. The condenser and absorber are often air cooled with generator temperatures in the range of 125 to 170°C. When water cooling is used, generator temperatures are in the range of 95 to 120°C. The generator temperatures are usually too high for flat-plate solar collectors. Again, focusing solar collectors are a possibility but are not yet well developed. Most work has been directed at development of cycles using higher concentrations of NH₃ to lower the generator temperatures.

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Drug delivery

Amit K. Nayak , ... Mohammad S. Hasnain , in Applications of Nanocomposite Materials in Drug Delivery, 2018

12.3.3 Multiporous oral drug absorption system (Elan Corporation, Ireland)

Multiporous oral drug absorption system (MODAS) is surrounded by a nondisintegrating, timed-release coating, which after coming in contact with GI fluid is transformed into a semi-permeable membrane through which the drug diffuses in a rate-limiting manner [25]. The tablet contains a core of active drug plus excipients. This is then coated with a solution of insoluble polymers and soluble excipients. When the drug is ingested, the fluid present in GI tract dissolves the excipients which are soluble and leaves behind polymers, which are insoluble, forming a mesh like network acting as a passage for GI fluid to the interior of drug which is water soluble. It thereby dissolves the drug and resulting solution diffuse out in a controlled way. Addition of excipients like buffers can produce a micro environment within the tablet, which facilitates more expected rate of absorption and its release. Examples of MODAS products developed by Elan include Bron-12 [a 12   h multi component over-the-counter (OTC) cough and cold product] and once-daily potassium chloride [25].

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Exergy analyses of absorption cooling systems

Ibrahim Dincer , Marc A. Rosen , in Exergy (Third Edition), 2021

8.3.2 Double effect absorption cooling system (DEACS)

The double effect absorption system ( Fig. 8.2) can be viewed as an enhanced version of the single effect absorption cooling system. The double effect cooling system consists of two generators, two heat exchangers, a condenser heat exchanger, a condenser, an evaporator, an absorber, and a pump.

Fig. 8.2. Schematic of a double effect absorption cooling system (DEACS).

In the DEACS in Fig. 8.2, heat is provided to the high-temperature generator from an external energy source, raising the temperature and vaporizing the strong solution entering at state 4. The strong solution exits as a concentrated ammonia-water vapor (99.99% ammonia) at state 5 and as a weak solution at state 12. The weak solution passes through a high-temperature heat exchanger (HHX) and transfers heat to the incoming strong solution at state 20. After releasing heat in HHX, the weak solution leaves at state 13 to combine with the weak solution from the low-temperature generator (LTG) at state 23, yielding a weak solution at state 14. That weak solution enters the low temperature heat exchanger to release heat to the incoming strong solution at state 19 and leaves at state 15. This weak solution at state 15 passes through the expansion valve, where its pressure decreases and enters the absorber at state 16. The ammonia-water vapor at state 5 enters the low-temperature generator where it acts as a heat source for the strong solution entering the LTG at state 22. That strong solution is heated to an ammonia-water vapor at state 7 and is transferred to the condenser. The ammonia-water vapor at state 5, after transferring heat, leaves the LTG at state 6 and enters the condenser heat exchanger (CHX). The condenser heat exchanger reduces the temperature of the ammonia-water vapor entering at state 6 and increases the temperature of the strong solution from the solution pump at state 17. After transferring heat in the condenser heat exchanger, the ammonia-water vapor exits is conveyed at state 8 to the condenser. The heated strong solution leaves CHX at state 18 and combines with the strong solution coming at state 13, and enters the LTG at state 22 at a relatively higher temperature. The two ammonia-water vapor streams reject heat to the environment in the condenser and leave at state 9. This ammonia-water vapor stream enters the expansion valve, where its pressure and temperature decrease, and enters the evaporator at state 10. In the evaporator, heat is gained from the air-conditioned space and conveyed by the ammonia-water vapor, which leaves at state 11 as an ammonia-water mixture. The ammonia-water streams from states 16 and 11 enter the absorber where they reject heat to the environment exit as an ammonia-water solution which enters the solution pump at state 1.

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Sulfur Dioxide Removal

Arthur L. Kohl , Richard B. Nielsen , in Gas Purification (Fifth Edition), 1997

Absorption Step

Operating data for four absorption systems of plants utilizing the Cominco process are presented in Table 7-25. Observed sulfur dioxide removal efficiencies vary from 85 to 97%. The degree of sulfur dioxide removal attainable in a system of this type is obviously dependent upon a large number of variables. Chief among these are

Table 7-25. Cominco Process Absorber Operating Data

Plant Factors	Trail Lead Sinter Plant Gas	Trail Zinc Roaster Plant Gas	Trail Acid Plant Gas	Olin-Mathieson Acid Plant Gas
Gas volumetric flow, scfm	150,000	20,000 (avg)	50,000–60,000
Feed gas, % SO2	0.75	5.5	1.0	0.9
Tail gas, % SO2	0.10	<0.2	0.08	0.03
SO2 in rich sol., g/l	500	400–550
Gas velocity (superficial), ft/sec	4.0	1.7	2.9
No. of stages in series	3	4	1	2
Packing height per stage, ft.	17	17	25
Approx. circulation rate, gpm:
Stage 1 **	1,200–1,500	450	1,000
Stage 2	1,200–1,500	450		900 ***
Stage 3	600–800	450
Stage 4		450
SO2 removal efficiency, %	85	97	92	97

^* Per unit.

**

Gasfeed to stage 1.

***

For stages 1 and 2 combined.

1.

Height (and type) of packing in each stage

2.

Number of stages

3.

Solution-circulation rate in each stage

4.

Gas-flow rate

5.

Solution composition (with respect to both ammonia and sulfur dioxide) in each stage

6.

Temperature

Wood-slat packing is used in all of the absorbers described in Table 7-25. Packing of the lead-sinter plant and zinc-roaster plant gas-absorbers is described (Ontario, 1947) as 2- by 6-in. boards on edge, 2 in. apart, with each layer arranged at right angles to the one below it. At intervals, 2- by 8-in. boards are used in place of 2- by 6-in. boards so that the alternate layers are about 2 in. apart, permitting lateral flow of gas.

Aqueous ammonia of approximately 30% concentration is used as make-up in the Trail absorbers. Where several absorption stages are used, the fresh ammonia additive is divided so that a portion goes to the circulating stream of each tower to maintain the proper pH for optimum absorption with minimum ammonia loss. The pH values of the solution in the various absorption units range from about 4.1 to 5.4. The low figure represents the richest solution with regard to sulfur dioxide; this circulates in the first stage of the zinc-roaster system and contacts gas containing 5.5% sulfur dioxide. The pH of solutions in the last stages (with respect to gas) of the Trail absorption systems are approximately 5.1 for the lead-sinter plant unit, 5.2 for the zinc-roaster plant unit, and 5.4 for the acid-plant pretreatment tower (single stage).

Temperatures of absorption must be kept as low as possible to minimize ammonia loss and maintain a favorable equilibrium for absorption of sulfur dioxide. The heat of reaction is removed from the sulfur dioxide-absorption units handling smelter gases by passing the circulating streams of solution through aluminum-tube coolers so that the final gas-contact temperature is no higher than about 35°C (95°F). Temperature control is greatly simplified for the absorbers handling acid-plant tail gas, as this gas stream is so dry that evaporation of water to saturate it provides ample cooling if the gas does not contain more than about 1 % sulfur dioxide. A heat balance for a typical case is presented by Burgess (1956) and is based upon the following overall heat of reaction for the absorption of sulfur dioxide in a circulating solution to which 28% ammonia solution is added:

(7-71) $\text{S}{\text{O}}_{2\left(\text{g}\right)}+\text{N}{\text{H}}_{3\left(28\mathrm{\%\; aq}\right)}+{\text{H}}_2{\text{O}}_{\left(\mathrm{l}\right)}=\text{N}{\text{H}}_4\text{H}\text{S}{\text{O}}_{3\left(\text{aq}\right)}\text{Δ}\text{H}=-42,750\text{B}\text{t}\text{u}/\text{l}\text{b mole}$

Simple calculations show that for a gas containing 1% SO₂, the heat generated by reaction 7-71 is approximately equal to the heat required for the evaporation of water into vapor at 25°C (77°F), assuming that equilibrium is attained with regard to water, and that the vapor pressure of water over the solution is about 80% that of pure water.

Burgess (1956) points out the importance of feeding a clean gas into the absorber to minimize ammonia losses. The presence of an acid fog in the gas stream from H₂SO₄ plants can cause formation of an ammonium sulfate aerosol, which is not recovered by the scrubber solution, and can result in a tenfold increase in ammonia losses. Too high a pH in the absorbing solution can also cause a fogging condition due to the formation of ammonium sulfite in the gas stream.

As previously noted, the pH of the solution in the Cominco operation ranges from 4.1 to 5.4 with the lowest value for the most concentrated gas stream. Hein et al. (1955) found that, with a gas containing about 0.3% sulfur dioxide, essentially no absorption took place when the pH was 5.6 or less. Their work was conducted using a 2 ft inside diameter pilot-plant scrubber packed with 2-in. ceramic rings. As would be expected, increasing the pH gave increased SO₂ recovery, but also increased ammonia loss. To attain 80% SO₂ recovery with 8 ft of packed height, for example, a pH of about 6.4 was required, and an ammonia loss of about 5% occurred. The ammonia was found to be recoverable, however, by introducing a second-stage absorber.

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