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54 Energy Engineering Vol. 107, No. 4 2010

Potential of Process EnergyOptimization (PEO) on aSoda Ash Production Plant

T. de Lange and Prof. L.J. Grobler

Energy Cybernetics

Potchefstroom, South Africa

ABSTRACT

An energy audit was conducted on a soda ash plant in Botswana,and it was found that the performance of the plant could be improved.Further investigation revealed that the performance of the carbon diox-ide (CO2) production section was heavily inuenced by control problemsexperienced at the two coal-ring boilers. The CO2 production sectionconsumes energy in the form of steam and electricity and utilizes a sol-vent mono-ethanol-amine (MEA) to extract CO2from boiler ue gasses. Recommendations were made to repair and re-commission theboiler control system and to install an energy optimization system. An-nual savings that could be achieved by optimal control of steam from theboilers to the turbine, CO2plant, and processing plant (thereby control-ling the interactions between these sections) amounted to BWP 2, 6 mil-

lion. This constitutes 10% of the total annual coal bill. A total project life cycle of only 5 years was used, and the escalat-ing savings due to ination was disregarded. The IRR of the project wascalculated to be 114%. This article covers the results of the energy audit. It also discussesthe proposed solution and the savings potential that was identied re-garding the CO2plant.

INTRODUCTION

Soda ash is a chemical used in the manufacturing of glass anddetergents. It is also used in metallurgical applications and chemical


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manufacturing. The soda ash plant consists of a few processes or sec-

tions. This article will only focus on the process and operation of theCO2production section.

Description of the CO2 Production Process

CO2 in ue gas from the two coal-red boilers is extracted to beused in the carbonation of Sodium Carbonate (Na2CO3). In this exother-mic chemical reaction Sodium Bicarbonate (2NaHCO3) is formed. Figure 1 shows a block diagram of the CO2 plant. Flue gas fromthe boilers is scrubbed, using a soda ash-rich brine solution to remove

the sulphur dioxide (SO2) from the gas. SO2 is removed from ue gassince it reacts with and contaminates the MEA solution used in thesubsequent purication process. The scrubbed ue gas is then passed through a packed CO2absorp-tion column, where it contacts the MEA solution. In Heat Exchanger 1 (HE1) the CO2-rich MEA solution is preheatedbefore it is pumped to the top of the CO2 stripping column. CO2 isstripped by boiling the solution, using low pressure (LP) steam in the

two kettle-type re-boilers. Gaseous CO2 is compressed and pumped tothe carbonation section. The main energy consumers of this section are the re-boilers thatuse LP steam to heat the MEA. Some electrical energy is used for pump-ing and various other purposes; however, this article focuses on thesteam consumption of this section.

Purpose of the Audit

The purpose of the energy management (EM) and Process EnergyOptimization (PEO) study was to maximize production while minimiz-ing production costs and resources. This had to be accomplished withoutviolating process or equipment constraints. The audit revealed that plantperformance could be improved and that the CO2 production sectionwas consuming large amounts of energy.

CO2PRODUCTIONSECTION PERFORMANCE AND OPERATION

The rst step in the evaluation of the CO2production section wasthe analysis of this sections performance over a period of three months.


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Figu

re1.

Blockdiagram

ofCO2productionsection


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57

CO2Production Section Performance

The performance of the CO2production section can be seen in Fig-ure 2. Steam energy consumed at the re-boilers is shown as a functionof CO2production. The trend line in Figure 2 indicates the average performance of theCO2plant. As expected, the CO2production increases with an increase inenergy consumption. There is, however, a large scatter around the aver-age performance line. The base load is high, indicating that, although noCO2 is produced, the system consumes a lot of energy. It can thereforebe concluded that process or energy improvement is possible.

In Figure 2, Point A indicates that on one day 3,400 GJ of steamwas used to produce 56,550 m3 of CO2. On another day only 2,350 GJof steam energy was consumed (see Point B) to produce approximatelythe same amount of CO2. This constitutes a 30% difference in steamconsumption for production of the same amount of CO2. Possible causes of the large scatter or variance in performance aswell as the large base load were explored. From this it was found thatthe energy consumed by the CO2production section was a function of

the following [1]:

The amount of ue gas used to produce CO2 and the percentageCO2 it contained

Percentage MEA in solution The rate at which the MEA solution was pumped through the CO2

production section Heat exchanged at the re-boilers, depending on the amount of

steam to the re-boilers

Current Operation Methodology

Some difculty was experienced in controlling the coal-red boilersand cutting back on steam production when the soda ash plant was notat full production. According to plant personnel, the CO2re-boilers wereused as a heat sink at times when too much steam was produced. It was thought that the excess heat dumped at the re-boilerswould ultimately be dissipated through the two heat exchangers (HE 1and HE 2 in Figure 1). The excess heat would eventually heat up brinethat was pumped back to the ponds, thereby improving evaporationrates. In addition to heat being dumped at the CO2 re-boilers, another


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Figure2.

CO2productionsectionperformance


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59

decision was made by plant personnel to operate the CO2 plant at a

lower percentage of MEA in solution. This was done to save on MEAcosts.

Effect of Current Operation Methodology on the CO2Plant

When less MEA is used to extract CO2 from ue gasses, the rateat which the rich MEA solution is pumped from the CO2 absorptioncolumn to the CO2 stripping column, and the rate at which the leanMEA solution is pumped from the CO2 stripping column to the CO2absorption column should increase to produce the same amount of CO2.

These two rates should be equal, since both rates are controlled withthe levels of the two columns. If the rates (or volume per unit of time)of pumping the solution around increases, the amount of energy usedto boil the liquid increases. In Figure 3 the original MEA cost (1) and energy cost (2) lines areshown. The total cost line is line (3), and the optimum operation pointis at A. With the current operating methodology the following will occur:

Dumping heat will increase the energy cost; therefore, the total costwill rise (line 5). Optimum operation will now be at point B.

Lower percentage MEA in solution will cause the actual operatingpoint to move to point C.

Therefore, when energy is being dumped the percentage of MEAin solution should be increased to operate at optimum operation levels

(B), not decreased (C). The total cost will have increased with the shift(8) shown in Figure 3. Determining how much energy can be saved and the monetaryvalue of the savings was the next step in our analysis.

ENERGY SAVINGS OPPORTUNITY

The cumulative sum of differences (CUSUM) analysis is a tool usedby the audit team to monitor and track the performance of the CO2plant. Applying the CUSUM analysis revealed that periods of goodoperation and other periods of poor operation occurred, as can beseen from Figure 4.


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Figure3.M

EAandEnergycostvs.percentageMEAinsolution


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The dashed line in Figure 4 indicates periods of poor operation

while the dotted line represents periods of good operation. The solidline is a period of average operation. Figure 4 shows a long period of poor operation, after whichsomething changed and the operation improved for a few days. Op-eration alternated between modes of good, average, and poorthroughout the measured period. Energy could have been saved if the plant were primarily operatedin a mode of good operation, as had already happened some of thetime.

The amount of energy that could have been saved was calculatedby subtracting the performance line during the good performance pe-riods from the performance line during all periods. The savings resultingfrom changing average operation to good operation can be seen inFigure 5. Average performance of the CO2 production section during allmodes of operation is shown in the solid line. Figure 5 also shows theperformance line during the good operation periods in the dotted line.

Steam could be saved if the plant were operated at good operationlevels most of the time. Savings were calculated in terms of the coalrequired to generate that amount of steam. Table 1 shows the coal bill for the calculated base year(BWP 25,734,143). The annual savings calculated on this base year coalcost was BWP 2,560,795. If a total project life cycle of only 5 years is used and escalatingsavings due to ination is disregarded (a very conservative view), the

IRR of the project is calculated to be 114% with a payback period of 1.15years. Apart from these savings, dumping energy at the re-boilers affectsthe production process in various ways. These effects will be discussedin detail in the next section.

EFFECTS OF DUMPING ENERGY AT THE REBOILERS

Dumping of excess steam at the re-boilers directly affects the opera-tion/performance of the CO2plant in the following manner:

Temperature T1 (shown in Figure 1) will increase if not all of the


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Figure4.

CUSUMC

hart

ofCO2productionsection


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63

Figure5.

CO2

Production

vs.Steam

Energy


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Table 1. Estimated value of savings opportunity

excess heat is dissipated through HE 1&2. This in turn will causeless CO

2to be absorbed at the CO

2absorption column and a drop

in relative yield of CO2. Therefore, it indirectly affects the down-stream carbonation process.

The temperature in the CO2 stripping column will increase and alarger volume of gas will be boiled off, containing impurities. Theseimpurities will be compressed along with the CO2and pumped tothe carbonation section.

The rst step in solving these problems is the repair and re-com-missioning of the boiler control system. This will enable plant operatorsto cut back on steam production when the need arises. Process Energy Optimization (PEO) will enable operators toimprove the operational efciency of the CO2 production section. ThePEO solution can be deployed by installing a decision support system(DSS). The effect of a DSS on the performance scatter shown in Figure 2will be to narrow the scatter around the average performance line. It will

also enable operators to shift the performance from average performanceto good operation levels.

THE DECISION SUPPORT SYSTEM (DSS)

In essence the DSS will rstly aim to assist the operators of theplant in anticipating the steam demand of the plant. This demand should

be met in the most cost effective manner. The DSS will structure thedemand as follows:

Steam demand to the calcining section will be prioritized.


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Secondary to the steam supply to the calcining section will be that

of the turbine for generation purposes, and the supply to the MEAre-boilers for CO2production purposes.

The remaining amount of steam will be made available to othersteam users such as the MEA re-claimer and salt production sec-tion.

As far as possible the existing infrastructure (i.e. sensors and equip-ment) will be used and the introduction of new equipment (if any) kept

minimal. Some production-related problems and bottlenecks can be elimi-nated by the energy performance measurements and tracking system.This information must be made available to operators in real time tofacilitate their meeting production targets. The system will function as a DSS; thus, human intervention willnot be excluded from the control loop. This strategy will eliminate risk,and once the performance of the system has been veried the option to

close the control loop can be considered.

CONCLUSION

The Energy Audit conducted on the soda ash plant showed thatthe performance of the CO2plant could be improved. Dumping energy as a result of limited boiler control had an adverse

effect on the production process. It can be concluded that these problems are symptoms of controlproblems experienced at the boilers. Therefore, an integrated solutionwas proposed. The PEO solution aims to optimize and maintain theboilers, as well as the production section, at optimum levels. Annual energy savings for the whole project was calculated to beBWP 2, 6 million. This investment could be reclaimed in 1.15 years.

References

[1] Grobler, Prof. L.J., Bosman, I.E., Molefhi, B., De Lange, T. & Kaliswayo, L., Ener-

gy Audit Report, 2006.


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ABOUT THE AUTHORS

Tanya de Lange

holds a degree in chemical engineering from theNorth-West University, South Africa. She is an employee of the North-West University and Energy Cybernetics, where she focuses on theenergy engineering eld. She is a member of the Southern African Asso-ciation for Energy Efciency (SAEE). Tanya is a certied energy manager(CEM) as well as a certied measurement and verication professional(CMVP).

L.J. Grobleris a professor at the School of Mechanical Engineering,

North-West University, South Africa. He specializes in energy manage-ment and process energy optimization. He is certied by the Associationof Energy Engineers of the USA as a certied energy manager (CEM)and as a certied measurement and verication professional (CMVP).

Both can be reached at Energy Cybernetics, PostNet Suite 148,Private Bag X1277, Potchefstroom, 2520, South Africa.


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