Robust Design of an Evaporator Station as Applied to the Xinavane Rehabilitation Project

The rehabilitation ol' thc Xinavane sugar factory in Mozambique necessitated a redesign of the evaporator station. The particular arrangement at Xinavane proved to be extremely sensitive to changes i n operating parametcrs. Using this evaporator station as an example, the concept of design robustness (suitability under a range of operating conditions) is introduccd. A rigorous evaporator simulation program was used to optimise heating surface and temperature driving force distribution along the evaporator train, and to model and minimise the potentially considerable effect on evaporator performance of variations in bleed rate. The unique requirements of evaporator station design are discussed, and sorne of the details of the equipment designed for Xinavane are presented. Introduction Recent design work for rcl'urbishmcnt of the Xinavane factory required thc redesign of the evaporator station. Preliminary calculations highlighted the fact that some options for modifications would meet the new requirements for the selected design paramcters, but the performance would deteriorate rapidly if the design parameters were slightly different. This highlighted the need for a robust evaporator design which was not as sensitive to design parameters. Refurbishment of the Xinavane factory Xinavane sugar mill (known previously as Inkomati mill) is located i n southern Mozambique some 110 km from Maputo. In 1998 Tongaat-Hulett Sugar purchased a share i n the mill, with the remaining share being held by the Mozambican government. At its previous production peak, in the 197 1 season. the mill crushed 483 147 tons of cane. but the civil war had a devastating effect on production, and throughput i n the 1998 season was a mere 97 745 tons. The Technical Managcment Dcpartment ot Tongaat-Hulett Sugar was appointed to carry out the design and detailed engineering for the rehabilitation of thc mill to a nominal throughput of 120 tons canc per hour, with a view to a possible future doubling of capacity. The scope of the rehabilitation included upgrading of boilers, a new heavy duty shredder, a new 6 m wide diffuses, modification of the evaporator station. a new 42 m' batch A-pan, a new 43 m' continuous C-pan, two new 120 m' vertical crystallisers, two new 1 500 kg batch centrifugals and four new 1 300 mrti continuous centrii'ugals, as well as the refurbishment of most of the existing plant. The boiling house would be arranged to produce VHP sugar, using a threeboiling system with graining of A-pans and the usc of C-magma for B-seed. In addition, the process of recycle of clarifier mud to the diffuser was selected, obviating the need for refurbishment of the filtcr station or ancillaries. While various areas of the proccss design presented interesting challenges, one of the more absorbing came from an unexpected quarter the evaporator station. Although evaporator design is ostensibly a routine exercise, the specifics of the Xinavane dcsign forced consideration of the subtleties of evaporator dcsign optimisation. A prelimina~-y evaluation of the evaporator station indicated that, irrespcetive of the condition of the vessels, the existing station design was not appropriate for the expected duty i n the refurbished factory. In particular, thc first effect area was clearly too small and the distribution ol' heating surface between the vessels in the tail was far from optimum. This discrepancy can probably be explained by the following changes, which are part of the refurbishment and significantly affect vapour blecd requirements: thc elimination of the usc of exhaust steam for boiling pans (previously available for use on two calandria pans) the scrapping ol' old coil pans which used stcam at 400 kPa(g) a changed boiling scheme whcl-c B-sugar is fully remelted rather than being bagged higher levels of imbibition (selected to achieve higher extraclion). A further faclor is that the exisling evaporator station was modified during the civil war ycars, with the modifications defined by vesscls that were available from other non-operational mills. A detailcd investigation into the dcsign of the evaporator station was clearly requil-cd. It must be emphasised that evapo~.;ltor station design cannot bc conductcd independently but is very closcly linked to overall factory steam demand and the requirements 1'0s fuel cconomy. Proc S Afr Siig Teclulol Ass (1 999) 73 21 1 Requirements of evaporator station design On first consideration, it may seem that the requirement of an cvaporator station is simply to remove water fromjuicc to producc syrup. However, Inore dctailccl consideration will identify a significant number ol' rcquircmcnts and constraints. Good evapol-atolstation design involvcs balancing conflicting rcquil-cnicnts and constraints while attempting to approach each of them as closcly as possible. The major rcquircmcnts arc: Evaporate the required quantity of watcr from clear juice to producc syrup at the appropriate brix for pan boiling. Condense all exhaust steam horn the turbines (both primc movers and turbo altcrnato~-S), avoiding blow-01'1' uncl allowing a sul'l'icient let-down from HP to exhaus1 (to allow exhaust steam prcssurc contl-01). Have an efficiency dcsigncd in conjunction with the rest of the l'actory to eliminate (or at least minimise) cithcr unwanted bagassc sul-pluses or the need [or supplementary fuel (i.e. achicvc a 'I'ucl balance'). Supply vapour bleeds at thc rcquircd quantities ancl prcssurcs to meet the proccss demands of other sections of the I'actory. Supply thc rccluircd quality and cluantity of boiler feed water, primarily l'rom condensed exhaust steam, but supplemented with other acceptable condcnsatcs. Act as a reactor Ihr thc destruction of ?larch whcn su~tably dosed with the appropriate cnLymc (not necessary in diffuser factories). These must bc achieved subject to the following constraints: The evaporator dcsign must I'acilitatc stable operation and control. The thermal degradation ol' sucrose and reducing sugars must bc minimised. The capital cost must bc minimised a major factor i n achieving this being thc optimum distribution ol' heating surfacc between cffects. The dcsign should be robust, i.c. it should continue to pcrform acceptably ovcr a reasonable range of operating conditions. The design should be conipatiblc with I'uture cxpansion plans. Requirements of the Xinavane evaporator station The first stcp in clctcrniining the requiremcnts of the Xinavanc evaporator station was to pcllhl-m detailed stcam balances ovcr the entire factory. This was done using a con)putcr program called SLOB (Steam Load Overall Balance) which was developed in-housc by Tongaat Hulctt Sugar (Rein and Hockstl-a, 1994). These calculations were able to take into account I'actors spccific to Xinuvane that make it significantly different from the average South African sugar factory, viz: a low cane crush rate (avcragc ol' 120 tons canelh) lowclefficiency boilers (2 x 30 tonlh Dutch oven type boilers) I O W HP stcam prcssurc (2 l00 kPa(a)) lowclefficiency turbines an unrcliablc clcctricity supply I'rom the national grid a market Sor generation of clcctrical power (I'or irrigation) thc usc of local trees as the supplementary I'uel source. Unfortunately, bccausc of thc decline in performance of the mill during thc civil war in Mozamhicluc, Ihcrc is limited qu~llity data on plant pcrl'ormancc ancl many 'best estimates' havc bccn necessary as the basis for calculations of expected pcrformancc. Taking thcse I'aclors into account, steam balances for a range o f cxpcctcd operating conditions highlighted the following points spccil'ic to Xinavanc. When opcrat~ng w ~ t h a cluudruplc el'l'ect evaporator (V 1 bleed only): supplcmcntary I'ucl woultl bc rcquircd cvcn during the high I'ibrc portion ol. rhc scason the export ol'powcr would bc possihlc without the need to blow off cxhaust s t c ~ ~ m a reasonable quantily ol' HP to exhaust let-down would hcilitatc cxhaust steam prcssurc contl-ol. When operating with a quintuple cvaporator (with V2 blccd for ~ ~ ~ ' I ' L I s c I and primary juice heating): thcrc would be a fuel shortage only during the low I'ibrc 1mrtion ol'thc season, with a bagassc surplus during high 1.i hrc periods thc maximum gcncration of clcctricity Ior export would rcsulr i n cxhaust steam blow-01.1' during pcl-iods of low fibre throughput, thcrc would bc vcry lirtlc HP slcam to Ict down to cxhaust which could I-csult In unstable cxhaust steam p~cssurc control. Thc dil'l'iculty in dcciding between thcsc two options is compounticd by the possibility tlia1 the I'ucl shortage could be eliminated by an cxpansion in the ncar Suture, doubling lhe capacity ol' thc mill. Preliminary calculations indicated that thc lowcr spccil'ic stcam demand 01. u largcr l'actory would cl iminatc the ncccl for supplerncntary I'ucl even when opcrating ;I cluadruplc cvaporator. The added cxpcnsc of cl-cating a cjui~it~~plc el'l'ect evaporator as part of the rc1u1-bishnient would thcn be particularly dil'l'icult to justify, unless the design was I'ully compatible with thc cxpansion. Given thcsc unccrtuintics, i t was ncccssary to generate alternative designs Solrefurbishment 01thc cxisling evaporator station at Xinavanc us cithcr a cluadruplc 01quintuple effect evaporation station. To cvaluatc the s~~itabil i ty ol' both ol' thcsc dcsigns for future cxpansion. i t was also necessary to dcsign cvapol-ator stations lbr the cxpa~ision. This includes I'urthcr possibilities sincc the bcncl'its ol' generating export power I'or irrigation would rcclui~-c a cluintuple evaporator if a condensing turbine proved economic or a quadruplc cvaporator i l ' the condensing turbine was not an economic option. Proc S Afi. SLIS Tecll~lol Ass (1 999) 73 Robust design oj'arl e ~ ~ z l ~ o r n t o r st tioll as applierl to Xinavarle Design of an evaporator station Tongaat-Hulctt Sugar has for many years used a computer simulation program (known as Program for Evaporator Simulation and Testing, or PEST for short) for the design and evaluation ol' evaporator stations. This program was developed in-house (Hoekstra, 198 1) and provides detailed calculations which avoid many of the simplifying assumptions of evaporator calculations used i n standard sugaltexts describing evaporator calculations (Hugot, 1986). While PEST or other such programs will handle the details of the calculations, i t is important for the evaporator designer to understand the principles of evaporator operation and the underlying calculations to use the program effectively. Elucidating these principles does, however, require using many simplifying assumptions. A simple quadruple evaporator station (with vapour one bleed only) is shown in Figure 1 , as the basis for the following discussion. I 1 I At2 4 I At, 4 ( At4 4 Effective I AT, _I ( AT., Temperature ' Differences IAT, -I Figure 1. Representation of quadruple evaporator station with V1 bleed. F~rridari~erltc~ls oflletzt trtlrlsferThe fundan~ental heat transfer ecluation which describes how heat is transferred across a heating surface is: Q = U . A . A T where Q is the heat transferred i n W U is the heat transfer cocf'ficient in W/m2/K A is the hcating surface area in m? and A T is the temperature driving force in K. In the case ol'an evaporator, AT is the difference between the saturated temperatuse of the steani in the calandria and boiling temperature of the liquid. The convention used in this work is to calculate A and U based on the outside diameter of the evaporatoltubes and thc distance between the tube plates. This formula clearly shows the equivalent inlluence of the heat transfer cocfl'icient, U , and the heating surface arca, A, on hcnt tsansfcr. A 10% drop in heat transfer coefficient is directly equivalent to a 10% loss of heating surface area. The evaporation rate W in kg/s can be calculated from the heat transferl-ed as: where h is the enthalpy of evaporation in J/kg. This assumes that heat losses arc negligible and that there is no sub-cooling of condensate below its saturated temperature. The three principles of multiple-effect evaporation originally espoused by Rillicux (the pioneer of this technique) are (Spencer and Mcade, 1945): First principle In a multiple-efl'ect evaporator, for each kilogram of steam used. as many kilograms of evaporation will result as there are units i n the sct. Second principle If vapours are withdrawn from any unit of ;I multiple-effect evaporator to replace steam in a concurrent process, the saving of steani will be equal to the amount of vapour so used divided by the number of units in the set and multiplied by (he scquencc position of the unit from which the vapour has been withdrawn. Third principle In any apparatus in which steam or vapour is condensed, i t is necessary continuously to withdraw the accumulation of non-condensable gas which is unavoidably left in the heating sul-face compartment. The first and second principles arc useful approximations which relate to the el'ficiency oi' evaporation but ignore the effects of juice flashing and variations in cnthalpy of vaporisation (latent heat) with temperature. The nomenclature used in Figure 1 is based on the assumption that these principlec arc Isue. Distribiltiorl of'teri~perot~rre drivirlg,force over evaporators The temperature driving force available for achieving the required evaporation is the difference betwecn the saturated temperature of the exhaust steam and that of the vapour in the final effect. Thc exhaust steam pressure is normally sclected as 200 kPa(a) (being a compromise between the requirements Ibr power gcneration and evaporator heating surfacc recluirements). Smith and Tay lor (l 98 l ) have shown that the optimum pressure Ihr i'inal cl'fect vapour is between 16 and 20 kPa(a). This total available temperature difference will be reduced by thc elevation oi'hoiling point of the juice in each effect as a consequence of concentration and hydrostatic head. The net or cffective temperature difference will then be distributed between the etfects as per the heat transfcr ey uation. Proc S Afr Silg Teclzrlol Ass (1999) 73 Kob~lst desigrl of nil e\.trporrrtor~tntiorr os rrl>plied to Xiilcrvai~e DJ Love, D M Meadows & RC; Hoekstrn Effect of q~lantity o j vrrl7orrr hleed or1 evrrl7orrrtor .strrtiorl capacity Detailed evaporator calculations on many practical evaporator stations in sugar factories have shown that incrcasing the quantity of vapour bleed will increase the evaporation capacity or the evaporator station. This increase in capacity will be associated with a decrease i n blccd pressure. This phenomenon is not a natural consequence of multiple effect evaporator stations but only occurs when the effects before the bleed have a higher combination of area and heat transfer cocfficient than the effects further down the evaporation train. Appendix I givcs a derivation of this principle for a quadruple cvaporator with vapour one bleed only. Simply stated, if I we consider K, to be the resistance ol'an cvaporator effect to heat transfer. an increase in bleed will increase the cvaporator capacity if the resistance of the first effect is less than the average resistance of the effects in the tail. Effect of corrde~~snteflash on evaporator statioil ccyxrcitj, In contrast to the effect of vapour bleed on capacity, the return of condensate flash into vapour streams will normally increase vapour pressures, and therefore reduce capacity but improve steatn utilisation efficiency. This effect is, however, usually small. The use of vapo~rr tl~i.ottlirlg Vapour throttling (nor~nally used after the last vapour blccd) can be used as a control variable to reduce evaporator capacity without affecting evaporator efficiency. This will reduce capacity while increasing bleed pressures as opposed to the alternative of rcducing capacity by reducing exhaust steam pressure (which will reduce bleed pressures). Throttling can bc thought of as consuming driving forcc hut not stcam. Vessel arca prior to the vapour blccds should be installed to provide the required blccd prcssurcs. Excess arca installed here will result in bleed pressures above the required minimum values and, although this will increase the cvaporator capacity (by driving the tail harder). the extra arca would have been more effective had i t been installed in the tail. For effective distribution of heating surlace area betwccn the effects in the cvaporator tail (i.e. alicr the vapour blccd or bleeds) Buczolich and Zadori (1963) provtde guidelines 1'0s the optimum distribution of hcating surfacc, as summarised by Hoekstra (l 98 l). The criterion f'or optimum distribution of heating surface is that the ratio ol' healing surl'acc to cl'fcctivc tempcl-ature driving lhrcc should I-emain constant for each effect. Expressed mathcrnaticall y: ! L z C At; where i ~-eprcscllts each cl'l'ect and C is an arbitrary constant, termed here the arca efficiency criterion. Since heat transfelcoel'ficients not-mally decrease towards the last cl'l'ect, causing temperature diff'erences to increase, the hcating surface area will need to increase down thc tail for effective use of installed arca. On this basis existing cvaporator stations with equally sized vessels in the tail indicatc a design that is not optimal. Design for the refurbishn~ent of Xinavane evaporator station Existirlg rllcrporrrtor stcitiorl The existing quadruple cl'l'cct evaporator consists of the following vessels: Eflect Ty pc Area ( m 2 First Semi-Kcstncl1 523 Second Robcrt 703 Third Robert 402 Fourt h Robcr~ 877 Preliminary calculations showed this station to have significantly less than the required capacity. In particular the first effect area was too small to provide an adequate bleed pressure and the distribution of heating surlhcc between the cffccts i n the tail was t'ar from optimal. The heat transfer coefi'icients (listed below i n Table I ) used i n these calculations, and those for 311 modified configurations are based on cxtcnsivc mcasurcmcnts by Tongaat-Hulctt Sugar over many years and represent practically attainable design figures. Tahle l . Heat transfer coefficients used in evaporator simulations.