Evaporator Rebuilds for Increased Capacity and Condensate Segregation - Technical Paper

1 015.rpt EVAPORATOR REBUILDS FOR INCREASED CAPACITY AND CONDENSATE SEGREGATION James L. Straka Staff Engineer A. H. Lundberg Associates, Inc. Bellevue, Washington U.S.A. ABSTRACT The emphasis placed on improving the environmental efficiency of the pulping industry has impacted the configuration of the multiple effect evaporator plant. The evaporator is one of the largest contributors of BOD and TRS loading to the waste water treatment facilities within a mill. At the same time the evaporator often becomes the bottleneck when a production capacity increase is considered. The upgrading of an existing evaporator plant to increase capacity can be accomplished in conjunction with the implementation of condensate segregation to solve capacity concerns and environmental objectives. INTRODUCTION As pulp mill production is frequently increased incrementally, little attention is paid to the evaporator system or the system is assumed to be adequate. Further, as the mill's evaporators age, becoming dirty or corroded, the performance deteriorates. Process changes in pulp washing, etc., can have major consequences on the performance of the set of evaporators. These oversights can lead to the multiple effect evaporator set being one of the mill's bottlenecks. The increase in pulp mill capacity can overload the mill's effluent handling system which can lead to BOD discharges in excess of the permitted limits. In order to reduce the emission of both BOD and TRS to the treatment lagoons, many mills are resorting to condensate stripping of the foul condensates. One of the major contributors of BOD and TRS ladened condensate is the multiple effect evaporator system. The cost of treating the entire process condensate from the evaporators is prohibitive, so the highly fouled condensate is currently separated from the lightly contaminated condensate in new evaporator systems. Most existing evaporators must be modified to separate highly fouled condensates from the lightly contaminated ones. The process by which the BOD and TRS contaminants are concentrated into a relatively small volume of condensate is called "condensate segregation." Typically, to determine what changes need to be made to the evaporators and to implement them correctly, it is necessary to collect sufficient data to calculate a heat and material balance based upon actual performance. Using this collected data it is possible to determine where the system needs to be modified and what beneficial effect this will have on the system. 2 015.rpt Modifications to the evaporator system in order to increase capacity generally fall into three categories: 1. Reduction in the rate of scaling. This improves capacity by decreasing the frequency of washing, thus increasing up time and reducing liquor dilution with wash liquor. 2. Addition of condensate segregation equipment. These modifications increase capacity by the addition of heat transfer area as well as the potential for improved system vacuum. 3. Reconfiguration of the evaporator set. These modifications may include the addition of area, alteration of flow sequence of both liquor and steam, or changing the number of effects. EVAPORATOR TYPES Three basic types of evaporators are used within the industry. The work horse evaporator technology is the rising film or LTV (long tube vertical) design. The rising film evaporator relies on the change in density of the boiling fluid to drive the liquor through the tubes. This density change is dependent upon the shell side (steam side) temperature, the liquor temperature, and the percent liquor solids. The minimum temperature differential, or "delta T" (ÄT), under which a rising film evaporator will operate stably is about 7°C per body. When the ÄT is less than 7°C per body, the evaporator will surge. Scaling is a serious problem in the higher solids effects of a rising film evaporator. Scaling can be attributed to poor liquor distribution within the bodies and result in frequent boilouts. The second type of evaporator design, the falling film evaporator, reduces the scaling problem of the rising film design by using energy (pump horsepower) to force the liquor through the tubes. No longer is the liquor flow dependent upon the feed to a given effect, but rather, it is the designed flow dictated by the recirculation rate. Liquor is pumped to the top tube sheet and is distributed across the diameter to assure uniform wetting of each tube. Bodies with adequate recirculation and distribution have minimal scaling. The third type of evaporation system is the flooded forced circulation unit. This system is frequently used on solids concentrations above 65%. Flooded forced circulation evaporators combine the technology of fully submerged heating surfaces with flash tank capability. Since these units are designed to provide for completely wetted tubes, liquor distribution is not a problem. Velocity of the liquor within the tubes can be directly related to the length of time between washes. Since both falling film and flooded forced circulation technologies are relatively new applications in the pulp and paper industry, and feature many of the improvements to be discussed below, this paper will focus primarily on the rising film evaporator design currently employed in a majority of pulp mills. MODIFICATIONS TO MINIMIZE FOULING Many of the evaporator systems in the industry are of a rising film design. The advantage of rising film technology is its low electrical energy consumption. However, this is offset by poor turndown capability and the propensity to scale in the heavy liquor effects. Two types of scaling are present in a normal evaporator body. The first of these is soft scale which is due to organics such as soap and fiber. Soft scale is usually attributed to poor liquor distribution. The second type of scale is the 3 015.rpt hard scale which results from precipitation of inorganic salts and other compounds and is attributable to high solids and high temperatures. Additionally, the cause of fouling in the strong liquor effects is twofold: first is the reduced solubility of the inorganic components at the higher temperatures seen in the pressure effects of the evaporator; and second is the much lower flow rate of liquor in the individual tubes in the strong liquor effects. To start minimizing the scaling tendency of the evaporator, it is important to keep as many of the fouling components as possible out of the liquor: • The recovery boiler should be operated in a manner which maximizes the reduction efficiency. • The caustic plant should be operated to minimize the carry through of calcium in the form of carbonate. • Chemical additions from the tall oil plant or the chlorine dioxide plant should be made to the evaporation plant in the location that can best handle them, such as the concentrators. Even with concerted efforts to keep these components out of the liquor, scaling may still be severe. Additional scaling components come into the liquor cycle with the wood being pulped. This is particularly true of components such as calcium and silica. Silica may also enter the liquor cycle in the form of defoamer additive. It is well accepted within the industry that the scaling tendency is greatest when the high solids seen in the product effect of the evaporator are present in the live steam effect. In recently installed evaporators utilizing rising film technology, many of the systems are configured with final product liquor exiting the second effect of the evaporator. The second effect product configuration has the advantage of lower temperature product liquor. Furthermore, since the pressure in the second effect vapor head is lower than in the first effect, the specific volume of the vapor generated upon evaporation is much higher. Figure 1 illustrates the change in specific volume of vapor relative to operating temperatures. It can be seen that as the vapor temperature decreases from 256°F to 228°F the specific volume increases from 12.5 ft3/lb to 20.1 ft3/lb. Thus, for effects of equal area, the liquor velocity at the tube exit will nearly double when a second effect product conversion is performed. Incorporating a product effect change to an existing rising film evaporator system is relatively easy and cost effective. Two approaches may be taken to make the product effect change. The first is to change the liquor flow sequence; the second is to change the vapor flow sequence. Generally, changing the vapor flow is more cost effective than changing the liquor flow. This is because most old LTV sets have vapor pipe by-passing requiring only minor additions of vapor duct to make these changes. However, if the modifications are made through changes in vapor flow, care should be taken to assure that the new live steam body is rated for the pressure required. The process flow sheet for the product effect change is shown in Figure 2. The dashed lines 4 015.rpt indicate additional lines that are required for the second effect product modification. In this scheme the liquor flow sequence does not change; that is, the product liquor will still flow from the first body in line. However, live steam is now used on the second body, making it the first effect. Vapor from the new first effect is sent to the first body (old first effect) which now has become the new second effect. Most sets of evaporators have the ability to take live steam to either the first or second effect and many sets have vapor duct by-passing on at least the first two bodies, requiring minimal piping modifications. Since the vapor must flow from the second body back to the first body, it is necessary to install vapor duct from the outlet of the second body (old second effect) to the vapor inlet of the new second effect (old first body). The condensate piping on the first two bodies must also be modified to permit collection of live steam condensate from the second body and the transfer of vapor condensate from the first body to the third effect chest. Review of Figure 2 will also show several other recommended additions to the system when a product effect change is made. Quite important to the successful operation of the system is the first effect liquor preheater. When the liquor flow sequence is modified to second effect product, the liquor passes directly from the third to the first effect. This greatly increases the sensible heat load on the first effect. Without the new liquor heater, the sensible heat load incurred by the first effect will lead to a lower heat transfer coefficient and increased scaling. The liquor preheat eliminates the heating requirement in the first effect by accomplishing the preheating in a flooded unit specially designed for the liquor heating. The heater utilizes live steam to permit the heating of the liquor to the normal operating temperature in the first effect. The heater is a flooded unit with high liquor velocity within the tubes to minimize fouling. A product effect change can substantially increase the length of time between washings due to the decreased tendencies of hard scale formation. Soft scaling is caused by poor liquor distribution across the tube sheet. As the liquor is evaporated in each successive effect, its volume is decreased at a much greater rate than the area of the body. In many existing systems this was mitigated to a great extent by designing the product effect as an A/B body. The product effect body was made two pass and the liquor flow rate per tube was essentially doubled. Figure 3 illustrates the A/B evaporator configuration. Note that product from the "B" side is reintroduced into the second pass or "A" side of the evaporator body. While this helped the liquor distribution to a great extent, the problem of soft scaling remained. Soft scale is caused by insufficient liquor flow through individual tubes or blocks of tubes. When fouled heating surfaces are inspected, typically highly fouled soft scaled areas are evident in areas away from the liquor inlet. Because the liquor flow through the tubes is limited, the organics tend to build up on the wall. A number of existing evaporator systems have been modified with augmented liquor flow. The liquor is recycled from the vapor head back to the bottom liquor box. The amount of liquor recycled is more than sufficient to insure adequate distribution across the tube bundle. If an A/B baffle is present when a body is modified, it is removed and the heater is made one pass. 5 015.rpt A great deal of care must be taken when installing augmented flow on an evaporator set. The augmented flow is substantially greater than the nominal throughput of the system, so it is important in the design to be sure that the tube sheet will not be flooded after the modification. Further, in order for the recirculation pump to adequately recirculate the liquor, it is necessary to have sufficient liquor residence time at the suction of the pump. This frequently is not possible in the vapor head of the body alone and an external standpipe or surge tank is required. Without the added benefit of liquor recirculation or augmented flow, an evaporator system can become fouled on start-up by running the product effect dry. This is of even greater importance if a second effect product change is made. The addition of liquor recirculation provides a reservoir of liquor to minimize the potential for operator error during start-up. One drawback to this type of modification is that the additional liquor reservoir increases the time required to boil out the system. Typically, an evaporator requires two to three hours for a boilout. After the augmented flow modifications, a boilout will take four to five hours. this is normally offset by the increase in time between required boilouts. CONDENSATE SEGREGATION A large portion of a pulp mill's contaminated condensate comes from the multiple effect evaporator system. These condensates are contaminated with BOD compounds and TRS compounds which affect their reuse within the pulping process. About 75% of the volatile BOD and a much larger fraction of the TRS are stripped from the weak liquor in the first two stages of evaporation. If we consider a six effect evaporator system with the weak liquor feeding the fifth effect, then the condensate resulting from the vapor generated in the fifth and sixth effects will be the highest in contamination. The process of condensate segregation is shown in Figure 4. Generally, to gain the highest steam economy within a set of evaporators, the contaminated condensate is flashed downhill in each succeeding effect to recover the heat. Hence, the relatively clean condensates generated in the second through fourth effects of a six effect system are contaminated when they are flashed in the bodies of the fifth and sixth effects. The first step to minimize the amount of condensate to be handled is to remove the condensate from the fourth effect and flash it in external flash tanks with the vapors going to the shells of the fifth and sixth effects. This not only maintains the system's overall economy but the condensates which are flashed are cleaned of a portion of the volatile contaminants. This technology is applied to the evaporator system as shown in Figure 4 by adding additional heat exchangers and an auxiliary condenser to the system. In condensate segregation the liquor reheating is accomplished in external liquor heaters. Normally, condensate segregation is achieved on the fifth and sixth effects and the surface condenser in a six effect system. The vapor enters each evaporator heating element in a typical fashion. However, rather than vent the heater shell through a small vent, the heater shell is vented through a large nozzle into the external heater or the auxiliary condenser. The liquor to be reheated is somewhat cooler than the liquor in the body and therefore, as the liquor flows through the external liquor heater, it draws the vent vapors into it. The amount of vapor that may be drawn into the liquor heater is related to the heat and material 6 015.rpt balance of the set. Split feeding of the evaporator is detrimental to the performance of condensate segregation for two reasons. The first is that when the feed liquor is split, typically one of the liquor flows will flow forward as opposed to backwards; therefore, this liquor does not have the benefit of having two stages of evaporation and, as a result, some contaminants will not be removed. The second is that since some of the liquor flows forward directly from the feed body, there is not as much liquor to be reheated. Hence, the percentage of the vapor that is blown through to the external liquor heater is less and as such the separation efficiency will be lower. The amount of liquor reheat available in the external liquor heater is a direct function of the temperature distribution in the set and the flow rate of the liquor to be reheated. The segregation efficiency will be greatest on the fifth effect and least on the sixth effect in a six effect evaporator with the following liquor flow sequence: feed liquor to the fifth effect, then to the sixth effect, then to the fourth effect with liquor reheating with vapor from the sixth effect and then from the fifth effect (between the sixth and the fourth effects). This is because the liquor exiting the sixth effect can only be reheated to within about 7 °C of the vapor entering the body. This is only about a 5.5 °C rise in liquor temperature and represents only about 3 to 4 percent of the heat entering the sixth effect. The temperature rise of the liquor as it is reheated in the fifth effect external heater is on the order of 10 to 20 °C. This represents a substantial flow of vapor into the external liquor heater and, as a result, a high degree of BOD separation. 7 015.rpt Most of the BOD and TRS contaminants are sent to the surface condenser by way of the noncondensible gas vents off of the evaporator bodies and the external heaters. A similar approach to the segregation of these contaminants in the surface condenser area is taken as with the external heaters on the fifth and sixth effects. The vapor enters the existing surface condenser as it would normally. The vent to the ejector system is blanked off and a large vent is installed near the bottom tube sheet to vent to the secondary condenser. Since cooling water is used as the condensing media, it is possible to control the cooling water outlet temperature of the first surface condenser, insuring sufficient vapor is vented through to the auxiliary condenser which provides for adequate condensate segregation. Frequently, existing evaporator systems are operated with insufficient vacuum. This may be caused by insufficient condenser capacity or an overloaded ejector system. The addition of surface area in the form of an auxiliary condenser will frequently bring the vacuum back to where it should normally be, approximately 25-1/2" Hg. When this happens, the amount of condensing done in the auxiliary condenser may be greater than the normal 15 to 20 percent that would be recovered for stripping. Under conditions where this rate is greatly exceeded, the surface condenser would be baffled to allow separate collection of the foulest portion of the vapor condensate. Many older mills have small ejectors with barometric (direct contact) intercondensers. The amount of water that is contaminated in the barometric condenser is quite high. To reduce the volume of foul condensate, the inter condenser must be replaced with an indirect contact intercondenser. Typically, when replacement is required the entire ejector system is replaced to upgrade capacity. The exact capacity increase that may be realized by the addition of condensate segregation is a function of the performance and flow sequence of each evaporator system. When external liquor heaters are added, the area formerly used for heating internal to the evaporator set will become area for evaporation. The greatest benefit will be seen in mills where vacuum problems exist. When these are corrected either by the addition of more surface area and/or larger ejectors, capacity increases of 10% or greater may be realized. OTHER MODIFICATIONS FOR CAPACITY INCREASES There are a myriad of process schemes which can be devised to increase the capacity of an existing set. The list is too long to discuss them all here. However, there is one in particular that has proven to be quite effective and is, to a certain extent, a combination of both of the above modifications. Many mills operate sets of evaporators with doubled up bodies, i.e. a seven body six effect system or a six body five effect system. The first two effects are frequently combined. At several mills evaporators with combined bodies have been modified so that one of the combined bodies functions as a pre-concentrator. The vapor from this "preconcentrator" no longer enters the second effect but usually enters the third effect. When this modification is made, a set that has seven bodies and six effects becomes one of six bodies six effects and has a pre-concentrator acting with an economy of five effects. Figure 5 illustrates the flow diagram for this type of modification. After this modification is made, the liquor solids from the system operating as a six effect system 8 015.rpt may be reduced to approximately 38 to 42 percent which reduces the scaling tendency in the live steam body. The liquor from the six effect system then goes to the pre-concentrator which has a much greater temperature differential (?T) available. As a result, the steam pressure may be somewhat reduced; the lower pressure being directly related to lower temperature in the shell and reduced scaling of the liquor. Also, because of the greater ?T and the reduced scaling tendency, the overall evaporation capacity is somewhat increased. This modification does decrease the overall system steam economy because steam is used both on the first body of the six effect system and the preconcentrator which forms a five effect system. The additional capacity available will generally offset the loss in steam economy. As with most condensate segregation projects, an auxiliary surface condenser is often required to achieve the capacity increase anticipated. This modification increases the capacity of the evaporators by reducing scaling tendency, improving system vacuum which increases the available system temperature differential (?T). CONCLUSION The starting point in any planned evaporator modification is the up-front determination of the capacity increase desired. The ability to achieve this capacity increase is, to a certain extent, dependent upon the performance of the existing system. It is always possible to achieve a greater increase in capacity in a system that is performing poorly since there is more room for improvement. Frequently, the capacity increase modifications will include some or all of the modifications recommended above. Further, the cost of environmentally mandated system changes can be offset by a capacity increase in the evaporator.