To reduce operating costs, energy conservation and heat recovery should be paramount to the configuration of any unit operation — especially thermal drying. Numerous drying technologies benefit from energy recovery; however, this article looks at a dryer efficiency study focused on reducing utility consumption related to industrial conveyor drying. Drying super absorbent polymer (SAP) is used as an example of an application that is ideal for recovering energy and recycling heat within the dryer. SAP drying yields effluent streams with sufficient internal energy to financially justify modifying the configuration of the dryer for heat recovery.
Opportunities for heat recovery include cascading airflow, air-to-air heat recovery and condensate flashing. The dryer study suggests that a potential energy savings of 18.7 percent is possible while improving the thermal efficiency of the dryer by nearly 12.5 percent.
SAP production involves granulating sheets of long-chain polymers into particles that are able to absorb and retain liquids. Some of the current markets for SAP include disposable diapers, medical bandages and controlled-release mediums. The highly versatile material is a product of the polymerization reaction intended to form lightly cross-linked poly(acrylic acid). Once the reaction is complete, the SAP must be dried and milled into small white particles. Due to the physical properties of SAP, the product is limited to thermal drying as opposed to mechanical dewatering followed by thermal drying. After passing through the conveyor dryers, the rigid cake is pulverized in a hammer mill. Finally, the product is sorted by size using a series of sieve trays.
Thermally Processing SAP in a Dryer
Energy recovery can be accomplished by first identifying prospective drying applications that require a relatively large input of energy per quantity of water thermally evaporated. Drying wet material requires a large amount of energy/mass transfer to obtain the desired final moisture content of the product. Dryers for SAP are designed for high process temperatures (T ≥ 302°F [150°C]) and accelerated airflow rates through the stacked bed of product (V ≥ 4.9 ft/sec [1.5 m/sec]) in order to facilitate a high rate of evaporation. A typical SAP conveyor dryer configuration can be seen in figure 1.
Once a potential application has been recognized, an energy balance can be broken down to identify areas where energy can either be reduced or reused. Figure 2 shows an approximate energy consumption profile for a conveyor dryer producing SAP at 5 tons/hr. As seen in figure 2, water evaporation requires the most amount of energy due to the phase change. For this example, the most beneficial savings can be achieved by recovering the thermal energy used to evaporate the water and heat the air. A smaller portion of heat can also be recovered from the product via recycling cooling exhaust air. Last but not least, better mechanical practices help minimize heat losses.
The initial example required more than 6 MW of thermal energy to produce 5 tons/hr of SAP. Drying SAP is a typical application for heat recovery because the production and evaporation rates are frequently high (P > 4000 kgPROD/hr). The standard conditions for drying SAP require a high evaporative load. SAP enters the dryer with an average moisture content between 50 and 60 percent wwb and exits at 5 percent. Utilities are taxed heavily by generating 6 MW of thermal energy.
The most beneficial performance-to-cost ratio is achieved by processing low density SAP. Polymers that are heavily cross-linked cannot absorb as much liquid and are less desirable. Reducing operating costs also will improve the profits from the product. Utility costs will continue to increase as a direct result of the depleting nonrenewable natural resources. Energy conservation will help mitigate escalating costs associated with operating boilers for steam production. The price of steam varies depending upon a number of factors: boiler operating parameters, geography, fuel, etc. For the purpose of an economic analysis, the price of utility steam will be set at $0.013 per kg steam consumed.
The economics of the dryer configurations varied based on the energy recovery options. However, there were parameters that were constant throughout the simulations. The quality of the air entering the dryer was defined at ambient conditions (TDB = 68°F [20°C]) and sea level. Also, the general dryer configuration and process parameters were standardized while modeling the energy options.
The dryer conditions in table 1 were predetermined based on empirical testing of SAP. These conditions were universal throughout all of the simulator modules. As mentioned, the overall configuration of the dryer in regard to airflow direction, drying/cooling area and general arrangement was consistent. In terms of design, the dryers were single-pass, single-plenum, conveyor dryers. Each simulation had six drying zones with an extended cooling zone. The airflow direction in the first two zones was upflow followed by downflow air in the later sections of the dryer.
The simulations share many design features in common; however, the energy recovery features were unique to each design. The study focused primarily on three:
- Cascading airflow.
- Air-to-air heat recovery from exhaust to makeup.
- Condensate flashing to preheat makeup air.
Figure 3 provides illustrations of the equipment.
Drying Modeling and Testing
Module 1: Base Case. The first scenario that was modeled had no heat recovery in addition to the recirculated air (figure 4). The results of the energy and mass balance for Module 1 can be seen in table 2. The dryer’s exhaust and makeup air were optimized to maintain absolute humidity levels at approximately 0.15 kgH2O/kgDA.
Module 1 represents the most -energy-intensive and least cost-effective option from an operating perspective. Drying efficiency is defined as the ratio of energy required for moisture evaporation to the total energy supplied to the dryer. The definition has been simplified:
QP is the energy required to heat the product.
QEVAP is the energy required to evaporate the water.
QH2Ov is the energy required to heat the water vapor.
QSTEAM is the energy available from the saturated steam supply at the reference conditions (408.2°F [209°C]; 17 bar).
Based on previous studies, the drying efficiency for conveyor drying technology is between 40 and 60 percent. The theoretical results of the analysis yielded a drying efficiency of 53 percent, which lies in the range of the published literature.
Table 2 also shows the specific energy and steam consumption along with the specific moisture-extraction rate. These values frequently are used to compare similar drying applications but at varying sizes and production rates. They will be used as a baseline for the following energy modules.
Module 2: Cooling Exhaust Makeup Air. The second scenario focused on heat recovery from the cooling exhaust (figure 5). In this design, the air being exhausted from the cooling section was routed to the makeup air ducts entering each heat zone. The makeup air to each zone was no longer at ambient conditions; rather, it was preheated (TDB=86°F [30°C]). The cooling exhaust airflow rate was optimized based on the required makeup air mass flow rate. Similarly to Module 1, it was assumed that the quality of the air entering the cooler was at ambient conditions (TDB=68°F [20°C]) and sea level.
Again, table 2 shows the energy and mass results for Module 2. The drying efficiency improved by 2 percent. When compared to Module 1, there was also a 2 percent savings in energy usage. As expected, the specific energy and steam consumption are less because the same amount water was evaporated using less steam, i.e. energy. Similarly, the specific moisture-extraction rate increased because it is inversely proportional.
Module 3: Cascading Airflow. Cascading airflow was simulated in Module 3 (figure 6). Similarly to Module 2, the cooling exhaust is utilized as makeup air; however, it only enters the dryer in heat zones 1, 2 and 6. The makeup air is introduced in zone 6, and the exhaust from zone 6 internally “cascades” as the makeup air to zone 5. This design is continued to zone 3, where it is finally exhausted from the dryer. The cascading design conserves energy by reusing relatively dry preheated air.
Depending upon the drying characteristics of the product and the configuration of the dryer, some applications permit cascading airflow through all of the heat zones. A prime example of cascading airflow would be a rubber dryer.
As it relates to drying SAP, there is a high evaporation load in the first two heat zones in addition to the change in airflow direction from zones 2 to 3. Therefore, it is mechanically and thermodynamically impractical to cascade airflow from zone 6 to zone 1 for SAP applications.
Table 2 depicts the results of the energy analysis for cascading airflow. In comparison to Module 1, there is a 4 percent reduction in energy consumption and a 3 percent increase in drying efficiency.
Module 4 – Flash Steam. Flash steam is generated when high pressure condensate is released to low pressure. The temperature of the steam rapidly drops to the boiling temperature at the lower pressure. The excess latent heat causes some of the condensate to re-evaporate into steam. A design heuristic for flash steam systems requires that the low pressure flash steam be a minimum of 520 kPa below the high pressure condensate.
In the drying simulation (figure 7), the low pressure flash steam is utilized via an external steam coil. The exhaust air from the cooling section is ducted to the low pressure steam coil. After the air is preheated by the flash steam, it is distributed to each zone.
In the simulator, the condensate was leaving the dryer at 1765 kPa, and the flash tank was operating at 690 kPa. Based on the energy balance, the dryer produced condensate at a rate of 10,330 kg/hr. The flash steam system was able to recover nearly 500 kW of power from the high pressure condensate.
Table 2 lists the values from the flash steam energy balance. The flash steam modifications yielded energy savings of 10 percent when compared to the base-case scenario. The drying efficiency also improved by 6 percent. The flash steam option elevates the machine into a high efficiency range relative to similar conveyor drying technology.
Module 5: Air-to-Air Heat Exchanger. The air-to-air heat exchanger recovers heat from the exhaust streams that would have otherwise been expelled to the environment. Once again, the makeup air is supplied from the cooling exhaust/transfer fan. An illustration of the dryer configuration can be seen figure 8.
A counter-current parallel-plate heat exchanger is used to reduce pressure drop and improve approach temperatures between the hot exhaust air and cooler makeup air. Pressure drop becomes a concern while sizing transfer and exhaust fans. Typically, the pressure drop across the heat exchanger is between 620 and 750 Pa; thus, significantly increasing the horsepower of the fans is required. There are also design considerations in regards to the approach temperatures of the makeup air. For this application, the efficiency of the heat exchanger is limited by the formation of condensation in the exhaust stream. The humid exhaust air contains SAP particulate, which will foul the heat exchanger if water condenses along the heat transfer surface area. Therefore, the exhaust temperature must maintain at least a 19.8°F (11°C) buffer above the dewpoint temperature. Shown as an equation, that is:
TEX ≥ TDEW + 19.8°F (11°C)
This exhaust temperature setpoint is used as a design parameter for the heat exchanger.
Despite these design considerations, counter-current air-to-air heat exchangers are highly efficient, typically with efficiencies in excess of 90 percent.
Table 2 shows the benefits of recovering energy from the exhaust airstream to preheat the makeup air. Testing shows 11 percent energy savings; however, case studies show potential energy savings up to 16 percent. An air-to-air heat exchanger also improves the performance of the dryer so that it operates in the upper efficiency range.
Module 6: Combined Energy Conservation Measures. As shown in figure 9, Module 6 combined the energy features described in Modules 2 through 5. First, the exhaust air from the cooling section was passed through an air-to-air heat exchanger. Hot exhaust air from zones 1, 2 and 3 ran counter-current to the preheated makeup air. The second feature involved further heating the makeup air using flash steam coils. Then, the hot makeup air was ducted to zones 1, 2 and 6. Finally, the air was cascaded from zone 6 to 3, where it was exhausted from the dryer.
As shown in table 2, the combined energy savings from the heat recovery features was 19 percent when compared to Module 1. The drying efficiency exceeded the original amount by 13 percent, which is 6 percent above high efficiency conveyor dryer applications.
The specific steam consumption was determined to be 1.35 kgSteam/kgEvp. Similar results have been achieved for actual conveyor drying applications. According to recent energy audits, conveyor drying applications have a targeted specific heat consumption of 1.3 kgSteam/kgEvp.
In conclusion, the energy savings were calculated using a mass and energy balance at the design conditions. A nominal throughput of 5 tons/hr was selected as the production rate for the SAP dryer. Operating parameters were consistent with the industry’s standard.
The heat recovery options were compared separately to the base-case scenario, which had no energy saving features. Each energy module was not added incrementally to the same simulator. Since each energy-saving option was evaluated and optimized independently, there are overlapping savings between each option. Therefore, the combined savings are not cumulative and will not equal the sum of the individual options.
An economic analysis was performed to determine the payback period for each heat recovery option. Financial savings were calculated assuming a production schedule of 350 days/year, providing two weeks of equipment maintenance/downtime. Table 3 provides an estimate of the capital investment and return for each energy option.