Hydrogen production methods
INDUSTRIAL IMPLEMENTATION OF ENERGY-SAVING TECHNOLOGY FOR HYDROGEN PURIFICATION FROM CARBON DIOXIDE
I. L. Leites
Khimprom Today LLC, D. I. Mendeleyev Russian University of Chemical Technology, A-47, Miusskaya pl., Moscow, 125047, Russia Phone: (095) 357-70-16, mobile phone: 8-902-638-7924 Fax: (095) 114-58-68, (095) 114-59-00, e-mail: leites@rol.ru
The stage of hydrogen purification from CO2 is the most energy-consuming stage of hydrogen production using natural gas conversion. Therefore, the purification technology has changed considerably for the last 50 years and, in the first place, towards power consumption reduction. It is well-known that more than 20 domestically designed big ammonia production facilities were commissioned in the USSR over 1970s-1980s. However the production process fundamentally differed from Kellog technology, which was widely spread outside Russia in those years, only with respect to one stage, namely, the synthesis-gas purification from CO2. In spite of the fact that those facilities used monoethanolamine (MEA), the «as old as time» absorbent, the heat consumption for its regeneration was reduced for 2 times (in AM-70 facilities, 8 facilities) at first, and then for about 3 times more (in AM-76 facilities, 15 facilities including 1 factory in Bulgaria) as compared with the MEA purification technology adopted worldwide. It was achieved only owing to the implementation of new flow sheet and a new intensive and low-size mass-exchange apparatus. It proves that thermodynamic analysis is a powerful tool for the process improvement.
The results obtained have not been reproduces in other countries so far. However, the modernisation of purification from CO2 is pursued in many countries in
terms of substituting MEA with MDEA (methyldietha-nolamine), which is advantageous over MEA in certain respects, including in regard to heat consumption. Incidentally, the process used abroad would require large-scale facility refurbishment including additional equipment mounting and approximately tripling the electric power consumption for the absorbent circulation.
In Russia MEA was substituted for MDEA in the beginning of the 21st century based on domestic technical solutions at one of the factories that provided ~30% heat saving along with the absorbent circulation reduction rather than its increase. Three more factories are being prepared for modernisation. An expected reduction of specific heat consumption is about 1.7 times along with power increase and obtaining pure carbon dioxide for carbomide production.
A theoretical basis for passing over to the energy-saving processes of gas absorption purification from CO2 is presented by a consequence from the Second Law of thermodynamics, namely, a requirement to reduce the moving forces at all the process stages in all apparatus points. This requirement is often understood in a simplified manner, i.e. as equilibrium approximation of one or two apparatus ends. However, it would be possible to design much more efficient facilities, increase their height, use more active catalysts, etc. Nevertheless, if the work-
Table 1
Several parameters of industrial processes for fine purification of hydrogen-containing synthesis-gas from CO2 under the pressure of 2.8 MPa with 18% of the CO2 content in the initial gas
Absorbent Process features Specific heat consumption, kcal/m3 CO2
1. MEA, 20% Conventional 1 flow of a saturated solution (SS), 1 flow of a re-generated solution (RS). 2600-3000
2. MEA, 20% 3 SS flows, 2 RS flows. Partial combination of heat exchange with CO2 desorption. 1250
3. MEA, 20% Full combination of heat exchange with desorption, 2 RS flows. 1000
4. MDEA, 40% Intermediate desorption of a part of CO2, 2 SS flows, 1 RS flow. 750
ISJAEE Special issue (2003)
Second International Symposium «Safety and Economy of Hydrogen Transport»
IFSSEHT-2003
ing (or equilibrium) process diagram is curvilinear one and in other similar cases characterized by a non-uniform moving force, the equilibrium would be reached at the beginning only at one or two points, when reducing the moving force. In the other points the moving force and, consequently, the thermodynamic irreversibility and the total power consumption can be significant.
Therefore a necessity arises to use more complicated methods for the process moving forces reduction, including the following:
— the use of schemes with separated absorbent flows;
— combination of heat transfer and mass transfer processes;
— the use of special mass-exchange devices.
In the limit, heat and material flows should be brought in and out over the whole apparatus height.
The Table 1 shows certain characteristics of the above-mentioned processes.
References
1. Очистка технологических газов. Под редакцией Т. А. Семёновой и И. Л. Лейтеса. М.:, Химия, 1977, изд. 2.
2. Справочник азотчика., том 1, изд. 2, 1986.
3. А. К. Аветисов и др. Опыт модернизации отделения абсорбционной очистки агрегата синтеза аммиака АМ-70 на ОАО «Невинномысский Азот» с заменой МЭА-раствора на МДЭА-абсорбент российского производства. Химическая промышленность сегодня. № 2, 2003, стр. 22-24.
4. I. L. Leites. Some Thermodynamic Bases for Design of Energy Saving Chemical Processes. ECOS 2000. Proceedings. Universiteit Twente. Nederland. Part 3. P. 1235-1245.
Для дальнейшего прочтения статьи необходимо приобрести полный текст. Статьи высылаются в формате PDF на указанную при оплате почту. Время доставки составляет менее 10 минут. Стоимость одной статьи — 150 рублей.