Liquefaction process control

NIOSH. 1980a. Control technology assessment for coal gasification and liquefaction processes, General Electric Co., Corporate Research and Development Center, Coal Gasification Section, Schenectady, New York. Cincinnati, OH U.S. Department of Health and Human Services, National Institute for Occupational Safety and Health, Division of Physical Sciences and Engineering. NITS publication no. PB84-181890. 

Perhaps the most important components of reactor solids are those that are generated during processing rather than those that are derived from inert minerals (quartz, clays) and macerals (fu-sinites, etc.) in the feed coal (74). The retention of these formed materials is more difficult to predict from the characteristics of the feed and, hence, control in liquefaction processes. 

With three or more stages for each refrigerant, the power consumption for the cascade cycle is found to compete quite favourable with other liquefaction processes, especially in arctic conditions. This is mainly because of the low flow rate of refrigerant. The cascade cycle is also more flexible in operation, since each circuit of refrigerant can be separately controlled. For the classical cascade, however, an overlap may occur of refrigerants like methane and ethylene. Thus, as methane condenses at an elevated pressure, this will inherently cause some throttling losses. 

The control and optimization of a nitrogen precooled hydrogen liquefaction process. 

The two key temperatures of the liquefaction process (inversion and liquid product temperatures) are controlled by TC-25 and TC-26. TC-25 modulates the level set point of LC-24 of the LN2 accumulator, and TC-26 adjusts the level set point of FC-27 of the LH2 accumulator. The LH2 (or nitrogen) supplies to the evaporators are controlled by cascade loops that adjust the levels in the accumulators, which in turn vary the heat transfer area, and therefore, the rate of evaporation. 

Results are presented and discussed in this paper outlining procedures for estimating mole fractional abundances of specified sulfur compounds from two readily feasible measurements, viz., pH and redox potential. Voltammetric and/or enthalpimetric (calorimetric) methods are described for the determination of several important inorganic sulfur compounds, as well as for the quantitation of the sulfur heterocycle dibenzothiophene in coal liquefaction products. These new approaches transcend classical capabilities 0-6), are based on quantitative theoretical correlations and are amenable to in-plant process control. 

Any hydrogen contained witliin the clilorine from the electrolyzer is concentrated in the residual gas from the liquefaction process and must not be allowed to exceed the explosive concentration limit of 5%. Although hydrogen concentration can be controlled by adding dry air to the process,... 

In the 1960s, two direct coal liquefaction processes were under development in the U.S. the Exxon Donor Solvent (EDS) process and the H-Coal process. The distinguishing feature of the EDS process was a separate solvent hydrogenation step to carefully control the hydrogen donor characteristics of the solvent. The most important feature of the H-Coal process was the emulated bed reactor in the process. 

The exhaust gases contain CH4, N2, H2O, tar, acidic and basic compounds (NH3, HCN, H2S) considered as impurities. Tar conversion has to be controlled to maximize the reliability of mechanical equipments and to assure the operation of the successive clean-up catalytic steps for final hydrogen separation and purification [64]. This step involves the utilization of additional steam and selective catalysts, affecting the overall efficiency of the process [65]. The operation with oxygen instead of air may improve the efficiency of the process but it suffers the costs associated with air liquefaction process, necessary for O2/N2 separation. 

It is difficult to determine an optimum time or temperature for the liquefaction process because reaction time and temperature are interactive variables. Reaction temperature is the most important factor according to our research, in agreement with work reported by Corbett and Richards (68) in terms of the amount of volatile products formed from cellulose. Inclusion of lignin and hemicellulose in a wood substrate may alter the optimum temperature, but should not alter the fact that reaction temperature is controlling. However, oil yield itself may not be a suitable parameter for measuring the efficiency of the liquefaction process. A high yield of oil may... 

Obviously, in the field of metallurgical coke production, the coking potential of a coal can be determined within fairly accurate limits if the quantities and types of macerals in the coal are known. Consequently, as more data became available concerning the reactivity of the various macerals during gasification or pyrolysis (liquefaction), the more efficient will be the process control and degree of conversion. 

Furthermore, when solvent treatment is applied to coal as liquefaction process the challenge is to (1) reject the minaal matta, (2) reduce the sulfur content of the products, (3) control the microstructure or macrostructure, and (4) control the chanistry. 

Hydrochloric acid may be purchased or produced internally. It is a widely available commodity, easily obtained in good quality. HCl is available in the anhydrous form as well as in the form of aqueous acid (up to 23° Be or about 37% HCl). The use of aqueous acid is standard in the chlor-alkali industry, and we do not discuss anhydrous HCl here. Byproduct acids are available, sometimes at lower prices, and may be suitable for use in the chlor-alkali process. Their quality should be checked carefully, and testing may be advisable before use. When HCl is produced from chlorine liquefaction tail gas, the absorbing water is the most likely source of impurities. Demineralized water is the standard source when producing acid for use in a membrane-cell chlorine plant. A certain amount of chlorine tends to be present in burner acid. This can be minimized by process control, and a small bed packed with activated carbon (Section 7.5.9.3B) is a useful safeguard. Usually only the acid intended for use in the ion-exchange system need be treated in this way. 

The importance of the latter lies in the reduced probability of surge. If, say, a downstream liquefaction process operates at a controlled back-pressure and the pressure increases slightly for some reason, a shallow performance curve might allow enough decrease in the flow to put die compressor into surge. 

It appears that for each type of biomass, liquefying conditions must be developed. These conditions may include liquefying reagents, catalysts, temperature and time. Different types of biomass contain different functional groups, which determine the potential use for the liquefied biomass. Currently, most research has been centered on how effective the liquefaction process is in terms of yield, time and energy use. Little effort has been made to understand the processes and the chemical profiles of the liquefied biomass, not to mention controlling the processes to produce liquefied biomass with desirable chemical profiles. 

Thermochemical conversion processes use heat in an oxygen controlled environment that produce chemical changes in the biomass. The process can produce electricity, gas, methanol and other products. Gasification, pyrolysis, and liquefaction are thermochemical methods for converting biomass into energy. 

After World War II, direct liquefaction of coal became uneconomical as the use of lower-cost petroleum products became more widespread. However, the German process of indirect coal liquefaction, the Fischer-Tropsch process, continued to hold some interest. The Fischer-Tropsch process first involved production of a carbon monoxide and hydrogen-rich synthesis gas by the controlled gasification of coal followed by a catalytic reaction process to yield a valuable mixture of hydrocarbon products. Simplified Fischer-Tropsch reactions are shown by the following equations ... 

Energy demand, the implementation of sulfur oxide pollution controls, and the future commercialization of coal gasification and liquefaction have increased the potential for the development of considerable supplies of sulfur and sulfuric acid as a result of abatement, desulfurization and conversion processes. Lesser potential sources include shale oil, domestic tar sands and heavy oil, and unconventional sources of natural gas. Current supply sources of saleable sulfur values include refineries, sour natural gas processing and smelting operations. To this, Frasch sulfur production must be added. 

Purification of Air Prior to Liquefaction. Separation of air by cryogenic fractionation processes requires removal of water vapor and carbon dioxide to avoid heat exchanger freeze-up. Many plants today are using a 13X (Na-X) molecular sieve adsorbent to remove both water vapor and carbon dioxide from air in one adsorption step. Since there is no necessity for size selective adsorption, 13X molecular sieves are generally preferred over type A molecular sieves. The 13X molecular sieves have not only higher adsorptive capacities but also faster rates of C02 adsorption than type A molecular sieves. The rate of C02 adsorption in a commercial 13X molecular sieve seems to be controlled by macropore diffusion 37). The optimum operating temperature for C02 removal by 13X molecular sieve is reported as 160-190°K 38). 

C. D. Kalfadelis and E. M. Magee, Evaluation of Pollution Control in Fossil Fuel Conversion Processes, Liquefaction, Section 1, COED Process, EPA-650/2-74-009e, Environmental Protection Agency, Washington, D.C., 1975. 

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