Purge optimum rate

Because of the differences in the construction of various purge and trap devices, actual recoveries may vary significantly from those shown in Figure 3 and Table I. Therefore it is required that individual investigators determine recoveries of compounds to be measured as a function of flow rate with their apparatus. Operation in the optimum flow rate range will assure maximum sensitivity and precision for the compounds measured. 

The operational conditions of the purge and trap must be compatible with the configuration of the GC system. A high carrier gas (desorb gas) flow rate can be used with a packed GC column. The trap desorption time is short at the high flow rate, producing a narrowband injection. The optimum flow is about 50 mL/min. Capillary columns are generally preferred over packed columns for better resolution, but these columns require lower flow rate. 

Reineccius and Liardon [207] studied volatiles evolved from heated thiamine solutions. Samples of 2% thiamine hydrochloride in various 0.2M buffers were heated under various conditions. A temperature of 40°C and a sampling time of 45 min were found to minimize artifact formation and yet produce sufficient volatiles for analysis. Nitrogen was used as the purge gas at a flow rate of 50 ml/min. Several materials were evaluated as absorbents, with graphite found to be the optimum. A microwave desorption system was used to rapidly desorb the trapped volatiles onto a fused silica capillary column. Twenty-five compounds were identified in the headspace of the heated thiamine solutions. 

The mercury vapor produced by bubbling the sulfur dioxide through a mercurous nitrate solution was detected by the gold-coated crystal due to the formation of a mercury amalgam. The optimum conditions were temperature = 27°C flow rate = 55 ml/min [Hg2 ] = 4 x 10 M purge time = 10 min. The sensitivity of the detector depended on the sample size, and linear calibration curves in the concentration... 

The purge gas is burned as fuel in the primary reformer furnace, reducing the total fuel requirement for the reformer. Additional hydrogen must be produced to account for the purge gas loss and this increases the feed rate and the size of the reformer furnace. Nevertheless, the overall economics of the PSA unit, due in part to more efficient heat recovery, are superior to the conventional process with the low temperature shift and methanation reactors. Also, because the PSA unit removes all of the methane and carbon oxides from the hydrogen product, the operation of the reformer is independent of hydrogen product purity and, thus, can be operated at optimum conditions [4]. 

Apparatus—The structure of a TG-DTA-FTIR instrument is shown in Figure 2.40. For optimum performance the lowest purge gas flow rate possible increases the concentration of product gases, while avoiding secondary gas-phase... 

Experimental variables such as sample volume, stripping time and gas flow rates are interrelated, and depend on the dimensions of the purge vessel and column as well as on the design of the FPD. Thus, the performance of each analytical system should be assessed individually. Using the above specified equipment and 3-12 mL samples, the author obtained optimum system performance with the following settings, which may serve as a guideline for system optimisation. 

The purpose of temperature calibration is to match the thermocouple readings to the true temperature. In a TMA instrument, a single thermocouple is used to control and measure the temperature of the sample in the furnace. The conditions at which the temperature calibration is carried out [i.e., the measurement mode (compression, tension, etc.), heating rate, flow rate of the purge gas, the purge gas itself, etc.] should be identical to those of the measurements on actual samples. Also, the geometry of the calibration standard should be as similar as possible to the geometry of the samples. This means that for optimum calibration, film standards must be used for measurements on films, pellets for pellets, and fibers for fibers (Lotti and Canevarolo 1998 Mano and Cahon 2004). 

Methane and inert gases, mainly argon, accumulate in the loop and suppress the partial pressure of the reactants. The optimum inerts level is a balance between increased power consumption with a high inerts concentration and reduced feedstock efficiency if the inerts level is lowered by increasing the purge rate. 

In. systems where the sulfur is reslurried and sent to a melter, there are no filtercake solution losses, but some blowdown is necessary to purge salts from the system. Since the required blowdown rate depends on the process tendency to form undesirable by-products, mainly thiosulfate, the selection of operating conditions that lower by-product formation will push the optimum iron content of the solution toward higher concentration values and lower circulation rates (Hardison and Ramshaw, 1990). 

Control of many modern ammonia plants is accomplished via gas analyzers and on-line computers to adjust conditions for optimum performance. The control strategy includes adjustments to the H2/N2 ratio, purge rate, and reformer furnace control. Heat recovery during regeneration of the catalysts also is included. 

---