Purge number

In order to capture the impact of the purge stream on the dynamics of this generic process, let us define the purge number. 

Definition 4.1. The purge number of a material recycle loop in an integrated process is a process-wide dimensionless number expressed as the ratio of the steady-state flow rate of the purge and the process throughput, as captured by the flow rate of the process feed stream ... 

Assumption 4.1. The nominal flow rate of the purge stream is much smaller than that of the feed stream, i.e., the purge number of the process is very small ... 

Let us concentrate on the model in Equation (4.18) and consider the limit case of a zero purge number, i.e., e —> 0. Physically, this limit corresponds to setting the... 

It is also easy to verify that the total inventory of I is influenced by the inflow of impurity, its net removal rate in the separator, and the flow rate of the purge stream. In the limit of an infinitely low purge number, these are set to zero and, as can be seen from Equation (4.20), clearly have no influence on the dynamics... 

Let us consider again the limit of an infinitely small purge number (e —> 0), this time in the newly defined slow time scale. This yields the algebraic equations... 

We showed that this dynamic behavior originates, in effect, from the large discrepancies between the flow rates of the different material streams in the process (captured by the recycle and purge numbers), thereby establishing a clear and causal connection between the steady-state design and the dynamic response of an integrated process. 

Feed purification. Impurities that enter with the feed inevitably cause waste. If feed impurities undergo reaction, then this causes waste from the reactor, as already discussed. If the feed impurity does not undergo reaction, then it can be separated out from the process in a number of ways, as discussed in Sec. 4.1. The greatest source of waste occurs when we choose to use a purge. Impurity builds up in the recycle, and we would like it to build up to a high concentration to minimize waste of feed materials and product in the purge. However, two factors limit the extent to which the feed impurity can be allowed to build up ... 

There has been an increasing interest in utilising off-gas technology to produce ammonia. A number of ammonia plants have been built that use methanol plant purge gas, which consists typically of 80% hydrogen. A 1250 t/d methanol plant can supply a sufficient amount of purge gas to produce 544 t/d of ammonia. The purge gas is first subjected to a number of purification steps prior to the ammonia synthesis. 

Table 4 summarizes commercial and precommercial gas separation appHcations (86,87). The first large-scale commercial appHcation of gas separation was the separation of hydrogen from nitrogen ia ammonia purge gas streams. This process, launched ia 1980 by Monsanto, was followed by a number of similar appHcations, such as hydrogen—methane separation ia refinery off-gases and hydrogen—carbon monoxide adjustment ia oxo-chemical synthetic plants. 

Ethylene is recycled, some of which is purged, to eliminate the accumulation of ethane. The olefins are distilled into even-carbon-number fractions... 

The designer usually wants to specify stream flow rates or parameters in the process, but these may not be directly accessible. For example, the desired separation may be known for a distiUation tower, but the simulation program requires the specification of the number of trays. It is left up to the designer to choose the number of trays that lead to the desired separation. In the example of the purge stream/ reactor impurity, a controller module may be used to adjust the purge rate to achieve the desired reactor impurity. This further complicates the iteration process. 

Typical pilot-plant top-suspended baskets are 305 mm (12 in) diameter by 127 mm (5 in) deep. Commercial machines are available in sizes from 508-mm (20-in) diameter by 305-mm (l2-in) depth to 1524-mm (60-in) diameter by 1016-mm (40-in) depth and develop up to 1800g in the smaller and intermediate sizes. Except in the sugar apphcation, operation with a two-speed motor (half speed for loading and full speed for purging) is typical. Hydraulic drives with variable-speed capability are commonly used in the chemical industiy To maximize the number of cycles per hour, a combination of electrical and mechanical braking is employed to minimize the deceleration period, which is a transition period of no value to the process. 

To determine the number of purge cycles and achieve a specified component concentration after j purge cycles of pressure (or vacuum) and relief [29] ... 

Fuel gas to fuel gas purging The purge may be carried out by burning the gas under supervision on an appliance or appliances. If the two fuel gases have different burning characteristics and Wobbe numbers, correct burner selection is important. 

Shown in Figure 15.5 are the temperature dependent XRD data for the 5% Pd-1% Sn catalyst. As noted above, the scans were offset in the order that they were obtained (the Time axis, as shown, is the scan sequence number and not the actual temperature). The inset of Figure 15.5 illustrates the temperature profile for the scan sequence. The first scan was obtained at room temperature, at which time hydrogen was introduced into the chamber at 500 Torr. The temperature was then ramped in 10°C increments to 160°C and XRD scans were taken after each increment. The sample was held at 160°C for I/2 hour, and then cooled to room temperature. After I/2 hour at room temperature, the sample was purged with dry nitrogen. 

Figure 4.3 Schematic of an off-line purge-and-trap apparatus. After Cole and Woolfenden [208a]. Reprinted with permission from LC.GC, Vol. 10, Number 2, February 1992, pp. 76-82. LC.GC is a copyrighted publication of Advanstar Communications Inc. All rights reserved...

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