Material balance closure

A total material balance assay is a Fischer assay in which the retort gases are collected. A complete material balance closure and yields in excess of those expected from Fischer assay results are achieved. More complete descriptions of both the Fischer assay and the Tosco material balance assay methods have been reported (9). 

During this preparation stage, analysts will frequently find that there is insufficient quantity or quality of measurements to close the material balance. Analysts should make eveiy effort to measure aU stream flows and compositions for the actual test. They should not rely upon closing material balances by back-calculating missing streams. Tne material balance closure will provide a check on the vahdity of the measurements. This preparatoiy material balance will help to identify additional measurements and schedule the installation of the additional instruments. 

The second section of the spreadsheet contains the overall flows, the calculated component flows, and the material balance closure of each. The weighted nonclosure can be calculated using the random error calculated above, and a constraint test can be done with each component constraint if desired. Whether the measurement test is done or not, the nonclosure of the material balance for each component gives an indication of the validity of the overall flows and the compositions. If particiilar components are found to have significant constraint error, discussions with laboratory personnel about sampling and analysis and with instrument personnel about flow-measurement errors can take place before any extensive computations begin. 

We turn now to the issue of material balance closure. Material balances can be perfect when one of the flow rates and one of the components is unmeasured. The keen experimenter for Examples 7.1 and 7.2 measured the outlet concentration of both reactive components and consequently obtained a less-than-perfect balance. Should the measured concentrations be adjusted to achieve closure and, if so, how should the adjustment be done The general rule is that a material balance should be closed if it is reasonably possible to do so. It is necessary to know the number of inlet and outlet flow streams and the various components in these streams. The present example has one inlet stream, one outlet stream, and three components. The components are A, B, and I, where I represents all inerts. 

There is an extensive literature, particularly the Russian literature, on material balance closures. The procedures can be quite complicated. The following paper includes a review of die literature. 

Step 7 Write an Overall Material Balance. Remember that the P2 audit focuses on a unit process, but that there are individual unit operations that make up this process. The team will need to develop a series of material balances for each unit operation, and an overall material balance about the entire unit process, to bring closure to a solution of parameters of interest. The individual or component material balances developed may be summed to give a balance for the whole process, production area, or factory. 

Material balances during the depolymerization step were within 5% closure for all the experiments reported. 

In textbooks, the closure of every steady-state material balance [defined as (output/input) x 100%] is 100%. In practice, measurement imprecision and inaccurate assumptions may lead to closures that differ—possibly significantly—from 100%. Also, in practice there is no such thing as true steady state variable values always fluctuate or drift to some extent. 

There are other possibilities, but you get the idea. The point is that no matter how carefully you design a process, you cannot predict exactly what the real process will do. Approximations and assumptions must be made for every process design closures on real process material balances are never exactly 100% nothing can be measured with complete accuracy and everyone sometimes makes mistakes. 

Using the preliminary test run, perform material balances on the unit and each column Ensure that these close within 5 percent. Do not proceed with the performance test if the closure is worse than 10 percent (118). Leaking valves and exchanger tubes will affect the material balance, and these leaks need to be identified prior to the final test. Meters have happened to read incorrectly even at times when the instrument people were sure they were... 

Carry out an energy balance An incorrect reflux meter will not show up in the material balance. Ensure closure is similar to the material balance. 

Let us look at the overall material balance for the tower. The measured product rates in barrels per day are 18,130 of naphtha, 12,200 of kerosene, 9,968 of diesel, and 9,968 of unconverted oil, which gives a total of 50,266bpd. The difference between the measured total of 50,266 and actual feed rate of 51,548 is 3vol%. As the light end products are not measured, it usually accounts for around 3 vol% of feed. Thus, the material balance for the tower is in a good closure at less than 1% of uncertainty. 

Some useful and appropriate determinations for the feed-reject and permeate are as follows, albeit no material balances (and their closure)... 

It may be added that there is not closure on the overall material balances, since B/D as determined from the stream flow rates or fluxes does not agree with that determined from a mole fraction balance. Of course,... 

The pyrolysis experiments were conducted in an electrically heated, once-through tubular flow reactor, designed to simulate the time-temperature history experienced in commercial steam-cracking operations. Reactor effluent compositions were ascertained by gas chromatograph and mass spectrometer analyses. Material and hydrogen balances could always be effected, with typical closures of 98 2 wt %. 

Element mass balances were determined using both 1) the direct air flow (rotameters) and fuel feed rate measurements, and 2) by closing the carbon balance. The extent to which the element balances are closed by the two methods appears in Table 7 (closure on the carbon balance for the second technique is necessarily 100%). Excess air was S7% by the direct method and S6% via carbon balance using CO and CO2 concentrations measured by the continuous gas analyzer. Small amounts of ash were found in the bed material and most ash was deposited in the ash dropouts below the horizontal pass and the cyclone. A small fraction of carbon was found in ash and spent bed material. 

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