Basis and Space Velocity

Although the intent of this chapter is not detailed design, it is in order to state what is included in a proper design basis, for example at least these items  

Rate of reaction, preferably in equation form, relating it to composition, temperature, pressure, impurities, catalysts and so on. Alternately tabular or graphical data relating compositions to time and the other variables listed. 

The most common versions of space velocities in typical units are  

GHSV (gas hourly space velocity) = (volumes of feed as gas at STP/hr)/(volume of the reactor or its content of catalyst) = (SCFH gas feed)/cuft. 

In practical cases reaction times vary from fractions of a second to many hours. The compilation of Table 17.1 of some commercial practices may be a basis for choosing by analogy an order of magnitude of reactor sizes for other processes. 

For ease of evaluation and comparison, an apparent residence time often is used instead of the true one it is defined as the ratio of the reactor volume to the inlet volumetric flow rate, 

On the other hand, the true residence time must be found by integration. 

---

Design Basis and Space Velocity 549 Design Basis 549... 

This accounting procedure was verified by repeating a set of experiments conducted early in the run (at constant CO partial pressure, Pco = 4.93 atm, H2/CO = 2.4 and space velocities = 15.0, 12.5, 10.0, 8.0,5.0 and 3.0 SL/h/gcat) and at the end of the kinetic study with identical conditions. The deactivation-adjusted CO hydrogenation rates are very similar to the activity of the starting set of experiments (Table 4). The CO conversion and product distributions from the raw reaction data were adjusted for each individual sample to that of a fresh catalyst basis. Since reactor wax samples were not representative due to the short time at each reaction condition, the rate of C5+... 

An examination of some laboratory runs with diluted C150-1-02 catalyst can illustrate this problem. In one run with 304°C at inlet, 314 °C at exit, and 97,297 outlet dry gas space velocity, the following results were obtained after minor corrections for analytical errors. Of the CO present (out of an inlet 2.04 mole % ), 99.9885% disappeared in reaction while the C02 present (from an initial 1.96%) increased by over 30%. Equilibrium carbon oxides for both methanation reactions were essentially zero whereas the equilibrium CO based on the water-gas shift reaction at the exit composition was about one-third the actual CO exit of 0.03 mole %. From these data, activities for the various reactions may be estimated on the basis of various assumptions (see Table XIX for the effect of two different assumptions). 

Space Velocity. Most of our experimental data were developed with operation at a wet outlet space velocity of approximately 10,000 vol/vol hour. However, we do have data at space velocities of up to 25,000/hr. The pilot plant will operate at a space velocity of 5,000/hr while processing 1 million scf raw syngas/day. With operation on a once-through basis without recycle and at the indicated space velocities, catalyst volumes are minimum compared with other processes when identical over-design factors are used. 

Conversion efficiencies of the dynamometer-aged catalysts were measured in a standard A/F sweep test on an engine dynamometer [6]. The sweep experiments were carried out at 450 and 85,000 h space velocity (volumetric basis standard conditions). The sweep ranged from 0.5 A/F lean of stoichiometry to 0.5 A/F rich of stoichiometry with imposed A/F perturbations of+.0.5 A/F at 1 Hz. After sweep evaluation, small samples of catalyst were renrroved from the front region of the brick for chemisorption and flow reactor experiments. 

The MDA experiments were performed in a continuous down-flow fixed bed reactor at 700°C, atmospheric pressure, and a space velocity of 1500 em3/(gcat h). Catalysts were pretreated in He flow at 700°C for 30 min before feeding a CH4 N2 mixture in a 9 1 voEvol ratio (N2 used as internal standard). Unreacted methane, the reference N2, and the reaction products were analyzed on line in a gas chromatograph (HP-GC6890) as detailed in [6]. Product selectivities are given on a carbon basis. The use of N2 as internal standard allows to obtain the amount of carbonaceous residues as the amount of carbon required to close the mass carbon balances to 100%. 

However, the detailed description of the FT product distribution together with the reactant conversion is a very important task for the industrial practice, being an essential prerequisite for the industrialization of the process. In this work, a detailed kinetic model developed for the FTS over a cobalt-based catalyst is presented that represents an evolution of the model published previously by some of us.10 Such a model has been obtained on the basis of experimental data collected in a fixed bed microreactor under conditions relevant to industrial operations (temperature, 210-235°C pressure, 8-25 bar H2/CO feed molar ratio, 1.8-2.7 gas hourly space velocity, (GHSV) 2,000-7,000 cm3 (STP)/h/gcatalyst), and it is able to predict at the same time both the CO and H2 conversions and the hydrocarbon distribution up to a carbon number of 49. The model does not presently include the formation of alcohols and C02, whose selectivity is very low in the FTS on cobalt-based catalysts. 

When the best catalyst has been chosen and found to fulfil the requirements with respect to activity, strength, pressure drop, production and profitability, a procedure must be developed for calculation of the catalyst volume required to obtain a given SO2 conversion in an industrial reactor. In its simplest form, the calculation basis can be a table or an expression for space velocity (NHSV) as a function of feed gas properties and final conversion. A more detailed approach is used for design of catalytic reactors at Haldor Topsoe, where a rate expression of the form... 

In most of what follows, we deal with the space-velocity and space-time based on feed at actual entering conditions however, the change to any other basis is easily made. 

Catalytic evaluation of the different pillared clays was performed using a microactivity test (MAT) and conditions described in detail elsewhere (5). The weight hourly space velocity (WHSV) was 14-15 the reactor temperature was 510 C. A catalyst-to-oil ratio of 3.5-3.8 was used. The chargestock s slurry oil (S.O., b.p. >354 C), light cycle oil (LCGO, 232 C < b.p. <354 C) and gasoline content were 62.7 vol%, 33.1 vol% and 4.2 vol% respectively. Conversions were on a vol% fresh feed (FF) basis and were defined as [VfVp/V ] x 100, where is the volume of feed... 

For the activity comparisons, the heteroatom removals (and CCR reduction) are plotted versus reactor temperature at a liquid hourly space velocity of 1.0. Consequently, catalyst activity can be compared on the basis of temperature requirements for achieving specific liquid product heteroatom (or CCR) contents. 

When data are collected under several different sets of conditions, and those conditions can be expressed in quantitative terms, effective data summarization often takes the form offitting an approximate equation to the data. As the basis of a simple example of this, consider the data in Table 5.2. The variable x, hydrocarbon liquid hourly space velocity, specifies the conditions under which information on the response variable y, a measure of isobutylene conversion, was obtained in a study involving the direct hydration of olefins. 

By using the reciprocal space velocity as an independent variable, the adiabatic reactor can be treated as a one-dimensional reactor. Equations (3-31) and (3-8) provide the basis for the calculation. Put in terms of the virtual conversions, these equations are... 

A catalytic process was designed to make 150 metric tons per year of product with a net profit of 0.25/lb of product. The catalyst for the process costs 10/lb and it takes 2 months to shut down the process, empty the old catalyst, reload fresh catalyst, and restart the process. The feed and product recovery and purification sections can be pushed to make as much as 120% of design basis capacity. The reactor section is sized with sufficient catalyst to make 100% of design basis when operated with fresh catalyst at 500°F. The reactor can be operated at temperatures only up to 620°F, for safety reasons. The reactor weight hourly space velocity (lb of product per hour per lb of catalyst) is given by the equation... 

Catalytic reactions were carried out with 2 g catalyst placed in a fixed-bed continuous-flow reactor at the gas space velocity (F/W) of 1440 ml/g h under the reaction pressure of 200 KPa. The products were withdrawn periodically from the outlet of the reactor and analyzed by gas chromatography with a 4 m long squalane column and detected by a hydrogen flame ionization detector. The conversion and selectivity were calculated on the carbon number basis. 

However, the benzene and xylenes were measured and pumped separately into the system as liquids by means of a proportioning pump. Hence the space velocity was reported on a liquid-hourly basis that is, as the ratio of the feed rate, in cubic centimeters of liquid per hour, to the total volume of the reactor, in cubic Centimeters. The feed consisted of an equimolal mixture of reactants, and the liquid rates were corrected to 60°F before reporting the following information ... 

---