CHEMCAD and Solids The process of crystallization, fusion, and melting are too complex to approach with general rules. For most systems, only an empirical model is accurate. As such, process simulators do not inherently address solids. so lids. CHEMCAD provides two approaches to dealing d ealing with solids. A third approach is recommended as an analog model that is often used as a workaround Defined Solids Select Thermophysical > Solids > Identify Solid Components to define a component as a solid. The vapor pressure of solid components co mponents is ignored for flash calculations. In separation units, the solid components will always leave w ith the heavy liquid stream. Heat capacity data is taken for the solid, if present. You can select Thermophysical > Solids > Particle Size Distribution to specify the particle part icle size distribution (PSD) of a solid. The various solids handling unit operations o perations can be used with defined solids. Several solid separation UnitOps in CHEMCAD use the PSD for solid separation. Electrolyte Salts Using the true species electrolytes model, you can define an electrolyte as a po ssible precipitate. The electrolyte model will determine whether the salt precipitates precipitates during dur ing the flash calculation of a stream. A precipitated salt follows the same beha vior as a defined solid. If you have solubility data for a salt, it can be regressed into the electrolyte system. The heat o f solution for an electrolyte salt can be regressed into the e lectrolyte system. Analog Model If you have temperature-dependent solubility solubility data dat a for the system of your process, pro cess, you can regress temperature vs. solubility data into a kinetic expression. You can create a separate component as a solid; use a kinetic reactor to change the component to a solid component.
Baghouse
Filter (BAGH)
The BAGH module simulates or rates the operation o f a standard fabric filter dust collector. The module calculates the collection co llection efficiency and pressure drop or flow through the dust bed and fabric. Collection efficiency is defined by e ntering a particle size versus efficiency table. By defining the flow resistances of the bed and fabric and the cleaning cycle, the user may calculate
either the flow or the pressure drop. Specifications Tab Select Calculation Mode 0 =
Specify number of cells and time of filtration. Calculate pressure drop.
1 =
Specify number of cells and allowable pressure drop. Calculate filtration time.
2 =
Specify filtration time and allowable pressure drop. Calculate number of cells.
Number of cells The number of cells is required for rating calculations (mode 0 or 1). Time of filtration Filtration time is required for mode 0 or 2. Default is 30 minutes. Units are from engineering units setting. Pressure drop Maximum allowable pressure drop is required for mode 1 or 2. Cloth resist. factor Default value is 0.84 (in. H2O) / (centipoise) / (ft./min.). Pressure drop through the cloth is expressed as: Dc = Kc * vis * Vf where Dc
=
cloth pressure drop, in. H2O.
vis
=
fluid viscosity, centipoise.
Vf
=
superficial velocity of gas through cloth, ft/min.
Dust resist. factor Default value is 0.1 (in. H2O) / (centipoise) / (grains/ft2) / (ft/min). Pressure drop through the dust layer is expressed as: Dd = Kd * vis * M s * Vf
where Dd
=
dust pressure drop, in. H2O.
vis
=
fluid viscosity, centipoise.
Ms
=
Mass of solids per unit of cloth area, grains/ft2.
Vf
=
Superficial velocity of gas through cloth, ft/min.
Cells cleaned At any given time, a defined number of cells may be out of service for cleaning. This number is required for both design and rating calculations. Default value = 1. Bags
per cell
Required input, default = 78. Bag
diameter
Required input, default = 0.5 ft. Filter area per bag Required input, default = 16 ft2. Calculated Results Pressure drop Calculated pressure drop for mode 0. Efficiency Calculated overall efficiency, solids collected / solids in feed. Floor space Calculated floor space, all modes. Gas velocity Calculated gas velocity through active filters, all modes.
Performance table The baghouse model uses an empirical efficiency model since there is no usable theoretical model. You can enter a table of collection efficiencies versus particle size on Screen no. 2; otherwise, the Sylvan chart with the following efficiency data will be used: Particle Size (microns) 0.3 0.45 1.0 1.5 3.5 7.5 11.0 24.0
Efficiency 0.50 0.60 0.75 0.80 0.90 0.95 0.97 0.99
Topology A BAGH unit has one inlet and two outlets. Cleaned gas exits in the first outlet. Collected dust exits in the second outlet. Methods The method involves summing the pressure drops through the cloth (Dc) and the dust (Dd) according to the following equat ion: Dp = Dc + Dd Given the required input flow and its calculated viscosity, you can specify two o f the three variables: pressure drop (Dp), number of cells, or filtration time. Cell and bag geometry will convert flow rate into velocity for use in the pressure drop equations. Specific resistance parameters (Kc and Kd) are usually part of the users' and vendors' art and experience, but Perry's Handbook can provide some guidance.
