Back-blending

In the production of dissolved pulp, special emphasis is placed on the quality of the pulp, especially with respect to fibre length. Unlike many other processes, where back-blending of product can ensure adherence to product quality requirements, this is not possible in the case of dissolved pulp production. 

Hands-on workshops on model building using back-blending procedure (Section 2.11) and investigating new product profiles with new product demands (Section 2.12)... 

The most important factor in the success of any model of crade columns is an accurate representation of the crude feed. There are two ways to quantify the crude feed to the unit The first method relies on the availability of cmde assays and knowledge of the ratios of the crude mixes fed to the unit This is particularly useful when a column only processes a few types of crude. The other method uses current column product yield and qualities to back-mix or back-blend these products. The goal o( back-blending is to recover the composition of the crude fed to the column. This method is very useful when we have little information of the crude fed to the column or the assays are too old and unreliable. Kaes (1) provides methods to estimate missing data when back-blending data for modeling. 

This section documents some of the key steps required to model the representative crude unit described in the previous section. We use a 50%-50% mixture of the assays presented in Table 2.5 to Table 2.8 as the feed to the column. In the workshop examples accompanying this chapter, we will simulate this column on the basis of back-blending alone and compare the results. 

Product yields It is not possible to match yields given a poor representation of feed. Feed crude assays may be too old or inaccurate to represent current operation. Back-blending the products may be the only way to correctly represent column feed. 

Workshop 2.1 -Rebuild Model Using Back-blending Procedure... 

Workshop 2.7 -Rebuild Model Using Back-blending" Procedure 99... 

Table 2.14 Product yield and properties required for back-blending".

Figure 2.78 Comparison of temperature profile from back-blending" and traditional assay procedure.

Matching plant results with back-blending is generally more difficult The most direct way to improve results is to obtain a distillation curve for the residue product The residue product is a significant portion of the crude unit effluent and is quite heavy. These heavy components can affect the distribution of light components through all plant cuts. 

High-quality assay data for the atmospheric residue are always desirable in modeling a VDU. For VDU simulation, there are three ways to obtain assay data of the atmospheric residue (1) stream results of ADU simulation ifwe build the ADU and VDU models together (2) analysis of the atmospheric residue and (3) back-blending the assay data of VDU product streams if product analyses are available. 

We need to consider a number of issues when applying each approach to represent the feed for modeling purposes. Specifically, when we build the ADU and VDU models together, the representation of the atmospheric residue is reliable if a detailed feed representation of crude oil is available (by either crude assay or back-blending product analysis) and the ADU model performs correctly. 

Figure 3.4 Comparison between feed analysis and back-blend of product analyses.

Figure 4.9 Back-blending products to reconstitute FCC reactor effluent.

Using the back-blending concept from the previous section, develop a FCC effluent TB P curve from a reference set of product yields. These yields include all liquid products such as light and heavy naphtha, light and heavy cycle oil or diesel, slurry or decant oil. 

Fit a cumulative beta distribution to this back-blended reference TBP curve and obtain the best values for the cumulative beta distribution fit. We calculate this initial set of parameters only once. 

Develop fractionation models by back-blending the measured reactor products... 

Workshop 1 from Chapter 2 for back-blending crude feeds... 

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