Reaction principle of chemical pulping-2

Residual lignin in pulp is not easy to measure directly because of its complex and volatile chemical structure. However, there is a good relationship between the lignin content and the number of certain oxidants consumed, especially those that react with double bonds but do not oxidize or dissolve carbohydrates. Such oxidants are potassium permanganate and chlorine. There are several internationally accepted methods of expressing residual lignin in pulp by measuring the consumption of oxidants under standard laboratory conditions.

The potassium permanganate value and the kappa value indicate the amount of potassium permanganate consumed by the lignin in the pulp. A commonly used method for determining residual lignin is the kappa value method. It measures the number of milliliters of 0.1 mol/L potassium permanganate solution consumed by 1 g of dry pulp at room temperature (25°C) under acidic conditions for 10 min. The measured value is converted to the value when 50% of the KMnO4 is consumed. The kappa values apply to all chemical slurries with yields below 70%. It should be noted that HexA, which is produced by polypentose during cooking, contributes to the kappa value because of the oxidation reaction between potassium permanganate and HexA. Therefore, the HexA content of hardwood pulp should be determined to facilitate the calculation of an accurate kappa value.

The degree of degradation of cellulose and hemicellulose retained in the pulp during the pulping process affects the strength of the fibers and also affects the papermaking properties. Determination of the characteristic viscosity of the dissolved solution of the pulp reflects the degree of chemical reaction of the fibers during the pulping process. Viscosity reflects the (average) degree of polymerization (DP) of cellulosic polymers, however, it does not give an indication of their distribution. Conversely, when the operating conditions of a process variable, the measured (average) DP gives a rough indication of the potential strength of the pulp, however, the relationship must be established empirically. However, since the relationship between strength and viscosity is not exact, DP cannot be used as a standard for comparing the strength of pulp from different fiber materials or different pulping methods.

The measurement of viscosity is based on the dissolution of lignin-free pulp in a solvent, usually a standard copper ethylenediamine solution. The degree of polymerization (average molecular size) can be estimated by measuring the characteristic viscosity of the solution with a standard capillary viscometer.

Several definitions are applied: the relative viscosity ηr is η/η0, where η is the viscosity of the solution and η0 is the viscosity of the solvent. The relative viscosity increase (sometimes referred to as specific viscosity) is defined as ηi = ηr – 1. The characteristic viscosity [η] represents the viscosity of the solution extrapolated to zero sample concentration (where the sample concentration tends to zero). The relationship between them is expressed by the Martin equation.

Here C is the concentration of dissolved pulp, K = 0.13.

The approximate relationship between characteristic viscosity and mean molar mass or degree of polymerization (DP) can be calculated by the Mark-Houwink equation.

Here and α are constants, depending on the molecular structure and solvent. For cellulose dissolved in copper ethylenediamine solution, α = 0.905, = 1.33.

The relationship between the different standard methods of measuring viscosity (DP) is shown in the figure below.

Relationship between pulp viscosity and degree of polymerization measured by different measuring methods.

Note: CED solution refers to copper ethylenediamine solution.

Accurate determination of the pulp yield can only be done by laboratory cooking. It is recommended to determine the total yield of a cleaned and homogeneous pulp, screen it with a laboratory standard screen of 0.15 mm or 0.25 mm, and determine the number of rejects. Thus the loss of fibers in the screening process can be avoided. The lower the screen residue, the better the homogeneity of the pulping.

Comparing different pulping methods and their effects on average fiber length and fiber length distribution is an effective way to evaluate their paper-making properties. There are several methods for determining fiber length. They are all based on optical measurements and image analysis of very small concentrations of pulp fiber suspensions as they flow through the capillary.

The analysis of the paper copying performance of the pulp is based on the simulation of a paper copying procedure to determine the performance of hand-copied paper sheets under standard laboratory conditions. The following diagram shows the laboratory paper copying procedure and the associated analysis and testing. The stock is pulped to different pulping degrees using standard pulping equipment. The degree of pulping can be determined directly from the stock

 (e.g., filter resistance or free degree). The wet strength can be measured by wet pressing the sheet. Standard manuscript sheets are made from pulped pulp samples and dried under certain conditions. The hand-copied sheets are cut into strips of paper and the strength is then measured. The dried sheet can continue to be calendered to determine the response of the pulp to high line pressure. There are several special methods for evaluating the papermaking properties of specialty pulp.

Pulp simulation test program

Certain properties of chemical pulp are usually measured. The Canadian Standard Freeboard reflects the filter resistance of the pulp. Wet tensile strength, tear strength, and tensile strength are the most common strength test items for paper. Structural properties, apparent density, permeability, and water absorption, as well as surface properties (after calendering), are the most important physical properties to be measured. Optical properties include ISO brightness, light absorption (coefficient), and light scattering (coefficient).

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