Use of \Citet{} within biblatex fails

Question

I am writing my thesis with by use of the classicthesis style by use of biblatex for my bibliography. biblatex offers a compatibility mode for natbib and all of the basic commands shall work according to the documentation of biblatex. However, the command \Citet{} which is important for capital letters of authors like "van Krevelen" or "de Nevers". In my case \Citet{} does the same like \citet{}

   \documentclass[
    twoside,openright,titlepage,numbers=noenddot,headinclude,
    footinclude=true,cleardoublepage=empty,
    headsepline,
    dottedtoc,
    BCOR=5mm,paper=a4,fontsize=11pt, 
    ngerman,american,
    ]{scrreprt} 


   \PassOptionsToPackage{utf8}{inputenc}
   \usepackage{inputenc}
   \usepackage[american]{babel}

   \PassOptionsToPackage{%
    backend=biber,
    isbn = false,
    doi = false,
    language=auto,
    style=authoryear-comp,
    sorting=nyt,
    maxbibnames=1,
    natbib=true
    }{biblatex}
    \usepackage{biblatex}

    \addbibresource{example.bib}

    \usepackage{classicthesis} 

    \begin{document}
        When citing with "Citet" one obtains for example: 
        \Citet{Krevelen.1950}.
    \end{document}

with example.bib:

@article{Krevelen.1950,
    author = {{van Krevelen}, D.W. and Hoftijzer, P.J.},
    year = {1950},
    title = {Studies of Gas Bubble Formation},
    volume = {46},
    number = {1},
    journal = {Chemical Engineering Progress}
}

Answer 1

You need to change two things.

You have specified author={{van Krevelen}, D.W.}. The braces mean that biblatex treats the author's family name as "van Krevelen" and will it sort under v (rater than k). biblatex assumes that the family name need not be capitalised even in \Citet, so nothing happens. Instead you want author={van Krevelen, D.W.}.

Then in order to get the "van" to print in your citation and get it to sort under v in your bibliography, pass the useprefix option to biblatex.

Aside: biblatex prefers the date field and the journaltitle field over year and journal.

MWE

\documentclass{article}
\usepackage{filecontents}
\begin{filecontents}{\jobname.bib}
@article{Krevelen.1950,
  author = {van Krevelen, D.W. and Hoftijzer, P.J.},
  date = {1950},
  title = {Studies of Gas Bubble Formation},
  volume = {46},
  number = {1},
  journaltitle = {Chemical Engineering Progress}
}
\end{filecontents}
\usepackage[american]{babel}
\usepackage{csquotes}
\usepackage[
  useprefix,
  isbn = false,
  doi = false,
  language=auto,
  style=authoryear-comp,
  sorting=nyt,
  maxbibnames=1,
  natbib]{biblatex}
\addbibresource{\jobname.bib}
\begin{document}
When citing with ``\verb+\Citet+'' one obtains for example: 
\Citet{Krevelen.1950}.
\end{document}